Evidence of unconventional pairing in the quasi-two-dimensional CuIr2-x Rux Te4 superconductor

The CuIr2-xRuxTe4 superconductors (with a Tc around 2.8 K) can host charge-density waves, whose onset and interplay with superconductivity are not well known at a microscopic level. Here, we report a comprehensive study of the x=0 and 0.05 cases, whose superconductivity was characterized via electrical-resistivity, magnetization, and heat-capacity measurements, while their microscopic superconducting properties were studied via muon-spin rotation and relaxation (μSR). In CuIr2-xRuxTe4, both the temperature-dependent electronic specific heat and the superfluid density (determined via transverse-field μSR) are best described by a two-gap (s+d)-wave model, comprising a nodeless gap and a gap with nodes. The multigap superconductivity is also supported by the temperature dependence of the upper critical field Hc2(T). However, under applied pressure, a charge-density-wave order starts to develop and, as a consequence, the superconductivity of CuIr2Te4 achieves a more conventional s-wave character. Our series of experiments provides ample evidence that the CuIr2-xRuxTe4 family belongs to the rare cases where an unconventional superconducting pairing is found near a charge-density-wave quantum critical point.