Title: 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.