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Combined experimental and theoretical studies are reported of the Fermi surface, band structure, generalized magnetic susceptibility, electron-phonon enhancement factor λ and superconducting transition temperature TT of LaB6. Whereas the unusually large λ values, ranging from 1.0 to 2.5, are expected to result in high TT values, TT is observed to be only 0.122°K. These results further emphasize the need for appropriate theoretical formulations for these systems.

A magnetic field penetrates a superconductor through an array of ‘vortices’, each of which carries one quantum of flux that is surrounded by a circulating supercurrent. In this vortex state, the resistivity is determined by the dynamical properties of the vortex ‘matter’. For the high-temperature copper oxide superconductors (see ref.1 for a theoretical review), the vortex phase can be a ‘solid’, in which the vortices are pinned, but the solid can ‘melt’ into a ‘liquid’ phase, in which their mobility gives rise to a finite resistance. (This melting phenomenon is also believed to occur in conventional superconductors, but in an experimentally inaccessible part of the phase diagram2.) For the case of YBa2Cu3O7, there are indications of the existence of a critical point, at which the character of the melting changes3,4,5,6,7,8,9,10. But neither the thermodynamic nature of the melting, nor the phase diagram in the vicinity of the critical point, has been well established. Here we report measurements of specific heat and magnetization that determine the phase diagram in this material to 26 T, well above the critical point. Our results reveal the presence of a reversible second-order transition above the critical point. An unusual feature of this transition—namely, that the high-temperature phase is the less symmetric in the sense of the Landau theory11—is in accord with theoretical predictions12,13,14 of a transition to a second vortex-liquid phase.

Durch Elektrokristallisation (2 uA) von (I) und Tetrabutylammonium-Isr, in THF entsteht an den Pt-Elekvroden eine a- und eine β-Phase des Titelkomplexes, von denen nur die β-Phase (RG: PT, Z = l) supraleitend wird (Sprungtempetatur 2.3-2.7 K bzw. 4.2 K fur einen metastabilen Zustand).

A remarkable increase in T/sub c/ (superconducting transition temperature) to temperatures near 90 K has been reported recently for a phase of the Y-Ba-Cu-O system, variously identified as YBa2Cu3O/sub 9-delta/ (delta = 2.1), YBa2C3O/sub 6+x/ (x = 0.5), and YBa2Cu3O/sub 6.81/. Relatively sharp superconducting transitions occur for this material with onset T/sub c/'s at 93-98 K, with zero resistance at 80-94 K, and with substantial Meissner effects measured at temperatures not far below zero resistance. This superconducting oxide is a compound similar in metal stoichiometry to Y2Cu2O5 on the basis of the total noncopper metal-to-copper atomic ratios. It is well documented that, in addition to Y, the lanthanide ions, LnT , with more than seven 4f electrons form similar compounds, Ln2Cu2O5. Consequently, one speculates that superconducting oxides with T/sub c/'s near 90 K might also occur for compositions LnBa2Cu3O/sub 6+x/, where LnT is any one of the heavier lanthanide ions. An interesting question in this case is the effect of the magnetic moment on superconductivity. The presence of the moment decreases T/sub c/ through exchange interactions and, if magnetic ordering occurs, may lead to the coexistence of superconductivity and magnetism, or reentrant superconductivity (reentrant means return to the normal state).more » They have chosen to explore the synthesis and superconducting properties of HoBa2Cu3O/sub 6+x/ because HoT has the same radius as nonmagnetic YT (0.89 A)Y and, along with DyT , has the largest magnetic moment (10.4 /sub B/) of the LnT ions. In the course of this work, conflicting reports have appeared which indicate that an uncharacterized phase (or phases) of the Ho-Ba-Cu-O system is superconducting with T/sub c/'s of approx. 46, 70, or 93 K. In this communication they report on bulk superconductivity near 91 K without reentrant behavior down to 0.55 K for HoBa2Cu3O/sub 6+x/.« less