Observation of superconductivity and enhanced upper critical field of η-carbide-type oxide Zr4Pd2O

Abstract We report the first observation of bulk superconductivity of a η-carbide-type oxide Zr4Pd2O. The crystal structure and the superconducting properties were studied through synchrotron X-ray diffraction, magnetization, electrical resistivity, and specific heat measurement. The superconducting transition was observed at T c = 2.73 K. Our measurement revealed that the η-carbide-type oxide superconductor Zr4Pd2O shows an enhanced upper critical field μ 0 H c2(0) = 6.72 T, which violates the Pauli-Clogston limit μ 0 H P = 5.29 T. On the other hand, we found that the enhanced upper critical field is absent in a Rh analogue Zr4Rh2O. The large μ 0 H c2(0) of Zr4Pd2O would be raised from strong spin–orbit coupling with Pd-4d electrons. The discovery of new superconducting properties for Zr4Pd2O would shed light on the further development of η-carbide-type oxide superconductors