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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↗

arXiv2000

Freezing of dynamical exponents in low dimensional random media

A particle in a random potential with logarithmic correlations in dimensions $d=1,2$ is shown to undergo a dynamical transition at $T_{dyn}>0$. In $d=1$ exact results demonstrate that $T_{dyn}=T_c$, the static glass transition temperature, and that the dynamical exponent changes from $z(T)=2 + 2 (T_c/T)^2$ at high temperature to $z(T)= 4 T_c/T$ in the glass phase. The same formulae are argued to hold in $d=2$. Dynamical freezing is also predicted in the 2D random gauge XY model and related systems. In $d=1$ a mapping between dynamics and statics is unveiled and freezing involves barriers as well as valleys. Anomalous scaling occurs in the creep dynamics.

cond-mat↗

arXiv2002

Critical phenomena of thick branes in warped spacetimes

We have investigated the effects of a generic bulk first-order phase transition on thick Minkowski branes in warped geometries. As occurs in Euclidean space, when the system is brought near the phase transition an interface separating two ordered phases splits into two interfaces with a disordered phase in between. A remarkable and distinctive feature is that the critical temperature of the phase transition is lowered due to pure geometrical effects. We have studied a variety of critical exponents and the evolution of the transverse-traceless sector of the metric fluctuations.

hep-th↗

arXiv2002

Three body problem in a dilute Bose-Einstein condensate

We derive the explicit three body contact potential for a dilute condensed Bose gas from microscopic theory. The three body coupling constant exhibits the general form predicted by T.T. Wu [Phys. Rev. 113, 1390 (1959)] and is determined in terms of the amplitudes of two and three body collisions in vacuum. In the present form the coupling constant becomes accessible to quantitative studies which should provide the crucial link between few body collisions and the stability of condensates with attractive two body forces.

cond-mat↗

arXiv2003

The nature of Feshbach molecules in Bose-Einstein condensates

We discuss the long range nature of the molecules produced in recent experiments on molecular Bose-Einstein condensation. The properties of these molecules depend on the full two-body Hamiltonian and not just on the states of the system in the absence of interchannel couplings. The very long range nature of the state is crucial to the efficiency of production in the experiments. Our many-body treatment of the gas accounts for the full binary physics and describes properly how these molecular condensates can be directly probed.

cond-mat↗

arXiv2001

Information and Computation: Classical and Quantum Aspects

Quantum theory has found a new field of applications in the realm of information and computation during the recent years. This paper reviews how quantum physics allows information coding in classically unexpected and subtle nonlocal ways, as well as information processing with an efficiency largely surpassing that of the present and foreseeable classical computers. Some outstanding aspects of classical and quantum information theory will be addressed here. Quantum teleportation, dense coding, and quantum cryptography are discussed as a few samples of the impact of quanta in the transmission of information. Quantum logic gates and quantum algorithms are also discussed as instances of the improvement in information processing by a quantum computer. We provide finally some examples of current experimental realizations for quantum computers and future prospects.

quant-ph↗

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↗

arXiv1999

Recent Experiments with Bose-Condensed Gases at JILA

We consider a binary mixture of two overlapping Bose-Einstein condensates in two different hyperfine states of \Rb87 with nearly identical magnetic moments. Such a system has been simply realized through application of radiofrequency and microwave radiation which drives a two-photon transition between the two states. The nearly identical magnetic moments afford a high degree of spatial overlap, permitting a variety of new experiments. We discuss some of the conditions under which the magnetic moments are identical, with particular emphasis placed on the requirements for a time-averaged orbiting potential (TOP) magnetic trap.

cond-mat↗

arXiv2000

Collective Phenonema in the Interaction of Radiation with Matter

The aim of this communication is to present in a concentrated form the main ideas of a method, developed by the author, for treating strongly nonequilibrium collective phenomena typical of the interaction of radiation with matter, as well as to give a survey of several applications of the method. The latter is called the Scale Separation Approach since its basic techniques rely on the possibility of separating different space-time scales in nonequilibrium statistical systems. This approach is rather general and can be applied to diverse physical problems, several of which are discussed here. These problems are: Superradiance of nuclear spins, filamentation in resonant media, semiconfinement of neutral atoms, negative electric current, and collective liberation of light.

cond-mat↗

arXiv2001

Dynamic Critical Phenomena in Trapped Bose Gases

Nonlinear dynamics of a trapped Bose-Einstein condensate, subject to the action of a resonant external field, is studied. This field produces a spatio-temporal modulation of the trapping potential with the frequency close to the transition frequency between the ground state and a higher energy level. The evolution equations of fractional populations display a kind of critical phenomena at a critical line on the manifold of the system parameters. It is demonstrated that there exists a direct analogy between dynamical instability at this line and critical phenomena at a critical line of a related averaged system.

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↗

arXiv2001

CP_N Solitons in Quantum Hall Systems

We will present here an elementary pedagogical introduction to $CP_N$ solitons in quantum Hall systems. We begin with a brief introduction to both $CP_N$ models and to quantum Hall (QH) physics. Then we focus on spin and layer-spin degrees of freedom in QH systems and point out that these are in fact $CP_N$ fields for N=1 and N=3. Excitations in these degrees of freedom will be shown to be topologically non-trivial soliton solutions of the corresponding $CP_N$ field equations. This is followed by a brief summary of our own recent work in this area, done with Sankalpa Ghosh. [ Invited Plenary Lecture at the International Conference on Geometry, Integrability and Nonlinearity in Condensed Matter & Soft Condensed Matter Physics held at Bansko, Bulgaria, July 15-20, 2001. ]

cond-mat↗

arXiv2002

Loading of a cold atomic beam into a magnetic guide

We demonstrate experimentally the continuous and pulsed loading of a slow and cold atomic beam into a magnetic guide. The slow beam is produced using a vapor loaded laser trap, which ensures two-dimensional magneto-optical trapping, as well as cooling by a moving molasses along the third direction. It provides a continuous flux larger than $10^9$ atoms/s with an adjustable mean velocity ranging from 0.3 to 3 m/s, and with longitudinal and transverse temperatures smaller than $100 μ$K. Up to $3 10^8$ atoms/s are injected into the magnetic guide and subsequently guided over a distance of 40 cm.

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

Waveguides coupled through a semitransparent barrier: a Birman-Schwinger analysis

The paper is devoted to a model of a mesoscopic system consisting of a pair of parallel planar waveguides separated by an infinitely thin semitransparent boundary modeled by a transverse delta interaction. We develop the Birman-Schwinger theory for the corresponding generalized Schroedinger operator. The spectral properties become nontrivial if the barrier coupling is not invariant with respect to longitudinal translations, in particular, there are bound states if the barrier is locally more transparent in the mean and the coupling parameter reaches the same asymptotic value in both directions along the guide axis. We derive the weak-coupling expansion of the ground-state eigenvalue for the cases when the perturbation is small in the supremum and the L^1-norms. The last named result applies to the situation when the support of the leaky part shrinks: the obtained asymptotics differs from that of a double guide divided by a pierced Dirichlet barrier. We also derive an upper bound on the number of bound states.

math-ph↗

arXiv2001

A simple solid-on-solid model of epitaxial film growth: submonolayer substrate coverage

In this work we investigate influence of substrate temperature on the surface morphology for substrate coverage below one monolayer. The model of film growth is based on random deposition enriched by limited surface diffusion. Also anisotropy in the growth is involved. We found from computer simulations for simple cubic lattice and solid-on-solid model, that the surface morphology changes with increasing temperature from isotropically distributed isolated small islands, through anisotropic 1-D stripes to larger 2-D anisotropic islands and again randomly distributed single atoms. The transition is also marked in height-height correlation function dependence on temperature, as directly seen by snapshots from simulations. The results are in good qualitative agreement with already published results of kinetic Monte Carlo simulations, as well as with some experimental data.

cond-mat↗