Possible Bragg-Bose glass phase in vortex states of high- T _c superconductors with sparse and weak columnar defects

Abstract

Phase diagram of vortex states of high-Tc superconductors with sparse and weak columnar defects is obtained by large-scale Monte Carlo simulations of the three-dimensional anisotropic, frustrated XY model. The Bragg-Bose glass phase characterized by hexagonal Bragg spots and the diverging tilt modulus is observed numerically for the first time at low density of columnar defects. As the density of defects increases, the melting temperature increases owing to "selected pinning" of flux lines. When the density of defects further increases, the transition to the Bose glass phase occurs. The interstitial liquid region is observed between these two glass phases and the vortex liquid phase. PACS numbers: 74.25.Qt, 74.62.Dh, 74.25.Dw In vortex states of high-T c superconductors, columnar defects introduced by heavy-ion irradiation are strong pinning centers of flux lines, and dramatically enhance the critical current.[1] Nelson and Vinokur [2] analyzed these defects and derived a phase transition between the Bose glass (BG) and vortex liquid (VL) phases. They also showed that this system can be mapped to twodimensional interacting bosons in a random potential and that this phase transition corresponds to a localization transition of interacting bosons. In the case of dense and strong columnar defects, every flux line is trapped by randomly-distributed columnar defects at low temperatures, and the BG phase naturally appears. Many numerical studies [3,4,5] have been made in this case.In the case of sparse and strong columnar defects, part of flux lines are not trapped by defects, and physical properties are more complicated. At low temperatures untrapped flux lines are also frozen owing to interactions with trapped flux lines, and the BG phase is stable. At intermediate temperatures the defects still trap some flux lines but other interstitial ones are freely moving around, and the resistivity becomes finite. This region is called as the interstitial liquid (IL).[6] At high temperatures all flux lines are depinned and the VL phase appears. It is not clarified yet whether there exists a phase transition or merely a crossover between the IL and VL. A numerical simulation was attempted [7] in this case, and the existence of the BG phase was confirmed. Two kinds of BG phases for dense or sparse columnar defects are divided by the Mott insulator phase at the matching field. Recently the transition between these two (strong and weak) BG phases was observed numerically. [8] In pure systems, the triangular flux-line lattice (FLL) is formed below the melting temperature T m . Is such order completely destroyed by introducing infinitesimal columnar defects? In vortex states with point defects, the long-range order of vortex positions is destroyed by ; Electronic address: nonomura.yoshihiko@nims.go.jp infinitesimal defects, but the Bragg glass phase [9,10,11] characterized by the quasi long-range order of vortex positions and Bragg spots of the structure factor is stable up to a considerable density of point defects. In the presen...

Collective pinning of the vortex lattice by columnar defects in layered superconductors

“…As discussed previously at the end of section II, grain boundaries do not occur in the vortex lattice of the frustrated XY model (2) used here [9][22], since it describes incompressible vortex matter. More generally however, these measurements find that the vortex solid melts through a second-order phase transition at perpendicular magnetic fields above a certain critical point [3].…”

“…This makes them ideal candidates to be a host for interstitial vortex matter. Monte Carlo simulations of the frustrated XY model have been performed recently in order to study extremely type-II layered superconductors in perpendicular magnetic field with sparse columnar pins located at random [9]. These simulations find evidence for the existence of an interstitial liquid/glass phase as described above.…”

Phase diagram of vortex matter in layered superconductors with tilted columnar pinning centers

“…Effects of random columnar pinning produced by heavy-ion bombardment have been studied theoretically 2,3 , experimentally 4 and numerically 5,6,7,8,9 for the situation in which both the columnar pins and the magnetic field are perpendicular to the layers. In this geometry, if the areal concentration of columnar pins exceeds that of vortex lines (i.e.…”

“…The pinned vortices delocalize at a "depinning crossover" that occurs at a higher temperature. Numerical studies 6,7,8 also indicate the occurrence of a topologically ordered phase, analogous to the Bragg glass (BrG) phase 10,11 in systems with random point pinning, at low temperatures if the relative concentration of columnar pins is sufficiently small.…”

Comparatively high in-field critical current in type-II superconductors from heterogeneous columnar pins: A molecular dynamics study

“…Theoretical arguments and numerical Monte Carlo simulations indicate, in particular, that a hexatic Bose glass state can exist at low temperature in such a case [19] [20]. Here the vortex lattice is threaded by isolated lines of edge dislocations parallel to the lines of relatively weak pins/anti-pins.…”