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

用约瑟夫森效应和点接触谱研究了CeCu2Si2，CeCoIn5，CeIrIn5重费米子超导体的d波序参数。制作了这些材料与Al之间的约瑟夫森结，并测量了约瑟夫森临界电流随磁场的变化关系。对于CeIrIn5，观察到了[100]，[110]和[001]方向的夫琅和费衍射图，这可能表明了传统的类BCS超导电性。相反，CeCoIn5显示出沿[001]方向的结振荡图案，这与dx_2-y_2态并不矛盾。多晶CeCu2Si2也表现出衍射图，尽管结合区含有几十个微晶。如果晶体的方向是随机的，这一结果就不能用d波超导电性来理解。对于点接触光谱分析，用商用的压电式致动器轻轻地将铂针压在样品上。CeCoIn_5上的一些接触显示出典型的隧道效应光谱。由光谱得到的能隙Δ在报告值的散射范围内，并且Δ的温度依赖关系不同于传统的BCS超导体。另一方面，CeIrIn5只给出了反映体阻超导转变的光谱，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.