Snorre Ballière Farner

Specific Heat of Superconducting Sr2RuO4

Master thesis of Snorre Farner, NTH (now NTNU), 3 April 1995. My supervisor was Dr. Yoshiteru Maeno, who is now Professor at Kyoto University (and was an excellent supervisor).


Superconductors are materials that exhibit zero electrical resistance and peculiar magnetic properties when they are frozen to a sufficiently low critical temperature Tc. Until 1986, the materials were metal-based, and Tc was confined below about 30 K or -243°C. Then came the discovery of superconductivity in a certain ceramic material at higher temperatures. Furthermore, it behaved differently and was called type II superconductivity. Since then, more similar materials have been tested, and quite a few have shown the same properties and superconductivity has been achieved up to about 125 K (-148°C).

Superconductivity is a quantum phenomenon, which insures the absolute zero resistivity and a repulsion or locking of magnetic fields. When superconductivity appears, the electronic contribution to the specific heat of the material, γ0, increases abruptly (if pure) at Tc and then falls systematically towards as the temparature approaches 0 K. The curve it follows characterizes the mechanism governing the type-II superconductivity.

The objective of my master work was to measure the specific heat of a 0.5 cm2 small crystalline superconducting sample of Sr2RuO4 with a Tc around 1 K (-272°C) in order to determine its type of superconductivity. The work thus consisted in development of apparatus, a simple heat-flow model, and a program (in BASIC) to control the experimental equipments as well as acquisition and temporary processing of data in real time.

The apparatus (cylinder of about 3 cm diameter and 8 cm length) was mounted on a He-3 refridgerator, which cool down to 0.35 K, and the temperature variation in the sample inside the apparatus was measured. The parameters of the heat-transfer model could then be fitted to the temperature variation, the heat capacity of the sample be calculated, and the electronic specific heat be deduced.

I calibrated the apparatus with an aluminium sample (Tc = 1.2 K), then measured the electronic specific heat of the sample of Sr2RuO4. The sample was not sufficiently pure crystalline to allow an unambiguous extrapolation toward 0 K, but after my departure from Hiroshima University, better crystals were succeeded produced, and the results indicated that Sr2RuO4 is a type-II superconductor with a very low Tc of about 1.2 K.