Phase diagram and spectroscopic signatures of a supersolid in the quantum ising magnet K2Co(SeO3)2
T L Chen, Alireza Ghasemi, Junyi Zhang, Liyu Shi, Zhenisbek Tagay, Youzhe Chen, Lei Chen, E. S. Choi, M. Jaime, Minseong Lee, Yiqing Hao, Huibo Cao, Barry L. Winn, Andrey A. Podlesnyak, Daniel M. Pajerowski, Ruidan Zhong, Xianghan Xu, N. P. Armitage, Robert Cava, C. Broholm
Abstract
Supersolid phases are quantum-entangled states of matter exhibiting the dual characteristics of superfluidity and solidity. Theory predicts that hard-core bosons on a triangular lattice can form such phases at half filling and near complete filling. Leveraging an exact mapping between bosons and spin-$$\frac{1}{2}$$ degrees of freedom, here we show that these phases are realized in the triangular-lattice antiferromagnet K2Co(SeO3)2. At zero field, neutron diffraction reveals the development of quasi-two-dimensional $$\sqrt{3}\times \sqrt{3}$$ magnetic order with Z3 translational symmetry breaking (solidity), though with reduced amplitude indicating strong quantum fluctuations. These fluctuations manifest as equidistant bands of continuum neutron scattering, where the lowest-energy mode is gapless at K $$(\frac{1}{3}\frac{1}{3})$$, consistent with broken U(1) spin rotational symmetry (superfluidity). For c-axis-oriented magnetic fields near saturation, we find a second phase consistent with a high-field supersolid. These two supersolids are separated by a pronounced 1/3 magnetization plateau phase that supports coherent spin waves, from which we determine the underlying spin Hamiltonian. Supersolid phases are quantum-entangled states of matter exhibiting the dual characteristics of superfluidity and solidity. Here, the authors map the phase diagram of K2Co(SeO3)2. and identify signatures of two supersolid phases separated by a 1/3 magnetization plateau.