Time-reversal symmetry breaking in the noncentrosymmetric Zr3Ir superconductor

We report the discovery of Zr3Ir as a structurally different type of unconventional noncentrosymmetric superconductor (with T-c = 2.3 K), here investigated mostly via muon-spin rotation/relaxation (mu SR) techniques. Its superconductivity was characterized using magnetic susceptibility, electrical resistivity, and heat capacity measurements. The low-temperature superfluid density, determined via transverse-field mu SR and electronic specific heat, suggests a fully gapped superconducting state. The spontaneous magnetic fields, revealed by zero-field mu SR below T-c, indicate the breaking of time-reversal symmetry in Zr3Ir and hence the unconventional nature of its superconductivity. By using symmetry arguments and electronic-structure calculations we obtain a superconducting order parameter that is fully compatible with the experimental observations. Hence, our results clearly suggest that Zr3Ir represents a structurally different member of noncentrosymmetric superconductors with broken time-reversal symmetry.