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Abstract:
Competing interactions in the so-called spin-ice compounds stabilize a
frustrated ground state with finite zero-point entropy and,
interestingly, emergent magnetic monopole excitations. The properties of
these monopoles are at the focus of recent research with particular
emphasis on their quantum dynamics. It is predicted that each monopole
also possesses an electric dipole moment, which allows to investigate
their dynamics via the dielectric function epsilon(v). Here we report on
broadband spectroscopic measurements of epsilon(v) in Dy2Ti2O7 down to
temperatures of 200 mK with a specific focus on the critical end point
present for a magnetic field along the crystallographic [111] direction.
Clear critical signatures are revealed in the dielectric response when,
similarly as in the liquid-gas transition, the density of monopoles
changes in a critical manner. The dielectric relaxation time tau
exhibits a critical speeding-up with a significant enhancement of 1/tau
as the temperature is lowered towards the critical temperature. Besides
demonstrating the magnetoelectric character of the emergent monopole
excitations, our results corroborate the unique critical dynamics near
the monopole condensation transition.
