Normal and Superconducting Heat Capacities of Lanthanum

Abstract

The heat capacities of three samples of lanthanum have been measured in the temperature range 1.6 to to 6.5°K. A four-constant formula was found which represented to high precision the resistance-temperature relation of the carbon composition resistance thermometer from 1.6 to 7.2°K. Two superconducting transitions were found in each sample: one at 4.8°K and the other at 5.9°K. These are associated respectively with the hexagonal close-packed and face-centered cubic modifications of the metal. Below 2.5°K, a magnetic field of 10 000 gauss was found insufficient to quench completely the superconducting phase. The values of the normal heat capacity constants for the purest sample, averaged over the two crystal structures present, were determined by a thermodynamic analysis of the data to be y = (24.1 ±0.6) X10~4 cal/mole (°K) 2 , 0=142±3°K. The data are further analyzed for evidence of a law of corresponding states among superconductors.

Law of Corresponding States for fcc and d-hcp La

“…12 * 13 The over-all behavior of these data is consistent with earlier measurement on La samples with a large fraction of both fee and d-hep phases. 14,15 There is no evidence in any of these results for a Schottky-like anomaly which might result from thermal excitation of electrons into a narrow / band just above the Fermi surface. Both d-hep and fee La show a superconducting specific heat which is similar to tin, niobium, or lead.…”

Effect of Pressure on the Superconducting Transition Temperature of Lanthanum

“…Such an increase is quite feasible if the Fermi energy of lanthanum occurs near a peak in the density-of-states curve and this is not unreasonable since lanthanum is an element with a high density of states. 13 However, a high density of states alone is not sufficient to account for the superconducting behavior of lanthanum since scandium, yttrium and lutetium have roughly the same value for their densities of states 19 and are not superconducting above 0.1 °K. We think, therefore, that the amount of / admixture in the wave functions at the Fermi energy must be an important feature in the superconductivity of lanthanum.…”

The specific heats of metals at low temperatures

“…T h e transition temperature is not well defined owing to the difficulty of preparing strain free samples of a single crystalline form ; they are usually mixed face-centred cubic and hexagonal close-packed. It has a comparatively high linear term in the specific heat below 5°K which corresponds to electrons in the 5d band : y = 8.4 x joule mol-1 deg-2 (Berman, Zemansky andBoorse 1955, Parkinson, Simon andSpedding 1951). The latter also report measurements on cerium, praseodymium and neodymium.…”