Title: Upper critical field and pairing mechanism in ferromagnetic superconductor UCoGe
When: Friday 11 november 2016
Where: Seminar room, Modulo 03, pta 5, Departamento de Fisica de la Materia Condensada, Facultad de Ciencias UAM
Who: Beilun WU, Univ. Grenoble Alpes, CEA, INAC, SPSMS, Grenoble, France

We present our transport study on the upper critical field of the orthorhombic ferromagnetic superconductor UCoGe. The bulk upper critical field curves at ambient pressure are obtained by thermal conductivity measurements for magnetic field H along all three crystal axes. For H//b in particular, the S-shape re-entrance phenomenon is well observed, and even more strongly than on resistivity measurements [1]. Surprisingly, for field above 8T along the b axis, bulk superconductivity appears to be “more robust” than observed on resistivity measurements: this is reminiscent of observations on cuprates High-Tc superconductors, where it was taken as a demonstration of the existence of a (resistive) vortex liquid phase.
The Hc2 in UCoGe is a very strange case in terms of classical theories for superconductivity. We show that the numbers of features in Hc2 in UCoGe, including the huge anisotropy and the anomalous curvatures of the Hc2 along different field directions, can be consistently understood with a simple quantitative model for the field dependence of the pairing strength[2,3]. The consistency of the model is further checked by specific heat measurements. It points out the major role of ferromagnetic fluctuations as a mechanism for superconductivity in this system. [4] [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)

Work made in collaboration with G.Bastien1, D.Braithewaite1, M. Taupin1,2, D. Aoki1,3, J.Flouquet1 and J.-P. Brison1
1 Univ. Grenoble Alpes, CEA, INAC, SPSMS, Grenoble, France
2 Low Temperature Laboratory, Aalto University, P.O. Box 13500, FI-00076 Aalto, Finland
3 Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313, Japan