Uncommonly high upper critical field of the pyrochlore superconductor KOs2O6 below the enhanced paramagnetic limit

The entire temperature dependence of the upper critical field H-c2 in the beta-pyrochlore KOs2O6 is obtained from high-field resistivity and magnetic measurements. Both techniques identically give H-c2(T similar or equal to 0 K) not only surprisingly high (similar to 33 T), but also the approach to it is unusually temperature linear all the way below T-c(=9.6 K). We show that, while H-c2(0) exceeds a simple spin-singlet paramagnetic limit H-P, it is well below an H-P enhanced due to the missing spatial inversion symmetry reported recently in KOs2O6, ensuring that the pair breaking here is executed by orbital degrees. Ab initio calculations of orbital H-c2 show that an unusual temperature dependence is reproduced if dominant s-wave superconductivity resides on the smaller closed Fermi surfaces.