【Date and Time】11-November-2025 14:00 (Beijing time)

【Venue】Room 526

【Host】Katsumi Tanigaki (BAQIS)

【Title】 Spin-triplet superconductivity and the field-induced phenomena of UTe₂

【Speaker】 

Dai Aoki received his PhD in Physics from Osaka University in 2000 and then spent two years as a postdoctoral fellow at CEA and CNRS in Grenoble, France. During this period, he discovered the first ferromagnetic superconductor at ambient pressure, URhGe. After returning to Japan as an assistant professor at Tohoku University, he studied transuranium compounds and discovered the first neptunium-based heavy-fermion superconductor, NpPd₅Al₂.

In 2007, he obtained a permanent research position at CEA in Grenoble, where he continued his work on ferromagnetic superconductors and other heavy-fermion systems. Since 2012, he has been a full professor at Tohoku University, focusing on the crystal growth of uranium compounds and the exploration of their physical properties under extreme conditions.

【Abstract】

Unconventional superconductivity in the heavy fermion paramagnet Ute2 has emerged as one of the most compelling topics in the field of strongly correlated electron systems [1]. Superconductivity sets in below Tc = 1.5–2.1 K, accompanied by a large specific heat jump indicative of a strong-coupling regime. The system exhibits a highly anisotropic and exceptionally large upper critical field Hc2, reminiscent of that found in ferromagnetic superconductors such as URhGe and UCoGe. Although ferromagnetic fluctuations in UTe? have not yet been confirmed, inelastic neutron scattering has revealed the presence of antiferromagnetic fluctuations with an incommensurate q-vector.

A striking feature of Ute2 is its field-reentrant superconductivity, which persists up to the metamagnetic transition field, Hm ≈ 35 T, when the magnetic field is applied along the b-axis of the orthorhombic crystal structure. In addition, multiple superconducting phases emerge at high fields and at high pressure, implying the realization of distinct superconducting order parameters. These experimental findings collectively support a spin-triplet superconducting state, which arises from the spin and orbital degrees of freedom.

A key ongoing challenge is elucidating the Fermi surface topology via quantum oscillation measurements below and above Hm, as well as in the antiferromagnetic state above Pc, using ultraclean single crystals [2]. In this talk, I will present our latest results on Ute2, focusing on Fermi surface properties [3,4], the nature of multiple superconducting phases, and the intriguing regime of field-reentrant superconductivity [5,6].

[1] See a review paper, for example, D. Aoki, J. P. Brison, J. Flouquet, K. Ishida, G. Knebel, Y. Tokunaga, and Y. Yanase, J. Phys.: Condens. Matter 34, 243002 (2022).

[2] D. Aoki, J. Phys. Soc. Jpn. 93, 043703 (2024).

[3] D. Aoki, H. Sakai, P. Opletal, Y. Tokiwa, J. Ishizuka, Y. Yanase, H. Harima, A. Nakamura, D. Li, Y. Homma, Y. Shimizu, G. Knebel, J. Flouquet, and Y. Haga, J .Phys. Soc. Jpn. 91, 083704 (2022).

[4] D. Aoki, I. Sheikin, A. McCollam, J. Ishizuka, Y. Yanase, G. Lapertot, J. Flouquet, and G. Knebel, J. Phys. Soc. Jpn. 92, 065002 (2023).

[5] D. Aoki, I. Sheikin, N. Marquardt, G. Lapertot, J. Flouquet, and G. Knebel, J. Phys. Soc. Jpn., 93, 123702 (2024).

[6] T. Vasina, D. Aoki, A. Miyake, G. Seyfarth, A. Pourret, C. Marcenat, M. Amano Patino, G. Lapertot, J. Flouquet, J.-P. Brison, D. Braithwaite, and G. Knebel, Phys. Rev. Lett. 134, 096501 (2025).