We examine a vortex phase diagram for high-T c superconductor Bi 2 Sr 2 CaCu 2 O 8 when columnar defects are introduced by heavy-ion irradiation, by using Monte Carlo simulation based on the Lawrence-Doniach model. At finite temperatures, we numerically find a magnetic-field-driven discontinuous transition in the trapping rate of "pancake" vortices to the defects, at a critical field of B͞B F Ӎ 1͞3 below the matching field B F . The transition in the vortex liquid phase is accompanied by a discontinuous jump of the interlayer coherence, where the decoupled vortex liquid appears to partially couple along the defects. [S0031-9007(98)05690-7] PACS numbers: 74.60. Ge, 74.40. + k, 74.80.Dm In understanding the mixed state of high-T c superconductors, it is a challenge to explore the possible phase transitions between different vortex phases such as the vortex solids (or glasses) and liquids [1]. The nature of transitions is determined by a complicated interplay between thermal fluctuations, disorder, and large anisotropy. In clean samples of YBa 2 Cu 3 O 72d and Bi 2 Sr 2 CaCu 2 O 8 (BSCCO), experimental evidence for the first-order melting transition of an Abrikosov vortex lattice was obtained by observing discontinuous jumps in the magnetization and the specific heat [2,3]. Furthermore, in highly anisotropic BSCCO, Josephson plasma resonance (JPR) measurements [4], which directly probe the interlayer coherence of the vortex state [5][6][7], revealed that the vortex liquid above the melting line is not a linelike but decoupled liquid along the c axis.On the other hand, in irradiated samples, the linear tracks of heavy ions produce columnar defects (CDs), which act as strong pinning centers, and these are expected to improve the c-axis correlation of the vortex lines. The theoretical prediction [8], followed by experimental works [9], showed that a strongly pinned (localized) vortex phase called the Bose glass (BG) appears at low temperatures, and the BG phase can be discriminated from the vortex liquid by a melting curve with the second-order transition character. Yet, very recently, the JPR measurements on irradiated BSCCO single crystals [10,11] have provided anomalous double resonant peaks, implying that a change from decoupled vortices to coupled ones occurs with increasing magnetic field, in the vortex liquid regime above the irreversibility line (IL). However, the physical origin of such a phase change in the liquid phase is not clear, and the important questions remain, whether it is a sharp transition [10] or a crossover [11], which are controversial in both theoretical and experimental aspects.In this Letter, we provide numerically clear evidence that a discontinuous transition from decoupled to coupled vortices occurs above the BG melting line, via a computer simulation study of a BSCCO model with dense and randomly distributed CDs (k c axis) of the matching field B F 1 tesla (T). The singularity of this transition manifests itself in the vicinity of B F ͞3 not as a function of temperature but magnetic f...

By molecular dynamics simulation, we have investigated classical Heisenberg spins, which are arrayed on a finite simple cubic lattice and interact with each other only by the dipoledipole interaction, and have found its peculiar from-Edge-to-interior freezing process. As the temperature is decreased, spins on each edge predominantly start to freeze in a ferromagnetic alignment parallel to the edge around the corresponding bulk transition temperature, then from each edges grow domains with short-range orders similar to the corresponding bulk orders, and the system ends up with a unique multi-domain ground state at the lowest temperature. We interpret this freezing characteristics is attributed to the anisotropic and long-range nature of the dipole-dipole interaction combined with a finite-size effect.In the last decade systems consisting of arrayed ferromagnetic nanoparticles have attracted much attention as a possible element with the high storage density. 1 Magnetic properties of such systems have been analyzed based on various theoretical model, in which the magnetic moment of each nanoparticle is represented by a classical Heisenberg spin with a proper magnitude. The spins are interacting with each other by the dipole-dipole interaction, and each spin suffers from the magnetic anisotropy energy which represents the shape and/or bulk lattice anisotropies of the original nanoparticle magnetic moments. There appeared studies on the susceptibility, 2 magnetic hysteresis, 3, 4 and energy relaxation 5 in such models of a finite size. However, there have been little work on systems only with the dipole-dipole interaction which we call here simply dipolar systems. Considering that to clarify magnetic properties of dipolar systems of a finite size is of importance as the first step in researches of arrayed magnetic nanoparticles as well as of interest from a theoretical viewpoint in statistical physics, *

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