Borocarbide Superconductors, Materials and Properties

用各种实验探头研究了碳化硼超导体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，其上、下临界场分别为-10T和-0.1T。尽管有层状结构，但在临界场中没有观察到与NiB平面平行和垂直的任何显著的各向异性，这表明碳化硼在电子上是三维的，而不是二维的。由LUNI_2B_2C随温度变化的比热估算出一个相对较大的电子比系数Gamma=18mJ/Molk^2，这意味着在费米能级上存在一个较窄的Ni3d能带，这意味着高的态密度是高T_c的部分原因。另一方面，高的态密度表明电子关联效应的重要性。事实上，我们已经通过实验证明了强电子关联…更多的元素确实在碳化硼中起着非常重要的作用。在低温下，LUNI_2B_2C和YNi_2B_2C单晶的电阻率与温度呈T^2关系，其系数A满足Kadowaki-wood关系，该关系适用于各种相关的费米液体。我们发现用光电子能谱(Fujimori基团)测得的YNi_2B_2C的Ni3d带宽比由能带计算得到的结果窄20%，这可以归因于关联效应。LUNI_2B_2C和YNi_2B_2C(带有Yasuoka基团)的^&lt；11&gt；B和^&lt；61&gt；Ni的核磁共振测量表明，(T_1T)^&lt；-1&gt；与弱的温度相关的奈特移位相反。因此，碳化硼是一种具有很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Cooper对的对称性。用弛豫量热计测量了LUNI_2B_2C的比热随磁场的变化。我们发现，电子比热系数伽马依赖于磁场H，即伽马*Roo&lt；H&gt；，这与具有各向同性带隙的常规超导体所期望的伽马*h相反。这种反常的场依赖关系可以通过假设费米面上存在无间隙区域来理解。考虑到化合物中存在显著的自旋涨落，因此我们建议该化合物具有-d波超导电性。较少

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