Complete field-induced spectral response of the spin-1/2 triangular-lattice antiferromagnet CsYbSe$_2$

TL;DR

This study provides a comprehensive experimental and theoretical analysis of the spectral response of the spin-1/2 triangular-lattice antiferromagnet CsYbSe$_2$ under various magnetic fields, revealing complex excitations and continuum features.

Contribution

It combines neutron scattering experiments with MPS calculations to accurately characterize field-induced excitations in CsYbSe$_2$, a model frustrated quantum magnet.

Findings

Extensive continuum features near the 120°-ordered state

Agreement between experimental spectra and MPS calculations

Identification of multi-magnon bound and scattering states

Abstract

Fifty years after Anderson's resonating valence-bond proposal, the spin-1/2 triangular-lattice Heisenberg antiferromagnet (TLHAF) remains the ultimate platform to explore highly entangled quantum spin states in proximity to magnetic order. Yb-based delafossites are ideal candidate TLHAF materials, which allow experimental access to the full range of applied in-plane magnetic fields. We perform a systematic neutron scattering study of CsYbSe$_2$, first proving the Heisenberg character of the interactions and quantifying the second-neighbour coupling. We then measure the complex evolution of the excitation spectrum, finding extensive continuum features near the 120$^{\circ}$-ordered state, throughout the 1/3-magnetization plateau and beyond this up to saturation. We perform cylinder matrix-product-state (MPS) calculations to obtain an unbiased numerical benchmark for the TLHAF and spectacular agreement with the experimental spectra. The measured and calculated longitudinal spectral functions reflect the role of multi-magnon bound and scattering states. These results provide valuable insight into unconventional field-induced spin excitations in frustrated quantum materials.