Critical speeding-up in the magnetoelectric response of spin-ice near its monopole liquid–gas transition

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 e(n). Here we report on broadband spectroscopic measurements of e(n) in Dy 2 Ti 2 O 7 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 t exhibits a critical speeding-up with a significant enhancement of 1/t 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.

Optimal Cycles for Low-Dissipation Heat Engines

“…This condition is notably milder than the analogous condition α−zν ≥ 1 found for the Otto cycle proposed in [34]. Examples of physical systems where (21) is satisfied are also provided in [34], particularly in the presence of critical speed-ups of thermalisation where z < 0 [54][55][56]. Besides efficiency and power, another crucial aspect of a heat engine is its reliability, i.e.…”

Multiferroicity in spin ice: Towards magnetic crystallography ofTb2Ti2O7in a field

“…In particular it is tempting to speculate whether the glassy behaviour observed in some samples [48][49][50] [70], non-Kondo resistivity minimum [71][72][73][74] and a new kind of quantum criticality [75] in iridates), we should expect the coupling to an additional, ferroic, degree of freedom to bring a new flavor to spin ice and spin liquids, both at equilibrium and dynamically [76]. Experiments on Dy 2 Ti 2 O 7 and Ho 2 Ti 2 O 7 already suggest the presence of magneto-electric effects [77][78][79][80], which could be enhanced or even qualitatively modified by doping and chemical pressure [81][82][83] or with the inclusion of an electric field. More generally, multiferroicity offers a promising mechanism to control topological defects in magnets.…”

Suppression of Pauling's residual entropy in the dilute spin ice(Dy1−xYx)2Ti2Scharffe1, Breunig2, Cho3 et al. 2015Phys. Rev. B

“…As a consequence of the ground-state degeneracy, each pair fractionalizes into two individual excitations that can be described as magnetic (anti-)monopoles propagating independently through the lattice [3,4,11,12]. The dynamics of these monopole excitations is subject of intense research [5,[13][14][15][16].Experimental evidence for Pauling's residual entropy in spin-ice systems stems from specific heat measurements [17][18][19][20] reporting a practically temperatureindependent entropy S ex (T ≈ 0.4 K) ≃ S P . More recently, however, extremely slow relaxation phenomena were observed for Dy 2 Ti 2 O 7 in low-temperature measurements of, e.g., the magnetization [21,22], ac susceptibility [23], thermal transport [24,25] or the specific heat [24,26,27].…”

“…As a consequence of the ground-state degeneracy, each pair fractionalizes into two individual excitations that can be described as magnetic (anti-)monopoles propagating independently through the lattice [3,4,11,12]. The dynamics of these monopole excitations is subject of intense research [5,[13][14][15][16].…”