Excitation spectrum and magnetic field effects in a quantum critical spin-orbital system:FeSc$_2$S$_4$

TL;DR

This paper refines the theoretical understanding of FeSc2S4, a spin-orbital liquid, by analyzing its excitation spectrum, phase diagram under magnetic fields, and the effects of exchange interactions near a quantum critical point.

Contribution

It introduces a refined model including first neighbor exchange and magnetic field effects, providing detailed phase diagrams and susceptibility calculations for FeSc2S4.

Findings

Inclusion of J1 induces incommensurate phase near critical point

Magnetic field suppresses magnetic ordering

Computed field-dependent magnetic susceptibility and phase behavior

Abstract

The orbitally degenerate A-site spinel compound FeSc2S4 has been experimentally identified as a ``spin-orbital liquid'', with strong fluctuations of both spins and orbitals. Assuming that the second neighbor spin exchange J2 is the dominant one, we argued in a recent theoretical study [Chen et al., Phys. Rev. Lett. 102, 096406 (2009)] that FeSc2S4 is in a local ``spin-orbital singlet'' state driven by spin orbit coupling, close to a quantum critical point, which separates the ``spin-orbital singlet'' phase from a magnetically and orbitally ordered phase. In this paper, we refine further and develop this theory of FeSc2S4 . First, we show that inclusion of a small first neighbor exchange J1 induces a narrow region of incommensurate phase near the quantum critical point. Next, we derive the phase diagram in the presence of an external magnetic field B, and show that the latter suppresses the ordered phase. Lastly, we compute the field dependent dynamical magnetic susceptibility $\chi(k, \omega; B)$, from which we extract a variety of physical quantities. Comparison with and suggestions for experiment are discussed.