Phase diagram and spectroscopic signatures of a supersolid in the quantum ising magnet K<sub>2</sub>Co(SeO<sub>3</sub>)<sub>2</sub>.
basic_science · Level V
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- Record sourced from PubMed, PMID 41708616.
- Also identified by DOI 10.1038/s41467-026-69661-0 and PMC identifier 13031379.
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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- <math xmlns="http://www.w3.org/1998/Math/MathML"> <mfrac><mrow><mn>1</mn></mrow> <mrow><mn>2</mn></mrow> </mfrac> </math> degrees of freedom, here we show that these phases are realized in the triangular-lattice antiferromagnet K<sub>2</sub>Co(SeO<sub>3</sub>)<sub>2</sub>. At zero field, neutron diffraction reveals the development of quasi-two-dimensional <math xmlns="http://www.w3.org/1998/Math/MathML"> <msqrt><mrow><mn>3</mn></mrow> </msqrt> <mo>×</mo> <msqrt><mrow><mn>3</mn></mrow> </msqrt> </math> magnetic order with Z<sub>3</sub> 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 <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>(</mo> <mrow> <mfrac><mrow><mn>1</mn></mrow> <mrow><mn>3</mn></mrow> </mfrac> <mfrac><mrow><mn>1</mn></mrow> <mrow><mn>3</mn></mrow> </mfrac> </mrow> <mo>)</mo></mrow> </math> , 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.