Few electron mobility measurements across mm long transport line on thin He film

Electron transport over thin helium films opens the door to probing new 2D electron phase transitions because of the higher electron densities that van der Waals (vdW) enhanced films can support. Thin film transport is difficult, however, due to large potential barriers that arise from rough metal surfaces or gaps between gates. Here, we demonstrate a device structure which measures electron mobility across a continuous 5.6 um wide, 4 mm long wire coated with a 34 nm vdW helium film. The wire is fabricated using amorphous, resistive (0.8 MΩ at 1.9 K) NbSi which is advantageous for transport due to its ultrasmooth surface and low surface work function variation. To measure electron mobilities, we conduct time-of-flight measurements by pulsing superconducting Nb “door” gates positive and negative, controlling the movement of electrons in and out of the thin film region. To eliminate potential complications from transferring a high electron density system, we add small measurement gates to allow for working with and measuring few electrons using a HEMT-based cryogenic cascode amplifier. We measure a time-of-flight mobility limited by He vapor atom scattering of 4680 cm2/Vs over the thin film by transferring small packets of ~200 electrons at 1.9 K.