Superfluidity and internal parameters in helium II

We derive a model describing some of the main properties of liquid helium at temperatures below the lambda point, in the framework of the modified thermodynamics with internal state variables. For the present derivation we introduce one scalar field whose spatial gradient, interpreted as an internal state variable is directly related to the counter-flow velocity. As a result the hyperbolic system of field equations has been obtained and the corresponding five speeds of waves have been calculated.