106 research outputs found
Transport properties in network models with perfectly conducting channels
We study the transport properties of disordered electron systems that contain
perfectly conducting channels. Two quantum network models that belong to
different universality classes, unitary and symplectic, are simulated
numerically. The perfectly conducting channel in the unitary class can be
realized in zigzag graphene nano-ribbons and that in the symplectic class is
known to appear in metallic carbon nanotubes. The existence of a perfectly
conducting channel leads to novel conductance distribution functions and a
shortening of the conductance decay length.Comment: 4 pages, 6 figures, proceedings of LT2
Multifractality and Conformal Invariance at 2D Metal-Insulator Transition in the Spin-Orbit Symmetry Class
We study the multifractality (MF) of critical wave functions at boundaries
and corners at the metal-insulator transition (MIT) for noninteracting
electrons in the two-dimensional (2D) spin-orbit (symplectic) universality
class. We find that the MF exponents near a boundary are different from those
in the bulk. The exponents at a corner are found to be directly related to
those at a straight boundary through a relation arising from conformal
invariance. This provides direct numerical evidence for conformal invariance at
the 2D spin-orbit MIT. The presence of boundaries modifies the MF of the whole
sample even in the thermodynamic limit.Comment: 5 pages, 4 figure
Conductance distributions in disordered quantum spin-Hall systems
We study numerically the charge conductance distributions of disordered
quantum spin-Hall (QSH) systems using a quantum network model. We have found
that the conductance distribution at the metal-QSH insulator transition is
clearly different from that at the metal-ordinary insulator transition. Thus
the critical conductance distribution is sensitive not only to the boundary
condition but also to the presence of edge states in the adjacent insulating
phase. We have also calculated the point-contact conductance. Even when the
two-terminal conductance is approximately quantized, we find large fluctuations
in the point-contact conductance. Furthermore, we have found a semi-circular
relation between the average of the point-contact conductance and its
fluctuation.Comment: 9 pages, 17 figures, published versio
Boundary criticality and multifractality at the 2D spin quantum Hall transition
Multifractal scaling of critical wave functions at a disorder-driven
(Anderson) localization transition is modified near boundaries of a sample.
Here this effect is studied for the example of the spin quantum Hall plateau
transition using the supersymmetry technique for disorder averaging. Upon
mapping of the spin quantum Hall transition to the classical percolation
problem with reflecting boundaries, a number of multifractal exponents
governing wave function scaling near a boundary are obtained exactly. Moreover,
additional exact boundary scaling exponents of the localization problem are
extracted, and the problem is analyzed in other geometries.Comment: v2, 17 pages, 10 figures, published versio
Multifractality at the quantum Hall transition: Beyond the parabolic paradigm
We present an ultra-high-precision numerical study of the spectrum of
multifractal exponents characterizing anomalous scaling of wave
function moments at the quantum Hall transition. The result
reads , with and . The central finding is that the spectrum
is not exactly parabolic, . This rules out a class of theories of
Wess-Zumino-Witten type proposed recently as possible conformal field theories
of the quantum Hall critical point.Comment: 4 pages, 4 figure
Disorder-Induced Multiple Transition involving Z2 Topological Insulator
Effects of disorder on two-dimensional Z2 topological insulator are studied
numerically by the transfer matrix method. Based on the scaling analysis, the
phase diagram is derived for a model of HgTe quantum well as a function of
disorder strength and magnitude of the energy gap. In the presence of sz
non-conserving spin-orbit coupling, a finite metallic region is found that
partitions the two topologically distinct insulating phases. As disorder
increases, a narrow-gap topologically trivial insulator undergoes a series of
transitions; first to metal, second to topological insulator, third to metal,
and finally back to trivial insulator. We show that this multiple transition is
a consequence of two disorder effects; renormalization of the band gap, and
Anderson localization. The metallic region found in the scaling analysis
corresponds roughly to the region of finite density of states at the Fermi
level evaluated in the self-consistent Born approximation.Comment: 5 pages, 5 figure
Point-Contact Conductance in Asymmetric Chalker-Coddington Network Model
We study the transport properties of disordered two-dimensional electron
systems with a perfectly conducting channel. We introduce an asymmetric
Chalker-Coddington network model and numerically investigate the point-contact
conductance. We find that the behavior of the conductance in this model is
completely different from that in the symmetric model. Even in the limit of a
large distance between the contacts, we find a broad distribution of
conductance and a non-trivial power law dependence of the averaged conductance
on the system width. Our results are applicable to systems such as zigzag
graphene nano-ribbons where the numbers of left-going and right-going channels
are different.Comment: 6 pages, 11 figures, final versio
Boundary multifractality at the integer quantum Hall plateau transition: implications for the critical theory
We study multifractal spectra of critical wave functions at the integer
quantum Hall plateau transition using the Chalker-Coddington network model. Our
numerical results provide important new constraints which any critical theory
for the transition will have to satisfy. We find a non-parabolic multifractal
spectrum and we further determine the ratio of boundary to bulk multifractal
exponents. Our results rule out an exactly parabolic spectrum that has been the
centerpiece in a number of proposals for critical field theories of the
transition. In addition, we demonstrate analytically exact parabolicity of
related boundary spectra in the 2D chiral orthogonal `Gade-Wegner' symmetry
class.Comment: 4 pages, 3 figures, v2, published versio
Critical level statistics and anomalously localized states at the Anderson transition
We study the level-spacing distribution function at the Anderson
transition by paying attention to anomalously localized states (ALS) which
contribute to statistical properties at the critical point. It is found that
the distribution for level pairs of ALS coincides with that for pairs of
typical multifractal states. This implies that ALS do not affect the shape of
the critical level-spacing distribution function. We also show that the
insensitivity of to ALS is a consequence of multifractality in tail
structures of ALS.Comment: 8 pages, 5 figure
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