Kelvin Probe Measurement of High Desnity, Mobile Electrons on a Thin Helium Film Supported by a Metal Substrate

Transport of electrons over thin helium films covering a metal substrate has proven difficult due in part to the metal’s rough surface profile and potential barriers due to discrete grain boundaries. Additionally, surface state electrons on a thin film are prevented from crossing between neighboring metal gates due to the potential barrier arising from the lack of a metal image charge directly below the electron when in the gap. Using an ultra-smooth amorphous metal with a thin central channel connecting two large area gates, we observe evidence of mobile surface state electrons at high electron density. By changing the potential of these gates, we measure density changes corresponding to electron transfer from one plate to the other. Because the gates are connected by a single channel, the electrons never cross gaps in the metal surface and because the metal alloy used (Nb₁₆Si₈₄) is highly resistive at cryogenic temperatures, the helium film is not significantly heated while a potential difference is applied to the connected gates. The electrons on either side of the channel are measured with a Kelvin probe technique, which can selectively measure the density of electrons on each plate.