Experiments on the self-organized critical state of 4He

Citation

Chatto, Andrew Rosenberg (2006) Experiments on the self-organized critical state of 4He. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-06062006-094705

Abstract

When a heat flux is applied downwards through a sample of 4He near the superfluid transition temperature T_lambda, the gradient in the temperature self-organizes to the gradient in T_lambda caused by gravity. This creates the Self-Organized Critical (SOC) state. Previous experiments have observed the state, measured the self-organization temperature T_SOC vs. heat flux, and investigated a remarkable wave that only travels upwards against the flow of the heat flux~cite{Moeur:1997,Sergatskov:2004,Lee:2004}.

We report the first results of the heat capacity of the SOC state, C_gradT, for heat fluxes 60 nW/cm^2 < Q < 13 uW/cm^2 and corresponding temperatures 9 nK > T_SOC-T_lambda > -1.1 uK. We find that C_gradT tracks the static (i.e., zero heat flux) unrounded (i.e., in zero gravity) heat capacity C_0 with two exceptions. The first is that within 250 nK of T_lambda, C_gradT is depressed relative to C_0 and the maximum in C_gradT is shifted to 50 nK below T_lambda. The second difference is that at high heat flux, C_gradT is again depressed relative to C_0 with the departure starting at about 650 nK below T_lambda.

We present the most extensive measurements of the speed and attenuation of the SOC wave to date. We report wave speed measurements taken over our full experimental range 30 nW/cm^2 < Q < 13 uW/cm^2 and attenuation results over the limited range that produced enough attenuation to measure. We also report the first accurate calculation of the speed of the SOC wave.

Thesis Files