Giant increase in critical current density of KxFe2-ySe2 single crystals

By using post-annealing and quenching technique, we show that the critical current density Jc,ab of KxFe2-ySe2 single crystals can be enhanced more than one order of magnitude up to ~ 2.1 \times 10^4 A/cm^2 at 1.8 K. The scaling law between normalized pinning force and reduced field for all measured temperatures was observed, reflecting the presence of only one dominant pinning mechanism. Analysis indicates presence of 3D point-like normal core pinning sources in quenched KxFe2-ySe2 samples whereas dominant vortex interaction with pinning centers is via spatial variations in Tc ("deltaTc pinning").Comment: 3 figures, 4 page

Linear magnetoresistance on the topological surface

A positive, non-saturating and dominantly linear magnetoresistance is demonstrated to occur in the surface state of a topological insulator having a wavevector-linear energy dispersion together with a finite positive Zeeman energy splitting. This linear magnetoresistance shows up within quite wide magnetic-field range in a spatially homogenous system of high carrier density and low mobility in which the conduction electrons are in extended states and spread over many smeared Landau levels, and is robust against increasing temperature, in agreement with recent experimental findings in Bi$_2$Se$_3$ nanoribbons.Comment: 7 pages, 4 figure

On positive solutions and the Omega limit set for a class of delay differential equations

This paper studies the positive solutions of a class of delay differential equations with two delays. These equations originate from the modeling of hematopoietic cell populations. We give a sufficient condition on the initial function for $t\leq 0$ such that the solution is positive for all time $t>0$. The condition is "optimal". We also discuss the long time behavior of these positive solutions through a dynamical system on the space of continuous functions. We give a characteristic description of the $\omega$ limit set of this dynamical system, which can provide informations about the long time behavior of positive solutions of the delay differential equation.Comment: 15 pages, 2 figure

Thermoelectric studies of KxFe2-ySe2: weakly correlated superconductor

We report thermal transport measurement of KxFe2-ySe2 superconducting single crystal. Significant peak anomaly in thermal conductivity is observed at nearly TC/2 indicating a large phonon mean-free-path in the superconducting state. The zero-temperature extrapolated thermoelectric power is smaller than the value in typical strongly correlated superconductors, implying large normalized Fermi temperature. In contrast to other iron superconductors, thermoelectric power in our sample does not exhibit significant anomalies. These findings indicate that KxFe2-ySe2 is a weakly or intermediately correlated superconductor without significant Fermi surface nesting.Comment: Revised version, 5 pages, 5 figures, Will appear in Physical Review

Screening of point charge impurities in highly anisotropic metals: application to μ+\mu^+ spin relaxation in underdoped cuprates

We calculate the screening charge density distribution due to a point charge, such as that of a positive muon ($\mu^+$), placed between the planes of a highly anisotropic layered metal. In underdoped hole cuprates the screening charge converts the charge density in the metallic-plane unit cells in the vicinity of the $\mu^+$ to nearly its value in the insulating state. The current-loop ordered state observed by polarized neutron diffraction then vanishes in such cells, and also in nearby cells over a distance of order the intrinsic correlation length of the loop-ordered state. This in turn strongly suppresses the loop-current field at the $\mu^+$ site. We estimate this suppressed field in underdoped YBa$_2$Cu$_3$O$_{6+x}$ and La$_{2-x}$Sr$_x$CuO$_4$, and find consistency with the observed 0.2--0.3 G field in the former case and the observed upper bound of $\sim$0.2 G in the latter case. This resolves the controversy between the neutron diffraction and $\mu$SR experiments. The screening calculation also has relevance for the effect of other charge impurities in the cuprates, such as the dopants themselves

Thermal Fluctuations in a Lamellar Phase of a Binary Amphiphile-Solvent Mixture: A Molecular Dynamics Study

We investigate thermal fluctuations in a smectic A phase of an amphiphile-solvent mixture with molecular dynamics simulations. We use an idealized model system, where solvent particles are represented by simple beads, and amphiphiles by bead-and-spring tetramers. At a solvent bead fraction of 20 % and sufficiently low temperature, the amphiphiles self-assemble into a highly oriented lamellar phase. Our study aims at comparing the structure of this phase with the predictions of the elastic theory of thermally fluctuating fluid membrane stacks [Lei et al., J. Phys. II 5, 1155 (1995)]. We suggest a method which permits to calculate the bending rigidity and compressibility modulus of the lamellar stack from the simulation data. The simulation results are in reasonable agreement with the theory