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. doi:10.7907/NN9C-1A08. https://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_λ, the gradient in the temperature self-organizes to the gradient in T_λ 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.

We report the first results of the heat capacity of the SOC state, C_∇T, for heat fluxes 60 nW/cm² < Q < 13 uW/cm² and corresponding temperatures 9 nK > T_SOC-T_λ > -1.1 uK. We find that C_∇T 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_λ, C_gradT is depressed relative to C₀ and the maximum in C_∇T is shifted to 50 nK below T_λ. The second difference is that at high heat flux, C_∇T is again depressed relative to C₀ with the departure starting at about 650 nK below T_λ.

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/cm² < Q < 13 uW/cm² 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.

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