Magnetic field dependence of excitations near spin-orbital quantum criticality

TL;DR

This study investigates how magnetic excitations in FeSc₂S₄, a material near a spin-orbital quantum critical point, respond to magnetic fields, revealing unique entangled spin-orbital behaviors distinct from spin-only systems.

Contribution

It provides the first comprehensive neutron scattering analysis of magnetic excitations in FeSc₂S₄ under magnetic fields, highlighting the role of spin-orbital entanglement in its quantum critical behavior.

Findings

Magnetic excitations are well described by spin-orbital triplon models.

Spectral weight shifts to higher energies with increasing magnetic field.

Contrasts with spin-only systems where the spin gap decreases under field.

Abstract

The spinel FeSc$_2$S$_4$ has been proposed to realize a near-critical spin-orbital singlet (SOS) state, where entangled spin and orbital moments fluctuate in a global singlet state on the verge of spin and orbital order. Here we report powder inelastic neutron scattering measurements that observe the full bandwidth of magnetic excitations and we find that spin-orbital triplon excitations of an SOS state can capture well key aspects of the spectrum in both zero and applied magnetic fields up to 8.5 T. The observed shift of low-energy spectral weight to higher energies upon increasing applied field is naturally explained by the entangled spin-orbital character of the magnetic states, a behavior that is in strong contrast to spin-only singlet ground state systems, where the spin gap decreases upon increasing applied field.