Superconductivity is a quantum mechanical phenomenon, where metals show zero electrical resistivity at low temperatures. It is carried by electron pairs which form a superconducting state. The attractive force for pairing is mediated by lattice vibrations, where electrons are coupled so as to cancel the dynamical charge imbalance due to phonons. On the other hand, it is believed that magnetic fluctuations play an important role in the pairing mechanism of high-Tc cuprate and/or uranium (U) superconductors. In these systems, superconductivity exists in a magnetically ordered phase or in the vicinity of a quantum critical point with strong magnetic fluctuations. Conduction electrons have strong repulsion; thus, pairing without overlap of the electron wave functions is energetically favorable. This results in a non-BCS type of superconductivity with an anisotropic energy gap which closes on a line, or at point nodes.
Very recently, the new superconductor PrOs4Sb12, having the filled-skutterudite structure shown in Fig. 7-7, has been discovered. This compound exhibits superconductivity below Tc =1.85K with no magnetic ordering down to 50mK. An ordered phase has been reported to occur on application of a high magnetic field. Neutron scattering experiments have been carried out in order to investigate this field-induced ordered phase.
As shown in Fig. 7-8, a clear antiferromagnetic peak is observed with the application of a field H=8 T at 0.25 K, while no trace of this peak is found for H=0 T. The magnetic structure is shown in Fig. 7-7. This antiferromagnetic structure and its temperature and field dependence have been explained in terms of field-induced antiferroquadrupolar ordering based on an f electron model with a singlet ground state and a triplet excited state.
The phase diagram of PrOs4Sb12 is shown schematically in Fig. 7-9 (a). The superconducting phase occurs very close to the antiferroquadrupolar phase. It should be noted that strong magnetic and quadrupolar fluctuations take place in the paramagnetic phase. This f electron state with multipolar degree of freedom could be the origin of the heavy fermion superconducting state, where the superconductivity might possibly be mediated by quadrupole fluctuations. This is suggested by a comparison of the phase diagrams of high-Tc and U superconductors shown in Fig. 7-9 (b), where superconductivity occurs in the vicinity of a magnetically ordered phase having strong magnetic fluctuations. Further studies are necessary to shed light on the microscopic mechanism of the superconductivity in PrOs4Sb12. |
