Abstract
Triangular lattice of rare-earth ions with interacting effective spin- local moments is an ideal platform to explore the physics of quantum spin liquids (QSLs) in the presence of strong spin-orbit coupling, crystal electric fields, and geometrical frustration. The Yb delafossites, (, S, Se) with Yb ions forming a perfect triangular lattice, have been suggested to be candidates for QSLs. Previous thermodynamics, nuclear magnetic resonance, and powder-sample neutron scattering measurements on have supported the suggestion of the QSL ground states. The key signature of a QSL, the spin excitation continuum, arising from the spin quantum number fractionalization, has not been observed. Here we perform both elastic and inelastic neutron scattering measurements as well as detailed thermodynamic measurements on high-quality single-crystal samples to confirm the absence of long-range magnetic order down to 40 mK, and further reveal a clear signature of magnetic excitation continuum extending from 0.1 to 2.5 meV. The comparison between the structure of the magnetic excitation spectra and the theoretical expectation from the spinon continuum suggests that the ground state of is a QSL with a spinon Fermi surface.
- Received 23 December 2020
- Revised 30 March 2021
- Accepted 8 April 2021
DOI:https://doi.org/10.1103/PhysRevX.11.021044
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
In a quantum spin liquid (QSL), electrons do not assume any regular magnetic order even at zero temperature. This sought-after state may hold secrets to high-temperature superconductivity and have properties conducive to quantum computing. However, observation of a QSL is exceedingly difficult. Here, we present strong evidence of a QSL in a 2D triangular lattice material.
The only way to positively identify a QSL is by firing neutrons at the material. In an inelastic neutron scattering experiment, the neutrons create pairs of spinlike quasiparticles called spinons that, for a given neutron momentum, have a continuous range of energies (as opposed to ordinary magnets, where the spin excitations end up with discrete energies). While there have been a few reports of such a spinon continuum, these may have alternate interpretations and could be due to disorder in the materials.
In our experiments, we scatter neutrons off of high-quality samples of , a material long suggested as a QSL candidate and one that has much less disorder compared with other 2D systems. We not only confirm the absence of magnetic order down to 40 mK but also see clear evidence of a spinon continuum—the hallmark of a QSL.
Although hints of a QSL state have been reported in the same family as , those experiments were on powders of the material, not crystals, which left interpretation of those results somewhat ambiguous. Therefore, we believe that our measurements provide strong evidence for a QSL signature in a 2D triangular lattice—almost 50 years after their first prediction.