Search results for "statistical [methods]"
showing 10 items of 1664 documents
Inverse problem for the Landau-Zener effect
2002
We consider the inverse Landau-Zener problem which consists in finding the energy-sweep functions W(t)=E1(t)-E2(t) resulting in the required time dependences of the level populations for a two-level system crossing the resonance one or more times during the sweep. We find sweep functions of particular forms that let manipulate the system in a required way, including complete switching from the state 1 to the state 2 and preparing the system at the exact ground and excited states at resonance.
Nonlocal density correlations as a signature of Hawking radiation from acoustic black holes
2008
We have used the analogy between gravitational systems and nonhomogeneous fluid flows to calculate the density-density correlation function of an atomic Bose-Einstein condensate in the presence of an acoustic black hole. The emission of correlated pairs of phonons by Hawking-like process results into a peculiar long-range density correlation. Quantitative estimations of the effect are provided for realistic experimental configurations.
Dimensionality effects in restricted bosonic and fermionic systems
2000
The phenomenon of Bose-like condensation, the continuous change of the dimensionality of the particle distribution as a consequence of freezing out of one or more degrees of freedom in the low particle density limit, is investigated theoretically in the case of closed systems of massive bosons and fermions, described by general single-particle hamiltonians. This phenomenon is similar for both types of particles and, for some energy spectra, exhibits features specific to multiple-step Bose-Einstein condensation, for instance the appearance of maxima in the specific heat. In the case of fermions, as the particle density increases, another phenomenon is also observed. For certain types of sing…
Finite-temperature correlations in the one-dimensional trapped and untrapped Bose gases
2003
We calculate the dynamic single-particle and many-particle correlation functions at non-zero temperature in one-dimensional trapped repulsive Bose gases. The decay for increasing distance between the points of these correlation functions is governed by a scaling exponent that has a universal expression in terms of observed quantities. This expression is valid in the weak-interaction Gross-Pitaevskii as well as in the strong-interaction Girardeau-Tonks limit, but the observed quantities involved depend on the interaction strength. The confining trap introduces a weak center-of-mass dependence in the scaling exponent. We also conjecture results for the density-density correlation function.
Superradiance from crystals of molecular nanomagnets
2002
We show that crystals of molecular nanomagnets can exhibit giant magnetic relaxation due to the Dicke superradiance of electromagnetic waves. Rigorous treatment of the superradiance induced by a field pulse is presented.
Condensation of classical nonlinear waves
2005
We study the formation of a large-scale coherent structure (a condensate) in classical wave equations by considering the defocusing nonlinear Schr\"odinger equation as a representative model. We formulate a thermodynamic description of the condensation process by using a wave turbulence theory with ultraviolet cut-off. In 3 dimensions the equilibrium state undergoes a phase transition for sufficiently low energy density, while no transition occurs in 2 dimensions, in analogy with standard Bose-Einstein condensation in quantum systems. Numerical simulations show that the thermodynamic limit is reached for systems with $16^3$ computational modes and greater. On the basis of a modified wave tu…
Transmission of torque at the nanoscale
2018
In macroscopic mechanical devices torque is transmitted through gearwheels and clutches. In the construction of devices at the nanoscale, torque and its transmission through soft materials will be a key component. However, this regime is dominated by thermal fluctuations leading to dissipation. Here we demonstrate the principle of torque transmission for a disc-like colloidal assembly exhibiting clutch-like behaviour, driven by $27$ particles in optical traps. These are translated on a circular path to form a rotating boundary that transmits torque to additional particles confined to the interior. We investigate this transmission and find that it is determined by solid-like or fluid-like be…
Dynamic Self-assembly of Non-Brownian Spheres.
2017
International audience; Granular self-assembly of confined non-Brownian spheres under gravity is studied by Molecular Dynamics simulations. Starting from a disordered phase, dry or cohesive spheres organize, by vibrational an-nealing into BCT or FCC structures, respectively. During the self-assembling process, isothermal and isodense points are observed. The existence of such points indicates that both granular temperature and packing fraction undergo an inversion process. Around the isothermal point, a sudden growth of beads having the maximum coordination number takes place. We show by a density fluctuation analysis that a transition form a disordered phase to a crystalline structure may …
Location- and observation time-dependent quantum-tunneling
2009
We investigate quantum tunneling in a translation invariant chain of particles. The particles interact harmonically with their nearest neighbors, except for one bond, which is anharmonic. It is described by a symmetric double well potential. In the first step, we show how the anharmonic coordinate can be separated from the normal modes. This yields a Lagrangian which has been used to study quantum dissipation. Elimination of the normal modes leads to a nonlocal action of Caldeira-Leggett type. If the anharmonic bond defect is in the bulk, one arrives at Ohmic damping, i.e. there is a transition of a delocalized bond state to a localized one if the elastic constant exceeds a critical value $…
Dynamical heterogeneities in a supercooled Lennard-Jones liquid
1997
We present the results of a large scale molecular dynamics computer simulation study in which we investigate whether a supercooled Lennard-Jones liquid exhibits dynamical heterogeneities. We evaluate the non-Gaussian parameter for the self part of the van Hove correlation function and use it to identify ``mobile'' particles. We find that these particles form clusters whose size grows with decreasing temperature. We also find that the relaxation time of the mobile particles is significantly shorter than that of the bulk, and that this difference increases with decreasing temperature.