1. James Franck Institute and Department of Physics, University of Chicago, Chicago, Illinois 60637, USA
2. London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, UK, and NEC Laboratories, 4 Independence Way, Princeton, New Jersey 08540, USA
3. Department of Physics, University of Wisconsin, Madison, Wisconsin 53706, USA

Correspondence to: T. F. Rosenbaum¹ Email: t-rosenbaum@uchicago.edu

Abstract

Free magnetic moments usually manifest themselves in Curie laws, where weak external magnetic fields produce magnetizations that vary as the reciprocal of the temperature (1/T). For a variety of materials that do not display static magnetism, including doped semiconductors¹ and certain rare-earth intermetallics², the 1/T law is replaced by a power law T^- with < 1. Here we show that a much simpler material system—namely, the insulating magnetic salt LiHo_xY_1-xF₄—can also display such a power law. Moreover, by comparing the results of numerical simulations of this system with susceptibility and specific-heat data³, we show that both energy-level splitting and quantum entanglement are crucial to describing its behaviour. The second of these quantum mechanical effects—entanglement, where the wavefunction of a system with several degrees of freedom cannot be written as a product of wavefunctions for each degree of freedom—becomes visible for remarkably small tunnelling terms, and is activated well before tunnelling has visible effects on the spectrum. This finding is significant because it shows that entanglement, rather than energy-level redistribution, can underlie the magnetic behaviour of a simple insulating quantum spin system.

1. James Franck Institute and Department of Physics, University of Chicago, Chicago, Illinois 60637, USA
2. London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, UK, and NEC Laboratories, 4 Independence Way, Princeton, New Jersey 08540, USA
3. Department of Physics, University of Wisconsin, Madison, Wisconsin 53706, USA

Correspondence to: T. F. Rosenbaum¹ Email: t-rosenbaum@uchicago.edu