Evidence of unconventional pairing in the quasi two-dimensional CuIr${}_2$Te${}_4$ superconductor

Abstract

The CuIr${}_{2-x}$Ru${}_x$Te${}_4$ superconductors (with a $T_c$ 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 ($\hat{I}¼$SR). In CuIr${}_{2-x}$Ru${}_x$Te${}_4$, both the temperature-dependent electronic specific heat and the superfluid density (determined via transverse-field $\hat{I}¼$SR) 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 $H_{c2}(T)$. However, under applied pressure, a charge-density-wave order starts to develop and, as a consequence, the superconductivity of CuIr${}_2$Te${}_4$ achieves a more conventional s-wave character. From a series of experiments, we provide ample evidence that the CuIr${}_{2-x}$Ru${}_x$Te${}_4$ family belongs to the rare cases, where an unconventional superconducting pairing is found near a charge-density-wave quantum critical point.

10 pages, 12 figures