| Research Abstract |
Physical properties of borocarbide superconductors, LuNi_2B_2C and YNi_2B_2C,have been investigated by using a variety of experimental probes. These compounds were found to be clean limit type II superconductors with the upper and lower critical fields, -10T and -0.1T,respectively. Despite the layred structure, any significant anisotropy in the critical fields with fields parallel and perpendicular to the NiB plane was not observed, indicating that the borocarbides are electronically three dimensional rather than two dimensional. A relatively large electronic specific coefficient gamma=18mJ/molK^2 was estimated from the temperature dependent specific heat of LuNi_2B_2C,implying that the high density of states, associated with the presence of a narrow Ni3d band at the Fermi level, is part of the reason for the high-Tc.
The high density of state, on the other hand, suggests the importance of electron correlation effects. Indeed, we have shown experimentally that the strong electron correl
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ation indeed plays a very important role in the borocarbides. The resistivities of LuNi_2B_2C and YNi_2B_2C single crystals were found to show T^2-dependence at low temperatures, with the coefficient A satisfying Kadowaki-woods relationship which holds for a variey of correlated Fermi liquids. We found that the Ni 3d bandwidth of YNi_2B_2C determined by photoemission (with Fujimori group) is 20% narrower than the one estimated from the band calculation, which can be ascribed to the correlation efect. The ^<11>B and ^<61>Ni NMR measurements on LuNi_2B_2C and YNi_2B_2C (with Yasuoka group) indicated a strong enhancement of (T_1T)^<-1> in contrast to the weakly temperature dependent Knight shift. The borocarbide is therefore a superconductors with strong antiferromagnetic spin fluctuations.
Symmetry of Cooper pairs have been investigated through the quasiparticle states within the vortex core. Magnetic field dependence of the specific heat of LuNi_2B_2C was measured by a relaxation calorimeter. We found that the electronic specific heat coefficient gamma depends on the magnetic field H as gamma *ROO<H>, in contrast to those expected for conventional superconductors with an isotropic gap, gamma*H.This anomalous field dependence of gamma can be understood by assuming the presence of a gapless region on the Fermi surface. Considering the presence of significant spin fluctuations in the compound, we therefore suggest-d-wave superconductivity in this compound. Less
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