凝聚态物理
We present experimental evidence and theoretical arguments showing that the time-evolution of freely decaying 2-d turbulence is governed by a {\it discrete} time scale invariance rather than a continuous time scale invariance. Physically,…
The properties of S = 1 anisotropic Heisenberg models with nondiagonal exchange between axial and planar spin components are investigated using Monte Carlo techniques. The quantum nature is taken into account in a semi-classical…
Charge dynamics of the two-dimensional Hubbard model is investigated. Lancz$\ddot{\rm o}$s-diagonalization results for the optical conductivity and the Drude weight of this model are presented. Near the Mott transition, large incoherence…
The effects of the Zeeman term on the de Haas-van Alphen oscillation is studied in the magnetic breakdown system. We find that the amplitude of the oscillation with the frequencies of $f_{\beta} + f_{\alpha}$ and $f_{\beta} + 2f_{\alpha}$…
A recent technique, proposed to alleviate the ``sign problem disease'', is discussed in details. As well known the ground state of a given Hamiltonian $H$ can be obtained by applying the imaginary time propagator $e^{-H \tau}$ to a given…
The basic theory of the linear and nonlinear ac susceptibilities of type-II superconductors is outlined for various geometries, e.g. circular and elliptic disks, rings, and strips of thin films or of thicker platelets in a perpendicular…
A thin superconductor platelet with elliptic shape in a perpendicular magnetic field is considered. Using a method originally applied to circular disks, we obtain an approximate analytic solution for the two-dimensional critical state of…
We use a diffusion Monte Carlo method to calculate the lowest energy state of a uniform gas of bosons interacting through different model potentials, both strictly repulsive and with an attractive well. We explicitly verify that at low…
We make the first steps towards a generic theory for energy spreading and quantum dissipation. The Wall formula for the calculation of friction in nuclear physics and the Drude formula for the calculation of conductivity in mesoscopic…
We investigate the structure and collective modes of a planar surface of a trapped Bose-Einstein condensed gas at zero temperature. In the long-wavelength limit we find a mode similar to the gravity wave on the surface of a fluid with the…
We study the dynamics of a magnetically trapped Bose-Einstein condensate in the presence of an external electromagnetic field coupling trapped, untrapped and antitrapped Zeeman sublevels. For large condensates an approximate analytical…
We apply the self-consistent harmonic approximation (SCHA) to study static and dynamic properties of the two-dimensional classical Heisenberg model with easy-axis anisotropy. The static properties obtained are magnetization and spin wave…
We present a Monte Carlo study of various universal amplitude ratios of the two dimensional q=4 Potts model. We simulated the model close to criticality in both phases taking care to keep the systematic errors, due to finite size effects…
In the 2-dimensional anisotropic Heisenberg model with XY-symmetry there are non-planar vortices which exhibit a localized structure of the z-components of the spins around the vortex center. We study how thermal noise induces a transition…
We study how thermal fluctuations affect the dynamics of vortices in the two-dimensional classical, ferromagnetic, anisotropic Heisenberg model depending on their additive or multiplicative character. Using a collective coordinate theory,…
We investigate the collective excitations of a single-species Bose gas at T=0 in a harmonic trap where the confinement undergoes some splitting along one spatial direction. We mostly consider onedimensional potentials consisting of two…
In the context of an effective model for doped antiferromagnets, whereby the charge carriers are treated as hard-core bosons, we demonstrate that the ground state energy close to half-filling is an even periodic function of the external…
A condensate with two internal states coupled by external electromagnetic radiation, is described by coupled Gross Pitaevskii equations, whose eigenstates are analogous to the dressed states of quantum optics. We solve for these eigenstates…
We study the consequences of random mass, random scalar potential and random vector potential on the line of clean fixed points between integer/fractional quantum Hall states and an insulator. This line of fixed points was first identified…
Simulations of a rotationally stirred condensate show that a regime of simple behaviour occurs in which a single vortex cycles in and out of the condensate. We present a simple quantitative model of this behaviour, which accurately…