2,986 research outputs found
Replica Monte Carlo Simulation (Revisited)
In 1986, Swendsen and Wang proposed a replica Monte Carlo algorithm for spin
glasses [Phys. Rev. Lett. 57 (1986) 2607]. Two important ingredients are
present, (1) the use of a collection of systems (replicas) at different of
temperatures, but with the same random couplings, (2) defining and flipping
clusters. Exchange of information between the systems is facilitated by fixing
the tau spin (tau=sigma^1\sigma^2) and flipping the two neighboring systems
simultaneously. In this talk, we discuss this algorithm and its relationship to
replica exchange (also known as parallel tempering) and Houdayer's cluster
algorithm for spin glasses. We review some of the early results obtained using
this algorithm. We also present new results for the correlation times of
replica Monte Carlo dynamics in two and three dimensions and compare them with
replica exchange.Comment: For "Statistical Physics of Disordered Systems and Its Applications",
12-15 July 2004, Shonan Village Center, Hayama, Japan, 7 page
Nonequilibrium Green's function approach to mesoscopic thermal transport
We present a formulation of a nonequilibrium Green's function method for
thermal current in nanojunction atomic systems with nonlinear interactions.
This first-principle approach is applied to the calculation of the thermal
conductance in carbon nanotube junctions. It is shown that nonlinearity already
becomes important at low temperatures. Nonlinear interactions greatly suppress
phonon transmission at room temperature. The peak of thermal conductance is
found to be around 400K, in good agreement with experiments. High-order phonon
scattering processes are important for diffusive heat transport.Comment: 4 pages, 4 figure
Interfacial thermal transport in atomic junctions
We study ballistic interfacial thermal transport across atomic junctions.
Exact expressions for phonon transmission coefficients are derived for thermal
transport in one-junction and two-junction chains, and verified by numerical
calculation based on a nonequilibrium Green's function method. For a
single-junction case, we find that the phonon transmission coefficient
typically decreases monotonically with increasing freqency. However, in the
range between equal frequency spectrum and equal acoustic impedance, it
increases first then decreases, which explains why the Kapitza resistance
calculated from the acoustic mismatch model is far larger than the experimental
values at low temperatures. The junction thermal conductance reaches a maximum
when the interfacial coupling equals the harmonic average of the spring
constants of the two semi-infinite chains. For three-dimensional junctions, in
the weak coupling limit, we find that the conductance is proportional to the
square of the interfacial coupling, while for intermediate coupling strength
the conductance is approximately proportional to the interfacial coupling
strength. For two-junction chains, the transmission coefficient oscillates with
the frequency due to interference effects. The oscillations between the two
envelop lines can be understood analytically, thus providing guidelines in
designing phonon frequency filters.Comment: 10 pages, 13 figures. Accepted by Phys. Rev.
Semiquantum key distribution using entangled states
Recently, Boyer et al. presented a novel semiquantum key distribution
protocol [M. Boyer, D. Kenigsberg, and T. Mor, Phys. Rev. Lett. 99, 140501
(2007)], by using four quantum states, each of which is randomly prepared by Z
basis or X basis. Here we present a semiquantum key distribution protocol by
using entangled states in which quantum Alice shares a secret key with
classical Bob. We also show the protocol is secure against eavesdropping.Comment: 6 page
Non-Markovian finite-temperature two-time correlation functions of system operators: beyond the quantum regression theorem
An extremely useful evolution equation that allows systematically calculating
the two-time correlation functions (CF's) of system operators for non-Markovian
open (dissipative) quantum systems is derived. The derivation is based on
perturbative quantum master equation approach, so non-Markovian open quantum
system models that are not exactly solvable can use our derived evolution
equation to easily obtain their two-time CF's of system operators, valid to
second order in the system-environment interaction. Since the form and nature
of the Hamiltonian are not specified in our derived evolution equation, our
evolution equation is applicable for bosonic and/or fermionic environments and
can be applied to a wide range of system-environment models with any factorized
(separable) system-environment initial states (pure or mixed). When applied to
a general model of a system coupled to a finite-temperature bosonic environment
with a system coupling operator L in the system-environment interaction
Hamiltonian, the resultant evolution equation is valid for both L = L^+ and L
\neq L^+ cases, in contrast to those evolution equations valid only for L = L^+
case in the literature. The derived equation that generalizes the quantum
regression theorem (QRT) to the non-Markovian case will have broad applications
in many different branches of physics. We then give conditions on which the QRT
holds in the weak system-environment coupling case, and apply the derived
evolution equation to a problem of a two-level system (atom) coupled to a
finite-temperature bosonic environment (electromagnetic fields) with L \neq
L^+.Comment: To appear in the Journal of Chemical Physics (12 pages, 1 figure
Geometrical effects on spin injection: 3D spin drift diffusion model
We discuss a three-dimensional (3D) spin drift diffusion (SDD) model to
inject spin from a ferromagnet (FM) to a normal metal (N) or semiconductor
(SC). Using this model we investigate the problem of spin injection into
isotropic materials like GaAs and study the effect of FM contact area and SC
thickness on spin injection. We find that in order to achieve detectable spin
injection a small contact area or thick SC samples are essential for direct
contact spin injection devices. We investigate the use of thin metal films (Cu)
proposed by S.B. Kumar et al. and show that they are an excellent substitute
for tunnelling barriers (TB) in the regime of small contact area. Since most
tunnelling barriers are prone to pinhole defects, we study the effect of
pinholes in AlO tunnelling barriers and show that the reduction in the
spin-injection ratio () is solely due to the effective area of the
pinholes and there is no correlation between the number of pinholes and the
spin injection ratio.Comment: 5 pages, 6 figures. Accepted by JA
- …