arXiv subjects

L. Schweitzer

Publications and source records attributed to L. Schweitzer.

35 records · Page 2Linked to original sources

arXiv2001

Influence of Correlated Disorder Potentials on the Levitation of Current Carrying States in the Quantum Hall Effect

The disorder driven quantum Hall to insulator transition is investigated for a two-dimensional lattice system. We consider a Gaussian correlated random potential, study the behaviour of the current carrying states and trace their energetical position when the disorder strength is increased. Our results qualitatively resemble those obtained previously for exponentially correlated disorder potentials. We find both the downward movement of the anti-Chern states as well as the floating up of the Chern states across the Landau gap which is sometimes masked by the global broadening of the tight binding band.

cond-mat.mes-hall↗

arXiv2001

Levitation of Current Carrying States in the Lattice Model for the Integer Quantum Hall Effect

The disorder driven quantum Hall to insulator transition is investigated for a two-dimensional lattice model. The Hall conductivity and the localization length are calculated numerically near the transition. For uncorrelated and weakly correlated disorder potentials the current carrying states are annihilated by the negative Chern states originating from the band center. In the presence of correlated disorder potentials with correlation length larger than approximately half the lattice constant the floating up of the critical states in energy without merging is observed. This behavior is similar to the levitation scenario proposed for the continuum model.

cond-mat.mes-hall↗

arXiv2001

Floating of critical states and the QH to insulator transition

The transition from the quantum Hall state to the insulator is considered for non-interacting electrons in a two-dimensional disordered lattice model with perpendicular magnetic field. Using correlated random disorder potentials the floating up of the critical states can be observed in a similar way as in the continuum model. Thus, the peculiar behaviour of the lattice models reported previously originates in the special choice of uncorrelated random disorder potentials.

cond-mat.mes-hall↗

arXiv2001

Frequency dependent transport in the integer quantum Hall effect

The frequency dependent transport is investigated for a two-dimensional disordered system under QHE conditions. The real and imaginary parts of the conductivity are calculated numerically in linear response using a recursive Green function technique. The energy dependence of $σ_{xx}(E,ω)$ is obtained within the lowest Landau band. When the width of the system exceeds the characteristic length, $L_ω=(\hbarωρ(E))^{-1/2}$, the maximum of the real part of $σ_{xx}(E,ω)$ decays with frequency almost linearly which is different from the classical Drude behaviour.

cond-mat.mes-hall↗

arXiv1999

Localization of electrons in two-dimensional spatially-correlated random magnetic fields

The localization properties of electrons moving in a plane perpendicular to a spatially-correlated static magnetic field of random amplitude and vanishing mean are investigated. We apply the method of level statistics to the eigenvalues and perform a multifractal analysis for the eigenstates. From the size and disorder dependence of the variance of the nearest neighbor energy spacing distribution, $P_{W,L}(s)$, a single branch scaling curve is obtained. Contrary to a recent claim, we find no metal-insulator-transition in the presence of diagonal disorder. Instead, as in the uncorrelated random magnetic field case, conventional unitary behavior (all states are localized) is observed. The eigenstates at the band center, which in the absence of diagonal disorder are believed to belong to the chiral unitary symmetry class, are shown to exhibit a $f(α)$-distribution for not too weak random fields. The corresponding generalized multifractal dimensions are calculated and found to be different from the results known for a QHE-system.

cond-mat.dis-nn↗

arXiv1999

Frequency dependent conductivity in the integer quantum Hall effect

Frequency dependent electronic transport is investigated for a two-dimensional disordered system in the presence of a strong perpendicular static magnetic field. The ac-conductivity is calculated numerically from Kubo's linear response theory using a recursive Green's function technique. In the tail of the lowest Landau band, we find a linear frequency dependence for the imaginary part of $σ_{xx}(ω)$ which agrees well with earlier analytical calculations. On the other hand, the frequency dependence of the real part can not be expressed by a simple power law. The broadening of the $σ_{xx}$-peak with frequency in the lowest Landau band is found to exhibit a scaling relation from which the critical exponent can be extracted.

cond-mat.dis-nn↗

arXiv1998

Critical level statistics of a quantum Hall system with Dirichlet boundary conditions

We investigate numerically the influence of Dirichlet boundary conditions on the nearest neighbor level spacing distribution $P(s)$ of a two-dimensional disordered tight-binding model in the presence of a strong perpendicular magnetic field. From the calculation of the second moment of $P(s)$ it is shown that for Dirichlet boundary conditions, due to the presence of edge states, the position of the critical energy shifts with increasing system size to the location of the critical energy for periodic boundary conditions. An extrapolation to infinite system size results in different critical (scale independent) $P(s)$ distributions for periodic and Dirichlet boundary conditions.

cond-mat.dis-nn↗

arXiv1998

Extended Fractons and Localized Phonons on Percolation Clusters

We investigate the localization behavior of vibrational modes of infinite percolation clusters above the critical concentration in two and three dimensions using the method of level statistics. We find that all eigenstates are localized in d=2, including the low frequency phonon states. In d=3 we obtain evidence for a localization-delocalization transition. But contrary to the common view this transition occurs for frequencies above the phonon fracton crossover giving rise to a new regime of extended fracton states.

cond-mat.dis-nn↗

arXiv1998

Influence of boundary conditions on level statistics and eigenstates at the metal insulator transition

We investigate the influence of the boundary conditions on the scale invariant critical level statistics at the metal insulator transition of disordered three-dimensional orthogonal and two-dimensional unitary and symplectic tight-binding models. The distribution of the spacings between consecutive eigenvalues is calculated numerically and shown to be different for periodic and Dirichlet boundary conditions whereas the critical disorder remains unchanged. The peculiar correlations of the corresponding critical eigenstates leading to anomalous diffusion seem not to be affected by the change of the boundary conditions.

cond-mat.dis-nn↗

arXiv1998

Quantum-Hall to insulator transition

The crossover from the quantum Hall regime to the Hall-insulator is investigated by varying the strength of the diagonal disorder in a 2d tight-binding model. The Hall and longitudinal conductivities and the behavior of the critical states are calculated numerically. We find that with increasing disorder the current carrying states close to the band center disappear first. Simultaneously, the quantized Hall conductivity drops monotonically to zero also from higher quantized values.

cond-mat.dis-nn↗

arXiv1998

Dependence of critical level statistics on the sample shape

The level-spacing distribution of consecutive energy eigenvalues is calculated numerically at the metal insulator transition for 3d systems with different cuboid shapes. It is found that the scale independent critical $P_c(s)$ changes as a function of the aspect ratio of the samples while the critical disorder $W_c/V=16.4$ remains the same. We use our data to test whether an expression for the small-$s$ behaviour of the level statistics proposed by Kravtsov and Mirlin for the metallic regime is applicable also at the critical point. For this reason, a shape dependent dimensionless critical conductance $g_c$ has been extracted from the small-$s$ behaviour of the critical level statistics. Our result for a cubic sample, $g_c=0.112\pm 0.005$, is in good agreement with a value obtained previously from calculations using the Kubo-formula.

cond-mat.dis-nn↗

arXiv1997

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-$s$ behavior of the nearest neighbor level spacing distribution $P(s)$ and from a finite size scaling analysis we find only localized states in the suggested energy and disorder range.

cond-mat.mes-hall↗

arXiv1997

Scaling of level-statistics and critical exponent of disordered two-dimensional symplectic systems

The statistics of the energy eigenvalues at the metal-insulator-transition of a two-dimensional disordered system with spin-orbit interaction is investigated numerically. The critical exponent $ν$ is obtained from the finite-size scaling of the number $J_0$ which is related to the probability $Q_{n}(s)$ of having $n$ energy levels within an interval of width $s$. In contrast to previous estimates, we find $ν=2.32\pm 0.14$ close to the value of the two-dimensional quantum Hall system.

cond-mat.dis-nn↗

arXiv1996

Level statistics in the quantum Hall regime

The statistical properties of energy eigenvalues in the critical regime of the lowest Landau band are investigated. The nearest neighbour level spacing distribution $P(s)$ and the Mehta quantity $I_0$, from which the scaling exponent $ν$ of the correlation length can be derived, are calculated. While $P(s)$ disagrees with recent analytical predictions, $ν$ is found to be in agreement with theoretical and experimental results.

cond-mat↗

arXiv1996

Crossover from critical orthogonal to critical unitary statistics at the Anderson transition

We report a novel scale-independent, Aharonov-Bohm flux controlled crossover from critical orthogonal to critical unitary statistics at the disorder induced metal insulator transition. Our numerical investigations show that at the critical point the level statistics are definitely distinct and determined by fundamental symmetries. The latter is similar to the behavior of the metallic phase known from random matrix theory. The Aharonov-Bohm flux dependent crossover is characteristic of the critical ensemble.

cond-mat↗

arXiv1995

Critical level spacing distribution of two-dimensional disordered systems with spin-orbit coupling

The energy level statistics of 2D electrons with spin-orbit scattering are considered near the disorder induced metal-insulator transition. Using the Ando model, the nearest-level-spacing distribution is calculated numerically at the critical point. It is shown that the critical spacing distribution is size independent and has a Poisson-like decay at large spacings as distinct from the Gaussian asymptotic form obtained by the random-matrix theory.

cond-mat↗

arXiv1994

Relation between the Correlation Dimensions of Multifractal Wavefunctions and Spectral Measures in Integer Quantum Hall Systems

We study the time evolution of wavepackets of non-interacting electrons in a two-dimensional disordered system in strong magnetic field. For wavepackets built from states near the metal-insulator transition in the center of the lowest Landau band we find that the return probability to the origin $p(t)$ decays algebraically, $p(t) \sim t^{-D_2/2}$, with a non-conventional exponent $D_2/2$. $D_2$ is the generalized dimension describing the scaling of the second moment of the wavefunction. We show that the corresponding spectral measure is multifractal and that the exponent $D_2/2$ equals the generalized dimension $\widetilde{D}_2$ of the spectral measure.

cond-mat↗