Fluctuation contributions to the heat capacity of superconducting thin films
"This thesis is a report of experimental investigations into the nature of the phase transition of a two-dimensional superconductor between its normal and superconducting states. The heat capacity and electrical resistivity have been measured for a series of amorphous Bismuth-Antimony films of various thicknesses. Mean field theories predict a discontinuous jump in heat capacity at the transition, and this has ""been observed in experiments on pure bulk superconductors. The presence of thermodynamic fluctuations causes a breakdown of mean field theory if one gets close enough to the transition temperature, T. For a pure bulk superconductor, the temperature region over which thermodynamic fluctuations become important is inuneasurably small. For dirty superconductors in restricted geometries (i.e., films or filaments), fluctuations are important over a much wider range in temperature. Various experimentalists have measured the electrical conductivity of thin films and have observed an extra fluctuation conductivity above Tc which varies as (T - Tc )-1 as the transition temperature is approached. A number of theorists have treated fluctuation effects to first order and have calculated the fluctuation conductivity, finding the (T - Tc )-1 depenc dence and even producing the observed magnitude. Similar calculations applied to the heat capacity of a dirty thin film superconductor predict an extra fluctuation heat capacity above T that varies as (T - Tc )-1 and is important over the same temperature range as is the fluctuation conductivity. For the amorphous thin films examined in this experiment, the electrical conductivity measured was in agreement with theory and with other experiments. The heat capacity measurements showed an extra contribution that was consistent with the (T - Tc)-1 behavior predicted. The width in temperature of the heat capacity transition was approximately the same as for the resistive transition. A recent calculation by Masker, Marcelja, and Parks, treating fluctuations to second order, predicts a sharp peak in heat capacity just below T. Such an effect should have been observable with the resolution c of the present experiment, but no such peak was seen."
fluctuation contributions, heat capacity, superconducting thin films
University of Illinois - Urbana-Champaign
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