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

我们从理论上再现了有机超导体κ-(BETS)2FeBr4在温度和磁场平面上的相图。在该化合物中，不仅在高磁场中存在场致超导性，而且在低磁场中也存在超导性，在低磁场中反铁磁长程序与超导性共存。我们将FISC的Jaccarino-Peter机制扩展到反铁磁超导体。根据从FISC的临界场确定的模型参数值，我们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机制发生在真实材料中。根据这一机制，我们可以预测在某种反铁磁超导体中，上临界场可以提高到远远大于泡利顺磁极限的值。此外，我们还研究了该化合物中FFLO状态的可能性。研究发现，与λ-(BETS)2FeCl4化合物相比，FFLO在FISC中几乎不发生，而在低场超导中可能发生。我们研究了重费米子超导体中超导的各向异性和配对机制。UPd2Al3。我们指出在该化合物中，层内配对被共存的反铁磁长程序所禁止。此外，我们还指出，稳定反铁磁长程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，并且这种单线态序参数根本不影响共存的反铁磁序。这些结果与最近的热导率和核磁共振数据一致。

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.