Variations on the Supersymmetric Q6 Model of Flavor

We observe that a recently proposed supersymmetric model with Q6 flavor symmetry admits a new CP violating ground state. A new sum rule for the quark mixing parameters emerges, which is found to be consistent with data. Simple extensions of the model to the neutrino sector suggest an inverted hierarchical mass spectrum with nearly maximal CP violation (|delta_{MNS}| simeq pi/2). Besides reducing the number of parameters in the fermion sector, these models also provide solutions to the SUSY flavor problem and the SUSY CP problem. We construct a renormalizable scalar potential that leads to the spontaneous breaking of CP symmetry and the family symmetry.Comment: 22 pages, 7 figure

Double-q\it q Order in a Frustrated Random Spin System

We use the three-dimensional Heisenberg model with site randomness as an effective model of the compound Sr(Fe$_{1-x}$Mn$_x$)O$_2$. The model consists of two types of ions that correspond to Fe and Mn ions. The nearest-neighbor interactions in the ab-plane are antiferromagnetic. The nearest-neighbor interactions along the c-axis between Fe ions are assumed to be antiferromagnetic, whereas other interactions are assumed to be ferromagnetic. From Monte Carlo simulations, we confirm the existence of the double-$\boldsymbol{q}$ ordered phase characterized by two wave numbers, $(\pi\pi\pi)$ and $(\pi\pi0)$. We also identify the spin ordering pattern in the double-$\boldsymbol{q}$ ordered phase.Comment: 5pages, 3figure

Magnetic properties of the spin-1/2 XXZ model on the Shastry-Sutherland lattice: Effect of long-range interactions

We study magnetic properties of the $S=1/2$ Ising-like XXZ model on the Shastry-Sutherland lattices with long-range interactions, using the quantum Monte Carlo method. This model shows magnetization plateau phases at one-half and one-third of the saturation magnetization when additional couplings are considered. We investigate the finite temperature transition to one-half and one-third plateau phases. The obtained results suggest that the former case is of the first order and the latter case is of the second order. We also find that the system undergoes two successive transitions with the 2D Ising model universality, although there is a single phase transition in the Ising limit case. Finally, we estimate the coupling ratio to explain the magnetization process observed in ${\rm TmB_4}$Comment: 5 pages, 6 figure

Generalization of the Fortuin-Kasteleyn transformation and its application to quantum spin simulations,

We generalize the Fortuin-Kasteleyn (FK) cluster representation of the partition function of the Ising model to represent the partition function of quantum spin models with an arbitrary spin magnitude in arbitrary dimensions. This generalized representation enables us to develop a new cluster algorithm for the simulation of quantum spin systems by the worldline Monte Carlo method. Because the Swendsen-Wang algorithm is based on the FK representation, the new cluster algorithm naturally includes it as a special case. As well as the general description of the new representation, we present an illustration of our new algorithm for some special interesting cases: the Ising model, the antiferromagnetic Heisenberg model with $S=1$, and a general Heisenberg model. The new algorithm is applicable to models with any range of the exchange interaction, any lattice geometry, and any dimensions.Comment: 46 pages, 10 figures, to appear in J.Stat.Phy

Traveling waves for models of phase transitions of solids driven by configurational forces

This article is concerned with the existence of traveling wave solutions, including standing waves, to some models based on configurational forces, describing respectively the diffusionless phase transitions of solid materials, e.g., Steel, and phase transitions due to interface motion by interface diffusion, e.g., Sintering. These models were proposed by Alber and Zhu in [3]. We consider both the order-parameter-conserved case and the non-conserved one, under suitable assumptions. Also we compare our results with the corresponding ones for the Allen-Cahn and the Cahn-Hilliard equations coupled with linear elasticity, which are models for diffusion-dominated phase transitions in elastic solids

Asymptotic stability of rarefaction wave for the navier-stokes equations for a compressible fluid in the half space

This paper is concerned with the asymptotic stability towards a rarefaction wave of the solution to an outflow problem for the Navier-Stokes equations in a compressible fluid in the Eulerian coordinate in the half space. This is the second one of our series of papers on this subject. In this paper, firstly we classify completely the time-asymptotic states, according to some parameters, that is the spatial-asymptotic states and boundary conditions, for this initial boundary value problem, and some pictures for the classification of time-asymptotic states are drawn in the state space. In order to prove the stability of the rarefaction wave, we use the solution to Burgers' equation to construct a suitably smooth approximation of the rarefaction wave and establish some time-decay estimates in Lp-norm for the smoothed rarefaction wave. We then employ the L2-energy method to prove that the rarefaction wave is non-linearly stable under a small perturbation, as time goes to infinity. © Springer-Verlag 2008

Quantum Monte Carlo Study on Magnetization Processes

A quantum Monte Carlo method combining update of the loop algorithm with the global flip of the world line is proposed as an efficient method to study the magnetization process in an external field, which has been difficult because of inefficiency of the update of the total magnetization. The method is demonstrated in the one dimensional antiferromagnetic Heisenberg model and the trimer model. We attempted various other Monte Carlo algorithms to study systems in the external field and compared their efficiency.Comment: 5 pages, 9 figures; added references for section 1, corrected typo

High pressure high temperature (HPHT) synthesis and magnetization of Magneto-Superconducting RuSr2(LnCe2)Cu2O12.25 (Ru-1232) compounds (Ln = Y and Dy)

RuSr2(LnCe2)Cu2O12.25 (Ru-1232) compounds with Ln = Y and Dy being synthesized by high pressure high temperature (6GPa, 12000C) solid state synthesis route do crystallize in space group P4/mmm in near single phase form with small quantities of SrRuO3 and RuSr2(RE1.5Ce0.5)Cu2O10 (Ru-1222). Both samples exhibit magnetic transitions (Tmag.) at ~90 K with significant branching of zfc (zero-field-cooled) and fc (field-cooled) magnetization and a sharp cusp in zfc at ~ 70 K, followed by superconducting transitions at ~ 30 K. Both compounds show typical ferromagnetic hysteresis loops in magnetic moment (M) versus field (H) magnetization right upto Tmag. i.e. < 90K. To our knowledge these are the first successfully synthesized Ru-1232 compounds in near single phase with lanthanides including Y and Dy. The results are compared with widely reported Gd/Ru-1222 and Ru-1212 (RuSr2GdCu2O8) compounds. In particular, it seems that the Ru moments magnetic ordering temperature (Tmag.) scales with the c-direction distance between magnetic RuO6 octahedras in Ru-1212/1222 or 1232 systems.Comment: 15 pages of TEXT and Fig