Evidence for a spinon Fermi surface in the triangular S=1 quantum spin liquid Ba$_3$NiSb$_2$O$_9$

TL;DR

This study provides experimental evidence for a spinon Fermi surface in a spin-1 triangular lattice quantum spin liquid, using neutron scattering to reveal gapless excitations consistent with a fermionic spinon model.

Contribution

First experimental identification of a spinon Fermi surface in a spin-1 quantum spin liquid on a triangular lattice.

Findings

Observation of broad, gapless, nondispersive excitation continua

Data incompatible with most existing theoretical models

Support for a quantum spin liquid with three flavors of fermionic spinons

Abstract

Inelastic neutron scattering is used to study the low-energy magnetic excitations in the spin-1 triangular lattice of the 6H-B phase of Ba$_3$NiSb$_2$O$_9$. We study two powder samples: Ba$_3$NiSb$_2$O$_9$ synthesized under high pressure and Ba$_{2.5}$Sr$_{0.5}$NiSb$_2$O$_9$ in which chemical pressure stabilizes the 6H-B structure. The measured excitation spectra show broad gapless and nondispersive continua at characteristic wave vectors. Our data rules out most theoretical scenarios that have previously been proposed for this phase, and we find that it is well described by an exotic quantum spin liquid with three flavors of unpaired fermionic spinons, forming a large spinon Fermi surface.