  Condensed matter systems offer the opportunity to maximize the number of
spins, probed in magnetic resonance experiments that search for physics beyond
the standard model. EuCl$_3\cdot 6$H$_2$O is a promising solid-state platform
for such searches. The narrow inhomogeneous linewidth of the Eu optical
$^7\hspace{-0.1em}F_0 \leftrightarrow\,^5\hspace{-0.1em}D_0$ transition in this
material opens the opportunity to optically pump the $^{153}$Eu nuclear spin
ensemble into a single hyperfine sublevel. The closely-spaced opposite-parity
nuclear energy levels, and the low-energy collective octupole mode, or a
possible static octupole deformation, enhance the $^{153}$Eu nuclear Schiff
moment. The optimistic estimate of the effective electric field is
$E^*=10\,$MV/cm. With this estimate, the CASPEr-electric search for the gluon
interaction of axion dark matter can reach QCD axion sensitivity with a 3~cm
sample.

Other Condensed Matter