‘Heat from Above’ Heat Capacity Measurements in Liquid ^4He

Abstract

We have made heat capacity measurements of superfluid ^4He at temperatures very close to the lambda point, T_λ, in a constant heat flux, Q, when the helium sample is heated from above. In this configuration the helium enters a self-organized (SOC) heat transport state at a temperature T soc(Q), which for Q≥100 nW/cm^2 lies below T_λ. At low Q we observe little or no deviation from the Q=0 heat capacity up to T_(SOC)(Q); beyond this temperature the heat capacity appears to be sharply depressed, deviating dramatically from its bulk behaviour. This marks the formation and propagation of a SOC/superfluid two phase state, which we confirm with a simple model. The excellent agreement between data and model serves as an independent confirmation, of the existence of the SOC state. As Q is increased (up to 6 µW/cm^2) we observe a Q dependent depression in the heat capacity that occurs just below T_(SOC)(Q), when the entire sample is still superfluid, This is due to the emergence of a large thermal resistance in the sample, which we have measured and used to model the observed heat capacity depression. Our measurements of the superfluid thermal resistivity are a factor of ten larger than previous measurements by Baddar et al.