221 research outputs found

### Correlation functions for time-dependent calculation of linear-response functions

We emphasize the importance of choosing an appropriate correlation function
to reduce numerical errors in calculating the linear-response function as a
Fourier transformation of a time-dependent correlation function. As an example
we take dielectric functions of silicon crystal calculated with a
time-dependent method proposed by Iitaka et al. [Phys. Rev. E 56, 1222 (1997)].Comment: to be published in Phys.Rev.E 01 Dec 1997, 2 pages, 4 figures, more
information at http://espero.riken.go.jp

### Enhancement of entanglement transfer in a spin chain by phase shift-control

We study the effect of a phase shift on the amount of transferrable two-spin
entanglement in a spin chain. We consider a ferromagnetic Heisenberg/XY spin
chain, both numerically and analytically, and two mechanisms to generate a
phase shift, the Aharonov-Casher effect and the Dzyaloshinskii-Moriya
interaction. In both cases, the maximum attainable entanglement is shown to be
significantly enhanced, suggesting its potential usefulness in quantum
information processing.Comment: 7 pages, 5 figures. v2: a fig added, the main text modified a bi

### Temperature dependence of ESR intensity for the nanoscale molecular magnet V15

The electron spin resonance (ESR) of nanoscale molecular magnet ${\rm
V}_{15}$ is studied. Since the Hamiltonian of ${\rm V}_{15}$ has a large
Hilbert space and numerical calculations of the ESR signal evaluating the Kubo
formula with exact diagonalization method is difficult, we implement the
formula with the help of the random vector technique and the Chebyshev
polynominal expansion, which we name the double Chebyshev expansion method. We
calculate the temperature dependence of the ESR intensity of ${\rm V}_{15}$ and
compare it with the data obtained in experiment. As another complementary
approach, we also implement the Kubo formula with the subspace iteration method
taking only important low-lying states into account. We study the ESR
absorption curve below $100{\rm K}$ by means of both methods. We find that side
peaks appear due to the Dzyaloshinsky-Moriya interaction and these peaks grows
as temperature decreases.Comment: 9 pages, 4 figures. To appear in J. Phys. Soc. Jpn. Supp

### Fast Algorithm for Finding the Eigenvalue Distribution of Very Large Matrices

A theoretical analysis is given of the equation of motion method, due to
Alben et al., to compute the eigenvalue distribution (density of states) of
very large matrices. The salient feature of this method is that for matrices of
the kind encountered in quantum physics the memory and CPU requirements of this
method scale linearly with the dimension of the matrix. We derive a rigorous
estimate of the statistical error, supporting earlier observations that the
computational efficiency of this approach increases with matrix size. We use
this method and an imaginary-time version of it to compute the energy and the
specific heat of three different, exactly solvable, spin-1/2 models and compare
with the exact results to study the dependence of the statistical errors on
sample and matrix size.Comment: 24 pages, 24 figure

### Calculating response functions in time domain with non-orthonormal basis sets

We extend the recently proposed order-N algorithms (cond-mat/9703224) for
calculating linear- and nonlinear-response functions in time domain to the
systems described by nonorthonormal basis sets.Comment: 4 pages, no figure

### 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

### Fast and stable method for simulating quantum electron dynamics

A fast and stable method is formulated to compute the time evolution of a
wavefunction by numerically solving the time-dependent Schr{\"o}dinger
equation. This method is a real space/real time evolution method implemented by
several computational techniques such as Suzuki's exponential product, Cayley's
form, the finite differential method and an operator named adhesive operator.
This method conserves the norm of the wavefunction, manages periodic conditions
and adaptive mesh refinement technique, and is suitable for vector- and
parallel-type supercomputers. Applying this method to some simple electron
dynamics, we confirmed the efficiency and accuracy of the method for simulating
fast time-dependent quantum phenomena.Comment: 10 pages, 35 eps 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 $\chi(\vec{q},\omega)$ 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

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