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

摘要

Ba$_8$CoNb$_6$O$_{24}$ presents a system whose Co$^{2+}$ ions have aneffective spin 1/2 and construct a regular triangular-lattice antiferromagnet(TLAFM) with a very large interlayer spacing, ensuring purely two-dimensionalcharacter. We exploit this ideal realization to perform a detailed experimentalanalysis of the $S = 1/2$ TLAFM, which is one of the keystone models infrustrated quantum magnetism. We find strong low-energy spin fluctuations andno 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 unexpected magnetically disorderedstate, which is a candidate quantum spin liquid. We establish the $(H,T)$ phasediagram, mapping in detail the quantum fluctuation corrections to the availabletheoretical analysis. These include a strong upshift in field of the maximumordering temperature, qualitative changes to both low- and high-field phaseboundaries, and an ordered regime apparently dominated by the collinear"up-up-down" state. Ba$_8$CoNb$_6$O$_{24}$ therefore offers fresh input for thedevelopment of theoretical approaches to the field-induced quantum phasetransitions of the $S = 1/2$ Heisenberg TLAFM.