Effective Magnetic Hamiltonian at Finite Temperatures for Rare Earth Chalcogenides

TL;DR

This paper develops an effective magnetic Hamiltonian that includes crystal electric field effects to accurately describe the thermodynamics of rare-earth chalcogenides at finite temperatures, exemplified by NaYbSe2.

Contribution

It introduces a comprehensive model incorporating CEF excitations and spin interactions, enabling precise analysis of magnetic properties and crossover behavior in rare-earth quantum spin liquids.

Findings

Determined anisotropic exchange coupling energies for NaYbSe2.

Identified a crossover temperature (~25 K) where CEF effects diminish.

Explained ESR spectral width temperature dependence successfully.

Abstract

Alkali metal rare-earth chalcogenide $ARECh2$ (A=alkali or monovalent metal, RE=rare earth, Ch=O, S, Se, Te), is a large family of quantum spin liquid (QSL) candidates we discovered recently. Unlike $YbMgGaO4$, most members in the family except for the oxide ones, have relatively small crystalline electric-field (CEF) excitation levels, particularly the first ones. This makes the conventional Curie-Weiss analysis at finite temperatures inapplicable and CEF excitations may play an essential role in understanding the low-energy spin physics. Here we considered an effective magnetic Hamiltonian incorporating CEF excitations and spin-spin interactions, to accurately describe thermodynamics in such a system. By taking $NaYbSe2$ as an example, we were able to analyze magnetic susceptibility, magnetization under pulsed high fields and heat capacity in a systematic and comprehensive way. The analysis allows us to produce accurate anisotropic exchange coupling energies and unambiguously determine a crossover temperature ($\sim$25 K in the case of $NaYbSe2$), below which CEF effects fade away and pure spin-spin interactions stand out. We further validated the effective picture by successfully explaining the anomalous temperature dependence of electron spin resonance (ESR) spectral width. The effective scenario in principle can be generalized to other rare-earth spin systems with small CEF excitations.