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Professor Kemp Plumb, Dept of Physics, Brown University Rhode Island NY

Event Date: 
Wednesday, November 21, 2018 - 1:30pm to 2:30pm

Emergent States in Frustrated Magnets, Investigations of a Coulomb Spin Liquid

Conventional magnets are characterized by symmetry breaking and the formation long-range magnetic order at low temperatures. New and unanticipated phases of matter can arise when such symmetry breaking is inhibited by highly frustrated magnetic interactions. A spectacular example of this is the pyrochlore lattice with isotropic antiferromagnetic interactions, where the collective behavior of fluctuating magnetic moments is described by an emergent divergence free field.  The microscopic magnetic degrees of freedom continue to fluctuate even at zero temperature, as if in a liquid state, but all configurations must obey the divergence condition; hence, this phase is termed a Coulomb spin liquid. In this talk, I will discuss a new material, NaCaNi2F7, which realizes the isotropic Coulomb spin liquid with the complication of random Na+ - Ca2+ charge disorder in the crystal structure. I will present neutron scattering and calorimetric measurements that were used to uncover the magnetic correlations in this material and fully determine the magnetic interactions. The ionic disorder creates a rugged energy landscape that acts to freeze a small fraction of the magnetic degrees of freedom in time. However, the energy scale set by this disorder is small, and the Heisenberg interactions prevail. Only a small fraction of the available moment is frozen, and the magnetism in NaCaNi2F7 is dominated by a persistently fluctuating component. These measurements demonstrate a beautiful realization of the Coulomb spin liquid and provide new insight into the interplay between disorder and magnetic exchange interactions in highly frustrated magnets.

Kemp Plumb is an Assistant Professor of Physics at Brown University. He received his Ph.D. from the University of Toronto in 2014. He then spent three years as a post-doctoral fellow at the Johns Hopkins Institute for Quantum matter before joining the faculty at Brown in the fall of 2017.  Kemp uses neutron and x-ray scattering to study all forms of magnetism in strongly correlated quantum materials.  His research focus is to discover and understand quantum spin liquids through their collective excitations. The current emphasis of his work is on magnetic frustration and heavy transition metal compounds, where electronic correlations and spin-orbit coupling have an equal footing.

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