Spin-orbiton and quantum criticality in FeSc2S4

TL;DR

This study investigates FeSc2S4, revealing a spin-orbiton and signs of quantum criticality through spectroscopic analysis, highlighting its status as a spin-orbital liquid near a quantum-critical point.

Contribution

It provides the first spectroscopic evidence of a spin-orbiton and explores quantum critical behavior in FeSc2S4, advancing understanding of spin-orbital liquids.

Findings

Identification of a spin-orbiton excitation that intensifies at low temperatures.

Detection of a continuum suggestive of quantum criticality.

FeSc2S4 remains disordered due to spin-orbital competition.

Abstract

In FeSc2S4 spin-orbital exchange competes with strong spin-orbit coupling, suppressing long-range spin and orbital order and, hence, this material represents one of the rare examples of a spin-orbital liquid ground state. Moreover, it is close to a quantum-critical point separating the ordered and disordered regimes. Using THz and FIR spectroscopy we study low-lying excitations in FeSc2S4 and provide clear evidence for a spin-orbiton, an excitation of strongly entangled spins and orbitals. It becomes particularly well pronounced upon cooling, when advancing deep into the quantum-critical regime. Moreover, indications of an underlying structureless excitation continuum are found, a possible signature of quantum criticality.