737 research outputs found
One-parameter Superscaling at the Metal-Insulator Transition in Three Dimensions
Based on the spectral statistics obtained in numerical simulations on three
dimensional disordered systems within the tight--binding approximation, a new
superuniversal scaling relation is presented that allows us to collapse data
for the orthogonal, unitary and symplectic symmetry () onto a
single scaling curve. This relation provides a strong evidence for
one-parameter scaling existing in these systems which exhibit a second order
phase transition. As a result a possible one-parameter family of spacing
distribution functions, , is given for each symmetry class ,
where is the dimensionless conductance.Comment: 4 pages in PS including 3 figure
Shape Analysis of the Level Spacing Distribution around the Metal Insulator Transition in the Three Dimensional Anderson Model
We present a new method for the numerical treatment of second order phase
transitions using the level spacing distribution function . We show that
the quantities introduced originally for the shape analysis of eigenvectors can
be properly applied for the description of the eigenvalues as well. The
position of the metal--insulator transition (MIT) of the three dimensional
Anderson model and the critical exponent are evaluated. The shape analysis of
obtained numerically shows that near the MIT is clearly different
from both the Brody distribution and from Izrailev's formula, and the best
description is of the form , with
. This is in good agreement with recent analytical results.Comment: 14 pages in plain TeX, 6 figures upon reques
Does a magnetic field modify the critical behaviour at the metal-insulator transition in 3-dimensional disordered systems?
The critical behaviour of 3-dimensional disordered systems with magnetic
field is investigated by analyzing the spectral fluctuations of the energy
spectrum. We show that in the thermodynamic limit we have two different
regimes, one for the metallic side and one for the insulating side with
different level statistics. The third statistics which occurs only exactly at
the critical point is {\it independent} of the magnetic field. The critical
behaviour which is determined by the symmetry of the system {\it at} the
critical point should therefore be independent of the magnetic field.Comment: 10 pages, Revtex, 4 PostScript figures in uuencoded compressed tar
file are appende
Anderson localization vs. Mott-Hubbard metal-insulator transition in disordered, interacting lattice fermion systems
We review recent progress in our theoretical understanding of strongly
correlated fermion systems in the presence of disorder. Results were obtained
by the application of a powerful nonperturbative approach, the Dynamical
Mean-Field Theory (DMFT), to interacting disordered lattice fermions. In
particular, we demonstrate that DMFT combined with geometric averaging over
disorder can capture Anderson localization and Mott insulating phases on the
level of one-particle correlation functions. Results are presented for the
ground-state phase diagram of the Anderson-Hubbard model at half filling, both
in the paramagnetic phase and in the presence of antiferromagnetic order. We
find a new antiferromagnetic metal which is stabilized by disorder. Possible
realizations of these quantum phases with ultracold fermions in optical
lattices are discussed.Comment: 25 pages, 5 figures, typos corrected, references update
Relation between Energy Level Statistics and Phase Transition and its Application to the Anderson Model
A general method to describe a second-order phase transition is discussed. It
starts from the energy level statistics and uses of finite-size scaling. It is
applied to the metal-insulator transition (MIT) in the Anderson model of
localization, evaluating the cumulative level-spacing distribution as well as
the Dyson-Metha statistics. The critical disorder and the critical
exponent are computed.Comment: 9 pages, Latex, 6 PostScript figures in uuencoded compressed tar file
are appende
Interference in interacting quantum dots with spin
We study spectral and transport properties of interacting quantum dots with
spin. Two particular model systems are investigated: Lateral multilevel and two
parallel quantum dots. In both cases different paths through the system can
give rise to interference. We demonstrate that this strengthens the multilevel
Kondo effect for which a simple two-stage mechanism is proposed. In parallel
dots we show under which conditions the peak of an interference-induced orbital
Kondo effect can be split.Comment: 8 pages, 8 figure
Anomalous diffusion at the Anderson transitions
Diffusion of electrons in three dimensional disordered systems is
investigated numerically for all the three universality classes, namely,
orthogonal, unitary and symplectic ensembles. The second moment of the wave
packet at the Anderson transition is shown to behave as . From the temporal autocorrelation function , the
fractal dimension is deduced, which is almost half the value of space
dimension for all the universality classes.Comment: Revtex, 2 figures, to appear in J. Phys. Soc. Jpn.(1997) Fe
Anderson transition in three-dimensional disordered systems with symplectic symmetry
The Anderson transition in a 3D system with symplectic symmetry is
investigated numerically. From a one-parameter scaling analysis the critical
exponent of the localization length is extracted and estimated to be . The level statistics at the critical point are also analyzed
and shown to be scale independent. The form of the energy level spacing
distribution at the critical point is found to be different from that
for the orthogonal ensemble suggesting that the breaking of spin rotation
symmetry is relevant at the critical point.Comment: 4 pages, revtex, to appear in Physical Review Letters. 3 figures
available on request either by fax or normal mail from
[email protected] or [email protected]
Zero-Bias Conductance Through Side-Coupled Double Quantum Dots
Low temperature zero-bias conductance through two side-coupled quantum dots
is investigated using Wilson's numerical renormalization group technique. A
low-temperature phase diagram is computed. Near the particle-hole symmetric
point localized electrons form a spin-singlet associated with weak conductance.
For weak inter-dot coupling we find enhanced conductance due to the two-stage
Kondo effect when two electrons occupy quantum dots. When quantum dots are
populated with a single electron, the system enters Kondo regime with enhanced
conductance. Analytical expressions for the width of the Kondo regime and the
Kondo temperature in this regime are given.Comment: to be published in the Proceedings of the NATO Advanced Research
Workshop on "Electron Correlations in New Materials and Nanosystems" held in
Yalta, Ukraine, 19 - 23 September 2005 (NATO Science Series II, Springer
2006
Electron Transport through T-Shaped Double-Dots System
Correlation effects on electron transport through a system of T-shaped
double-dots are investigated, for which only one of the dots is directly
connected to the leads. We evaluate the local density of states and the
conductance by means of the non-crossing approximation at finite temperatures
as well as the slave-boson mean field approximation at zero temperature. It is
found that the dot which is not directly connected to the leads considerably
influences the conductance, making its behavior quite different from the case
of a single-dot system. In particular, we find a novel phenomenon in the Kondo
regime with a small inter-dot coupling, i.e.
Fano-like suppression of the Kondo-mediated conductance, when two dot levels
coincide with each other energetically.Comment: 6 pages,7 figure
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