Quantum fluctuations in Frustrated Magnets

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 and thermomagnetic measurements 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 is an archetypical frustrated system with finite entropy at . In highly two dimensional materials realizations of this model, finite transverse interactions  lift degeneracy and produce emergent collective states of matter. We have explored this quantum many body physics in the spin-1/2 triangular-lattice antiferromagnet K2Co(SeO3)2. At zero field, we find the gradual development for  K of quasi-two-dimensional  magnetic order with strongly reduced sublattice magnetization. We examine theoretical predictions that this phase may be described as a super-solid state of interacting bosons. For c-oriented magnetic fields in the range  that almost saturate the magnetization, we find an additional phase that is predicted to be a supersolid. Between these phases there is a pronounced 1/3 magnetization plateau that supports coherent spin waves from which we accuerately determine the spin Hamiltonian.

.* This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0024469 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.