Oscillatory behavior of vortex-lattice melting transition line in mesoscopic Bi_{2}Sr_{2}CaCu_{2}O_{8+y} superconductors.

The vortex-lattice melting transition of a limited number of vortices confined in mesoscopic square superconductors was studied by c-axis resistance measurements using stacks of intrinsic Josephson junctions in Bi_{2}Sr_{2}CaCu_{2}O_{8+y}. In contrast to the melting transition in bulk crystals, we have first found a clear oscillatory behavior in the field dependence of the melting temperature in small samples of 5-10 µm square. The periods of the oscillations roughly obey the regularity of the matching conditions of square vortex lattices surrounded by a square boundary and the melting temperatures are enhanced around the vortex number of i^{2} (where i is an integer). The results suggest that a confinement effect by the square boundary stabilizes square lattice structures which are realized around i^{2} vortex number even in competition with the favorable Abrikosov triangular lattice in the bulk.

Tunable terahertz emission from the intrinsic Josephson junctions in acute isosceles triangular Bi2Sr2CaCu2O8+δ mesas.

In order to determine if the mesa geometry might affect the properties of the coherent terahertz (THz) radiation emitted from the intrinsic Josephson junctions in mesas constructed from single crystals of the high-temperature superconductor, Bi2Sr2CaCu2O8+δ, we studied triangular mesas. For equilateral triangular mesas, the observed emission was found to be limited to the single mesa TM(1,0) mode. However, tunable radiation over the range from 0.495 to 0.934 THz was found to arise from an acute isosceles triangular mesa. This 47% tunability is the widest yet observed from the outer current-voltage characteristic branch of such mesas of any geometry. Although the radiation at a few of the frequencies in the tunable range appear to have been enhanced by cavity resonances, most frequencies are far from such resonance frequencies, and can only be attributed to the ac-Josephson effect.

Lattice effect of strong electron correlation: implication for ferroelectricity and superconductivity.

Much theoretical work has been devoted to understanding the role of strong electron correlations in high-temperature superconductivity mainly through magnetic interactions, but the possible role of electron correlation in ferroelectricity of metal oxides has not received attention. Diagonalization of a simple many-body, tight-binding Hamiltonian shows that the electron-lattice interaction is dramatically enhanced in some cases by strong electron correlation because of deformation-induced charge transfer. This effect may be closely related to ferroelectricity and superconductivity in transition metal oxides.

Possible tricritical point in phase diagrams of interlayer josephson-vortex systems in high- T(c) superconductors

A critical value in the product of the anisotropy parameter and the magnetic field is observed in interlayer Josephson-vortex systems by extensive Monte Carlo simulations. Below (above) this critical value the thermodynamic phase transition between the normal and the superconducting states upon temperature sweeping is first (second) order. It is discussed that the origin of this tricritical point is the highly anisotropic layered structure of high- T(c) superconductors.