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

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 $\hat{I}”$, 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 $\hat{I}”$, $\partial S^m/\partial \hat{I}”$. 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 $\hat{I}”$.

4 pages, 4 figures now identical with finally published version