Possible Thermodynamical Phase Slips in Superfluid 4He Confined in a 2.5-nm Channel of FSM

We have measured the superfluid response of 4He confined in a 2.5-nm channel and have compared with that of 4He in a 2.8-nm channel. In the 2.5-nm channel, the superfluid growth is gradual regardless of areal density and pressure. In contrast, 4He in a 2.8-nm channel shows a sharp superfluid onset at low pressure, although it shows a gradual temperature dependence with increasing pressure. The gradual temperature dependence in both channels is well fitted to the function of static superfluid density based on the Tomonaga–Luttinger liquid model. It indicates that the thermal excitation of phase slips plays an important role in the observed superfluid response.