2,432 research outputs found
Superconductivity of Quasi-One-Dimensional Electrons in Strong Magnetic Field
The superconductivity of quasi-one-dimensional electrons in the magnetic
field is studied. The system is described as the one-dimensional electrons with
no frustration due to the magnetic field. The interaction is assumed to be
attractive between electrons in the nearest chains, which corresponds to the
lines of nodes of the energy gap in the absence of the magnetic field. The
effective interaction depends on the magnetic field and the transverse
momentum. As the magnetic field becomes strong, the transition temperature of
the spin-triplet superconductivity oscillates, while that of the spin-singlet
increases monotonically.Comment: 15 pages, RevTeX, 3 PostScript figures in uuencoded compressed tar
file are appende
Martin boundary of a reflected random walk on a half-space
The complete representation of the Martin compactification for reflected
random walks on a half-space is obtained. It is shown that the
full Martin compactification is in general not homeomorphic to the ``radial''
compactification obtained by Ney and Spitzer for the homogeneous random walks
in : convergence of a sequence of points to a
point of on the Martin boundary does not imply convergence of the sequence
on the unit sphere . Our approach relies on the large
deviation properties of the scaled processes and uses Pascal's method combined
with the ratio limit theorem. The existence of non-radial limits is related to
non-linear optimal large deviation trajectories.Comment: 42 pages, preprint, CNRS UMR 808
Dimensional crossover and metal-insulator transition in quasi-two-dimensional disordered conductors
We study the metal-insulator transition (MIT) in weakly coupled disordered
planes on the basis of a Non-Linear Sigma Model (NLM). Using two
different methods, a renormalization group (RG) approach and an auxiliary field
method, we calculate the crossover length between a 2D regime at small length
scales and a 3D regime at larger length scales. The 3D regime is described by
an anisotropic 3D NLM with renormalized coupling constants. We obtain
the critical value of the single particle interplane hopping which separates
the metallic and insulating phases. We also show that a strong parallel
magnetic field favors the localized phase and derive the phase diagram.Comment: 16 pages (RevTex), 4 poscript figure
Superconductivity of Quasi-One and Quasi-Two Dimensional Tight-Binding Electrons in Magnetic Field
The upper critical field of the tight-binding electrons in the
three-dimensional lattice is investigated.
The electrons make Cooper pairs between the eigenstates with the same energy
in the strong magnetic field. The transition lines in the quasi-one dimensional
case are shown to deviate from the previously obtained results where the
hopping matrix elements along the magnetic field are neglected. In the absence
of the Pauli pair breaking the transition temperature of the quasi-two
dimensional electrons is obtained to oscillationally increase as the magnetic
field becomes large and reaches to in the strong field as in the
quasi-one dimensional case.Comment: 4pages,4figures,to be published in J.Phys.Soc.Jp
Large Deviations Analysis for Distributed Algorithms in an Ergodic Markovian Environment
We provide a large deviations analysis of deadlock phenomena occurring in
distributed systems sharing common resources. In our model transition
probabilities of resource allocation and deallocation are time and space
dependent. The process is driven by an ergodic Markov chain and is reflected on
the boundary of the d-dimensional cube. In the large resource limit, we prove
Freidlin-Wentzell estimates, we study the asymptotic of the deadlock time and
we show that the quasi-potential is a viscosity solution of a Hamilton-Jacobi
equation with a Neumann boundary condition. We give a complete analysis of the
colliding 2-stacks problem and show an example where the system has a stable
attractor which is a limit cycle
A Renormalization group approach for highly anisotropic 2D Fermion systems: application to coupled Hubbard chains
I apply a two-step density-matrix renormalization group method to the
anisotropic two-dimensional Hubbard model. As a prelude to this study, I
compare the numerical results to the exact one for the tight-binding model. I
find a ground-state energy which agrees with the exact value up to four digits
for systems as large as . I then apply the method to the
interacting case. I find that for strong Hubbard interaction, the ground-state
is dominated by magnetic correlations.
These correlations are robust even in the presence of strong frustration.
Interchain pair tunneling is negligible in the singlet and triplet channels and
it is not enhanced by frustration. For weak Hubbard couplings, interchain
non-local singlet pair tunneling is enhanced and magnetic correlations are
strongly reduced. This suggests a possible superconductive ground state.Comment: 8 pages, 11 figures, expanded version of cond-mat/060856
Renyi generalizations of the conditional quantum mutual information
The conditional quantum mutual information of a tripartite state
is an information quantity which lies at the center of many
problems in quantum information theory. Three of its main properties are that
it is non-negative for any tripartite state, that it decreases under local
operations applied to systems and , and that it obeys the duality
relation for a four-party pure state on systems . The
conditional mutual information also underlies the squashed entanglement, an
entanglement measure that satisfies all of the axioms desired for an
entanglement measure. As such, it has been an open question to find R\'enyi
generalizations of the conditional mutual information, that would allow for a
deeper understanding of the original quantity and find applications beyond the
traditional memoryless setting of quantum information theory. The present paper
addresses this question, by defining different -R\'enyi generalizations
of the conditional mutual information, some of which we can
prove converge to the conditional mutual information in the limit
. Furthermore, we prove that many of these generalizations
satisfy non-negativity, duality, and monotonicity with respect to local
operations on one of the systems or (with it being left as an open
question to prove that monotoniticity holds with respect to local operations on
both systems). The quantities defined here should find applications in quantum
information theory and perhaps even in other areas of physics, but we leave
this for future work. We also state a conjecture regarding the monotonicity of
the R\'enyi conditional mutual informations defined here with respect to the
R\'enyi parameter . We prove that this conjecture is true in some
special cases and when is in a neighborhood of one.Comment: v6: 53 pages, final published versio
Large deviations for polling systems
Related INRIA Research report available at : http://hal.inria.fr/docs/00/07/27/62/PDF/RR-3892.pdfInternational audienceWe aim at presenting in short the technical report, which states a sample path large deviation principle for a resealed process n-1 Qnt, where Qt represents the joint number of clients at time t in a single server 1-limited polling system with Markovian routing. The main goal is to identify the rate function. A so-called empirical generator is introduced, which consists of Q t and of two empirical measures associated with S t the position of the server at time t. The analysis relies on a suitable change of measure and on a representation of fluid limits for polling systems. Finally, the rate function is solution of a meaningful convex program
Efficient Quantum Polar Coding
Polar coding, introduced 2008 by Arikan, is the first (very) efficiently
encodable and decodable coding scheme whose information transmission rate
provably achieves the Shannon bound for classical discrete memoryless channels
in the asymptotic limit of large block sizes. Here we study the use of polar
codes for the transmission of quantum information. Focusing on the case of
qubit Pauli channels and qubit erasure channels, we use classical polar codes
to construct a coding scheme which, using some pre-shared entanglement,
asymptotically achieves a net transmission rate equal to the coherent
information using efficient encoding and decoding operations and code
construction. Furthermore, for channels with sufficiently low noise level, we
demonstrate that the rate of preshared entanglement required is zero.Comment: v1: 15 pages, 4 figures. v2: 5+3 pages, 3 figures; argumentation
simplified and improve
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