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