Study on magnetic excitons in uranium-based heavy fermion superconductors

(1)在UPd_2Al_3中磁激子介导的超导电性我们成功地制备了高质量的单晶，并测量了这些晶体的物理性质，以获得磁性和超导电性的信息。我们发现非弹性中子散射谱中存在两个分量，并且我们安全地将较低能量分量归因于准粒子激发，而较高能量分量归因于磁激子。我们提出了一个新的模型，即“5 f电子的对偶性”，来分析数据。我们终于发现了两个分量之间的强耦合。我们还发现，光谱与Eliashberg方程框架内的隧道实验得到的光谱是一致的。从这些结果中，我们指出超导吸引相互作用是通过玻色磁激子交换来实现的。该结果报道在Nature，410（2001）340-343中。(2)UNi_2Al_3In的磁性和超导电性 ...更多信息 与UPd_2Al_3的晶体结构相同，反铁磁有序态也与超导电性共存。我们进行了核磁共振实验，发现在超导转变温度以下，Knight位移不发生还原，这与UPd_2Al_3的各向同性还原形成鲜明对比。从这一结果以及超导上临界场的温度依赖性中不存在泡利限制效应，我们得出结论，形成了自旋三重态（奇宇称）配对。(3)UGe_2和URu_2Si_2的磁性和超导电性我们制备了单晶体来研究UGe_2的铁磁性和超导电性之间的关系。我们测量了高压下的直流磁化强度和交流磁化率，发现超导体积分数随压力的增加而增加。我们还对URu_2Si_2单晶进行了高压下的膨胀测量。我们发现了一个异常的热膨胀系数，对应于隐藏和反铁磁顺序之间的相变。少

(1) Magnetic exciton mediated superconductivity in UPd_2Al_3We succeeded in preparing single crystals of high quality, and measured physical properties of those crystals to obtain information of the magnetism and superconductivity. We discovered the existence of two components in inelastic neutron scattering spectra, and we safely ascribed the lower energy component to quasiparticle excitations and the higher energy one to magnetic excitons. We proposed a new model, that is called "dual nature of 5f electrons", to analyze the data. We finally found the strong coupling between the two components. We also found that the spectra are consistent with spectra obtained from tunneling experiments within the Eliashberg equation framework. From these results, we pointed out that the superconducting attractive interaction is mediated by exchange of bosonic magnetic excitons. This result was reported in Nature, 410(2001)340-343.(2) Magnetism and superconductivity in UNi_2Al_3In UNi_2Al_3 that has … More the same crystal structure as UPd_2Al_3, the antiferromagnetic ordered state also coexists with superconductivity. We carried out NMR experiments, and found that the Knight shift shows no reduction below the superconducting transition temperature, in remarkably contrast with the isotropic reduction in UPd_2Al_3. From this result together with the absence of the Pauli limiting effect in the temperature dependence of the superconducting upper critical field, we concluded that the spin-triplet (odd parity) paring is formed.(3) Magnetism and superconductivity in UGe_2 and URu_2Si_2We prepared single crystals to investigate the correlation of ferromagnetism and superconductivity in UGe_2. We measured the dc magnetization and ac magnetic susceptibility under high pressures, and we found that the superconducting volume fraction grows with increasing pressure. From this, we suggested that the superconductivity coexists in a competitive way with the ferromagnetism.We also carried out the dilatation measurements under high pressure for single crystals of URu_2Si_2. We found an anomaly in the thermal expansion coefficient that corresponds to the phase transition between hidden and antiferromagnetic order. Less