Novel equilibrium phase diagram and dynamics of interlayer Josephson vortices in high-T_c superconductors - computer simulation approach

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(1) Possible phases and phase transitions of interlayer Josephson vortex-lines are explored by computer simulation and the density functional theory. In high magnetic fields where vortices reside in every layer (m=1), computer simulations reveal a novel KT phase in which the intralayer vortex correlations are quasi-long-ranged while the interlayer ones are short-ranged. This novel phase transforms into low-temperature 3D lattice phase via a 2^<nd> order phase transition and into high-temperature liquid phase via a KT transition. As the magnetic field decreases, a vortex smectic phase, which shows a short-ranged intralayer vortex correlations while the block layers with denser vortices appear in every other layers (period m=2) in a long-ranged way, is stabilized by layer pinning in materials such as YBCO, accompanied by a sequential second- and first-order transitions upon cooling. In low magnetic fields, the effect of layer pinning is suppressed drastically because of high transition t … More emperature and large discrepancy between the wave-length of layer pinning and the vortex lattice constant, which naturally recovers the anisotropy scaling of melting line. These results are reported in many international conferences as invited talks, and attracting considerable attentions. Experiments designed for testing these theoretical results are scheduled in Europe, USA and Japan.(2) By means of computer simulation we find that when the angle between the magnetic field and the ab plane is not so large the distorted triangular lattice of Josephson vortices dominates the system such that pancake vortices look like to sit on chains as observed in experiment. As the tilt angle increases, the flux lines are aligned essentially in the same way as Abrikosov vortex lattice. There is a first-order phase transition between the two structures.(3) Combining group theoretical analysis of the hexagonal system with energetics based on fermiology, we found within a single band picture, that among the likely pairing states is a novel chiral spin-triplet state, which takes advantage of the nesting among different segments of the Fermi surface. This is similar to the pairing mechanism proposed for the tetragonal material SrRuO_4, though the hexagonal case would benefit further from the nesting effect. More recent developments incorporate orbital effects. There again, the physical picture of pairing induced by scattering between Fermi segments, resulting in a triplet state continues to be an important possibility pursued both experimentally and theoretically. Less