A second phase transition and superconductivity in the beta-pyrochlore oxide KOs2O6

Abstract

Another phase transition that is probably of first order is found in the beta-pyrochlore oxide superconductor KOs2O6 with a superconducting transition temperature Tc of 9.6 K. It takes place at Tp=7.5 K in the superconducting state in a zero magnetic field. By applying magnetic fields of up to 140 kOe, the Tc gradually decreased to 5.2 K, while Tp changed little, eventually breaking through the Hc2 line at approximately 65 kOe in the H-T diagram. Both the normal-state resistivity and Hc2 change slightly but significantly across the second phase transition. It is suggested that the transition is associated with the rattling of potassium ions located in an oversized cage of osmium and oxide ions.