674 research outputs found
Electromagnetic wave propagation in spatially homogeneous yet smoothly time-varying dielectric media
We explore the propagation and transformation of electromagnetic waves
through spatially homogeneous yet smoothly time-dependent media within the
framework of classical electrodynamics. By modelling the smooth transition,
occurring during a finite period {\tau}, as a phenomenologically realistic and
sigmoidal change of the dielectric permittivity, an analytically exact solution
to Maxwell's equations is derived for the electric displacement in terms of
hypergeometric functions. Using this solution, we show the possibility of
amplification and attenuation of waves and associate this with the decrease and
increase of the time-dependent permittivity. We demonstrate, moreover, that
such an energy exchange between waves and non-stationary media leads to the
transformation (or conversion) of frequencies. Our results may pave the way
towards controllable light-matter interaction in time-varying structures.Comment: 5 figure
Towards a quantum Hall effect for atoms using electric fields
An atomic analogue of Landau quantization based on the Aharonov-Casher (AC)
interaction is developed. The effect provides a first step towards an atomic
quantum Hall system using electric fields, which may be realized in a
Bose-Einstein condensate
Steady-state crystallization of Rydberg excitations in an optically driven lattice gas
We study resonant optical excitations of atoms in a one-dimensional lattice
to the Rydberg states interacting via the van der Waals potential which
suppresses simultaneous excitation of neighboring atoms. Considering two- and
three-level excitation schemes, we analyze the dynamics and stationary state of
the continuously-driven, dissipative many-body system employing time-dependent
density-matrix renormalization group (t-DMRG) simulations. We show that
two-level atoms can exhibit only nearest neighbor correlations, while
three-level atoms under dark-state resonant driving can develop finite-range
crystalline order of Rydberg excitations. We present an approximate rate
equation model whose analytic solution yields qualitative understanding of the
numerical results.Comment: 5 pages,3 figure
Work fluctuation theorems for harmonic oscillators
The work fluctuations of an oscillator in contact with a thermostat and
driven out of equilibrium by an external force are studied experimentally and
theoretically within the context of Fluctuation Theorems (FTs). The oscillator
dynamics is modeled by a second order Langevin equation. Both the transient and
stationary state fluctuation theorems hold and the finite time corrections are
very different from those of a first order Langevin equation. The periodic
forcing of the oscillator is also studied; it presents new and unexpected short
time convergences. Analytical expressions are given in all cases
Experimental evidence of non-Gaussian fluctuations near a critical point.
submitted to PRLInternational audienceThe orientation fluctuations of the director of a liquid crystal are measured, by a sensitive polarization interferometer, close to the Fréedericksz transition, which is a second order transition driven by an electric field. We show that near the critical value of the field the spatially averaged order parameter has a generalized Gumbel distribution instead of a Gaussian one. The latter is recovered away from the critical point. The relevance of slow modes is pointed out. The parameter of generalized Gumbel is related to the effective number of degrees of freedom
Anomalous latent heat in non-equilibrium phase transitions
We study first-order phase transitions in a two-temperature system, where due
to the time-scale separation all the basic thermodynamical quantities (free
energy, entropy, etc) are well-defined. The sign of the latent heat is found to
be counterintuitive: it is positive when going from the phase where the
temperatures and the entropy are higher to the one where these quantities are
lower. The effect exists only out of equilibrium and requires conflicting
interactions. It is displayed on a lattice gas model of ferromagnetically
interacting spin-1/2 particles.Comment: 4 pages, 2 figure
Experimental study of work fluctuations in a harmonic oscillator
International audienceThe work fluctuations of a harmonic oscillator in contact with a thermostat and driven out of equilibrium by an external force are studied experimentally. For the work both the transient and stationary state fluctuation theorems hold. The finite time corrections are very different from those of a first order Langevin equation. The heat and work fluctuations are studied when a periodic forcing is applied to the oscillator. The importance of the choice of the ''good work'' to compute the free energy from the Jarzinsky equality is discussed
Classification of Singular Fibres on Rational Elliptic Surfaces in Characteristic Three
We determine and list all possible configurations of singular fibres on
rational elliptic surfaces in characteristic three. In total, we find that 267
distinct configurations exist. This result complements Miranda and Persson's
classification in characteristic zero, and Lang's classification in
characteristic two.Comment: 40 Pages. Minor typos correcte
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