Unconventional superconductivity in Y5Rh6Sn18 probed by muon spin relaxation

<jats:title>Abstract</jats:title><jats:p>Conventional superconductors are robust diamagnets that expel magnetic fields through the Meissner effect. It would therefore be unexpected if a superconducting ground state would support spontaneous magnetics fields. Such broken time-reversal symmetry states have been suggested for the high—temperature superconductors, but their identification remains experimentally controversial. We present magnetization, heat capacity, zero field and transverse field muon spin relaxation experiments on the recently discovered caged type superconductor Y<jats:sub>5</jats:sub>Rh<jats:sub>6</jats:sub>Sn<jats:sub>18</jats:sub> ( <jats:italic>T</jats:italic><jats:sub><jats:italic>C</jats:italic></jats:sub>= 3.0 K). The electronic heat capacity of Y<jats:sub>5</jats:sub>Rh<jats:sub>6</jats:sub>Sn<jats:sub>18</jats:sub> shows a <jats:italic>T</jats:italic><jats:sup>3</jats:sup> dependence below <jats:italic>T</jats:italic><jats:sub><jats:italic>c</jats:italic></jats:sub> indicating an anisotropic superconducting gap with a point node. This result is in sharp contrast to that observed in the isostructural Lu<jats:sub>5</jats:sub>Rh<jats:sub>6</jats:sub>Sn<jats:sub>18</jats:sub> which is a strong coupling <jats:italic>s</jats:italic>—wave superconductor. The temperature dependence of the deduced superfluid in density Y<jats:sub>5</jats:sub>Rh<jats:sub>6</jats:sub>Sn<jats:sub>18</jats:sub> is consistent with a BCS <jats:italic>s</jats:italic>—wave gap function, while the zero-field muon spin relaxation measurements strongly evidences unconventional superconductivity through a spontaneous appearance of an internal magnetic field below the superconducting transition temperature, signifying that the superconducting state is categorized by the broken time-reversal symmetry.</jats:p>