UCoGe is a unique substance that exhibits a superconducting transition inside ferromagnetism. Based on our group’s previous studies, it is considered that large spin fluctuations in the c- axis direction (states close to the ferromagnetic transition) are essential for the development of superconductivity in this system. Since it has a close relationship with ferromagnetism, it has been expected that in this ferromagnetic superconducting state, a spin-triplet pair of spins of the Cooper pair will be realized. In fact, this hypothesis is supported by the fact that the upper critical field greatly exceeds the Pauli critical field obtained by simple calculation, and the NMR Knight shift does not decrease. On the other hand, even if the ferromagnetism disappears due to the application of pressure, the superconducting phase survives, and since the upper critical magnetic field is also large, the spin-triplet pair is a strong hypothesis, but there are few research cases and superconducting symmetry. The details are not clear yet.
We performed 59 Co nuclear magnetic resonance (NMR) at 1.09 GPa, where ferromagnetism disappears, on a UCoGe single-crystal sample. As a result, it was clarified that the knight shift did not decrease greatly with the superconducting transition as compared with the spin component, and was consistent with the hypothesis of the spin-triplet pair. Furthermore, a closer look at the spectrum revealed that there are two components that increase and decrease the night shift below the transition temperature (Figure). This suggests that there is a component that increases as well as decreases the spin susceptibility, and this behavior can be said to be an extremely peculiar phenomenon not seen in the spin-singlet pairs realized in many superconductors. In this measurement, the spectrum was obtained by making the magnetic field orthogonal to the c- axis. Considering this fact and the anisotropy of the magnetic susceptibility, the d vector, which is the order variable of the spin-triplet pair, should be measured at least this time. It was pointed out that under the applied magnetic field, it may be oriented in the direction close to the a-axis, which is the difficult axis of magnetization.
This research is collaborative research with Nagoya University and Tohoku University Institute for Materials Research. The results are published in Physical Review B as a Regular Article. Click here for the preprint .
Article information
Masahiro Manago, Shunsaku Kitagawa, Kenji Ishida, Kazuhiko Deguchi, Noriaki K. Sato, and Tomoo Yamamura ” Spin-triplet superconductivity in the paramagnetic UCoGe under pressure studied by 59 Co NMR ” Phys. Rev. B 100, 035203 (Jul. 2019).
