A Supercooled Magnetic Liquid State In The Frustrated Pyrochlore Dy2Ti2O7
A "supercooled" liquid forms when a liquid is cooled below its ordering temperature while avoiding a phase transition to a global ordered ground state. Upon further cooling its microscopic relaxation times diverge rapidly, and eventually the system becomes a glass that is non-ergodic on experimental timescales. Supercooled liquids exhibit a common set of characteristic phenomena: there is a broad peak in the specific heat below the ordering temperature; the complex dielectric function has a Kohlrausch-Williams-Watts (KWW) form in the time domain and a Havriliak-Negami (HN) form in the frequency domain; and the characteristic microscopic relaxation times diverge rapidly on a Vogel-Tamman-Fulcher (VTF) trajectory as the liquid approaches the glass transition. The magnetic pyrochlore Dy2 Ti2 O7 has attracted substantial recent attention as a potential host of deconfined magnetic Coulombic quasiparticles known as "monopoles". To study the dynamics of this material we introduce a highprecision, boundary-free experiment in which we study the time-domain and frequency-domain dynamics of toroidal Dy2 Ti2 O7 samples. We show that the EMF resulting from internal field variations can be used to robustly test the predictions of different parametrizations of magnetization transport, and we find that HN relaxation without monopole transport provides a self-consistent description of our AC measurements. Furthermore, we find that KWW relaxation provides an excellent parametrization of our DC time-domain measurements. Using these complementary measurement techniques, we show that the temperature dependence of the microscopic relaxation times in Dy2 Ti2 O7 has a VTF form. It follows that Dy2 Ti2 O7 , a crystalline material with very low structural disorder, hosts a supercooled magnetic liquid at low temperatures. The formation of such a state in a system without explicit disorder has become a subject of considerable theoretical interest. Recent numerical work suggests that the unconventional glassy magnetic dynamics in Dy2 Ti2 O7 may result from interacting clusters of spins that evolve according to the general principles of Hierarchical Dynamics proposed 30 years ago. In the absence of disorder this may fall analytically into the realm of Many-Body Localization, a relatively new theory that is currently under intense development. Dy2 Ti2 O7 could therefore push forward our understanding of the glass transition and bring together theories both old and new.
Spin Ice; Supercooled Liquids; Periodic Boundary Conditions
Lawler,Michael J.; Parpia,Jeevak M
Ph.D. of Physics
Doctor of Philosophy
dissertation or thesis