Torsional oscillator experiment on superfluid 4He confined in a porous alumina nanopore array

We studied superfluidity of liquid 4He confined in an array of well-characterized straight nanopores of porous alumina (PA). The PA plate sample of 45 nm pore size is set in an annular flow channel and the superflow is detected by torsional oscillator (TO) technique. Superfluid transition Tc in the nanopores is suppressed by 3.5 mK from the bulk λ point. Tc is consistent with the temperature at which the healing length is equal to the pore radius. We have observed many anti-crossing anomalies in the TO frequency associated with dissipation peaks, which are attributed to the coupling to second sound resonances.