70 research outputs found
Mesoscopic fluctuations of nonlinear conductance of chaotic quantum dots
The nonlinear dc conductance of a two-terminal chaotic cavity is
investigated. The fluctuations of the conductance (anti)symmetric with respect
to magnetic flux inversion through multichannel cavities are found analytically
for arbitrary temperature, magnetic field, and interaction strength. For
few-channel dots the effect of dephasing is investigated numerically. A
comparison with recent experimental data is provided.Comment: 4 pages, 2 figures, v.2-notations correcte
Toward a theory of the integer quantum Hall transition: continuum limit of the Chalker-Coddington model
An N-channel generalization of the network model of Chalker and Coddington is
considered. The model for N = 1 is known to describe the critical behavior at
the plateau transition in systems exhibiting the integer quantum Hall effect.
Using a recently discovered equality of integrals, the network model is
transformed into a lattice field theory defined over Efetov's sigma model space
with unitary symmetry. The transformation is exact for all N, no saddle-point
approximation is made, and no massive modes have to be eliminated. The naive
continuum limit of the lattice theory is shown to be a supersymmetric version
of Pruisken's nonlinear sigma model with couplings sigma_xx = sigma_xy = N/2 at
the symmetric point. It follows that the model for N = 2, which describes a
spin degenerate Landau level and the random flux problem, is noncritical. On
the basis of symmetry considerations and inspection of the Hamiltonian limit, a
modified network model is formulated, which still lies in the quantum Hall
universality class. The prospects for deformation to a Yang-Baxter integrable
vertex model are briefly discussed.Comment: 25 pages, REVTEX, calculation of sigma_xx correcte
Duality, the Semi-Circle Law and Quantum Hall Bilayers
There is considerable experimental evidence for the existence in Quantum Hall
systems of an approximate emergent discrete symmetry, . The evidence consists of the robustness of the tests of a suite a
predictions concerning the transitions between the phases of the system as
magnetic fields and temperatures are varied, which follow from the existence of
the symmetry alone. These include the universality of and quantum numbers of
the fixed points which occur in these transitions; selection rules governing
which phases may be related by transitions; and the semi-circular trajectories
in the Ohmic-Hall conductivity plane which are followed during the transitions.
We explore the implications of this symmetry for Quantum Hall systems involving
{\it two} charge-carrying fluids, and so obtain predictions both for bilayer
systems and for single-layer systems for which the Landau levels have a spin
degeneracy. We obtain similarly striking predictions which include the novel
new phases which are seen in these systems, as well as a prediction for
semicircle trajectories which are traversed by specific combinations of the
bilayer conductivities as magnetic fields are varied at low temperatures.Comment: 12 pages, 8 figures; discussion of magnetic field dependence modified
and figures and references updated in v
Theory of quantum paraelectrics and the metaelectric transition
We present a microscopic model of the quantum paraelectric-ferroelectric
phase transition with a focus on the influence of coupled fluctuating phonon
modes. These may drive the continuous phase transition first order through a
metaelectric transition and furthermore stimulate the emergence of a textured
phase that preempts the transition. We discuss two further consequences of
fluctuations, firstly for the heat capacity, and secondly we show that the
inverse paraelectric susceptibility displays T^2 quantum critical behavior, and
can also adopt a characteristic minimum with temperature. Finally, we discuss
the observable consequences of our results.Comment: 5 pages, 2 figure
Spin-splitting in the quantum Hall effect of disordered GaAs layers with strong overlap of the spin subbands
With minima in the diagonal conductance G_{xx} and in the absolute value of
the derivative |dG_{xy}/dB| at the Hall conductance value G_{xy}=e^{2}/h,
spin-splitting is observed in the quantum Hall effect of heavily Si-doped GaAs
layers with low electron mobility 2000 cm^2/Vs in spite of the fact that the
spin-splitting is much smaller than the level broadening. Experimental results
can be explained in the frame of the scaling theory of the quantum Hall effect,
applied independently to each of the two spin subbands.Comment: 4 pages, 4 figure
The quantum paraelectric behavior of SrTiO_{3} revisited: relevance of the structural phase transition temperature
It has been known for a long time that the low temperature behavior shown by
the dielectric constant of quantum paraelectric can not be fitted
properly by Barrett's formula using a single zero point energy or saturation
temperature (). As it was originally shown [K. A. M\"{u}ller and H.
Burkard, Phys. Rev. B {\bf 19}, 3593 (1979)] a crossover between two different
saturation temperatures (=77.8K and =80K) at is
needed to explain the low and high temperature behavior of the dielectric
constant. However, the physical reason for the crossover between these two
particular values of the saturation temperature at is unknown. In
this work we show that the crossover between these two values of the saturation
temperature at can be taken as a direct consequence of (i) the
quantum distribution of frequencies associated
with the complete set of low-lying modes and (ii) the existence of a definite
maximum phonon frequency given by the structural transition critical
temperature .Comment: 8 pages, 3 figure
Theory of Anomalous Quantum Hall Effects in Graphene
Recent successes in manufacturing of atomically thin graphite samples
(graphene) have stimulated intense experimental and theoretical activity. The
key feature of graphene is the massless Dirac type of low-energy electron
excitations. This gives rise to a number of unusual physical properties of this
system distinguishing it from conventional two-dimensional metals. One of the
most remarkable properties of graphene is the anomalous quantum Hall effect. It
is extremely sensitive to the structure of the system; in particular, it
clearly distinguishes single- and double-layer samples. In spite of the
impressive experimental progress, the theory of quantum Hall effect in graphene
has not been established. This theory is a subject of the present paper. We
demonstrate that the Landau level structure by itself is not sufficient to
determine the form of the quantum Hall effect. The Hall quantization is due to
Anderson localization which, in graphene, is very peculiar and depends strongly
on the character of disorder. It is only a special symmetry of disorder that
may give rise to anomalous quantum Hall effects in graphene. We analyze the
symmetries of disordered single- and double-layer graphene in magnetic field
and identify the conditions for anomalous Hall quantization.Comment: 13 pages (article + supplementary material), 5 figure
Critical dynamics and effective exponents of magnets with extended impurities
We investigate the asymptotic and effective static and dynamic critical
behavior of (d=3)-dimensional magnets with quenched extended defects,
correlated in dimensions (which can be considered as the
dimensionality of the defects) and randomly distributed in the remaining
dimensions. The field-theoretical renormalization group
perturbative expansions being evaluated naively do not allow for the reliable
numerical data. We apply the Chisholm-Borel resummation technique to restore
convergence of the two-loop expansions and report the numerical values of the
asymptotic critical exponents for the model A dynamics. We discuss different
scenarios for static and dynamic effective critical behavior and give values
for corresponding non-universal exponents.Comment: 12 pages, 6 figure
Critical behavior of two-dimensional cubic and MN models in the five-loop renormalization-group approximation
The critical thermodynamics of the two-dimensional N-vector cubic and MN
models is studied within the field-theoretical renormalization-group (RG)
approach. The beta functions and critical exponents are calculated in the
five-loop approximation and the RG series obtained are resummed using the
Borel-Leroy transformation combined with the generalized Pad\'e approximant and
conformal mapping techniques. For the cubic model, the RG flows for various N
are investigated. For N=2 it is found that the continuous line of fixed points
running from the XY fixed point to the Ising one is well reproduced by the
resummed RG series and an account for the five-loop terms makes the lines of
zeros of both beta functions closer to each another. For the cubic model with
N\geq 3, the five-loop contributions are shown to shift the cubic fixed point,
given by the four-loop approximation, towards the Ising fixed point. This
confirms the idea that the existence of the cubic fixed point in two dimensions
under N>2 is an artifact of the perturbative analysis. For the quenched dilute
O(M) models ( models with N=0) the results are compatible with a stable
pure fixed point for M\geq1. For the MN model with M,N\geq2 all the
non-perturbative results are reproduced. In addition a new stable fixed point
is found for moderate values of M and N.Comment: 26 pages, 3 figure
Effect of Tilted Magnetic Field on the Anomalous H=0 Conducting Phase in High-Mobility Si MOSFETs
The suppression by a magnetic field of the anomalous H=0 conducting phase in
high-mobility silicon MOSFETs is independent of the angle between the field and
the plane of the 2D electron system. In the presence of a parallel field large
enough to fully quench the anomalous conducting phase, the behavior is similar
to that of disordered GaAs/AlGaAs heterostructures: the system is insulating in
zero (perpendicular) field and exhibits reentrant insulator-quantum Hall
effect-insulator transitions as a function of perpendicular field. The results
demonstrate that the suppression of the low-T phase is related only to the
electrons' spin.Comment: 4 pages, including 3 figures. We corrected several typos in the
figures and caption
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