Sign change of the Grueneisen parameter and magnetocaloric effect near quantum critical points

TL;DR

This paper analyzes how the Grueneisen parameter and magnetocaloric effect behave near quantum critical points, highlighting their sign changes and divergences as key indicators for identifying quantum criticality in materials.

Contribution

It provides a detailed theoretical framework describing the sign change and divergence of the Grueneisen parameter and magnetocaloric effect near quantum critical points, including finite temperature phase transition lines.

Findings

Sign change of the Grueneisen parameter signals entropy accumulation.

Divergences of the Grueneisen parameter occur near quantum critical points.

Thermal expansion and magnetocaloric effect are effective probes for quantum criticality.

Abstract

We consider the Grueneisen parameter and the magnetocaloric effect near a pressure and magnetic field controlled quantum critical point, respectively. Generically, the Grueneisen parameter (and the thermal expansion) displays a characteristic sign change close to the quantum-critical point signaling an accumulation of entropy. If the quantum critical point is the endpoint of a line of finite temperature phase transitions, T_c \propto (p_c-p)^Psi, then we obtain for p<p_c, (1) a characteristic increase \Gamma \sim T^{-1/(\nu z)} of the Grueneisen parameter Gamma for T>T_c, (2) a sign change in the Ginzburg regime of the classical transition, (3) possibly a peak at T_c, (4) a second increase Gamma \sim -T^{-1/(nu z)} below T_c for systems above the upper critical dimension and (5) a saturation of Gamma \propto 1/(p_c-p). We argue that due to the characteristic divergencies and sign changes the thermal expansion, the Grueneisen parameter and magnetocaloric effect are excellent tools to detect and identify putative quantum critical points.