Mermin-Wagner physics, (H,T) phase diagram, and candidate quantum spin-liquid phase in the spin-1/2 triangular-lattice antiferromagnet Ba8CoNb6O24

TL;DR

This study investigates Ba8CoNb6O24, a nearly ideal two-dimensional spin-1/2 triangular antiferromagnet, revealing strong quantum fluctuations, absence of magnetic order, and potential quantum spin liquid behavior, along with detailed phase diagram mapping.

Contribution

The paper provides the first detailed experimental analysis of a nearly ideal 2D spin-1/2 triangular antiferromagnet, including its phase diagram and evidence for a candidate quantum spin liquid state.

Findings

No magnetic order down to 0.1 K despite diverging correlation length.

Identification of a candidate quantum spin liquid state below 0.1 K.

Detailed (H,T) phase diagram showing quantum fluctuation effects.

Abstract

Ba$_8$CoNb$_6$O$_{24}$ presents a system whose Co$^{2+}$ ions have an effective spin 1/2 and construct a regular triangular-lattice antiferromagnet (TLAFM) with a very large interlayer spacing, ensuring purely two-dimensional character. We exploit this ideal realization to perform a detailed experimental analysis of the $S = 1/2$ TLAFM, which is one of the keystone models in frustrated quantum magnetism. We find strong low-energy spin fluctuations and no magnetic ordering, but a diverging correlation length down to 0.1 K, indicating a Mermin-Wagner trend towards zero-temperature order. Below 0.1 K, however, our low-field measurements show an nexpected magnetically disordered state, which is a candidate quantum spin liquid. We establish the $(H,T)$ phase diagram, mapping in detail the quantum fluctuation corrections to the available theoretical analysis. These include a strong upshift in field of the maximum ordering temperature, qualitative changes to both low- and high-field phase boundaries, and an ordered regime apparently dominated by the collinear "up-up-down" state. Ba$_8$CoNb$_6$O$_{24}$ therefore offers fresh input for the development of theoretical approaches to the field-induced quantum phase transitions of the $S = 1/2$ Heisenberg TLAFM.