Borocarbide Superconductors, Materials and Properties

用不同的实验探针研究了硼碳化物超导体LuNi_2B_2C和YNi_2B_2C的物理性质。这些化合物是清洁极限II型超导体，上临界场为-10T，下临界场为-0.1T。尽管具有层状结构，但在平行和垂直于NiB平面的关键场中没有观察到任何显著的各向异性，这表明硼碳化物在电子上是三维的而不是二维的。从LuNi_2B_2C的温度相关比热估计出了一个相对较大的电子比系数γ =18mJ/molK^2，这意味着在费米能级上存在窄Ni3d带的高密度态是高tc的部分原因。另一方面，态的高密度表明了电子相关效应的重要性。事实上，我们已经通过实验证明，在硼碳化物中，更多的离子化确实起着非常重要的作用。发现LuNi_2B_2C和YNi_2B_2C单晶的电阻率在低温下与T^2相关，且系数A满足Kadowaki-woods关系，该关系适用于各种相关费米液体。我们发现YNi_2B_2C的Ni三维带宽（带有Fujimori基团）比波段计算估计的带宽窄20%，这可归因于相关效应。在LuNi_2B_2C和YNi_2B_2C（与Yasuoka组）上的^<11>B和^<61>Ni NMR测量表明（T_1T）^<-1>强烈增强，而不是弱温度依赖性的Knight位移。因此硼碳化物是一种具有强反铁磁自旋涨落的超导体。通过涡旋核内的准粒子态研究了库柏对的对称性。用弛豫量热计测定了LuNi_2B_2C的比热与磁场的关系。我们发现，电子比热系数取决于磁场H，即γ *ROO<H>，这与具有各向同性间隙的传统超导体的期望相反，伽马*H。通过假设费米表面上存在一个无间隙区域，可以理解伽马的这种异常场依赖性。考虑到化合物中存在显著的自旋波动，因此我们认为该化合物具有-d波超导性。少

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 … More 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