Study on magnetic excitons in uranium-based heavy fermion superconductors

(1)磁激子在UPd_2Al_3中的超导电性我们成功地制备了高质量的单晶，并测量了这些单晶的物理性质，以获得磁性和超导电性的信息。我们发现在非弹性中子散射谱中存在两种成分，并将能量较低的成分归因于准粒子激发，将能量较高的成分归因于磁激子。我们提出了一个新的模型，称为“5f电子的对偶性质”，来分析数据。我们最终发现了这两个组件之间的强耦合。我们还发现，这些光谱与在Eliashberg方程框架下的隧穿实验获得的光谱是一致的。根据这些结果，我们指出超导吸引相互作用是通过玻色子磁激子的交换来实现的。这一结果发表在《自然》杂志第410(2001)340-343中。(2)含有…的UnI_2Al_3中的Ni2Al_3的磁性和超导电性在与UPd_2Al_3晶体结构相同的情况下，反铁磁有序态也与超导共存。我们进行了核磁共振实验，发现Knight位移在超导转变温度以下没有还原，这与UPd_2Al_3的各向同性还原形成了鲜明的对比。根据这一结果，再加上超导上临界场的温度依赖关系中没有Pauli限制效应，我们得出了自旋三重态(奇宇称)配对的结论。(3)研究了UGe_2和Uru_2Si_2We单晶的磁性和超导电性，研究了UGe_2的铁磁性和超导电性之间的关系。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着压力的增加而增加。在此基础上，我们提出了超导电性与铁磁性是以竞争的方式共存的观点。我们还对单晶进行了高压膨胀测量。我们发现热膨胀系数出现了一个反常现象，对应于隐藏有序和反铁磁有序之间的相变。较少

(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