  Conventional superconductors are robust diamagnets that expels 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 \yrhsn\ ($T_{\bf c}$ = 3.0 K). The
electronic heat capacity of \yrhsn\ shows a $T^3$ dependence below $T_{\bf c}$
indicating an anisotropic superconducting gap with a point node. This result is
in sharp contrast to that observed in the isostructural Lu$_5$Rh$_6$Sn$_{18}$
which is a strong coupling $s-$wave superconductor. The temperature dependence
of the deduced superfluid density \yrhsn\ 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.
