Absence of Spin Frustration in the Kagom\'e Layers of Cu2+ Ions in Volborthite Cu3V2O7(OH)2x2H2O and Observation of the Suppression and Re-entrance of Specific Heat Anomalies in Volborthite Under External Magnetic Field

TL;DR

This study reveals that the kagome layers in volborthite are not spin-frustrated and behave as weakly interacting antiferromagnetic chains, with specific heat anomalies influenced by external magnetic fields and entropy spectra.

Contribution

The paper provides a detailed analysis of the spin exchanges in volborthite, showing the absence of spin frustration and introducing the concept of weakly interacting S=1/2 pseudospin chains in kagome layers.

Findings

Kagome layers are hardly spin-frustrated and behave as weakly interacting antiferromagnetic chains.

Specific heat anomalies are suppressed or re-entrant under external magnetic fields.

Three sets of entropy spectra govern the energy exchange and field dependence of specific heat anomalies.

Abstract

We determined the spin exchanges between the Cu2+ ions in the kagome layers of volborthite, Cu3V2O7(OH)2x2H2O, by performing the energy-mapping analysis based on DFT+U calculations, to find that the kagom\'e layers of Cu2+ ions are hardly spin-frustrated, and the magnetic properties of volborthite below ~75 K should be described by very weakly interacting antiferromagnetic uniform chains made up of effective S=1/2 pseudospin units. This conclusion was verified by synthesizing single crystals of not only Cu3V2O7(OH)2x2H2O but also its deuterated analogue Cu3V2O7(OD)2x2D2O and then by investigating their magnetic susceptibilities and specific heats. Each kagome layer consists of intertwined two-leg spin ladders with rungs of linear spin trimers. With the latter acting as S=1/2 pseudospin units, each two-leg spin ladder behaves as a chain of S=1/2 pseudospins. Adjacent two-leg spin ladders in each kagome layer interact very weakly, so it is required that all nearest-neighbor spin exchange paths of every two-leg spin ladder remain antiferromagnetically coupled in all spin ladder arrangements of a kagome layer. This constraint imposes three sets of entropy spectra with which each kagome layer can exchange energy with the surrounding on lowering the temperature below ~1.5 K and on raising the external magnetic field B. We discovered that the specific heat anomalies of volborthite observed below ~1.5 K at B = 0 are suppressed by raising the magnetic field B to ~4.2 T, that a new specific heat anomaly occurs when B is increased above ~5.5 T, and that the imposed three sets of entropy spectra are responsible for the field-dependence of the specific heat anomalies.