publicationDate stringlengths 10 10 | title stringlengths 17 233 | abstract stringlengths 20 3.22k | id stringlengths 9 12 |
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2003-12-28 | Optical properties of pyrochlore oxide $Pb_{2}Ru_{2}O_{7-δ}$ | We present optical conductivity spectra for $Pb_{2}Ru_{2}O_{7-{\delta}}$
single crystal at different temperatures. Among reported pyrochlore ruthenates,
this compound exhibits metallic behavior in a wide temperature range and has
the least resistivity. At low frequencies, the optical spectra show typical
Drude responses, but with a knee feature around 1000 \cm. Above 20000 \cm, a
broad absorption feature is observed. Our analysis suggests that the low
frequency responses can be understood from two Drude components arising from
the partially filled Ru $t_{2g}$ bands with different plasma frequencies and
scattering rates. The high frequency broad absorption may be contributed by two
interband transitions: from occupied Ru $t_{2g}$ states to empty $e_{g}$ bands
and from the fully filled O 2p bands to unoccupied Ru $t_{2g}$ states. | 0312662v1 |
2004-01-31 | Unprecedented Superconductivity in the beta-Pyrochlore Osmate KOs2O6 | Superconductivity in the potassium osmium oxide KOs2O6 crystallizing in the
beta-pyrochlore structure is studied by means of electrical resistivity,
magnetic susceptibility and specific heat. It is the second superconductor in
the family of pyrochlore oxides, following the alfa-type pyrochlore oxide
Cd2Re2O7 which is believed to be a conventional s-wave superconductor. The
superconducting transition temperature Tc of KOs2O6 is 9.6 K, almost one order
higher than the Tc = 1.0 K of Cd2Re2O7. Moreover, the superconductivity of
KOs2O6 is remarkably robust under high magnetic fields, with a large upper
critical magnetic field Hc2 of about 38 T, which seems to exceed Pauli's limit
expected for conventional superconductivity. This is also in contrast to the
case of Cd2Re2O7, in which the Hc2 is 0.29 T, much smaller than the
corresponding Pauli's limit. These distinct contrasts strongly suggest that the
mechanism of superconductivity is essentially different between the two
pyrochlore oxides. | 0402006v3 |
2004-03-31 | Spin-Transfer Effects in Nanoscale Magnetic Tunnel Junctions | We report measurements of magnetic switching and steady-state magnetic
precession driven by spin-polarized currents in nanoscale magnetic tunnel
junctions with low-resistance, < 5 Ohm-micron-squared, barriers. The current
densities required for magnetic switching are similar to values for
all-metallic spin-valve devices. In the tunnel junctions, spin-transfer-driven
switching can occur at voltages that are high enough to quench the tunnel
magnetoresistance, demonstrating that the current remains spin-polarized at
these voltages. | 0404002v2 |
2004-08-16 | Structural, transport, magnetic properties and Raman spectroscopy of orthorhombic Y$1-x$CaxMnO3(0 <= x <= 0.5) | Orthorhombic Y$_{1-x}$Ca$_x$MnO$_3$ ($0 \leq x \leq 0.5$) was prepared under
high pressure and the variations with $x$ of its structural, magnetic,
electrical properties and the polarized Raman spectra were investigated. The
lattice parameters change systematically with $x$. Although there are strong
indications for increasing disorder above $x = 0.20$, the average structure
remains orthorhombic in the whole substitutional range. Ca doping increases
conductivity, but temperature dependence of resistivity $\rho$(T) remains
semiconducting for all $x$. The average magnetic exchange interaction changes
from antiferromagnetic for $x < 0.08$ to ferromagnetic for $x > 0.08$. The
evolution with $x$ of the Raman spectra provides evidence for increasingly
disordered oxygen sublattice at $x \geq 0.10$, presumably due to quasistatic
and/or dynamical Jahn-Teller distortions. | 0408360v1 |
2004-09-08 | Evidence for charge Kondo effect in superconducting Tl-doped PbTe | We report results of low-temperature thermodynamic and transport measurements
of Pb_{1-x}Tl_{x}Te single crystals for Tl concentrations up to the solubility
limit of approximately x = 1.5%. For all doped samples, we observe a
low-temperature resistivity upturn that scales in magnitude with the Tl
concentration. The temperature and field dependence of this upturn are
consistent with a charge Kondo effect involving degenerate Tl valence states
differing by two electrons, with a characteristic Kondo temperature T_K ~ 6 K.
The observation of such an effect supports an electronic pairing mechanism for
superconductivity in this material and may account for the anomalously high T_c
values. | 0409174v2 |
2004-09-08 | Tailoring of ferromagnetic Pr0.85Ca0.15MnO3/ferroelectric Ba0.6Sr0.4TiO3 superlattices for multiferroic properties | Superlattices composed of ferromagnetic Pr0.85Ca0.15MnO3 and ferroelectric
Ba0.6Sr0.4TiO3 layers were fabricated on (100) SrTiO3 substrates by a
pulsed-laser deposition method. The capacitance and resistive parts of the
samples were analyzed from the complex impedance measurements, performed on the
samples using a special experimental set-up. The superlattice with larger
ferroelectric thickness shows unique characteristics which are not present in
the parent ferromagnetic thin film. The superlattice show both ferromagnetic
and ferroelectric transitions which is an evidence for the coexistence of both
the properties. The high magnetoresistance (40 % at 80K) shown by the
superlattice can be attributed to the coupling between ferromagnetic and
ferroelectric layers, i.e, to the magnetoelectric effect. | 0409182v1 |
2004-09-20 | Effect of Joule heating in current-driven domain wall motion | It was found that high current density needed for the current-driven domain
wall motion results in the Joule heating of the sample. The sample temperature,
when the current-driven domain wall motion occurred, was estimated by measuring
the sample resistance during the application of a pulsed-current. The sample
temperature was 750 K for the threshold current density of 6.7 x 10^11 A/m2 in
a 10 nm-thick Ni81Fe19 wire with a width of 240 nm. The temperature was raised
to 830 K for the current density of 7.5 x 10^11 A/m2, which is very close to
the Curie temperature of bulk Ni81Fe19. When the current density exceeded 7.5 x
10^11 A/m2, an appearance of a multi-domain structure in the wire was observed
by magnetic force microscopy, suggesting that the sample temperature exceeded
the Curie temperature. | 0409494v1 |
2004-11-12 | Observation of abrupt first-order metal-insulator transition in GaAs-based two-terminal device | An abrupt first-order metal-insulator transition (MIT) as a jump of the
density of states is observed for Be doped GaAs, which is known as a
semiconductor, by inducing very low holes of approximately n_p=5x10^{14}
cm^{-3} into the valence band by the electric field; this is anomalous. In a
higher hole doping concentration of n_p=6x10^{16} cm^{-3}, the abrupt MIT is
not observed at room temperature, but measured at low temperature. A large
discontinuous decrease of photoluminescence intensity at 1.43 eV energy gap and
a negative differential resistance are also observed as further evidence of the
MIT. The abrupt MIT does not undergo a structural phase transition and is
accompanied with inhomogeneity. The upper limit of the temperature allowing the
MIT is deduced to be approximately 440K from experimental data. The abrupt MIT
rather than the continuous MIT is intrinsic and can explain the "breakdown"
phenomenon (unsolved problem) incurred by a high electric field in
semiconductor devices. | 0411328v2 |
2004-11-13 | Large magnetoresistance anomalies in Dy7Rh3 | The compound Dy7Rh3 ordering antiferromagnetically below (TN=) 59 K has been
known to exhibit a temperature (T) dependent electrical resistivity (rho)
behavior in the paramagnetic state unusual for intermetallic compounds in the
sense that there is a broad peak in rho(T) in the paramagnetic state (around
130 K) as though there is a semi-conductor to metal transition. In addition,
there is an upturn below T_N due to magnetic super-zone gap effects. Here we
report the influence of external magnetic field (H) on the rho(T) behavior of
this compound below 300 K. The rise of rho(T) found below TN could be
suppressed at very high fields (>> 60 kOe), thus resulting in a very large
magnetoresistance (MR) in the magnetically ordered state. The most notable
finding is that the magnitude of MR is large for moderate applications of H
(say 80 kOe) in a temperature range far above T_N as well, which is untypical
of intermetallic compounds. Thus, this compound is characterized by large MR
anomalies in the entire T range of investigation. | 0411360v1 |
2004-11-24 | Large magnetoresistance in the magnetically ordered state as well as in the paramagnetic state near 300 K in an intermetallic compound,Gd7Rh3 | We report the response of electrical resistivity $\rho$ to the application of
magnetic fields (H) up to 140 kOe in the temperature interval 1.8-300 K for the
compound, Gd7Rh3, ordering antiferromagnetically below 150 K. We find that
there is an unusually large decrease of $\rho$ for moderate values of H in the
close vicinity of room temperature uncharacteristic of paramagnets, with the
magnitude of the magnetoresistance increasing with decreasing temperature as
though the spin-order contribution to $\rho$ is temperature dependent. In
addition, this compound exhibits giant magnetoresistance behaviour at rather
high temperatures (above 77 K) in the magnetically ordered state due to a
metamagnetic transition. | 0411608v1 |
2005-04-05 | Coherent transport in homojunction between excitonic insulator and semimetal | From the solution of a two-band model, we predict that the thermal and
electrical transport across the junction of a semimetal and an excitonic
insulator will exhibit high resistance behavior and low entropy production at
low temperatures, distinct from a junction of a semimetal and a normal
semiconductor. This phenomenon, ascribed to the dissipationless exciton flow
which dominates over the charge transport, is based on the much longer length
scale of the change of the effective interface potential for electron
scattering due to the coherence of the condensate than in the normal state. | 0504121v1 |
2005-04-25 | Impurity scattering and quantum confinement in giant magnetoresistance systems | Ab initio calculations for the giant magnetoresistance (GMR) in Co/Cu, Fe/Cr,
and Fe/Au multilayers are presented. The electronic structure of the
multilayers and the scattering potentials of point defects therein are
calculated self-consistently. Residual resistivities are obtained by solving
the quasi-classical Boltzmann equation including the electronic structure of
the layered system, the anisotropic scattering cross sections derived by a
Green's function method and the vertex corrections. Furthermore, the influence
of scattering centers at the interfaces and within the metallic layers is
incorporated by averaging the scattering cross sections of different impurities
at various sites. An excellent agreement of experimental and theoretical
results concerning the general trend of GMR in Co/Cu systems depending on the
type and the position of impurities is obtained. Due to the quantum confinement
in magnetic multilayers GMR can be tailored as a function of the impurity
position. In Co/Cu and Fe/Au systems impurities in the magnetic layer lead to
high GMR values, whereas in Fe/Cr systems defects at the interfaces are most
efficient to increase GMR. | 0504634v1 |
2005-05-16 | Novel procedure to prepare cadmium stannate films using spray pyrolysis technique for solar cell applications | Thin films of cadmium stannate was prepared using low cost cadmium acetate
and tin (II) chloride precursors by spray pyrolysis technique at three
different substrate temperatures of 400, 450 and 5000 C. A novel procedure of
simultaneously forming additional layer, introduced for the first time in this
work, on the already coated cadmium stannate film reduced the sheet resistance
from 160 ohms/sq to 15 ohms/sq. Further, it is identified that the formation of
additional layer does not affect the structural and optical properties of the
cadmium stannate films, but improves the electrical property; thus the
formation of additional layer seems to be an effective alternate for annealing
the films at high temperature in the presence of Ar, CdS, Ar/CdS mixture,
hydrogen or nitrogen to improve the structural, electrical and optical
properties of the cadmium stannate films as has been reported in the
literature. The maximum optical transmittance value of the prepared cadmium
stannate film is about 99.8 % and the optical band gap energy value is about
2.9 eV. | 0505383v1 |
2005-05-30 | Mechanisms limiting the coherence time of spontaneous magnetic oscillations driven by DC spin-polarized currents | The spin-transfer torque from a DC spin-polarized current can generate
highly-coherent magnetic precession in nanoscale magnetic-multilayer devices.
By measuring linewidths of spectra from the resulting resistance oscillations,
we argue that the coherence time can be limited at low temperature by thermal
deflections about the equilibrium magnetic trajectory, and at high temperature
by thermally-activated transitions between dynamical modes. Surprisingly, the
coherence time can be longer than predicted by simple macrospin simulations. | 0505733v2 |
2005-06-05 | On the unusual behavior of nitride compounds | This report presents consistent insight into the mechanism behind the unusual
behavior of nitride compounds from the perspective of tetrahedron bond
formation and its consequence on valence density of states. An extension of the
recent bond-band-barrier (BBB) correlation mechanism for oxidation [Sun CQ,
Prog Mater Sci 2003;48:521-685] to the electronic process of nitridation has
led to the essentiality of sp-orbital hybridization for a nitrogen atom upon
interacting with atoms in solid phase of arbitrary less-electronegative
element. In the process of nitridation, a nitrogen atom forms a
quasi-tetrahedron with surrounding host atoms through bonding and nonbonding
interaction associated with production of electronic holes and antibonding
dipoles, which add corresponding density of states to the valence band of the
host. It is suggested that the valance alteration of the system takes the
responsibility for the blue shift in photoluminescence, lowered work function
for cold cathode field emission, corrosion and wear resistant, high elasticity,
and magnetic modulation as well. | 0506109v1 |
2005-06-10 | Anisotropic Magnetoresistance and Magnetic Anisotropy in High-quality (Ga,Mn)As Films | We have performed a systematic investigation of magnetotransport of a series
of as-grown and annealed Ga1-xMnxAs samples with 0.011 <= x <= 0.09. We find
that the anisotropic magnetoresistance (AMR) generally decreases with
increasing magnetic anisotropy, with increasing Mn concentration and on low
temperature annealing. We show that the uniaxial magnetic anisotropy can be
clearly observed from AMR for the samples with x >= 0.02. This becomes the
dominant anisotropy at elevated temperatures, and is shown to rotate by 90o on
annealing. We find that the in-plane longitudinal resistivity depends not only
on the relative angle between magnetization and current direction, but also on
the relative angle between magnetization and the main crystalline axes. The
latter term becomes much smaller after low temperature annealing. The planar
Hall effect is in good agreement with the measured AMR indicating the sample is
approximately in a single domain state throughout most of the magnetisation
reversal, with a two-step magnetisation jump ascribed to domain wall nucleation
and propagation. | 0506250v1 |
2005-06-14 | The O-M-O triatomic molecule: Basic unit of cuprates & manganates | The O(oxygen)-M(metal)-O(oxygen) molecule is a basic unit of high-temperature
superconducting cuprates and colossal magnetoresistance exhibiting manganates.
This molecule can be regarded either as an element of a linear chain or as an
ingredient of the corresponding cuprate or manganate lattice. The symmetry of
the unit being different in the two approaches, group theory imposes different
limitations on conceivable vibrational modes and atomic otbitals that control
its transport and optical properties. We now calculate the electron hopping
energies along Cu(P0-O(A) bonds, sites for nonlocal electron-vibrational mode
coupling. We find the electric transport along the O(A)-Cu(P)-O(A) molecule
dominated by scattering from bond polarons which is reflected in the two-branch
character of the temperature dependence of its electric resistance. | 0506308v1 |
2005-09-27 | Critical currents and vortex dynamics in percolative superconductors containing fractal clusters of a normal phase | The effect of fractal clusters on magnetic and transport properties of
percolative superconductors is considered. The superconductor contains
percolative superconducting cluster, carrying a transport current, as well as
clusters of a normal phase, which act as pinning centers. A prototype of such a
structure is a high-temperature superconducting wire. It is found that
normal-phase clusters can have fractal features that affect the vortex
dynamics. The fractal dimension of the normal-phase cluster boundary in YBCO
films is estimated. Depinning and transport of vortices in fractal
superconducting structures are investigated. The current-voltage
characteristics of superconductors containing fractal clusters are obtained.
Dependencies of free vortex density on the fractal dimension as well as the
resistance on the transport current are studied. It is found that the
fractality of the cluster boundary intensifies pinning. This feature enables
the current-carrying capability of a superconductor to be enhanced without
changing of its chemical composition. | 0509701v1 |
2005-10-18 | Surface Roughness and Hydrodynamic Boundary Conditions | We report results of investigations of a high-speed drainage of thin aqueous
films squeezed between randomly nanorough surfaces. A significant decrease in
hydrodynamic resistance force as compared with predicted by Taylor's equation
is observed. However, this reduction in force does not represents the slippage.
The measured force is exactly the same as that between equivalent smooth
surfaces obeying no-slip boundary conditions, but located at the intermediate
position between peaks and valleys of asperities. The shift in hydrodynamic
thickness is shown to be independent on the separation and/or shear rate. Our
results disagree with previous literature data reporting very large and
shear-dependent boundary slip for similar systems. | 0510462v2 |
2005-10-20 | Current-driven magnetization switching in CoFeB/MgO/CoFeB magnetic tunnel junctions | Current-driven magnetization switching in low-resistance
Co40Fe40B20/MgO/Co40Fe40B20 magnetic tunnel junctions (MTJs) is reported. The
critical-current densities Jc required for current-driven switching in samples
annealed at 270C and 300C are found to be as low as 7.8 x 10^5 A/cm^2 and 8.8 x
10^5 A/cm^2 with accompanying tunnel magnetoresistance (TMR) ratios of 49% and
73 %, respectively. Further annealing of the samples at 350C increases TMR
ratio to 160 %, while accompanying Jc increases to 2.5 x 10^6 A/cm^2. We
attribute the low Jc to the high spin-polarization of tunnel current and small
MsV product of the CoFeB single free layer, where Ms is the saturation
magnetization and V the volume of the free layer. | 0510538v1 |
2006-02-13 | Doping of Ce in T-La2CuO4: Rigorous test for electron-hole symmetry for high-Tc superconductivity | We report that Ce doping was achieved in La2CuO4 with the K2NiF4 (T)
structure for the first time by molecular beam epitaxy. A synthesis temperature
of as low as ~ 630C and an appropriate substrate choice, i.e., (001)LaSrGaO4 (a
\~ 3.843 A), enebled us to incorporate Ce into K2NiF4 lattice and to obtain
Ce-doped T-La2-xCexCuO4 up to x ~ 0.06. The doping of Ce makes T-La2CuO4 more
insulating, which is in sharp contrast to Sr (or Ba) doping in T-La2CuO4, which
make the compound metallic and superconducting. The observed smooth increase in
resistivity from the hole-doped side (T-La2-xSrxCuO4) to the electron-doped
side (T-La2-xCexCuO4) indicates that the electron-hole symmetry is broken in
the T-phase materials. | 0602303v1 |
2006-03-02 | Coupled dynamics of electrons and phonons in metallic nanotubes: current saturation from hot phonons generation | We show that the self-consistent dynamics of both phonons and electrons is
the necessary ingredient for the reliable description of the hot phonons
generation during electron transport in metallic single-wall carbon nanotubes
(SWNTs). We solve the coupled Boltzmann transport equations to determine in a
consistent way the current vs. voltage (IV) curve and the phonon occupation in
metallic SWNTs which are lying on a substrate. We find a good agreement with
measured IV curves and we determine an optical phonon occupation which
corresponds to an effective temperature of several thousands K (hot phonons),
for the voltages typically used in experiments. We show that the high-bias
resistivity strongly depends on the optical phonon thermalization time. This
implies that a drastic improvement of metallic nanotubes performances can be
achieved by increasing the coupling of the optical phonons with a
thermalization source. | 0603046v1 |
2006-03-05 | Electro-Thermal Transport in Metallic Single-Wall Carbon Nanotubes for Interconnect Applications | This work represents the first electro-thermal study of metallic single-wall
carbon nanotubes (SWNTs) for interconnect applications. Experimental data and
careful modeling reveal that self-heating is of significance in short (1 < L <
10 um) nanotubes under high-bias. The low-bias resistance of micron scale SWNTs
is also found to be affected by optical phonon absorption (a scattering
mechanism previously neglected) above 250 K. We also explore length-dependent
electrical breakdown of SWNTs in ambient air. Significant self-heating in SWNT
interconnects can be avoided if power densities per unit length are limited to
less than 5 uW/um. | 0603110v1 |
2006-03-24 | Single-crystalline nanopillars for spin-transfer measurements | We report on current-induced magnetization switching (CIMS) in single-
crystalline nanopillars. Fe(14 nm)/Cr(0.9 nm)/Fe(10 nm)/Ag(6 nm)/Fe(2 nm)
multilayers are deposited by molecular-beam epitaxy. The central Fe layer is
coupled to the thick one by interlayer exchange coupling over Cr. The topmost
Fe layer is decoupled (free layer). Nanopillars with 150 nm diameter are
prepared by optical and e-beam lithography. The opposite spin scattering
asymmetries of Fe/Cr and Fe/Ag interfaces enable us to observe CIMS at small
magnetic fields and opposite current polarity in a single device. At high
magnetic fields, step-like resistance changes are measured at positive currents
and are attributed to current-driven magnetic excitations. | 0603662v1 |
2006-07-03 | Knight shift detection using gate-induced decoupling of the hyperfine interaction in quantum Hall edge channels | A method for the observation of the Knight shift in nanometer-scale region in
semiconductors is developed using resistively detected nuclear magnetic
resonance (RDNMR) technique in quantum Hall edge channels. Using a gate-induced
decoupling of the hyperfine interaction between electron and nuclear spins, we
obtain the RDNMR spectra with or without the electron-nuclear spin coupling. By
a comparison of these two spectra, the values of the Knight shift can be given
for the nuclear spins polarized dynamically in the region between the relevant
edge channels in a single two-dimensional electron system, indicating that this
method has a very high sensitivity compared to a conventional NMR technique. | 0607028v1 |
2006-12-01 | New Type Phase Transition of Li2RuO3 with Honeycomb Structure | A new-type structural transition has been found in Li2RuO3 with honeycomb
lattice of edge-sharing RuO6-octahedra. With decreasing temperature T, the
electrical resistivity exhibits an anomalous increase at T=Tc~540 K, suggesting
the (metal to insulator)-like transition and the magnetic susceptibility also
shows a sharp decrease. Detailed structure analyses have revealed that the high
temperature space group C2/m changes to P21/m at Tc. The most striking fact is
that a significant reduction of the bond lengths is found between two of six
Ru-Ru pairs of the hexagon in the low temperature phase, indicating a new type
phase transition by the mechanism of the formation of molecular orbits of these
Ru-Ru pairs. | 0612026v1 |
2006-12-26 | Transport properties of n-type ultrananocrystalline diamond films | We investigate transport properties of ultrananocrystalline diamond films for
a broad range of temperatures. Addition of nitrogen during plasma-assisted
growth increases the conductivity of ultrananocrystalline diamond films by
several orders of magnitude. We show that films produced at low concentration
of nitrogen in the plasma are very resistive and electron transport occurs via
a variable range hopping mechanism while in films produced at high nitrogen
concentration the electron states become delocalized and the transport
properties of ultrananocrystalline diamond films can be described using the
Boltzmann formalism. We discuss the critical concentration of carriers at which
the metal to insulator transition in ultrananocrystalline diamond films occurs
and compare our results with available experimental data. | 0612633v1 |
2007-01-29 | Parameters of the Dzyaloshinsky-Moriya type weak ferromagnetism for some perovskite compounds | Compounds with distorted perovskite structure of the 4-f and 3-d transition
metals with the common formula LnTO3 (where Ln is rare-earth element, T is an
element from the Fe group) are the most multifold binary oxides of these two
groups elements. Wide range of stability for this structure allows the
realization of combinations of the Lanthanides with all the transition metals
except the Nickel. Quite interesting physical phenomena take place in these
oxides like charge and orbital ordering; relatively independent magnetic
lattices of the both metals; particular magnetic structures; high optical
indicators; giant magneto-resistance; peculiar dielectric and ferroelectric
properties etc. We have investigated the magnetic properties of pure ferrites
and chromium-based materials as well as of some mixed type oxides like
HoxTb1-xO3, HoMnxFe1-xO3, HoMnxCr1-xO3 and DyFexCr1-xO3. | 0701708v1 |
2007-02-20 | Organic small molecule field-effect transistors with Cytop(TM) gate dielectric: eliminating gate bias stress effects | We report on organic field-effect transistors with unprecedented resistance
against gate bias stress. The single crystal and thin-film transistors employ
the organic gate dielectric Cytop(TM). This fluoropolymer is highly water
repellent and shows a remarkable electrical breakdown strength. The single
crystal transistors are consistently of very high electrical quality: near zero
onset, very steep subthreshold swing (average: 1.3 nF V/(dec cm2)) and
negligible current hysteresis. Furthermore, extended gate bias stress only
leads to marginal changes in the transfer characteristics. It appears that
there is no conceptual limitation for the stability of organic semiconductors
in contrast to hydrogenated amorphous silicon. | 0702472v1 |
2007-02-25 | Broadband dielectric microwave microscopy on $μ$m length scales | We demonstrate that a near-field microwave microscope based on a transmission
line resonator allows imaging in a substantially wide range of frequencies, so
that the microscope properties approach those of a spatially-resolved impedance
analyzer. In the case of an electric probe, the broadband imaging can be used
in a direct fashion to separate contributions from capacitive and resistive
properties of a sample at length scales on the order of one micron. Using a
microwave near-field microscope based on a transmission line resonator we
imaged the local dielectric properties of a Focused Ion Beam (FIB) milled
structure on a high-dielectric-constant Ba_{0.6}Sr_{0.4}TiO_3 (BSTO) thin film
in the frequency range from 1.3 GHz to 17.4 GHz. The electrostatic
approximation breaks down already at frequencies above ~10 GHz for the probe
geometry used, and a full-wave analysis is necessary to obtain qualitative
information from the images. | 0702573v1 |
2007-06-05 | Structure-Property Relationship in the Ordered-Perovskite- Related Oxide Sr3.12Er0.88Co4O10.5 | Synchrotron X-ray diffraction patterns were measured and analyzed for a
polycrystalline sample of the room-temperature ferromagnet Sr3.12Er0.88Co4O10.5
from 300 to 650 K, from which two structural phase transitions were found to
occur successively. The higher-temperature transition at 509 K is driven by
ordering of the oxygen vacancies, which is closely related to the metallic
state at high temperatures. The lower-temperature transition at 360 K is of
first order, at which the ferromagnetic state suddenly appears with exhibiting
a jump in magnetization and resistivity. Based on the refined structure,
possible spin and orbital models for the magnetic order are proposed. | 0706.0605v2 |
2007-08-14 | Origin of the Non-Linear Pressure Effects in Perovskite Manganites: Buckling of Mn-O-Mn Bonds and Jahn-Teller Distortion of the MnO6 Octahedra Induced by Pressure | High-pressure resistivity and x-ray diffraction have been measured on
La0.85MnO3-d. At low pressures the metal-insulator transition temperature (TMI)
increases linearly with pressure up to a critical pressure, P* ~ 3.4 GPa, which
is followed by reduction of TMI with increasing pressure. Analysis of the bond
distances and bond angles reveal that a bandwidth increase drives the in-crease
of TMI for pressure below P*. The reduction of TMI at higher pressures is found
to result from Jahn-Teller distortions of the MnO6 octahedra. The role of
anharmonic interatomic potential is discussed. | 0708.1963v2 |
2007-08-15 | Insulator to semiconductor transition and magnetic properties of the one-dimensional S = 1/2 system In_2VO_5 | We report structural, magnetization, electrical resistivity and nuclear- and
electron spin resonance data of the complex transition metal oxide In_2VO_5 in
which structurally well-defined V-O chains are realized. An itinerant character
of the vanadium d-electrons and ferromagnetic correlations, revealed at high
temperatures, are contrasted with the insulating behavior and predominantly
antiferromagnetic exchange between the localized V^{4+} S = 1/2-magnetic
moments which develop below a certain characteristic temperature T* ~ 120 K.
Eventually the compound exhibits short-range magnetic order at $T_SRO ~ 20 K.
We attribute this crossover occurring around T* to the unusual anisotropic
thermal contraction of the lattice which changes significantly the overlap
integrals and the character of magnetic intra- and interchain interactions. | 0708.2088v1 |
2007-11-15 | Recrystallization of glass: homogeneous vs. heterogeneous nucleation in La(0.5)Ca(0.5)MnO3 | We probe through magnetization and resistivity measurements a kinetically
arrested glass-like but long-range ordered magnetic state. The transformation
kinetics of the magnetic field-temperature induced broad first-order transition
from ferromagnetic-metallic (FMM) to antiferromagnetic-insulating (AFI) state
gets hindered at low temperature in a La(0.5)Ca(0.5)MnO3 sample. A fraction of
high-temperature FMM phase persists to the lowest temperature, albeit as a
non-ergodic state. We present a phenomenology for this glass-like but
long-range order FMM phase which devitrifies on heating and converts to
equilibrium AFI phase. The residual kinetically arrested FMM phase can be
`recrystallized' to AFI state by annealing and more efficiently by successive
annealing, presumably by heterogeneous nucleation. This glass-like state shows
a stimulating feature that when the fraction of glass is larger the
`recrystallization' is easier. | 0711.2347v1 |
2007-11-28 | Radiation Damage in Polarized Ammonia Solids | Solid NH3 and ND3 provide a highly polarizable, radiation resistant source of
polarized protons and deuterons and have been used extensively in high
luminosity experiments investigating the spin structure of the nucleon. Over
the past twenty years, the UVA polarized target group has been instrumental in
producing and polarizing much of the material used in these studies, and many
practical considerations have been learned in this time. In this discussion, we
analyze the polarization performance of the solid ammonia targets used during
the recent JLab Eg4 run. Topics include the rate of polarization decay with
accumulated charge, the annealing procedure for radiation damaged targets to
recover polarization, and the radiation induced change in optimum microwave
frequency used to polarize the sample. We also discuss the success we have had
in implementing frequency modulation of the polarizing microwave frequency. | 0711.4413v1 |
2008-03-06 | Field emission from single multi-wall carbon nanotubes | Electron field emission characteristics of individual multiwalled carbon
nanotubes have been investigated by a piezoelectric nanomanipulation system
operating inside a scanning electron microscopy chamber. The experimental setup
ensures a high control capability on the geometric parameters of the field
emission system (CNT length, diameter and anode-cathode distance). For several
multiwalled carbon nanotubes, reproducible and quite stable emission current
behaviour has been obtained with a dependence on the applied voltage well
described by a series resistance modified Fowler-Nordheim model. A turn-on
field of about 30 V/um and a field enhancement factor of around 100 at a
cathode-anode distance of the order of 1 um have been evaluated. Finally, the
effect of selective electron beam irradiation on the nanotube field emission
capabilities has been extensively investigated. | 0803.0810v1 |
2008-04-10 | Electronic properties and phase transitions in low-dimensional semiconductors | We present the first review of the current state of the literature on
electronic properties and phase transitions in TlX and TlMX2 (M = Ga, In; X =
Se, S, Te) compounds. These chalcogenides belong to a family of the
low-dimensional semiconductors possessing chain or layered structure. They are
of significant interest because of their highly anisotropic properties, semi-
and photoconductivity, non-linear effects in their I-V characteristics
(including a region of negative differential resistance), switching and memory
effects, second harmonic optical generation, relaxor behavior and potential
applications for optoelectronic devices. We review the crystal structure of TlX
and TlMX2 compounds, their transport properties under ambient conditions,
experimental and theoretical studies of the electronic structure, transport
properties and semiconductor-metal phase transitions under high pressure, and
sequences of temperature-induced structural phase transitions with intermediate
incommensurate states. Electronic nature of the ferroelectric phase transitions
in the above-mentioned compounds, as well as relaxor behavior, nanodomains and
possible occurrence of quantum dots in doped and irradiated crystals is
discussed. | 0804.1639v2 |
2008-04-23 | Superconductivity at 53.5 K in GdFeAsO1-delta | Here we report the fabrication and superconductivity of the iron-based
arsenic-oxide GdFeAsO1-delta compound with oxygen-deficiency, which has an
onset resistivity transition temperature at 53.5 K. This material has a same
crystal structure as the newly discovered high-Tc ReFeAsO1-delta family (Re =
rare earth metal) and a further reduced crystal lattice, while the Tc starts to
decrease compared with the SmFeAsO1-delta system. | 0804.3727v3 |
2008-05-28 | Effect of pressure on the superconducting critical temperature of La[O_{0.89}F_{0.11}]FeAs and Ce[O_{0.88}F_{0.12}]FeAs | We have performed several high-pressure resistivity experiments on the
recently discovered superconductors La[O_{0.89}F_{0.11}]FeAs and
Ce[O_{0.88}F_{0.12}]FeAs. At ambient pressure, these materials have
superconducting onset temperatures T_c of 28 K and 44 K, respectively. While
the T_c of La[O_{0.89}F_{0.11}]FeAs goes through a maximum between 10-68 kbar,
in qualitative agreement with a recent report by Takahashi et al., the T_c of
Ce[O_{0.88}F_{0.12}]FeAs decreases monotonically over the measured pressure
range. At 265 kbar, the T_c of the cerium-based compound has been suppressed
below 1.1 K. | 0805.4372v3 |
2008-06-18 | UV-photon and electrically driven resistance switching in ZnO nanotube arrays | Vertically aligned ZnO nanotube arrays fabricated on an ITO substrate are
found to exhibit strong persistent photoconductivity (PPC) effect and
electrically driven conductance switching behavior, though the latter shows a
gradual decay from high conductance state to a low conductance state. Unlike
the electrical switching, the PPC cannot be reset or reversed by an electrical
pulse. Excitation wavelength dependent conductance measurement indicates the
presence of the defect localized states (DLS) ~ 240meV above the valence band
edge, in support of the hypothesis that the doubly ionization of these DLS are
responsible for the PPC effect. | 0806.2907v2 |
2008-06-20 | Ferromagnetic spin fluctuation in LaFeAsO1-xFx | The F doped LaFeAsO, a recently discovered superconductor with the high Tc of
26 K, has been studied by the resistivity, magnetic susceptibility, and heat
capacity measurements in the F doping range from 0 to 0.14 (x in LaFeAsO1-xFx).
In the low temperature region, a T3lnT term in the heat capacity and a T2 term
in the magnetic susceptibility, which are derived from the spin fluctuation,
are observed. The nearly ferromagnetic nature evidenced by a large Wilson ratio
(6.5 for x = 0, and 11.2 for x = 0.025) suggests that the superconductivity in
the LaFeAsO system is mediated by ferromagnetic spin fluctuation. | 0806.3304v1 |
2008-06-27 | Magnetoresistance oscillations and relaxation effects at the SrTiO3-LaAlO3 interface | We present low-temperature and high-field magnetotransport data on
SrTiO3-LaAlO3 interfaces. The resistance shows hysteresis in magnetic field and
a logarithmic relaxation as a function of time. Oscillations in the
magnetoresistance are observed, showing a square root periodicity in the
applied magnetic field, both in large-area unstructured samples as well as in a
structured sample. An explanation in terms of a commensurability condition of
edge states in a highly mobile two-dimensional electron gas between substrate
step edges is suggested. | 0806.4450v1 |
2008-07-24 | Fishtail effect and the vortex phase diagram of single crystal Ba0.6K0.4Fe2As2 | By measuring the magnetization hysteresis loops of superconducting
Ba0.6K0.4Fe2As2 single crystals, we obtained the high upper critical field and
large current carrying ability, which point to optimistic applications. The
fishtail (or second peak) effect is also found in the material, and the
position of the vortex pinning force shows a maximum at 1/3 of the reduced
field, being consistent with the picture of vortex pinning by small size normal
cores in the sample. Together with the resistive measurements, for the first
time the vortex phase diagram is obtained for superconductor Ba0.6K0.4Fe2As2. | 0807.3786v3 |
2008-07-25 | Adaptation of the Bridgman anvil cell to liquid pressure mediums | The advantage of Bridgman anvil pressure cells is their wide pressure range
and the large number of wires which can be introduced into the pressure
chamber. In these pressure cells soft solid pressure mediums like steatite are
used. We have succeeded in adapting the Bridgman cell to liquid pressure
mediums. With this breakthrough it is now possible to measure in very good
hydrostatic pressure conditions up to 7 GPa, which is about twice the pressure
attainable in piston-cylinder cells. The pressure gradient in the cell,
estimated from the superconducting transition width of lead, is reduced by a
factor of five in the liquid medium with respect to steatite. By the use of
non-magnetic materials for the anvils and the clamp and due to the small
dimensions of the latter, our device is specially suitable for
magneto-transport measurements in dilution fridges. This pressure cell has been
developed to measure very fragile and brittle samples like organic conductors.
Resistivity measurements of (TMTTF)2BF4 performed in a solid and liquid
pressure medium demonstrate the necessity of hydrostatic pressure conditions
for the study of organic conductors at high pressures. | 0807.4137v1 |
2008-08-15 | Localization and the Anomalous Hall Effect in a "Dirty" Metallic Ferromagnet | We report magnetoresistance measurements over an extensive temperature range
(0.1 K $\leq T \leq$ 100 K) in a disordered ferromagnetic semiconductor (\gma).
The study focuses on a series of metallic \gma~ epilayers that lie in the
vicinity of the metal-insulator transition ($k_F l_e\sim 1$). At low
temperatures ($T < 4$ K), we first confirm the results of earlier studies that
the longitudinal conductivity shows a $T^{1/3}$ dependence, consistent with
quantum corrections from carrier localization in a ``dirty'' metal. In
addition, we find that the anomalous Hall conductivity exhibits universal
behavior in this temperature range, with no pronounced quantum corrections. We
argue that observed scaling relationship between the low temperature
longitudinal and transverse resistivity, taken in conjunction with the absence
of quantum corrections to the anomalous Hall conductivity, is consistent with
the side-jump mechanism for the anomalous Hall effect. In contrast, at high
temperatures ($T \gtrsim 4$ K), neither the longitudinal nor the anomalous Hall
conductivity exhibit universal behavior, indicating the dominance of inelastic
scattering contributions down to liquid helium temperatures. | 0808.2079v2 |
2008-08-22 | Phase Coherence Effects in the Vortex Transport Entropy | Nernst and electrical resistivity measurements in superconducting YBCO and
BSCCO with and without columnar defects show a distinctive thermodynamics of
the respective liquid vortex matter. At a field dependent high temperature
region in the phase diagram the Nernst signal is independent of structural
defects in both materials. At lower temperatures, in YBCO, defects contribute
only to the vortex mobility and the transport entropy is that of a system of
vortex lines. The transition to lower temperatures in BSCCO has a different
origin, the maximum in the Nernst signal when decreasing temperature is not
associated with transport properties but with the entropy behavior of pancake
vortices in the presence of structural defects. | 0808.3015v1 |
2009-02-05 | Itinerant antiferromagnetism in BaCr$_2$As$_2$ | We report single crystal synthesis, specific heat and resistivity
measurements and electronic structure calculations for BaCr$_2$As$_2$. This
material is a metal with itinerant antiferromagnetism, similar to the parent
phases of Fe-based high temperature superconductors, but differs in magnetic
order. Comparison of bare band structure density of states and the low
temperature specific heat implies a mass renormalization of $\sim$ 2.
BaCr$_2$As$_2$ shows stronger transition metal - pnictogen covalency than the
Fe compounds, and in this respect is more similar to BaMn$_2$As$_2$. This
provides an explanation for the observation that Ni and Co doping is effective
in the Fe-based superconductors, but Cr or Mn doping is not. | 0902.0945v2 |
2009-04-05 | Superconductivity in Ir-doped LaFe1-xIrxAsO | We report the realization of superconductivity by 5d element Ir doping in
LaFeAsO, a prototype parent compound of high-temperature iron based
superconductors. X-ray diffraction patterns indicate that the material has
formed the ZrCuSiAs-type structure with a space group P4/nmm. The systematic
evolution of the lattice constants demonstrated that the Fe ions were
successfully replaced by the Ir. Both electrical resistance and magnetization
measurements show superconductivity up to 11 K in LaFe1-xIrxAsO. The
superconducting transitions at different magnetic fields were also measured
yielding a slope of -dHc2/dT = 6.7 T / K near Tc, and the upper critical field
at zero K is found to be about 55 T. This is the first example of bulk
superconductivity induced by replacing the Fe sites with higher d-orbital
electrons in FeAs-1111 family, which should add more ingredients to the
underlying physics of the iron-based superconductors. | 0904.0772v4 |
2009-04-06 | Gap opening in the zeroth Landau level of graphene | We have measured a strong increase of the low-temperature resistivity
$\rho_{xx}$ and a zero-value plateau in the Hall conductivity $\sigma_{xy}$ at
the charge neutrality point in graphene subjected to high magnetic fields up to
30 T. We explain our results by a simple model involving a field dependent
splitting of the lowest Landau level of the order of a few Kelvin, as extracted
from activated transport measurements. The model reproduces both the increase
in $\rho_{xx}$ and the anomalous $\nu=0$ plateau in $\sigma_{xy}$ in terms of
coexisting electrons and holes in the same spin-split zero-energy Landau level. | 0904.0948v1 |
2009-07-09 | Agglomeration and filtration of colloidal suspensions with DVLO interactions in simulation and experiment | Cake filtration is a widely used solid-liquid separation process. However,
the high flow resistance of the nanoporous filter cake lowers the efficiency of
the process significantly. The structure and thus the permeability of the
filter cakes depend on the compressive load acting on the particles, the
particles size, and the agglomeration of the particles. The latter is
determined by the particle charge and the ionic strength of the suspension, as
described by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. In this paper,
we propose a combined stochastic rotation dynamics (SRD) and molecular dynamics
(MD) methodology to simulate the cake formation. The simulations give further
insight into the dependency of the filter cakes' structure on the agglomeration
of the particles, which cannot be accessed experimentally. The permeability, as
investigated with lattice Boltzmann (LB) simulations of flow through the
discretized cake, depends on the particle size and porosity, and thus on the
agglomeration of the particles. Our results agree qualitatively with
experimental data obtained from colloidal boehmite suspensions. | 0907.1551v2 |
2009-07-12 | Vortex-like state observed in ferromagnetic contacts | Point-contacts (PC) offer a simple way to create high current densities, 10^9
A/cm^2 and beyond, without substantial Joule heating. We have shown recently
(Nano Letters, 7 (2007) 927) that conductivity of nanosized PCs between a
normal and ferromagnetic metals exhibits bi-stable hysteretic states versus
both bias current and external magnetic field - the effect typical for
spin-valve structures. Here we report that apart from the bi-stable state a
third intermediate-resistance state is occasionally observed. We interpret this
state as due to a spin-vortex in the PC, nucleated either by Oersted field of
the bias current and/or by the circular geometry of PC. The observed
three-level-states in the PC conductivity testify that the interface spins are
both weakly coupled to the spins in the bulk and have depressed exchange
interaction within the surface layer. | 0907.2027v2 |
2009-07-16 | Dynamical correlations in electronic transport through a system of coupled quantum dots | Current auto- and cross-correlations are studied in a system of two
capacitively coupled quantum dots. We are interested in a role of Coulomb
interaction in dynamical correlations, which occur outside the Coulomb blockade
region (for high bias). After decomposition of the current correlation
functions into contributions between individual tunneling events, we can show
which of them are relevant and lead to sub-/supper-Poissonian shot noise and
negative/positive cross-correlations. The results are differentiated for a weak
and strong inter-dot coupling. Interesting results are for the strong coupling
case when electron transfer in one of the channel is strongly correlated with
charge drag in the second channel. We show that cross-correlations are
non-monotonic functions of bias voltage and they are in general negative
(except some cases with asymmetric tunnel resistances). This is effect of local
potential fluctuations correlated by Coulomb interaction, which mimics the
Pauli exclusion principle. | 0907.2879v1 |
2009-07-30 | Solvothermal Reduction of Chemically Exfoliated Graphene Sheets | Graphene has attracted much attention due to its interesting properties and
potential applications. Chemical exfoliation methods have been developed to
make graphene recently, aimed at large-scale assembly and applications such as
composites and Li ion batteries. Although efficient, the chemical exfoliation
methods involve oxidation of graphene and introduce defects in the as-made
sheets. Hydrazine reduction at 100 has shown to partially restore the structure
and conductance of graphite oxide. However, the reduced GO still shows strong
defect peaks in Raman spectra with higher resistivity than pristine graphene by
2 to 3 orders of magnitude. It is important to produce much less defective
graphene sheets than GO, and develop more effective graphene reduction.
Recently, we reported a mild exfoliation-reintercalation-expansion method to
form high-quality GS with higher conductivity and lower oxidation degree than
GO.5 Here, we present a 180 solvothermal reduction method for our GS and GO.
The solvothermal reduction is more effective than the earlier reduction methods
in lowering the oxygen and defect levels in GS, increasing the graphene
domains, and bringing the conductivity of GS close to pristine graphene. The
reduced GS possess the highest degree of pristinity among chemically derived
graphene. | 0907.5417v1 |
2009-09-23 | Intermediate-valence behavior of the transition-metal oxide CaCu$_3$Ru$_4$O$_{12}$ | The transition--metal oxide CaCu$_3$Ru$_4$O$_{12}$ with perovskite--type
structure shows characteristic properties of an intermediate--valence system.
The temperature--dependent susceptibility exhibits a broad maximum around $150
- 160$ K. At this temperature, neutron powder diffraction reveals a small but
significant volume change whereby the crystal structure is preserved. Moreover,
the temperature--dependent resistivity changes its slope. NMR Knight shift
measurements of Ru reveal a cross--over from high temperature paramagnetic
behavior of localized moments to itinerant band states at low temperatures.
Additional density--functional theory calculations can relate the structural
anomaly with the $d$--electron number. The different experimental and
calculational methods result in a mutually consistent description of
CaCu$_3$Ru$_4$O$_{12}$ as an intermediate--valent system in the classical sense
of having low--energy charge fluctuations. | 0909.4208v1 |
2009-10-09 | Observation of a d-wave nodal liquid in highly underdoped Bi_2Sr_2CaCu_2O_{8+δ} | We use angle resolved photoemission spectroscopy to probe the electronic
excitations of the non-superconducting state that exists between the
antiferromagnetic Mott insulator at zero doping and the superconducting state
at larger dopings in Bi_2Sr_2CaCu_2O_{8+\delta}. We find that this state is a
nodal liquid whose excitation gap becomes zero only at points in momentum
space. Despite exhibiting a resistivity characteristic of an insulator and the
absence of coherent quasiparticle peaks, this material has the same gap
structure as the d-wave superconductor. We observe a smooth evolution of the
spectrum across the insulator-to-superconductor transition, which suggests that
high temperature superconductivity emerges when quantum phase coherence is
established in a non-superconducting nodal liquid. | 0910.1648v1 |
2009-10-26 | Wafer-scale synthesis and transfer of graphene films | We developed means to produce wafer scale, high-quality graphene films as
large as 3 inch wafer size on Ni and Cu films under ambient-pressure and
transfer them onto arbitrary substrates through instantaneous etching of metal
layers. We also demonstrated the applications of the large-area graphene films
for the batch fabrication of field-effect transistor (FET) arrays and
stretchable strain gauges showing extraordinary performances. Transistors
showed the hole and electron mobilities of the device of 1,100 cm2/Vs and 550
cm2/Vs at drain bias of -0.75V, respectively. The piezo-resistance gauge factor
of strain sensor was ~6.1. These methods represent a significant step toward
the realization of graphene devices in wafer scale as well as application in
optoelectronics, flexible and stretchable electronics. | 0910.4783v1 |
2009-11-25 | RETGEM with polyvinylchloride (PVC) electrodes | This paper presents a new design of the RETGEM (Resistive Electrode Thick
GEM) based on electrodes made of a polyvinylchloride material (PVC). Our device
can operate with gains of 10E5 as a conventional TGEM at low counting rates and
as RPC in the case of high counting rates without of the transit to the violent
sparks. The distinct feature of present RETGEM is the absent of the metal
coating and lithographic technology for manufacturing of the protective
dielectric rms. The electrodes from PVC permit to do the holes by a simple
drilling machine. Detectors on a RETGEM basis could be useful in many fields of
an application requiring a more cheap manufacturing and safe operation, for
example, in a large neutrino experiments, in TPC, RICH systems. | 0911.4807v1 |
2009-12-03 | Atomic-scale patterning of hydrogen terminated Ge(001) by scanning tunneling microscopy | In this paper we demonstrate atomic-scale lithography on hydrogen terminated
Ge(001. The lithographic patterns were obtained by selectively desorbing
hydrogen atoms from a H resist layer adsorbed on a clean, atomically flat
Ge(001) surface with a scanning tunneling microscope tip operating in
ultra-high vacuum. The influence of the tip-to-sample bias on the lithographic
process have been investigated. Lithographic patterns with feature-sizes from
200 nm to 1.8 nm have been achieved by varying the tip-to-sample bias. These
results open up the possibility of a scanning-probe lithography approach to the
fabrication of future atomic-scale devices in germanium. | 0912.0754v1 |
2009-12-10 | Highly Uniform 300 mm Wafer-Scale Deposition of Single and Multilayered Chemically Derived Graphene Thin Films | The deposition of atomically thin highly uniform chemically derived graphene
(CDG) films on 300 mm SiO2/Si wafers is reported. We demonstrate that the very
thin films can be lifted off to form uniform membranes than can be
free-standing or transferred onto any substrate. Detailed maps of thickness
using Raman spectroscopy and atomic force microscopy (AFM) height profiles
reveal that the film thickness is very uniform and highly controllable, ranging
from 1-2 layers up to 30 layers. After reduction using a variety of methods,
the CDG films are transparent and electrically active with FET devices yielding
exceptionally high mobilities of ~ 15 cm2/Vs and sheet resistance of ~ 1 k
Ohm/sq at ~ 70 % transparency. | 0912.2087v1 |
2009-12-21 | Microstructure and superconductivity of Ir-doped BaFe2As2 superconductor | Polycrystalline samples with nominal composition of Ba(Fe1-xIrx)2As2 (x=0.10,
0.15, and 0.20) were investigated by means of X-ray diffraction (XRD), scanning
electron microscopy (SEM), electrical resistivity, and magnetization
measurements. XRD and SEM results showed that almost single phase samples were
obtained. Bulk superconductivity with TC~28 K was observed in the x=0.10
sample. TC~28 K is the highest superconducting critical temperature among the
reported data for electron-doped AFe2As2-type (A=Ca, Sr, and Ba)
superconductors. The upper critical field Hc2(0) reaches as high as 65 T for
the x=0.10 sample. The underlying physics is discussed in connection with
Co-doping case. | 0912.4113v1 |
2010-01-21 | Photon assisted tunneling as an origin of the Dynes density of states | We show that the effect of a high-temperature environment in current
transport through a normal metal-insulator-superconductor tunnel junction can
be described by an effective density of states (DOS) in the superconductor. In
the limit of a resistive low-ohmic environment, this DOS reduces into the
well-known Dynes form. Our theoretical result is supported by experiments in
engineered environments. We apply our findings to improve the performance of a
single-electron turnstile, a potential candidate for a metrological current
source. | 1001.3853v3 |
2010-02-07 | Time-Dependent Transport Through Molecular Junctions | We investigate transport properties of molecular junctions under two types of
bias--a short time pulse or an AC bias--by combining a solution for the Green
functions in the time domain with electronic structure information coming from
ab initio density functional calculations. We find that the short time response
depends on lead structure, bias voltage, and barrier heights both at the
molecule-lead contacts and within molecules. Under a low frequency AC bias, the
electron flow either tracks or leads the bias signal (capacitive or resistive
response) depending on whether the junction is perfectly conducting or not. For
high frequency, the current lags the bias signal due to the kinetic inductance.
The transition frequency is an intrinsic property of the junctions. | 1002.1441v1 |
2010-03-16 | All Magnesium diboride Josephson Junctions with MgO and native oxide barriers | We present results on all-MgB2 tunnel junctions, where the tunnel barrier is
deposited MgO or native-oxide of base electrode. For the junctions with MgO,
the hysteretic I-V curve resembles a conventional underdamped Josephson
junction characteristic with critical current-resistance product nearly
independent of the junction area. The dependence of the critical current with
temperature up to 20 K agrees with the [Ambegaokar and Baratoff, Phys. Rev.
Lett. 10, 486 (1963)] expression. For the junctions with native-oxide,
conductance at low bias exhibits subgap features while at high bias reveals
thick barriers. As a result no supercurrent was observed in the latter, despite
the presence of superconducting-gaps to over 30 K. | 1003.3158v1 |
2010-03-31 | Magneto-transport Effects in Topological Insulator Bi$_2$Se$_3$ Nanoribbons | Magneto-resistance (MR) of Bi$_2$Se$_3$ nanoribbons is studied over a broad
range of temperature ($T$=300K-2K) and under various magnetic field ($B$)
orientations. The MR is strongly anisotropic with the perpendicular MR much
larger than the longitudinal and transverse MRs. The perpendicular MR exhibits
quadratic $B$-dependence in low fields and becomes linear at high $B$. However,
when $T$ increases, the perpendicular MR becomes linear over the whole magnetic
field range (0-9T) up to room temperature. This unusual linear MR is discussed
in the context of the linear quantum MR of the topological surface-states. We
also observe the boundary-scattering effect in MR at low temperatures, which
indicates that the out-of-plane Fermi momentum is much smaller the in-plane
Fermi momentum, excluding the simple three-dimensional Fermi surface picture. | 1003.6099v4 |
2010-04-05 | Hysteresis in the magneto-transport of Manganese-doped Germanium: evidence for carrier-mediated ferromagnetism | We report the fabrication of Ge:Mn ferromagnetic semiconductors by Mn-ion
implantation into Ge followed by pulsed laser annealing. Benefiting from the
short time annealing, the hole concentration in Mn-implanted Ge has been
increased by two orders of magnitude from 10$^{18}$ to over 10$^{20}$
cm$^{-3}$. Likely due to the high hole concentration, we observe that the
longitudinal and Hall resistances exhibit the same hysteresis as the
magnetization, which is usually considered as a sign of carrier-mediated
ferromagnetism. | 1004.0568v1 |
2010-04-21 | Effect of pressure on the magnetic, transport, and thermal-transport properties of the electron-doped manganite CaMn$_{1-x}$Sb$_{x}$O$_{3}$ | We have demonstrated the effect of hydrostatic pressure on magnetic and
transport properties, and thermal transport properties in electron-doped
manganites CaMn$_{1-x}$Sb$_{x}$O$_{3}$. The substitution of Sb$^{5+}$ ion for
Mn $^{4+}$site of the parent matrix causes one-electron doping with the
chemical formula CaMn$^{4+}_{1-2x}$Mn$^{3+}_{x}$Sb$^{5+}_{x}$O$_{3}$
accompanied by a monotonous increase in unit cell volume as a function of $x$.
Upon increasing the doping level of Sb, the magnitudes of both electrical
resistivity and negative Seebeck coefficient are suppressed at high
temperatures, indicating the electron doping. Anomalous diamagnetic behaviors
at $x=0.05$ and 0.08 are clearly observed in field cooled dc magnetization. The
effect of hydrostatic pressure on dc magnetization is in contrast to the
chemical pressure effect due to Sb doping. The dynamical effect of ac magnetic
susceptibility measurement points to the formation of the magnetically
frustrated clusters such as FM clusters embedded in canted AFM matrix. | 1004.3669v3 |
2010-05-06 | Theoretical and Experimental Studies of Schottky Diodes That Use Aligned Arrays of Single Walled Carbon Nanotubes | We present theoretical and experimental studies of Schottky diodes that use
aligned arrays of single walled carbon nanotubes. A simple physical model,
taking into account the basic physics of current rectification, can adequately
describe the single-tube and array devices. We show that for as grown array
diodes, the rectification ratio, defined by the
maximum-to-minimum-current-ratio, is low due to the presence of m-SWNT shunts.
These tubes can be eliminated in a single voltage sweep resulting in a high
rectification array device. Further analysis also shows that the channel
resistance, and not the intrinsic nanotube diode properties, limits the
rectification in devices with channel length up to ten micrometer. | 1005.0870v1 |
2010-06-16 | Perfect Domain-Lattice Matching Between MgB2 and Al2O3: Single-Crystal MgB2 Thin Films Grown on Sapphire | We have found that single-crystal films can be grown on (0001) Al2O3
substrates through the golden relation of a perfect lattice-matching ratio
between the a-axis lattice constants of MgB2 and Al2O3. Selected area electron
diffraction patterns evidently indicate hexagonal MgB2 film with a 30 degrees
in-plane rotation with respect to the Al2O3 substrate. The films grown on Al2O3
show a zero-resistance transition temperature of 39.6 K with a transition width
of 0.1 K. The critical current density at zero field is comparable to the
depairing critical current density and rapidly decreases with increasing
applied field due to the lack of pinning sites, as observed for high-quality
MgB2 single crystals. | 1006.3169v1 |
2010-07-01 | Chip-scale nanofabrication of single spins and spin arrays in diamond | We demonstrate a technique to nanofabricate nitrogen vacancy (NV) centers in
diamond based on broad-beam nitrogen implantation through apertures in electron
beam lithography resist. This method enables high-throughput nanofabrication of
single NV centers on sub-100 nm length scales. Secondary ion mass spectroscopy
(SIMS) measurements facilitate depth profiling of the implanted nitrogen to
provide three-dimensional characterization of the NV center spatial
distribution. Measurements of NV center coherence with on-chip coplanar
waveguides suggest a pathway for incorporating this scalable nanofabrication
technique in future quantum applications. | 1007.0240v2 |
2010-08-04 | Transport in gapped bilayer graphene: the role of potential fluctuations | We employ a dual-gated geometry to control the band gap \Delta in bilayer
graphene and study the temperature dependence of the resistance at the charge
neutrality point, RNP(T), from 220 to 1.5 K. Above 5 K, RNP(T) is dominated by
two thermally activated processes in different temperature regimes and exhibits
exp(T3/T)^{1/3} below 5 K. We develop a simple model to account for the
experimental observations, which highlights the crucial role of localized
states produced by potential fluctuations. The high temperature conduction is
attributed to thermal activation to the mobility edge. The activation energy
approaches \Delta /2 at large band gap. At intermediate and low temperatures,
the dominant conduction mechanisms are nearest neighbor hopping and
variable-range hopping through localized states. Our systematic study provides
a coherent understanding of transport in gapped bilayer graphene. | 1008.0783v1 |
2010-09-13 | Cooper Pair Writing at the LaAlO3/SrTiO3 Interface | The LaAlO3/SrTiO3 interface provides a unique platform for controlling the
electronic properties of the superconducting semiconductor SrTiO3. Prior
investigations have shown that two-dimensional superconductivity can be
produced at the LaAlO3/SrTiO3 interface and tuned electrostatically. The
recently demonstrated reversible control of the metal-insulator transition at
the same interface using conductive atomic force microscopy (c-AFM) raises the
question of whether this room-temperature technique can produce structures that
exhibit superconducting, normal metallic and insulating phases at sub-Kelvin
temperatures. Here we report low-temperature magnetotransport experiments on
conducting structures defined at an otherwise insulating LaAlO3/SrTiO3
interface. A quantum phase transition associated with the formation of Cooper
pairs is observed in these predefined structures at sub-Kelvin temperatures.
However, a finite resistance remains even at the lowest temperature. At higher
magnetic fields, interfaces with high mobility also exhibit strong Shubnikov-de
Haas oscillations as well as a larger Ginsburg-Landau coherence length. Cooper
pair localization, spin-orbit coupling, and finite-size effects may factor into
an explanation for some of the unusual properties observed. | 1009.2424v2 |
2010-09-16 | Overlapping-gate architecture for silicon Hall bar MOSFET devices in the low electron density regime | We report the fabrication and study of Hall bar MOSFET devices in which an
overlapping-gate architecture allows four-terminal measurements of low-density
2D electron systems, while maintaining a high density at the ohmic contacts.
Comparison with devices made using a standard single gate show that
measurements can be performed at much lower densities and higher channel
resistances, despite a reduced peak mobility. We also observe a voltage
threshold shift which we attribute to negative oxide charge, injected during
electron-beam lithography processing. | 1009.3109v1 |
2010-10-01 | Contactless measurement of electrical conductance of a thin film of amorphous germanium | We present a contactless method for measuring charge in a thin film of
amorphous germanium (a-Ge) with a nanoscale silicon MOSFET charge sensor. This
method enables the measurement of conductance of the a-Ge film even in the
presence of blocking contacts. At high bias voltage, the resistance of the
contacts becomes negligible and a direct measurement of current gives a
conductance that agrees with that from the measurement of charge. This
charge-sensing technique is used to measure the temperature- and
field-dependence of the conductance, and they both agree with a model of Mott
variable-range hopping. From the model, we obtain a density of states at the
Fermi energy of 1.6 x 10^18 eV^-1 cm^-3 and a localization length of 1.06 nm.
This technique enables the measurement of conductance as low as 10^-19 S. | 1010.0045v1 |
2010-10-05 | Optical property modification of ZnO: Effect of 1.2 MeV Ar irradiation | We report a systematic study on 1.2 MeV Ar^8+ irradiated ZnO by x-ray
diffraction (XRD), room temperature photoluminescence (PL) and
ultraviolet-visible (UV-Vis) absorption measurements. ZnO retains its wurtzite
crystal structure up to maximum fluence of 5 x 10^16 ions/cm^2. Even, the width
of the XRD peaks changes little with irradiation. The UV-Vis absorption spectra
of the samples, unirradiated and irradiated with lowest fluence (1 x 10^15
ions/cm^2), are nearly same. However, the PL emission is largely quenched for
this irradiated sample. Red shift of the absorption edge has been noticed for
higher fluence. It has been found that red shift is due to at least two defect
centers. The PL emission is recovered for 5 x 10^15 ions/cm^2 fluence. The
sample colour is changed to orange and then to dark brown with increasing
irradiation fluence. Huge resistivity decrease is observed for the sample
irradiated with 5 x 10^15 ions/cm^2 fluence. Results altogether indicate the
evolution of stable oxygen vacancies and zinc interstitials as dominant defects
for high fluence irradiation. | 1010.0753v1 |
2010-11-23 | Transport in Graphene Tunnel Junctions | We present a technique to fabricate tunnel junctions between graphene and Al
and Cu, with a Si back gate, as well as a simple theory of tunneling between a
metal and graphene. We map the differential conductance of our junctions versus
probe and back gate voltage, and observe fluctuations in the conductance that
are directly related to the graphene density of states. The conventional
strong-suppression of the conductance at the graphene Dirac point can not be
clearly demonstrated, but a more robust signature of the Dirac point is found:
the inflection in the conductance map caused by the electrostatic gating of
graphene by the tunnel probe. We present numerical simulations of our
conductance maps, confirming the measurement results. In addition, Al causes
strong n-doping of graphene, Cu causes a moderate p-doping, and in high
resistance junctions, phonon resonances are observed, as in STM studies. | 1011.5067v1 |
2010-12-14 | Reentrant spin-glass behavior in $TlFe_{2-x}Se_2$ with the $ThCr_2Si_2$-type structure | We investigated the physical properties of $TlFe_{2-x}Se_2$ single crystals.
The resistivity of $TlFe_{2-x}Se_2$ shows typical semiconductor behavior with
an activation energy of 25 meV. DC susceptibility indicates an
antiferromagnetic transition at about 450 K. Reentrant spin-glass (RSG)
behavior was found at about 130 K through DC and AC magnetic measurements. The
RSG behavior suggests the existence of a strong competition between
ferromagnetic (FM) and antiferromagnetic (AFM) interactions due to Fe
deficiencies. Strong electron-electron correlation may exist in this material
and it is possibly a candidate of parent compound for high $T_c$
superconductors. | 1012.2929v1 |
2010-12-23 | Effect of chromium disorder on the thermoelectric properties of Layered-antiferromagnet CuCrS2 | Layered-antiferromagnetic compound CuCrS2 has been prepared by different
methods. The analysis of X-ray diffraction patterns of different samples gave
significant amount of vacancy-disorder of Cr-atoms within the layers. Extended
period of sintering above 9000C increases the transfer of Cr-atoms to the
interstitial sites between the layers. This disorder has marginal effect on the
Antiferromagnetic properties. The electrical conductivity is increased and the
thermoelectric power remains positive and quite high between 150-400\muV/K in
the paramagnetic state around room temperature with increase in disorder in
different samples. We interpret the temperature dependence of electrical
resistivity and thermoelectric power due to the localization of carriers by
interstitial defects and the formation of magnetic polarons in the paramagnetic
phase of CuCrS2. | 1012.5147v1 |
2011-03-04 | Giant enhancement of spin accumulation and long-distance spin precession in metallic lateral spin valves | The nonlocal spin injection in lateral spin valves is highly expected to be
an effective method to generate a pure spin current for potential spintronic
application. However, the spin valve voltage, which decides the magnitude of
the spin current flowing into an additional ferromagnetic wire, is typically of
the order of 1 {\mu}V. Here we show that lateral spin valves with low resistive
NiFe/MgO/Ag junctions enable the efficient spin injection with high applied
current density, which leads to the spin valve voltage increased hundredfold.
Hanle effect measurements demonstrate a long-distance collective 2-pi spin
precession along a 6 {\mu}m long Ag wire. These results suggest a route to
faster and manipulable spin transport for the development of pure spin current
based memory, logic and sensing devices. | 1103.0852v2 |
2011-03-23 | Local Electrical Stress-Induced Doping and Formation of 2D Monolayer Graphene P-N Junction | We demonstrated doping in 2D monolayer graphene via local electrical
stressing. The doping, confirmed by the resistance-voltage transfer
characteristics of the graphene system, is observed to continuously tunable
from N-type to P-type as the electrical stressing level (voltage) increases.
Two major physical mechanisms are proposed to interpret the observed phenomena:
modifications of surface chemistry for N-type doping (at low-level stressing)
and thermally-activated charge transfer from graphene to SiO2 substrate for
P-type doping (at high-level stressing). The formation of P-N junction on 2D
graphene monolayer is demonstrated with complementary doping based on locally
applied electrical stressing. | 1103.4568v3 |
2011-03-25 | Giant Positive Magnetoresistance in Co@CoO Nanoparticle Arrays | We report the magnetotransport properties of self-assembled Co@CoO
nanoparticle arrays at temperatures below 100 K. Resistance shows thermally
activated behavior that can be fitted by the general expression of R
exp{(T/T0)^v}. Efros-Shklovskii variable range hopping (v=1/2) and simple
activation (hard gap, v=1) dominate the high and low temperature region,
respectively, with a strongly temperature-dependent transition regime in
between. A giant positive magnetoresistance of >1,400% is observed at 10K,
which decreases with increasing temperature. The positive MR and most of its
features can be explained by the Zeeman splitting of the localized states that
suppresses the spin dependent hopping paths in the presence of on-site Coulomb
repulsion. | 1103.5029v1 |
2011-05-06 | Optical excitation of Electron-Glasses | Electron-glasses can be readily driven far from equilibrium by a variety of
means. Several mechanisms to excite the system and their relative merits are
reviewed. In this study we focus on the process of exciting electron-glasses by
interaction with near infrared radiation. The efficiency of this protocol
varies considerably among different electron-glasses, but it only weakly
depends on their resistance at liquid helium temperatures. A dramatic
enhancement of the excitation efficiency is observed upon doping crystalline
indium-oxide with Au. Some enhancement is observed also in samples doped with
Pb but this enhancement fades away with time unlike the situation in the
Au-doped samples. Several structural and analytical tools are used to
characterize the changes in the materials that may be responsible for these
effects. Possible routes by which high-frequency electromagnetic fields take
the system far from equilibrium are discussed. | 1105.1350v1 |
2011-05-22 | Evidence for a capacitor network near the metal insulator transition in VO2 thin films probed by in-plane impedance spectroscopy | Impedance spectroscopy measurements were performed in high quality Vanadium
dioxide (VO2) thin films. This technique allows us investigate the resistive
and capacitive contribution to the dielectric response near the metal-insulator
transition (MIT). A non ideal RC behavior was found in our films from room
temperature up to 334 K. A decrease of the total capacitance was found in this
region, possibly due to interface effects. Above the MIT, the system behaves
like a metal as expected, and a modified equivalent circuit is necessary to
describe the impedance data adequately. Around the MIT, an increase of the
total capacitance is observed. | 1105.4308v1 |
2011-05-24 | Rectification at Graphene-Semiconductor Interfaces: Zero-Gap Semiconductor Based Diodes | Using current-voltage (I-V) and capacitance-voltage (C-V) measurements, we
report on the unusual physics and promising technical applications associated
with the formation of Schottky barriers at the interface of a one-atom-thick
zero-gap semiconductor (graphene) and conventional semiconductors. When
chemical vapor deposited graphene is transferred onto n-type Si, GaAs, 4H-SiC
and GaN semiconductor substrates, there is a strong van der Waals attraction
that is accompanied by charge transfer across the interface and the formation
of a rectifying (Schottky) barrier. Thermionic emission theory in conjunction
with the Schottky-Mott model within the context of bond-polarization theory
provides a surprisingly good description of the electrical properties.
Applications, such as to sensors where in forward bias there is exponential
sensitivity to changes in the Schottky barrier height due to the presence of
absorbates on the graphene or to analogue devices for which Schottky barriers
are integral components are promising because of graphene's mechanical
stability, its resistance to diffusion, its robustness at high temperatures and
its demonstrated capability to embrace multiple functionalities. | 1105.4811v2 |
2011-05-27 | Magnonic spin-transfer torque MRAM with low power, high speed, and error-free switching | A new class of spin-transfer torque magnetic random access memory (STT-MRAM)
is discussed, in which writing is achieved using thermally initiated magnonic
current pulses as an alternative to conventional electric current pulses. The
magnonic pulses are used to destabilize the magnetic free layer from its
initial direction, and are followed immediately by a bipolar electric current
exerting conventional spin-transfer torque on the free layer. The combination
of thermal and electric currents greatly reduces switching errors, and
simultaneously reduces the electric switching current density by more than an
order of magnitude as compared to conventional STT-MRAM. The energy efficiency
of several possible electro-thermal circuit designs have been analyzed
numerically. As compared to STT-MRAM with perpendicular magnetic anisotropy,
magnonic STT-MRAM reduces the overall switching energy by almost 80%.
Furthermore, the lower electric current density allows the use of thicker
tunnel barriers, which should result in higher tunneling magneto-resistance and
improved tunnel barrier reliability. The combination of lower power, improved
reliability, higher integration density, and larger read margin make magnonic
STT-MRAM a promising choice for future non-volatile storage. | 1105.5473v1 |
2011-05-27 | Temperature Dependence of the Intrinsic Anomalous Hall Effect in Nickel | We investigate the unusual temperature dependence of the anomalous Hall
effect in Ni. By varying the thickness of the MBE-grown Ni films, the
longitudinal resistivity is uniquely tuned without resorting to doping
impurities; consequently, the intrinsic and extrinsic contributions are cleanly
separated out. In stark contrast to other ferromagnets such as Fe, the
intrinsic contribution in Ni is found to be strongly temperature dependent with
a value of 1100 (ohm*cm)^(-1) at low temperatures and 500 (ohm*cm)^(-1) at high
temperatures. This pronounced temperature dependence, a cause of long-standing
confusion concerning the physical origin of the AHE, is likely due to the small
energy level splitting caused by the spin orbit coupling close to the Fermi
surface. Our result helps pave the way for the general claim of the Berry-phase
interpretation for the AHE. | 1105.5664v1 |
2011-05-31 | Interplay between topological insulators and superconductors | Topological insulators are insulating in the bulk but possess metallic
surface states protected by time-reversal symmetry. Here, we report a detailed
electronic transport study in high quality Bi2Se3 topological insulator thin
films contacted by superconducting (In, Al and W) electrodes. The resistance of
the film shows an abrupt and significant upturn when the electrodes become
superconducting. In turn, the Bi2Se3 film strongly weakens the
superconductivity of the electrodes, significantly reducing both their
transition temperatures and critical fields. A possible interpretation of these
results is that the superconducting electrodes are accessing the surface states
and the experimental results are the consequence of the interplay between the
Cooper pairs of the electrodes and the spin polarized current of the surface
states in Bi2Se3. | 1105.6174v3 |
2011-06-04 | Facile fabrication of lateral nanowire wrap-gate devices with improved performance | We present a simple fabrication technique for lateral nanowire wrap-gate
devices with high capacitive coupling and field-effect mobility. Our process
uses e-beam lithography with a single resist-spinning step, and does not
require chemical etching. We measure, in the temperature range 1.5-250 K, a
subthreshold slope of 5-54 mV/decade and mobility of 2800-2500 $cm^2/Vs$ --
significantly larger than previously reported lateral wrap-gate devices. At
depletion, the barrier height due to the gated region is proportional to
applied wrap-gate voltage. | 1106.0796v1 |
2011-06-04 | Mechanochemical reaction in graphane under uniaxial tension | The quantum-mechanochemical-reaction-coordinate simulations have been
performed to investigate the mechanical properties of hydrogen functionalized
graphene. The simulations disclosed atomically matched peculiarities that
accompany the deformation-failure-rupture process occurred in the body. A
comparative study of the deformation peculiarities related to equi-carbon-core
(5,5) nanographene and nanographane sheets exhibited a high stiffness of both
bodies that is provided by the related hexagon units, namely benzenoid and
cyclohexanoid, respectively. The two units are characterized by anisotropy in
the microscopic behavior under elongation along mechanochemical internal
coordinates when the later are oriented either along (zg) or normally (ach) to
the C-C bonds chain. The unit feature in combination with different
configuration of their packing with respect to the body C-C bond chains forms
the ground for the structure-sensitive mechanical behavior that is different
for zg and ach deformation modes. Hydrogenation of graphene drastically
influences behavior and numerical characteristics of the body making
tricotage-like pattern of the graphene failure less pronounced and inverting it
from the zg to ach mode as well as providing less mechanical resistance of
graphane it total. | 1106.0837v2 |
2011-07-09 | Huge Volume Expansion and Structural Transformation of Carbon Nanotube Aligned Arrays during Electrical Breakdown in Vacuum | We observed a huge volume expansion of aligned single walled carbon nanotube
(SWNT) arrays accompanied by structural transformation during electrical
breakdown in vacuum. The SWNT arrays were assembled between prefabricated Pd
source and drain electrodes of 2 \mu m separation on Si/SiO_2 substrate via
dielectrophoresis. At high electrical field, the SWNT arrays erupt into large
mushroom-like structure. Systematic studies with controlled electrical bias
show that above a certain field SWNTs swell and transform to nanoparticles and
flower-like structures with small volume increase. Further increase in
electrical bias and repeated sweeping results into amorphous carbon as
determined from scanning and transmission electron microscopy (TEM). Cross
sectional studies using focused ion beam and TEM show the height of 2-3 nm SWNT
array increased to about 1 \mu m with a volume gain of ~ 400 times. The
electron energy loss spectroscopy reveals that graphitic sp^2 networks of SWNTs
are transformed predominantly to sp^3. The current-voltage measurements also
show an increase in the resistance of the transformed structure. | 1107.1758v1 |
2011-07-20 | Macroscopic Superconducting Current through a Silicon Surface Reconstruction with Indium Adatoms: Si(111)-(R7$\times$R3)-In | Macroscopic and robust supercurrents are observed by direct electron
transport measurements on a silicon surface reconstruction with In adatoms
(Si(111)-(R7xR3)-In). The superconducting transition manifests itself as an
emergence of the zero resistance state below 2.8 K. $I-V$ characteristics
exhibit sharp and hysteretic switching between superconducting and normal
states with well-defined critical and retrapping currents. The two-dimensional
(2D) critical current density $J_\mathrm{2D,c}$ is estimated to be as high as
$1.8 \ \mathrm{A/m}$ at 1.8 K. The temperature dependence of $J_\mathrm{2D,c}$
indicates that the surface atomic steps play the role of strongly coupled
Josephson junctions. | 1107.3902v2 |
2011-07-30 | Indications of an Electronic Phase Transition in 2D YBa2Cu3O7-x Induced by Electrostatic Doping | We successfully tuned an underdoped ultrathin YBa2Cu3O7-x film into the
overdoped regime by means of electrostatic doping using an ionic liquid as a
dielectric material. This process proved to be reversible. Transport
measurements showed a series of anomalous features compared to chemically doped
bulk samples and a different two-step doping mechanism for electrostatic doping
was revealed. The normal resistance increased with carrier concentration on the
overdoped side and the high temperature (180 K) Hall number peaked at a doping
level of p$\sim$0.15. These anomalous behaviors suggest that there is an
electronic phase transition in the Fermi surface around the optimal doping
level. | 1108.0083v1 |
2011-08-01 | Evolution of Structural and Physical Properties of Sr3(Ru1-xMnx)2O7 with Mn Concentration | Layered ruthenates are prototype materials with strong structure-property
correlations. We report the structural and physical properties of
double-layered perovskite Sr3(Ru1-xMnx)2O7 single crystals with 0<=x<=0.7.
Single crystal x-ray diffraction refinements reveal that Mn doping on the Ru
site leads to the shrinkage of unit-cell volume and disappearance of (Ru/Mn)O6
octahedron rotation when x>0.16, while the crystal structure remains
tetragonal. Correspondingly, the electric and magnetic properties change with
x. The electrical resistivity reveals metallic character (d rho/d T>0) at high
temperatures but insulating behavior (d rho/d T<0) below a characteristic
temperature T_MIT. Interestingly, T_MIT is different from T_M, at which
magnetic susceptibility reaches maximum. T_MIT monotonically increases with
increasing x while T_M shows non-monotonic dependence with x. The difference
between T_MIT and T_M (T_MIT>T_M) becomes larger when x>0.16. The constructed
phase diagram consists of five distinct regions, demonstrating that the
physical properties of such a system can easily be tuned by chemical doping. | 1108.0392v1 |
2011-08-15 | Large conductance modulation of gold thin films by huge charge injection via electrochemical gating | By using an electrochemical gating technique with a new combination of
polymer and electrolyte, we were able to inject surface charge densities n_2D
as high as 3.5 \times 10^15 e/cm^2 in gold films and to observe large relative
variations in the film resistance, DeltaR/R', up to 10% at low temperature.
DeltaR/R' is a linear function of n_2D - as expected within a free-electron
model - if the film is thick enough (> 25 nm), otherwise a tendency to
saturation due to size effects is observed. The application of this technique
to 2D materials will allow extending the field-effect experiments to a range of
charge doping where giant conductance modulations and, in some cases, even the
occurrence of superconductivity are expected. | 1108.3099v1 |
2011-09-19 | Fe-doping induced superconductivity in charge-density-wave system 1T-TaS2 | We report the interplay between charge-density-wave (CDW) and
superconductivity of 1$T$-Fe$_{x}$Ta$_{1-x}$S$_{2}$ ($0\leq x \leq 0.05$)
single crystals. The CDW order is gradually suppressed by Fe-doping,
accompanied by the disappearance of pseudogap/Mott-gap as shown by the density
functional theory (DFT) calculations. The superconducting state develops at low
temperatures within the CDW state for the samples with the moderate doping
levels. The superconductivity strongly depends on $x$ within a narrow range,
and the maximum superconducting transition temperature is 2.8 K as $x=0.02$. We
propose that the induced superconductivity and CDW phases are separated in real
space. For high doping level ($x>0.04$), the Anderson localization (AL) state
appears, resulting in a large increase of resistivity. We present a complete
electronic phase diagram of 1$T$-Fe$_{x}$Ta$_{1-x}$S$_{2}$ system that shows a
dome-like $T_{c}(x)$. | 1109.3962v3 |
2011-09-28 | A combination of capillary assembly and dielectrophoresis for wafer scale integration of carbon nanotubes-based electrical and mechanical devices | The wafer scale integration of carbon nanotubes (CNT) remains a challenge for
electronic and electromechanical applications. We propose a novel CNT
integration process relying on the combination of controlled capillary assembly
and buried electrode dielectrophoresis (DEP). This process enables to monitor
the precise spatial localization of a high density of CNTs and their alignment
in a pre-defined direction. Large arrays of independent and low resistivity
(4.4 x 10^-5 {\Omega}.m) interconnections were achieved using this hybrid
assembly with double-walled carbon nanotubes (DWNT). Finally, arrays of
suspended individual CNT carpets have been realized and we demonstrate their
potential use as functional nano-electromechanical systems (NEMS) by monitoring
their resonance frequencies (ranging between 1.7 MHz to 10.5MHz) using a
Fabry-Perot interferometer. | 1109.6268v1 |
2011-11-14 | Thermoelectric properties of Ba-Cu-Si clathrates | Thermoelectric properties of the type-I clathrates Ba$_8$Cu$_x$Si$_{46-x}$
($3.6 \leq x \leq 7$, $x$ = nominal Cu content) are investigated both
experimentally and theoretically. The polycrystalline samples are prepared
either by melting, ball milling and hot pressing or by melt spinning, hand
milling and hot pressing techniques. Temperature-dependent electrical
resistivity, $\rho(T)$, and the Seebeck coefficient, $S(T)$, measurements
reveal metal-like behavior for all samples. For $x = 5$ and 6, density
functional theory calculations are performed for deriving the enthalpy of
formation and the electronic structure which is exploited for the calculation
of Seebeck coefficients and conductivity within Boltzmann's transport theory.
For simulating the properties of doped clathrates the rigid band model is
applied. On the basis of the density functional theory results the
experimentally observed compositional dependence of $\rho(T)$ and $S(T)$ of the
whole sample series is analyzed. The highest dimensionless thermoelectric
figure of merit $ZT$ of 0.28 is reached for a melt-spun sample at
$600^{\circ}$C. The relatively low $ZT$ values in this system are attributed to
the too high charge carrier concentrations. | 1111.3278v1 |
2011-12-13 | The Disordered Induced Interaction and the Phase Diagram of Cuprates | There are processes in nature that resemble a true force but arise due to the
minimization of the local energy. The most well-known case is the exchange
interaction that leads to magnetic order in some materials. We discovered a new
similar process occurring in connection with an electronic phase separation
transition that leads to charge inhomogeneity in cuprate superconductors. The
minimization of the local free energy, described here by the Cahn-Hilliard
diffusion equation, drives the charges into regions of low and high densities.
This motion leads to an effective potential with two-fold effect: creation of
tiny isolated regions or micrograins, and two-body attraction, which promotes
local or intra-grain superconducting pairing. Consequently, as in granular
superconductors, the superconducting transition appears in two steps. First,
with local intra-grain superconducting amplitudes and, at lower temperature,
the superconducting phase or resistivity transition is attained by intergrain
Josephson coupling. We show here that this approach reproduces the main
features of the cuprates phase diagram, gives a clear interpretation to the
pseudogap phase and yields the position dependent local density of states gap
$\Delta(\vec r)$ measured by tunnelling experiments. | 1112.2880v1 |
2012-05-08 | Normal and intrinsic anomalous Hall effect in Nb1-yFe2+y | The Hall effect on selected samples of the dilution series Nb1-yFe2+y is
studied. Normal and anomalous contributions are observed, with positive normal
Hall effect dominating at high temperatures. Consistent analysis of the
anomalous contribution is only possible for Fe-rich Nb0.985Fe2.015 featuring a
ferromagnetic ground state. Here, a positive normal Hall coefficient is found
at all temperatures with a moderate maximum at the spin-density-wave
transition. The anomalous Hall effect is consistent with an intrinsic
(Berry-phase) contribution which is constant below the ordering temperature TC
and continuously vanishes above TC. For stoichiometric NbFe2 and Nb-rich
Nb1.01Fe1.99 - both having a spin-density-wave ground state - an additional
contribution to the Hall resistivity impedes a complete analysis and indicates
the need for more sophisticated models of the anomalous Hall effect in
itinerant antiferromagnets. | 1205.1651v1 |
2012-05-10 | Physical properties and crystal chemistry of Ce2Ga12Pt | Single crystals of the new ternary compound Ce2Ga12Pt were prepared by the
self-flux technique. The crystal structure with the space group P4/nbm was
established from single-crystal X-ray diffraction data and presents a
derivative of the LaGa6Ni0.6 prototype. Magnetic susceptibility measurements
show Curie-Weiss behaviour due to local Ce^3+ moments. At high temperatures,
the magnetic anisotropy is dominated by the crystal-electric-field (CEF) effect
with the easy axis along the crystallographic c direction. Ce2Ga12Pt undergoes
two antiferromagnetic phase transitions at T_N,1 = 7.3K and T_N,2 = 5.5K and
presents several metamagnetic transitions for the magnetic field along c.
Specific-heat measurements prove the bulk nature of these magnetic transitions
and reveal a doublet CEF ground state. The 4f contribution to the resistivity
shows a broad maximum at T_max ~ 85K due to Kondo scattering off the CEF ground
state and excited levels. | 1205.2301v1 |
2012-05-13 | Voltage-Induced Ferromagnetic Resonance in Magnetic Tunnel Junctions | We demonstrate excitation of ferromagnetic resonance in CoFeB/MgO/CoFeB
magnetic tunnel junctions (MTJs) by the combined action of voltage-controlled
magnetic anisotropy (VCMA) and spin transfer torque (ST). Our measurements
reveal that GHz-frequency VCMA torque and ST in low-resistance MTJs have
similar magnitudes, and thus that both torques are equally important for
understanding high-frequency voltage-driven magnetization dynamics in MTJs. As
an example, we show that VCMA can increase the sensitivity of an MTJ-based
microwave signal detector to the sensitivity level of semiconductor Schottky
diodes. | 1205.2835v2 |
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