Continuum of quantum fluctuations in a three-dimensional S = 1 Heisenberg magnet

• Published in: Nature physics 2019-01, Vol.15 (1), p.54-59
• Main Authors: Plumb, K. W., Changlani, Hitesh J., Scheie, A., Zhang, Shu, Krizan, J. W., Rodriguez-Rivera, J. A., Qiu, Yiming, Winn, B., Cava, R. J., Broholm, C. L.
• Format: Article
• Language: English
• Subjects: 639/766; 639/766/119/997; Angular momentum; Antiferromagnetism; Atomic; Broken symmetry; Calcium ions; Classical and Continuum Physics; CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; Complex Systems; Condensed Matter Physics; Diffraction; Excitation; Heisenberg theory; Inelastic scattering; Lattice vibration; Low temperature; Magnets; Magnons; Mathematical and Computational Physics; Molecular; Neutron scattering; Neutrons; Optical and Plasma Physics; Physics; Physics and Astronomy; Quantum phenomena; Spin liquid; Statistical models; Theoretical; Wave dispersion; Weight
• ISSN: 1745-2473; 1745-2481
• DOI: 10.1038/s41567-018-0317-3

Conventional crystalline magnets are characterized by symmetry breaking and normal modes of excitation called magnons, with quantized angular momentum ħ . Neutron scattering correspondingly features extra magnetic Bragg diffraction at low temperatures and dispersive inelastic scattering associated with single magnon creation and annihilation. Exceptions are anticipated in so-called quantum spin liquids, as exemplified by the one-dimensional spin-1/2 chain, which has no magnetic order and where magnons accordingly fractionalize into spinons with angular momentum ħ /2. This is spectacularly revealed by a continuum of inelastic neutron scattering associated with two-spinon processes. Here, we report evidence for these key features of a quantum spin liquid in the three-dimensional antiferromagnet NaCaNi 2 F 7 . We show that despite the complication of random Na 1+ –Ca 2+ charge disorder, NaCaNi 2 F 7 is an almost ideal realization of the spin-1 antiferromagnetic Heisenberg model on a pyrochlore lattice. Magnetic Bragg diffraction is absent and 90% of the neutron spectral weight forms a continuum of magnetic scattering with low-energy pinch points, indicating NaCaNi 2 F 7 is in a Coulomb-like phase. Our results demonstrate that disorder can act to freeze only the lowest-energy magnetic degrees of freedom; at higher energies, a magnetic excitation continuum characteristic of fractionalized excitations persists. Despite of the charge disorder, the three-dimensional antiferromagnet NaCaNi 2 F 7 is an almost ideal realization of the spin-1 antiferromagnetic Heisenberg model on a pyrochlore lattice, showing key features of quantum spin liquid.