131 research outputs found
Charge-carrier transport properties of ultrathin Pb films
The charge-carrier transport properties of ultrathin metallic films are
analysed with ab-initio methods using the density functional theory (DFT) on
free-standing single crystalline slabs in the thickness range between 1 and 8
monolayers and compared with experiments for Pb films on Si(111). A strong
interplay between bandstructure, quantised in the direction normal to the
ultrathin film, charge-carrier scattering mechanisms and magnetoconduction was
found. Based on the bandstructure obtained from the DFT, we used standard
Boltzmann transport theory in two dimensions to obtain results for the
electronic transport properties of 2 to 8 monolayers thick Pb(111) slabs with
and without magnetic field. Comparison of calculations and experiment for the
thickness dependence of the dc conductivity shows that the dominant scattering
mechanism of electrons is diffuse elastic interface scattering for which the
assumption of identical scattering times for all subbands and directions, used
in this paper, is a good approximation. Within this model we can explain the
thickness dependences of the electric conductivity and of the Hall coefficient
as well as the anomalous behaviour of the first Pb layer.Comment: 7 pages incl. 9 figures, submitted to the European Physical Journal
Nonlinear elastic and electronic properties of Mo_6S_3I_6 nanowires
The properties of Mo_6S_3I_6 nanowires were investigated with ab initio
calculations based on the density-functional theory. The molecules build weakly
coupled one-dimensional chains with three sulfur atoms in the bridging planes
between the Mo octahedra, each dressed with six iodines. Upon uniaxial strain
along the wires, each bridging plane shows two energy minima, one in the ground
state with the calculated Young modulus Y=82 GPa, and one in the stretched
state with Y=94 GPa. Both values are at least four times smaller than the
experimental values and the origin of the discrepancy remains a puzzle. The
ideal tensile strength is about 8.4 GPa, the chains break in the Mo-Mo bonds
within the octahedra and not in the S bridges. The charge-carrier conductivity
is strongly anisotropic and the Mo_6S_3I_6 nanowires behave as
quasi-one-dimensional conductors in the whole range of investigated strains.
The conductivity is extremely sensitive to strain, making this material very
suitable for stain gauges. Very clean nanowires with good contacts may be
expected to behave as ballistic quantum wires over lengths of several m.
On the other hand, with high-impedance contacts they are good candidates for
the observation of Luttinger liquid behaviour. The pronounced 1D nature of the
Mo_6S_3I_6 nanowires makes them a uniquely versatile and user-friendly system
for the investigation of 1D physics.Comment: 7 pages, 8 figures include
Reconstructed Rough Growing Interfaces; Ridgeline Trapping of Domain Walls
We investigate whether surface reconstruction order exists in stationary
growing states, at all length scales or only below a crossover length, . The later would be similar to surface roughness in growing crystal
surfaces; below the equilibrium roughening temperature they evolve in a
layer-by-layer mode within a crossover length scale , but are always
rough at large length scales. We investigate this issue in the context of KPZ
type dynamics and a checker board type reconstruction, using the restricted
solid-on-solid model with negative mono-atomic step energies. This is a
topology where surface reconstruction order is compatible with surface
roughness and where a so-called reconstructed rough phase exists in
equilibrium. We find that during growth, reconstruction order is absent in the
thermodynamic limit, but exists below a crossover length , and that this local order fluctuates critically. Domain walls become
trapped at the ridge lines of the rough surface, and thus the reconstruction
order fluctuations are slaved to the KPZ dynamics
Magnetic versus nonmagnetic doping effects on the magnetic ordering in the Haldane chain compound PbNi2V2O8
A study of an impurity driven phase-transition into a magnetically ordered
state in the spin-liquid Haldane chain compound PbNi2V2O8 is presented. Both,
macroscopic magnetization as well as 51V nuclear magnetic resonance (NMR)
measurements reveal that the spin nature of dopants has a crucial role in
determining the stability of the induced long-range magnetic order. In the case
of nonmagnetic (Mg2+) doping on Ni2+ spin sites (S=1) a metamagnetic transition
is observed in relatively low magnetic fields. On the other hand, the magnetic
order in magnetically (Co2+) doped compounds survives at much higher magnetic
fields and temperatures, which is attributed to a significant anisotropic
impurity-host magnetic interaction. The NMR measurements confirm the predicted
staggered nature of impurity-liberated spin degrees of freedom, which are
responsible for the magnetic ordering. In addition, differences in the
broadening of the NMR spectra and the increase of nuclear spin-lattice
relaxation in doped samples, indicate a diverse nature of electron spin
correlations in magnetically and nonmagnetically doped samples, which begin
developing at rather high temperatures with respect to the antiferromagnetic
phase transition.Comment: 10 pages, 7 figure
Active beating of a reconstituted synthetic minimal axoneme
Propelling microorganisms through fluids and moving fluids along cellular surfaces are essential biological functions accomplished by long, thin structures called motile cilia and flagella, whose regular, oscillatory beating breaks the time-reversal symmetry required for transport. Although top-down experimental approaches and theoretical models have allowed us to broadly characterize such organelles and propose mechanisms underlying their complex dynamics, constructing minimal systems capable of mimicking ciliary beating and identifying the role of each component remains a challenge. Here we report the bottom-up assembly of a minimal synthetic axoneme, which we call a synthoneme, using biological building blocks from natural organisms, namely pairs of microtubules and cooperatively associated axonemal dynein motors. We show that upon provision of energy by ATP, microtubules undergo rhythmic bending by cyclic association-dissociation of dyneins. Our simple and unique beating minimal synthoneme represents a self-organized nanoscale biomolecular machine that can also help understand the mechanisms underlying ciliary beating
Anomalous thickness dependence of the Hall effect in ultrathin Pb layers on Si(111)
The magnetoconductive properties of ultrathin Pb films deposited on Si(111)
are measured and compared with density-functional electronic band-structure
calculations on two-dimensional, free-standing, 1 to 8 monolayers thick Pb(111)
slabs. A description with free-standing slabs is possible because it turned out
that the Hall coefficient is independent of the substrate and of the
crystalline order in the film. We show that the oscillations in sign of the
Hall coefficient observed as a function of film thickness can be explained
directly from the thickness dependent variations of the electronic
bandstructure at the Fermi energy.Comment: 4 pages incl. 3 figures, RevTeX, to appear in Phys. Rev.
Novel Phases and Finite-Size Scaling in Two-Species Asymmetric Diffusive Processes
We study a stochastic lattice gas of particles undergoing asymmetric
diffusion in two dimensions. Transitions between a low-density uniform phase
and high-density non-uniform phases characterized by localized or extended
structure are found. We develop a mean-field theory which relates
coarse-grained parameters to microscopic ones. Detailed predictions for
finite-size () scaling and density profiles agree excellently with
simulations. Unusual large- behavior of the transition point parallel to
that of self-organized sandpile models is found.Comment: 7 pages, plus 6 figures uuencoded, compressed and appended after
source code, LATeX, to be published as a Phys. Rev. Let
Phase Transition in the ABC Model
Recent studies have shown that one-dimensional driven systems can exhibit
phase separation even if the dynamics is governed by local rules. The ABC
model, which comprises three particle species that diffuse asymmetrically
around a ring, shows anomalous coarsening into a phase separated steady state.
In the limiting case in which the dynamics is symmetric and the parameter
describing the asymmetry tends to one, no phase separation occurs and the
steady state of the system is disordered. In the present work we consider the
weak asymmetry regime where is the system size and
study how the disordered state is approached. In the case of equal densities,
we find that the system exhibits a second order phase transition at some
nonzero .
The value of and the optimal profiles can be
obtained by writing the exact large deviation functional. For nonequal
densities, we write down mean field equations and analyze some of their
predictions.Comment: 18 pages, 3 figure
Effects of differential mobility on biased diffusion of two species
Using simulations and a simple mean-field theory, we investigate jamming
transitions in a two-species lattice gas under non-equilibrium steady-state
conditions. The two types of particles diffuse with different mobilities on a
square lattice, subject to an excluded volume constraint and biased in opposite
directions. Varying filling fraction, differential mobility, and drive, we map
out the phase diagram, identifying first order and continuous transitions
between a free-flowing disordered and a spatially inhomogeneous jammed phase.
Ordered structures are observed to drift, with a characteristic velocity, in
the direction of the more mobile species.Comment: 15 pages, 4 figure
Structure Factors and Their Distributions in Driven Two-Species Models
We study spatial correlations and structure factors in a three-state
stochastic lattice gas, consisting of holes and two oppositely ``charged''
species of particles, subject to an ``electric'' field at zero total charge.
The dynamics consists of two nearest-neighbor exchange processes, occuring on
different times scales, namely, particle-hole and particle-particle exchanges.
Using both, Langevin equations and Monte Carlo simulations, we study the
steady-state structure factors and correlation functions in the disordered
phase, where density profiles are homogeneous. In contrast to equilibrium
systems, the average structure factors here show a discontinuity singularity at
the origin. The associated spatial correlation functions exhibit intricate
crossovers between exponential decays and power laws of different kinds. The
full probability distributions of the structure factors are universal
asymmetric exponential distributions.Comment: RevTex, 18 pages, 4 postscript figures included, mistaken half-empty
page correcte
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