Second-sound velocity in rotating superfluid helium

We have investigated the second-sound velocity u₂ in rotating superfluid helium by observing the resonant modes of a rotating cavity. We have discovered an unexpected decrease in u₂. This effect has been measured in different second-sound resonators, and as a function of the second-sound frequency (10³≤ω≤10⁵ rad/sec), the HeII temperature (1.3≤T≤2.1 °K), and the angular velocity of rotation (Ω≤8 rad/sec). We have found that the decease in u₂ is proportional to Ω and is inversely proportional to the second-sound frequency. We show that this decrease may be explained within the the two-fluid model of Landau and Khalatnikov if we suppose the existence of a thermodynamic coupling between the dissipative heat flux and the mutual-friction force. The principal cause of this coupling, we suggest, is entropy transport by the moving vortices. We discuss the origin of this entropy transport using the idea, suggested by Hall and Vinen, of the moving vortices dragging the normal fluid. The theoretical expression we obtain for the decrease of u₂ in rotating helium is in reasonable agreement with our experimental results. We propose that this effect can be considered as the analog in rotating HeII of the thermomagnetic effect in the mixed state of type-II superconductors.