104 research outputs found
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
Conductance Distribution in Disordered Quantum Wires with a Perfectly Conducting Channel
We study the conductance of phase-coherent disordered quantum wires focusing
on the case in which the number of conducting channels is imbalanced between
two propagating directions. If the number of channels in one direction is by
one greater than that in the opposite direction, one perfectly conducting
channel without backscattering is stabilized regardless of wire length.
Consequently, the dimensionless conductance does not vanish but converges to
unity in the long-wire limit, indicating the absence of Anderson localization.
To observe the influence of a perfectly conducting channel, we numerically
obtain the distribution of conductance in both cases with and without a
perfectly conducting channel. We show that the characteristic form of the
distribution is notably modified in the presence of a perfectly conducting
channel.Comment: 7 pages, 16 figure
Correlation Exponent and Anomalously Localized States at the Critical Point of the Anderson Transition
We study the box-measure correlation function of quantum states at the
Anderson transition point with taking care of anomalously localized states
(ALS). By eliminating ALS from the ensemble of critical wavefunctions, we
confirm, for the first time, the scaling relation z(q)=d+2tau(q)-tau(2q) for a
wide range of q, where q is the order of box-measure moments and z(q) and
tau(q) are the correlation and the mass exponents, respectively. The influence
of ALS to the calculation of z(q) is also discussed.Comment: 6 pages, 3 figure
Non-chiral current algebras for deformed supergroup WZW models
We study deformed WZW models on supergroups with vanishing Killing form. The
deformation is generated by the isotropic current-current perturbation which is
exactly marginal under these assumptions. It breaks half of the global
isometries of the original supergroup. The current corresponding to the
remaining symmetry is conserved but its components are neither holomorphic nor
anti-holomorphic. We obtain the exact two- and three-point functions of this
current and a four-point function in the first two leading orders of a 1/k
expansion but to all orders in the deformation parameter. We further study the
operator product algebra of the currents, the equal time commutators and the
quantum equations of motion. The form of the equations of motion suggests the
existence of non-local charges which generate a Yangian. Possible applications
to string theory on Anti-de Sitter spaces and to condensed matter problems are
briefly discussed.Comment: 43 pages, Latex, one eps figure; v.2: minor corrections, a reference
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Quantum walks: a comprehensive review
Quantum walks, the quantum mechanical counterpart of classical random walks,
is an advanced tool for building quantum algorithms that has been recently
shown to constitute a universal model of quantum computation. Quantum walks is
now a solid field of research of quantum computation full of exciting open
problems for physicists, computer scientists, mathematicians and engineers.
In this paper we review theoretical advances on the foundations of both
discrete- and continuous-time quantum walks, together with the role that
randomness plays in quantum walks, the connections between the mathematical
models of coined discrete quantum walks and continuous quantum walks, the
quantumness of quantum walks, a summary of papers published on discrete quantum
walks and entanglement as well as a succinct review of experimental proposals
and realizations of discrete-time quantum walks. Furthermore, we have reviewed
several algorithms based on both discrete- and continuous-time quantum walks as
well as a most important result: the computational universality of both
continuous- and discrete- time quantum walks.Comment: Paper accepted for publication in Quantum Information Processing
Journa
p38 MAPK-Mediated Bmi-1 Down-Regulation and Defective Proliferation in ATM-Deficient Neural Stem Cells Can Be Restored by Akt Activation
A-T (ataxia telangiectasia) is a genetic disease caused by a mutation in the Atm (A-T mutated) gene that leads to neurodegeneration. Despite an increase in the numbers of studies in this area in recent years, the mechanisms underlying neurodegeneration in human A-T are still poorly understood. Previous studies demonstrated that neural stem cells (NSCs) isolated from the subventricular zone (SVZ) of Atm-/- mouse brains show defective self-renewal and proliferation, which is accompanied by activation of chronic p38 mitogen-activated protein kinase (MAPK) and a lower level of the polycomb protein Bmi-1. However, the mechanism underlying Bmi-1 down-regulation and its relevance to defective proliferation in Atm-/- NSCs remained unclear. Here, we show that over-expression of Bmi-1 increases self-renewal and proliferation of Atm-/- NSCs to normal, indicating that defective proliferation in Atm-/- NSCs is a consequence of down-regulation of Bmi-1. We also demonstrate that epidermal growth factor (EGF)-induced Akt phosphorylation renders Bmi-1 resistant to the proteasomal degradation, leading to its stabilization and accumulation in the nucleus. However, inhibition of the Akt-dependent Bmi-1 stabilizing process by p38 MAPK signaling reduces the levels of Bmi-1. Treatment of the Atm-/- NSCs with a specific p38 MAPK inhibitor SB203580 extended Bmi-1 posttranscriptional turnover and H2A ubiquitination in Atm-/- NSCs. Our observations demonstrate the molecular basis underlying the impairment of self-renewal and proliferation in Atm-/- NSCs through the p38 MAPK-Akt-Bmi-1-p21 signaling pathway
A Complex Cell Division Machinery Was Present in the Last Common Ancestor of Eukaryotes
Background: The midbody is a transient complex structure containing proteins involved in cytokinesis. Up to now, it has been described only in Metazoa. Other eukaryotes present a variety of structures implied in the last steps of cell division, such as the septum in fungi or the phragmoplast in plants. However, it is unclear whether these structures are homologous (derive from a common ancestral structure) or analogous (have distinct evolutionary origins). Recently, the proteome of the hamster midbody has been characterized and 160 proteins identified. Methodology/Principal Findings: Using phylogenomic approaches, we show here that nearly all of these 160 proteins (95%) are conserved across metazoan lineages. More surprisingly, we show that a large part of the mammalian midbody components (91 proteins) were already present in the last common ancestor of all eukaryotes (LECA) and were most likely involved in the construction of a complex multi-protein assemblage acting in cell division. Conclusions/Significance: Our results indicate that the midbodies of non-mammalian metazoa are likely very similar to the mammalian one and that the ancestor of Metazoa possessed a nearly modern midbody. Moreover, our analyses support the hypothesis that the midbody and the structures involved in cytokinesis in other eukaryotes derive from a large and complex structure present in LECA, likely involved in cytokinesis. This is an additional argument in favour of the idea of a comple
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