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At least 307 records · Page 17Linked to original sources

arXiv1998

The dynamics of component separation in a binary mixture of Bose-Einstein condensates

We present studies of the time-evolution of a two-component system of Bose-Einstein condensates (BEC) in the |F=1, mf=-1> and |2,1> spin states of Rb-87. The two condensates are created with a well-defined relative phase and complete spatial overlap. In subsequent evolution they undergo complex relative motions which tend to preserve the total density profile. the motions quickly damp out, leaving the condensates in a steady state with a nonnegligible (and adjustable) overlap region.

cond-mat↗

arXiv1998

Disordered XY models and Coulomb gases: renormalization via traveling waves

We present a novel RG approach to 2D random XY models using direct and replicated Coulomb gas methods. By including fusion of environments (charge fusion in the replicated CG) it follows the distribution of local disorder, found to obey a Kolmogorov non linear equation (KPP) with traveling wave solutions. At low T and weak disorder it yields a glassy XY phase with broad distributions and precise connections to Derrida's GREM. Finding marginal operators at the disorder-induced transition is related to the front velocity selection problem in KPP equations yielding new critical behaviour. The method is applied to critical random Dirac problems.

cond-mat↗

arXiv1998

Comment on ``Roughening Transition of Interfaces in Disordered Media''

Emig and Nattermann (Phys. Rev. Lett. 81, 1469 (1998)) have recently investigated the competition between lattice pinning and impurity pinning using a Renormalisation Group (RG) approach. For elastic objects of internal dimensions $2 < D < 4$, they find, at zero temperature, an interesting second order phase transition between a flat phase for small disorder and a rough phase for large disorder. These results contrast with those obtained using the replica variational approach for the same problem, where a first order transition between flat and rough phases was predicted. In this comment, we show that these results can be reconciled by analysing the RG flow for an arbitrary dimension $N$ for the displacement field.

cond-mat↗

arXiv1998

Thermodynamics of the one-dimensional SU(4) symmetric spin-orbital model

The ground state properties and the thermodynamics of the one-dimensional SU(4) symmetric spin system with orbital degeneracy are investigated using the quantum Monte Carlo loop algorithm. The spin-spin correlation functions exhibit a 4-site periodicity, and their low temperature behavior is controlled by two correlation lengths that diverge like the inverse temperature, while the entropy is linear in temperature and its slope is consistent with three gapless modes of velocity $π/2$. The physical implications of these results are discussed.

cond-mat↗

arXiv1999

Correlations in the Adiabatic Response of Chaotic Systems

Adiabatic variation of the parameters of a chaotic system results in a fluctuating reaction force. In the leading order in the adiabaticity parameter, a dissipative force, that is present in classical mechanics was found to vanish in quantum mechanics. On the time scale t, this force is proportional to I(t), the integral of the force-force correlation function over time t. In order to understand the crossover between the classical and the quantum mechanical behavior we calculated I(t) in random matrix theory. We found that for systems belonging to the Gaussian unitary ensemble this crossover takes place at a characteristic time (proportional to the Heisenberg time) and for longer times I(t) practically vanishes, resulting in vanishing dissipation. For systems belonging to the Gaussian orthogonal ensemble I(t) drops like 1/t and there is no such characteristic time. I(t) is calculated for various models and the relation to experiment is discussed.

chao-dyn↗

arXiv2001

High Pressure Insulator-Metal Transition in Molecular Fluid Oxygen

We report the first experimental evidence for a metallic phase in fluid molecular oxygen. Our electrical conductivity measurements of fluid oxygen under dynamic quasi-isentropic compression show that a non-metal/metal transition occurs at 3.4 fold compression, 4500 K and 1.2 Mbar. We discuss the main features of the electrical conductivity dependence on density and temperature and give an interpretation of the nature of the electrical transport mechanisms in fluid oxygen at these extreme conditions.

cond-mat↗

arXiv2001

New phasing method based on the principle of minimum charge

A new method of the phase determination in X-ray crystallography is proposed. The method is based on the so-called "minimum charge" principle, recently suggested by Elser. The electron density function $ρ$ is sought in the form $ρ({\bf x})=|ψ({\bf x})|^2$, where $ψ$ is an $n{-component}$ real function. The norm $\int|ψ({\bf x})|^2 d{\bf x}$ is minimized under the constraint imposed by the measured data on the amplitudes of Fourier harmonics of $ρ$. Compared to the straightforward implementation of the "minimum charge" scheme, the method attenuates the Gibbs phenomenon and is also capable of extrapolation of the diffraction data beyond the set of measured amplitudes. The method is applicable to quasicrystals under the condition that the number of components $n$ of the function $ψ$ is bigger than the dimensionality of the ``atomic surface''. It is successfully tested on synthetic data for Fibonacci chain and the octagonal tiling. In the latter case the reconstructed density map shows the shape of the atomic surface, despite relatively low data resolution.

cond-mat↗

arXiv1995

Evidence for charge-flux duality near the quantum Hall liquid to insulator transition

We examine the longitudinal, non-linear, current-voltage characteristics near the quantum Hall liquid to insulator transition and show that a simple mapping exists between the characteristics on the quantum Hall side and those on the insulating side of the transition. More precisely, at filling factors related by the law of corresponding states the current and voltage simply trade places. We interpret these observations as evidence for the existence, in the composite boson description, of charge-flux duality near disorder dominated transitions in quantum Hall systems. (Appearances notwithstanding, this is an experimental paper.)

cond-mat↗

arXiv1998

Critical Velocity in 3He-B Vibrating Wire Experiments as Analog of Vacuum Instability in a Slowly Oscillating Electric Field

The Lancaster experiments with a cylindrical wire moving in superfluid 3He-B are discussed, where the measured critical velocity of pair creation is much below the Landau critical velocity. The phenomenon is shown to be analogous to the instability of the electron-positron vacuum in an adiabatically alternating strong electric potential of both signs, where the positive- and negative-root levels cross and thus the instability treshold is twice less than in the conventional case of a single static potential well.

cond-mat↗

arXiv1995

Ginzburg-Landau Expansion and the Slope of the Upper Critical Field in Disordered Superconductors with Anisotropic Pairing

It is demonstrated that the slope of the upper critical field $|dH_{c2}/dT|_{T_{c}}$ in superconductors with $d$-wave pairing drops rather fast with concentration of normal impurities, while in superconductors with anisotropic $s$-wave pairing $|dH_{c2}/dT|_{T_{c}}$ grows, and in the limit of strong disorder is described by the known dependences of the theory of ``dirty'' superconductors. This allows to use the measurements of $H_{c2}$ in disordered superconductors to discriminate between these different types of pairing in high-temperature and heavy-fermion superconductors.

cond-mat↗

arXiv1993

Quantum Antiferromagnets in Two Dimensions

These are lectures presented at the summer course on ``Low Dimensional Quantum Field Theories for Condensed Matter Physicists'', 24 Aug. to 4 Sep. 1992, Trieste, Italy. I review recent work, performed in collaboration primarily with N. Read and Jinwu Ye, on the properties of quantum antiferromagnets in two dimensions. The emphasis is on the properties of the antiferromagnet in states which do not have any long-range magnetic order. The universal spin dynamics in the quantum critical region of number of frustrated and random antiferromagnets is studied; implications for neutron scattering experiments in the lightly-doped cuprates are noted. The nature of the quantum-disordered phase of non-random frustrated antiferromagnets is examined in some detail: the states found have ({\em i\/}) collinear spin correlations, spin-Peierls or valence-bond-solid order, and confined spinons, order and confined spinons or ({\em ii\/}) coplanar spin correlations, no spin-Peierls order and deconfined bosonic spinons.

cond-mat↗

arXiv2000

Boolean derivatives and computation of cellular automata

The derivatives of a Boolean function are defined up to any order. The Taylor and MacLaurin expansions of a Boolean function are thus obtained. The last corresponds to the ring sum expansion (RSE) of a Boolean function, and is a more compact form than the usual canonical disjunctive form. For totalistic functions the RSE allows the saving of a large number of Boolean operations. The algorithm has natural applications to the simulations of cellular automata using the multi site coding technique. Several already published algorithms are analized, and expressions with fewer terms are generally found.

cond-mat↗

arXiv1992

Fluctuating Interfaces, Surface Tension, and Capillary Waves: An Introduction

We present an introduction to modern theories of interfacial fluctuations and the associated interfacial parameters: surface tension and surface stiffness, as well as their interpretation within the capillary wave model. Transfer matrix spectrum properties due to fluctuation of an interface in a long-cylinder geometry are reviewed. The roughening transition and properties of rigid interfaces below the roughening temperature in 3d lattice models are surveyed with emphasis on differences in fluctuations and transfer matrix spectral properties of rigid vs. rough interfaces.

cond-mat↗

arXiv1996

Power Laws are Logarithmic Boltzmann Laws

Multiplicative random processes in (not necessaryly equilibrium or steady state) stochastic systems with many degrees of freedom lead to Boltzmann distributions when the dynamics is expressed in terms of the logarithm of the normalized elementary variables. In terms of the original variables this gives a power-law distribution. This mechanism implies certain relations between the constraints of the system, the power of the distribution and the dispersion law of the fluctuations. These predictions are validated by Monte Carlo simulations and experimental data. We speculate that stochastic multiplicative dynamics might be the natural origin for the emergence of criticality and scale hierarchies without fine-tuning.

adap-org↗

arXiv1992

Second-Order Dynamics in the Collective Evolution of Coupled Maps and Automata

We review recent numerical studies and the phenomenology of spatially synchronized collective states in many-body dynamical systems. These states exhibit thermodynamic noise superimposed on the collective, quasiperiodic order parameter evolution with typically one basic irrational frequency. We concentrate on the description of the global temporal properties in terms of second-order difference equations.

cond-mat↗

arXiv1996

Partially Solvable Anisotropic t-J Model with Long-Range Interactions

A new anisotropic t-J model in one dimension is proposed which has long-range hopping and exchange. This t-J model is only partially solvable in contrast to known integrable models with long-range interaction. In the high-density limit the model reduces to the XXZ chain with the long-range exchange. Some exact eigenfunctions are shown to be of Jastrow-type if certain conditions for an anisotropy parameter are satisfied. The ground state as well as the excitation spectrum for various cases of the anisotropy parameter and filling are derived numerically. It is found that the Jastrow-type wave function is an excellent trial function for any value of the anisotropy parameter.

cond-mat↗

arXiv1995

Detection of pairing from the extended Aharonov-Bohm period in strongly correlated electron systems

Inspired from Sutherland's work [Phys. Rev. Lett. {\bf 74}, 816 (1995)] on detecting bound spin waves, we propose that bound electron states can be detected from the dependence of interacting electron systems to the Aharonov-Bohm flux in the `extended zone' scheme, where the electron pairing halves the original period $N_a$ flux quanta in a system of linear size $N_a$. Along with the Bethe-ansatz analysis, a numerical implementation for keeping track of the adiabatic flow of energy levels is applied to the attractive/repulsive Hubbard models and the $t-J$ ladder.

cond-mat↗

arXiv1995

Spin Gap of Two-Dimensional Antiferromagnet Representing CaV$_4$O$_9$

We examined a two-dimensional Heisenberg model with two kinds of exchange energies, $J_e$ and $J_c$. This model describes localized spins at vanadium ions in a layer of CaV$_4$O$_9$, for which a spin gap is found by a recent experiment. Comparing the high temperature expansion of the magnetic susceptibility to experimental data, we determined the exchange energies as $J_e \simeq$ 610 K and $J_c \simeq$ 150 K. By the numerical diagonalization we estimated the spin gap as $Δ\sim 0.2J_e \simeq$ 120 K, which consists with the experimental value 107 K. Frustration by finite $J_c$ enhances the spin gap.

cond-mat↗