173 research outputs found
Finite-size Effects in a Two-Dimensional Electron Gas with Rashba Spin-Orbit Interaction
Within the Kubo formalism, we estimate the spin-Hall conductivity in a
two-dimensional electron gas with Rashba spin-orbit interaction and study its
variation as a function of disorder strength and system size. The numerical
algorithm employed in the calculation is based on the direct numerical
integration of the time-dependent Schrodinger equation in a spin-dependent
variant of the particle source method. We find that the spin-precession length,
L_s controlled by the strength of the Rashba coupling, establishes the critical
lengthscale that marks the significant reduction of the spin-Hall conductivity
in bulk systems. In contrast, the electron mean free path, inversely
proportional to the strength of disorder, appears to have only a minor effect.Comment: 5 pages, 3 figure
Algorithm for Linear Response Functions at Finite Temperatures: Application to ESR spectrum of s=1/2 Antiferromagnet Cu benzoate
We introduce an efficient and numerically stable method for calculating
linear response functions of quantum systems at finite
temperatures. The method is a combination of numerical solution of the
time-dependent Schroedinger equation, random vector representation of trace,
and Chebyshev polynomial expansion of Boltzmann operator. This method should be
very useful for a wide range of strongly correlated quantum systems at finite
temperatures. We present an application to the ESR spectrum of s=1/2
antiferromagnet Cu benzoate.Comment: 4 pages, 4 figure
Solid helium at high pressure: A path-integral Monte Carlo simulation
Solid helium (3He and 4He) in the hcp and fcc phases has been studied by
path-integral Monte Carlo. Simulations were carried out in the
isothermal-isobaric (NPT) ensemble at pressures up to 52 GPa. This allows one
to study the temperature and pressure dependences of isotopic effects on the
crystal volume and vibrational energy in a wide parameter range. The obtained
equation of state at room temperature agrees with available experimental data.
The kinetic energy, E_k, of solid helium is found to be larger than the
vibrational potential energy, E_p. The ratio E_k/E_p amounts to about 1.4 at
low pressures, and decreases as the applied pressure is raised, converging to
1, as in a harmonic solid. Results of these simulations have been compared with
those yielded by previous path integral simulations in the NVT ensemble. The
validity range of earlier approximations is discussed.Comment: 7 pages, 5 figure
Large scale simulation of quantum-mechanical molecular dynamics for nano-polycrystalline diamond
Quantum-mechanical molecular-dynamics simulations are carried out to explore
possible precursor states of nano-polycrystalline diamond, a novel ultra-hard
material produced directly from graphite. Large-scale simulation with 10^5
atoms is realized by using the ' order-N' simulation code 'ELSES'
(http://www.elses.jp). The simulation starts with a diamond structure that
contains initial structural defects and results in a mixture of
graphite(sp^2)-like and diamond(sp^3)-like regions as nano-meter-scale domains.
We speculate that the domains are metastable and are possible candidates of the
precursor structures.Comment: 4 pages 2 figures. A PDF file in better graphics is available at
http://www.elses.jp
Quantum Dynamics of Spin Wave Propagation Through Domain Walls
Through numerical solution of the time-dependent Schrodinger equation, we
demonstrate that magnetic chains with uniaxial anisotropy support stable
structures, separating ferromagnetic domains of opposite magnetization. These
structures, domain walls in a quantum system, are shown to remain stable if
they interact with a spin wave. We find that a domain wall transmits the
longitudinal component of the spin excitations only. Our results suggests that
continuous, classical spin models described by LLG equation cannot be used to
describe spin wave-domain wall interaction in microscopic magnetic systems
Truncated correlations in video microscopy of colloidal solids
Studies by video microscopy on fluctuating colloids measure the real-space
cross-correlations in particle motion. This set of correlations is then treated
as a matrix, in order to study the spectrum and mode structure. We show that in
general the modes are modified by the truncation of the full real-space
correlations. We perform a theoretical analysis of the truncation, find the
boundary conditions imposed by the truncation, and propose practical windowing
strategies to eliminate artefacts. We study the problem from various
perspectives, to compile a survey for experimentalists.Comment: 11 pages, 9 figure
Fast algorithm for calculating two-photon absorption spectra
We report a numerical calculation of the two-photon absorption coefficient of
electrons in a binding potential using the real-time real-space higher-order
difference method. By introducing random vector averaging for the intermediate
state, the task of evaluating the two-dimensional time integral is reduced to
calculating two one-dimensional integrals. This allows the reduction of the
computation load down to the same order as that for the linear response
function. The relative advantage of the method compared to the straightforward
multi-dimensional time integration is greater for the calculation of non-linear
response functions of higher order at higher energy resolution.Comment: 4 pages, 2 figures. It will be published in Phys. Rev. E on 1, March,
199
Time-dependent properties of proton decay from crossing single-particle metastable states in deformed nuclei
A dynamical study of the decay of a metastable state by quantum tunneling
through an anisotropic, non separable, two-dimensional potential barrier is
performed by the numerical solution of the time-dependent Schrodinger equation.
Initial quasi- stationary proton states are chosen in the framework of a
deformed Woods-Saxon single-particle model. The decay of two sets of states
corresponding to true and quasi levels-crossing is studied and the evolution of
their decay properties as a function of nuclear deformation is calculated
around the crossing point. The results show that the investigation of the
proton decay from metastable states in deformed nuclei can unambiguously
distinguish between the two types of crossing and determine the structure of
the nuclear states involved.Comment: 15 pages, 9 figures, submitted to Phys. Rev.
Origin of the Canonical Ensemble: Thermalization with Decoherence
We solve the time-dependent Schrodinger equation for the combination of a
spin system interacting with a spin bath environment. In particular, we focus
on the time development of the reduced density matrix of the spin system. Under
normal circumstances we show that the environment drives the reduced density
matrix to a fully decoherent state, and furthermore the diagonal elements of
the reduced density matrix approach those expected for the system in the
canonical ensemble. We show one exception to the normal case is if the spin
system cannot exchange energy with the spin bath. Our demonstration does not
rely on time-averaging of observables nor does it assume that the coupling
between system and bath is weak. Our findings show that the canonical ensemble
is a state that may result from pure quantum dynamics, suggesting that quantum
mechanics may be regarded as the foundation of quantum statistical mechanics.Comment: 12 pages, 4 figures, accepted for publication by J. Phys. Soc. Jp
An efficient scheme for numerical simulations of the spin-bath decoherence
We demonstrate that the Chebyshev expansion method is a very efficient
numerical tool for studying spin-bath decoherence of quantum systems. We
consider two typical problems arising in studying decoherence of quantum
systems consisting of few coupled spins: (i) determining the pointer states of
the system, and (ii) determining the temporal decay of quantum oscillations. As
our results demonstrate, for determining the pointer states, the
Chebyshev-based scheme is at least a factor of 8 faster than existing
algorithms based on the Suzuki-Trotter decomposition. For the problems of
second type, the Chebyshev-based approach has been 3--4 times faster than the
Suzuki-Trotter-based schemes. This conclusion holds qualitatively for a wide
spectrum of systems, with different spin baths and different Hamiltonians.Comment: 8 pages (RevTeX), 3 EPS figure
- …