Multiple superconducting phases and field-induced phenomena in spin-triplet superconductor UTe2

We present our recent results on the novel spin-triplet superconductor UTe2, which is at the verge of the ferromagnetic order. The huge upper critical field exceeding the Pauli limit indicates the spin-triplet state. For the field along b-axis, the field reentrant superconductivity is observed up to Hm~35T, where the first order metamagnetic transition occurs. The field reentrant superconductivity in UTe2 is similar to that observed in ferromagnetic superconductors, namely URhGe and UCoGe. Applying the pressure in UTe2, the superconducting transition temperature splits and the mulitple superconducting phase is detected in the AC calorimetry measurements as a thermodynamic response. We overview the results on UTe2 comparing with ferromagnetic superconductors, and show our perspective. This work was done in collaboration with G. Knebel, A. Pourret, J.P. Brison, D. Braithwaite, Q. Niu, M. Valiska, J. Flouquet, A. Nakamura, F. Honda, Y. Homma, D.X,Li, Y. Shimizu, Y. J. Sato, A. Miyake, S. Imajo, S. Kohama, M. Tokunaga, H. Harima, W. Knafo, I. Sheikin, C. Paulsen, Y. Tokunaga, S. Kambe, H. Sakai, S. Fujimori, K. Ishida, G. Nakamine, S. Kitagawa.