Nodeless superconductivity with broken time reversal symmetry

In unconventional superconductors, additional symmetries, e.g., rotational symmetry and time reveral symmetry (TRS), may be broken below the superconducting transition temperature. In the past, evidence for TRS breaking was observed in a few nodal superconductors with a possible triplet pairing state. Recently, there are a growing number of compounds which behave like a simple s-wave BCS superconductor, but break TRS below T_c. How to reconcile these seemingly contradictory feature is currently an interesting problem. In this presentation, we will present our recent results on a few Re- and La-based nodelss superconductors with broken TRS at T_c, including a few new examples. We will try to summrize their common features and discuss the possible pairing states and origin of the TRS breaking.

This work is in collaboration with M. Smidman, A, Wang, P. R. Zhang, Z. Y. Nie, W. Xie, G. M. Pang (Zhejiang University), T. Shang (PSI), R. P. Singh (IISER), J. Akimitsu (Aoyama-Gakuin U), J. Quintanilla, S. K. Ghosh (U. Kent) and J. Annett (U. Bristol).