76 research outputs found
Electrostatic- and Parallel Magnetic Field- Tuned Two Dimensional Superconductor-Insulator Transitions
The 2D superconductor-insulator transition in disordered ultrathin amorphous
bismuth films has been tuned both by electrostatic electron doping using the
electric field effect and by the application of parallel magnetic fields.
Electrostatic doping was carried out in both zero and nonzero magnetic fields,
and magnetic tuning was conducted at multiple strengths of electrostatically
induced superconductivity. The transitions were analyzed using finite size
scaling with critical exponent products nu*z = 0.65-0.7. The parallel critical
magnetic field increased with electron transfer as (dn_c-dn)^0.33, where dn is
the electron transfer and dn_c is its critical value, and the critical
resistance decreased linearly with dn. However at lower temperatures, in the
insulating regime, the resistance became larger than expected from
extrapolation of its temperature dependence at higher temperatures, and scaling
failed. These observations imply that although the electrostatic- and parallel
magnetic field- tuned superconductor-insulator transitions would appear to
belong to the same universality class and to be delineated by a robust phase
boundary that can be crossed either by tuning electron density or magnetic
field, in the case of the field-tuned transition at the lowest temperatures,
some different type of physical behavior turns on in the insulating regime.Comment: About 11 pages, with 14 figures. To be submitted to Phys Rev
Optical conductivity of a granular metal at not very low temperatures
We study the finite-temperature optical conductivity, sigma(omega,T), of a
granular metal using a simple model consisting of a array of spherical metallic
grains. It is necessary to include quantum tunneling and Coulomb blockade
effects to obtain the correct temperature dependence of sigma(omega, T), and to
consider polarization oscillations to obtain the correct frequency dependence.
We have therefore generalized the Ambegaokar-Eckern-Schoen (AES) model for
granular metals to obtain an effective field theory incorporating the
polarization fluctuations of the individual metallic grains. In contrast to the
DC conductivity, which is determined by inter-grain charge transfer and obeys
an Arrhenius law at low temperature, the AC conductivity is dominated by a
resonance peak for intra-grain polarization oscillations, which has a power-law
tail at low frequencies. More importantly, although the resonance frequency
agrees with the classical prediction, the resonance width depends on intergrain
quantum tunneling and Coulomb blockade parameters, in addition to the classical
Drude relaxation within the grain. This additional damping is due to inelastic
cotunneling of polarization fluctuations to neighbouring grains and it
qualitatively differs from the DC conductivity in its temperature dependence
quite unlike the expectation from Drude theory.Comment: Added figures, published version, 16 pages, REVTe
Apparent Metallic Behavior at B = 0 of a two-dimensional electron system in AlAs
We report the observation of metallic-like behavior at low temperatures and
zero magnetic field in two dimensional (2D) electrons in an AlAs quantum well.
At high densities the resistance of the sample decreases with decreasing
temperature, but as the density is reduced the behavior changes to insulating,
with the resistance increasing as the temperature is decreased. The effect is
similar to that observed in 2D electrons in Si-MOSFETs, and in 2D holes in SiGe
and GaAs, and points to the generality of this phenomenon
Breakdown of the Korringa Law of Nuclear Spin Relaxation in Metallic GaAs
We present nuclear spin relaxation measurements in GaAs epilayers using a new
pump-probe technique in all-electrical, lateral spin-valve devices. The
measured T1 times agree very well with NMR data available for T > 1 K. However,
the nuclear spin relaxation rate clearly deviates from the well-established
Korringa law expected in metallic samples and follows a sub-linear temperature
dependence 1/T1 ~ T^0.6 for 0.1 K < T < 10 K. Further, we investigate nuclear
spin inhomogeneities.Comment: 5 pages, 4 (color) figures. arXiv admin note: text overlap with
arXiv:1109.633
Effect of granularity on the insulator-superconductor transition in ultrathin Bi films
We have studied the insulator-superconductor transition (IST) by tuning the
thickness in quench-condensed films. The resistive transitions of the
superconducting films are smooth and can be considered to represent
"homogeneous" films. The observation of an IST very close to the quantum
resistance for pairs, on several substrates supports
this idea. The relevant length scales here are the localization length, and the
coherence length. However, at the transition, the localization length is much
higher than the superconducting coherence length, contrary to expectation for a
"homogeneous" transition. This suggests the invalidity of a purely fermionic
model for the transition. Furthermore, the current-voltage characteristics of
the superconducting films are hysteretic, and show the films to be granular.
The relevant energy scales here are the Josephson coupling energy and the
charging energy. However, Josephson coupling energies () and the charging
energies () at the IST, they are found to obey the relation .
This is again contrary to expectation, for the IST in a granular or
inhomogeneous, system. Hence, a purely bosonic picture of the transition is
also inconsistent with our observations. We conclude that the IST observed in
our experiments may be either an intermediate case between the fermioinc and
bosonic mechanisms, or in a regime of charge and vortex dynamics for which a
quantitative analysis has not yet been done.Comment: accepted in Physical Review
Effect of Tilted Magnetic Field on the Anomalous H=0 Conducting Phase in High-Mobility Si MOSFETs
The suppression by a magnetic field of the anomalous H=0 conducting phase in
high-mobility silicon MOSFETs is independent of the angle between the field and
the plane of the 2D electron system. In the presence of a parallel field large
enough to fully quench the anomalous conducting phase, the behavior is similar
to that of disordered GaAs/AlGaAs heterostructures: the system is insulating in
zero (perpendicular) field and exhibits reentrant insulator-quantum Hall
effect-insulator transitions as a function of perpendicular field. The results
demonstrate that the suppression of the low-T phase is related only to the
electrons' spin.Comment: 4 pages, including 3 figures. We corrected several typos in the
figures and caption
Defects in correlated metals and superconductors
In materials with strong local Coulomb interactions, simple defects such as
atomic substitutions strongly affect both macroscopic and local properties of
the system. A nonmagnetic impurity, for instance, is seen to induce magnetism
nearby. Even without disorder, models of such correlated systems are generally
not soluble in 2 or 3 dimensions, and so few exact results are known for the
properties of such impurities. Nevertheless, some simple physical ideas have
emerged from experiments and approximate theories. Here, we first review what
we can learn about this problem from 1D antiferromagnetically correlated
systems. We then discuss experiments on the high Tc cuprate normal state which
probe the effect of impurities on local charge and spin degrees of freedom, and
compare with theories of single impurities in correlated hosts, as well as
phenomenological effective Kondo descriptions. Subsequently, we review theories
of impurities in d-wave superconductors including residual quasiparticle
interactions, and compare with experiments in the superconducting state. We
argue that existing data exhibit a remarkable similarity to impurity-induced
magnetism in the 1D case, implying the importance of electronic correlations
for the understanding of these phenomena, and suggesting that impurities may
provide excellent probes of the still poorly understood ground state of the
cuprates.Comment: 66 pages, 48 figures, review articl
X-Ray Fluorescence Determination of Major Elements in Powder Chromium Ore Samples Prepared as Pressed Pellets
Предложен способ рентгенофлуоресцентного определения основных элементов
(Cr, Fe, Si, Al, Mg), а также Mn и Ti в порошковых пробах хромовых руд. Измерения проводили
на волноводисперсионном рентгенофлуоресцентном спектрометре S 4 Pioneer (Bruker AXS).
Подготовка проб включала дополнительное измельчение порошков калибровочных образцов
и анализируемых проб, которое обеспечивало размер частиц порошка менее 50 мкм. Для
измерения готовили таблетки прессованием порошков на подложке из борной кислоты.
Дополнительное измельчение позволило уменьшить влияние гранулометрического состава
порошков на погрешность определения главных полезных компонентов: хрома и железа. В качестве
калибровочной функции использовали эмпирическое уравнение Лукаса-Туса,
имеющееся в опциях
программного обеспечения спектрометра. Отклонения результатов рентгенофлуоресцентного
анализа от результатов химического анализа с фотометрическим или атомно-абсорбционным
окончанием для четырех контрольных проб составили менее: 0.34 % мас. для Cr2O3 (в диапазоне
18–56 % мас.), 0.16 % мас. для Fe2O3 (в диапазоне 10–26 % мас.), 0.29 % мас. для Al2O3 (в диапазоне
5–20 % мас.), 0.013 % мас. для MnO (в диапазоне 0.1–0.2 % мас.), 0.03 % мас. для TiO2 (в диапазоне
0.09–0.67 % мас.). Для MgO (в диапазоне 14–32 % мас.) и SiO2 (в диапазоне 2–22 % мас.) погрешность
определения может быть более 1 % мас., что превышает допустимую для количественного
определенияX‑ray fluorescence techniques is proposed for the determination of the main elements (Cr, Fe, Si, Al, Mg), as well as Mn and Ti, in powder samples of chromium ores. The measurements were carried out using wavelength- dispersive X‑ray fluorescence spectrometer S 4 Pioneer (Bruker AXS). Sample preparation included additional grinding of calibration and analyzed samples, which ensured a powder particle size of less than 50 μm. Pellets were prepared for measurements by pressing powders on a boric acid substrate. Additional grinding made it possible to reduce the influence of the granulometric composition of powders on the error in determining the main useful components: chromium and iron. The available in the software options of the spectrometer empirical Lucas- Tooth equation was used as a calibration function. Deviations between X‑ray fluorescence and chemical analysis (photometry or atomic absorption spectrometry) results for four test samples were less than: 0.34 wt.% for Cr2O3 (in the range of 18–56 wt.%), 0.16 wt.% for Fe2O3 (in the range of 10–26 wt.%), 0.29 wt.% for Al2O3 (in the range of 5–20 wt.%), 0.013 wt.% for MnO (in the range of 0.1–0.2 wt.%), 0.03 wt.% for TiO2 (in the range of 0.09–0.67 wt.%). For the MgO (in the range of 14–32 wt.%) and SiO2, (in the range of 2–22 wt.%), the error can be more than 1 wt.% that exceeds the allowable error for quantitative determinatio
The renormalization group for interacting fermions: from Fermi liquids to quantum dots
The renormalization group approach as developed by the author for Fermi
liquids is applied to clean Fermi liquids and ballistic quantum dots. In the
former case Landau theory is shown to be a fixed point and in the latter the
Universal Hamiltonian is shown to be a fixed point for weak coupling. The
strong coupling phase is analyzed using large N and Random Matrix methods.Comment: Lectures given at the Fifteenth Chris Engelbrecht Summer School South
Africa, January 2004. 6 eps figs and springer style file (svmult
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