11 research outputs found
Quantum Hall effects in layered disordered superconductors
Layered singlet paired superconductors with disorder and broken time reversal
symmetry are studied. The phase diagram demonstrates charge-spin separation in
transport. In terms of the average intergrain transmission and the interlayer
tunnelling we find quantum Hall phases with spin Hall coefficients of 0 and 2
separated by a spin metal phase. We identify a spin metal-insulator
localization exponent as well as a spin conductivity exponent of ~0.9. In
presence of a Zeeman term an additional phase with spin Hall coefficient of 1
appears.Comment: 4 pages, 4 figure
Level statistics for quantum Hall systems
Level statistics for two classes of disordered systems at criticality are analyzed in terms of different
realizations of the Chalker–Coddington network model. These include: 1) Re-examination
of the standard U(1) model describing dynamics of electrons on the lowest Landau level in the
quantum Hall effect, where it is shown that after proper local unfolding the nearest-neighbor spacing
distribution (NNSD) at the critical energy follows the Wigner surmise for Gaussian unitary ensembles
(GUE). 2) Quasi-particles in disordered superconductors with broken time reversal and
spin rotation invariance (in the language of random matrix theory this system is a representative of
symmetry class D in the classification scheme of Altland and Zirnbauer). Here again the NNSD
obeys the Wigner surmise for GUE, reflecting therefore only «basic» discrete symmetries of the
system (time reversal violation) and ignoring particle–hole symmetries and other finer details
(criticality). In the localized regime level repulsion is suppressed
Decoherence of a particle in a ring
We consider a particle coupled to a dissipative environment and derive a
perturbative formula for the dephasing rate based on the purity of the reduced
probability matrix. We apply this formula to the problem of a particle on a
ring, that interacts with a dirty metal environment. At low but finite
temperatures we find a dephasing rate , and identify dephasing
lengths for large and for small rings. These findings shed light on recent
Monte Carlo data regarding the effective mass of the particle. At zero
temperature we find that spatial fluctuations suppress the possibility of
having a power law decay of coherence.Comment: 5 pages, 1 figure, proofed version to be published in EP
Dephasing of a particle in a dissipative environment
The motion of a particle in a ring of length L is influenced by a dirty metal
environment whose fluctuations are characterized by a short correlation
distance . We analyze the induced decoherence process, and compare
the results with those obtained in the opposing Caldeira-Leggett limit (). A proper definition of the dephasing factor that does not depend on a
vague semiclassical picture is employed. Some recent Monte-Carlo results about
the effect of finite temperatures on "mass renormalization" in this system are
illuminated.Comment: 18 pages, 2 figures, some textual improvements, to be published in
JP
Thermal metal in network models of a disordered two-dimensional superconductor
We study the universality class for localization which arises from models of
non-interacting quasiparticles in disordered superconductors that have neither
time-reversal nor spin-rotation symmetries. Two-dimensional systems in this
category, which is known as class D, can display phases with three different
types of quasiparticle dynamics: metallic, localized, or with a quantized
(thermal) Hall conductance. Correspondingly, they can show a variety of
delocalization transitions. We illustrate this behavior by investigating
numerically the phase diagrams of network models with the appropriate symmetry,
and for the first time show the appearance of the metallic phase.Comment: 5 pages, 3 figure
Landau level mixing and spin degeneracy in the quantum Hall effect
We study dynamics of electrons in a magnetic field using a network model with
two channels per link with random mixing in a random intrachannel potential;
the channels represent either two Landau levels or two spin states. We consider
channel mixing as function of the energy separation of the two extended states
and show that its effect changes from repulsion to attraction as the energy
separation increases. For two Landau levels this leads to level floating at low
magnetic fields while for Zeeman split spin states we predict level attraction
at high magnetic fields, accounting for ESR data. We also study random mixing
of two degenerate channels, while the intrachannel potential is periodic
(non-random). We find a single extended state with a localization exponent
for real scattering at nodes; the general case has also a
single extended state, though the localized nature of nearby states sets in at
unusually large scales.Comment: 18 pages, 11 tex-files and 1 ps-file of figure
Spontaneous magnetization and Hall effect in superconductors with broken time-reversal symmetry
Broken time reversal symmetry (BTRS) in d wave superconductors is studied and
is shown to yield current carrying surface states. The corresponding
spontaneous magnetization is temperature independent near the critical
temperature Tc for weak BTRS, in accord with recent data. For strong BTRS and
thin films we expect a temperature dependent spontaneous magnetization with a
paramagnetic anomaly near Tc. The Hall conductance is found to vanish at zero
wavevector q and finite frequency w, however at finite q,w it has an unusual
structure.Comment: 7 pages, 1 eps figure, Europhysics Letters (in press
Spin quantum Hall transition in disordered superconductors
We study a delocalization transition for non-interacting quasiparticles moving in two dimensions, which belongs to a new symmetry class. This symmetry class can be realized in a dirty, gapless superconductor in which time-reversal symmetry for orbital motion is broken, but spin rotation symmetry is intact. We find a direct transition between two insulating phases with quantized Hall conductances of zero and two for the conserved quasiparticles. The energy of quasiparticles acts as a relevant, symmetry-breaking field at the critical point, which splits the direct transition into two conventional plateau transitions. (C) 2001 Elsevier Science B.V. All rights reserved
Quantum Hall plateau transitions in disordered superconductors
We study a delocalization transition for non-interacting quasiparticles moving in two dimensions, which belongs to a new symmetry class. This symmetry class can be realised in a dirty, gapless superconductor in which time reversal symmetry for orbital motion is broken, but spin rotation symmetry is intact. We find a direct transition between two insulating phases with quantized Hall conductances of zero and two for the conserved quasiparticles. The energy of quasiparticles acts as a relevant, symmetry-breaking field at the critical point, which splits the direct transition into two conventional plateau transitions
Level statistics for quantum Hall systems
Level statistics for two classes of disordered systems at criticality are analyzed in terms of different
realizations of the Chalker–Coddington network model. These include: 1) Re-examination
of the standard U(1) model describing dynamics of electrons on the lowest Landau level in the
quantum Hall effect, where it is shown that after proper local unfolding the nearest-neighbor spacing
distribution (NNSD) at the critical energy follows the Wigner surmise for Gaussian unitary ensembles
(GUE). 2) Quasi-particles in disordered superconductors with broken time reversal and
spin rotation invariance (in the language of random matrix theory this system is a representative of
symmetry class D in the classification scheme of Altland and Zirnbauer). Here again the NNSD
obeys the Wigner surmise for GUE, reflecting therefore only «basic» discrete symmetries of the
system (time reversal violation) and ignoring particle–hole symmetries and other finer details
(criticality). In the localized regime level repulsion is suppressed