Spin fluctuations, absence of magnetic order, and crystal electric field studies in the Yb$^{3+}$-based triangular lattice antiferromagnet Rb$_3$Yb(VO$_4$)$_2$

TL;DR

This study thoroughly investigates the magnetic and structural properties of Rb$_3$Yb(VO$_4$)$_2$, revealing a Kramers' doublet ground state, weak antiferromagnetic interactions, and no long-range order down to 1.6 K, positioning it as a candidate for quantum spin-liquid behavior.

Contribution

The paper provides comprehensive experimental and theoretical analysis of Rb$_3$Yb(VO$_4$)$_2$, establishing it as an ideal, disorder-free Yb-based triangular lattice antiferromagnet with potential quantum spin-liquid properties.

Findings

No magnetic long-range order down to 1.6 K.

Weak antiferromagnetic interactions with small Curie-Weiss temperature.

Large energy gap (~18.61 meV) between ground and excited doublets.

Abstract

We report a comprehensive experimental investigation of the structural, thermodynamic, static, and dynamic properties of a triangular lattice antiferromagnet Rb$_3$Yb(VO$_4$)$_2$. Through the analysis of magnetic susceptibility, magnetization, and specific heat, complemented by crystal electric field (CEF) calculations, we confirm the Kramers' doublet with effective spin $J_{\rm{eff}}=1/2$ ground state. Magnetic susceptibility and isothermal magnetization analysis reveal a weak antiferromagnetic interaction among the $J_{\rm{eff}}=1/2$ spins, characterized by a small Curie-Weiss temperature ($\theta_{\text{CW}}^{\text{LT}}\simeq-0.26$ K) or a reduced exchange coupling ($J/k_{\rm B} \simeq 0.18$ K). The $^{51}$V NMR spectra and spin-lattice relaxation rate ($1/T_1$) show no evidence of magnetic long-range-order down to 1.6 K but reflect strong influence of CEF excitations in the intermediate temperatures. At low temperatures, $1/T_1(T)$ shows pronounced frequency dependence and $1/T_1$ vs field in different temperatures follows the scaling behaviour, highlighting the role of paramagnetic fluctuations. The CEF calculations using the point charge approximation divulge a large energy gap ($\sim 18.61$ meV) between the lowest and second lowest energy doublets, further establishing Kramers' doublet as the ground state. Our calculations also reproduce the experimental magnetization and specific heat data and indicate an in-plane magnetic anisotropy. These findings position Rb$_3$Yb(VO$_4$)$_2$ as an ideal and disorder-free candidate to explore intrinsic quantum fluctuations and possible quantum spin-liquid physics in a Yb$^{3+}$-based triangular lattice antiferromagnet.