2,266 research outputs found
Size distribution of sputtered particles from Au nanoislands due to MeV self-ion bombardment
Nanoisland gold films, deposited by vacuum evaporation of gold onto Si(100)
substrates, were irradiated with 1.5 MeV Au ions up to a fluence of
ions cm and at incidence angles up to
with respect to the surface normal. The sputtered particles were collected on
carbon coated grids (catcher grid) during ion irradiation and were analyzed
with transmission electron microscopy and Rutherford backscattering
spectrometry. The average sputtered particle size and the areal coverage are
determined from transmission electron microscopy measurements, whereas the
amount of gold on the substrate is found by Rutherford backscattering
spectrometry. The size distributions of larger particles (number of
atoms/particle, 1,000) show an inverse power-law with an exponent of
-1 in broad agreement with a molecular dynamics simulation of ion impact
on cluster targets.Comment: 13 pages, 8 figures, Submitted for publication in JA
Escape Times in Fluctuating Metastable Potential and Acceleration of Diffusion in Periodic Fluctuating Potentials
The problems of escape from metastable state in randomly flipping potential
and of diffusion in fast fluctuating periodic potentials are considered. For
the overdamped Brownian particle moving in a piecewise linear dichotomously
fluctuating metastable potential we obtain the mean first-passage time (MFPT)
as a function of the potential parameters, the noise intensity and the mean
rate of switchings of the dichotomous noise. We find noise enhanced stability
(NES) phenomenon in the system investigated and the parameter region of the
fluctuating potential where the effect can be observed. For the diffusion of
the overdamped Brownian particle in a fast fluctuating symmetric periodic
potential we obtain that the effective diffusion coefficient depends on the
mean first-passage time, as discovered for fixed periodic potential. The
effective diffusion coefficients for sawtooth, sinusoidal and piecewise
parabolic potentials are calculated in closed analytical form.Comment: 10 pages, 2 figures. In press in Physica A, 2004. In press in Physica
A, 200
Super-shell structure in harmonically trapped fermionic gases and its semi-classical interpretation
It was recently shown in self-consistent Hartree-Fock calculations that a
harmonically trapped dilute gas of fermionic atoms with a repulsive two-body
interaction exhibits a pronounced {\it super-shell} structure: the shell
fillings due to the spherical harmonic trapping potential are modulated by a
beat mode. This changes the ``magic numbers'' occurring between the beat nodes
by half a period. The length and amplitude of the beating mode depends on the
strength of the interaction. We give a qualitative interpretation of the beat
structure in terms of a semiclassical trace formula that uniformly describes
the symmetry breaking U(3) SO(3) in a 3D harmonic oscillator potential
perturbed by an anharmonic term with arbitrary strength. We show
that at low Fermi energies (or particle numbers), the beating gross-shell
structure of this system is dominated solely by the two-fold degenerate
circular and (diametrically) pendulating orbits.Comment: Final version of procedings for the 'Nilsson conference
Thermal ratchet effects in ferrofluids
Rotational Brownian motion of colloidal magnetic particles in ferrofluids
under the influence of an oscillating external magnetic field is investigated.
It is shown that for a suitable time dependence of the magnetic field, a noise
induced rotation of the ferromagnetic particles due to rectification of thermal
fluctuations takes place. Via viscous coupling, the associated angular momentum
is transferred from the magnetic nano-particles to the carrier liquid and can
then be measured as macroscopic torque on the fluid sample. A thorough
theoretical analysis of the effect in terms of symmetry considerations,
analytical approximations, and numerical solutions is given which is in
accordance with recent experimental findings.Comment: 18 pages, 6 figure
Characteristics of the polymer transport in ratchet systems
Molecules with complex internal structure in time-dependent periodic
potentials are studied by using short Rubinstein-Duke model polymers as an
example. We extend our earlier work on transport in stochastically varying
potentials to cover also deterministic potential switching mechanisms,
energetic efficiency and non-uniform charge distributions. We also use currents
in the non-equilibrium steady state to identify the dominating mechanisms that
lead to polymer transportation and analyze the evolution of the macroscopic
state (e.g., total and head-to-head lengths) of the polymers. Several numerical
methods are used to solve the master equations and nonlinear optimization
problems. The dominating transport mechanisms are found via graph optimization
methods. The results show that small changes in the molecule structure and the
environment variables can lead to large increases of the drift. The drift and
the coherence can be amplified by using deterministic flashing potentials and
customized polymer charge distributions. Identifying the dominating transport
mechanism by graph analysis tools is found to give insight in how the molecule
is transported by the ratchet effect.Comment: 35 pages, 17 figures, to appear in Phys. Rev.
Transport and interaction blockade of cold bosonic atoms in a triple-well potential
We theoretically investigate the transport properties of cold bosonic atoms
in a quasi one-dimensional triple-well potential that consists of two large
outer wells, which act as microscopic source and drain reservoirs, and a small
inner well, which represents a quantum-dot-like scattering region. Bias and
gate "voltages" introduce a time-dependent tilt of the triple-well
configuration, and are used to shift the energetic level of the inner well with
respect to the outer ones. By means of exact diagonalization considering a
total number of six atoms in the triple-well potential, we find diamond-like
structures for the occurrence of single-atom transport in the parameter space
spanned by the bias and gate voltages. We discuss the analogy with Coulomb
blockade in electronic quantum dots, and point out how one can infer the
interaction energy in the central well from the distance between the diamonds.Comment: 18 pages, 6 figure
Finite sampling effects on generalized fluctuation-dissipation relations for steady states
We study the effects of the finite number of experimental data on the
computation of a generalized fluctuation-dissipation relation around a
nonequilibrium steady state of a Brownian particle in a toroidal optical trap.
We show that the finite sampling has two different effects, which can give rise
to a poor estimate of the linear response function. The first concerns the
accessibility of the generalized fluctuation-dissipation relation due to the
finite number of actual perturbations imposed to the control parameter. The
second concerns the propagation of the error made at the initial sampling of
the external perturbation of the system. This can be highly enhanced by
introducing an estimator which corrects the error of the initial sampled
condition. When these two effects are taken into account in the data analysis,
the generalized fluctuation-dissipation relation is verified experimentally
Brownian motion exhibiting absolute negative mobility
We consider a single Brownian particle in a spatially symmetric, periodic
system far from thermal equilibrium. This setup can be readily realized
experimentally. Upon application of an external static force F, the average
particle velocity is negative for F>0 and positive for F<0 (absolute negative
mobility).Comment: 4 pages, 3 figures, to be published in PR
Spectroscopy and dynamics of unoccupied electronic states of the topological insulators Sb2Te3 and Sb2Te2S
Time- and angle-resolved two-photon photoemission (2PPE) was used to study the electronic structure and ultrafast electron dynamics of the p-doped topological insulator Sb2Te3 and its derivative Sb2Te2S. Our 2PPE experiments directly reveal that the massless Dirac-cone like energy dispersion of topological surface states is realized above the Fermi energy in both materials. The observed bulk conduction bands of Sb2Te2S are found to be shifted to higher energies as compared to Sb2Te3. This shift has, however, surprisingly almost no influence on the electron dynamics in the topological surface state, which proceed on a picosecond time scale.We acknowledge funding by the Deutsche Forschungsgemeinschaft through SPP1666.Peer Reviewe
Current-spin-density functional study of persistent currents in quantum rings
We present a numerical study of persistent currents in quantum rings using
current spin density functional theory (CSDFT). This formalism allows for a
systematic study of the joint effects of both spin, interactions and impurities
for realistic systems. It is illustrated that CSDFT is suitable for describing
the physical effects related to Aharonov-Bohm phases by comparing energy
spectra of impurity-free rings to existing exact diagonalization and
experimental results. Further, we examine the effects of a symmetry-breaking
impurity potential on the density and current characteristics of the system and
propose that narrowing the confining potential at fixed impurity potential will
suppress the persistent current in a characteristic way.Comment: 7 pages REVTeX, including 8 postscript figure
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