Atomic Heats of Cesium, Rubidium, and Lithium Between 0.35 and 2°K

The electronic and lattice contributions to the atomic heat of lithium, rubidium, and cesium have been determined from heat capacity measurements on large high-purity samples between 0.35 and 2°K. The linear contribution to the heat capacity yields a value for $\gamma$ of 1.71 for Li, 4.07 for Rb, and 6.29 for Cs in mJ/mole-${\left(\mathrm{°}\mathrm{K}\right)}^2$. Interpreting this linear contribution to the heat capacity in terms of the density of single-excitation states at the Fermi level, a thermal effective mass $\frac{C_{\mathrm{el}}}{C_0}=\frac{\gamma }{{\gamma }_0}=\frac{m}{m_0}$ is obtained. The value is $\frac{m}{m_0}=2.31\mathrm{}$ for Li, 2.14 for Rb, and 2.82 for Cs. These values for the thermal effective mass suggest considerable anisotropy for the Fermi surface and are compared with the band calculations of F. S. Ham, and a calculation by Silverstein which combines the results of the band calculation with electron-electron interaction. The value for Li is in good agreement with that of Martin, but the effective mass for rubidium and cesium are considerably larger than the values of 1.26 and 1.43 obtained by Lien and Phillips. The lattice contribution to the very low-temperature heat capacity is described by a value of ${\Theta }_0=58.7\mathrm{}$ for Rb and ${\Theta }_0=40.6\mathrm{}$ for Cs. The dependence of $\Theta$ on temperature is given and the values at 2°K are in good agreement with the data of Lien and Phillips and the data of McCollum and Silsbee.