Effects of point defects on the phase diagram of vortex states in high-Tc superconductors in B//c axis

The phase diagram for the vortex states of high-$T_{\rm c}$ superconductors with point defects in $\vec{B} \parallel c$ axis is drawn by large-scale Monte Carlo simulations. The vortex slush (VS) phase is found between the vortex glass (VG) and vortex liquid (VL) phases. The first-order transition between this novel normal phase and the VL phase is characterized by a sharp jump of the density of dislocations. The first-order transition between the Bragg glass (BG) and VG or VS phases is also clarified. These two transitions are compared with the melting transition between the BG and VL phases.Comment: 4 pages, 9 eps figures (included in text), uses revtex.sty, overall changes with several additional data points, though conclusion is unchange

Generic phase diagram of active polar films

We study theoretically the phase diagram of compressible active polar gels such as the actin network of eukaryotic cells. Using generalized hydrodynamics equations, we perform a linear stability analysis of the uniform states in the case of an infinite bidimensional active gel to obtain the dynamic phase diagram of active polar films. We predict in particular modulated flowing phases, and a macroscopic phase separation at high activity. This qualitatively accounts for experimental observations of various active systems, such as acto-myosin gels, microtubules and kinesins in vitro solutions, or swimming bacterial colonies.Comment: 4 pages, 1 figur

A quantum Monte Carlo algorithm realizing an intrinsic relaxation

We propose a new quantum Monte Carlo algorithm which realizes a relaxation intrinsic to the original quantum system. The Monte Carlo dynamics satisfies the dynamic scaling relation $\tau\sim \xi^z$ and is independent of the Trotter number. Finiteness of the Trotter number just appears as the finite-size effect. An infinite Trotter number version of the algorithm is also formulated, which enables us to observe a true relaxation of the original system. The strategy of the algorithm is a compromise between the conventional worldline local flip and the modern cluster loop flip. It is a local flip in the real-space direction and is a cluster flip in the Trotter direction. The new algorithm is tested by the transverse-field Ising model in two dimensions. An accurate phase diagram is obtained.Comment: 9 pages, 4 figure

The antiferromagnetic order in an F-AF random alternating quantum spin chain : (CH_3)_2 CHNH_3 Cu(Cl_x Br_{1-x})_3

A possibility of the uniform antiferromagnetic order is pointed out in an S=1/2 ferromagnetic (F) - antiferromagnetic (AF) random alternating Heisenberg quantum spin chain compound: (CH_3)_2 CHNH_3 Cu(Cl_x Br_{1-x})_3. The system possesses the bond alternation of strong random bonds that take +/- 2J and weak uniform AF bonds of -J. In the pure concentration limits, the model reduces to the AF-AF alternation chain at x=0 and to the F-AF alternation chain at x=1. The nonequilibrium relaxation of large-scale quantum Monte Carlo simulations exhibits critical behaviors of the uniform AF order in the intermediate concentration region, which explains the experimental observation of the magnetic phase transition. The present results suggest that the uniform AF order may survive even in the presence of the randomly located ferromagnetic bonds.Comment: 4 pages, 3 figure

Entanglement Mean Field Theory and the Curie-Weiss Law

The mean field theory, in its different hues, form one of the most useful tools for calculating the single-body physical properties of a many-body system. It provides important information, like critical exponents, of the systems that do not yield to an exact analytical treatment. Here we propose an entanglement mean field theory (EMFT) to obtain the behavior of the two-body physical properties of such systems. We apply this theory to predict the phases in paradigmatic strongly correlated systems, viz. the transverse anisotropic XY, the transverse XX, and the Heisenberg models. We find the critical exponents of different physical quantities in the EMFT limit, and in the case of the Heisenberg model, we obtain the Curie-Weiss law for correlations. While the exemplary models have all been chosen to be quantum ones, classical many-body models also render themselves to such a treatment, at the level of correlations.Comment: 5 pages, 4 figure

Flux-line entanglement as the mechanism of melting transition in high-temperature superconductors in a magnetic field

The mechanism of the flux-line-lattice (FLL) melting in anisotropic high-T_c superconductors in ${\bf B}\parallel {\bf \hat{c}}$ is clarified by Monte Carlo simulations of the 3D frustrated XY model. The percentage of entangled flux lines abruptly changes at the melting temperature T_m, while no sharp change can be found in the number and size distribution of vortex loops around T_m. Therefore, the origin of this melting transition is the entanglement of flux lines. Scaling behaviors of physical quantities are consistent with the above mechanism of the FLL melting. The Lindemann number is also evaluated without any phenomenological arguments.Comment: 10 pages, 5 Postscript figures, RevTeX; changed content and figures, Phys. Rev. B Rapid Commun. in pres

A Novel RNA-Recognition-Motif Protein Is Required for Premeiotic G1/S-Phase Transition in Rice (Oryza sativa L.)

The molecular mechanism for meiotic entry remains largely elusive in flowering plants. Only Arabidopsis SWI1/DYAD and maize AM1, both of which are the coiled-coil protein, are known to be required for the initiation of plant meiosis. The mechanism underlying the synchrony of male meiosis, characteristic to flowering plants, has also been unclear in the plant kingdom. In other eukaryotes, RNA-recognition-motif (RRM) proteins are known to play essential roles in germ-cell development and meiosis progression. Rice MEL2 protein discovered in this study shows partial similarity with human proline-rich RRM protein, deleted in Azoospermia-Associated Protein1 (DAZAP1), though MEL2 also possesses ankyrin repeats and a RING finger motif. Expression analyses of several cell-cycle markers revealed that, in mel2 mutant anthers, most germ cells failed to enter premeiotic S-phase and meiosis, and a part escaped from the defect and underwent meiosis with a significant delay or continued mitotic cycles. Immunofluorescent detection revealed that T7 peptide-tagged MEL2 localized at cytoplasmic perinuclear region of germ cells during premeiotic interphase in transgenic rice plants. This study is the first report of the plant RRM protein, which is required for regulating the premeiotic G1/S-phase transition of male and female germ cells and also establishing synchrony of male meiosis. This study will contribute to elucidation of similarities and diversities in reproduction system between plants and other species

Disorder Driven Melting of the Vortex Line Lattice

We use Monte Carlo simulations of the 3D uniformly frustrated XY model, with uncorrelated quenched randomness in the in-plane couplings, to model the effect of random point pins on the vortex line phases of a type II superconductor. We map out the phase diagram as a function of temperature T and randomness strength p for fixed applied magnetic field. We find that, as p increases to a critical value p_c, the first order vortex lattice melting line turns parallel to the T axis, and continues smoothly down to low temperature, rather than ending at a critical point. The entropy jump across this line at p_c vanishes, but the transition remains first order. Above this disorder driven transition line, we find that the helicity modulus parallel to the applied field vanishes, and so no true phase coherent vortex glass exists.Comment: 4 pages, 6 eps figure

Bond-disordered spin systems: Theory and application to doped high-Tc compounds

We examine the stability of magnetic order in a classical Heisenberg model with quenched random exchange couplings. This system represents the spin degrees of freedom in high-$T_\textrm{c}$ compounds with immobile dopants. Starting from a replica representation of the nonlinear $\sigma$-model, we perform a renormalization-group analysis. The importance of cumulants of the disorder distribution to arbitrarily high orders necessitates a functional renormalization scheme. From the renormalization flow equations we determine the magnetic correlation length numerically as a function of the impurity concentration and of temperature. From our analysis follows that two-dimensional layers can be magnetically ordered for arbitrarily strong but sufficiently diluted defects. We further consider the dimensional crossover in a stack of weakly coupled layers. The resulting phase diagram is compared with experimental data for La$_{2-x}$Sr$_x$CuO$_4$.Comment: 12 pages, 5 figure