Condensed Matter Seminar: "Tunable Ground States in a Model Quantum Magnet"

Changing the ground state of a system via a non-thermal parameter such as doping or pressure has been a topic of great interest in condensed matter physics for decades, most notably in the study of quantum phase transitions and the associated quantum critical regime. The rare-earth fluoride LiHoF_4 and the related dilution series LiHo_{1-x}Y_xF4, long known to be a realization of the S=1/2 Transverse Ising Model, can be used to study state tuning as a function of several tuning parameters. In addition to the well-understood field-induced ferromagnet to paramagnet quantum phase transition in pure LiHoF_4, there are a number of subtler transitions which are seen in the diluted materials. For x~=0.5, application of a magnetic field transverse to the Ising axis transitions the system into a realization of the Random-Field Ising Model, an important generic model for studying the effects of disorder. In the more highly dilution limit, at x~0.05, ordered ferromagnetic states are no longer stable due to frustration effects. Instead, the system shows behavior consistent with either a spin glass or a non-freezing spin liquid, and can be tuned between these two regimes by varying the degree of thermal connectivity with an external heat bath or by tuning the rate of quantum tunneling via an external magnetic field.