Normal density and moment of inertia of a moving superfluid

TL;DR

This paper investigates the normal density and moment of inertia in a moving superfluid, revealing finite normal density at zero temperature and its implications for superfluidity and experimental measurements.

Contribution

It introduces the concept of finite normal density at zero temperature in moving superfluids and links it to moment of inertia quantization, advancing understanding of superfluid dynamics.

Findings

Finite normal density exists at zero temperature.

Normal density reaches total mass density at sound velocity.

Finite normal density leads to quantized moment of inertia.

Abstract

In this work, the normal density $\rho_n$ and moment of inertia of a moving superfluid are investigated. We find that, even at zero temperature, there exists a finite normal density for the moving superfluid. When the velocity of superfluid reaches sound velocity, the normal density becomes total mass density $\rho$, which indicates that the system losses superfluidity. At the same time, the Landau's critical velocity also becomes zero. The existence of the non-zero normal density is attributed to the coupling between the motion of superflow and density fluctuation in transverse directions. With Josephson relation, the superfluid density $\rho_s$ is also calculated and the identity $\rho_s+\rho_n=\rho$ holds. Further more, we find that the finite normal density also results in a quantized moment of inertia in a moving superfluid trapped by a ring. The normal density and moment of inertia at zero temperature could be verified experimentally by measuring the angular momentum of a moving superfluid in a ring trap.