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

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.

arXiv comment14 pages, 4 figures; replacement and qualitative extension of arXiv:1612.05217, now including full field-temperature phase diagram and sighting of anomalous QSL candidate

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