A theoretical study of superconductivity in layered type-II superconductors in strong parallel magnetic fields

本文从理论上再现了有机超导体κ-（BETS）_2FeBr_4在温度和磁场平面上的相图。在该化合物中，已知不仅在高磁场下发生场致超导（FISC），而且在低磁场下也发生超导，并且在低磁场下反铁磁长程有序与超导共存。我们将FISC的Jaccarino-Peter机制推广到反铁磁超导体。根据由FISC临界场确定的模型参数值，定量地再现了低温超导的上临界场。这一结果表明，扩展的Jaccarino-Peter机制发生在一个真实的材料。根据这一机制，我们可以预言，在某种反铁磁超导体中，上临界场可以被增强到远大于泡利顺磁极限的值。此外，我们还研究了该化合物中存在FFLO态的可能性。与λ-（BETS）_2FeCl_4化合物相比，FISC中几乎不发生FFLO，而在低场超导中可能发生FFLO.我们研究了重费米子超导体的各向异性和超导配对机制. UPd2A1 3.我们已经指出，在这种化合物层内配对是禁止共存的反铁磁长程有序。此外，我们还指出，稳定反铁磁长程有序的反铁磁交换相互作用，如在该化合物中观察到的，起到层间单线态配对相互作用的作用，这样的单线态序参数是不影响共存的反铁磁秩序在所有。这些结果与最近的热导率和核磁共振数据是一致的。

We have reproduced a phase diagram in temperature and magnetic field plane in the organic superconductor κ-(BETS)2FeBr4 theoretically. In this compound, it was known that not only field-induced superconductivity (FISC) in high magnetic field, but also superconductivity occurs in low magnetic fields, and at low fields antiferromagnetic long-range order coexists with superconductivity. We extended the Jaccarino-Peter mechanism of the FISC to antiferromagnetic superconductors. Based on values of model parameters that are determined from the critical fields of the FISC, we have reproduced upper critical fields of low temperature superconductivity quantitatively. This results suggests that the extended Jaccarino-Peter mechanism takes place in a real material. According to this mechanism, we could predict that upper critical field may be enhanced up to a value which is much larger than the Pauli paramagnetic limit in a certain kind of antiferromagnetic superconductors. In addition, we have examined possibility of the FFLO state in this compound. It has been found that the FFLO hardly occurs in the FISC in contrast to the λ-(BETS)2FeCl4 compound, while it may occur in low-field superconductivity.We have examined the anisotropy and the pairing mechanism of superconductivity in the heavy fermion superconductor. UPd2Al3. We have pointed out that in this compound intralayer pairing is prohibited by the coexisting antiferromagnetic long-range order. Furthermore, we have pointed out that the antiferromagnetic exchange interactions that stabilize the antiferromagnetic long-range order like one observed in this compound play a role of interlayer singlet pairing interactions, and such a singlet order parameter is not influenced the coexisting antiferromagnetic order at all. These results are consistent with recent thermalconductivity and nuclear magnetic resonance data.