arXiv
(422.153 recursos)
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Mostrando recursos 1 - 20 de 87.621
1.
Drag of superfluid current in bilayer Bose systems - Fil, D. V.; Shevchenko, S. I.
An effect of nondissipative drag of a superfluid flow in a system of two Bose
gases confined in two parallel quasi two-dimensional traps is studied. Using an
approach based on introduction of density and phase operators we compute the
drag current at zero and finite temperatures for arbitrary ratio of densities
of the particles in the adjacent layers. We demonstrate that in a system of two
ring-shape traps the "drag force" influences on the drag trap in the same way
as an external magnetic flux influences on a superconducting ring. It allows to
use the drag effect to control persistent current states in superfluids and
opens a possibility...
2.
Bifurcation analysis in an associative memory model - Kawamura, Masaki; Tokunaga, Ryuji; Okada, Masato
We previously reported the chaos induced by the frustration of interaction in
a non-monotonic sequential associative memory model, and showed the chaotic
behaviors at absolute zero. We have now analyzed bifurcation in a stochastic
system, namely a finite-temperature model of the non-monotonic sequential
associative memory model. We derived order-parameter equations from the
stochastic microscopic equations. Two-parameter bifurcation diagrams obtained
from those equations show the coexistence of attractors, which do not appear at
absolute zero, and the disappearance of chaos due to the temperature effect.
3.
Cooperativity in sandpiles: statistics of bridge geometries - Mehta, Anita; Barker, G. C.; Luck, J. M.
Bridges form dynamically in granular media as a result of spatiotemporal
inhomogeneities. We classify bridges as linear and complex, and analyse their
geometrical characteristics. In particular, we find that the length
distribution of linear bridges is exponential. We then turn to the analysis of
the orientational distribution of linear bridges and find that, in three
dimensions, they are {\it vertically diffusive but horizontally
superdiffusive}; thus, when they exist, long linear bridges form `domes'. Our
results are in good accord with Monte Carlo simulations of bridge structure; we
make predictions for quantities that are experimentally accessible, and suggest
that bridges are very closely related to force chains.
4.
Nonlinear alternating current responses of dipolar fluids - Huang, J. P.; Yu, K. W.; Karttunen, Mikko
The frequency-dependent nonlinear dielectric increment of dipolar fluids in
nonpolar fluids is often measured by using a stationary relaxation method in
which two electric fields are used: The static direct current (DC) field of
high strength causing the dielectric nonlinearity, and the probing alternating
current (AC) field of low strength and high frequency. When a nonlinear
composite is subjected to a sinusoidal electric field, the electric response in
the composite will, in general, consist of AC fields at frequencies of
higher-order harmonics. Based on the Froehlich model, we present a theory to
investigate nonlinear AC responses of dipolar fluids containing both
polarizable monomers and dimers. In the case of...
5.
Circulating-current states and ring-exchange interactions in cuprates - Normand, B.; Oles, Andrzej M.
We consider the consequences for circulating-current states of a cyclic,
four-spin, ``ring-exchange'' interaction of the type shown recently to be
significant in cuprate systems. The real-space Hartree-Fock approach is used to
establish the existence of charge-current and spin-current phases in a
generalized Hubbard model for the CuO_2 planes in cuprates. We compare the
results of the Hartree-Fock approximation with the correlated states
renormalized by Gutzwiller projection factors which allows us to gauge the
qualitative effects of projection to no double site occupancy. We find that
charge flux states may be competitive in cuprates, whereas spin flux states are
suppressed in the strongly correlated regime. We then include the ring-exchange
interaction...
6.
Local density approximation for confined bosons in an optical lattice - Bergkvist, Sara; Henelius, Patrik; Rosengren, Anders
We investigate local and global properties of the one-dimensional
Bose-Hubbard model with an external confining potential, describing an atomic
condensate in an optical lattice. Using quantum Monte Carlo techniques we
demonstrate that a local-density approximation, which relates the unconfined
and the confined model, yields quantitatively correct results in most of the
interesting parameter range. We also examine claims of universal behavior in
the confined system, and demonstrate the origin of a previously calculated fine
structure in the experimentally accessible momentum distribution.
7.
Metamagnetic Transition in Na$_{0.85}$CoO$_2$ Single Crystals - Luo, J. L.; Wang, N. L.; Liu, G. T.; Wu, D.; Jing, X. N.; Hu, F.; Xiang, T.
We report the magnetization, specific heat and transport measurements of high
quality Na$_{0.85}$CoO$_2$ single crystals in applied magnetic fields up to
14T. In high temperatures, the system is in a paramagnetic phase. It undergoes
a magnetic phase transition below about 20K. When the field is applied along
the c-axis, the measurement data of magnetization, specific heat and
magnetoresistance reveal a metamagnetic transition from an antiferromagnetic
state to a quasi-ferromagnetic state at about 8T in low temperatures. However,
no transition is observed in the magnetization measurements up to 14T when the
field is applied perpendicular to the c-axis. The low temperature magnetic
phase diagram of Na$_{0.85}$CoO$_2$ is determined.
8.
Analysis of the phonon spectrum in the titanium oxyhalide TiOBr - Caimi, G.; Degiorgi, L.; Lemmens, P.; Chou, F. C.
We present the electrodynamic response of TiOBr, which undergoes a transition
to a spin-singlet ground state for temperatures below T$_{C1}=28$ K. The
temperature evolution of the phonon anomalies indicates, like in \cl, an
extended fluctuation regime, extending well above T$_{C1}$. At low frequencies,
the spectral weight is progressively suppressed by decreasing the temperature,
suggesting the formation of a spin-gap and a considerable electron-phonon
coupling. A comparison of the two oxyhalides shows a weaker interplane coupling
and reduced dimensionality for TiOBr that enhances fluctuations and shifts the
ordering temperatures to lower values. In the fluctuation regime the
temperature dependencies of some phonon parameters track the temperature
evolution of the magnetic susceptibility.
9.
Self-Focussing Dynamics in Monopolarly Charged Suspensions - Dammer, Stephan M.; Wolf, Dietrich E.
The Smoluchowski equation for irreversible aggregation in suspensions of
equally charged particles is studied. Accumulation of charges during the
aggregation process leads to a crossover from power law to sub-logarithmic
cluster growth at a characteristic time and cluster size. For larger times the
suspension is usually called stable, although aggregation still proceeds
slowly. In this regime the size distribution evolves towards a {\em universal}
scaling form, independent of {\em any} parameter included in the theory. The
relative width falls off to a universal value $\sigma_{\rm
r}^{\infty}{\approx}0.2017$ that is much smaller than in the uncharged case. We
conjecture that $\sigma_{\rm r}^{\infty}$ is a lower bound for the asymptotic
relative width for...
10.
Stochastic properties of systems controlled by autocatalytic reactions II - Pal, L.
We analyzed the stochastic behavior of systems controlled by autocatalytic
reaction A+X -> X+X, X+X -> A+X, X -> B provided that the distribution of
reacting particles in the system volume is uniform, i.e. the point model of
reaction kinetics introduced in arXiv:cond-mat/0404402 can be applied. Assuming
the number of substrate particles A to be kept constant by a suitable
reservoir, we derived the forward Kolmogorov equation for the probability of
finding n=0,1,... autocatalytic particles X in the system at a given time
moment. We have shown that the stochastic model results in an equation for the
mean value of autocatalytic particles X which differs strongly from the...
11.
Topological properties of the mean field phi^4 model - Andronico, A.; Angelani, L.; Ruocco, G.; Zamponi, F.
We study the thermodynamics and the properties of the stationary points
(saddles and minima) of the potential energy for a phi^4 mean field model. We
compare the critical energy Vc (i.e. the potential energy V(T) evaluated at the
phase transition temperature Tc) with the energy V{theta} at which the saddle
energy distribution show a discontinuity in its derivative. We find that, in
this model, Vc >> V{theta}, at variance to what has been found in the
literature for different mean field and short ranged systems. By direct
calculation of the energy Vs(T) of the ``inherent saddles'', i.e. the saddles
visited by the equilibrated system at temperature T, we...
12.
Equilibrium properties of highly asymmetric star-polymer mixtures - Mayer, Christian; Likos, Christos N.; Löwen, Hartmut
We employ effective interaction potentials to study the equilibrium structure
and phase behavior of highly asymmetric mixtures of star polymers. We consider
in particular the influence of the addition of a component with a small number
of arms and a small size on a concentrated solution of large stars with a high
functionality. By employing liquid integral equation theories we examine the
evolution of the correlation functions of the big stars upon addition of the
small ones, finding a loss of structure that can be attributed to a weakening
of the repulsions between the large stars due to the presence of the small
ones. We analyze this phenomenon...
13.
Propagation and Relaxation of Tension in Stiff Polymers - Hallatschek, Oskar; Frey, Erwin; Kroy, Klaus
We present a unified theory for the longitudinal dynamic response of a stiff
polymer in solution to various external perturbations (mechanical excitations,
hydrodynamic flows, electrical fields, temperature quenches ...) that can be
represented as sudden changes of ambient/boundary conditions. The theory relies
on a comprehensive analysis of the non--equilibrium propagation and relaxation
of backbone stresses in a wormlike chain. We recover and substantially extend
previous results based on heuristic arguments. Intriguing new experimental
implications are pointed out.
14.
Anomalous Hall Effect and Skyrmion Number in Real- and Momentum-space - Onoda, Masaru; Tatara, Gen; Nagaosa, Naoto
We study the anomalous Hall effect (AHE) for the double exchange model with
the exchange coupling $|J_H|$ being smaller than the bandwidth $|t|$ for the
purpose of clarifying the following unresolved and confusing issues: (i) the
effect of the underlying lattice structure, (ii) the relation between AHE and
the skyrmion number, (iii) the duality between real and momentum spaces, and
(iv) the role of the disorder scatterings; which is more essential, $\sigma_H$
(Hall conductivity) or $\rho_H$ (Hall resistivity)? Starting from a generic
expression for $\sigma_H$, we resolve all these issues and classify the regimes
in the parameter space of $J_H \tau$ ($\tau$: elastic-scattering time), and
$\lambda_{s}$ (length scale of...
15.
Polaronic excitations in CMR manganite films - Hartinger, Ch.; Mayr, F.; Loidl, A.; Kopp, T.
In the colossal magnetoresistance manganites polarons have been proposed as
the charge carrier state which localizes across the metal-insulator transition.
The character of the polarons is still under debate. We present an assessment
of measurements which identify polarons in the metallic state of
La{2/3}Sr{1/3}MnO{3} (LSMO) and La{2/3}Ca{1/3}MnO{3} (LCMO) thin films. We
focus on optical spectroscopy in these films which displays a pronounced
resonance in the mid-infrared. The temperature dependent resonance has been
previously assigned to polaron excitations. These polaronic resonances are
qualitatively distinct in LSMO and LCMO and we discuss large and small polaron
scenarios which have been proposed so far. There is evidence for a large
polaron excitation in...
16.
Topological spin current - Schmeltzer, D.
We present an exact derivation for non-commuting coordinates induced by the
SU(2) transformation used to diagonalize the spin-orbit hamiltonian in two
dimension.As a result an exact non-dissipative Hall current less sensitive to
disorder and complementary to the dissipative conductivity is found.
We compute the non-dissipative charge and spin-Hall conductance for the
spin-orbit problem.We find that the spin-Hall conductance is quantized in units
of $\frac{eg\mu_{B}}{h}$ .In the presence of a Zeeman interaction the
charge-Hall conductivity is proportional to the magnetic field. We propose an
experiment to test our theory.
17.
Statistical characterization of the forces on spheres in an upflow of air - Ojha, R. P.; Abate, A. R.; Durian, D. J.
The dynamics of a sphere fluidized in a nearly-levitating upflow of air were
previously found to be identical to those of a Brownian particle in a
two-dimensional harmonic trap, consistent with a Langevin equation [Ojha {\it
et al.}, Nature {\bf 427}, 521 (2004)]. The random forcing, the drag, and the
trapping potential represent different aspects of the interaction of the sphere
with the air flow. In this paper we vary the experimental conditions for a
single sphere, and report on how the force terms in the Langevin equation scale
with air flow speed, sphere radius, sphere density, and system size. We also
report on the effective interaction potential...
18.
Dynamics at the angle of repose: jamming, bistability, and collapse - Luck, J. M.; Mehta, Anita
When a sandpile relaxes under vibration, it is known that its measured angle
of repose is bistable in a range of values bounded by a material-dependent
maximal angle of stability; thus, at the same angle of repose, a sandpile can
be stationary or avalanching, depending on its history. In the nearly jammed
slow dynamical regime, sandpile collapse to a zero angle of repose can also
occur, as a rare event. We claim here that fluctuations of {\it dilatancy} (or
local density) are the key ingredient that can explain such varied phenomena.
In this work, we model the dynamics of the angle of repose and of the density
fluctuations,...
19.
Numerical Renormalization Group for Quantum Impurities in a Bosonic Bath - Bulla, Ralf; Lee, Hyun-Jung; Tong, Ning-Hua; Vojta, Matthias
We present a detailed description of the recently proposed numerical
renormalization group method for models of quantum impurities coupled to a
bosonic bath. Specifically, the method is applied to the spin-boson model, both
in the Ohmic and sub-Ohmic cases. We present various results for static as well
as dynamic quantities and discuss details of the numerical implementation,
e.g., the discretization of a bosonic bath with arbitrary continuous spectral
density, the suitable choice of a finite basis in the bosonic Hilbert space,
and questions of convergence w.r.t. truncation parameters. The method is shown
to provide high-accuracy data over the whole range of model parameters and
temperatures, which are in agreement...
20.
Damping of acoustic waves in dilute polymer solutions - Tsori, Yoav; de Gennes, Pierre-Gilles
The shear viscosity eta_s(w) of dilute polymer solutions (of flexible coils)
has been classically measured and interpreted in terms of the Zimm modes. We
point out that the longitudinal viscosity eta_l(w) is the sum of two components
eta_l=4*eta_s/3+eta_p, where eta_p is due to an effect of pressure on the coil
size. We discuss eta_p for a crude model involving a dependence of both the
Kuhn length a and the Flory parameter chi on pressure. We find that eta_p is
comparable to eta_s for good solvent conditions, but that eta_p should dominate
over eta_s near the theta point.
1.
