Experimental apparatus to search for supersolidity in monolayer ⁴ He on graphite

We describe some details of a new experimental setup for torsional oscillator (TO) measurement of ⁴ He monolayers adsorbed on a graphite surface. This setup is designed to seek for the possible supersolid phase, in which crystalline order coexists with superfluidity, in two dimensional (2D) solid ⁴ He below 300 mK. Among such 2D solids, the commensurate phase in the second layer on graphite is the most hopeful candidate for the novel supersolid phase since it is the lowest-density quantum solid ever found. An exfoliated graphite substrate we used is ZYX which has at least ten times longer surface coherence length compared to Grafoil, an exfoliated graphite most commonly used in previous experiments. The first version of TO we made has the resonant frequency of 786.8 Hz and the Q value of 1.1 × 10⁵ at T ≤ 10 mK. The resonant frequency of this particular TO without any He samples (f_cell) showed unexpectedly large temperature variation and non-reproducibility below 1 K as well as sudden jumps when mechanical shocks are applied to the experimental apparatus. We found the stability of f _cell is highly correlated with the temperature stability of 1 K pot in dilution refrigerator.