Charge Modulation in L${\mathrm{a}}_{1.67}$S${\mathrm{r}}_{0.33}$Ni${\mathrm{O}}_4$: A Bulk Thermodynamic Study

We present ultrasonic $\left(\nu \right)$ and specific heat $\left(c_p\right)$ measurements performed on L${\mathrm{a}}_{1.67}$S${\mathrm{r}}_{0.33}$Ni${\mathrm{O}}_4$ to confirm and characterize the nature of the thermodynamic transition observed at $T_c=240\mathrm{K}$ in this material. The behavior of $\nu \left(T\sim T_c\right)$ suggests that this transition is not due to the long-range ordering of the uncompensated nickel spins. The possibility of independent hole-spin behavior is consistent with the observed entropy and measured anomalies in the resistivity, susceptibility, and ultrasound. A comparison of $\nu \left(T\right)$ and $c_p\left(T\right)$ suggests that the sound couples primarily to the order parameter energy.