Thermal transport of uncomventional superconductors at low temperatures

测量了YNi_2B_2C在涡旋状态下的低温热容C_p和微波表面阻抗Z_s。与传统的s波超导体相比，C_p显示出接近H^<0.5>的依赖关系。这种H^<0.5>的依赖性即使在引入柱状缺陷后仍然存在。柱状缺陷极大地改变了涡旋核区电子结构，并强烈地扰乱了规则的涡旋晶格。另一方面，由Z_s得到的磁通流电阻率几乎与H成正比，排除了涡旋核收缩的可能性。综上所述，这些结果表明YNi_2B_2C的涡旋态与传统的s波对应物有着根本的不同，涡旋核心周围的离域准粒子态起着更重要的作用，类似于d波超导体。为了阐明自旋三重态超导体Sr_2RuO_4的超导间隙结构，测量了二维RuO_2平面内热流相对取向、晶体轴和旋转磁场的平面内热导率。热导率的面内变化与任何线节点垂直于二维平面的模型都不兼容，表明存在水平节点。这些结果对试图解释三重态超导机制的模型施加了强烈的限制。

We have measured the low temperature heat capacity C_p and microwave surface impedance Z_s in the vortex state of YNi_2B_2C.In contrast to conventional s-wave superconductors, C_p shows a nearly H^<0.5>-dependence. This H^<0.5>-dependence persists even after the introduction of the columnar defects which change the electronic structure of the vortex core regime dramatically and strongly disturb the regular vortex lattice. On the other hand, flux flow resistivity obtained from Z_s is nearly proportional to H, ruling out the vortex core shrinking scenario. Taken together, these results indicate that the vortex state of YNi_2B_2C is fundamentally different from the conventional s-wave counterparts, in that the delocalized quasiparticle states around the vortex core play a much more important role, similar to d-wave superconductors.To clarify the superconducting gap structure of the spin-triplet superconductor Sr_2RuO_4, the in-plane thermal conductivity has been measured as a function of relative orientations of the thermal flow, the crystal axes, and a magnetic field rotating within the 2D RuO_2 planes. The in-plane variation of the thermal conductivity is incompatible with any model with line nodes vertical to the 2D planes and indicates the existence of horizontal nodes. These results place strong constraints on models that attempt to explain the mechanism of the triplet superconductivity.