Study of heavy-fermion d-wave superconductors using tunneling effect

用Josephson效应和点接触光谱学研究了重费米子超导体CeCu_2Si_2、CeCoIn_5和CeIrIn5的d波阶参数。制备了这些材料与Al之间的约瑟夫森结，并测量了约瑟夫森临界电流对磁场的依赖关系。对于CeIrIn5，在[100]，[110]和[001]方向上观察到弗劳恩霍夫衍射模式，可能表明传统的类bcs超导性。相比之下，CeCoIn5显示出沿[001]方向的结振荡模式，这与dx_2-y_2状态并不矛盾。多晶CeCu_2Si_2也显示出衍射模式，尽管在结区内包含了几十个晶体。如果晶体的方向是随机的，这个结果不能用d波超导性来理解。对于点接触光谱学，使用商业压电致动器将pt针轻轻压在样品上。CeCoIn_5上的一些触点显示出典型的隧穿效应光谱。从光谱中得到的能隙Δ在报道值的散射范围内，并且Δ的温度依赖性与传统BCS超导体不同。另一方面，CeIrIn5只给出了反映体电阻超导跃迁的光谱，因此无法确定能隙的精确值。

The d-wave order parameter of heavy-fermion superconductors have been investigated for CeCu_2Si_2, CeCoIn_5, CeIrIn5 using Josephson effect and point-contact spectroscopy. The Josephson junctions between these materials and Al were fabricated and the magnetic field dependence of Josephson critical current was measured. For CeIrIn5, Fraunhofer diffraction patterns have been observed for the [100], [110] and [001] directions, possibly suggesting conventional BCS-like superconductivity. In contrast, CeCoIn5 has shown an oscillating pattern for the junction along the [001] direction, which does not contradict with the dx_2-y_2 state. Polycrystalline CeCu_2Si_2 also showed a diffraction pattern in spite of several tens of crystallites included in the junction area. This result cannot be understood in terms of d-wave superconductivity, if the directions of the crystallites are at random. For point-contact spectroscopy, a Pt-needle was gently pressed onto the sample with a commercial piezo-electric actuator. Some of the contacts on CeCoIn_5 have shown spectra that are typical of tunneling effect. The energy gaps Δ derived from the spectra are within the scattering range of the reported values, and the temperature dependence of Δ differs from that of conventional BCS superconductors. CeIrIn5, on the other hand, only gave the spectra that reflect the superconducting transition of the bulk resistance, so that a precise value of the energy gap cannot be determined.