151 research outputs found
Aharonov-Bohm effect in higher genus materials
Flux periodicity of conducting electrons on a closed surface with genus two
(double torus) are investigated theoretically. We examine flux
periodicity of the ground-state energy and of the wave functions as a function
of applied magnetic field. A fundamental flux period of the ground-state energy
is twice a fundamental unit of magnetic flux for uniformly applied magnetic
field, which is shown to be valid for a simple ladder geometry and carbon
double torus. Flux periodicity of the wave functions in a double torus is
complicate as compared with a simple torus (), and an adiabatic addition
of magnetic fluxes does not provide a good quantum number for the energy
eigenstates. The results are extended to higher genus materials and the
implications of the results are discussed.Comment: 4 pages, 6 figure
Dynamics of grain ejection by sphere impact on a granular bed
The dynamics of grain ejection consecutive to a sphere impacting a granular
material is investigated experimentally and the variations of the
characteristics of grain ejection with the control parameters are
quantitatively studied. The time evolution of the corona formed by the ejected
grains is reported, mainly in terms of its diameter and height, and favourably
compared with a simple ballistic model. A key characteristic of the granular
corona is that the angle formed by its edge with the horizontal granular
surface remains constant during the ejection process, which again can be
reproduced by the ballistic model. The number and the kinetic energy of the
ejected grains is evaluated and allows for the calculation of an effective
restitution coefficient characterizing the complex collision process between
the impacting sphere and the fine granular target. The effective restitution
coefficient is found to be constant when varying the control parameters.Comment: 9 page
Granular Avalanches in Fluids
Three regimes of granular avalanches in fluids are put in light depending on
the Stokes number St which prescribes the relative importance of grain inertia
and fluid viscous effects, and on the grain/fluid density ratio r. In gas (r >>
1 and St > 1, e.g., the dry case), the amplitude and time duration of
avalanches do not depend on any fluid effect. In liquids (r ~ 1), for
decreasing St, the amplitude decreases and the time duration increases,
exploring an inertial regime and a viscous regime. These regimes are described
by the analysis of the elementary motion of one grain
Multiple Current States of Two Phase-Coupled Superconducting Rings
The states of two phase-coupled superconducting rings have been investigated.
Multiple current states have been revealed in the dependence of the critical
current on the magnetic field. The performed calculations of the critical
currents and energy states in a magnetic field have made it possible to
interpret the experiment as the measurement of energy states into which the
system comes with different probabilities because of the equilibrium and
non-equilibrium noises upon the transition from the resistive state to the
superconducting state during the measurement of the critical currentComment: 5 pages, 5 figure
Evidence for directed percolation universality at the onset of spatiotemporal intermittency in coupled circle maps
We consider a lattice of coupled circle maps, a model arising naturally in
descriptions of solid state phenomena such as Josephson junction arrays. We
find that the onset of spatiotemporal intermittency (STI) in this system is
analogous to directed percolation (DP), with the transition being to an unique
absorbing state for low nonlinearities, and to weakly chaotic absorbing states
for high nonlinearities. We find that the complete set of static exponents and
spreading exponents at all critical points match those of DP very convincingly.
Further, hyperscaling relations are fulfilled, leading to independent controls
and consistency checks of the values of all the critical exponents. These
results lend strong support to the conjecture that the onset of STI in
deterministic models belongs to the DP universality class.Comment: Submitted to Physical Review
Magneto-transport in periodic and quasiperiodic arrays of mesoscopic rings
We study theoretically the transmission properties of serially connected
mesoscopic rings threaded by a magnetic flux. Within a tight-binding formalism
we derive exact analytical results for the transmission through periodic and
quasiperiodic Fibonacci arrays of rings of two different sizes. The role played
by the number of scatterers in each arm of the ring is analyzed in some detail.
The behavior of the transmission coefficient at a particular value of the
energy of the incident electron is studied as a function of the magnetic flux
(and vice versa) for both the periodic and quasiperiodic arrays of rings having
different number of atoms in the arms. We find interesting resonance properties
at specific values of the flux, as well as a power-law decay in the
transmission coefficient as the number of rings increases, when the magnetic
field is switched off. For the quasiperiodic Fibonacci sequence we discuss
various features of the transmission characteristics as functions of energy and
flux, including one special case where, at a special value of the energy and in
the absence of any magnetic field, the transmittivity changes periodically as a
function of the system size.Comment: 9 pages with 7 .eps figures included, submitted to PR
Regular dendritic patterns induced by non-local time-periodic forcing
The dynamic response of dendritic solidification to spatially homogeneous
time-periodic forcing has been studied. Phase-field calculations performed in
two dimensions (2D) and experiments on thin (quasi 2D) liquid crystal layers
show that the frequency of dendritic side-branching can be tuned by oscillatory
pressure or heating. The sensitivity of this phenomenon to the relevant
parameters, the frequency and amplitude of the modulation, the initial
undercooling and the anisotropies of the interfacial free energy and molecule
attachment kinetics, has been explored. It has been demonstrated that besides
the side-branching mode synchronous with external forcing as emerging from the
linear Wentzel-Kramers-Brillouin analysis, modes that oscillate with higher
harmonic frequencies are also present with perceptible amplitudes.Comment: 15 pages, 23 figures, Submitted to Phys. Rev.
Persistent currents in mesoscopic rings: A numerical and renormalization group study
The persistent current in a lattice model of a one-dimensional interacting
electron system is systematically studied using a complex version of the
density matrix renormalization group algorithm and the functional
renormalization group method. We mainly focus on the situation where a single
impurity is included in the ring penetrated by a magnetic flux. Due to the
interplay of the electron-electron interaction and the impurity the persistent
current in a system of N lattice sites vanishes faster then 1/N. Only for very
large systems and large impurities our results are consistent with the
bosonization prediction obtained for an effective field theory. The results
from the density matrix renormalization group and the functional
renormalization group agree well for interactions as large as the band width,
even though as an approximation in the latter method the flow of the
two-particle vertex is neglected. This confirms that the functional
renormalization group method is a very powerful tool to investigate correlated
electron systems. The method will become very useful for the theoretical
description of the electronic properties of small conducting ring molecules.Comment: 9 pages, 8 figures include
Wave patterns generated by an axisymmetric obstacle in a two-layer flow
Gravity waves generated by a moving obstacle in a two-layer stratified fluid are investigated. The experimental configuration is three-dimensional with an axisymmetric obstacle which is towed in one of the two layers. The experimental method used in the present study is based on a stereoscopic technique allowing the 3D reconstruction of the interface between the two layers. Investigation into the wave pattern as a function of the Froude number, Fr, based on the relative density of the fluid layers and the velocity of the towed obstacle is presented. Specific attention is paid to the transcritical regime for which Fr is close to one. Potential energy trapped in the wave field patterns is also extracted from the experimental results and is analyzed as a function of both the Froude number, Fr, and the transcritical similarity parameter Γ. In particular, a remarkable increase in the potential energy around Fr = 1 is observed and a scaling allowing to assemble data resulting from different experimental parameters is proposed
Breakdown of universality in transitions to spatiotemporal chaos
We show that the transition from laminar to active behavior in extended chaotic systems can vary from a continuous transition in the universality class of directed percolation with infinitely many absorbing states to what appears as a first-order transition. The latter occurs when finite lifetime nonchaotic structures, called "solitons," dominate the dynamics. We illustrate this scenario in an extension of the deterministic Chaté-Manneville coupled map lattice model and in a soliton including variant of the stochastic Domany-Kinzel cellular automaton
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