Effects of Two Energy Scales in Weakly Dimerized Antiferromagnetic Quantum Spin Chains

TL;DR

This study investigates how two energy scales influence the thermodynamic properties of weakly dimerized antiferromagnetic spin chains, revealing a unique thermal expansion anomaly linked to the spin gap and magnetic entropy changes.

Contribution

It provides a quantitative analysis connecting the spin gap and magnetic entropy to thermal expansion anomalies in weakly dimerized antiferromagnetic chains, using T-DMRG calculations.

Findings

Thermal expansion anomaly linked to the spin gap Δ.

Magnetic entropy change ∂S^m/∂Δ explains the anomaly.

Weak-dimerization limit reveals excitation spectrum peculiarities.

Abstract

By means of thermal expansion and specific heat measurements on the high-pressure phase of (VO)$_2$P$_2$O$_7$, the effects of two energy scales of the weakly dimerized antiferromagnetic $S$ = 1/2 Heisenberg chain are explored. The low energy scale, given by the spin gap $\Delta$, is found to manifest itself in a pronounced thermal expansion anomaly. A quantitative analysis, employing T-DMRG calculations, shows that this feature originates from changes in the magnetic entropy with respect to $\Delta$, $\partial S^{m}/ \partial \Delta$. This term, inaccessible by specific heat, is visible only in the weak-dimerization limit where it reflects peculiarities of the excitation spectrum and its sensitivity to variations in $\Delta$.