2,660 research outputs found
Effect of electron and hole doping on the structure of C, Si, and S nanowires
We use ab initio density functional calculations to study the effect of
electron and hole doping on the equilibrium geometry and electronic structure
of C, Si, and S monatomic wires. Independent of doping, all these nanowires are
found to be metallic. In absence of doping, C wires are straight, whereas Si
and S wires display a zigzag structure. Besides two preferred bond angles of 60
deg and 120 deg in Si wires, we find an additional metastable bond angle of 90
deg in S wires. The equilibrium geometry and electronic structure of these
nanowires is shown to change drastically upon electron and hole doping.Comment: 5 pages including 5 figure
A new approach to dynamic finite-size scaling
In this work we have considered the Taylor series expansion of the dynamic
scaling relation of the magnetization with respect to small initial
magnetization values in order to study the dynamic scaling behaviour of 2- and
3-dimensional Ising models. We have used the literature values of the critical
exponents and of the new dynamic exponent to observe the dynamic
finite-size scaling behaviour of the time evolution of the magnetization during
early stages of the Monte Carlo simulation. For 3-dimensional Ising Model we
have also presented that this method opens the possibility of calculating
and separately. Our results show good agreement with the literature
values. Measurements done on lattices with different sizes seem to give very
good scaling.Comment: Latex file with six figures. Accepted for publication in IJM
Lattice simulations of real-time quantum fields
We investigate lattice simulations of scalar and nonabelian gauge fields in
Minkowski space-time. For SU(2) gauge-theory expectation values of link
variables in 3+1 dimensions are constructed by a stochastic process in an
additional (5th) ``Langevin-time''. A sufficiently small Langevin step size and
the use of a tilted real-time contour leads to converging results in general.
All fixed point solutions are shown to fulfil the infinite hierarchy of
Dyson-Schwinger identities, however, they are not unique without further
constraints. For the nonabelian gauge theory the thermal equilibrium fixed
point is only approached at intermediate Langevin-times. It becomes more stable
if the complex time path is deformed towards Euclidean space-time. We analyze
this behavior further using the real-time evolution of a quantum anharmonic
oscillator, which is alternatively solved by diagonalizing its Hamiltonian.
Without further optimization stochastic quantization can give accurate
descriptions if the real-time extend of the lattice is small on the scale of
the inverse temperature.Comment: 36 pages, 15 figures, Late
Dynamic SU(2) Lattice Gauge Theory at Finite Temperature
The dynamic relaxation process for the (2+1)--dimensional SU(2) lattice gauge
theory at critical temperature is investigated with Monte Carlo methods. The
critical initial increase of the Polyakov loop is observed. The dynamic
exponents and as well as the static critical exponent
are determined from the power law behaviour of the Polyakov loop, the
auto-correlation and the second moment at the early stage of the time
evolution. The results are well consistent and universal short-time scaling
behaviour of the dynamic system is confirmed. The values of the exponents show
that the dynamic SU(2) lattice gauge theory is in the same dynamic universality
class as the dynamic Ising model.Comment: 10 pages with 2 figure
Simulating nonequilibrium quantum fields with stochastic quantization techniques
We present lattice simulations of nonequilibrium quantum fields in
Minkowskian space-time. Starting from a non-thermal initial state, the
real-time quantum ensemble in 3+1 dimensions is constructed by a stochastic
process in an additional (5th) ``Langevin-time''. For the example of a
self-interacting scalar field we show how to resolve apparent unstable Langevin
dynamics, and compare our quantum results with those obtained in classical
field theory. Such a direct simulation method is crucial for our understanding
of collision experiments of heavy nuclei or other nonequilibrium phenomena in
strongly coupled quantum many-body systems.Comment: 4 pages, 4 figures, PRL version, minor change
Dynamics of ripple formation on silicon surfaces by ultrashort laser pulses in sub-ablation conditions
An investigation of ultrashort pulsed laser induced surface modification due
to conditions that result in a superheated melted liquid layer and material
evaporation are considered. To describe the surface modification occurring
after cooling and resolidification of the melted layer and understand the
underlying physical fundamental mechanisms, a unified model is presented to
account for crater and subwavelength ripple formation based on a synergy of
electron excitation and capillary waves solidification. The proposed
theoretical framework aims to address the laser-material interaction in
sub-ablation conditions and thus minimal mass removal in combination with a
hydrodynamics-based scenario of the crater creation and ripple formation
following surface irradiation with single and multiple pulses, respectively.
The development of the periodic structures is attributed to the interference of
the incident wave with a surface plasmon wave. Details of the surface
morphology attained are elaborated as a function of the imposed conditions and
results are tested against experimental data
Southward propagating auroral structure in meso-micro scale obtained from ground-based multiple observations at Poker Flat Research Range
第3回極域科学シンポジウム/第36回極域宙空圏シンポジウム 11月26日(月)、27日(火) 国立極地研究所 2階ラウン
Monte Carlo Simulation of the Short-time Behaviour of the Dynamic XY Model
Dynamic relaxation of the XY model quenched from a high temperature state to
the critical temperature or below is investigated with Monte Carlo methods.
When a non-zero initial magnetization is given, in the short-time regime of the
dynamic evolution the critical initial increase of the magnetization is
observed. The dynamic exponent is directly determined. The results
show that the exponent varies with respect to the temperature.
Furthermore, it is demonstrated that this initial increase of the magnetization
is universal, i.e. independent of the microscopic details of the initial
configurations and the algorithms.Comment: 14 pages with 5 figures in postscrip
Generalized Dynamic Scaling for Critical Magnetic Systems
The short-time behaviour of the critical dynamics for magnetic systems is
investigated with Monte Carlo methods. Without losing the generality, we
consider the relaxation process for the two dimensional Ising and Potts model
starting from an initial state with very high temperature and arbitrary
magnetization. We confirm the generalized scaling form and observe that the
critical characteristic functions of the initial magnetization for the Ising
and the Potts model are quite different.Comment: 32 pages with15 eps-figure
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