Vortex lattice melting in layered superconductors with periodic columnar pins

The melting transition of the vortex lattice in highly anisotropic, layered superconductors with commensurate, periodic columnar pins is studied in a geometry where magnetic field and columnar pins are normal to the layers. Thermodynamic properties and equilibrium density distributions are obtained from numerical minimizations of an appropriate free-energy functional. We find a line of first-order transitions that ends at a critical point as the pin concentration is increased. A simple Landau theory providing a semi-quantitative explanation of the numerical results is proposed.Comment: Four pages, 3 Figure

Comparative study between two quantum spin systems KCuCl3_{3} and TlCuCl3_{3}

We have performed an {\it ab initio} study of the electronic structure of two isostructural quantum spin systems, KCuCl$_{3}$ and TlCuCl$_{3}$, which have recently attracted much attention due to their unconventional magnetic properties. Our first-principles analysis shows unambiguously the role of Tl, as opposed to structural differences between the two compounds, in making TlCuCl$_{3}$ a {\it strongly} coupled s=1/2 dimer system compared to KCuCl$_{3}$ which shows a {\it weakly} coupled s=1/2 dimer behavior. Good agreement with the existing analysis of inelastic neutron scattering results has been observed.Comment: minor changes in text with additional references; in EPL forma

Non-classical Rotational Inertia in a Two-dimensional Bosonic Solid Containing Grain Boundaries

We study the occurrence of non-classical rotational inertia (NCRI) arising from superfluidity along grain boundaries in a two-dimensional bosonic system. We make use of a standard mapping between the zero-temperature properties of this system and the statistical mechanics of interacting vortex lines in the mixed phase of a type-II superconductor. In the mapping, the liquid phase of the vortex system corresponds to the superfluid bosonic phase. We consider numerically obtained polycrystalline configurations of the vortex lines in which the microcrystals are separated by liquid-like grain boundary regions which widen as the vortex system temperature increases. The NCRI of the corresponding zero-temperature bosonic systems can then be numerically evaluated by solving the equations of superfluid hydrodynamics in the channels near the grain boundaries. We find that the NCRI increases very abruptly as the liquid regions in the vortex system (equivalently, superfluid regions in the bosonic system) form a connected, system-spannig structure with one or more closed loops. The implications of these results for experimentally observed supersolid phenomena are discussed.Comment: Ten pages, including figure

Phase diagram of randomly pinned vortex matter in layered superconductors: dependence on the details of the point pinning

We study the thermodynamic and structural properties of the superconducting vortex system in high temperature layered superconductors, with magnetic field normal to the layers, in the presence of a small concentration of strong random point pinning defects via numerical minimization of a model free energy functional in terms of the time-averaged local density of pancake vortices. Working at constant magnetic induction and point pinning center concentration, we find that the equilibrium phase at low temperature ($T$) and small pinning strength ($s$) is a topologically ordered Bragg glass. As $T$ or $s$ is increased, the Bragg glass undergoes a first order transition to a disordered phase which we characterize as a ``vortex slush'' with polycrystalline structure within the layers and interlayer correlations extending to about twenty layers. This is in contrast with the pinned vortex liquid phase into which the Bragg glass was found to melt, using the same methods, in the case of a large concentration of weak pinning centers: that phase was amorphous with very little interlayer correlation. The value of the second moment of the random pinning potential at which the Bragg glass melts for a fixed temperature is very different in the two systems. These results imply that the effects of random point pinning can not be described only in terms of the second moment of the pinning potential, and that some of the unresolved contradictions in the literature concerning the nature of the low $T$ and high $s$ phase in this system are likely to arise from differences in the nature of the pinning in different samples, or from assumptions made about the pinning potential.Comment: 13 pages including 11 figures. Typos in HTML abstract corrected in v

Electronic and magnetic structure of CsV2_2O5_5

We have studied the electronic structure of the spin-gapped system CsV$_2$O$_5$ by means of an ab initio calculation. Our analysis and a re-examination of the susceptibility data indicate that the behavior of this system is much closer to that of an alternating spin-1/2 antiferromagnetic chain with significant inter-dimer coupling and weaker inter-chain couplings than that of isolated dimers as was initially proposed. Comparison to the vanadate family members $\alpha '$-NaV$_2$O$_5$, $\gamma$-LiV$_2$O$_5$ and isostructural compounds like (VO)$_{2}$P$_{2}$O$_{7}$ (VOPO) is discussed.Comment: revised version. To appear in Phys. Rev.

Order independent structural alignment of circularly permuted proteins

Circular permutation connects the N and C termini of a protein and concurrently cleaves elsewhere in the chain, providing an important mechanism for generating novel protein fold and functions. However, their in genomes is unknown because current detection methods can miss many occurances, mistaking random repeats as circular permutation. Here we develop a method for detecting circularly permuted proteins from structural comparison. Sequence order independent alignment of protein structures can be regarded as a special case of the maximum-weight independent set problem, which is known to be computationally hard. We develop an efficient approximation algorithm by repeatedly solving relaxations of an appropriate intermediate integer programming formulation, we show that the approximation ratio is much better then the theoretical worst case ratio of $r = 1/4$. Circularly permuted proteins reported in literature can be identified rapidly with our method, while they escape the detection by publicly available servers for structural alignment.Comment: 5 pages, 3 figures, Accepted by IEEE-EMBS 2004 Conference Proceeding

Entropic Origin of the Growth of Relaxation Times in Simple Glassy Liquids

Transitions between ``glassy'' local minima of a model free-energy functional for a dense hard-sphere system are studied numerically using a ``microcanonical'' Monte Carlo method that enables us to obtain the transition probability as a function of the free energy and the Monte Carlo ``time''. The growth of the height of the effective free energy barrier with density is found to be consistent with a Vogel-Fulcher law. The dependence of the transition probability on time indicates that this growth is primarily due to entropic effects arising from the difficulty of finding low-free-energy saddle points connecting glassy minima.Comment: Four pages, plus three postscript figure