On the Stereochemistry of the Cations in the Doping Block of Superconducting Copper-Oxides

Metal-oxygen complexes containing Cu,- Tl-, Hg-, Bi- and Pb-cations are electronically active in superconducting copper-oxides by stabilizing single phases with enhanced $T_c$, whereas other metal-oxygen complexes deteriorate copper-oxide superconductivity. Cu, Tl, Hg, Bi, Pb in their actual oxidation states are closed shell $d^{10}$ or inert $s^2$ pair ions. Their electronic configurations have a strong tendency to polarize the oxygen environment. The closed shell $d$ ions with low lying $nd^{10}\leftrightarrow nd^9(n+1)s$ excitations form linear complexes through $d_{z^2}-s$ hybridization polarizing the apical oxygens. Comparatively low $nd^9(n+1)s$ excitation energies distinguish $\rm Cu^{1+,3+}, Tl^{3+}, Hg^{2+}$ from other closed shell $d^{10}$ ions deteriorating copper-oxide superconductivity, {\it e.g.} $\rm Zn^{2+}$.Comment: 5 pages, uses REVTEX. To be published in: J. Superconductivity, Proc. Int. Workshop on "Phase Separation, Electronic Inhomogenities and Related Mechanisms for High T_c Superconductors", Erice (Sicily) 9-15 July 199

Band Gaps for Atoms in Light based Waveguides

The energy spectrum for a system of atoms in a periodic potential can exhibit a gap in the band structure. We describe a system in which a laser is used to produce a mechanical potential for the atoms, and a standing wave light field is used to shift the atomic levels using the Autler-Townes effect, which produces a periodic potential. The band structure for atoms guided by a hollow optical fiber waveguide is calculated in three dimensions with quantised external motion. The size of the band gap is controlled by the light guided by the fiber. This variable band structure may allow the construction of devices which can cool atoms. The major limitation on this device would be the spontaneous emission losses.Comment: 7 pages, four postscript figures, uses revtex.sty, available through http://online.anu.edu.au/Physics/papers/atom.htm

Relaxation of a test particle in systems with long-range interactions: diffusion coefficient and dynamical friction

We study the relaxation of a test particle immersed in a bath of field particles interacting via weak long-range forces. To order 1/N in the $N\to +\infty$ limit, the velocity distribution of the test particle satisfies a Fokker-Planck equation whose form is related to the Landau and Lenard-Balescu equations in plasma physics. We provide explict expressions for the diffusion coefficient and friction force in the case where the velocity distribution of the field particles is isotropic. We consider (i) various dimensions of space $d=3,2$ and 1 (ii) a discret spectrum of masses among the particles (iii) different distributions of the bath including the Maxwell distribution of statistical equilibrium (thermal bath) and the step function (water bag). Specific applications are given for self-gravitating systems in three dimensions, Coulombian systems in two dimensions and for the HMF model in one dimension

Atom cooling and trapping by disorder

We demonstrate the possibility of three-dimensional cooling of neutral atoms by illuminating them with two counterpropagating laser beams of mutually orthogonal linear polarization, where one of the lasers is a speckle field, i.e. a highly disordered but stationary coherent light field. This configuration gives rise to atom cooling in the transverse plane via a Sisyphus cooling mechanism similar to the one known in standard two-dimensional optical lattices formed by several plane laser waves. However, striking differences occur in the spatial diffusion coefficients as well as in local properties of the trapped atoms.Comment: 11 figures (postscript

Cooling of a single atom in an optical trap inside a resonator

We present detailed discussions of cooling and trapping mechanisms for an atom in an optical trap inside an optical cavity, as relevant to recent experiments. The interference pattern of cavity QED and trapping fields in space makes the trapping wells distinguishable from one another. This adds considerable flexibility to creating effective trapping and cooling conditions and to detection possibilities. Friction and diffusion coefficients are calculated in and beyond the low excitation limit and full 3-D simulations of the quasiclassical motion of a Cs atom are performed.Comment: One more figure and one more autho

Skeletal Muscle–Derived Cell Implantation for the Treatment of Fecal Incontinence: A Randomized, Placebo-Controlled Study

Background and Aims: Fecal incontinence (FI) improvement following injection of autologous skeletal muscle–derived cells has been previously suggested. This study aimed to test the efficacy and safety of said cells through a multicenter, placebo-controlled study, to determine an appropriate cell dose, and to delineate the target patient population that can most benefit from cell therapy. Methods: Patients experiencing FI for at least 6 months were randomized to receive a cell-free medium or low or high dose of cells. All patients received pelvic floor electrical stimulation before and after treatment. Incontinence episode frequency (IEF), FI quality of life, FI burden assessed on a visual analog scale, Wexner score, and parameters reflecting anorectal physiological function were all assessed for up to 12 months. Results: Cell therapy improved IEF, FI quality of life, and FI burden, reaching a preset level of statistical significance in IEF change compared with the control treatment. Post hoc exploratory analyses indicated that patients with limited FI duration and high IEF at baseline are most responsive to cells. Effects prevailed or increased in the high cell count group from 6 to 12 months but plateaued or diminished in the low cell count and control groups. Most physiological parameters remained unaltered. No unexpected adverse events were observed. Conclusions: Injection of a high dose of autologous skeletal muscle–derived cells followed by electrical stimulation significantly improved FI, particularly in patients with limited FI duration and high IEF at baseline, and could become a valuable tool for treatment of FI, subject to confirmatory phase 3 trial(s). (ClinicalTrialRegister.eu; EudraCT Number: 2010-021463-32)

Guiding Neutral Atoms with a Wire

We demonstrate guiding of cold neutral atoms along a current carrying wire. Atoms either move in Kepler-like orbits around the wire or are guided in a potential tube on the side of the wire which is created by applying an additional homogeneous bias field. These atom guides are very versatile and promising for applications in atom optics.Comment: 4 pages, 6 figures, submitted to PR

Kinetic theory of point vortices in two dimensions: analytical results and numerical simulations

We develop the kinetic theory of point vortices in two-dimensional hydrodynamics and illustrate the main results of the theory with numerical simulations. We first consider the evolution of the system "as a whole" and show that the evolution of the vorticity profile is due to resonances between different orbits of the point vortices. The evolution stops when the profile of angular velocity becomes monotonic even if the system has not reached the statistical equilibrium state (Boltzmann distribution). In that case, the system remains blocked in a sort of metastable state with a non standard distribution. We also study the relaxation of a test vortex in a steady bath of field vortices. The relaxation of the test vortex is described by a Fokker-Planck equation involving a diffusion term and a drift term. The diffusion coefficient, which is proportional to the density of field vortices and inversely proportional to the shear, usually decreases rapidly with the distance. The drift is proportional to the gradient of the density profile of the field vortices and is connected to the diffusion coefficient by a generalized Einstein relation. We study the evolution of the tail of the distribution function of the test vortex and show that it has a front structure. We also study how the temporal auto-correlation function of the position of the test vortex decreases with time and find that it usually exhibits an algebraic behavior with an exponent that we compute analytically. We mention analogies with other systems with long-range interactions

Self-gravitating Brownian particles in two dimensions: the case of N=2 particles

We study the motion of N=2 overdamped Brownian particles in gravitational interaction in a space of dimension d=2. This is equivalent to the simplified motion of two biological entities interacting via chemotaxis when time delay and degradation of the chemical are ignored. This problem also bears some similarities with the stochastic motion of two point vortices in viscous hydrodynamics [Agullo & Verga, Phys. Rev. E, 63, 056304 (2001)]. We analytically obtain the density probability of finding the particles at a distance r from each other at time t. We also determine the probability that the particles have coalesced and formed a Dirac peak at time t (i.e. the probability that the reduced particle has reached r=0 at time t). Finally, we investigate the variance of the distribution and discuss the proper form of the virial theorem for this system. The reduced particle has a normal diffusion behaviour for small times with a gravity-modified diffusion coefficient =r_0^2+(4k_B/\xi\mu)(T-T_*)t, where k_BT_{*}=Gm_1m_2/2 is a critical temperature, and an anomalous diffusion for large times ~t^(1-T_*/T). As a by-product, our solution also describes the growth of the Dirac peak (condensate) that forms in the post-collapse regime of the Smoluchowski-Poisson system (or Keller-Segel model) for T<T_c=GMm/(4k_B). We find that the saturation of the mass of the condensate to the total mass is algebraic in an infinite domain and exponential in a bounded domain.Comment: Revised version (20/5/2010) accepted for publication in EPJ

Synchronization of Hamiltonian motion and dissipative effects in optical lattices: Evidence for a stochastic resonance

We theoretically study the influence of the noise strength on the excitation of the Brillouin propagation modes in a dissipative optical lattice. We show that the excitation has a resonant behavior for a specific amount of noise corresponding to the precise synchronization of the Hamiltonian motion on the optical potential surfaces and the dissipative effects associated with optical pumping in the lattice. This corresponds to the phenomenon of stochastic resonance. Our results are obtained by numerical simulations and correspond to the analysis of microscopic quantities (atomic spatial distributions) as well as macroscopic quantities (enhancement of spatial diffusion and pump-probe spectra). We also present a simple analytical model in excellent agreement with the simulations