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2016-10-18 | Light emission, light detection and strain sensing with nanocrystalline graphene | Graphene is of increasing interest for optoelectronic applications exploiting
light detection, light emission and light modulation. Intrinsically light
matter interaction in graphene is of a broadband type. However by integrating
graphene into optical micro cavities also narrow band light emitters and
detectors have been demonstrated. The devices benefit from the transparency,
conductivity and processability of the atomically thin material. To this end we
explore in this work the feasibility of replacing graphene by nanocrystalline
graphene, a material which can be grown on dielectric surfaces without catalyst
by graphitization of polymeric films. We have studied the formation of
nanocrystalline graphene on various substrates and under different
graphitization conditions. The samples were characterized by resistance,
optical transmission, Raman, X-ray photoelectron spectroscopy, atomic force
microscopy and electron microscopy measurements. The conducting and transparent
wafer-scale material with nanometer grain size was also patterned and
integrated into devices for studying light-matter interaction. The measurements
show that nanocrystalline graphene can be exploited as an incandescent emitter
and bolometric detector similar to crystalline graphene. Moreover the material
exhibits piezoresistive behavior which makes nanocrystalline graphene
interesting for transparent strain sensors. | 1610.05513v1 |
2017-02-01 | Pore ordering in mesoporous matrices induced by different directing agents | Mesoporous silica particles of MCM-41 type were synthesized by sol-gel method
from tetraethyl orthosilicate (TEOS) in 2-methoxyethanol and deionized water
mixture in base conditions at room temperature. Ammonia or sodium hydroxides
were used as catalysts and cetyl-trimethylammonium bromide (CTAB) and
n-dodecyl-trimethylammonium bromide (DTAB) as structure directing agents. The
porosities and the ordered structure have been analyzed using transmission and
scanning electron microscopy, small angle neutron and X-ray diffraction,
nitrogen adsorption, thermal analysis and FTIR spectroscopy. The samples
consist of spherical particles of sub-micrometer size, with radially arranged
pores. The comparison of the effect of the different surfactants and catalysts
shows that by varying the surfactant type and their proportion, the pore sizes
can be controlled. As compared to the commonly used ammonia catalyst, the use
of NaOH as catalyst results in a much smaller porosity of the as-prepared
materials. These materials are not resisting to the heat treatment at 700 C
used for the template removal, and the ordered porous structure is completely
lost. | 1702.00366v1 |
2017-10-27 | Materials considerations for forming the topological insulator phase in InAs/GaSb heterostructures | In an ideal InAs/GaSb bilayer of appropriate dimension in-plane electron and
hole bands overlap and hybridize, and a topologically non-trivial, or quantum
spin Hall (QSH) insulator, phase is predicted to exist. The in-plane
dispersion's potential landscape, however, is subject to microscopic
perturbations originating from material imperfections. In this work, the effect
of disorder on the electronic structure of InAs/GaSb bilayers was studied by
the temperature and magnetic field dependence of the resistance of a dual-gated
heterostructures gate-tuned through the inverted to normal gap regimes.
Conduction in the inverted (predicted topological) regime was qualitatively
similar to behavior in a disordered two-dimensional system. The impact of
charged impurities and interface roughness on the formation of topologically
protected edge states and an insulating bulk was estimated. The experimental
evidence and estimates of disorder in the potential landscape indicated the
potential fluctuations in state-of-the-art films are sufficiently strong such
that conduction in the predicted topological insulator (TI) regime was
dominated by a symplectic metal phase rather than a TI phase. The implications
are that future efforts must address disorder in this system and focus must be
placed on the reduction of defects and disorder in these heterostructures if a
TI regime is to be achieved. | 1710.10095v1 |
2014-08-05 | Emergent surface superconductivity of nanosized Dirac puddles in a topological insulator | Surfaces of three-dimensional topological insulators have emerged as one of
the most remarkable states of condensed quantum matter1-5 where exotic
electronic phases of Dirac particles should arise1,6-8. Here we report a
discovery of surface superconductivity in a topological material (Sb2Te3) with
resistive transition at a temperature of ~9 K induced through a minor tuning of
growth chemistry that depletes bulk conduction channels. The depletion shifts
Fermi energy towards the Dirac point as witnessed by about two orders of
magnitude reduction of carrier density and by very large (~25,000 cm^2/Vs)
carrier mobility. Direct evidence from scanning tunneling spectroscopy and from
magnetic response show that the superconducting condensate forms in surface
puddles at unprecedentedly higher temperatures, near 60 K and above. The new
superconducting state we observe to emerge in puddles can be tuned by the
topological material's parameters such as Fermi velocity and mean free path
through band engineering; it could potentially become a hunting ground for
Majorana modes6 and lead to a disruptive paradigm change9 in how quantum
information is processed. | 1408.1046v1 |
2019-07-31 | Coexisting 1T/2H polymorphs, reentrant resistivity behavior, and charge distribution in MoS2-hBN 2D/2D composite thin films | In view of their immensely intriguing properties, two dimensional materials
are being intensely researched in search of novel phenomena and diverse
application interests, however, studies on the realization of nanocomposites in
the application-worthy thin-film platform are rare. Here we have grown MoS2-hBN
composite thin films on different substrates by the pulsed laser deposition
technique and made comparative studies with the pristine MoS2 and hBN films.
The Raman, XPS and HRTEM confirm the concomitant presence of both the 1T
(conducting) and 2H (semiconducting) polymorphs of MoS2 in the composite film.
Interestingly, a peculiar reentrant semiconductor-metal-insulator transition is
seen in the composite film which is absent in the MoS2 film, and it correlates
well with the signatures of phonon softening seen in temperature-dependent
Raman spectroscopy. Furthermore, electrostatic force microscopy reveals the
presence of three distinct regions (metallic, semiconducting, and insulating)
in the composite film with differing contact potentials and enhanced propensity
for charge transfer with respect to pristine MoS2. A triboelectric
nanogenerator device containing biphasic composite film as an electron acceptor
exhibits more than twofold (sixfold) enhancement in peak-to-peak output voltage
as compared to the pristine MoS2 (hBN) film. These observations bring out the
potential of nanocomposite thin films for unfolding emergent phenomena and
technological applications. | 1907.13522v1 |
2022-02-21 | Towards fully two-dimensional spintronic devices | Within the field of spintronics major efforts are directed towards developing
applications for spin-based transport devices made fully out of two-dimensional
(2D) materials. In this work we present an experimental realization of a
spin-valve device where the generation of the spin signal is exclusively
attributed to the spin-dependent conductivity of the magnetic graphene
resulting from the proximity of an interlayer antiferromagnet, CrSBr. We
clearly demonstrate that the usage of the conventional 3D magnetic contacts,
that are commonly air-sensitive and incompatible with practical technologies,
can be fully avoided when graphene/CrSBr heterostructures are employed.
Moreover, apart from providing exceptionally long spin relaxation length, the
usage of graphene for both generation and transport of the spin allows to
automatically avoid the conductivity mismatch between the source and the
channel circuits that has to be considered when using conventional
low-resistive contacts. Our results address a necessary step in the engineering
of spintronic circuitry out of layered materials and precede further
developments in the area of complex spin-logic devices. Moreover, we introduce
a fabrication procedure where we designed and implemented a recipe for the
preparation of electrodes via a damage-free technique that offers an immediate
advantage in the fields of air-sensitive and delicate organic materials. | 2202.09972v1 |
2016-03-10 | Möbius Kondo Insulators | Heavy fermion materials have recently attracted attention for their potential
to combine topological protection with strongly correlated electron physics. To
date, the ideas of topological protection have been restricted to the heavy
fermion or "Kondo" insulators with the simplest point-group symmetries. Here we
argue that the presence of nonsymmorphic crystal symmetries in many heavy
fermion materials opens up a new family of topologically protected heavy
electron systems. Re-examination of archival resistivity measurements in
nonsymmorphic heavy fermion insulators Ce$_3$Bi$_4$Pt$_3$ and CeNiSn reveals
the presence of low temperature conductivity plateau, making them candidate
members of the new class of material. We illustrate our ideas with a specific
model for CeNiSn, showing how glide symmetries generate surface states with a
novel Mobius braiding that can be detected by ARPES or non-local conductivity
measurements. One of the interesting effects of strong correlation, is the
development of partially localization or "Kondo breakdown" on the surfaces,
which transforms Mobius surface states into quasi-one dimensional conductors,
with the potential for novel electronic phase transitions. | 1603.03435v4 |
2017-07-14 | Mesoscale simulations of confined Nafion thin films | The morphology and transport properties of thin films of the ionomer Nafion,
with thicknesses on the order of the bulk cluster size, have been investigated
as a model system to explain the anomalous behaviour of
catalyst/electrode-polymer interfaces in membrane-electrode assemblies. We have
employed dissipative particle dynamics (DPD) to investigate the interaction of
water and fluorocarbon chains with carbon and quartz as confining materials for
a wide range of operational water contents and film thicknesses. We found
confinement-induced clustering of water perpendicular to the thin film.
Hydrophobic carbon forms a water depletion zone near the film interface,
whereas hydrophilic quartz results in a zone with excess water. There are, on
average, oscillating water-rich and fluorocarbon-rich regions, in agreement
with experimental results from neutron reflectometry. Water diffusivity shows
increasing directional anisotropy of up to 30% with decreasing film thickness,
depending of the confining material. The percolation analysis revealed
significant differences in water clustering and connectivity with the confining
material. These findings indicate the fundamentally different nature of ionomer
thin films, compared to membranes, and suggest explanations for increased ionic
resistances observed in the catalyst layer. | 1707.04604v3 |
2017-07-27 | SRF Theory Developments from the Center for Bright Beams | We present theoretical studies of SRF materials from the Center for Bright
Beams. First, we discuss the effects of disorder, inhomogeneities, and
materials anisotropy on the maximum parallel surface field that a
superconductor can sustain in an SRF cavity, using linear stability in
conjunction with Ginzburg-Landau and Eilenberger theory. We connect our
disorder mediated vortex nucleation model to current experimental developments
of Nb$_3$Sn and other cavity materials. Second, we use time-dependent
Ginzburg-Landau simulations to explore the role of inhomogeneities in
nucleating vortices, and discuss the effects of trapped magnetic flux on the
residual resistance of weakly- pinned Nb$_3$Sn cavities. Third, we present
first-principles density-functional theory (DFT) calculations to uncover and
characterize the key fundamental materials processes underlying the growth of
Nb$_3$Sn. Our calculations give us key information about how, where, and when
the observed tin-depletedregions form. Based on this we plan to develop new
coating protocols to mitigate the formation of tin depleted regions. | 1707.09025v1 |
2019-12-13 | Strength, transformation toughening and fracture dynamics of rocksalt-structure Ti1-xAlxN (0 <= x <= 0.75) alloys | Ab initio-calculated ideal strength and toughness describe the upper limits
for mechanical properties attainable in real systems and can, therefore, be
used in selection criteria for materials design. We employ density-functional
ab initio molecular dynamics (AIMD) to investigate the mechanical properties of
defect-free rocksalt-structure (B1) TiN and B1 Ti1-xAlxN (x = 0.25, 0.5, 0.75)
solid solutions subject to [001], [110], and [111] tensile deformation at room
temperature. We determine the alloys' ideal strength and toughness, elastic
responses, and ability to plastically deform up to fracture as a function of
the Al content. Overall, TiN exhibits greater ideal moduli of resilience and
tensile strengths than TiAlN solid solutions. Nevertheless, AIMD modelingshows
that, irrespective of the strain direction, the binary compound systematically
fractures by brittle cleavage at its yield point. The simulations also indicate
that Ti0.5Al0.5N and Ti0.25Al0.75N solid solutions are inherently more
resistant to fracture and possess much greater toughness than TiN, due to the
activation of local structural transformations (primarily of B1 -> wurtzite
type) beyond the elastic-response regime. In sharp contrast, TiAlN alloys with
25% Al exhibit similar brittleness as TiN. The results of this work are
examples of the limitations of elasticity-based criteria for prediction of
strength, brittleness, ductility, and toughness in materials able to undergo
phase transitions with loading. Furthermore, comparing present and previous
findings, we suggest a general principle for design of hard ceramic solid
solutions that are thermodynamically inclined to dissipate extreme mechanical
stresses via transformation toughening mechanisms. | 1912.06367v2 |
2015-04-02 | Thermoelectric Signal Enhancement by Reconciling the Spin Seebeck and Anomalous Nernst Effects in Ferromagnet/Non-magnet Multilayers | The utilization of ferromagnetic (FM) materials in thermoelectric devices
allows one to have a simpler structure and/or independent control of electric
and thermal conductivities, which may further remove obstacles for this
technology to be realized. The thermoelectricity in FM/non-magnet (NM)
heterostructures using an optical heating source is studied as a function of NM
materials and a number of multilayers. It is observed that the overall
thermoelectric signal in those structures which is contributed by spin Seebeck
effect and anomalous Nernst effect (ANE) is enhanced by a proper selection of
NM materials with a spin Hall angle that matches to the sign of the ANE.
Moreover, by an increase of the number of multilayer, the thermoelectric
voltage is enlarged further and the device resistance is reduced,
simultaneously. The experimental observation of the improvement of
thermoelectric properties may pave the way for the realization of magnetic-(or
spin-) based thermoelectric devices. | 1504.00642v1 |
2018-07-02 | Electron-electron interactions of the multi-Cooper-pairs in the 1D limit and their role in the formation of global phase coherence in quasi-one-dimensional superconducting nanowire arrays | Nanostructuring of superconducting materials to form dense arrays of thin
parallel nanowires with significantly large transverse Josephson coupling has
proven to be an effective way to increase the upper critical field of
superconducting elements by as much as two orders of magnitude as compared to
the corresponding bulk materials and, in addition, may cause considerable
enhancements in their critical temperatures. Such materials have been realized
in the linear pores of mesoporous substrates or exist intrinsically in the form
of various quasi-1D crystalline materials. The transverse coupling between the
superconducting nanowires is determined by the size-dependent coherence length
E0. In order to obtain E0 over the Langer-Ambegaokar- McCumber-Halperin (LAMH)
theory, extensive experimental fitting parameters have been required over the
last 40 years. We propose a novel Monte Carlo algorithm for determining E0 of
the multi-Cooper pair system in the 1D limit. The concepts of uncertainty
principle, Pauli-limit, spin flip mechanism, electrostatic interaction, thermal
perturbation and co-rotating of electrons are considered in the model. We use
Pb nanowires as an example to monitor the size effect of E0 as a result of the
modified electron-electron interaction without the need for experimental
fitting parameters. We investigate how the coherence length determines the
transverse coupling of nanowires in dense arrays. This determines whether or
not a global phase-coherent state with zero resistance can be formed in such
arrays. Our Monte Carlo results are in very good agreement with experimental
data from various types of superconducting nanowire arrays | 1807.00611v1 |
2018-07-30 | Tunable Thermal Energy Transport across Diamond Membranes and Diamond-Si Interfaces by Nanoscale Graphoepitaxy | The development of electronic devices, especially those that involve
heterogeneous integration of materials, has led to increased challenges in
addressing their thermal operational-temperature demands. The heat flow in
these systems is significantly influenced or even dominated by thermal boundary
resistance at interface between dissimilar materials. However, controlling and
tuning heat transport across an interface and in the adjacent materials has so
far drawn limited attention. In this work, we grow chemical-vapor-deposited
(CVD) diamond on silicon substrates by graphoepitaxy and experimentally
demonstrate tunable thermal transport across diamond membranes and
diamond-silicon interfaces. We observed the highest diamond-silicon thermal
boundary conductance (TBC) measured to date and increased diamond thermal
conductivity due to strong grain texturing in the diamond near the interface.
Additionally, non-equilibrium molecular-dynamics (NEMD) simulations and a
Landauer approach are used to understand the diamond-silicon TBC. These
findings pave the way for tuning or increasing thermal conductance in
heterogeneously integrated electronics that involve polycrystalline materials
and will impact applications including electronics thermal management and
diamond growth. | 1807.11400v2 |
2019-10-28 | Tailored Graphenic Structures Directly Grown on Titanium Oxide Boost the Interfacial Charge Transfer | The successful application of titanium oxide-graphene hybrids in the fields
of photocatalysis, photovoltaics and photodetection strongly depends on the
interfacial contact between both materials. The need to provide a good coupling
between the enabling conductor and the photoactive phase prompted us to
directly grow conducting graphenic structures on TiO2 crystals. We here report
on the direct synthesis of tailored graphenic structures by using Plasma
Assisted Chemical Vapour Deposition that present a clean junction with the
prototypical titanium oxide (110) surface. Chemical analysis of the interface
indicates chemical bonding between both materials. Photocurrent measurements
under UV light illumination manifest that the charge transfer across the
interface is efficient. Moreover, the influence of the synthesis atmosphere,
gas precursor (C2H2) and diluents (Ar, O2), on the interface and on the
structure of the as-grown graphenic material is assessed. The inclusion of O2
promotes vertical growth of partially oxidized carbon nanodots/rods with
controllable height and density. The deposition with Ar results in continuous
graphenic films with low resistivity (6.8x10-6 ohm x m). The synthesis
protocols developed here are suitable to produce tailored carbon-semiconductor
structures on a variety of practical substrates as thin films, pillars or
nanoparticles. | 1910.12667v1 |
2020-08-23 | Nanofibril-mediated Fracture Resistance of Bone | Natural hard composites like human bone possess a combination of strength and
toughness that exceeds that of their constituents and of many engineered
composites. This augmentation is attributed to their complex hierarchical
structure, spanning multiple length scales; in bone, characteristic dimensions
range from nanoscale fibrils to microscale lamellae to mesoscale osteons and
macroscale organs. The mechanical properties of bone have been studied, with
the understanding that the isolated microstructure at micro- and nano-scales
gives rise to superior strength compared to that of whole tissue, and the
tissue possesses an amplified toughness relative to that of its nanoscale
constituents. Nanoscale toughening mechanisms of bone are not adequately
understood at sample dimensions that allow for isolating salient
microstructural features, because of the challenge of performing fracture
experiments on small-sized samples. We developed an in-situ three-point bend
experimental methodology that probes site-specific fracture behavior of
micron-sized specimens of hard material. Using this, we quantify crack
initiation and growth toughness of human trabecular bone with sharp fatigue
pre-cracks and blunt notches. Our findings indicate that bone with fatigue
cracks is two times tougher than that with blunt cracks. In-situ
data-correlated electron microscopy videos reveal this behavior arises from
crack-bridging by nanoscale fibril structure. The results reveal a transition
between fibril-bridging (~1 $\mu$m) and crack deflection/twist (~500 $\mu$m) as
a function of length-scale, and quantitatively demonstrate hierarchy-induced
toughening in a complex material. This versatile approach enables quantifying
the relationship between toughness and microstructure in various complex
material systems and provides direct insight for designing biomimetic
composites. | 2008.09955v1 |
2020-10-28 | Statistical analysis of the material, geometrical and imperfection characteristics of structural stainless steels and members | Traditional member-based two-step design approaches included in current
structural codes for steel structures, as well as more recent system-based
direct-design alternatives, require building rigorous structural reliability
frameworks for the calibration of partial coefficients (resistance factors) to
achieve specified target reliability requirements. Key design parameters
affecting the strength of structures and their random variations are generally
modelled by nominal or characteristic values in design standards, which are
combined with partial coefficients that need to be calibrated from measurements
on real samples. While the statistical characterization of material and
geometric properties of structural steels has been consolidated over the last
decades, information about the characterization of structural stainless steels
is virtually non-existent due to the limited pool of available data. Thus, this
paper presents the basic ingredient for developing reliability frameworks for
stainless steel structures and components by statistically characterizing the
main random parameters affecting their strength through a comprehensive
database collected from the literature. Based on the collected data,
appropriate probabilistic models are proposed for geometric properties,
material properties, imperfections and residual stresses of different stainless
steel alloys and cross-section or product types. The data is equally applicable
to member-based reliability analyses as described in existing codes and
system-based analyses targeted at the direct-design of stainless steel
structures by advanced analysis. | 2010.14777v2 |
2020-11-03 | Fully-Compensated Ferrimagnetic Spin Filter Materials within the Cr$\textit{M}\textit{N}$Al Equiatomic Quaternary Heusler Alloys | XX'YZ equiatomic quaternary Heusler alloys (EQHA's) containing Cr, Al, and
select Group IVB elements ($\textit{M}$ = Ti, Zr, Hf) and Group VB elements
($\textit{N}$ = V, Nb, Ta) were studied using state-of-the-art density
functional theory to determine their effectiveness in spintronic applications.
Each alloy is classified based on their spin-dependent electronic structure as
a half-metal, a spin gapless semiconductor, or a spin filter material. We
predict several new fully-compensated ferrimagnetic spin filter materials with
small electronic gaps and large exchange splitting allowing for robust spin
polarization with small resistance. CrVZrAl, CrVHfAl, CrTiNbAl, and CrTiTaAl
are identified as particularly robust spin filter candidates with an exchange
splitting of $\sim 0.20$ eV. In particular, CrTiNbAl and CrTiTaAl have
exceptionally small band gaps of $\sim 0.10$ eV. Moreover, in these compounds,
a spin asymmetric electronic band gap is maintained in 2 of 3 possible atomic
arrangements they can take, making the electronic properties less susceptible
to random site disorder. In addition, hydrostatic stress is applied to a subset
of the studied compounds in order to determine the stability and tunability of
the various electronic phases. Specifically, we find the CrAlV$\textit{M}$
subfamily of compounds to be exceptionally sensitive to hydrostatic stress,
yielding transitions between all spin-dependent electronic phases. | 2011.01389v1 |
2020-12-02 | Strain-induced anion ordering in perovskite oxyfluoride films | Anionic ordering is a promising route to engineer physical properties in
functional heteroanionic materials. A central challenge in the study of
anion-ordered compounds lies in developing robust synthetic strategies to
control anion occupation and in understanding the resultant implications for
electronic structure. Here, we show that epitaxial strain induces preferential
occupation of F and O on the anion sites in perovskite oxyfluoride SrMnO2.5-dFg
films grown on different substrates. Under compressive strain, F tends to take
the apical-like sites, which was revealed by F and O K-edge linearly polarized
x-ray absorption spectroscopy and density functional theory calculations,
resulting in an enhanced c-axis expansion. Under tensile strain, F tends to
take the equatorial-like sites, enabling the longer Mn-F bonds to lie within
the plane. The anion ordered oxyfluoride films exhibit a significant orbital
polarization of the 3d electrons, distinct F-site dependence to their valence
band density of states, and an enhanced resistivity when F occupies the
apical-like anion site compared to the equatorial-like site. By demonstrating a
general strategy for inducing anion-site order in oxyfluoride perovskites, this
work lays the foundation for future materials design and synthesis efforts that
leverage this greater degree of atomic control to realize new polar or
quasi-two-dimensional materials. | 2012.01221v1 |
2021-02-06 | Multiple charge density waves and superconductivity nucleation at antiphase domain walls in the nematic pnictide Ba$_{1-x}$Sr$_{x}$Ni$_{2}$As$_{2}$ | How superconductivity interacts with charge or nematic order is one of the
great unresolved issues at the center of research in quantum materials.
Ba$_{1-x}$Sr$_{x}$Ni$_{2}$As$_{2}$ (BSNA) is a charge ordered pnictide
superconductor recently shown to exhibit a six-fold enhancement of
superconductivity due to nematic fluctuations near a quantum phase transition
(at $x_c=0.7$). The superconductivity is, however, anomalous, with the
resistive transition for $0.4 < x< x_c$ occurring at a higher temperature than
the specific heat anomaly. Using x-ray scattering, we discovered a new charge
density wave (CDW) in BSNA in this composition range. The CDW is commensurate
with a period of two lattice parameters, and is distinct from the two CDWs
previously reported in this material. We argue that the anomalous transport
behavior arises from heterogeneous superconductivity nucleating at antiphase
domain walls in this CDW. We also present new data on the incommensurate CDW,
previously identified as being unidirectional, showing that is a rotationally
symmetric, "4$Q$" state with $C_4$ symmetry. Our study establishes BSNA as a
rare material containing three distinct CDWs, and an exciting testbed for
studying coupling between CDW, nematic, and SC orders. | 2102.03592v2 |
2021-04-29 | Competing magnetic interactions and magnetoresistance anomalies in cubic intermetallic compounds, Gd4RhAl and Tb4RhAl, and enhanced magnetocaloric effect for the Tb case | We report complex magnetic, magnetoresistance (MR) and magnetocaloric
properties of Gd4RhAl and Tb4RhAl forming in the Gd4RhIn type cubic structure.
Though the synthesis of the compounds was reported long ago, to our knowledge,
no attempt was made to investigate the properties of these compounds. The
present results of ac and dc magnetization, electrical resistivity and
heat-capacity measurements down to 1.8 K establish that these compounds undergo
antiferromagnetic order initially, followed by complex spin-glass features with
decreasing temperature. These characteristic temperatures are: For Gd case, TN
is about 46K and TG is about 21 K, and for Tb, about 32 and 28 K respectively.
Additionally, there are field induced magnetic effects, interestingly leading
to non-monotonic variations in MR. There is a significant MR over a wide
temperature range above TN, similar to the behavior of magnetocaloric effect
(MCE) as measured by isothermal entropy change (DeltaS). An intriguing finding
we made is that DeltaS at the onset of magnetic order is significantly larger
for the Tb compound than that observed for the Gd analogue near its TN. On the
basis of this observation in a cubic material, we raise a question whether
aspherical nature of the 4f orbital can play a role to enhance MCE under
favorable circumstances, a clue that could be useful to find materials for
magnetocaloric applications. | 2104.14521v1 |
2021-10-12 | Bridging the Band Gap: What Device Physicists Need to Know About Machine Learning | This article surveys the landscape of semiconductor materials and devices
research for the acceleration of machine learning (ML) algorithms. We observe a
disconnect between the semiconductor and device physics and engineering
communities, and the digital logic and computer hardware architecture
communities. The article first provides an overview of the principles of
computational complexity and fundamental physical limits to computing and their
relation to physical systems. The article then provides an introduction to ML
by presenting three key components of ML systems: representation, evaluation,
and optimisation. The article then discusses and provides examples of the
application of emerging technologies from the demiconductor and device physics
domains as solutions to computational problems, alongside a brief overview of
emerging devices for computing applications. The article then reviews the
landscape of ML accelerators, comparing fixed-function and reprogrammable
digital logic with novel devices such as memristors, resistive memories,
magnetic memories, and probabilistic bits. We observe broadly lower performance
of ML accelerators based on novel devices and materials when compared to those
based on digital complimentary metal-oxide semiconductor (CMOS) technology,
particularly in the MNIST optical character recognition task, a common ML
benchmark, and also highlight the lack of a trend of progress in approaches
based on novel materials and devices. Lastly, the article proposes figures of
merit for meaningful evaluation and comparison of different ML implementations
in the hope of fostering a dialogue between the materials science, device
physics, digital logic, and computer architecture communities by providing a
common frame of reference for their work. | 2110.05910v2 |
2021-11-15 | In situ Bragg coherent X-ray diffraction imaging of corrosion in a Co-Fe alloy microcrystal | Corrosion is a major concern for many industries, as corrosive environments
can induce structural and morphological changes that lead to material
dissolution and accelerate material failure. The progression of corrosion
depends on nanoscale morphology, stress, and defects present. Experimentally
monitoring this complex interplay is challenging. Here we implement in situ
Bragg coherent X-ray diffraction imaging (BCDI) to probe the dissolution of a
Co-Fe alloy microcrystal exposed to hydrochloric acid (HCl). By measuring five
Bragg reflections from a single isolated microcrystal at ambient conditions, we
compare the full three-dimensional (3D) strain state before corrosion and the
strain along the [111] direction throughout the corrosion process. We find that
the strained surface layer of the crystal dissolves to leave a progressively
less strained surface. Interestingly, the average strain closer to the centre
of the crystal increases during the corrosion process. We determine the
localised corrosion rate from BCDI data, revealing the preferential dissolution
of facets more exposed to the acid stream, highlighting an experimental
geometry effect. These results bring new perspectives to understanding the
interplay between crystal strain, morphology, and corrosion; a prerequisite for
the design of more corrosion-resistant materials. | 2111.07903v5 |
2021-12-16 | Observation and manipulation of a phase separated state in a charge density wave material | The 1T polytype of TaS$_\textrm{2}$ has been studied extensively as a
strongly correlated system. As 1T-TaS$_\textrm{2}$ is thinned towards the 2D
limit, its phase diagram shows significant deviations from that of the bulk
material. Optoelectronic maps of ultrathin 1T-TaS$_\textrm{2}$ have indicated
the presence of non-equilibrium charge density wave phases within the
hysteresis region of the nearly commensurate (NC) to commensurate (C)
transition. We perform scanning tunneling microscopy on exfoliated ultrathin
flakes of 1T-TaS$_\textrm{2}$ within the NC-C hysteresis window, finding
evidence that the observed non-equilibrium phases consist of intertwined,
irregularly shaped NC-like and C-like domains. After applying lateral
electrical signals to the sample we image changes in the geometric arrangement
of the different regions. We use a phase separation model to explore the
relationship between electronic inhomogeneity present in ultrathin
1T-TaS$_\textrm{2}$ and its bulk resistivity. These results demonstrate the
role of phase competition morphologies in determining the properties of 2D
materials. | 2112.09240v1 |
2022-03-17 | Quasi-static crack front deformations in cohesive materials | When a crack interacts with material heterogeneities, its front distorts and
adopts complex tortuous configurations that are reminiscent of the energy
barriers encountered during crack propagation. As such, the study of crack
front deformations is key to rationalize the effective failure properties of
micro-structured solids and interfaces. Yet, the impact of a localized
dissipation in a finite region behind the crack front, called the process zone,
has often been overlooked. In this work, we derive the equation ruling 3D
coplanar crack propagation in heterogeneous cohesive materials where the
opening of the crack is resisted by some traction in its wake. We show that the
presence of a process zone results in two competing effects on the deformation
of crack fronts: (i) it makes the front more compliant to small-wavelength
perturbations, and (ii) it smooths out local fluctuations of strength and
process zone size, from which emerge heterogeneities of fracture energy. Their
respective influence on front deformations is shown to strongly impact the
stability of perturbed crack fronts, as well as their stationary shapes when
interacting with arrays of tough obstacles. Overall, our theory provides a
unified framework to predict the variety of front profiles observed in
experiments, even when the small-scale yielding hypothesis of linear elastic
fracture mechanics breaks down. | 2203.09317v2 |
2022-05-11 | Reversible Tuning of Superconductivity in Ion-Gated NbN Ultrathin Films by Self-Encapsulation with a High-$κ$ Dielectric Layer | Ionic gating is a powerful technique for tuning the physical properties of a
material via electric field-induced charge doping, but is prone to introduce
extrinsic disorder and undesired electrochemical modifications in the gated
material beyond pure electrostatics. Conversely, reversible, volatile and
electrostatic modulation is pivotal in the reliable design and operation of
novel device concepts enabled by the ultrahigh induced charge densities
attainable via ionic gating. Here we demonstrate a simple and effective method
to achieve reversible and volatile gating of surface-sensitive ultrathin
niobium nitride films via controlled oxidation of their surface. The resulting
niobium oxide encapsulation layer exhibits a capacitance comparable to that of
non-encapsulated ionic transistors, withstands gate voltages beyond the
electrochemical stability window of the gate electrolyte, and enables a
fully-reversible tunability of both the normal-state resistivity and the
superconducting transition temperature of the encapsulated films. Our approach
should be transferable to other materials and device geometries where more
standard encapsulation techniques are not readily applicable. | 2205.05491v3 |
2022-06-07 | Observation of surface superconductivity in a three-dimensional Dirac material | Superconductivity becomes more interesting when it encounters dimensional
constraint or topology, because it is of importance for exploring exotic
quantum phenomena or developing superconducting electronics. Here we report the
coexistence of naturally formed surface superconducting state and
three-dimensional topological Dirac state in single crystals of BaMg$_2$Bi$_2$.
The electronic structure obtained from the first-principles calculations
demonstrates that BaMg$_2$Bi$_2$ is an ideal Dirac material, in which the Dirac
point is very close to the Fermi level and no other energy band crosses the
Fermi level. Superconductivity up to 4.77 K can be observed under ambient
pressure in the measurements of resistivity. The angle dependent
magnetoresistance reveals the two-dimensional characteristic of
superconductivity, indicating that superconductivity occurs on the surface of
the sample and is absent in the bulk state. Our study not only provides
BaMg$_2$Bi$_2$ as a suitable platform to study the interplay between
superconductivity and topological Dirac state, but also indicates that
MgBi-based materials may be a promising system for exploring new
superconductors. | 2206.03405v1 |
2022-10-03 | Interwoven atypical quantum states in CeLiBi$_2$ | We report the discovery of CeLiBi$_2$, the first example of a material in the
tetragonal Ce$TX_2$ ($T$ = transition metal; $X$ = pnictogen) family wherein an
alkali cation replaces the typical transition metal. Magnetic susceptibility
and neutron powder diffraction measurements are consistent with a crystal-field
$\Gamma_6$ ground state Kramers doublet that orders antiferromagnetically below
$T_N = 3.4$ K with an incommensurate propagation wave vector ${\bf{k}} = (0,
0.0724(4), 0.5)$ that generates a nanometric modulation of the magnetic
structure. The best model of the ordered state is an elliptical cycloid with Ce
moments primarily residing in the $ab$ plane. This is highly unusual, as all
other $\Gamma_6$ Ce$TX_2$ members order ferromagnetically. Further, we observe
an atypical hard-axis metamagnetic transition at $2$ T in magnetostriction,
magnetization, and resistivity measurements. CeLiBi$_2$ is a rare example of a
highly conductive material with dominant skew scattering leading to a large
anomalous Hall effect. Quantum oscillations with five frequencies arise in
magnetostriction and magnetic susceptibility data to $T = 30$ K and $\mu_0H =
55$ T, which indicate small Fermi pockets of light carriers with effective
masses as low as 0.07$m_e$. Density functional theory calculations indicate
that square-net Dirac-like Bi$-p$ bands are responsible for these ultralight
carriers. Together, our results show that CeLiBi$_2$ enables multiple atypical
magnetic and electronic properties in a single clean material. | 2210.01031v2 |
2022-11-27 | Ionic Peltier Effect in Li-Ion Electrolytes | The coupled transport of charge and heat provide fundamental insights into
the microscopic thermodynamics and kinetics of materials. We describe a
sensitive ac differential resistance bridge that enables measurements of the
temperature difference on two sides of a coin cell with a resolution of better
than 10 uK. We use this temperature difference metrology to determine the ionic
Peltier coefficients of symmetric Li-ion electrochemical cells as a function of
Li salt concentration, solvent composition, electrode material, and
temperature. The Peltier coefficients {\Pi} are negative, i.e., heat flows in
the direction opposite to the drift of Li ions in the applied electric field,
large, 30 kJ mol-1, and increase with increasing temperature at T > 300 K. The
Peltier coefficient is approximately constant on time scales that span the
characteristic time for mass diffusion across the thickness of the electrolyte,
suggesting that heat of transport plays a minor role in comparison to the
changes in partial molar entropy of Li at the interface between the electrode
and electrolyte. Our work demonstrates a new platform for studying the
non-equilibrium thermodynamics of electrochemical cells and provides a window
into the transport properties of electrochemical materials through measurements
of temperature differences and heat currents that complement traditional
measurements of voltages and charge currents. | 2211.14949v2 |
2023-03-28 | A model for critical current effects in point-contact Andreev-reflection spectroscopy | It is well known that point-contact Andreev reflection spectroscopy provides
reliable measurements of the energy gap(s) in a superconductor when the contact
is in the ballistic or nearly-ballistic regime. However, especially when the
mean free path of the material under study is small, obtaining ballistic
contacts can be a major challenge. One of the signatures of a Maxwell
contribution to the contact resistance is the presence of "dips" in the
differential conductance, associated to the sudden appearance of a Maxwell
term, in turn due to the attainment of the critical current of the material in
the contact region. Here we show that, using a proper model for the $R(I)$ of
the material under study, it is possible to fit the experimental curves
(without the need of normalization) obtaining the correct values of the gap
amplitudes even in the presence of such dips, as well as the temperature
dependence of the critical current in the contact. We present a test of the
procedure in the case of Andreev-reflection spectra in
Mg$_{0.85}$Al$_{0.15}$B$_2$ single crystals. | 2303.15968v2 |
2023-05-12 | Striation lines in intermittent fatigue crack growth in an Al alloy | Fatigue failure of crystalline materials is a difficult problem in science
and engineering, and recent results have shown that fatigue crack growth can
occur in intermittent jumps which have fat-tailed distributions. As fatigue
crack propagation is known to leave markings -- called striations -- on the
fracture surface, the distances between these should also have fat-tailed
distributions, if the crack propagation is intermittent. Here, we combine
macroscale crack tip tracking in fatigue crack growth measurements of aluminum
5005 samples with \emph{post-mortem} scanning electron microscopy imaging of
the striation lines. We introduce two different methods for extracting the
striation line spacing from the images. What we find is a similar distribution
of striation spacings as jump sizes using one of our methods, but the average
striation spacing does not correlate with the crack growth rate. We conclude
that we observe avalanche-like crack propagation, reflected in both the
macroscale crack tip tracking as well as the analysis of the fracture surfaces.
Our results show that the fracture surfaces can be used to study the
intermittency of fatigue crack propagation and in development of
crack-resistant materials. The advantages and disadvantages of the two methods
introduced are discussed. | 2305.07460v1 |
2023-07-07 | Observation of the anomalous Hall effect in a layered polar semiconductor | Progress in magnetoelectric materials is hindered by apparently contradictory
requirements for time-reversal symmetry broken and polar ferroelectric
electronic structure in common ferromagnets and antiferromagnets. Alternative
routes could be provided by recent discoveries of a time-reversal symmetry
breaking anomalous Hall effect in noncollinear magnets and altermagnets, but
hitherto reported bulk materials are not polar. Here, we report the observation
of a spontaneous anomalous Hall effect in doped AgCrSe$_2$, a layered polar
semiconductor with an antiferromagnetic coupling between Cr spins in adjacent
layers. The anomalous Hall resistivity 3 $\mu\Omega$ cm is comparable to the
largest observed in compensated magnetic systems to date, and is rapidly
switched off when the angle of an applied magnetic field is rotated to $\sim
80^{\circ}$ from the crystalline $c$-axis. Our ionic gating experiments show
that the anomalous Hall conductivity magnitude can be enhanced by modulating
the $p$-type carrier density. We also present theoretical results that suggest
the anomalous Hall effect is driven by Berry curvature due to noncollinear
antiferromagnetic correlations among Cr spins, which are consistent with the
previously suggested magnetic ordering in AgCrSe$_2$. Our results open the
possibility to study the interplay of magnetic and ferroelectric-like responses
in this fascinating class of materials. | 2307.03541v1 |
2023-08-23 | Plastic deformation mechanisms during nanoindentation of W, Mo, V body-centered cubic single crystals and their corresponding W-Mo, W-V equiatomic random solid solutions | Deformation plasticity mechanisms in alloys and compounds may unveil the
material capacity towards optimal mechanical properties. We conduct a series of
molecular dynamics (MD) simulations to investigate plasticity mechanisms due to
nanoindentation in pure tungsten, molybdenum and vanadium body-centered cubic
single crystals, as well as the also body-centered cubic, equiatomic, random
solid solutions (RSS) of tungsten--molybdenum and tungsten--vanadium alloys.
Our analysis focuses on a thorough, side-by-side comparison of dynamic
deformation processes, defect nucleation, and evolution, along with
corresponding stress--strain curves. We also check the surface morphology of
indented samples through atomic shear strain mapping. As expected, the presence
of Mo and V atoms in W matrices introduces lattice strain and distortion,
increasing material resistance to deformation and slowing down dislocation
mobility of dislocation loops with a Burgers vector of 1/2 $\langle 111
\rangle$. Our side-by-side comparison displays a remarkable suppression of the
plastic zone size in equiatomic W--V RSS, but not in equiatomic W--Mo RSS
alloys, displaying a clear prediction for optimal hardening response equiatomic
W--V RSS alloys. If the small-depth nanoindentation plastic response is
indicative of overall mechanical performance, it is possible to conceive a
novel MD-based pathway towards material design for mechanical applications in
complex, multi-component alloys. | 2308.12206v1 |
2023-09-08 | Intralayer Negative Poisson's Ratio in Two-Dimensional Black Arsenic by Strain Engineering | Negative Poisson's ratio as the anomalous characteristic generally exists in
artificial architectures, such as re-entrant and honeycomb structures. The
structures with negative Poisson's ratio have attracted intensive attention due
to their unique auxetic effect and many promising applications in shear
resistant and energy absorption fields. However, experimental observation of
negative Poisson's ratio in natural materials barely happened, although various
two-dimensional layered materials are predicted in theory. Herein, we report
the anisotropic Raman response and the intrinsic intralayer negative Poisson's
ratio of two-dimensional natural black arsenic (b-As) via strain engineering
strategy. The results were evident by the detailed Raman spectrum of b-As under
uniaxial strain together with density functional theory calculations. It is
found that b-As was softer along the armchair than zigzag direction. The
anisotropic mechanical features and van der Waals interactions play essential
roles in strain-dependent Raman shifts and negative Poisson's ratio in the
natural b-As along zigzag direction. This work may shed a light on the
mechanical properties and potential applications of two-dimensional puckered
materials. | 2309.04058v1 |
2023-10-04 | Three-Sensor 2ω Method with Multi-directional Layout: A General Methodology for Measuring Thermal Conductivity of Solid Materials | Anisotropic thermal transport plays a key role in both theoretical study and
engineering practice of heat transfer, but accurately measuring anisotropic
thermal conductivity remains a significant challenge. To address this issue, we
propose the three-sensor 2{\omega} method in this study, which is capable of
accurately measuring the isotropic or anisotropic thermal conductivity of solid
materials. In this method, several three-sensor groups following the design
guidelines are fabricated upon the sample along different characteristic
directions, and each group consists of three parallel metal sensors with
unequal widths and distances optimally designed based on sensitivity analysis.
Among the three sensors, the outer two serve as AC heaters and the middle one
as a DC detector. The 2{\omega} voltage signals across the detector in each
three-sensor group are measured, and then the data are processed by the
proposed Intersection Method to derive the thermal conductivities along
directions of interest. The application of the detector's 2{\omega} instead of
the heater's 3{\omega} voltage signals eliminates the errors introduced by the
uncertainties of thermal resistance in superficial structures (metal layer,
insulation layer, interface, etc.). Meanwhile, by replacing the fitting
algorithm with the Intersection Method, the local optimum trap of multivariate
fitting is avoided. To verify the accuracy and reliability, four typical
monocrystalline semiconductors, i.e., Si, GaN, AlN, and {\beta -Ga _2 O _3},
are measured, and the results are consistent with the literature. This method
will provide a comprehensive and versatile solution for the thermal
conductivity measurements of solid materials. | 2310.02846v1 |
2023-10-25 | Non-Destructive Imaging of Breakdown Process in Ferroelectric Capacitors Using \textit{In-situ} Laser-Based Photoemission Electron Microscopy | HfO$_2$-based ferroelectrics are one of the most actively developed
functional materials for memory devices. However, in HfO$_2$-based
ferroelectric devices, dielectric breakdown is a main failure mechanism during
repeated polarization switching. Elucidation of the breakdown process may
broaden the scope of applications for the ferroelectric HfO$_2$. Here, we
report direct observations of a breakdown process in HfO$_2$-based
ferroelectric capacitors, by \textit{in-situ} laser-based photoemission
electron microscopy (laser-PEEM). We have not only clearly visualized the hard
dielectric breakdown (HDB) spot, but also observed the regions responsible for
the soft dielectric breakdown (SDB) which is a precursor phenomenon to HDB. It
was found that the low-resistance region formed after SDB is wider than the
conduction path formed after HDB. Furthermore, our spectromicroscopic analysis
revealed that the photoelectron spectrum after SDB shows an enhancement in
intensity without spectral-shape modulation, interpreted that the initially
existed defects are increased. In the HDB spot, however, an additional shoulder
structure was observed. These results provide spectroscopic evidence that the
electronic states responsible for the conduction path after SDB are different
from those after HDB. Through this work, we propose this microscopic approach
as a versatile tool for studying buried materials as they are, accelerating the
development of material engineering for advanced electronic devices. | 2310.16275v1 |
2023-11-16 | Properties of Nb\_xTi\_{(1-x)}N thin films deposited on 300 mm silicon wafers for upscaling superconducting digital circuits | Scaling superconducting digital circuits requires fundamental changes in the
current material set and fabrication process. The transition to 300 mm wafers
and the implementation of advanced lithography are instrumental in facilitating
mature CMOS processes, ensuring uniformity, and optimizing the yield. This
study explores the properties of NbxTi(1-x)N films fabricated by magnetron DC
sputtering on 300 mm Si wafers. As a promising alternative to traditional Nb in
device manufacturing, NbxTi(1-x)N offers numerous advantages, including
enhanced stability and scalability to smaller dimensions, in both processing
and design. As a ternary material, NbxTi(1-x)N allows engineering material
parameters by changing deposition conditions. The engineered properties can be
used to modulate device parameters through the stack and mitigate failure
modes. We report characterization of NbxTi(1-x)N films at less than 2%
thickness variability, 2.4% Tc variability and 3% composition variability. The
films material properties such as resistivity (140-375 {\Omega}cm) and critical
temperature Tc (4.6 K - 14.1 K) are correlated with stoichiometry and
morphology of the films. Our results highlight the significant influence of
deposition conditions on crystallographic texture along the films and its
correlation with Tc. | 2311.09772v2 |
2024-03-18 | Primary Defect Production in Doped Iron Grain Boundaries during Low Energy Collision Cascades | This study explores the intricate interactions between grain boundaries (GBs)
and irradiation-induced defects in nanocrystalline iron, highlighting the role
of dopants like copper. Utilizing molecular dynamics simulations, the research
delineates how GB properties, such as GB energy and defect formation energies,
influence the formation and evolution of defects in low energy collision
cascades. It reveals that GBs not only augment defect production but also show
a marked preference for interstitials over vacancies, a behavior significantly
modulated by the cascade's proximity to the GB. The presence of dopants is
shown to alter GB properties, affecting both the rate and type of defect
production, thereby underscoring the complex interplay between GB
characteristics, dopant elements, and defect dynamics. Moreover, the
investigation uncovers that the structural characteristics of GBs play a
crucial role in cascade evolution and defect generation, with certain GB
configurations undergoing reconfiguration in response to cascades. For
instance, the reconfiguration of one pure Fe twist GB suggests that GB geometry
can significantly influence defect generation mechanisms. These findings point
to the potential of GB engineering in developing materials with enhanced
radiation tolerance, advocating for a nuanced approach to material design. By
tailoring GB properties and selectively introducing dopant elements, materials
can be optimized to exhibit superior resistance to radiation-induced damage,
offering insights for applications in nuclear reactors and other
radiation-prone environments. | 2403.12257v1 |
2024-05-21 | Engineering band structures of two-dimensional materials with remote moire ferroelectricity | The stacking order and twist angle provide abundant opportunities for
engineering band structures of two-dimensional materials, including the
formation of moire bands, flat bands, and topologically nontrivial bands. The
inversion symmetry breaking in rhombohedral-stacked transitional metal
dichalcogenides (TMDCs) endows them with an interfacial ferroelectricity
associated with an out-of-plane electric polarization. By utilizing twist angle
as a knob to construct rhombohedral-stacked TMDCs, antiferroelectric domain
networks with alternating out-of-plane polarization can be generated. Here, we
demonstrate that such spatially periodic ferroelectric polarizations in
parallel-stacked twisted WSe2 can imprint their moire potential onto a remote
bilayer graphene. This remote moire potential gives rise to pronounced
satellite resistance peaks besides the charge-neutrality point in graphene,
which are tunable by the twist angle of WSe2. Our observations of ferroelectric
hysteresis at finite displacement fields suggest the moire is delivered by a
long-range electrostatic potential. The constructed superlattices by moire
ferroelectricity represent a highly flexible approach, as they involve the
separation of the moire construction layer from the electronic transport layer.
This remote moire is identified as a weak potential and can coexist with
conventional moire. Our results offer a comprehensive strategy for engineering
band structures and properties of two-dimensional materials by utilizing moire
ferroelectricity. | 2405.12811v1 |
1996-07-08 | Dynamic Conductance in Quantum Hall Systems | In the framework of the edge-channel picture and the scattering approach to
conduction, we discuss the low frequency admittance of quantized Hall samples
up to second order in frequency. The first-order term gives the leading order
phase-shift between current and voltage and is associated with the displacement
current. It is determined by the emittance which is a capacitance in a
capacitive arrangement of edge channels but which is inductive-like if edge
channels predominate which transmit charge between different reservoirs. The
second-order term is associated with the charge relaxation. We apply our
results to a Corbino disc and to two- and four-terminal quantum Hall bars, and
we discuss the symmetry properties of the current response. In particular, we
calculate the longitudinal resistance and the Hall resistance as a function of
frequency. | 9607051v1 |
1997-02-12 | Kondo Effect in High-T_c Cuprates | We study the Kondo effect due to the nonmagnetic impurity, e.g., Zn, in
high-T_c cuprates based on the spin-change separated state. In the optimal or
overdoped case with the Kondo screening, the residual resistivity is dominated
by the spinons while the T-dependent part determined by the holons. This gives
$\rho(T) = { {4 \hbar} /{e^2}} { { n_{imp.}} /{(1-x)}} + {{\alpha T} / x}$ (x:
hole concentration,$n_{imp.}$: impurity concentration, $\alpha$: constant ),
which is in agreement with experiments. In the underdoped region with the
pseudo spin gap, an SU(2) formulation predicts that the holon phase shift is
related to the formation of the local spin moment, and hence the residual
resistivity is given by $\rho_{res.} = { {4 \hbar} /{ e^2}} { { n_{imp.}}/{x}}
$, which is also consistent with the experiments. The magnetic impurity case,
e.g., Ni, is also discussed. | 9702103v2 |
1999-03-11 | Anisotropic States of Two-Dimensional Electron Systems in High Landau Levels: Effect of an In-Plane Magnetic Field | We report the observation of an acute sensitivity of the anisotropic
longitudinal resistivity of two-dimensional electron systems in half-filled
high Landau levels to the magnitude and orientation of an in-plane magnetic
field. In the third and higher Landau levels, at filling fractions nu=9/2,
11/2, etc., the in-plane field can lead to a striking interchange of the "hard"
and "easy" transport directions. In the second Landau level the normally
isotropic resistivity and the weak nu=5/2 quantized Hall state are destroyed by
a large in-plane field and the transport becomes highly anisotropic. | 9903196v2 |
1999-06-23 | Micro-Raman and resistance measurements of epitaxial La0.7Sr0.3MnO3 films | The Channel-Spark method was used for deposition of highly oriented
ferromagnetic La0.7Sr0.3MnO3 films on NdGaO3 substrates. It was found that
additional oxygen decreases the film quality suppressing the Curie temperature
and metal-insulator transition below the room temperature. To achieve the best
quality of the films the samples were either annealed in high vacuum at
deposition temperature or even deposited in argon atmosphere with no oxygen
annealing. For such films the resistive measurements showed a metallic
behaviour in the interval 10-300 K in accordance with the high Curie point (Tc
350 K). Micro-Raman analysis indicate that the La0.7Sr0.3MnO3 films are well
ordered, while some outgrowths show stoichiometrical deviations. | 9906348v1 |
1999-07-19 | Quantum Theory of Quantum-Hall Smectics | We propose a quantum stripe (smectic) coupled-Luttinger-liquid model for the
anisotropic states which occur in two-dimensional electron systems with
high-index partial Landau level filling, $\nu^{*} = \nu - \lbrack\nu\rbrack$.
Perturbative renormalization group calculations establish that interaction
terms neglected in this model are relevant - probably driving the system into
an anisotropic Wigner crystal---but for $0.4 \lesssim \nu^{*} \lesssim 0.6$
only below temperatures which are outside of the experimentally accessible
range. We argue that the Hall conductance of the ground state flows toward
$\lbrack\nu\rbrack e^{2}/h$ and $(\lbrack\nu\rbrack + 1) e^{2}/h$ respectively,
on the low and high filling factor sides of this range, consistent with recent
observations. A semiclassical theory of smectic state transport properties,
which incorporates Luttinger liquid effects in the evaluation of scattering
amplitudes, accounts for the magnitude of the dissipative resistivities at
$\nu^{*}=1/2$, for their $\nu^{*}$-dependence, and for the observation of
non-linearities of opposite sign in easy and hard direction resistivities. | 9907278v1 |
1999-10-14 | Electrical Noise From Phase Separation In Pr2/3Ca1/3MnO3 Single Crystal | Low frequency electrical noise measurements have been used to probe the
electronic state of the perovskite-type manganese oxide Pr2/3Ca1/3MnO3 versus
temperature and in the vicinity of the field-induced transition from the
insulating, charge-ordered state (I-CO) to the metallic, ferromagnetic state
(M-F). At high temperature we have observed a high level of the excess noise
with mainly a gaussian distribution of the resistance fluctuations, and the
associated power spectral density has a standard 1/f dependence. However, in
the hysteretic region, where the electrical resistance depends dramatically on
the sample history, we have observed a huge non-gaussian noise characterized by
two level fluctuator-like switching (TLS) in the time domain. We discuss the
origin of the noise in terms of percolative behavior of the conductivity. We
speculate that the dominant fluctuators are manganese clusters switching
between the M-F and the I-CO phases. | 9910204v1 |
1999-12-20 | Oxygen isotope effects in high-quality thin films of manganites: Quantitative constraints on the physics of manganites | Oxygen isotope effects on the transport properties have been studied in
high-quality epitaxial thin films of La_{0.75}Ca_{0.25}MnO_{3} and
Nd_{0.7}Sr_{0.3}MnO_{3}. In the paramagnetic state, the resistivity can be well
fitted by \rho (T) = (A/\sqrt{T})\exp(E_{\rho}/k_{B}T) with the parameters A
and E_{a} depending strongly on the oxygen isotope mass. The resistivity below
80 K almost perfectly follows \rho = \rho_{o}+
B\omega_{s}/\sinh^{2}(\hbar\omega_{s}/2k_{B}T) with \hbar\omega_{s}/k_{B} \sim
100 K. Both \rho_{o} and B increase by about 15(3)% upon raplacing $^{16}$O by
$^{18}$O. The results provide quantitative constraints on the basic physics of
manganites. | 9912355v1 |
2000-05-03 | The doping dependence of T* - what is the real high-Tc phase diagram? | Underdoped high-Tc superconductors are frequently characterised by a
temperature, T*, below which the normal-state pseudogap opens. Two different
"phase diagrams" based on the doping (p) dependence of T* are currently
considered: one where T* falls to zero at a critical doping state and the other
where T* merges with Tc in the overdoped region. By examining the temperature
dependence of the NMR Knight shift and relaxation rate, entropy, resistivity,
infrared conductivity, Raman scattering, ARPES and tunnelling data it is
concluded that the second scenario is not at all supported. Neither can one
distinguish a small and a large pseudogap as is often done. T* is an energy
scale which falls abruptly to zero at p=0.19. | 0005063v2 |
2000-07-13 | Exclusion of quantum coherence as the origin of the 2D metallic state in high-mobility silicon inversion layers | The temperature and density dependence of the phase coherence time
$\tau_\phi$ in high-mobility silicon inversion layers was determined from the
magnetoresistivity due to weak localization. The upper temperature limit for
single-electron quantum interference effects was delineated by comparing
$\tau_\phi$ with the momentum relaxation time $\tau$. A comparison between the
density dependence of the borders for quantum interference effects and the
strong resistivity drop reveals that theses effects are not related to each
other. As the strong resistivity drop occurs in the Drude regime, the apparent
metallic behavior can not be caused by quantum coherent effects. | 0007230v3 |
2001-02-22 | Magnetoresistivity and Complete $H_{c2}(T)$ in $MgB_2$ | Detailed magneto-transport data on dense wires of $MgB_2$ are reported for
applied magnetic fields up to 18 T. The temperature and field dependencies of
the electrical resistivity are consistent with $MgB_2$ behaving like a simple
metal and following a generalized form of Kohler's rule. In addition, given the
generally high $T_c$ values and narrow resistive transition widths associated
with $MgB_2$ synthesized in this manner, combined with applied magnetic fields
of up to 18 T, an accurate and complete $H_{c2}(T)$ curve could be determined.
This curve agrees well with curves determined from lower field measurements on
sintered pellets and wires of $MgB_2$. $H_{c2}(T)$ is linear in $T$ over a wide
range of temperature (7 K $\le~T~\le$ 32 K) and has an upward curvature for $T$
close to $T_c$. These features are similar to other high $\kappa$, clean limit,
boron-bearing intermetallics: $YNi_2B_2C$ and $LuNi_2B_2C$. | 0102413v2 |
2001-08-17 | Entropy of vortex cores on the border of the superconductor-to-insulator transition in an underdoped cuprate | We present a study of Nernst effect in underdoped $La_{2-x}Sr_xCuO_4$ in
magnetic fields as high as 28T. At high fields, a sizeable Nernst signal was
found to persist in presence of a field-induced non-metallic resistivity. By
simultaneously measuring resistivity and the Nernst coefficient, we extract the
entropy of vortex cores in the vicinity of this field-induced
superconductor-insulator transition. Moreover, the temperature dependence of
the thermo-electric Hall angle provides strong constraints on the possible
origins of the finite Nernst signal above $T_c$, as recently discovered by Xu
et al. | 0108277v2 |
2002-03-07 | Pseudogap and Conduction Dimensionalities in High-T_c Superconductors | The nature of normal state charge-carriers' dynamics and the transition in
conduction and gap dimensionalities between 2D and 3D for YBa_2 Cu_3
O_{7-delta} and Bi_2 Sr_2 Ca_{1-x} Y_x Cu_2 O_8 high-T_c superconductors were
described by computing and fitting the resistivity curves, rho(T,delta,x).
These were carried out by utilizing the 2D and 3D Fermi liquid (FL) and
ionization energy (E_I) based resistivity models coupled with charge-spin (CS)
separation based t-J model [Phys. Rev. B 64, 104516 (2001)]. rho(T,delta,x)
curves of Y123 and Bi2212 samples indicate the beginning of the transition of
conduction and gap from 2D to 3D with reduction in oxygen content (7-delta) and
Ca^{2+} (1-x) as such, c-axis pseudogap could be a different phenomenon from
superconductor and spin gaps. These models also indicate that the recent MgB_2
superconductor is at least not Y123 or Bi2212 type. | 0203164v8 |
2002-03-07 | The Onset of Anisotropic Transport of Two-Dimensional Electrons in High Landau Levels: An Isotropic-to-Nematic Liquid Crystal Phase Transition? | The recently discovered anisotropy of the longitudinal resistance of
two-dimensional electrons near half filling of high Landau levels is found to
persist to much higher temperatures T when a large in-plane magnetic field B||
is applied. Under these conditions we find that the longitudinal resistivity
scales quasi-linearly with B||/T. These observations support the notion that
the onset of anisotropy at B||=0 does not reflect the spontaneous development
of charge density modulations but may instead signal an isotropic-to-nematic
liquid crystal phase transition. | 0203174v1 |
2003-05-21 | Magnetoresistive response of a high mobility 2DES under electromagnetic wave excitation | Oscillations of the resistance observed under electromagnetic wave excitation
in the high mobility GaAs/AlGaAs 2DES are examined as a function of the
radiation frequency and the power, utilizing an empirical lineshape based on
exponentially damped sinusoids. The fit-analysis indicates the resistance
oscillation frequency, F, increases with the radiation frequency, n, at the
rate dF/dn = 2.37 mTesla/GHz; the damping parameter, a, is approximately
independent of n at constant power; and the amplitude, A, of the oscillations
grows slowly with the incident power, at a constant temperature and frequency.
The lineshape appears to provide a good description of the data. | 0305507v2 |
2003-07-15 | Bolometric technique for high-resolution broadband microwave spectroscopy of ultra-low-loss samples | A novel low temperature bolometric method has been devised and implemented
for high-precision measurements of the microwave surface resistance of small
single-crystal platelet samples having very low absorption, as a continuous
function of frequency. The key to the success of this non-resonant method is
the in-situ use of a normal metal reference sample that calibrates the absolute
rf field strength. The sample temperature can be controlled independently of
the 1.2 K liquid helium bath, allowing for measurements of the temperature
evolution of the absorption. However, the instrument's sensitivity decreases at
higher temperatures, placing a limit on the useful temperature range. Using
this method, the minimum detectable power at 1.3 K is 1.5 pW, corresponding to
a surface resistance sensitivity of $\approx$1 $\mu\Omega$ for a typical 1
mm$\times$1 mm platelet sample. | 0307340v1 |
2003-09-26 | Metastable Resistance Anisotropy Orientation of Two-Dimensional Electrons in High Landau Levels | In half-filled high Landau levels, two-dimensional electron systems possess
collective phases which exhibit a strongly anisotropic resistivity tensor. A
weak, but as yet unknown, rotational symmetry-breaking potential native to the
host semiconductor structure is necessary to orient these phases in macroscopic
samples. Making use of the known external symmetry-breaking effect of an
in-plane magnetic field, we find that the native potential can have two
orthogonal local minima. It is possible to initialize the system in the higher
minimum and then observe its relaxation toward equilibrium. | 0309625v2 |
2003-12-31 | Single-electron transistors in electromagnetic environments | The current-voltage (I-V) characteristics of single-electron transistors
(SETs) have been measured in various electromagnetic environments. Some SETs
were biased with one-dimensional arrays of dc superconducting quantum
interference devices (SQUIDs). The purpose was to provide the SETs with a
magnetic-field-tunable environment in the superconducting state, and a
high-impedance environment in the normal state. The comparison of SETs with
SQUID arrays and those without arrays in the normal state confirmed that the
effective charging energy of SETs in the normal state becomes larger in the
high-impedance environment, as expected theoretically. In SETs with SQUID
arrays in the superconducting state, as the zero-bias resistance of the SQUID
arrays was increased to be much larger than the quantum resistance R_K = h/e^2
= 26 kohm, a sharp Coulomb blockade was induced, and the current modulation by
the gate-induced charge was changed from e periodic to 2e periodic at a bias
point 0<|V|<2D_0/e, where D_0 is the superconducting energy gap. The author
discusses the Coulomb blockade and its dependence on the gate-induced charge in
terms of the single Josephson junction with gate-tunable junction capacitance. | 0312726v2 |
2004-08-19 | Charge dynamics of Ca_{2-x}Na_{x}CuO_{2}Cl_{2} as a correlated electron system with the ideal tetragonal lattice | We report the reflectivity and the resistivity measurement of
Ca_{2-x}Na_{x}CuO_{2}Cl_{2} (CNCOC), which has a single-CuO2-plane lattice with
no orthorhombic distortion. The doping dependence of the in-plane optical
conductivity spectra for CNCOC is qualitatively the same to those of other
cuprates, but a slight difference between CNCOC and LSCO, i.e., the absence of
the 1.5 eV peak in CNCOC, can be attributed to the smaller charge-stripe
instability in CNCOC. The temperature dependence of the optical onductivity
spectra of CNCOC has been analyzed both by the two-component model
(Drude+Lorentzian) and by the one-component model (extended-Drude analysis).
The latter analysis gives a universal trend of the scattering rate Gamma(omega)
with doping. It was also found that Gamma(omega) shows a saturation behavior at
high frequencies, whose origin is the same as that of resistivity saturation at
high temperatures. | 0408423v1 |
2004-11-17 | Spin-fluctuation dominated electrical transport of Ni3Al at high pressure | We present the first study of a magnetic quantum phase transition in the
itinerant-electron ferromagnet Ni3Al at high pressures. Electrical resistivity
measurements in a diamond anvil cell at hydrostatic pressures up to 100 kbar
and temperatures as low as 50 mK indicate that the Curie temperature collapses
towards absolute zero at a critical pressure pc=82(2) kbar. Over wide ranges in
pressure and temperature, both in the ferromagnetic and paramagnetic states,
the temperature variation of the resistivity is found to deviate from the
conventional Fermi-liquid form. We consider the extent to which this deviation
can be understood in terms of a mean-field model of enhanced spin fluctuations
on the border of ferromagnetism in three dimensions. | 0411451v1 |
2004-12-27 | Fiske steps studied with flux-flow resistance oscillation in a narrow stack of Bi2Sr2CaCu2O8+d junctions | We have experimentally investigated the fluxon dynamics in a narrow
Bi2Sr2CaCu2O8+d stack with junction length L~1.8 um. As an evidence of
high-frequency excitation by a collective cavity mode, under an (in-plane)
external magnetic field, the current-voltage characteristics show prominent
Fiske steps with the corresponding resonance frequencies of 75-305 GHz. Further
study of flux-flow resistance oscillation with various c-axis currents
clarifies the correlation with Fiske steps by distinguishing two different
regions i.e., static flux-flow region at low bias current level and dynamic
Fiske step region at high bias current level. | 0412690v3 |
2005-03-07 | Quantum Hall Effect at 40 kelvin: Evidence of MacroscopicQuantization in the Extreme Soft Limit | Evidence of both fractional and integer quantum hall effects (QHE) in three
dimensional bulk replica opal (250nm diameter) structures of non-crystalline
carbon are presented. In a remarkably soft quantum limit of ~ 40K temperature
and about one tesla of magnetic field clear hall steps, such as n= 2/3, 4/5, 1
and others were observed to be coordinated with the minima of longitudinal
magneto-resistance. This behavior is indicative of macroscopic quantum
phenomenon associated with electronic condensation into a strongly correlated
quantum liquid (QL). For other systems, such as very high mobility,
two-dimensional, electron (hole)-gas or (TDEG) these effects typically arise
under high magnetic fields (B) and at low temperatures (T), i.e., in the
extreme quantum limit (B/T>1). Currently, QHE is applied as calibration
benchmark, international resistance standard, and a characterization technique
for semiconductor heterostructures. We believe that applications can be
widespread if the devices and the operating conditions were more accessible. | 0503166v1 |
2005-04-01 | On the origin of multiple ordered phases in PrFe4P12 | The nature of multiple electronic orders in skutterudite PrFe_4P_{12} is
discussed on the basis of a model with antiferro-quadrupole (AFQ) interaction
of \Gamma_3 symmetry. The high-field phase can be reproduced qualitatively
provided (i) ferro-type interactions are introduced between the dipoles as well
as between the octupoles of localized f-electrons, and (ii) separation is
vanishingly small between the \Gamma_1-\Gamma_4^{(1)} crystalline electric
field (CEF) levels. The high-field phase can have either the same ordering
vector q=(1,0,0) as in the low-field phase, or a different one q=0 depending on
the parameters. In the latter case, distortion of the crystal perpendicular to
the (111) axis is predicted. The corresponding anomaly in elastic constants
should also appear. The electrical resistivity is calculated with account of
scattering within the CEF quasi-quartet. It is found that the resistivity as a
function of the direction of magnetic field shows a sharp maximum around the
(111) axis at low temperatures because of the level crossing. | 0504014v2 |
2005-04-27 | Influence of a Parallel Magnetic Field on Microwave Photoconductivity in a High-Mobility 2D Electron System | We have studied experimentally the influence of a parallel magnetic field
($B_{//}$) on microwave-induced resistance oscillations (MIRO) and
zero-resistance states (ZRS) previously discovered in a high-mobility 2D
electron system. We have observed a strong suppression of MIRO/ZRS by a modest
$B_{//}\sim 0.5$ T. In Hall bar samples, magnetoplasmon resonance (MPR) has
also been observed concurrently with the MIRO/ZRS. In contrast to the
suppression of MIRO/ZRS, the MPR peak is found to be enhanced by $B_{//}$.
These findings have not been addressed by current models proposed to explain
the microwave-induced effects. | 0504715v1 |
2005-05-25 | Anisotropy, disorder, and superconductivity in CeCu2Si2 under high pressure | Resistivity measurements were carried out up to 8 GPa on single crystal and
polycrystalline samples of CeCu2Si2 from differing sources in the homogeneity
range. The anisotropic response to current direction and small uniaxial
stresses was explored, taking advantage of the quasi-hydrostatic environment of
the Bridgman anvil cell. It was found that both the superconducting transition
temperature Tc and the normal state properties are very sensitive to uniaxial
stress, which leads to a shift of the valence instability pressure Pv and a
small but significant change in Tc for different orientations with respect to
the tetragonal c-axis. Coexistence of superconductivity and residual
resistivity close to the Ioffe-Regel limit around 5 GPa provides a compelling
argument for the existence of a valence-fluctuation mediated pairing
interaction at high pressure in CeCu2Si2. | 0505613v1 |
2005-07-06 | Magnetoresistance and spin polarization in the insulating regime of a Si two-dimensional electron system | We have studied the magnetoresistance in a high-mobility Si inversion layer
down to low electron concentrations at which the longitudinal resistivity
$\rho_{xx}$ has an activated temperature dependence. The angle of the magnetic
field was controlled so as to study the orbital effect proportional to the
perpendicular component $B_\perp$ for various total strengths $B_{\rm tot}$. A
dip in $\rho_{xx}$, which corresponds to the Landau level filling factor of
$\nu=4$, survives even for high resistivity of $\rho_{xx} \sim 10^8 \Omega$ at
$T= 150 {\rm mK}$. The linear $B_{\rm tot}$-dependence of the value of
$B_\perp$ at the dip for low $B_{\rm tot}$ indicates that a ferromagnetic
instability does not occur even in the far insulating regime. | 0507136v1 |
2006-05-22 | Radiation-induced magnetoresistance oscillations in two-dimensional electron systems under bichromatic irradiation | We analyze the magnetoresistance $R_{xx}$ oscillations in high-mobility
two-dimensional electron systems induced by the combined driving of two
radiation fields of frequency $\omega_1$ and $\omega_2$, based on the
balance-equation approach to magnetotransport for high-carrier-density systems
in Faraday geometry. It is shown that under bichromatic irradiation of
$\omega_2\sim 1.5 \omega_1$, most of the characterstic peak-valley pairs in the
curve of $R_{xx}$ versus magnetic field in the case of monochromatic
irradiation of either $\omega_1$ or $\omega_2$ disappear, except the one around
$\omega_1/\omega_c\sim 2$ or $\omega_2/\omega_c\sim 3$. $R_{xx}$ oscillations
show up mainly as new peak-valley structures around other positions related to
multiple photon processes of mixing frequencies $\omega_1+\omega_2$,
$\omega_2-\omega_1$, etc. Many minima of these resistance peak-valley pairs can
descend down to negative with enhancing radiation strength, indicating the
possible bichromaticzero-resistance states. | 0605521v1 |
2006-06-19 | Metallic State in Cubic FeGe beyond its Quantum Phase Transition | We report on results of electrical resistivity and structural investigations
on the cubic modification of FeGe under high pressure. The long-wavelength
helical order ($T_C=280$ K) is suppressed at a critical pressure $p_c\approx
19$ GPa. An anomaly in the resistivity data at $T_X(p)$ and strong deviations
from a Fermi-liquid behavior in a wide pressure range above $p_c$ suggest that
the suppression of $T_C$ disagrees with the standard notion of a quantum
critical phase transition. The metallic ground state persisting at high
pressure can be described by band-structure calculations if structural disorder
due to zero-point motion is included. Discontinuous changes in the pressure
dependence of the shortest Fe-Ge interatomic distance occurring close to the
$T_C(p)$ phase line could be interpreted as a symmetry-conserving transition of
first order. | 0606493v1 |
2007-03-23 | Giant asymmetry of the longitudinal magnetoresistance in high-mobility two-dimensional electron gas on a cylindrical surface | A giant asymmetry in the magnetoresistance was revealed in high-mobility,
two-dimensional electron gas on a cylindrical surface. The longitudinal
resistance along the magnetic-field gradient impressed by the surface curvature
was found to vanish if measured along one of the edges of the curved Hall bar.
If the external magnetic field is reversed, then the longitudinal resistance
vanishes at the opposite edge of the Hall bar. This asymmetry is analyzed
quantitatively in terms of the Landauer-Buettiker formalism. | 0703623v1 |
2006-12-17 | A New GEM-like Imaging Detector with Electrodes Coated with Resistive Layers | We have developed and tested several prototypes of GEM-like detectors with
electrodes coated with resistive layers: CuO or CrO. These detectors can
operate stably at gains close to 10E5 and they are very robust. We discovered
that the cathodes of these detectors could be coated by CsI layers and in such
a way the detectors gain high efficiency for the UV photons. We also
demonstrated that such detectors can operate stably in the cascade mode and
high overall gains (~10E6) are reachable. This opens applications in several
areas, for example in RICH or in noble liquid TPCs. Results from the first
applications of these devices for UV photon detection at room and cryogenic
temperatures are given. | 0612166v1 |
1997-02-07 | The Landauer Resistance and Band Spectra for the Counting Quantum Turing Machine | The generalized counting quantum Turing machine (GCQTM) is a machine which,
for any N, enumerates the first $2^{N}$ integers in succession as binary
strings. The generalization consists of associating a potential with read-1
steps only. The Landauer Resistance (LR) and band spectra were determined for
the tight binding Hamiltonians associated with the GCQTM for energies both
above and below the potential height. For parameters and potentials in the
electron region, the LR fluctuates rapidly between very high and very low
values as a function of momentum. The rapidity and extent of the fluctuations
increases rapidly with increasing N. For N=18, the largest value considered,
the LR shows good transmission probability as a function of momentum with
numerous holes of very high LR values present. This is true for energies above
and below the potential height. It is suggested that the main features of the
LR can be explained by coherent superposition of the component waves reflected
from or transmitted through the $2^{N-1}$ potentials in the distribution. If
this explanation is correct, it provides a dramatic illustration of the effects
of quantum nonlocality. | 9702021v1 |
2008-06-17 | Structural transition and anisotropic properties of single crystalline SrFe2As2 | Plate-like single crystals of SrFe2As2 as large as 3x3x0.5 mm3 have been
grown out of Sn flux. The SrFe2As2 single crystals show a structural phase
transition from a high temperature tetragonal phase to a low temperature
orthorhombic phase at To = 198 K, and do not show any sign of superconductivity
down to 1.8 K. The structural transition is accompanied by an anomaly in the
electrical resistivity, Hall resistivity, specific heat, and the anisotropic
magnetic susceptibility. In an intermediate temperature range from 198 K to 160
K, single crystal X-ray diffraction suggests a coexistence of the
high-temperature tetragonal and the low-temperature orthorhombic phases. | 0806.2711v1 |
2008-08-29 | On collisions driven negative energy waves and Weibel instability of a relativistic electron beam in a quasi-neutral plasma | A new quasi-neutral model describing the Weibel instability of a high-current
relativistic beam propagating through a resistive plasma is developed. It
treats beam electrons as kinetic particles, and ambient plasma as a
non-relativistic fluid. For a finite-temperature beam, a new class of negative
energy magneto-sound waves is identified, which can possess negative energy.
Their growth due to collisional dissipation in the cold return current
destabilizes the beam-plasma system even for high beam temperatures. We perform
detailed two- and three-dimensional particle-in-cell (PIC) simulations of the
thermal beam and collisional plasma. It is shown that in three dimensions, the
Weibel instability persists even for physically collisionless background
plasma. The anomalous plasma resistivity is then caused by the two-stream
instability. | 0808.4078v2 |
2008-10-05 | Anisotropic magnetic and superconducting properties of pure and Co-doped CaFe$_2$As$_2$ single crystals | We report anisotropic dc magnetic susceptibility $\chi(T)$, electrical
resistivity $\rho(T)$, and heat capacity $C(T)$ measurements on the single
crystals of CaFe$_{2-x}$Co$_x$As$_2$ for $x$ = 0 and 0.06. Large sized single
crystals were grown by the high temperature solution method with Sn as the
solvent. For the pure compound with $x$ = 0, a high temperature transition at
170 K is observed which is attributed to a combined spin density wave (SDW)
ordering and a structural phase transition. On the other hand, for the Co-doped
samples for $x$ = 0.06, the SDW transition is suppressed while
superconductivity is observed at $\simeq$17 K. The superconducting transition
has been confirmed from the magnetization and electrical resistivity studies.
The $^{57}$Fe M\"ossbauer spectrum in CaFe$_2$As$_2$ indicates that the SDW
ordering is incommensurate. In the Co-doped sample, a prominent paramagnetic
line at 4.2 K is observed indicating a weakening of the SDW state. | 0810.0848v2 |
2008-11-17 | Possible superconductivity above 25 K in single crystalline Co-doped BaFe$_{2}$As$_{2}$ | We present superconducting properties of single crystalline
Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$ by measuring magnetization, resistivity,
upper critical field, Hall coefficient, and magneto-optical images. The
magnetization measurements reveal fish-tail hysteresis loop at high
temperatures and relatively high critical current density above $J_{c}=10^{5}$
A/cm$^{2}$ at low temperatures. Upper critical field determined by resistive
transition is anisotropic with anisotropic parameter $\sim$ 3.5. Hall effect
measurements indicate that Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$ is a
multiband system and the mobility of electron is dominant. The magneto-optical
imaging reveals prominent Bean-like penetration of vortices although there is a
slight inhomogeneity in a sample. Moreover, we find a distinct
superconductivity above 25 K, which leads us to speculate that higher
transition temperature can be realized by fine tuning Co-doping level. | 0811.2621v1 |
2008-12-05 | High-T_c superconductivity induced by doping rare earth elements into CaFeAsF | We have successfully synthesized the fluoride-arsenide compounds
Ca$_{1-x}$RE$_x$FeAsF (RE=Nd, Pr; x=0, 0.6). The x-ray powder diffraction
confirmed that the main phases of our samples are Ca$_{1-x}$RE$_x$FeAsF with
the ZrCuSiAs structure. By measuring resistivity, superconductivity was
observed at 57.4 K in Nd-doped and 52.8 K in Pr-doped samples with x=0.6. Bulk
superconductivity was also proved by the DC magnetization measurements in both
samples. Hall effect measurements revealed hole-like charge carriers in the
parent compound CaFeAsF with a clear resistivity anomaly below 118 K, while the
Hall coefficient $R_H$ in the normal state is negative for the superconducting
samples Ca$_{0.4}$Nd$_{0.6}$FeAsF and Ca$_{0.4}$Pr$_{0.6}$FeAsF. This indicates
that the rare earth element doping introduces electrons into CaFeAsF which
induces the high temperature superconductivity. | 0812.1192v1 |
2009-01-14 | AC Josephson effect in the long voltage-biased SINIS junction | Theory of non-stationary coherent effects is developed for
superconductor-normal-superconductor (SNS) structures with relatively strong
normal scattering on S/N interfaces (interface resistance is large compared to
intrinsic resistance of N metal). Analitical expressions are found for the
time-dependent anomalous Green functions induced in the N region under the
fixed-voltage-bias. The amplitude of the current oscillations is determined in
non-equilibrium conditions. Non-stationary correction to the distribution
function is calculated in high-temperature limit and found to be slowly
decreasing with the temperature, leading to the dominance of the
second-harmonic term in the Josepshon current at high temperatures and low
voltage. | 0901.1966v2 |
2009-03-04 | Observation of a non-Ohmic Hall resistivity above the critical temperature in the high-temperature superconductor YBa$_{2}$Cu$_{3}$O$_{7-δ}$ | Investigations of the resistivity and the Hall effect as a function of
electric field and temperature in the normal state and upper part of the
superconducting transition of an optimally doped, very thin film of
YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ are reported. Using a fast pulsed-current
technique allowed to reduce the self-heating of the sample and to reach
electric fields up to 1 kV/cm. An intrinsic non-Ohmic behavior of the Hall
conductivity above the critical temperature that appears to originate from two
different, partially counteracting effects is revealed. The major contribution
stems from the suppression of Aslamazov-Larkin superconducting fluctuations in
high electric fields. | 0903.0725v1 |
2009-04-17 | EuFe$_2$As$_2$ under high pressure: an antiferromagnetic bulk superconductor | We report the ac magnetic susceptibility $\chi_{ac}$ and resistivity $\rho$
measurements of EuFe$_2$As$_2$ under high pressure $P$. By observing nearly
100% superconducting shielding and zero resistivity at $P$ = 28 kbar, we
establish that $P$-induced superconductivity occurs at $T_c \sim$~30 K in
EuFe$_2$As$_2$. $\rho$ shows an anomalous nearly linear temperature dependence
from room temperature down to $T_c$ at the same $P$. $\chi_{ac}$ indicates that
an antiferromagnetic order of Eu$^{2+}$ moments with $T_N \sim$~20 K persists
in the superconducting phase. The temperature dependence of the upper critical
field is also determined. | 0904.2618v2 |
2009-06-29 | Spontaneous non-steady magnetic reconnection within the solar environment | This work presents a 2.5-dimensional simulation study of the instability of
current-sheets located in a medium with a strong density variation along the
current layer. The initial force-free configuration is observed to undergo a
two-stage evolution consisting of an abrupt regime transition from a slow to a
fast reconnection process leading the system to a final chaotic configuration.
Yet, the onset of the fast phase is not determined by the presence of any
anomalous enhancement in plasma's local resistivity, but rather is the result
of a new mechanism discovered in Lapenta (2008)* and captured only when
sufficient resolution is present. Hence, the effects of the global resistivity,
the global viscosity and the plasma beta on the overall dynamics are
considered. This mechanism allowing the transition from slow to fast
reconnection provides a simple but effective model of several processes taking
place within the solar atmosphere from the high chromosphere up to the low
corona. In fact, the understanding of a spontaneous transition to a
self-feeding fast reconnection regime as well as its macroscopic evolution is
the first and fundamental step to produce realistic models of all those
phenomena requiring fast (and high power) triggering events (* Lapenta G. 2008,
Phys. Rev. Lett., 100, 235001). | 0906.5382v1 |
2009-07-17 | Performances of linseed oil-free bakelite RPC prototypes with cosmic ray muons | A comparative study has been performed on Resistive Plate Chambers (RPC) made
of two different grades of bakelite paper laminates, produced and commercially
available in India. The chambers, operated in the streamer mode using argon,
tetrafluroethane and isobutane in 34:59:7 mixing ratio, are tested for the
efficiency and the stability with cosmic rays. A particular grade of bakelite
(P-120, NEMA LI-1989 Grade XXX), used for high voltage insulation in humid
conditions, was found to give satisfactory performance with stable efficiency
of > 96% continuously for more than 130 days. A thin coating of silicone fluid
on the inner surfaces of the bakelite RPC is found to be necessary for
operation of the detector. | 0907.2976v1 |
2009-10-21 | "Bad Metal" Conductivity of Hard Core Bosons | Two dimensional hard core bosons suffer strong scattering in the high
temperature resistive state at half filling. The dynamical conductivity is
calculated using non perturbative tools such as continued fractions, series
expansions and exact diagonalization. We find a large temperature range with
linearly increasing resistivity and broad dynamical conductivity, signaling a
breakdown of Boltzmann-Drude quasiparticle transport theory. At zero
temperature, a high frequency peak in the dynamical conductivity appears above
a "Higgs mass" gap, and corresponds to order parameter magnitude fluctuations.
We discuss the apparent similarity between conductivity of hard core bosons and
phenomenological characteristics of cuprates, including the universal scaling
of Homes et. al. (Nature 430, 539 (2004)). | 0910.4158v2 |
2009-12-16 | Distributed control of reactive power flow in a radial distribution circuit with high photovoltaic penetration | We show how distributed control of reactive power can serve to regulate
voltage and minimize resistive losses in a distribution circuit that includes a
significant level of photovoltaic (PV) generation. To demonstrate the
technique, we consider a radial distribution circuit with a single branch
consisting of sequentially-arranged residential-scale loads that consume both
real and reactive power. In parallel, some loads also have PV generation
capability. We postulate that the inverters associated with each PV system are
also capable of limited reactive power generation or consumption, and we seek
to find the optimal dispatch of each inverter's reactive power to both maintain
the voltage within an acceptable range and minimize the resistive losses over
the entire circuit. We assume the complex impedance of the distribution circuit
links and the instantaneous load and PV generation at each load are known. We
compare the results of the optimal dispatch with a suboptimal local scheme that
does not require any communication. On our model distribution circuit, we
illustrate the feasibility of high levels of PV penetration and a significant
(20% or higher) reduction in losses. | 0912.3281v1 |
2010-03-26 | Strong carrier-scattering in iron-pnictide superconductors with highest Tc obtained from charge transport experiments | Characteristic normal-state charge transport is found in the oxygen-deficient
iron-arsenides LnFeAsO1-y (Ln: La and Nd) with the highest Tc's among known
Fe-based superconductors. The effect of "doping" in this system is mainly on
the carrier scattering, quite distinct from that in high-Tc cuprates. In the
superconducting regime of the La system with maximum Tc = 28 K, the
low-temperature resistivity is dominated by a T^2 term. On the other hand, in
the Nd system with Tc higher than 40 K, the carriers are subject to stronger
scattering showing T-linear resistivity and small magnetoresistance. Such
strong scattering appears crucial for high-Tc superconductivity in the
iron-based system. | 1003.5039v1 |
2010-04-14 | Electron-hole coexistence in disordered graphene probed by high-field magneto-transport | We report on magneto-transport measurement in disordered graphene under
pulsed magnetic field of up to 57T. For large electron or hole doping, the
system displays the expected anomalous Integer Quantum Hall Effect (IQHE)
specific to graphene up to filling factor $\nu=2$. In the close vicinity of the
charge neutrality point, the system breaks up into co-existing puddles of holes
and electrons, leading to a vanishing Hall and finite longitudinal resistance
with no hint of divergence at very high magnetic field. Large resistance
fluctuations are observed near the Dirac point. They are interpreted as the the
natural consequence of the presence of electron and hole puddles. The magnetic
field at which the amplitude of the fluctuations are the largest is directly
linked to the mean size of the puddles. | 1004.2356v1 |
2010-05-30 | Loss-resistant state teleportation and entanglement swapping using a quantum-dot spin in an optical microcavity | We present a scheme for efficient state teleportation and entanglement
swapping using a single quantum-dot spin in an optical microcavity based on
giant circular birefringence. State teleportation or entanglement swapping is
heralded by the sequential detection of two photons, and is finished after the
spin measurement. The spin-cavity unit works as a complete Bell-state analyzer
with a built-in spin memory allowing loss-resistant repeater operation. This
device can work in both the weak coupling and the strong coupling regime, but
high efficiencies and high fidelities are only achievable when the side leakage
and cavity loss is low. We assess the feasibility of this device, and show it
can be implemented with current technology. We also propose a spin manipulation
method using single photons, which could be used to preserve the spin coherence
via spin echo techniques. | 1005.5545v2 |
2010-09-28 | Vortex Phase Diagram of Layered Superconductor Cu0.03TaS2 for H || c | The magnetization and anisotropic electrical transport properties have been
measured in high quality Cu0.03TaS2 single crystal. A pronounced peak effect
has been observed, indicating that the high quality and homogeneity are vital
to peak effect. A kink has been observed in the magnetic field H dependence of
the in-plane resistivity {\rho}ab for H || c, which corresponds to a transition
from activated to diffusive behavior of vortex liquid phase. In the diffusive
regime of the vortex liquid phase, the in-plane resistivity {\rho}ab shows
{\rho}ab $\propto$ H0.3 relation, which does not follow the Bardeen-Stephen law
for free flux flow. Finally, a simplified vortex phase diagram of Cu0.03TaS2
for H || c is given. | 1009.5449v2 |
2010-11-27 | Performance of Glass Resistive Plate Chambers for a high granularity semi-digital calorimeter | A new design of highly granular hadronic calorimeter using Glass Resistive
Plate Chambers (GRPCs) with embedded electronics has been proposed for the
future International Linear Collider (ILC) experiments. It features a 2-bit
threshold semi-digital read-out. Several GRPC prototypes with their electronics
have been successfully built and tested in pion beams. The design of these
detectors is presented along with the test results on efficiency, pad
multiplicity, stability and reproducibility. | 1011.5969v2 |
2010-12-13 | Self-consistent calculation of the single particle scattering rate in high $Tc$ cuprates | The linear temperature dependence of the resistivity above the optimal doping
is a longstanding problem in the field of high temperature superconductivity in
cuprates. In this paper, we investigate the effect of gauge fluctuations on the
momentum relaxation time and the transport scattering rate within the slave
boson method. We use a more general slave treatment to resolve the ambiguity of
decomposing the Heisenberg exchange term. We conclude that this term should be
decomposed only in the Cooper channel. This results in the spinon mass
inversely proportional to the doping. It is showed that solving the equation
for the transport scattering rate self-consistently, we find a crossover
temperature above which we obtain the linear temperature dependence of the
electrical resistivity as well as the single particle scattering rate. It is
also shown that this linear temperature dependence of the scattering rate in
the pseudogap region explains the existence of the Fermi arcs with a length
that linearly varies with temperature. | 1012.2764v1 |
2011-03-17 | Quantum-Classical Crossover and Apparent Metal-Insulator Transition in a Weakly Interacting 2D Fermi Liquid | We report the observation of a parallel magnetic field induced
metal-insulator transition (MIT) in a high-mobility two-dimensional electron
gas (2DEG) for which spin and localization physics most likely play no major
role. The high-mobility metallic phase at low field is consistent with the
established Fermi liquid transport theory including phonon scattering, whereas
the insulating phase at higher field shows a large negative temperature
dependence at resistances much smaller than the quantum of resistance, $h/e^2$.
We argue that this observation is a direct manifestation of a quantum-classical
crossover arising predominantly from the magneto-orbital coupling between the
finite width of the 2DEG and the in-plane magnetic field. | 1103.3496v2 |
2011-05-05 | Pairing Fluctuations and Anomalous Transport Above the BCS-BEC Crossover in the Two Dimensional Attractive Hubbard Model | A Fermi liquid with weak attractive interaction undergoes a BCS transition to
a superconductor with reducing temperature. With increasing interaction
strength, the thermal transition is progressively modified as the high
temperature `metallic' phase develops a pseudogap due to pairing fluctuations
and the resistivity above T_c shows insulating behaviour. The crossover to
insulating character occurs much before the system can be considered to be in
the BEC regime of preformed fermion pairs. We use a new Monte Carlo tool to map
out the BCS-BEC crossover in the attractive Hubbard model on large two
dimensional lattices and explicitly compute the resistivity to demonstrate how
the metal to superconductor (MS) thermal transition at weak coupling crosses
over to an insulator to superconductor (IS) transition at intermediate
coupling. Our high resolution access to the single particle and optical
spectrum at finite temperature allows us to completely describe the transport
crossover in this longstanding problem. | 1105.1156v1 |
2011-11-28 | Evolution of a metastable phase with a magnetic phase coexistence phenomenon and its unusual sensitivity to magnetic field cycling in the alloys Tb5-xLuxSi3 (x <= 0.7) | Recently, we reported an anomalous enhancement of the positive
magnetoresistance beyond a critical magnetic field in Tb5Si3 in the
magnetically ordered state, attributable to 'inverse metamagnetism'. This
results in unusual magnetic hysteresis loops for the pressurized specimens,
which are relevant to the topic of 'electronic phase separation'. In this
paper, we report the influence of small substitutions of Lu for Tb, to show the
evolution of these magnetic anomalies. We find that, at low temperatures, the
high-field high-resistivity phase could be partially stabilized on returning
the magnetic field to zero in many of these Lu substituted alloys, as measured
through the electrical resistivity ({\rho}). Also, the relative fractions of
this phase and the virgin phase appear to be controlled by a small tuning of
the composition and temperature. Interestingly, at 1.8 K a sudden 'switch-over'
of the value of {\rho} for this mixed phase to that for the virgin phase for
some compositions is observed at low fields after a few field cycles,
indicating metastability of this mixed phase. | 1111.6368v1 |
2012-10-09 | Interdependence of Electric Discharge and The Magnetorotational Instability in Protoplanetary Disks | We study how the magnetorotational instability (MRI) in protoplanetary disks
is affected by the electric discharge caused by the electric field in the
resistive MHD. We have performed three-dimensional shearing box simulations
with various values of plasma beta and electrical breakdown models. We find the
self-sustainment of the MRI in spite of the high resistivity. The instability
gives rise to the large electric field that causes the electrical breakdown,
and the breakdown maintains the high ionization degree required for the
instability.
The condition for this self-sustained MRI is set by the balance between the
energy supply from the shearing motion and the energy consumed by the Ohmic
dissipation. We apply the condition to various disk models and study where the
active, self-sustained, and dead zones of MRI are. In the fiducial minimum-mass
solar nebula (MMSN) model, the newly-found sustained zone occupies only the
limited volume of the disk. In the late-phase gas-depleted disk models,
however, the sustained zone occupies larger volume of the disk. | 1210.2508v2 |
2012-12-18 | Microwave properties of superconducting atomic-layer deposited TiN films | We have grown superconducting TiN films by atomic layer deposition with
thicknesses ranging from 6 to 89 nm. This deposition method allows us to tune
the resistivity and critical temperature by controlling the film thickness. The
microwave properties are measured, using a coplanar-waveguide resonator, and we
find internal quality factors above a million, high sheet inductances (5.2-620
pH), and pulse response times up to 100 \mu s. The high normal state
resistivity of the films (> 100 \mu\Omega cm) affects the superconducting state
and thereby the electrodynamic response. The microwave response is modeled
using a quasiparticle density of states modified with an effective
pair-breaker,consistently describing the measured temperature dependence of the
quality factor and the resonant frequency. | 1212.4434v1 |
2013-04-22 | Persistent non-metallic behavior in Sr2IrO4 and Sr3Ir2O7 at high pressures | Iridium-based 5d transition-metal oxides are attractive candidates for the
study of correlated electronic states due to the interplay of enhanced
crystal-field, Coulomb and spin-orbit interaction energies. At ambient
pressure, these conditions promote a novel Jeff = 1/2 Mott insulating state,
characterized by a gap of the order of ~0.1 eV. We present high-pressure
electrical resistivity measurements of single crystals of Sr2IrO4 and Sr3Ir2O7.
While no indications of a pressure-induced metallic state up to 55 GPa were
found in Sr2IrO4, a strong decrease of the gap energy and of the resistance of
Sr3Ir2O7 between ambient pressure and 104 GPa confirm that this compound is in
the proximity of a metal-insulator transition. | 1304.5864v2 |
2013-08-28 | Normal-state charge dynamics in doped BaFe2As2: Roles of doping and necessary ingredients for superconductivity | We carried out a comparative study of the in-plane resistivity and optical
spectrum of doped BaFe2As2 and investigated the doping evolution of the charge
dynamics. For BaFe2As2, charge dynamics is incoherent at high temperatures.
Electron (Co) and isovalent (P) doping into BaFe2As2 increase coherence of the
system and transform the incoherent charge dynamics into highly coherent one.
On the other hand, charge dynamics remains incoherent for hole (K) doping. It
is found in common with any type of doping that superconductivity with high
transition temperature emerges when the normal-state charge dynamics maintains
incoherence and when the resistivity associated with the coherent channel
exhibits dominant temperature-linear dependence. | 1308.6133v1 |
2014-04-07 | Crystal structure and physical properties of EuPtIn$_{4}$ intermetallic antiferromagnet | We report the synthesis of EuPtIn$_{4}$ single crystalline platelets by the
In-flux technique. This compound crystallizes in the orthorhombic Cmcm
structure with lattice parameters $a=4.542(1)$ \AA, $b=16.955(2)$ \AA$\,$ and
$c=7.389(1)$ \AA. Measurements of magnetic susceptibility, heat capacity,
electrical resistivity, and electron spin resonance (ESR) reveal that
EuPtIn$_{4}$ is a metallic Curie-Weiss paramagnet at high temperatures and
presents antiferromagnetic (AFM) ordering below $T_{N}=13.3$ K. In addition, we
observe a successive anomaly at $T^{*} = 12.6$ K and a spin-flop transition at
$H_{c} \sim 2.5$ T applied along the $ac$-plane. In the paramagnetic state, a
single Eu$^{2+}$ Dysonian ESR line with a Korringa relaxation rate of $b =
4.1(2)$ Oe/K is observed. Interestingly, even at high temperatures, both ESR
linewidth and electrical resistivity reveal a similar anisotropy. We discuss a
possible common microscopic origin for the observed anisotropy in these
physical quantities likely associated with an anisotropic magnetic interaction
between Eu$^{2+}$ 4$f$ electrons mediated by conduction electrons. | 1404.1660v1 |
2014-04-14 | High temperature superconducting FeSe films on SrTiO3 substrates | Interface enhanced superconductivity at two dimensional limit has become one
of most intriguing research directions in condensed matter physics. Here, we
report the superconducting properties of ultra-thin FeSe films with the
thickness of one unit cell (1-UC) grown on conductive and insulating SrTiO3
(STO) substrates. For the 1-UC FeSe on conductive STO substrate (Nb-STO), the
magnetization versus temperature (M-T) measurement shows a diamagnetic signal
at 85 K, suggesting the possibility of superconductivity appears at this high
temperature. For the FeSe films on insulating STO substrate, systematic
transport measurements were carried out and the sheet resistance of FeSe films
exhibits Arrhenius TAFF behavior with a crossover from a single-vortex pinning
region to a collective creep region. More intriguing, sign reversal of Hall
resistance with temperature is observed, demonstrating a crossover from hole
conduction to electron conduction above Tc in 1-UC FeSe films. | 1404.3464v2 |
2014-04-16 | Magnetic Ordering at Anomalously High Temperatures in Dy at Extreme Pressures: a New Kondo-Lattice State? | In an attempt to destabilize the magnetic state of the heavy lanthanides Dy
and Gd, extreme pressures were applied in an electrical resistivity measurement
to 157 GPa over the temperature range 5 - 295 K. The magnetic ordering
temperature $T_{\text{o}}$ and spin-disorder resistance $R_{sd}$ of Dy, as well
as the superconducting pair-breaking effect $\Delta T_{c}$ in Y(1 at.\% Dy),
are found to track each other in a highly non-monotonic fashion as a function
of pressure, all three increasing sharply above 73 GPa, the critical pressure
for a 6\% volume collapse in Dy. At 157 GPa $T_{\text{o}}$ is estimated to
reach temperatures in the range 370 - 500 K, the highest magnetic ordering
temperature of any lanthanide. In contrast, $T_{\text{o}% }(P)$ for Gd shows no
such sharp increase to 105 GPa. Taken together, these results suggest that
pressures greater than 73 GPa transform Dy from a conventional magnetic
lanthanide into a Kondo lattice system with an anomalously high magnetic
ordering temperature. | 1404.4256v3 |
2014-05-14 | Theory of universal incoherent metallic transport | In an incoherent metal, transport is controlled by the collective diffusion
of energy and charge rather than by quasiparticle or momentum relaxation. We
explore the possibility of a universal bound $D \gtrsim \hbar v_F^2/(k_B T)$ on
the underlying diffusion constants in an incoherent metal. Such a bound is
loosely motivated by results from holographic duality, the uncertainty
principle and from measurements of diffusion in strongly interacting
non-metallic systems. Metals close to saturating this bound are shown to have a
linear in temperature resistivity with an underlying dissipative timescale
matching that recently deduced from experimental data on a wide range of
metals. This bound may be responsible for the ubiquitous appearance of high
temperature regimes in metals with $T$-linear resistivity, motivating direct
probes of diffusive processes and measurements of charge susceptibilities. | 1405.3651v3 |
2014-05-14 | Linear Resistivity from Non-Abelian Black Holes | Starting with the holographic p-wave superconductor, we show how to obtain a
finite DC conductivity through a non-abelian gauge transformation. The
translational symmetry is preserved. We obtain phenomenological similarities
with high temperature cuprate superconductors. Our results suggest that a
lattice or impurities are not essential to produce a finite DC resistivity with
a linear temperature dependence. An analogous field theory calculation for free
fermions, presented in the appendix, indicates our results may be a special
feature of strong interactions. | 1405.3714v4 |
2014-06-10 | Nonlinear transport and noise thermometry in quasi-classical ballistic point contacts | We study nonlinear transport and non-equilibrium current noise in
quasi-classical point contacts (PCs) defined in a low-density high-quality
two-dimensional electron system in GaAs. At not too high bias voltages $V$
across the PC the noise temperature is determined by a Joule heat power and
almost independent on the PC resistance that can be associated with a
self-heating of the electronic system. This commonly accepted scenario breaks
down at increasing $V$, where we observe extra noise accompanied by a strong
decrease of the PC's differential resistance. The spectral density of the extra
noise is roughly proportional to the nonlinear current contribution in the PC
$\delta S\approx2F^*|e\delta I|\sim V^2$ with the effective Fano factor
$F^*<1$, indicating that a random scattering process is involved. A small
perpendicular magnetic field is found to suppress both $\delta I$ and $\delta
S$. Our observations are consistent with a concept of a drag-like mechanism of
the nonlinear transport mediated by electron-electron scattering in the leads
of quasi-classical PCs. | 1406.2481v2 |
2014-12-11 | Transition-Metal Substitutions in Iron Chalcogenides | The $ab$-plane resistivity and Hall effect are studied in
Fe$_{1-y}$M$_y$Te$_{0.65}$Se$_{0.35}$ single crystals doped with two transition
metal elements, M = Co or Ni, over a wide doping range, $0 \leq y \leq 0.2$.
The superconducting transition temperature, $T_{c}$, reaches zero for Co at $y
\simeq 0.14$ and for Ni at $y \simeq 0.032$, while the resistivity at the
$T_{c}$ onset increases weakly with Co doping, and strongly with Ni doping. The
Hall coefficient $R_H$, positive for $y$ = 0, remains so at high temperatures
for all $y$, while it changes sign to negative at low $T$ for $y > 0.135$ (Co)
and $y > 0.06$ (Ni). The analysis based on a two band model suggests that at
high $T$ residual hole pockets survive the doping, but holes get localized upon
the lowering of $T$, so that the effect of the electron doping on the transport
becomes evident. The suppression of the $T_c$ by Co impurity is related to
electron doping, while in case of the Ni impurity strong electron localization
most likely contributes to fast decrease of the $T_c$. | 1412.3599v1 |
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