# Superfluid Helium Or He II: Landau-Tisza Model

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Superfluid helium or He II is a macroscopic quantum fluid that exhibits extraordinary properties.
%By applying the limit of low flow velocities,
The behavior of the fluid can be understood using a Landau - Tisza model, where He II is considered to be a two-component fluid with independent velocity fields: the inviscid superfluid of density $\rho_s$, and the normal fluid of density $\rho_n$, where the total density $\rho$ =
$\rho_s$ + $\rho_n$ \cite{Tisza,Landau}. The superfluid has neither viscosity nor entropy, and the entire heat content of He II is carried by the normal component. This simplified picture is described by two fluid equations of motion.
%The important outcomes of these equations are not only the existence of density fluctuations relative to ordinary or first sound but also a prediction of the second sound, a wave described by temperature fluctuations.
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\textit{Recent experiments \cite{Rybalko_LTP,Rybalko_JLTP} reported another exceptional property of superfluid $^4$He, such as electrical activity that appears in the relative oscillatory motion of the normal fluid and superfluid components, or second sound wave.}
A standing half-wave of second sound was generated by a heater, which was placed on one end of a dielectric resonator.
The AC potential difference between an electrode that was placed on the opposite end of the resonator and the ground was recorded.
The resonance frequency of the electric response corresponded to the frequency of the second sound