Nearly-Resonant Crystalline-Phononic Coupling in Quantum Spin Liquid Candidate CsYbSe$_2$

TL;DR

This study reveals strong coupling between crystal electric field excitations and phonons in CsYbSe2, a quantum spin liquid candidate, leading to complex vibronic states and high-order mode combinations, opening avenues for controlling QSL states.

Contribution

It uncovers nearly-resonant CEF-phonon interactions and mesoscale interplay in CsYbSe2, a novel insight into QSL candidate materials.

Findings

Identification of CEF and phonon modes via Raman spectroscopy

Observation of strong CEF-phonon mixing and vibronic bound states

Detection of up to fifth-order combination modes and 17 total modes

Abstract

CsYbSe$_2$, a recently identified quantum spin liquid (QSL) candidate, exhibits strong crystal electric field (CEF) excitations. Here, we identify phonon and CEF modes with Raman spectroscopy and observe strong CEF-phonon mixing resulting in a vibronic bound state. Complex, mesoscale interplay between phonon modes and CEF modes is observed in real space, and an unexpected nearly resonant condition is satisfied, yielding up to fifth-order combination modes, with a total of 17 modes identified in the spectra. This study paves the way to coherent control of possible QSL ground states with optically accessible CEF-phonon manifolds and mesoscale engineering of CEF-phonon interactions.