Enhanced Heat Capacity and a New Temperature Instability in Superfluid ⁴He in the Presence of a Constant Heat Flux Near T_λ

We present the first experimental evidence that the heat capacity of superfluid ⁴He, at temperatures very close to the lambda point T_λ, is enhanced by a constant heat flux Q. The heat capacity at constant Q, C_Q, is predicted to diverge at a temperature T_c(Q)<T_λ at which superflow becomes unstable. In agreement with previous measurements, we find that dissipation enters our cell at a temperature, T_DAS(Q), below the theoretical value, T_c(Q). We argue that T_DAS(Q) can be accounted for by a temperature instability at the cell wall, and is therefore distinct from T_c(Q). The excess heat capacity we measure has the predicted scaling behavior as a function of T and Q, but it is much larger than predicted by current theory.