Niels Bohr Lecture by Collin Broholm

Quantum fluctuations on triangular lattices of Ising spins

Abstract: In the quest to chart qualitatively new states of matter, frustrated magnets offer a variety of opportunities. A frustrated magnet is a crystalline solid where the leading interactions between atomic magnetic moments are incompatible with an ordered magnetic state. The result is a degenerate manifold of spin configurations from which novel collective states of matter can emerge. I shall describe our use of magnetic neutron scattering to characterize these and to determine the frustrated interactions for deep engagement with quantum many body theory.

The particular example I focus on is the triangular lattice of spins-1/2 with uniaxial (Ising) antiferromagnetic interactions, which indeed has finite entropy at . In the cobalt based model systems that we have examined [1], weak transverse interactions introduce quantum fluctuations that lift this degeneracy and produce a rich magnetic phase diagram. We provide experimental evidence for compressible (gapless) and incompressible (gap-full) phases with spontaneous translational and rotational symmetry breaking. We associate the gapless phases near zero and full magnetization with super-solid states of hard core bosons on the triangular lattice.

* This work was supported as part of the Institute for Quantum Matter, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0019331 and by the Gordon and Betty Moore foundation through the EPIQS program GBMF9456.

[1] “Phase Diagram and Spectroscopic Evidence of Supersolids in Quantum Ising Magnet K₂Co(SeO₃)₂,” Tong Chen, Alireza Ghasemi, Junyi Zhang, Liyu Shi, Zhenisbek Tagay, Lei Chen, Eun-Sang Choi, Marcelo Jaime, Minseong Lee, Yiqing Hao, Huibo Cao, Barry Winn, Ruidan Zhong, Xianghan Xu, N. P. Armitage, Robert Cava, and Collin Broholm,

https://doi.org/10.48550/arXiv.2402.15869.

About the speaker:

COLLIN LESLIE BROHOLM is the Gerhard H. Dieke Professor in the Department of Physics and Astronomy at the Johns Hopkins University. He earned his Ph.D. from the University of Copenhagen in 1988, was a post doc at AT&T Bell Laboratories from 1988-1990 and joined Johns Hopkins in 1990.
An experimental condensed matter physicist, Dr. Broholm is interested in anomalous forms of magnetism, superconductivity, and their interplay. Of particular interest are crystalline materials where quantum effects are enhanced on account of competing interactions (frustration) or low dimensionality.

The main experimental tool is neutron scattering and Dr. Broholm has a long-standing involvement in development of the corresponding instrumentation. He has built two spectrometers at the NIST Center for Neutron Research and has served on various committees overseeing instrumentation development at National facilities for Neutron Scattering.

Dr. Broholm received the Presidential Faculty Fellowship in 1994, became a fellow of the American Physical Society in 2004, and received the Sustained Research Award of the Neutron Scattering Society of America in 2010. He was selected for funding by the Gordon and Betty Moore foundation as an experimentalists in the area of quantum materials in 2014 and 2019.

Coffee, tea and cake will be served outside Aud. 3 at 15:45