One- and three-dimensional quantum phase transitions and anisotropy in Rb$_{2}$Cu$_{2}$Mo$_{3}$O$_{12}$

TL;DR

This study investigates the complex quantum phase transitions in Rb$_{2}$Cu$_{2}$Mo$_{3}$O$_{12}$, revealing anisotropic phase behavior and a helical magnetic structure through comprehensive magnetic, thermodynamic, and ESR measurements.

Contribution

It provides the first detailed phase diagram of Rb$_{2}$Cu$_{2}$Mo$_{3}$O$_{12}$, highlighting its anisotropic quantum phase transitions and identifying a helical magnetic structure.

Findings

Quantum phase transitions between gapped, ordered, and saturated phases observed.

The gapped phase exhibits three-dimensional character, while the saturated phase shows one-dimensional fluctuations.

The phase diagram is highly anisotropic with critical fields varying significantly with field direction.

Abstract

Single crystal samples of the frustrated quasi one-dimensional quantum magnet Rb$_{2}$Cu$_{2}$Mo$_{3}$O$_{12}$ are investigated by magnetic, thermodynamic, and electron spin resonance (ESR) measurements. Quantum phase transitions between the gapped, magnetically ordered and fully saturated phases are observed. Surprisingly, the former has a distinctive three-dimensional character, while the latter is dominated by one-dimensional quantum spin fluctuations. The entire $H$-$T$ phase diagram is mapped out and found to be substantially anisotropic. In particular, the lower critical fields differ by over 50\% depending on the direction of applied field, while the upper ones are almost isotropic, as is the magnetization above saturation. The ESR spectra are strongly dependent on field orientation and point to a helical structure with a rigidly defined spin rotation plane.