Quantum disordered magnets

Moshe Schechter

The interest in magnetism, and specifically quantum magnetism, is twofold. Firstly, magnetic materials are of immense significance for the advance of technology. With reduced size of current and future devices, quantum effects become relevant, and their understanding is crucial to further advance. At the same time, magnetic systems are an ideal tool in the study of various physical phenomena, as they allow the realization of theoretical models with negligible extraneous interactions, and with the ability to tune the relevant parameters.

Perhaps the most studied model for interacting systems is the Ising model. With the addition of a transverse field term and a random field term the model is described by the Hamiltonian

\(H_{Is}=-\sum_{ij}J_{ij}S_i^zS_j^z-\Gamma\sum_iS_i^x-\sum_ih_iS_i^z\)

and allows the study of the interplay of interactions with quantum fluctuations and disorder. This interplay is of much recent interest, as it is essential in phenomena such as the superconducting-insulator transition, the quantum Hall effect, and high superconductivity. Recently we have shown that this model is realized in anisotropic dipolar magnets, allowing its experimental study in new regimes. This both led to an understanding of existing experiments, and motivated new experiments which were recently done and raised new questions regarding the disordering of spin glasses and ferromagnets by random fields. Current questions of interest include the transition between the ferromagnetic and glassy phase as function of disorder, random fields in nano magnetic grains, and entanglement of different degrees of freedom near the quantum phase transition.