Uncommonly high upper critical field of the pyrochlore superconductor K Os2 O6 below the enhanced paramagnetic limit

The entire temperature dependence of the upper critical field Hc2 in the β -pyrochlore K Os2 O6 is obtained from high-field resistivity and magnetic measurements. Both techniques identically give Hc2 (T 0 K) not only surprisingly high (∼33 T), but also the approach to it is unusually temperature linear all the way below Tc (=9.6 K). We show that, while Hc2 (0) exceeds a simple spin-singlet paramagnetic limit HP, it is well below an HP enhanced due to the missing spatial inversion symmetry reported recently in K Os2 O6, ensuring that the pair breaking here is executed by orbital degrees. Ab initio calculations of orbital Hc2 show that an unusual temperature dependence is reproduced if dominant s -wave superconductivity resides on the smaller closed Fermi surfaces. © 2006 The American Physical Society.