293 research outputs found
Crossover of Level Statistics between Strong and Weak Localization in Two Dimensions
We investigate numerically the statistical properties of spectra of
two-dimensional disordered systems by using the exact diagonalization and
decimation method applied to the Anderson model. Statistics of spacings
calculated for system sizes up to 1024 1024 lattice sites exhibits a
crossover between Wigner and Poisson distributions. We perform a self-contained
finite-size scaling analysis to find a single-valued one-parameter function
which governs the crossover. The scaling parameter is
deduced and compared with the localization length. does {\em
not} show critical behavior and has two asymptotic regimes corresponding to
weakly and strongly localized states.Comment: 4 pages in revtex, 3 postscript figure
Energy-level statistics and localization of 2d electrons in random magnetic fields
Using the method of energy-level statistics, the localization properties of
electrons moving in two dimensions in the presence of a perpendicular random
magnetic field and additional random disorder potentials are investigated. For
this model, extended states have recently been proposed to exist in the middle
of the band. In contrast, from our calculations of the large- behavior of
the nearest neighbor level spacing distribution and from a finite size
scaling analysis we find only localized states in the suggested energy and
disorder range.Comment: 4 pages LaTeX, 4 eps-figures. to appear in Physica
Spin and orbital effects in a 2D electron gas in a random magnetic field
Using the method of superbosonization we consider a model of a random
magnetic field (RMF) acting on both orbital motion and spin of electrons in two
dimensions. The method is based on exact integration over one particle degrees
of freedom and reduction of the problem to a functional integral over
supermatrices . We consider a general case when
both the direction of the RMF and the g-factor of the Zeeman splitting are
arbitrary. Integrating out fast variations of we come to a standard
collisional unitary non-linear -model. The collision term consists of
orbital, spin and effective spin-orbital parts. For a particular problem of a
fixed direction of RMF, we show that additional soft excitations identified
with spin modes should appear. Considering % -correlated weak RMF and
putting g=2 we find the transport time . This time is 2 times
smaller than that for spinless particles.Comment: 9 pages, no figure
Critical conductance of two-dimensional chiral systems with random magnetic flux
The zero temperature transport properties of two-dimensional lattice systems
with static random magnetic flux per plaquette and zero mean are investigated
numerically. We study the two-terminal conductance and its dependence on
energy, sample size, and magnetic flux strength. The influence of boundary
conditions and of the oddness of the number of sites in the transverse
direction is also studied. We confirm the existence of a critical chiral state
in the middle of the energy band and calculate the critical exponent nu=0.35
+/- 0.03 for the divergence of the localization length. The sample averaged
scale independent critical conductance _c turns out to be a function of the
amplitude of the flux fluctuations whereas the variance of the respective
conductance distributions appears to be universal. All electronic states
outside of the band center are found to be localized.Comment: to appear in Phys. Rev.
Comment on ``Critical Behavior in Disordered Quantum Systems Modified by Broken Time--Reversal Symmetry''
In a recent Letter [Phys. Rev. Lett. 80, 1003 (1998)] Hussein and Pato
employed the maximum entropy principle (MEP) in order to derive interpolating
ensembles between any pair of universality classes in random matrix theory.
They apply their formalism also to the transition from random matrix to Poisson
statistics of spectra that is observed for the case of the Anderson-type
metal-insulator transition. We point out the problems with the latter
procedure.Comment: 1 page in PS, to appear in PRL Sept. 2
Spectral statistics of disordered metals in the presence of several Aharonov-Bohm fluxes
The form factor for spectral correlations in a diffusive metal is calculated
in the presence of several Aharonov-Bohm fluxes. When the fluxes are
equal, the correlations are universal functions of where is
the dimensionless conductance and is the number of applied fluxes. This
explains recent flux dependence of the correlations found numerically at the
metal-insulator transition.Comment: 3 pages, Latex, 1 figure, to appear in Phys. Rev. B Rapid Com
Magnetic Field Effect for Two Electrons in a Two Dimensional Random Potential
We study the problem of two particles with Coulomb repulsion in a
two-dimensional disordered potential in the presence of a magnetic field. For
the regime, when without interaction all states are well localized, it is shown
that above a critical excitation energy electron pairs become delocalized by
interaction. The transition between the localized and delocalized regimes goes
in the same way as the metal-insulator transition at the mobility edge in the
three dimensional Anderson model with broken time reversal symmetry.Comment: revtex, 7 pages, 6 figure
Probability distribution of the conductance at the mobility edge
Distribution of the conductance P(g) at the critical point of the
metal-insulator transition is presented for three and four dimensional
orthogonal systems. The form of the distribution is discussed. Dimension
dependence of P(g) is proven. The limiting cases and are
discussed in detail and relation in the limit is proven.Comment: 4 pages, 3 .eps figure
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