Field-orientation dependence of quantum phase transitions in the S=1/2 triangular-lattice antiferromagnet Ba$_3$CoSb$_2$O$_9$

TL;DR

This study investigates how quantum phase transitions in the triangular-lattice antiferromagnet Ba$_3$CoSb$_2$O$_9$ depend on magnetic field orientation, revealing new features like a narrow 1/3-magnetization plateau and high-field transition.

Contribution

It provides the first detailed analysis of the field-angle dependence of quantum phase transitions in Ba$_3$CoSb$_2$O$_9$ using experimental and theoretical methods.

Findings

Identified a narrow 1/3-magnetization plateau for H parallel to c

Discovered a high-field transition with a small magnetization jump

Compared experimental phase diagrams with theoretical models

Abstract

Ba$_3$CoSb$_2$O$_9$ approximates the two-dimensional spin-1/2 triangular-lattice Heisenberg antiferromagnet. This compound displays magnetic-field-induced quantum phase transitions, including the 1/3-magnetization-plateau, but its magnetization processes for the magnetic field $H$ parallel and perpendicular to the $c$ axis are different due to the weak easy-plane anisotropy and the weak interlayer antiferromagnetic exchange interaction. To elucidate how the quantum phase transitions change between these two field directions, we measured the field-angle dependence of the magnetization process in Ba$_3$CoSb$_2$O$_9$ using pulsed high magnetic fields. We compared obtained magnetic field-field angle phase diagram with those obtained by the large-size cluster mean-field method combined with a scaling scheme and the semiclassical theory. We also found a narrow 1/3-magnetization plateau and a high-field transition with a small magnetization jump for $H\,{\parallel}\,c$, not observed in the previous studies.