12,233 research outputs found
Universal Conductance Fluctuations in Mesoscopic Systems with Superconducting Leads: Beyond the Andreev Approximation
We report our investigation of the sample to sample fluctuation in transport
properties of phase coherent normal metal-superconductor hybrid systems.
Extensive numerical simulations were carried out for quasi-one dimensional and
two dimensional systems in both square lattice (Fermi electron) as well as
honeycomb lattice (Dirac electron). Our results show that when the Fermi energy
is within the superconducting energy gap , the Andreev conductance
fluctuation exhibits a universal value (UCF) which is approximately two times
larger than that in the normal systems. According to the random matrix theory,
the electron-hole degeneracy (ehD) in the Andreev reflections (AR) plays an
important role in classifying UCF. Our results confirm this. We found that in
the diffusive regime there are two UCF plateaus, one corresponds to the
complete electron-hole symmetry (with ehD) class and the other to conventional
electron-hole conversion (ehD broken). In addition, we have studied the Andreev
conductance distribution and found that for the fixed average conductance
the Andreev conductance distribution is a universal function that depends only
on the ehD. In the localized regime, our results show that ehD continues to
serve as an indicator for different universal classes. Finally, if normal
transport is present, i.e., Fermi energy is beyond energy gap , the AR
is suppressed drastically in the localized regime by the disorder and the ehD
becomes irrelevant. As a result, the conductance distribution is that same as
that of normal systems
Stabilization of Quantum Spin Hall Effect by Designed Removal of Time-Reversal Symmetry of Edge States
The quantum spin Hall (QSH) effect is known to be unstable to perturbations
violating time-reversal symmetry. We show that creating a narrow ferromagnetic
(FM) region near the edge of a QSH sample can push one of the
counterpropagating edge states to the inner boundary of the FM region, and
leave the other at the outer boundary, without changing their spin
polarizations and propagation directions. Since the two edge states are
spatially separated into different "lanes", the QSH effect becomes robust
against symmetry-breaking perturbations.Comment: 5 pages, 4 figure
Baryon Fields with U_L(3) \times U_R(3) Chiral Symmetry: Axial Currents of Nucleons and Hyperons
We use the conventional F and D octet and decimet generator matrices to
reformulate chiral properties of local (non-derivative) and one-derivative
non-local fields of baryons consisting of three quarks with flavor SU(3)
symmetry that were expressed in SU(3) tensor form in Ref. [12]. We show
explicitly the chiral transformations of the [(6,3)\oplus(3,6)] chiral
multiplet in the "SU(3) particle basis", for the first time to our knowledge,
as well as those of the (3,\bar{3}) \oplus (\bar{3}, 3), (8,1) \oplus (1,8)
multiplets, which have been recorded before in Refs. [4,5]. We derive the
vector and axial-vector Noether currents, and show explicitly that their zeroth
(charge-like) components close the SU_L(3) \times SU_R(3) chiral algebra. We
use these results to study the effects of mixing of (three-quark) chiral
multiplets on the axial current matrix elements of hyperons and nucleons. We
show, in particular, that there is a strong correlation, indeed a definite
relation between the flavor-singlet (i.e. the zeroth), the isovector (the
third) and the eighth flavor component of the axial current, which is in decent
agreement with the measured ones.Comment: one typo correction, and accepted by PR
Multifractal detrending moving average cross-correlation analysis
There are a number of situations in which several signals are simultaneously
recorded in complex systems, which exhibit long-term power-law
cross-correlations. The multifractal detrended cross-correlation analysis
(MF-DCCA) approaches can be used to quantify such cross-correlations, such as
the MF-DCCA based on detrended fluctuation analysis (MF-X-DFA) method. We
develop in this work a class of MF-DCCA algorithms based on the detrending
moving average analysis, called MF-X-DMA. The performances of the MF-X-DMA
algorithms are compared with the MF-X-DFA method by extensive numerical
experiments on pairs of time series generated from bivariate fractional
Brownian motions, two-component autoregressive fractionally integrated moving
average processes and binomial measures, which have theoretical expressions of
the multifractal nature. In all cases, the scaling exponents extracted
from the MF-X-DMA and MF-X-DFA algorithms are very close to the theoretical
values. For bivariate fractional Brownian motions, the scaling exponent of the
cross-correlation is independent of the cross-correlation coefficient between
two time series and the MF-X-DFA and centered MF-X-DMA algorithms have
comparative performance, which outperform the forward and backward MF-X-DMA
algorithms. We apply these algorithms to the return time series of two stock
market indexes and to their volatilities. For the returns, the centered
MF-X-DMA algorithm gives the best estimates of since its
is closest to 0.5 as expected, and the MF-X-DFA algorithm has the
second best performance. For the volatilities, the forward and backward
MF-X-DMA algorithms give similar results, while the centered MF-X-DMA and the
MF-X-DFA algorithms fails to extract rational multifractal nature.Comment: 15 pages, 4 figures, 2 matlab codes for MF-X-DMA and MF-X-DF
Implementation of quantum gates based on geometric phases accumulated in the eigenstates of periodic invariant operators
We propose a new strategy to physically implement a universal set of quantum
gates based on geometric phases accumulated in the nondegenerate eigenstates of
a designated invariant operator in a periodic physical system. The system is
driven to evolve in such a way that the dynamical phase shifts of the invariant
operator eigenstates are the same (or {\it mod} ) while the corresponding
geometric phases are nontrivial.
We illustrate how this strategy to work in a simple but typical NMR-type
qubit system.Comment: 4 page
A full parametrization of the 6 X 6 flavor mixing matrix in the presence of three light or heavy sterile neutrinos
In addition to three active neutrinos, one or more light sterile neutrinos
have been conjectured to account for the LSND, MiniBooNE and reactor
antineutrino anomalies (at the sub-eV mass scale) or for warm dark matter in
the Universe (at the keV mass scale). Heavy Majorana neutrinos at or above the
TeV scale have also been assumed in some seesaw models. Such hypothetical
particles can weakly mix with active neutrinos, and thus their existence can be
detected at low energies. In the (3+3) scenario with three sterile neutrinos we
present a full parametrization of the 6 X 6 flavor mixing matrix in terms of
fifteen rotation angles and fifteen phase angles. We show that this standard
parametrization allows us to clearly describe the salient features of some
problems in neutrino phenomenology, such as (a) possible contributions of light
sterile neutrinos to the tritium beta decay and neutrinoless double-beta decay;
(b) leptonic CP violation and deformed unitarity triangles of the 3 X 3 flavor
mixing matrix of three active neutrinos; (c) a reconstruction of the 6 X 6
neutrino mass matrix in the type-(I+II) seesaw mechanism; and (d) flavored and
unflavored leptogenesis scenarios in the type-I seesaw mechanism with three
heavy Majorana neutrinos.Comment: RevTex 18 pages, 1 figure. Minor changes, references updated.
Accepted for publication in Phys. Rev.
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