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2020-09-04 | Thermal conductivity of PbTe-CoSb3 bulk polycrystalline composite: the role of microstructure and interface thermal resistance | Systematic experimental and theoretical research on the role of
microstructure and interface thermal resistance on the thermal conductivity of
the PbTe-CoSb3 bulk polycrystalline composite is presented. In particular, the
correlation between the particle size of the dispersed phase and interface
thermal resistance (R_{int}) on the phonon thermal conductivity (\kappa_{ph})
is discussed. With this aim, a series of PbTe-CoSb_3 polycrystalline composite
materials with the different particle sizes of CoSb_3 was prepared. The
structural (XRD) and microstructural analysis (SEM/EDXS) confirmed assumed
chemical and phase compositions. The acoustic impedance difference (\Delta Z)
was determined from measured sound velocities in PbTe and CoSb_3 phases. The
interface thermal resistance (R_{int}) was calculated using the Debye model and
agrees with the experimental R_{int}. It is shown that the \kappa_{ph} of the
composite may be reduced when the particle size of the dispersed phase (CoSb_3)
is smaller than the critical value of ~230nm. This relationship was concluded
to be crucial for controlling the heat transport phenomena in composite
thermoelectric materials. The selection of the components with different
elastic properties (acoustic impedance) and particle size smaller than the
Kapitza radius leads to a new direction in the engineering of composite TE
materials with designed thermal properties | 2009.02190v2 |
2020-09-20 | Development of a computational software in Python, used to study the materials resistance in beams | In this research, we do a software writing in Python to calculate the
efforts, bending moments and deformations in beams of different materials. This
computational tool, that we developed, is of great help in area of
computational physical, more exactly in resistance of materials, which serves
as support for researchers and teachers especially in physics and civil
engineering, of the part of statics, who wish to carry out the modeling of the
functions, involved in the calculation of resistance in beams in a practical
and simple way, using the software presented in this article. In order to carry
out this software, we are going to use the following methods: the
double-integration method and the conjugate-beam method, which will serve as
the basis of calculation to find the mathematical expressions involved in the
analysis of resistance in beams, then we will perform the implementation of the
aforementioned methods, using Python as the programming language. As a final
step in this project, the graphical interface of said calculation tool will be
made using the Python 3.0 Tkinter library. In this work, we show the results of
the graphs of the stress profiles, bending moments and deformations, for the
case of different types of beams, load and force distributions applied to them.
Where we were also able to conclude that a calculation software was
successfully built, dedicated to the analysis of efforts and deformations in
beams made of different materials. | 2009.09448v2 |
2022-07-01 | High-throughput analysis of Fröhlich-type polaron models | The electronic structure of condensed matter can be significantly affected by
the electron-phonon interaction, leading to important phenomena such as
electrical resistance, superconductivity or the formation of polarons. This
interaction is often neglected in band structure calculations but can have a
strong impact on band gaps or optical spectra. Commonly used frameworks for
electron-phonon energy corrections are the Allen-Heine-Cardona theory and the
Fr\"ohlich model. While the latter shows qualitative agreement with experiment
for many polar materials, its simplicity should bring hard limits to its
applicability in real materials. Improvements can be made by introducing a
generalized version of the model, which considers anisotropic and degenerate
electronic bands, and multiple phonon branches. In this work, we search for
trends and outliers on over a thousand materials in existing databases of
phonon and electron band structures. We use our results to identify the limits
of applicability of the standard Fr\"olich model by comparing to the
generalized version, and by testing its basic hypothesis of a large radius for
the polaronic wavefunction and the corresponding atomic displacement cloud.
Among our extended set of materials, most exhibit large polaron behavior as
well as validity of the perturbative treatment. For the valence band, there is
also a significant fraction of the materials for which the perturbative
treatment cannot be applied and/or for which the size of the self-trapping
region is close to the atomic repetition distance. We find a large variety of
behaviors, and employ much more accurate, fully ab initio Allen-Heine-Cardona
calculations to understand extreme cases, where the Fr\"ohlich model should
fail and unusually large zero-point renormalization energies occur. | 2207.00364v1 |
2013-03-26 | Molecular Memory with Atomically-Smooth Graphene Contacts | We report the use of bilayer graphene as an atomically-smooth contact for
nanoscale devices. A two-terminal Bucky ball (C60) based molecular memory is
fabricated with bilayer graphene as a contact on the polycrystalline nickel
electrode. Graphene provides an atomically-smooth covering over an otherwise
rough metal surface. The use of graphene additionally prohibits the
electromigration of nickel atoms into the C60 layer. The devices exhibit a
low-resistance state in the first sweep cycle and irreversibly switch to a high
resistance state at 0.8-1.2 V bias. The reverse sweep has a hysteresis behavior
as well. In the subsequent cycles, the devices retain the high-resistance
state, thus making it write-once read-many memory (WORM). The ratio of current
in low-resistance to high-resistance state is lying in 20-40 range for various
devices with excellent retention characteristics. Control sample without the
bilayer graphene shows random hysteresis and switching. | 1303.6603v1 |
2020-04-10 | Vortex Dynamics and Dissipation Under High-amplitude Microwave Drive | In this paper, we describe the vortex dynamics under high-amplitude microwave
drive and its effect on the surface resistance of superconductors. The vortex
surface resistance is calculated with a Montecarlo approach, where the vortex
motion equation is solved for a collection of vortex flux lines each
oscillating within a random pinning landscape. This approach is capable of
providing a detailed description of the microscopic vortex dynamics and in turn
important insights into the microwave field amplitude dependence of the vortex
surface resistance. The numerical simulations are compared against experimental
data of vortex surface resistance at high microwave amplitude measured by means
of bulk niobium superconducting-radio frequency cavities operating at 1.3 GHz.
The good qualitative agreement of simulations and experiments suggests that the
non-linear dependence of the trapped flux surface resistance with the microwave
field amplitude is generated by progressive microwave depinning and vortex
jumps. | 2004.05083v2 |
2015-07-17 | Resistive thrust production can be as crucial as added mass mechanisms for inertial undulatory swimmers | In this paper, we address a crucial point regarding the description of
moderate to high Reynolds numbers aquatic swimmers. For decades, swimming
animals have been classified in two different families of propulsive mechanisms
based on the Reynolds number: the "resistive" swimmers, using local friction to
produce the necessary thrust force for locomotion at low Reynolds number and
the "reactive" swimmers, lying in the high Reynolds range, and using added mass
acceleration (described by perfect fluid theory). However, inertial swimmers
are also systems that dissipate energy, due to their finite size, therefore
involving strong resistive contributions, even for high Reynolds numbers. Using
a complete model for the hydrodynamic forces, involving both reactive and
resistive contributions, we revisit here the physical mechanisms responsible
for the thrust production of such swimmers. We show, for instance, that the
resistive part of the force balance is as crucial as added mass effects in the
modeling of the thrust force, especially for elongated species. The conclusions
brought by this work may have significant contributions to the understanding of
complex swimming mechanisms, especially for the future design of artificial
swimmers. | 1507.04952v1 |
2022-07-13 | Memristive and tunneling effects in 3D interconnected silver nanowires | Due to their memristive properties nanowire networks are very promising for
neuromorphic computing applications. Indeed, the resistance of such systems can
evolve with the input voltage or current as it confers a synaptic behaviour to
the device. Here, we propose a network of silver nanowires (Ag-NWs) which are
grown in a nanopourous membrane with interconnected nanopores by
electrodeposition. This bottom-up approach fabrication method gives a
conducting network with a 3D architecture and a high density of Ag-NWs. The
resulting 3D interconnected Ag-NW network exhibits a high initial resistance as
well as a memristive behavior. It is expected to arise from the creation and
the destruction of conducting silver filaments inside the Ag-NW network.
Moreover, after several cycles of measurement, the resistance of the network
switches from a high resistance regime, in the GOhm range, with a tunnel
conduction to a low resistance regime, in the kOhm range. | 2207.06338v1 |
2011-05-06 | Giant microwave photoresistivity in a high-mobility quantum Hall system | We report the observation of a remarkably strong microwave photoresistivity
effect in a high-mobility two-dimensional electron system subject to a weak
magnetic field and low temperature. The effect manifests itself as a giant
microwave-induced resistivity peak which, in contrast to microwave-induced
resistance oscillations, appears only near the second harmonic of the cyclotron
resonance and only at sufficiently high microwave frequencies. Appearing in the
regime linear in microwave intensity, the peak can be more than an order of
magnitude stronger than the microwave-induced resistance oscillations and
cannot be explained by existing theories. | 1105.1390v1 |
2014-11-19 | Towards pristine graphene-metal interface and microstructures: Laser assisted direct patterning on Epitaxial graphene | Graphene-metal contact resistance is governed by both intrinsic and extrinsic
factors. Intrinsically, both the density of states bottleneck near the Dirac
point and carrier reflection at the graphene-metal interface lead to a high
contact resistance. Moreover, graphene exhibits insulating behavior for
out-of-the-plane conduction. Extrinsically, surface contamination introduced by
photoresist residue or different adsorbed species during standard lithography
processing alters graphene's intrinsic properties by uncontrolled doping and
increased scattering which results in high and inconsistent contact resistance.
Here we demonstrate a femto-second laser assisted direct patterning of graphene
microstructures that enables us to study both intrinsic and extrinsic effects
on the graphene-metal interface. We show that a clean graphene-metal interface
is not sufficient to obtain contact resistance approaching the intrinsic limit
set by the quantum resistance. We also demonstrated that unlike CVD graphene,
edge state conduction (or end-contact) is not spontaneously formed by metal
deposition in case of graphene grown on SiC(0001). We conclude that for
epitaxial graphene, intentional end-contact formation is necessary to obtain
contact resistance near the quantum contact resistance limit. | 1411.5114v1 |
2023-11-08 | An attention-based deep learning network for predicting Platinum resistance in ovarian cancer | Background: Ovarian cancer is among the three most frequent gynecologic
cancers globally. High-grade serous ovarian cancer (HGSOC) is the most common
and aggressive histological type. Guided treatment for HGSOC typically involves
platinum-based combination chemotherapy, necessitating an assessment of whether
the patient is platinum-resistant. The purpose of this study is to propose a
deep learning-based method to determine whether a patient is platinum-resistant
using multimodal positron emission tomography/computed tomography (PET/CT)
images. Methods: 289 patients with HGSOC were included in this study. An
end-to-end SE-SPP-DenseNet model was built by adding Squeeze-Excitation Block
(SE Block) and Spatial Pyramid Pooling Layer (SPPLayer) to Dense Convolutional
Network (DenseNet). Multimodal data from PET/CT images of the regions of
interest (ROI) were used to predict platinum resistance in patients. Results:
Through five-fold cross-validation, SE-SPP-DenseNet achieved a high accuracy
rate and an area under the curve (AUC) in predicting platinum resistance in
patients, which were 92.6% and 0.93, respectively. The importance of
incorporating SE Block and SPPLayer into the deep learning model, and
considering multimodal data was substantiated by carrying out ablation studies
and experiments with single modality data. Conclusions: The obtained
classification results indicate that our proposed deep learning framework
performs better in predicting platinum resistance in patients, which can help
gynecologists make better treatment decisions. Keywords: PET/CT, CNN, SE Block,
SPP Layer, Platinum resistance, Ovarian cancer | 2311.04769v1 |
2000-08-17 | Development of Gold Contacted Flip-chip Detectors with IMARAD CZT | We present initial results from our evaluation of a gold-contacted pixellated
detector using cadmium zinc telluride substrate produced by IMARAD Imaging
Systems. The Horizontal Bridgman (HB) grown crystals from IMARAD have been
shown to produce high resolution photopeaks, but they are also seen to have
large leakage current. Our previous tests with IMARAD CZT showed that the use
of indium anodes and gold cathode improved the resistivity compared to the
standard indium-contacted detectors. We seek to test whether simple evaporated
gold contacts alone could also reduce the leakage current and thus improve the
spectral resolution, especially in the 10-100 keV energy range. We have
fabricated several metal-semiconductor-metal (MSM) detectors with a 4x4 array
of pixels on 10x10 mm substrates. Measurements of the detectors' leakage
current, spectral response, and temperature sensitivity are presented and
compared to IMARAD's ohmic contact detector and gold contact MSM detectors made
of High Pressure Bridgman (HPB) material. Finally, we show preliminary results
from a tiled flip-chip pixellated detector made using the IMARAD detectors. | 0008275v1 |
1997-09-21 | Microwave properties of $(Pr_xY_{1-x})Ba_2Cu_3O_{7-δ}$ : Influence of magnetic scattering | We report measurements of the surface impedance $Z_s=R_s+iX_s$ of
$(Pr_xY_{1-x})Ba_2Cu_3O_{7-\delta}$, $(x=0,0.15,0.23,0.3,0.4,0.5)$. Increasing
$Pr$ concentration leads to some striking results not observed in samples doped
by non-magnetic constituents. The three principal features of the $R_s(T)$ data
- multiple structure in the transition, a high residual resistance and, at high
$Pr$ concentrations, an upturn of the low $T$ data, are all characteristic of
the influence of magnetic scattering on superconductivity, and appear to be
common to materials where magnetism and superconductivity coexist. The low $T$
behavior of $\lambda (T)$ appears to change from $T$ to $T^4$ at large $Pr$
doping, and provides evidence of the influence of magnetic pairbreaking of the
$Pr$. | 9709232v1 |
1997-12-08 | High frequency magneto-electrodynamics of La_(1-x)Sr_xMnO3 single crystals | The radio frequency (RF) response of La1-xSrxMnO3 single crystals reveal a
variety of features associated with the structural, electronic and magnetic
properties of the system. The resonance technique operating at ~ 4 MHz employed
in this study is sensitive to small changes in both the magnetic susceptibility
and resistivity of the samples. Very sharp changes in frequency are observed at
the ferromagnetic (FM) and structural phase transitions in both the metallic (x
= 0.175) and insulating (0.125) crystals studied. In addition to the known
transitions identified as FM and orthorhombic distortions, our experiments show
rich structures which are not observed in conventional DC magnetization and
transport experiments. Our results demonstrate that RF experiments are ideally
suited to investigate the complex phase diagram in the manganites. The colossal
frequency change that we observe at the FM transition in the La1-xSrxMnO3
crystals is indicative of the enormous potential for using these materials in
high frequency switching applications. | 9712093v1 |
1999-04-28 | Structure and properties of a novel fulleride Sm6C60 | A novel fulleride Sm6C60 has been synthesized using high temperature solid
state reaction. The Rietveld refinement on high resolution synchrotron X-ray
powder diffraction data shows that Sm6C60 is isostructural with body-centered
cubic A6C60 (A=K, Ba). Raman spectrum of Sm6C60 is similar to that of Ba6C60,
and the frequencies of two Ag modes in Sm6C60 are nearly the same as that of
Ba6C60, suggesting that Sm is divalent and hybridization between C60 molecules
and the Sm atom could exist in Sm6C60. Resistivity measurement shows a weak
T-linear behavior above 180 K, the transport at low temperature is mainly
dominated by granular-metal theory. | 9904404v1 |
2000-04-10 | Magnetotransport study of the charged stripes in high-T_c cuprates | We present a study of the in-plane and out-of-plane magnetoresistance (MR) in
heavily-underdoped, antiferromagnetic YBa_{2}Cu_{3}O_{6+x}, which reveals a
variety of striking features. The in-plane MR demonstrates a "d-wave"-like
anisotropy upon rotating the magnetic field H within the ab plane. With
decreasing temperature below 20-25 K, the system acquires memory: exposing a
crystal to the magnetic field results in a persistent in-plane resistivity
anisotropy. The overall features can be explained by assuming that the CuO_2
planes contain a developed array of stripes accommodating the doped holes, and
that the MR is associated with the field-induced topological ordering of the
stripes. | 0004135v1 |
2000-07-11 | Grain boundary effects on magnetotransport in bi-epitaxial films of La$_{0.7}$Sr$_{0.3}$MnO$_3$ | The low field magnetotransport of La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) films
grown on SrTiO$_3$ substrates has been investigated. A high qualtity LSMO film
exhibits anisotropic magnetoresistance (AMR) and a peak in the
magnetoresistance close to the Curie temperature of LSMO. Bi-epitaxial films
prepared using a seed layer of MgO and a buffer layer of CeO$_2$ display a
resistance dominated by grain boundaries. One film was prepared with seed and
buffer layers intact, while a second sample was prepared as a 2D square array
of grain boundaries. These films exhibit i) a low temperature tail in the low
field magnetoresistance; ii) a magnetoconductance with a constant high field
slope; and iii) a comparably large AMR effect. A model based on a two-step
tunneling process, including spin-flip tunneling, is discussed and shown to be
consistent with the experimental findings of the bi-epitaxial films. | 0007194v1 |
2001-02-23 | Temperature Dependence of Low-Lying Electronic Excitations of LaMnO_3 | We report on the optical properties of undoped single crystal LaMnO_3, the
parent compound of the colossal magneto-resistive manganites. Near-Normal
incidence reflectance measurements are reported in the frequency range of
20-50,000 cm-1 and in the temperature range 10-300 K. The optical conductivity,
s_1(w), is derived by performing a Kramers-Kronig analysis of the reflectance
data. The far-infrared spectrum of s_1(w) displays the infrared active optical
phonons. We observe a shift of several of the phonon to high frequencies as the
temperature is lowered through the Neel temperature of the sample (T_N = 137
K). The high-frequency s_1(w) is characterized by the onset of absorption near
1.5 eV. This energy has been identified as the threshold for optical
transitions across the Jahn-Teller split e_g levels. The spectral weight of
this feature increases in the low-temperature state. This implies a transfer of
spectral weight from the UV to the visible associated with the paramagnetic to
antiferromagnetic state. We discuss the results in terms of the double exchange
processes that affect the optical processes in this magnetic material. | 0102437v1 |
2001-02-27 | Anisotropic three-dimentional magnetic fluctuations in heavy fermion CeRhIn5 | CeRhIn5 is a heavy fermion antiferromagnet that orders at 3.8 K. The
observation of pressure-induced superconductivity in CeRhIn5 at a very high Tc
of 2.1 K for heavy fermion materials has led to speculations regarding to its
magnetic fluctuation spectrum. Using magnetic neutron scattering, we report
anisotropic three-dimensional antiferromagnetic fluctuations with an energy
scale of less than 1.7 meV for temperatures as high as 3Tc. In addition, the
effect of the magnetic fluctuations on electrical resistivity is well described
by the Born approximation. | 0102503v1 |
2001-12-21 | First order transition from ferromagnetism to antiferromagnetism in Ce(Fe$_{0.96}$Al$_{0.04}$)$_2$: a magnetotransport study | The magnetotransport behaviour is investigated in detail across the first
order magnetic phase transition from ferromagnetic to antiferromagnetic state
in polycrystalline Ce(Fe$_{0.96}$Al$_{0.04}$)$_2$ sample. The study clearly
brings out various generic features associated with a first order transition,
viz., hysteresis, phase coexistence, supercooling and superheating, presence
and limits of the metastable regimes. These results of magnetotransport study
exhibit and support all the interesting thermomagnetic history effects that
were observed in our earlier dc-magnetisation study on the same sample. Most
notable here is the initial (or virgin) resistivity vs. field curve lying
outside the hysteretic "butterfly shaped" magnetoresistivity loops obtained on
cyclying the magnetic field between high enough positive and negative
strengths. These findings, bearing one-to-one similarity with the data obtained
in their magnetic counterpart (i.e., dc-magnetisation), have been ascribed an
origin due to the arresting of this first order transition kinetics at low
temperature and high magnetic field. | 0112408v1 |
2002-08-07 | Transport critical current densities and n factors in mono- and multifilamentary MgB2/Fe tapes and wires using fine powders | Mono- and multifilamentary MgB2/Fe tapes and wires with high transport
critical current densities have been prepared using the powder-in-tube (PIT)
process. The fabrication details are described. The effect of powder grain
sizes and recrystallization temperature on jc has been investigated. At 25K and
1 T, jc values close to 105 A/cm2 were measured, while jc of 106 A/cm2 were
extrapolated for 4.2K/0T in our monofilamentary tape. MgB2/Fe tapes exhibit
high exponential n factors for the resistive transition: n ~ 80 and 40 were
found at 5 T and 7 T, respectively. The highest transport jc values obtained so
far in MgB2/Fe wires with 7 filaments were 1.1 * 105 A/cm2 at 4.2 K and in a
field of 2 T, which is still lower than for monofilamentary tapes. Improved
deformation and recovering processing is expected to lead to higher jc values. | 0208133v1 |
2004-02-20 | Metal-insulator transition and glassy behavior in two-dimensional electron systems | Studies of low-frequency resistance noise demonstrate that glassy freezing
occurs in a two-dimensional electron system in silicon in the vicinity of the
metal-insulator transition (MIT). The width of the metallic glass phase, which
separates the 2D metal and the (glassy) insulator, depends strongly on
disorder, becoming extremely small in high-mobility (low-disorder) samples. The
glass transition is manifested by a sudden and dramatic slowing down of the
electron dynamics, and by a very abrupt change to the sort of statistics
characteristic of complicated multistate systems. In particular, the behavior
of the second spectrum, an important fourth-order noise statistic, indicates
the presence of long-range correlations between fluctuators in the glassy
phase, consistent with the hierarchical picture of glassy dynamics. | 0402535v1 |
2004-04-21 | Formation and Thermal Stability of sub-10 nm Carbon Templates on Si(100) | We report a lithographic process for creating high-resolution (<10 nm) carbon
templates on Si(100). A scanning electron microscope, operating under low
vacuum (10E-6 mbar), produces a carbon-containing deposit ("contamination
resist") on the silicon surface via electron-stimulated dissociation of ambient
hydrocarbons, water and other adsorbed molecules. Subsequent annealing at
temperatures up to 1320 K in ultra-high vacuum removes SiO2 and other
contaminants, with no observable change in dot shape. The annealed structures
are compatible with subsequent growth of semiconductors and complex oxides.
Carbon dots with diameter as low as 3.5 nm are obtained with a 200 us
electron-beam exposure time. | 0404505v1 |
2005-04-19 | High temperature magnetic ordering in La2RuO5 | Magnetic susceptibility, heat capacity and electrical resistivity
measurements have been carried out on a new ruthenate, La2RuO5 (monoclinic,
space group P21/c) which reveal that this compound is a magnetic semiconductor
with a high magnetic ordering temperature of 170K. The entropy associated with
the magnetic transition is 8.3 J/mole-K -close to that expected for the low
spin (S=1) state of Ru4+ ions. The low temperatures specific heat coefficient g
is found to be nearly zero consistent with the semiconducting nature of the
compound. The magnetic ordering temperature of La2RuO5 is comparable to the
highest known Curie temperature of another ruthenate, namely, metallic SrRuO3,
and in both these compounds the nominal charge state of Ru is 4+. | 0504489v1 |
2006-05-11 | Multifilament YBa2Cu3O6+x -coated conductors with minimized coupling losses | We report an experimental approach to making multifilament coated conductors
with low losses in applied time-varying magnetic field. Previously, the
multifilament conductors obtained for that purpose by laser ablation suffered
from high coupling losses. Here we report how this problem can be solved. When
the substrate metal in the grooves segregating the filaments is exposed to
oxygen, it forms high resistivity oxides that electrically insulate the stripes
from each other and from the substrate. As the result, the coupling loss has
become negligible over the entire range of tested parameters (magnetic field
amplitudes B and frequencies f) available to us. | 0605313v1 |
2006-06-17 | The thermally activated motion of vortex bundles and the anomalous Hall effect in type-II conventional and high-Tc superconductors | The anomalous Hall effect in type-II conventional and high-Tc superconductors
is investigated based on the proposed novel theory of the thermally activated
motion of vortex bundles over a directional-dependent energy barrier. Our
calculations demonstrate clearly that anomalous Hall effect is induced by the
competition between the Magnus force and the random collective pinning force of
the vortex bundle. The Hall as well as the longitudinal resistivity for
constant temperature and constant applied magnetic field of type-II
superconducting films and bulk materials are calculated. The reentry phenomenon
is also investigated. All the results are in good agreement with the
experiments. | 0606464v2 |
1999-11-30 | Analysis of trap spectra in LEC and epitaxial GaAs | Different methods of trap parameter measurement are analysed. Transient
photoconductivity and thermally stimulated effects were used to investigate the
influence of traps in LEC SI-GaAs and high resistivity epitaxial GaAs. The
peculiarities of the TSC were analysed and shown to be related to the influence
of crystal micro-inhomogeneities. | 9911048v1 |
2006-08-25 | The Effects of Preheating of a Fine Tungsten Wire and the Polarity of a High-Voltage Electrode on the Energy Characteristics of an Electrically Exploded Wire in Vacuum | Results obtained from experimental and numerical studies of tungsten wires
electrical explosion in vacuum are presented. The experiments were performed
both with and without preheating of the wires, using positive or negative
polarity of a high-voltage electrode. Preheating is shown to increase energy
deposition in the wire core due to a longer resistive heating stage. The effect
was observed both in single wire and wire array experiments. The evolution of
the phase state of the wire material during explosion was examined by means of
one-dimensional numerical simulation using a semiempirical wide-range equation
of state describing the properties of tungsten with allowance made for melting
and vaporization. | 0608251v1 |
2007-03-11 | A low-noise ferrite magnetic shield | Ferrite materials provide magnetic shielding performance similar to commonly
used high permeability metals but have lower intrinsic magnetic noise generated
by thermal Johnson currents due to their high electrical resistivity.
Measurements inside a ferrite shield with a spin-exchange relaxation-free
atomic magnetometer reveal a noise level of 0.75 fT Hz^(-1/2), 25 times lower
than what would be expected in a comparable mu-metal shield. The authors
identify a 1/f component of the magnetic noise due to magnetization
fluctuations and derive general relationships for the Johnson current noise and
magnetization noise in cylindrical ferromagnetic shields in terms of their
conductivity and complex magnetic permeability. | 0703115v2 |
2007-08-14 | The zero-energy state in graphene in a high magnetic field | The fate of the charge-neutral Dirac point in graphene in a high magnetic
field $H$ has been investigated at low temperatures ($T\sim$ 0.3 K). In samples
with small $V_0$ (the gate voltage needed to access the Dirac point), the
resistance $R_0$ at the Dirac point diverges steeply with $H$, signalling a
crossover to an insulating state in intense field. The approach to the
insulating state is highly unusual. Despite the steep divergence in $R_0$, the
profile of $R_0$ vs. $T$ in fixed $H$ saturates to a $T$-independent value
below 2 K, consistent with charge carrying gapless excitations. | 0708.1959v2 |
2008-03-25 | Superconductivity at 43 K in Samarium-arsenide Oxides $SmFeAsO_{1-x}F_x$ | Since the discovery of high-transition temperature ($T_c$) superconductivity
in layered copper oxides, extensive efforts have been devoted to explore the
higher $T_c$ superconductivity. However, the $T_c$ higher than 40 K can be
obtained only in the copper oxide superconductors so far. The highest reported
value of $T_c$ for non-copper-oxide bulk superconductivity is 39 K in
$MgB_2$.\cite{jun} The $T_c$ of about 40 K is close to or above the theoretical
value predicted from BCS theory.\cite{mcmillan} Therefore, it is very
significant to search for non-copper oxide superconductor with the transition
temperature higher than 40 K to understand the mechanism of high-$T_c$
superconductivity. Here we report the discovery of bulk superconductivity in
samarium-arsenide oxides $SmFeAsO_{1-x}F_x$ with ZrCuAiAs type structure.
Resistivity and magnetization measurements show strong evidences for transition
temperature as high as 43 K. $SmFeAsO_{1-x}F_x$ is the first non-copper oxide
superconductor with $T_c$ higher than 40 K. The $T_c$ higher than 40 K may be a
strong argument to consider $SmFeAsO_{1-x}F_x$ as an unconventional
superconductor. | 0803.3603v2 |
2008-04-21 | CeFePO: A Heavy Fermion Metal with Ferromagnetic Correlations | The ground state properties of CeFePO, a homologue of the new high
temperature superconductors RFePnO(1-x)Fx, were studied by means of
susceptibility, specific heat, resistivity, and NMR measurements on
polycrystals. All the results demonstrate that this compound is a magnetically
non-ordered heavy Fermion metal with a Kondo temperature TK~10K, a Sommerfeld
coefficient gamma=700mJ/molK2 and a mass enhancement factor of the order of
200. The absence of a Fe-contribution to the effective moment at high
temperatures indicates that the magnetism in CeFePO is completely dominated by
the effect of Ce. Thus the strong electronic correlation effects originate from
the Ce-4f electrons rather than from the Fe-3d electrons. An enhanced
Sommerfeld-Wilson ratio R=5.5 as well as a Korringa product S0/T1TK2~0.065 well
below 1 indicate the presence of ferromagnetic correlations. Therefore, CeFePO
appears to be on the non-magnetic side of a ferromagnetic instability. | 0804.3250v2 |
2008-07-05 | Growth and characterization of A_{1-x}K_xFe_2As_2 (A = Ba, Sr) single crystals with x=0 - 0.4 | Single crystals of A$_{1-x}$K$_x$Fe$_2$As$_2$ (A=Ba, Sr) with high quality
have been grown successfully by FeAs self-flux method. The samples have sizes
up to 4 mm with flat and shiny surfaces. The X-ray diffraction patterns suggest
that they have high crystalline quality and c-axis orientation. The
non-superconducting crystals show a spin-density-wave (SDW) instability at
about 173 K and 135 K for Sr-based and Ba-based compound, respectively. After
doping K as the hole dopant into the BaFe$_2$As$_2$ system, the SDW transition
is smeared, and superconducting samples with the compound of
Ba$_{1-x}$K$_x$Fe$_2$As$_2$ (0 $< x \leqslant$ 0.4) are obtained. The
superconductors characterized by AC susceptibility and resistivity measurements
exhibit very sharp superconducting transition at about 36 K, 32 K, 27 K and 23
K for x= 0.40,0.28,0.25 and 0.23, respectively. | 0807.0759v3 |
2008-08-18 | A Ni-based Superconductor: the Heusler Compound ZrNi$_2$Ga | This work reports on the novel Heusler superconductor ZrNi2Ga. Compared to
other nickel-based superconductors with Heusler structure, ZrNi2Ga exhibits a
relatively high superconducting transition temperature of Tc=2.9 K and an upper
critical field of 1.5 T. Electronic structure calculations show that this
relatively high transition temperature is caused by a van Hove singularity,
which leads to an enhanced density of states at the Fermi energy. The van Hove
singularity originates from a higher order valence instability at the L-point
in the electronic structure. The enhanced density of states at the Fermi level
was confirmed by specific heat and susceptibility measurements. Although many
Heusler compounds are ferromagnetic, our measurements of ZrNi2Ga indicate a
paramagnetic state above Tc and could not reveal any traces of magnetic order
down to temperatures of at least 0.35 K. We investigated in detail the
superconducting state with specific heat, magnetization, and resistivity
measurements. The resulting data show the typical behavior of a conventional,
weakly coupled BCS (s-wave) superconductor. | 0808.2356v1 |
2008-10-03 | Small anisotropy, weak thermal fluctuations, and high field superconductivity in Co-doped iron pnictide Ba(Fe1-xCox)2As2 | We performed high-field magnetotransport and magnetization measurements on a
single crystal of the 122-phase iron pnictide Ba(Fe1-xCox)2As2. Unlike the HTS
cuprates and 1111-phase oxypnictides, Ba(Fe1-xCox)2As2 showed practically no
broadening of the resistive transitions under magnetic fields up to 45 T. The
mass anisotropy gamma = Hc2ab/Hc2c deduced from the slopes of the upper
critical field dHc2ab/dT = 4.9T/K and dHc2c/dT = 2.5T/K decreases from ~2 near
Tc, to ~1.5 at lower temperatures. We observed the irreversibility field close
to Hc2, and a rather unusual symmetric volume pinning force curve Fp(H)
suggestive of strong pinning nano-structure. | 0810.0699v2 |
2009-05-01 | Highly segmented thin microstrip detector with data-driven fast readout | In September 2008 the Slim5 collaboration submitted a low material budget
silicon demonstrator to test with 12 GeV/c protons, at the PS-T9 test-beam at
CERN. Inside the reference telescope, two different detectors were placed as
device under test (DUT). The first was a 4k-Pixel Matrix of Deep N Well MAPS,
developed in a 130 nm CMOS Technology, providing digital sparsified readout.
The other one was a high resistivity double sided silicon detector, with short
strips at 45-degree angle to the detector's edge, read out by the FSSR2 chip.
The FSSR2 is a 128 channel data-driven fast readout chip developed by Fermilab
and INFN. In this paper we describe the main features of latter sensor, the
striplet. The primary goal of the test was to measure the efficiency and the
resolution of the striplets. The data-driven approach of the FSSR2 readout
chips has been fully exploited by the DAQ system. | 0905.0083v3 |
2009-10-08 | Structural and transport properties of Sr2VO{3-delta}FeAs superconductors with different oxygen deficiencies | Sr2VO{3-delta}FeAs superconductors with different oxygen deficiencies have
been successfully fabricated. It is found that the superconducting transition
temperature drops down monotonically with the increase of oxygen deficiency.
The diminishing of superconductivity is accompanied by the enhancement of
residual resistivity, indicating an unraveled scattering effect induced by the
oxygen deficiency. The highest superconducting transition temperature at about
40 K is achieved near the stoichiometrical sample Sr$_2$VO$_{3}$FeAs.
Surprisingly, the X-ray photoelectron spectroscopy (XPS) shows that the
vanadium has a "5+" valence state in the samples. The Hall effect measurements
reveal that the density of charge carriers (electron-like here) varies
qualitatively with the increase of oxygen deficiency. Magnetotransport
measurements show that the superconducting transition changes from
one-step-like shape at low fields to two-step-like one at high fields,
indicating a high anisotropy. | 0910.1537v2 |
2010-04-22 | Optimizing Transistor Performance of Percolating Carbon Nanotube Networks | In percolating networks of mixed metallic and semiconducting CNTs, there is a
tradeoff between high on-current (dense networks) and high on/off ratio (sparse
networks in which the metallic CNT fraction is not percolating). Experiments on
devices in a transistor configuration and Monte Carlo simulations were
performed to determine the scaling behavior of device resistivity as a function
of channel length (L) for CNT density p in the range 0.04 - 1.29 CNT/{\mu}m^2
in the on- and off-states. Optimized devices with field-effect mobility up to
50 cm^2/Vs at on/off ratio > 10^3 were obtained at W = 50 {\mu}m, L > 70 {\mu}m
for p = 0.54 - 0.81 CNTs/{\mu}m^2. | 1004.4009v1 |
2010-06-25 | Spin and charge dynamics in [TbPc$_2$]$^0$ and [DyPc$_2$]$^0$ single molecule magnets | Magnetization, AC susceptibility and $\mu$SR measurements have been performed
in neutral phthalocyaninato lanthanide ([LnPc$_2]^0$) single molecule magnets
in order to determine the low-energy levels structure and to compare the
low-frequency spin excitations probed by means of macroscopic techniques, such
as AC susceptibility, with the ones explored by means of techniques of
microscopic character, such as $\mu$SR. Both techniques show a high temperature
thermally activated regime for the spin dynamics and a low temperature
tunneling one. While in the activated regime the correlation times for the spin
fluctuations estimated by AC susceptibility and $\mu$SR basically agree, clear
discrepancies are found in the tunneling regime. In particular, $\mu$SR probes
a faster dynamics with respect to AC susceptibility. It is argued that the
tunneling dynamics probed by $\mu$SR involves fluctuations which do not yield a
net change in the macroscopic magnetization probed by AC susceptibiliy. Finally
resistivity measurements in [TbPc$_2]^0$ crystals show a high temperature
nearly metallic behaviour and a low temperature activated behaviour. | 1006.4919v1 |
2010-09-17 | Graphene Transport at High Carrier Densities using a Polymer Electrolyte Gate | We report the study of graphene devices in Hall-bar geometry, gated with a
polymer electrolyte. High densities of 6 $\times 10^{13}/cm^{2}$ are
consistently reached, significantly higher than with conventional back-gating.
The mobility follows an inverse dependence on density, which can be correlated
to a dominant scattering from weak scatterers. Furthermore, our measurements
show a Bloch-Gr\"uneisen regime until 100 K (at 6.2 $\times10^{13}/cm^{2}$),
consistent with an increase of the density. Ubiquitous in our experiments is a
small upturn in resistivity around 3 $\times10^{13}/cm^{2}$, whose origin is
discussed. We identify two potential causes for the upturn: the renormalization
of Fermi velocity and an electrochemically-enhanced scattering rate. | 1009.3367v2 |
2010-10-28 | Poole-Frenkel Effect and Phonon-Assisted Tunneling in GaAs Nanowires | We present electronic transport measurements of GaAs nanowires grown by
catalyst-free metal-organic chemical vapor deposition. Despite the nanowires
being doped with a relatively high concentration of substitutional impurities,
we find them inordinately resistive. By measuring sufficiently high
aspect-ratio nanowires individually in situ, we decouple the role of the
contacts and show that this semi-insulating electrical behavior is the result
of trap-mediated carrier transport. We observe Poole-Frenkel transport that
crosses over to phonon-assisted tunneling at higher fields, with a tunneling
time found to depend predominantly on fundamental physical constants as
predicted by theory. By using in situ electron beam irradiation of individual
nanowires we probe the nanowire electronic transport when free carriers are
made available, thus revealing the nature of the contacts. | 1010.6086v1 |
2010-11-23 | NaIrO3 - A pentavalent post-perovskite | Sodium iridium(V) oxide, NaIrO3, was synthesized by a high pressure solid
state method and recovered to ambient conditions. It is found to be
isostructural with CaIrO3, the much-studied structural analogue of the
high-pressure post-perovskite phase of MgSiO3. Among the oxide
post-perovskites, NaIrO3 is the first example with a pentavalent cation. The
structure consists of layers of corner- and edge-sharing IrO6 octahedra
separated by layers of NaO8 bicapped trigonal prisms. NaIrO3 shows no magnetic
ordering and resistivity measurements show non-metallic behavior. The crystal
structure, electrical and magnetic properties are discussed and compared to
known post-perovskites and pentavalent perovskite metal oxides. | 1011.5125v2 |
2010-12-11 | Transport Spin Polarization of High-Curie Temperature MnBi Films | We report on the study of the structural, magnetic and transport properties
of highly textured MnBi films with the Curie temperature of 628K. In addition
to detailed measurements of resistivity and magnetization, we measure transport
spin polarization of MnBi by Andreev reflection spectroscopy and perform fully
relativistic band structure calculations of MnBi. A spin polarization from
51\pm1 to 63\pm1% is observed, consistent with the calculations and with an
observation of a large magnetoresistance in MnBi contacts. The band structure
calculations indicate that, in spite of almost identical densities of states at
the Fermi energy, the large disparity in the Fermi velocities leads to high
transport spin polarization of MnBi. The correlation between the values of
magnetization and spin polarization is discussed. | 1012.2486v1 |
2011-10-06 | High-Precision Tuning of State for Memristive Devices by Adaptable Variation-Tolerant Algorithm | Using memristive properties common for the titanium dioxide thin film
devices, we designed a simple write algorithm to tune device conductance at a
specific bias point to 1% relative accuracy (which is roughly equivalent to
7-bit precision) within its dynamic range even in the presence of large
variations in switching behavior. The high precision state is nonvolatile and
the results are likely to be sustained for nanoscale memristive devices because
of the inherent filamentary nature of the resistive switching. The proposed
functionality of memristive devices is especially attractive for analog
computing with low precision data. As one representative example we demonstrate
hybrid circuitry consisting of CMOS summing amplifier and two memristive
devices to perform analog multiply and accumulate computation, which is a
typical bottleneck operation in information processing. | 1110.1393v1 |
2011-10-19 | Evaluating the robustness of top coatings comprising plasma-deposited fluorocarbons in electrowetting systems | Thin dielectric stacks comprising a main insulating layer and a hydrophobic
top coating are commonly used in low voltage electrowetting systems. However,
in most cases, thin dielectrics fail to endure persistent electrowetting
testing at high voltages, namely beyond the saturation onset, as electrolysis
indicates dielectric failure. Careful sample inspection via optical microscopy
revealed possible local delamination of the top coating under high electric
fields. Thus, improvement of the adhesion strength of the hydrophobic top
coating to the main dielectric is attempted through a plasma-deposited
fluorocarbon interlayer. Interestingly enough the proposed dielectric stack
exhibited a) resistance to dielectric breakdown, b) higher contact angle
modulation range, and c) electrowetting cycle reversibility. Appearance of
electrolysis in the saturation regime is inhibited, suggesting the use of this
hydrophobic dielectric stack for the design of more efficient electrowetting
systems. The possible causes of the improved performance are investigated by
nanoscratch characterization. | 1110.4238v1 |
2011-10-21 | P-T phase diagram of iron arsenide superconductor NdFeAsO0.88F0.12 | NdFeAsO0.88F0.12 belongs to the recently discovered family of high-TC
iron-based superconductors. The influence of high pressure on transport
properties of this material has been studied. Contrary to La-based compounds,
we did not observe a maximum in TC under pressure. Under compression, TC drops
rapidly as a linear function of pressure with the slope k = -2.8 \pm 0.1 K /
GPa. The extrapolated value of TC at zero pressure is about TC (0) = 51.7 \pm
0.4 K. At pressures higher than ~18.4 GPa, the superconducting state disappears
at all measured temperatures. The resistance changes slope and shows a turn-up
behavior, which may be related to the Kondo effect or a weak localization of
two-dimensional carriers below ~45 K that is above TC and thus competing with
the superconducting phase. The behavior of the sample is completely reversible
at the decompression. On the bases of our experimental data, we propose a
tentative P-T phase diagram of NdFeAsO0.88F0.12. | 1110.4909v1 |
2011-11-20 | Flame Synthesis of Graphene Films in Open Environments | Few-layer graphene (FLG) is grown on copper and nickel substrates at high
rates using a novel flame synthesis method in open-atmosphere environments.
Transmittance and resistance properties of the transferred films are similar to
those grown by other methods, but the concentration of oxygen, as assessed by
XPS, is actually less than that for CVD-grown graphene under near vacuum
conditions. The method involves utilizing a multi-element
inverse-diffusion-flame burner, where post-flame species and temperatures are
radially-uniform upon deposition at a substrate. Advantages of the flame
synthesis method are scalability for large-area surface coverage, increased
growth rates, high purity and yield, continuous processing, and reduced costs
due to efficient use of fuel as both heat source and reagent. Additionally, by
adjusting local growth conditions, other carbon nanostructures (i.e. nanotubes)
are readily synthesized. | 1111.4699v1 |
2012-07-25 | Conductivity crossover in nano-crystalline diamond films: Realization of a disordered superlattice-like structure | We present the electrical transport characteristics of a batch of
nano-crystalline diamond films of varying nitrogen concentrations and explain
the conduction mechanism by the disordered quasi-superlattice model applied to
semiconductor heterostructures. Synthesized by the hot filament chemical vapour
deposition technique, the degree of structural disorder in the films, confirmed
from Raman spectroscopy, is found to be controllable, resulting in the
transition of conduction mechanism from localized and activated to the metallic
conduction regime. Hence through high field magneto-resistance measurements at
low temperatures we firmly establish a conductivity crossover from hopping to
3D weak localization. The long electronic dephasing time and its weak
temperature dependence suggest the possibility for diamond-based high-speed
device applications. | 1207.5916v1 |
2012-10-23 | Impact of the various spin and orbital ordering processes on multiferroic properties of orthovanadate DyVO3 | The orthovanadate DyVO3 crystal, known to exhibit multiple structural, spin
and orbital ordering transitions, is presently investigated on the basis of
magnetization, heat capacity, resistivity, dielectric and polarization
measurements. Our main result is experimental evidence for the existence of
multiferroicity below a high TC of 108 K over a wide temperature range
including different spin-orbital ordered states. The onset of ferroelectricity
is found to coincide with the antiferromagnetic C-type spin ordering transition
taking place at 108 K, which indicates that DyVO3 belongs to type II
multiferroics exhibiting a coupling between magnetism and ferroelectricity.
Some anomalies detected on the temperature dependence of electric polarization
are discussed with respect to the nature of the spin-orbital ordered states of
the V sublattice and the degree of spin alignment in the Dy sublattice. The
orthovanadates RVO3 (R = rare earth or Y) form an important new category for
searching for high-TC multiferroics. | 1210.6373v3 |
2013-04-04 | Atomic Calligraphy: The Direct Writing of Nanoscale Structures using MEMS | We present a micro-electromechanical system (MEMS) based method for the
resist free patterning of nano-structures. Using a focused ion beam (FIB) to
customize larger MEMS machines, we fabricate apertures as small as 50 nm on
plates that can be moved with nanometer precision over an area greater than
20x20 {\mu}m^2. Depositing thermally evaporated gold atoms though the apertures
while moving the plate results in the deposition of nanoscale metal patterns.
Adding a shutter only microns above the aperture, enables high speed control of
not only where but also when atoms are deposited. Using a shutter, different
sized apertures can be selectively opened and closed for nano-structure
fabrication with features ranging from nano- to micrometers in scale. The
ability to evaporate materials with high precision, and thereby fabricate
circuits and structures in situ, enables new kinds of experiments based on the
interactions of a small number of atoms and eventually even single atoms. | 1304.1334v1 |
2013-04-04 | Substrate-Independent Catalyst-Free Synthesis of High-Purity Bi2Se3 Nanostructures | We describe a catalyst-free vapor-solid synthesis of bismuth selenide
(Bi2Se3) nanostructures at ambient pressure with hydrogen as a carrier gas. The
nanostructures were synthesized on glass, silicon and mica substrates and the
method yields a variety of nanostructures: nanowires, nanoribbons,
nanoplatelets and nanoflakes. The materials analysis shows high chemical purity
in all cases, without sacrificing the crystalline structure of Bi2Se3.
Low-temperature measurements of the nanostructures indicate contributions from
the surface states with a tunable carrier density. Samples synthesized on
flexible mica substrates show no significant change in resistance upon bending,
indicating robustness of as-grown Bi2Se3 nanostructures and their suitability
for device applications. | 1304.1388v1 |
2013-07-14 | Voltage-triggered Ultra-fast Metal-insulator Transition in Vanadium Dioxide Switches | Electrically driven metal-insulator transition in vanadium dioxide (VO2) is
of interest in emerging memory devices, neural computation, and high speed
electronics. We report on the fabrication of out-of-plane VO2
metal-insulator-metal (MIM) structures and reproducible high-speed switching
measurements in these two-terminal devices. We have observed a clear
correlation between electrically-driven ON/OFF current ratio and
thermally-induced resistance change during metal-insulator transition. It is
also found that sharp metal-insulator transition could be triggered by external
voltage pulses within 2 ns at room temperature and the achieved ON/OFF ratio is
greater than two orders of magnitude with good endurance. | 1307.3774v1 |
2013-07-17 | Dielectric Coatings for IACT Mirrors | Imaging Atmospheric Cherenkov Telescopes for very-high energy gamma-ray
astronomy need mirror with high reflectance roughly in the wavelength between
300 and 550 nm. The current standard reflective layer of such mirrors is
aluminum. Being permanently exposed to the environment they show a constant
degradation over the years. New and improved dielectric coatings have been
developed to enhance their resistance to environmental impact and to extend
their possible lifetime. In addition, these customized coatings have an
increased reflectance of over 95% and are designed to significantly lower the
night-sky background contribution. The development of such coatings for mirrors
with areas up to 2 m2 and low application temperatures to suite the composite
materials used for the new mirror susbtrates of the Cherenkov Telescope Array
(CTA) and the results of extensive durability tests are presented. | 1307.4557v1 |
2013-08-14 | Perspectives of Micro-Pattern Gaseous Detector Technologies for Future Physics Projects | A centenary after the invention of the basic principle of gas amplification,
gaseous detectors - are still the first choice whenever the large area coverage
with low material budget is required. Advances in photo-lithography and
micro-processing techniques in the chip industry during the past two decades
triggered a major transition in the field of gas detectors from wire structures
to Micro-Pattern Gas Detector (MPGD) concepts, revolutionizing cell-size
limitations for many gas detector applications. The high radiation resistance
and excellent spatial and time resolution make them an invaluable tool to
confront future detector challenges at the frontiers of research. The design of
the new micro-pattern devices appears suitable for industrial production. In
2008, the RD51 collaboration at CERN has been established to further advance
technological developments of MPGDs and associated electronic-readout systems,
for applications in basic and applied research. This review provides an
overview of the state-of-the-art of the MPGD technologies and summarizes recent
activities for the next generation of colliders within the framework of the
RD51 collaboration. | 1308.3047v2 |
2013-08-18 | Pulsed Laser Deposition of High-Quality Thin Films of the Insulating Ferromagnet EuS | High-quality thin films of the ferromagnetic-insulator europium(II) sulfide
(EuS) were fabricated by pulsed laser deposition on Al2O3 (0001) and Si (100)
substrates. A single orientation was obtained with the [100] planes parallel to
the substrates, with atomic-scale smoothness indicates a near-ideal surface
topography. The films exhibit uniform ferromagnetism below 15.9 K, with a
substantial component of the magnetization perpendicular to the plane of the
films. Optimization of the growth condition also yielded truly insulating films
with immeasurably large resistance. This combination of magnetic and electric
properties open the gate for novel devices that require a true ferromagnetic
insulator. | 1308.3820v2 |
2014-02-19 | Monte Carlo Simulation of RPC-based PET with GEANT4 | The Resistive Plate Chambers (RPC) are low-cost charged-particle detectors
with good timing resolution and potentially good spatial resolution. Using RPC
as gamma detector provides an opportunity for application in positron emission
tomography (PET). In this work, we use GEANT4 simulation package to study
various methods improving the detection efficiency of a realistic RPC-based PET
model for 511keV photons, by adding more detection units, changing the
thickness of each layer, choosing different converters and using multi-gaps RPC
(MRPC) technique. Proper balance among these factors are discussed. It's found
that although RPC with materials of high atomic number can reach a higher
efficiency, they may contribute to a poor spatial resolution and higher
background level. | 1402.4544v1 |
2014-03-04 | Molecular weight dependent bimolecular recombination in organic solar cells | Charge carrier recombination is studied in operational organic solar cells
made from the polymer:fullerene system PCDTBT:PC71BM
(poly[N-9"-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]
: [6,6]-phenyl-C$_{70}$-butyric acid methyl ester). A newly developed technique
High Intensity Resistance dependent PhotoVoltage (HI-RPV) is presented for
reliably quantifying the bimolecular recombination coefficient independently of
variations in experimental conditions, thereby resolving key limitations of
previous experimental approaches. Experiments are performed on solar cells of
varying thicknesses and varying polymeric molecular weights. It is shown that
solar cells made from low molecular weight PCDTBT exhibit Langevin
recombination, whereas suppressed (non-Langevin) recombination is found in
solar cells made with high molecular weight PCDTBT. | 1403.0972v2 |
2015-01-27 | Transition from semiconducting to metallic-like conducting and weak antilocalization effect in single crystals of LuPtSb | High quality half-Heusler single crystals of LuPtSb have been synthesized by
a Pb flux method. The temperature dependent resistivity and Hall effects
indicate that the LuPtSb crystal is a p-type gapless semiconductor showing a
transition from semiconducting to metallic conducting at 150 K. Moreover, a
weakly temperature-dependent positive magnetoresistance (MR) as large as 109 %
and high carrier mobility up to 2950 cm2/Vs are experimentally observed at
temperatures below 150 K. The low-field MR data shows evidence for weak
antilocalization (WAL) effect at temperatures even up to 150 K. Analysis of the
temperature and angle dependent magnetoconductance manifests that the WAL
effect originates from the bulk contribution owing to the strong spin-orbital
coupling. | 1501.06714v1 |
2015-05-22 | Synthesis of monoclinic IrTe2 under high pressure and its physical properties | In a pressure-temperature (P-T) diagram for synthesizing IrTe2 compounds, the
well-studied trigonal (H) phase with the CdI2-type structure is stable at low
pressures. The superconducting cubic (C) phase can be synthesized under higher
temperatures and pressures. A rhombohedral phase with the crystal structure
similar to the C phase can be made at ambient pressure; but the phase contains
a high concentration of Ir deficiency. In this paper, we report that a rarely
studied monoclinic (M) phase can be stabilized in narrow ranges of pressure and
temperature in this P-T diagram. The peculiar crystal structure of the M-IrTe2
eliminates the tendency to form Ir-Ir dimers found in the H phase. The M phase
has been fully characterized by structural determination and measurements of
electrical resistivity, thermoelectric power, DC magnetization, and specific
heat. These physical properties have been compared with those in the H and C
phases of Ir1-xTe2. Moreover, magnetic and transport properties and specific
heat of the M-IrTe2 can be fully justified by calculations with the
density-functional theory presented in this paper. | 1505.06124v1 |
2015-08-12 | Direct Observation of Re-entrant Multiferroic CuO at High Pressures | We have carried out a detailed experimental investigation on CuO using
dielectric constant, ac resistance, Raman spectroscopy and X-ray diffraction
measurements at high pressures and room temperature. Both dielectric constant
and dielectric loss show an anomalous peak in the pressure range 3.4-4 GPa
indicating a ferroelectric transition. Raman studies show anomalous behaviour
of the Ag mode with a slope change in the mode frequency and a minimum in the
mode FWHM at 3.4 GPa indicating a strong spin phonon coupling along [1 0 -1]
direction. A step like behaviour in the intensity of the Ag mode is observed at
3.4 GPa, indicating a change in the polarization of the mode. A maximum in the
intensity of (2,0,-2)Bragg peak at 3.4 GPa points the occurrence of critical
scattering due to emergence of magnetic exchange interaction. All our
experimental evidences show to the presence of re-entrant type-II multiferroic
behaviour in CuO at about 4 GPa. | 1508.02874v1 |
2015-10-21 | Titanium Nitride as a Seed Layer for Heusler Compounds | Titanium nitride (TiN) shows low resistivity at room temperature, high
thermal stability and thus has the potential to serve as seed layer in magnetic
tunnel junctions. High quality TiN thin films with regard to the
crystallographic and electrical properties were grown and characterized by
X-ray diffraction and 4-terminal transport measurements. Element specific X-ray
absorption spectroscopy revealed pure TiN in the bulk. To investigate the
influence of a TiN seed layer on a ferro(i)magnetic bottom electrode, an
out-of-plane magnetized Mn2.45Ga as well as in- and out-of-plane magnetized
Co2FeAl thin films were deposited on a TiN buffer, respectively. The magnetic
properties were investigated using a superconducting quantum interference
device (SQUID) and anomalous Hall effect (AHE) for Mn2.45Ga. Magneto optical
Kerr effect (MOKE) measurements were carried out to investigate the magnetic
properties of Co2FeAl. TiN buffered Mn2.45Ga thin films showed higher
coercivity and squareness ratio compared to unbuffered samples. The Heusler
compound Co2FeAl showed already good crystallinity when grown at room
temperature. | 1510.06256v3 |
2016-04-19 | Insulating and metallic spin glass in K$_{x}$Fe$_{2-δ-y}$Ni$_{y}$Se$_{2}$ (0.06 $\leq$ $y$ $\leq$ 1.44) single crystals | We report electron doping effects by Ni in
K$_{x}$Fe$_{2-\delta-y}$Ni$_{y}$Se$_{2}$ (0.06 $\leq$ $y$ $\leq$ 1.44) single
crystal alloys. A rich ground state phase diagram is observed. Small amount of
Ni ($\sim$ 4\%) suppressed superconductivity below 1.8 K, inducing insulating
spin glass magnetic ground state for higher Ni content. With further Ni
substitution, metallic resistivity is restored. For high Ni concentration in
the lattice the unit cell symmetry is high symmetry $I4/mmm$ with no phase
separation whereas both $I4/m + I4/mmm$ space groups were detected in the phase
separated crystals when concentration of Ni $<$ Fe. The absence of
superconductivity coincides with the absence of crystalline Fe vacancy order. | 1604.05713v1 |
2016-05-16 | Growth and characterization of CaFe$_{1-x}$Co$_x$AsF single crystals by CaAs flux method | Millimeter sized single crystals of CaFe$_{1-x}$Co$_x$AsF were grown using a
self-flux method. It is found that high-quality single crystals can be grown
from three approaches with different initial raw materials. The chemical
compositions and crystal structure were characterized carefully. The c-axis
lattice constant is suppressed by the Co substitution. Superconductivity with
the critical transition $T_c$ as high as 21 K was confirmed by both the
resistivity and magnetic susceptibility measurements in the sample with $x$ =
0.12. Moreover, it is found that $T_c$ can be enhanced for about 1 K under the
very small hydrostatic pressure of 0.22 GPa, which is more quickly than that
reported in the polycrystalline samples. Our results is a promotion for the
physical investigations of 1111 phase iron-pnictide superconductors. | 1605.04642v1 |
2016-06-02 | High Current Density 2D/3D Esaki Tunnel Diodes | The integration of two-dimensional materials such as transition metal
dichalcogenides with bulk semiconductors offer interesting opportunities for
2D/3D heterojunction-based novel device structures without any constraints of
lattice matching. By exploiting the favorable band alignment at the GaN/MoS2
heterojunction, an Esaki interband tunnel diode is demonstrated by transferring
large area, Nb-doped, p-type MoS2 onto heavily n-doped GaN. A peak current
density of 446 A/cm2 with repeatable room temperature negative differential
resistance, peak to valley current ratio of 1.2, and minimal hysteresis was
measured in the MoS2/GaN non-epitaxial tunnel diode. A high current density of
1 kA/cm2 was measured in the Zener mode (reverse bias) at -1 V bias. The
GaN/MoS2 tunnel junction was also modeled by treating MoS2 as a bulk
semiconductor, and the electrostatics at the 2D/3D interface was found to be
crucial in explaining the experimentally observed device characteristics. | 1606.00509v1 |
2016-06-24 | Thermal insulator transition induced by interface scattering | We develop an effective medium model of thermal conductivity that accounts
for both percolation and interface scattering. This model accurately explains
the measured increase and decrease of thermal conductivity with loading in
composites dominated by percolation and interface scattering, respectively. Our
model further predicts that strong interface scattering leads to a sharp
decrease in thermal conductivity, or an insulator transition, at high loadings
when conduction through the matrix is restricted and heat is forced to diffuse
through particles with large interface resistance. The accuracy of our model
and its ability to predict transitions between insulating and conducting states
suggest it can be a useful tool for designing materials with low or high
thermal conductivity for a variety of applications. | 1606.07851v2 |
2016-06-29 | Large linear magnetoresistance from neutral defects in Bi$_2$Se$_3$ | The chalcogenide Bi$_2$Se$_3$ can attain the three dimensional (3D) Dirac
semimetal state under the influence of strain and microstrain. Here we report
the presnece of large linear magnetoresistance in such a Bi$_2$Se$_3$ crystal.
The magnetoresistance has quadratic form at low fields which crossovers to
linear above 4 T. The temperature dependence of magnetoresistance scales with
carrier mobility and the crossover field scales with inverse of mobility. Our
analysis suggest that the linear magnetoresistance in our system has a
classical origin and arises from the scattering of high mobility 3D Dirac
electrons from crystalline inhomogeneities. We observe that the charged
selenium vacancies are strongly screened by high mobility Dirac electrons and
the neutral crystalline defects are the main scattering center for transport
mechanism. Our analysis suggests that both the resistivity and the
magnetoresistance have their origin in scattering of charge carriers from
neutral defects. | 1606.09059v1 |
2016-10-03 | Thallium under extreme compression | We present a combined theoretical and experimental study of the high-pressure
behavior of thallium. X-ray diffraction experiments have been carried out at
room temperature up to 125 GPa using diamond-anvil cells, nearly doubling the
pressure range of previous experiments. We have confirmed the hcp-fcc
transition at 3.5 GPa and determined that the fcc structure remains stable up
to the highest pressure attained in the experiments. In addition, HP-HT
experiments have been performed up to 8 GPa and 700 K by using a combination of
x-ray diffraction and a resistively heated diamond-anvil cell. Information on
the phase boundaries is obtained, as well as crystallographic information on
the HT bcc phase. The equation of state for different phases is reported. Ab
initio calculations have also been carried out considering several potential
high-pressure structures. They are consistent with the experimental results and
predict that, among the structures considered in the calculations, the fcc
structure of thallium is stable up to 4.3 TPa. Calculations also predict the
post-fcc phase to have a close-packed orthorhombic structure above 4.3 TPa. | 1610.00435v1 |
2016-10-19 | Applications and non-idealities of submicron Al-AlOx-Nb tunnel junctions | We have developed a technique to fabricate sub-micron, 0.6 um x 0.6 um
Al-AlOx-Nb tunnel junctions using a standard e-beam resist, angle evaporation
and double oxidation of the tunneling barrier, resulting in high quality
niobium, as determined by the the high measured values of the critical
temperature TC 7.5 K and the gap 1.3 meV. The devices show great promise for
local nanoscale thermometry in the temperature range 1 - 7.5 K. Electrical
characterization of the junctions was performed at sub-Kelvin temperatures both
with and without an external magnetic field, which was used to suppress
superconductivity in Al and thus bring the junction into a
normal-metal-insulator-superconductor (NIS) configuration. We observed excess
sub-gap current, which could not be explained by the standard tunneling theory.
Evidence points towards materials science issues of the barrier or Nb/AlOx
interface as the culprit. | 1610.05903v1 |
2017-06-28 | Super-Hydrophobic Stearic Acid Layer Formed on Anodized High Purified Magnesium for Improving Corrosion Resistance of Biodegradable Implants | Magnesium and its alloys are ideal candidates for biodegradable implants.
However, they can dissolve too rapidly in the human body for most applications.
In this research, high purified magnesium (HP-Mg) was coated with stearic acid
in order to slow the corrosion rate of magnesium in simulated body fluid at
37{\deg}C. HP-Mg was anodized to form an oxide/hydroxide layer, then it was
immersed in a stearic acid solution. Electrochemical impedance spectroscopy and
potentiodynamic polarization were used to estimate the corrosion rate of HP-Mg
specimens. The results confirm that the hydrophobic coating can temporarily
decrease the corrosion rate of HP-Mg by 1000x. | 1706.09508v1 |
2017-10-19 | Design of high-strength refractory complex solid-solution alloys | Nickel-based superalloys and near-equiatomic high-entropy alloys containing
Molybdenum are known for higher temperature strength and corrosion resistance.
Yet, complex solid-solution alloys offer a huge design space to tune for
optimal properties at slightly reduced entropy. For refractory Mo-W-Ta-Ti-Zr,
we showcase KKR electronic-structure methods via the coherent-potential
approximation to identify alloys over 5-dimensional design space with improved
mechanical properties and necessary global (formation enthalpy) and local
(short-range order) stability. Deformation is modeled with classical molecular
dynamic simulations, validated from our first-principles data. We predict
complex solid-solution alloys of improved stability with greatly enhanced
modulus of elasticity ($3\times$ at 300 K) over near-equiatomic cases, as
validated experimentally, and with higher moduli above 500~K over commercial
alloys ($2.3\times$ at 2000 K). We also show that optimal complex
solid-solution alloys are not described well by classical potentials due to
critical electronic effects. | 1710.06983v2 |
2017-11-16 | Superconductivity at 7.3 K in the 133-type Cr-based RbCr3As3 single crystals | Here we report the preparation and superconductivity of the 133-type Cr-based
quasi-one-dimensional (Q1D) RbCr3As3 single crystals. The samples were prepared
by the deintercalation of Rb+ ions from the 233-type Rb2Cr3As3 crystals which
were grown from a high-temperature solution growth method. The RbCr3As3
compound crystallizes in a centrosymmetric structure with the space group of
P63/m (No. 176) different with its non-centrosymmetric Rb2Cr3As3
superconducting precursor, and the refined lattice parameters are a = 9.373(3)
{\AA} and c = 4.203(7) {\AA}. Electrical resistivity and magnetic
susceptibility characterizations reveal the occurrence of superconductivity
with an interestingly higher onset Tc of 7.3 K than other Cr-based
superconductors, and a high upper critical field Hc2(0) near 70 T in this
133-type RbCr3As3 crystals. | 1711.06133v1 |
2017-12-01 | Radiation damage and thermal shock response of carbon-fiber-reinforced materials to intense high-energy proton beams | A comprehensive study on the effects of energetic protons on carbon-fiber
composites and compounds under consideration for use as low-Z pion production
targets in future high-power accelerators and low-impedance collimating
elements for intercepting TeV-level protons at the Large Hadron Collider has
been undertaken addressing two key areas, namely, thermal shock absorption and
resistance to irradiation damage. | 1712.00509v1 |
2018-01-08 | Automated Instrumentation for the Determination of the High-Temperature Thermoelectric Figure-of-Merit | In this work, we report the fabrication of a high temperature measurement
setup to measure Figure of merit (ZT). This setup facilitates the simultaneous
measurement of Seebeck coefficient ({\alpha}), thermal conductivity (\kappa),
and electrical resistivity (\rho) required to calculate ZT. Measurement of
temperature, as well as voltages using same thermocouples, simplified the
design of the setup by minimizing sensors and wires. Limited components used in
the sample holder further simplify the design and make it small in size and
lightweight. The dedicated thin heater is made, which minimizes the heat loss.
Further, low heat loss is achieved by optimizing the insulator dimension. To
measure power delivered to the heater, 4-wire technique is used. Low cost and
commonly available materials used in the fabrication of various components make
it more accessible to the user as any parts can be easily replaced in case of
any damage occurs. A dedicated program is built in the Python programming
language to automate the whole measurement process. p-type Bi0.36Sb1.45Te3
sample is used to calibrate this measurement setup. The data collected are
found to be in good agreement with the reported data. | 1801.02360v1 |
2018-01-30 | Structural and electronic phase transitions in FePS$_3$ under the application of pressure | Two-dimensional materials have proven to be a prolific breeding ground of new
and unstudied forms of magnetism and unusual metallic states, particularly when
tuned between their insulating and metallic phases. In this paper we present
work on a new metal to insulator transition system FePS$_3$ . This compound is
a two-dimensional van-der-Waals antiferromagnetic Mott insulator. Here we
report the discovery of an insulator-metal transition in FePS$_3$, as evidenced
by x-ray diffraction and electrical transport measurements, using high pressure
as a tuning parameter. Two structural phase transitions are observed in the
x-ray diffraction data as a function of pressure and resistivity measurements
show evidence of onset of a metallic state at high pressures. We propose models
for the two new structures that can successfully explain the x-ray diffraction
patterns. | 1801.10089v2 |
2018-03-14 | A 1.6:1 Bandwidth Two-Layer Antireflection Structure for Silicon Matched to the 190-310 GHz Atmospheric Window | Although high-resistivity, low-loss silicon is an excellent material for THz
transmission optics, its high refractive index necessitates antireflection
treatment. We fabricated a wide-bandwidth, two-layer antireflection treatment
by cutting subwavelength structures into the silicon surface using multi-depth
deep reactive ion etching (DRIE). A wafer with this treatment on both sides has
<-20 dB (<1%) reflectance over 190-310 GHz. We also demonstrated that bonding
wafers introduces no reflection features above the -20 dB level, reproducing
previous work. Together these developments immediately enable construction of
wide-bandwidth silicon vacuum windows and represent two important steps toward
gradient-index silicon optics with integral broadband antireflection treatment. | 1803.05168v2 |
2018-04-26 | Large effective mass and interaction-enhanced Zeeman splitting of $K$-valley electrons in MoSe$_2$ | We study the magnetotransport of high-mobility electrons in monolayer and
bilayer MoSe$_2$, which show Shubnikov-de Haas (SdH) oscillations and quantum
Hall states in high magnetic fields. An electron effective mass of 0.8$m_e$ is
extracted from the SdH oscillations' temperature dependence; $m_e$ is the bare
electron mass. At a fixed electron density the longitudinal resistance shows
minima at filling factors (FFs) that are either predominantly odd, or
predominantly even, with a parity that changes as the density is tuned. The SdH
oscillations are insensitive to an in-plane magnetic field, consistent with an
out-of-plane spin orientation of electrons at the $K$-point. We attribute the
FFs parity transitions to an interaction enhancement of the Zeeman energy as
the density is reduced, resulting in an increased Zeeman-to-cyclotron energy
ratio. | 1804.10104v2 |
2018-05-31 | Unusual magnetotransport in holmium monoantimonide | We report the magnetotransport properties of HoSb, a semimetal with
antiferromagnetic ground state. HoSb shows extremely large magnetoresistance
(XMR) and Shubnikov-de Haas (SdH) oscillation at low temperature and high
magnetic field. Different from previous reports in other rare earth
monopnictides, kinks in $\rho(B)$ and $\rho_{xy}(B)$ curves and the field
dependent resistivity plateau are observed in HoSb, which result from the
magnetic phase transitions. The fast Fourier transform analysis of the SdH
oscillation reveals the split of Fermi surfaces induced by the nonsymmetric
spin-orbit interaction. The Berry phase extracted from SdH oscillation
indicates the possible nontrivial electronic structure of HoSb in the presence
of magnetic field. The Hall measurements suggest that the XMR originates from
the electron-hole compensation and high mobility. | 1805.12496v1 |
2018-06-28 | Tribological Properties of Ultrananocrystalline Diamond Nanowire Thin Film: Influence of Sliding Ball Counterbodies | Ultrananocrystalline Diamond Nanowire (UNCD NW) thin film was deposited on
mirror polished silicon substrate (100) using Microwave Plasma Enhanced
Chemical Vapor Deposition (MPECVD) System with optimized deposition parameters
in CH4 (6%)/N2 plasma media. The film exhibited wire like morphology with
randomly oriented and homogeneously distributed ultranano diamond grains
separated by an interphase boundary of graphitic and amorphous carbon (a-C)
phases. Micro-tribological studies of film were carried out against Al2O3, SiC
and steel balls in ambient atmospheric conditions. Initially, the friction
coefficient was found to be high for UNCD NW/SiC and UNCD NW/Steel sliding
pairs which gradually decreased to low value. While, in UNCD NW/Al2O3 sliding
combination, the ultralow value of friction coefficient was maintained
throughout the whole sliding process. High wear resistant properties of the
film were observed in UNCD NW/SiC and UNCD NW/Steel pairs. In UNCD NW/Al2O3
case, ball counterbody showed negligible wear dimension. Such kind of
tribological behavior was attributed to the different type of mechanical and
chemical interactions of ball counterbodies with UNCD NW thin film. | 1806.10765v1 |
2018-09-13 | Anisotropic hybridization in a new Kondo lattice compound CeCoInGa$_3$ | We report a detailed and comparative study of the single crystal CeCoInGa$_3$
in both experiment and theory. Resistivity measurements reveal the typical
behavior of Kondo lattice with the onset temperature of coherence, $T^*\approx
50\,$K. The magnetic specific heat can be well fitted using a spin-fluctuation
model at low temperatures, yielding a large Sommerfeld coefficient,
$\gamma\approx172\,$mJ/mol K$^2$ at 6 K, suggesting that this is a
heavy-fermion compound with a pronounced coherence effect. The magnetic
susceptibility exhibits a broad field-independent peak at $T_{\chi}$ and shows
an obvious anisotropy within the $bc$ plane, reflecting the anisotropy of the
coherence effect at high temperatures. These are compared with strongly
correlated calculations combining first-principles band structure calculations
and dynamical mean-field theory. Our results confirm the onset of coherence at
about 50 K and reveal a similar anisotropy in the hybridization gap, pointing
to a close connection between the hybridization strength of the low-temperature
Fermi-liquid state and the high-temperature coherence effect. | 1809.04735v1 |
2018-11-18 | In situ high-cycle fatigue reveals the importance of grain boundary structure in nanocrystalline Cu-Zr | Nanocrystalline metals typically have high fatigue strengths, but low
resistance to crack propagation. Amorphous intergranular films are disordered
grain boundary complexions that have been shown to delay crack nucleation and
slow crack propagation during monotonic loading by diffusing grain boundary
strain concentrations, suggesting they may also be beneficial for fatigue
properties. To probe this hypothesis, in situ transmission electron microscopy
fatigue cycling is performed on Cu-1 at.% Zr thin films thermally treated to
have either only ordered grain boundaries or to contain amorphous intergranular
films. The sample with only ordered grain boundaries experienced grain
coarsening at crack initiation followed by unsteady crack propagation and
extensive nanocracking, whereas the sample containing amorphous intergranular
films had no grain coarsening at crack initiation followed by steady crack
propagation and distributed plastic activity. Microstructural design for
control of these behaviors through simple thermal treatments can allow for the
improvement of nanocrystalline metal fatigue toughness. | 1811.07263v1 |
2019-09-02 | Thermoelastic equation of state and melting of Mg metal at high pressure and high temperature | The p-V-T equation of state of magnesium metal has been measured up to 20 GPa
and 1500 K using both multianvil and opposite anvil techniques combined with
synchrotron X-ray diffraction. To fit the experimental data, the model of
Anderson-Gr\"uneisen has been used with fixed parameter {\delta}T. The 300-K
bulk modulus of B0 = 32.5(1) GPa and its first pressure derivative, B0' =
3.73(2), have been obtained by fitting available data up to 20 GPa to Murnaghan
equation of state. Thermal expansion at ambient pressure has been described
using second order polynomial with coefficients a = 25(2)x10-6 K-1 and b =
9.4(4)x10-9 K-2. The parameter describing simultaneous pressure and temperature
impact on thermal expansion coefficient (and, therefore, volume) is {\delta}T =
1.5(5). The good agreement between fitted and experimental isobars has been
achieved to relative volumes of 0.75. The Mg melting observed by X-ray
diffraction and in situ electrical resistivity measurements confirms previous
results and additionally confirms the p-T estimations in the vicinity of
melting. | 1909.00645v1 |
2014-08-28 | Crossover of conduction mechanism in Sr2IrO4 epitaxial thin films | High quality epitaxial Sr2IrO4 thin films with various thicknesses (9-300 nm)
have been grown on SrTiO3 (001) substrates, and their electric transport
properties have been investigated. All samples showed the expected insulating
behavior with a strong resistivity dependence on film thickness, that can be as
large as three orders of magnitude at low temperature. A close examination of
the transport data revealed interesting crossover behaviors for the conduction
mechanism upon variation of thickness and temperature. While Mott variable
range hopping (VRH) dominated the transport for films thinner than 85 nm, high
temperature thermal activation behavior was observed for films with large
thickness, which was followed by a crossover from Mott to Efros-Shklovskii (ES)
VRH in the low temperature range. This low temperature crossover from Mott to
ES VRH indicates the presence of a Coulomb gap (~3 meV). Our results
demonstrate the competing and tunable conduction in Sr2IrO4 thin films, which
in turn would be helpful for understanding the insulating nature related to
strong spin-orbit-coupling of the 5d iridates. | 1408.6798v1 |
2017-03-01 | Tunnel-injected sub-260 nm ultraviolet light emitting diodes | We report on tunnel-injected deep ultraviolet light emitting diodes (UV LEDs)
configured with a polarization engineered Al0.75Ga0.25N/ In0.2Ga0.8N tunnel
junction structure. Tunnel-injected UV LED structure enables n-type contacts
for both bottom and top contact layers. However, achieving Ohmic contact to
wide bandgap n-AlGaN layers is challenging and typically requires high
temperature contact metal annealing. In this work, we adopted a compositionally
graded top contact layer for non-alloyed metal contact, and obtained a low
contact resistance of Rc=4.8x10-5 Ohm cm2 on n-Al0.75Ga0.25N. We also observed
a significant reduction in the forward operation voltage from 30.9 V to 19.2 V
at 1 kA/cm2 by increasing the Mg doping concentration from 6.2x1018 cm-3 to
1.5x1019 cm-3. Non-equilibrium hole injection into wide bandgap Al0.75Ga0.25N
with Eg>5.2 eV was confirmed by light emission at 257 nm. This work
demonstrates the feasibility of tunneling hole injection into deep UV LEDs, and
provides a novel structural design towards high power deep-UV emitters. | 1703.00117v1 |
2019-02-27 | High-performance dendritic metamaterial absorber for broadband and near-meter wave radar | Absorbing materials in ultra-high frequency (UHF) band has constantly been a
major challenge. The size of the absorber in UHF band is large, whereas the
resonant frequency band is narrow. According to Rozanov's theory, two kinds of
composite metamaterial absorbers are designed to realize the requirements of
low-frequency broadband metamaterial microwave absorber: the
magnetic-metamaterial composite absorber1 (MA1) and the dielectric-metamaterial
composite absorber 2 (MA2). In the range of approximately 300-1000MHz, both
absorbers achieve absorption of over 90% and feature good adaptability to the
incident angle of the incident wave. The absorbers also present good absorption
rate of over 80% in the range of 0-45 degree. Processing samples of indium tin
oxide (ITO) resistance film and polymethacrylimide (PMI) foam board feature
simple preparation and low cost, and the most important thing is to consider
the weight problem, which features certain advantages in terms of use. | 1902.10398v1 |
2019-07-29 | Mechanical Characterisation of the Protective Al$_2$O$_3$ Scale in Cr$_2$AlC MAX phases | MAX phases have great potential under demands of both high-temperature and
high-stress performance, with their mixed atomic bonding producing the
temperature and oxidation resistance of ceramics with the mechanical resilience
of metals. Here, we measure the mechanical properties up to 980C by
nanoindentation on highly dense and pure Cr$_2$AlC, as well as after oxidation
with a burner rig at 1200C for more than 29 hours. Only modest reductions in
both hardness and modulus up to 980C were observed, implying no change in
deformation mechanism. Furthermore, micro-cantilever fracture tests were
carried out at the Cr$_2$AlC/Cr$_7$C$_3$ and Cr$_7$C$_3$/Al$_2$O$_3$ interfaces
after the oxidation of the Cr$_2$AlC substrates with said burner rig. The
values are typical of ceramic-ceramic interfaces, below 4 MPa/m, leading to the
hypothesis that the excellent macroscopic behaviour is due to a combination of
low internal strain due to the match in thermal expansion coefficient as well
as the convoluted interface. | 1907.12341v1 |
2019-03-21 | Stabilization and heteroepitaxial growth of metastable tetragonal FeS thin films by pulsed laser deposition | Pulsed laser deposition, a non-equilibrium thin-film growth technique, was
used to stabilize metastable tetragonal iron sulfide (FeS), the bulk state of
which is known as a superconductor with a critical temperature of 4 K.
Comprehensive experiments revealed four important factors to stabilize
tetragonal FeS epitaxial thin films: (i) an optimum growth temperature of 300
{\deg}C followed by thermal quenching, (ii) an optimum growth rate of ~7
nm/min, (iii) use of a high-purity bulk target, and (iv) use of a
single-crystal substrate with small in-plane lattice mismatch (CaF2).
Electrical resistivity measurements indicated that none of all the films
exhibited superconductivity. Although an electric double-layer transistor
structure was fabricated using the tetragonal FeS epitaxial film as a channel
layer to achieve high-density carrier doping, no phase transition was observed.
Possible reasons for the lack of superconductivity include lattice strain,
off-stoichiometry of the film, electrochemical etching by the ionic liquid
under gate bias, and surface degradation during device fabrication. | 1903.08820v1 |
2019-03-26 | Gate tunable quantum Hall effects in defect-suppressed Bi2Se3 films | Despite many years of efforts, attempts to reach the quantum regime of
topological surface states (TSS) on an electrically tunable topological
insulator (TI) platform have so far failed on binary TI compounds such as
Bi2Se3 due to high density of interfacial defects. Here, utilizing an optimal
buffer layer on a gatable substrate, we demonstrate the first electrically
tunable quantum Hall effects (QHE) on TSS of Bi2Se3. On the n-side,
well-defined QHE shows up, but it diminishes near the charge neutrality point
(CNP) and completely disappears on the p-side. Furthermore, around the CNP the
system transitions from a metallic to a highly resistive state as the magnetic
field is increased, whose temperature dependence indicates presence of an
insulating ground state at high magnetic fields. | 1903.10945v2 |
2019-04-09 | Alloy Design for Mechanical Properties: Conquering the Length Scales | Predicting the structural response of advanced multiphase alloys and
understanding the underlying microscopic mechanisms that are responsible for it
are two critically important roles modeling plays in alloy development. An
alloys demonstration of superior properties, such as high strength, creep
resistance, high ductility, and fracture toughness, is not sufficient to secure
its use in widespread application. Still, a good model is needed, to take
measurable alloy properties, such as microstructure and chemical composition,
and forecast how the alloy will perform in specified mechanical deformation
conditions, including temperature, time, and rate. In this bulletin, we
highlight recent achievements by multiscale modeling in elucidating the coupled
effects of alloying, microstructure, and the dynamics of mechanisms on the
mechanical properties of polycrystalline alloys. Much of the understanding
gained by these efforts relied on integration of computational tools that
varied over many length and time scales, from first principles density
functional theory, atomistic simulation methods, dislocation and defect theory,
micromechanics, phase field modeling, single crystal plasticity, and
polycrystalline plasticity. | 1904.04569v1 |
2019-08-13 | Investigating the real-time dissolution of a compositionally complex alloy using inline ICP and correlation with XPS | The real-time dissolution of the single-phase compositionally complex alloy
(CCA), Al1.5TiVCr, was studied using an inline inductively coupled plasma
method. Compositionally complex alloys (CCAs), a term encompassing high entropy
alloys (HEAs) or multi-principal element alloys (MPEAs), are - in general -
noted for their inherently high corrosion resistance. In order to gain an
insight into the dissolution of Al1.5TiVCr alloy, atomic emission
spectroelectrochemistry was utilised in order to measure the ion dissolution of
the alloy during anodic polarisation. It was revealed that incongruent
dissolution occurred, with preferential dissolution of Al, and essentially no
dissolution of Ti, until the point of alloy breakdown. Results were correlated
with X-ray photoelectron spectroscopy, which revealed a complex surface oxide
inclusive of unoxidised metal, and metal oxides in disproportion to the bulk
alloying element ratio. | 1908.04493v1 |
2012-06-01 | Quantum Superinductor with Tunable Non-Linearity | We report on the realization of a superinductor, a dissipationless element
whose microwave impedance greatly exceeds the resistance quantum. The design of
the superinductor, implemented as a ladder of nanoscale Josephson junctions,
enables tuning of the inductance and its nonlinearity by a weak magnetic field.
The Rabi decay time of the superinductor-based qubit exceeds 1 microsecond. The
high kinetic inductance and strong nonlinearity offer new types of
functionality, including the development of qubits protected from both flux and
charge noises, fault tolerant quantum computing, and high-impedance isolation
for electrical current standards based on Bloch oscillations. | 1206.0307v2 |
2015-07-10 | Antibunched photons from inelastic Cooper-pair tunneling | We demonstrate theoretically that charge transport across a Josephson
junction, voltage-biased through a resistive environment, produces antibunched
photons. We develop a continuous-mode description of the emitted radiation
field in a semi-infinite transmission line terminated by the Josephson
junction. Within a perturbative treatment in powers of the tunneling coupling
across the Josephson junction, we capture effects originating in charging
dynamics of consecutively tunneling Cooper pairs. We find that within a
feasible experimental setup the Coulomb blockade provided by high
zero-frequency impedance can be used to create antibunched photons at a very
high rate and in a very versatile frequency window ranging from a few GHz to a
THz. | 1507.02885v1 |
2018-07-20 | High-Pressure Phase Diagram of NdFeAsO$_{0.9}$F$_{0.1}$: Disappearance of superconductivity on the verge of ferromagnetism from nd moments | We investigated transport and magnetic properties of NdFeAsO$_{0.9}$F$_{0.1}$
single crystal under hydrostatic pressures up to 50\,GPa. The ambient pressure
superconductivity at $T_{c} \sim$ 45.4\,K is fully suppressed at $P_{c} \sim$
21 GPa. Upon further increase of the pressure, the ferromagnetism associated
with the order of rare-earth subsystem is induced at the border of
superconductivity. Our finding is supported by the hysteresis in the
magnetization $M$($H$) loops and the strong increase in the field cooled data,
$M$($T$), toward low temperatures. We also show that the temperature evolution
of the electrical resistivity as a function of pressure is consistent with a
crossover from a Fermi-liquid to non-Fermi-liquid to Fermi-liquid. These
results give access to the high-pressure side of the superconducting phase
diagram in 1111 type of materials. | 1807.07729v1 |
2018-12-19 | Benchmarking vdW-DF first principle predictions against Coupled Electron-Ion Monte Carlo for high pressure liquid hydrogen | We report first principle results for nuclear structure and optical responses
of high pressure liquid hydrogen along two isotherms in the region of molecular
dissociation. We employ Density Functional Theory with the vdW-DF approximation
(vdW) and we benchmark the results against existing predictions from Coupling
Electron-Ion Monte Carlo (CEIMC). At fixed density and temperature, we find
that pressure from vdW is higher than pressure from CEIMC by about 10 GPa in
the molecular insulating phase and about 20 GPa in the dissociated metallic
phase. Molecules are found to be overstabilized using vdW, with a slightly
shorter bond length, and with a stronger resistance to compression. As a
consequence, pressure dissociation along isotherms using vdW is more
progressive than computed with CEIMC. Below the critical point, the
liquid-liquid phase transition is observed with both theories in the same
density region but the one predicted by vdW has a smaller density
discontinuity, i.e. a smaller first order character. The optical conductivity
computed using Kubo-Greenwood is rather similar for the two systems and
reflects the slightly more pronounced molecular character of vdW. | 1812.07818v1 |
2019-10-01 | In-situ detection of nucleation in high temperature solutions | The state of a sample during crystal growth from high temperature solutions
is not accessible in conventional furnace systems. An optimization of the
growth parameters often requires arduous trial and error procedures in
particular in case of novel multicomponent systems with unknown phase diagrams.
Here we present a measurement technique based on lock-in amplification that
allows for in-situ detection of the liquidus and solidus temperatures as well
as structural phase transitions. A thin, metallic measurement wire is mounted
in close vicinity to the melt. Characteristic anomalies in the time-dependent
electrical resistivity of this wire allow for the detection of latent heat
release without using a reference crucible. The method is implemented in a
'feedback furnace' and enables an adjustment of the temperature profile based
on the occurrence or absence of phase transitions. The absolute temperature
serves as an additional source of information. Obtained phase transition
temperatures are in good agreement with differential thermal analysis (DTA). | 1910.00316v1 |
2019-10-04 | Improvement of temperature uniformity of induction-heated T-shape susceptor for high-temperature MOVPE | The induction heating is a common method applied in metalorganic vapor phase
epitaxy (MOVPE) especially for higher-temperature growth conditions. However,
compared to the susceptor heated by the multiple-zone resistant heater, the
inductive-heated susceptor could suffer from severe thermal non-uniformity
issue. In this simulation study, we propose to employ a T-shape susceptor
design with various geometric modifications to significantly improve the
substrate temperature uniformity by manipulating thermal transfer.
Specifically, the thermal profile can be tailored by horizontal expansion and
vertical elongation of the susceptor, or forming a cylindrical hollow structure
at the susceptor bottom cylinder. Three optimized designs are shown with
different temperature uniformity as well as various induction heating
efficiencies. The temperature variation of the entire substrate surface can be
less than 5 {\deg}C at ~1900 {\deg}C with high induction heating efficiency
after applying the proposed techniques. | 1910.02024v1 |
2019-10-21 | Harnessing energy landscape exploration to control the buckling of cylindrical shells | The complexity and unpredictability of postbuckling responses in even simple
thin shells have raised great challenges to emerging technologies exploiting
buckling transitions. Here we comprehensively survey the buckling landscapes to
show the full complexity of the stable buckling states and the transition
mechanisms between each of them. This is achieved by combining a simple and
versatile triangulated lattice model for modelling the shell morphologies with
efficient high-dimensional free-energy minimisation and transition path finding
algorithms. We show how the simple free energy landscapes of short, lightly
compressed cylinders become vastly more complex at high compressive strains or
aspect ratios. We then exploit these landscapes to introduce an effective
method for targeted design - landscape biasing. This is used to inform
thickness modifications enabling landscape redesign, and the development of
structures which are highly resistant to lateral perturbations. Our methods are
general, and can be extended to studying postbuckling responses of other
geometries. | 1910.09210v1 |
2019-11-07 | Long- to short-junction crossover and field-reentrant critical current in Al/Ag-nanowires/Al Josephson junctions | We have probed the superconducting proximity effect through long high-quality
monocrystalline Ag nanowires, by realizing Josephson junctions of different
lengths, with different superconducting materials. Thanks to the high number of
junctions probed, both the contact resistance and electron diffusion constant
could be determined, enabling a comparison of the measured critical current to
theoretical expectation, over the entire regime from short to long diffusive
junction. Although the length dependence of the critical current is as
expected, the amplitude of the $R_{N}I_c$ product is smaller than predicted by
theory. We also address the magnetic field dependence of the critical current.
The quasi-gaussian decay of the critical current with field expected of a long
narrow junction is observed for all superconducting contacts we used except for
aluminum. We present the striking non-monotonous effect of field on the
critical current of junctions with aluminum contacts, and analyze it in terms
of improved quasiparticle thermalization by a magnetic field. | 1911.02962v1 |
2019-11-13 | Giant enhancement of cryogenic thermopower by polar structural instability in the pressurized semimetal MoTe2 | We found that a high mobility semimetal 1T'-MoTe2 shows a significant
pressure-dependent change in the cryogenic thermopower in the vicinity of the
critical pressure, where the polar structural transition disappears. With the
application of a high pressure of 0.75 GPa, while the resistivity becomes as
low as 10 {\mu}{\Omega}cm, thermopower reached the maximum value of 60
{\mu}VK-1 at 25 K, leading to a giant thermoelectric power factor of 300
{\mu}WK-2cm-1. Based on semiquantitative analyses, the origin of this behavior
is discussed in terms of inelastic electron-phonon scattering enhanced by the
softening of zone center phonon modes associated with the polar structural
instability. | 1911.05269v1 |
2019-11-26 | One-directional thermal transport in densely aligned single-wall carbon nanotube films | Individual carbon nanotubes (CNTs) possess extremely high thermal
conductivities. However, the thermal conductivities and their anisotropy of
macroscopic assemblies of CNTs have so far remained small. Here, we report
results of directional thermal transport measurements on a nearly-perfectly
aligned CNT film fabricated via controlled vacuum filtration. We found the
thermal conductivity to be 43 +- 2.2 W m^-1 K^-1 with a record-high thermal
anisotropy of 500. From the temperature dependence of the thermal conductivity
and its agreement with the atomistic phonon transport calculation, we conclude
that the effect of intertube thermal resistance on heat conduction in the
alignment direction is negligible because of the large contact area between
CNTs. These observations thus represent ideal unidirectional thermal transport,
i.e., the thermal conductivity of the film is determined solely by that of
individual CNTs. | 1911.11340v1 |
2020-05-05 | Nanofilm Materials for Devices of Magnetic Field Measurement in Radiation Environment | The prospects of using nanofilms of indium-containing III-V semiconductors,
gold and single-layer graphene in magnetic field sensors, intended for
application in radiation environment were evaluated on the results of testing
in neutron fluxes. Semiconductor sensors are capable of withstanding radiation
levels typical for the ITER-type fusion reactors, while gold sensors are stable
even under environment expected in the first fusion power plant DEMO. Graphene
is promising for creating sensors that combine high magnetic field sensitivity
and high irradiation resistance. | 2005.02075v1 |
2020-11-11 | Synthesis of high-entropy-alloy-type superconductors (Fe,Co,Ni,Rh,Ir)Zr2 with tunable transition temperature | We report on the synthesis and superconductivity of high-entropy-alloy-type
(HEA-type) compounds TrZr2 (Tr = Fe, Co, Ni, Rh, Ir), in which the Tr site
satisfies the criterion of HEA. Polycrystalline samples of HEA-type TrZr2 with
four different compositions at the Tr site were synthesized by arc melting
method. The phase purity and crystal structure were examined by Rietveld
refinement of X-ray diffraction profile. It has been confirmed that the
obtained samples have a CuAl2-type tetragonal structure. From analyses of
elemental composition and mixing entropy at the Tr site, the HEA state for the
Tr site was confirmed. The physical properties of obtained samples were
characterized by electrical resistivity and magnetization measurements. All the
samples show bulk superconductivity with various transition temperature (Tc).
The Tc varied according to the compositions and showed correlations with the
lattice constant c and Tr-Zr bond lengths. Introduction of an HEA site in TrZr2
is useful to achieve systematic tuning of Tc with a wide temperature range,
which would be a merit for superconductivity application. | 2011.05590v1 |
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