131 research outputs found
Annealed disorder, rare regions, and local moments: A novel mechanism for metal-insulator transitions
Local magnetic moments in disordered sytems can be described in terms of
annealed magnetic disorder, in addition to the underlying quenched disorder. It
is shown that for noninteracting electron systems at zero temperature, the
annealed disorder leads to a new mechanism, and a new universality class, for a
metal-insulator transition. The transition is driven by a vanishing of the
thermodynamic density susceptibility rather than by localization effects. The
critical behavior near two-dimensions is determined, and the underlying physics
is discussed.Comment: 4 pp., LaTeX, no figs., final version as publishe
Revisiting the Theory of Finite Size Scaling in Disordered Systems: \nu Can Be Less Than 2/d
For phase transitions in disordered systems, an exact theorem provides a
bound on the finite size correlation length exponent: \nu_{FS}<= 2/d. It is
believed that the true critical exponent \nu of a disorder induced phase
transition satisfies the same bound. We argue that in disordered systems the
standard averaging introduces a noise, and a corresponding new diverging length
scale, characterized by \nu_{FS}=2/d. This length scale, however, is
independent of the system's own correlation length \xi. Therefore \nu can be
less than 2/d. We illustrate these ideas on two exact examples, with \nu < 2/d.
We propose a new method of disorder averaging, which achieves a remarkable
noise reduction, and thus is able to capture the true exponents.Comment: 4 pages, Latex, one figure in .eps forma
Typical-Medium Theory of Mott-Anderson Localization
The Mott and the Anderson routes to localization have long been recognized as
the two basic processes that can drive the metal-insulator transition (MIT).
Theories separately describing each of these mechanisms were discussed long
ago, but an accepted approach that can include both has remained elusive. The
lack of any obvious static symmetry distinguishing the metal from the insulator
poses another fundamental problem, since an appropriate static order parameter
cannot be easily found. More recent work, however, has revisited the original
arguments of Anderson and Mott, which stressed that the key diference between
the metal end the insulator lies in the dynamics of the electron. This physical
picture has suggested that the "typical" (geometrically averaged) escape rate
from a given lattice site should be regarded as the proper dynamical order
parameter for the MIT, one that can naturally describe both the Anderson and
the Mott mechanism for localization. This article provides an overview of the
recent results obtained from the corresponding Typical-Medium Theory, which
provided new insight into the the two-fluid character of the Mott-Anderson
transition.Comment: to be published in "Fifty Years of Anderson localization", edited by
E. Abrahams (World Scientific, Singapore, 2010); 29 pages, 22 figures
Field-induced breakdown of the quantum Hall effect
A numerical analysis is made of the breakdown of the quantum Hall effect
caused by the Hall electric field in competition with disorder. It turns out
that in the regime of dense impurities, in particular, the number of localized
states decreases exponentially with the Hall field, with its dependence on the
magnetic and electric field summarized in a simple scaling law. The physical
picture underlying the scaling law is clarified. This intra-subband process,
the competition of the Hall field with disorder, leads to critical breakdown
fields of magnitude of a few hundred V/cm, consistent with observations, and
accounts for their magnetic-field dependence \propto B^{3/2} observed
experimentally. Some testable consequences of the scaling law are discussed.Comment: 7 pages, Revtex, 3 figures, to appear in Phys. Rev.
Quantum Hall Effect induced by electron-electron interaction in disordered GaAs layers with 3D spectrum
It is shown that the observed Quantum Hall Effect in epitaxial layers of
heavily doped n-type GaAs with thickness (50-140 nm) larger the mean free path
of the conduction electrons (15-30 nm) and, therefore, with a three-dimensional
single-particle spectrum is induced by the electron-electron interaction. The
Hall resistance R_xy of the thinnest sample reveals a wide plateau at small
activation energy E_a=0.4 K found in the temperature dependence of the
transverse resistance R_xx. The different minima in the transverse conductance
G_xx of the different samples show a universal temperature dependence
(logarithmic in a large range of rescaled temperatures T/T_0) which is
reminiscent of electron-electron-interaction effects in coherent diffusive
transport.Comment: 6 pages, 3 figures, 1 tabl
The RKKY interactions and the Mott Transition
A two-site cluster generalization of the Hubbard model in large dimensions is
examined in order to study the role of short-range spin correlations near the
metal-insulator transition (MIT). The model is mapped to a two-impurity
Kondo-Anderson model in a self-consistently determined bath, making it possible
to directly address the competition between the Kondo effect and RKKY
interactions in a lattice context. Our results indicate that the RKKY
interactions lead to qualitative modifications of the MIT scenario even in the
absence of long range antiferromagnetic ordering.Comment: 10 pages, 10 figures; to appear in Phys. Rev. B (1999
What Does The Korringa Ratio Measure?
We present an analysis of the Korringa ratio in a dirty metal, emphasizing
the case where a Stoner enhancement of the uniform susceptibilty is present. We
find that the relaxation rates are significantly enhanced by disorder, and that
the inverse problem of determining the bare density of states from a study of
the change of the Knight shift and relaxation rates with some parameter, such
as pressure, has rather constrained solutions, with the disorder playing an
important role. Some preliminary applications to the case of chemical
substitution in the RbKC family of superconductors is
presented and some other relevant systems are mentioned.Comment: 849, Piscataway, New Jersey 08855 24 June 199
Anomalous NMR Spin-Lattice Relaxation in SrB_{6} and Ca_{1-x}La_{x}B_{6}
We report the results of {11}B nuclear magnetic resonance (NMR) measurements
of SrB_{6} and Ca_{0.995}La_{0.05}B_{6} below room temperature. Although the
electrical resistivities of these two materials differ substantially, their
{11}B-NMR responses exhibit some strikingly common features. Both materials
exhibit ferromagnetic order, but their {11}B-NMR spectra reveal very small
hyperfine fields at the Boron sites. The spin lattice relaxation T_{1}^{-1}
varies considerably with external field but changes with temperature only below
a few K. We discuss these unusual results by considering various different
scenarios for the electronic structure of these materials.Comment: Accepted for publication in Phys. Rev. B Rapid communication, 4
pages, 3 figures. This manuscript replaces an earlier version and includes
some minor changes in the text and in Fig.
Magnetoresistance of composite fermions at \nu=1/2
We have studied temperature dependence of both diagonal and Hall resistivity
in the vicinity of . Magnetoresistance was found to be positive and
almost independent of temperature: temperature enters resistivity as a
logarithmic correction. At the same time, no measurable corrections to the Hall
resistivity has been found. Neither of these results can be explained within
the mean-field theory of composite fermions by an analogy with conventional
low-field interaction theory. There is an indication that interactions of
composite fermions with fluctuations of the gauge field may reconcile the
theory and experiment.Comment: 9 pages, 4 figure
Giant Magnetoresistance Oscillations Induced by Microwave Radiation and a Zero-Resistance State in a 2D Electron System with a Moderate Mobility
The effect of a microwave field in the frequency range from 54 to 140
on the magnetotransport in a GaAs quantum well with AlAs/GaAs
superlattice barriers and with an electron mobility no higher than
is investigated. In the given two-dimensional system under
the effect of microwave radiation, giant resistance oscillations are observed
with their positions in magnetic field being determined by the ratio of the
radiation frequency to the cyclotron frequency. Earlier, such oscillations had
only been observed in GaAs/AlGaAs heterostructures with much higher mobilities.
When the samples under study are irradiated with a 140- microwave
field, the resistance corresponding to the main oscillation minimum, which
occurs near the cyclotron resonance, appears to be close to zero. The results
of the study suggest that a mobility value lower than
does not prevent the formation of zero-resistance states in magnetic field in a
two-dimensional system under the effect of microwave radiation.Comment: 4 pages, 2 figur
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