Thermal expansion of the spin-1/2 Heisenberg-chain compound Cu(C$_4$H$_4$N$_2$)(NO$_3$)$_2$

TL;DR

This study investigates the thermal expansion behavior of a spin-1/2 Heisenberg-chain compound near a quantum phase transition, providing experimental evidence for quantum critical phenomena predicted by theory.

Contribution

First measurements of thermal expansion and magnetostriction in Cu(C$_4$H$_4$N$_2$)(NO$_3$)$_2$ near its quantum critical point, confirming theoretical predictions.

Findings

Sign change of thermal expansion across ground states

Thermal expansion diverges as 1/√T at critical field

Data aligns with quantum critical behavior expectations

Abstract

Compounds containing magnetic subsystems representing simple model spin systems with weak magnetic coupling constants are ideal candidates to test theoretical predictions for the generic behavior close to quantum phase transitions. We present measurements of the thermal expansion and magnetostriction of the spin-1/2-chain compound copper pyrazine dinitrate Cu(C$_4$H$_4$N$_2$)(NO$_3$)$_2$. Of particular interest is the low-temperature thermal expansion close to the saturation field $H_c \simeq 13.9 \mathrm{T}$, which defines a quantum phase transition from the gapless Luttinger liquid state to the fully saturated state with a finite excitation gap. We observe a sign change of the thermal expansion for the different ground states, and at the quantum critical point $H_c$ the low-temperature expansion approaches a $1/\sqrt{T}$ divergence. Thus, our data agree very well with the expected quantum critical behaviour.