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Abstract:
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
Y5Rh6Sn18 (T-C=3.0 K). The electronic heat capacity of Y5Rh6Sn18 shows a
T-3 dependence below T-c indicating an anisotropic superconducting gap
with a point node. This result is in sharp contrast to that observed in
the isostructural Lu5Rh6Sn18 which is a strong coupling s-wave
superconductor. The temperature dependence of the deduced superfluid in
density Y5Rh6Sn18 is consistent with a BCS s-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.
