Nonmonotonic superconducting transition temperature and large bulk modulus in the alloy superconductor Nb4Rh2C1−

We revised normal-state and superconducting properties of an alloy superconductor Nb4Rh2C1−δ through electrical transport, magnetic susceptibility, and specific heat. Our results reveal that it is a Pauli paramagnetic bad metal, a strongly coupled superconductor with specific heat jump ΔCe/γnTc∼1.65 and double s-wave gap symmetry. The evolution of superconducting transition temperature Tc was investigated by high-pressure electrical transport. It was found that Tc exhibits a nonmonotonic pressure dependence with a maximum Tcmax∼11 K, which is a record for the η-carbide alloy superconductors. Zero-temperature upper critical field Bc2(0) reduces smoothly from beyond to below the Pauli limit in 40–50 GPa. In situ synchrotron x-ray diffraction reveals a relatively large bulk modulus B0 ∼ 278.6 GPa in comparison with other alloy superconductors, which can explain the small-pressure coefficient dTc/dP ∼ −0.045 K/GPa above 20 GPa. The observations of high Tc and large B0 make Nb4Rh2C1−δ a promising superhard alloy superconductor. The pressure evolution of Bc2(0) is discussed in terms of Fermi surface reconstructions and the ratio of superconducting coherence length and electron mean free path of polycrystalline superconductors. locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon Physics Subject Headings (PhySH)Electrical conductivityFermi surfaceHigh-pressure studiesSuperconductivity