Seminar：Upper critical field in ferromagnetic superconductor UCoGe

Title: Upper critical field in ferromagnetic super-conductor UCoGe

Speaker: Beilun Wu

UCoGe is among the few compounds where superconductivity is found on the border of ferromagnetism. In such systems ferromagnetism and superconductivity may not only coexist microscopically, they are considered as two closely related phenomena as well.

Here we present our upper critical field study in the system of UCoGe. The upper critical field curves at ambient pressure are obtained by thermal conductivity measurements for magnetic field H along all three crystal axes. These bulk transition measurements confirm the numbers of particularities in upper critical field previously observed by resistivity measurements in UCoGe (the very large anisotropy in Hc2, the peculiar curvature in Hc2 for field along all three field directions…) [1] We also present our resistivity results under pressure up to 10.5GPa in a magnetic field along the easy magnetization axis H//c. The superconductivity domain extends up to 4GPa and the upper critical field is probed in the whole SC phase.  

All these results are very difficult to analyze in the framework of the usual orbital and paramagnetic limitation of the upper critical field. We show instead that, as suggested by various theoretical models for ferromagnetic superconductors, (see theories by with the theories of Y.Tada [2] and V.Mineev [3]), these results strongly support a magnetic field dependence of the SC coupling constant. We show that the anisotropy and the anomalous curvatures of the Hc2 curves at zero pressure, as well and the evolution of Hc2 // c under pressure, are consistently understood with a simple quantitative model for the field dependence of the pairing strength. It points out the major role of ferromagnetic fluctuations as a mechanism for superconductivity in this system.

[1] Aoki, D.; Matsuda, T.D.; Taufour, V.; Hassinger, E.; Knebel, G. & Flouquet J., J. Phys. Soc. Jpn. 78, 113709 (2009).

[2] Y. Tada, S. Fujimoto, N. Kawakami, T. Hattori, Y. Ihara, K.Ishida, K. Deguchi, N. K. Sato, and I. Satoh, Journal of Physics: Conference Series 449 (2013) 012029

[3] V. P. Mineev, PRB 83, 064515 (2011)

[4] Hattori, T.; Ihara, Y.; Nakai, Y.; Ishida, K.; Tada, Y.; Fujimoto, S.; Kawakami, N.; Osaki, E.; Deguchi, K.; Sato, N. K. & Satoh, I., PRL 108, 066403 (2012)