12 research outputs found
Transport between edge states in multilayer integer quantum Hall systems: exact treatment of Coulomb interactions and disorder
A set of stacked two-dimensional electron systems in a perpendicular magnetic
field exhibits a three-dimensional version of the quantum Hall effect if
interlayer tunneling is not too strong. When such a sample is in a quantum Hall
plateau, the edge states of each layer combine to form a chiral metal at the
sample surface. We study the interplay of interactions and disorder in
transport properties of the chiral metal, in the regime of weak interlayer
tunneling. Our starting point is a system without interlayer tunneling, in
which the only excitations are harmonic collective modes: surface
magnetoplasmons. Using bosonization and working perturbatively in the
interlayer tunneling amplitude, we express transport properties in terms of the
spectrum for these collective modes, treating electron-electron interactions
and impurity scattering exactly. We calculte the conductivity as a function of
temperature, finding that it increases with increasing temperature as observed
in recent experiments. We also calculate the autocorrelation function of
mesoscopic conductance fluctuations induced by changes in a magnetic field
component perpendicular to the sample surface, and its dependence on
temperature. We show that conductance fluctuations are characterised by a
dephasing length that varies inversely with temperature.Comment: 13 pages, 10 figures, minor changes made for publicatio
The fractional quantum Hall effect in infinite layer systems
Stacked two dimensional electron systems in transverse magnetic fields
exhibit three dimensional fractional quantum Hall phases. We analyze the
simplest such phases and find novel bulk properties, e.g., irrational braiding.
These phases host ``one and a half'' dimensional surface phases in which motion
in one direction is chiral. We offer a general analysis of conduction in the
latter by combining sum rule and renormalization group arguments, and find that
when interlayer tunneling is marginal or irrelevant they are chiral semi-metals
that conduct only at T > 0 or with disorder.Comment: RevTeX 3.0, 4p., 2 figs with epsf; reference to the detailed
companion paper cond-mat/0006506 adde
Metal-insulator transition in a multilayer system with a strong magnetic field
We study the Anderson localization in a weakly coupled multilayer system with
a strong magnetic field perpendicular to the layers. The phase diagram of 1/3
flux quanta per plaquette is obtained. The phase diagram shows that a
three-dimensional quantum Hall effect phase exists for a weak on-site disorder.
For intermediate disorder, the system has insulating and normal metallic phases
separated by a mobility edge. At an even larger disorder, all states are
localized and the system is an insulator. The critical exponent of the
localization length is found to be .Comment: Latex file, 3 figure
The transverse magnetoresistance of the two-dimensional chiral metal
We consider the two-dimensional chiral metal, which exists at the surface of
a layered, three-dimensional sample exhibiting the integer quantum Hall effect.
We calculate its magnetoresistance in response to a component of magnetic field
perpendicular to the sample surface, in the low temperature, but macroscopic,
regime where inelastic scattering may be neglected. The magnetoresistance is
positive, following a Drude form with a field scale,
, given by the transverse field strength at which
one quantum of flux, , passes through a rectangle with sides set by the
layer-spacing, , and the elastic mean free path, .
Experimental measurement of this magnetoresistance may therefore provide a
direct determination of the elastic mean free path in the chiral metal.Comment: submitted to Phys Rev
Dynamics and Critical Behaviour of the q-model
The -model, a random walk model rich in behaviour and applications, is
investigated. We introduce and motivate the -model via its application
proposed by Coppersmith {\em et al.} to the flow of stress through granular
matter at rest. For a special value of its parameters the -model has a
critical point that we analyse. To characterise the critical point we imagine
that a uniform load has been applied to the top of the granular medium and we
study the evolution with depth of fluctuations in the distribution of load.
Close to the critical point explicit calculation reveals that the evolution of
load exhibits scaling behaviour analogous to thermodynamic critical phenomena.
The critical behaviour is remarkably tractable: the harvest of analytic results
includes scaling functions that describe the evolution of the variance of the
load distribution close to the critical point and of the entire load
distribution right at the critical point, values of the associated critical
exponents, and determination of the upper critical dimension. These results are
of intrinsic interest as a tractable example of a random critical point. Of the
many applications of the q-model, the critical behaviour is particularly
relevant to network models of river basins, as we briefly discuss. Finally we
discuss circumstances under which quantum network models that describe the
surface electronic states of a quantum Hall multilayer can be mapped onto the
classical -model. For mesoscopic multilayers of finite circumference the
mapping fails; instead a mapping to a ferromagnetic supersymmetric spin chain
has proved fruitful. We discuss aspects of the superspin mapping and give a new
elementary derivation of it making use of operator rather than functional
methods.Comment: 34 pages, Revtex, typo correcte
Thermal activation between Landau levels in the organic superconductor -(BEDT-TTF)SFCHCFSO
We show that Shubnikov-de Haas oscillations in the interlayer resistivity of
the organic superconductor -(BEDT-TTF)SF
CHCFSO become very pronounced in magnetic fields ~60~T.
The conductivity minima exhibit thermally-activated behaviour that can be
explained simply by the presence of a Landau gap, with the
quasi-one-dimensional Fermi surface sheets contributing negligibly to the
conductivity. This observation, together with complete suppression of chemical
potential oscillations, is consistent with an incommensurate nesting
instability of the quasi-one-dimensional sheets.Comment: 6 pages, 4 figure
Are Directed Waves Multifractal?
Wave propagation is studied in a sufficiently anisotropic random medium that
backscattering along one direction can be neglected. A Fokker-Planck equation
is derived the solution to which would provide a complete statistical
description of such directed waves. The Fokker-Planck equation is mapped onto
an su(1,1) ferromagnet and its symmetries are identified. Using the symmetries
asymptotic wave function distributions are computed and used to show that
directed wave functions fill space uniformly and do not have multifractal
character.Comment: 5 pages. Submitted to Phys Rev Let
Quantum Hall Effect in Three Dimensional Layered Systems
Using a mapping of a layered three-dimensional system with significant
inter-layer tunneling onto a spin-Hamiltonian, the phase diagram in the strong
magnetic field limit is obtained in the semi-classical approximation. This
phase diagram, which exhibit a metallic phase for a finite range of energies
and magnetic fields, and the calculated associated critical exponent,
, agree excellently with existing numerical calculations. The
implication of this work for the quantum Hall effect in three dimensions is
discussed.Comment: 4 pages + 4 figure
Edge electron states for quasi-one-dimensional organic conductors in the magnetic-field-induced spin-density-wave phases
We develop a microscopic picture of the electron states localized at the
edges perpendicular to the chains in the Bechgaard salts in the quantum Hall
regime. In a magnetic-field-induced spin-density-wave state (FISDW)
characterized by an integer N, there exist N branches of chiral gapless edge
excitations. Localization length is much longer and velocity much lower for
these states than for the edge states parallel to the chains. We calculate the
contribution of these states to the specific heat and propose a time-of-flight
experiment to probe the propagating edge modes directly.Comment: 4 pages, 2 figures. V.2: Minor changes to the final version published
in PR
Critical State Behaviour in a Low Dimensional Metal Induced by Strong Magnetic Fields
We present the results of magnetotransport and magnetic torque measurements
on the alpha-(BEDT-TTF)2KHg(SCN)4 charge-transfer salt within the high magnetic
field phase, in magnetic fields extending to 33 T and temperatures as low as 27
mK. While the high magnetic field phase (at fields greater than ~ 23 T) is
expected, on theoretical grounds, to be either a modulated charge-density wave
phase or a charge/spin-density wave hybrid, the resistivity undergoes a
dramatic drop below ~ 3 K within the high magnetic field phase, falling in an
approximately exponential fashion at low temperatures, while the magnetic
torque exhibits pronounced hysteresis effects. This hysteresis, which occurs
over a broad range of fields, is both strongly temperature-dependent and has
several of the behavioural characteristics predicted by critical-state models
used to describe the pinning of vortices in type II superconductors in strong
magnetic fields. Thus, rather than exhibiting the usual behaviour expected for
a density wave ground state, both the transport and the magnetic properties of
alpha-(BEDT-TTF)2KHg(SCN)4, at high magnetic fields, closely resembles those of
a type II superconductor