Bose-Einstein condensation of a two-magnon bound state in a spin-one triangular lattice

Abstract

In ordered magnets, the elementary excitations are spin waves (magnons), which obey Bose-Einstein statistics. Similarly to Cooper pairs in superconductors, magnons can be paired into bound states under attractive interactions. The Zeeman coupling to a magnetic field is able to tune the particle density through a quantum critical point (QCP), beyond which a "hidden order" is predicted to exist. Here we report direct observation of the Bose-Einstein condensation (BEC) of the two-magnon bound state in Na${}_2$BaNi(PO${}_4$)${}_2$. Comprehensive thermodynamic measurements confirmed the two-dimensional BEC-QCP at the saturation field. Inelastic neutron scattering experiments were performed to establish the microscopic model. An exact solution revealed stable 2-magnon bound states that were further confirmed by electron spin resonance and nuclear magnetic resonance experiments, demonstrating that the QCP is due to the pair condensation and the phase below saturation field is likely the long-sought-after spin nematic phase.

53 pages, 31 figures. Accepted by Nature Materials

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Cited by in corpus (15)

• Continuum of spin excitations in an ordered magnet
• From continuum excitations to sharp magnons via transverse magnetic field in the spin-1/2 Ising-like triangular lattice antiferromagnet Na2BaCo(PO4)2
• Possible Observation of Quadrupole Waves in Spin Nematics
• Observation and Control of Chiral Spin Frustration in BiYIG Thin Films
• Spin structures and phase diagrams of the spin-$\frac{5}{2}$ triangular-lattice antiferromagnet Na${}_2$BaMn(PO${}_4$)${}_2$ under magnetic field
• Stability of chiral magnon condensates in collinear antiferromagnetic insulators
• Emergent Spin Supersolids in Frustrated Quantum Materials
• Frustration and chirality in three-dimensional trillium lattices: Insights and Perspectives
• Dissipationless dynamics of spin supersolid states in a spin-1/2 triangular antiferromagnet with impurities
• Possible Bose-Einstein condensation of magnons in a S = 5/2 honeycomb lattice
• Longitudinal magnons in large-$S$ easy-axis magnets
• Quantum entanglement of XY-type spin dimers in Shastry-Sutherland lattice
• Strongly frustrated 2D magnetism in a 3D hexagonal perovskite
• Phononic enhancement and detection of hidden spin-nematicity and dynamics in quantum magnets
• Ferroelectricity in a magnon Bose-Einstein condensate: Nonreciprocal superfluidity, exceptional points, and Majorana bosons