Electron mobility measurements on thin helium film across mm long transport line

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. However, thin film transport is often difficult due to large potential barriers that arise from rough metal surfaces or gaps between gates. Here, we demonstrate a device which allows for mobility measurements as electrons move 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 1.7K) NbSi which is advantageous for transport due to its ultrasmooth surface and lack of grain boundaries. To measure electron mobilities, we conduct time-of-flight measurements by pulsing “door” gates positive and negative, controlling the movement of electrons in and out of the thin film region. Earlier measurements with this variety of device showed evidence of mobilities on the order of 100,000 cm²/Vs. We report on new devices with superconducting Nb door gates for faster pulsed measurements and additional small measurement gates to allow for working with fewer electrons to eliminate possible space charge effects.