Observation of superconductivity and enhanced upper critical field of $\eta$-carbide-type oxide Zr${}_4$Pd${}_2$O

Abstract

We report the first observation of bulk superconductivity of a $\eta$-carbide-type oxide Zr${}_4$Pd${}_2$O. 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 $T_{\mathrm{c}}$ = 2.73 K. Our measurement revealed that the $\eta$-carbide-type oxide superconductor Zr${}_4$Pd${}_2$O shows an enhanced upper critical field ${\mu }_0{H}_{c2}(0)$ = 6.72 T, which violates the Pauli-Clogston limit ${\mu }_0{H}_{\mathrm{P}}$ = 5.29 T. On the other hand, we found that the enhanced upper critical field is absent in a Rh analogue Zr${}_4$Rh${}_2$O. The large ${\mu }_0{H}_{c2}(0)$ of Zr${}_4$Pd${}_2$O would be raised from strong spin-orbit coupling with Pd-4$d$ electrons. The discovery of new superconducting properties for Zr${}_4$Pd${}_2$O would shed light on the further development of $\eta$-carbide-type oxide superconductors.

12 pages, 4 figures, 3 tables