Equilibrium spin-glass transition of magnetic dipoles with random anisotropy axes on a site diluted lattice

We study partially occupied lattice systems of classical magnetic dipoles which point along randomly oriented axes. Only dipolar interactions are taken into account. The aim of the model is to mimic collective effects in disordered assemblies of magnetic nanoparticles. From tempered Monte Carlo simulations, we obtain the following equilibrium results. The zero temperature entropy approximately vanishes. Below a temperature T_c, given by k_B T_c= (0.95 +- 0.1)x e_d, where e_d is a nearest neighbor dipole-dipole interaction energy and x is the site occupancy rate, we find a spin glass phase. In it, (1) the mean value , where q is the spin overlap, decreases algebraically with system size N as N increases, and (2) D|q| = 0.5 (T/x)^1/2, independently of N, where D|q| is the root mean square deviation of |q|.Comment: 7 LaTeX pages, 7 eps figures. Submitted to PRB on 30 December 200

A multiquark description of the DsJ(2860)D_{sJ}(2860) and DsJ(2700)D_{sJ}(2700)

Within a theoretical framework that accounts for all open-charm mesons, including the $D_0^*(2308)$, the $D_{sJ}^*(2317)$ and the $D_{sJ}(2460)$, we analyze the structure and explore possible quantum number assignments for the $D_{sJ}(2860)$ and the $D_{sJ}(2700)$ mesons reported by BABAR and Belle Collaborations. The open-charm sector is properly described if considered as a combination of conventional quark-antiquark states and four--quark components. All negative parity and $2^+$ states can be understood in terms only of $q\bar q$ components, however the description of the $0^+$ and $1^+$ mesons is improved whenever the mixing between two-- and four--quarks configurations is included. We analyze all possible quantum number assignments for the $D_{sJ}(2860)$ in terms of both $c\bar s$ and $cn\bar s\bar n$ configurations. We discuss the role played by the electromagnetic and strong decay widths as basic tools to distinguish among them. The broad structure reported by BABAR near 2.7 GeV is also analyzed.Comment: 10 pages, 5 table

Unconventional critical activated scaling of two-dimensional quantum spin-glasses

We study the critical behavior of two-dimensional short-range quantum spin glasses by numerical simulations. Using a parallel tempering algorithm, we calculate the Binder cumulant for the Ising spin glass in a transverse magnetic field with two different short-range bond distributions, the bimodal and the Gaussian ones. Through an exhaustive finite-size scaling analysis, we show that the universality class does not depend on the exact form of the bond distribution but, most important, that the quantum critical behavior is governed by an infinite randomness fixed point.Comment: 6 pages, 6 figure