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

We report the first observation of bulk superconductivity of a eta-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 eta-carbide-type oxide superconductor Zr4Pd2O shows an enhanced upper critical field mu H-0(c2)(0) = 6.72 T, which violates the Pauli-Clogston limit mu H-0(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 mu H-0(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 eta-carbide-type oxide superconductors.