Method for Analyzing Second-Order Phase Transitions: Application to the Ferromagnetic Transition of a Polaronic System

A new method for analyzing second-order phase transitions is presented and applied to the polaronic system ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$. It utilizes heat capacity and thermal expansion data simultaneously to correctly predict the critical temperature’s pressure dependence. Analysis of the critical phenomena reveals second-order behavior and an unusually large heat capacity exponent.

Figure 1

(a) Molar heat capacity $C_P$, molar entropy $S$, and magnetic entropy $S_{\mathrm{MAG}}$ (inset) versus $T$. (b) Expansion coefficient $\mu$ and $\Delta l/l_0$ (inset); $l_0$ is the length at 300 K. Lines denote the jumps for an ideal second-order phase transition.

Figure 2

Molar heat capacity after subtracting a linear term, $C_P^{*}$ (open symbols), and $7000\mu T$ (closed symbols) versus $T$ illustrating the scaling suggested by Eq. (3).

Figure 3

$C_P^{*}-A_{\pm }$ (open symbols) and $7000\mu T-A_{\pm }$ (closed symbols) versus $t\equiv (T-T_c)/T_c$ on a log-log scale.

Figure 4

Electrical resistivity and ac susceptibility (upper inset) versus $T$ under pressure $P$. Lines illustrate $T_c$ determination. Lower inset shows $T_c$ and $T_{\mathrm{MI}}$ versus $P$.