arXiv subjects

H. Potempa

Publications and source records attributed to H. Potempa.

8 recordsLinked to original sources

arXiv2002

Metal-insulator transition in two-dimensional disordered systems with power-law transfer terms

We investigate a disordered two-dimensional lattice model for noninteracting electrons with long-range power-law transfer terms and apply the method of level statistics for the calculation of the critical properties. The eigenvalues used are obtained numerically by direct diagonalization. We find a metal-insulator transition for a system with orthogonal symmetry. The exponent governing the divergence of the correlation length at the transition is extracted from a finite size scaling analysis and found to be $ν=2.6\pm 0.15$. The critical eigenstates are also analyzed and the distribution of the generalized multifractal dimensions is extrapolated.

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↗

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↗

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

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↗