Surface-State Electrons on Thin Helium Films with Amorphous Metal Substrates

Abstract

Electrons bound to the surface of superfluid helium form a very clean two-dimensional (2D)electron gas owing to the smooth helium surface and the electrons’ relative isolation inside the vacuum. Because of this, the electrons on helium system has been utilized by experimentalists to better understand 2D physics for the last fifty years. Much of this work has been done with relatively thick helium (depths of a micron or more) because thicker films keep the electrons farthest away from the imperfect surfaces below the helium. The study of electrons on thin films of helium is a subject that is comparatively less explored, especially in the case of a metallic substrate. This is because, with metallic substrates, peaks on the rough surface reduce the otherwise high electron mobility and can lead to tunneling of electrons through the helium. However, there are many reasons that overcoming these limitations and investigating such a system is desirable, including the possibility of observing quantum melting of the 2D Wigner crystal and initializing electron spins via interactions with Johnson noise currents for some implementations of quantum computers. In this thesis, we make use of unique substrate materials to further our understanding of these surface-state electrons on thin films of helium. The materials used are amorphous metal alloys which are deposited into ultra-smooth layers to be used as the substrate under the helium films. Because the metal is smooth there are no protruding peaks to act as tunneling centers, and because the metal is amorphous there are no grain boundaries to create barriers for surface-state electrons. In one group of experiments, we demonstrate that by using these substrates, high densities of electrons (up to 5.83×1011 cm−2) can be supported on thin films of helium only a few nanometers thick. We employ a Kelvin probe measurement technique to determine the electron density and show that these surface electrons are mobile at high densities. For the temperature at which these measurements are made, the electron system is Fermi degenerate or the electrons are tightly trapped. It is possible that the electron system is not crystalline due to screening effects from the nearby metallic substrate. While not conclusively shown in this work, this is a promising lead toward an eventual observation of quantum melting of the 2D Wigner crystal. In a second group of experiments, we characterize the transport of electrons across a thin helium film. We are able to move the surface-state electrons between two measurement regions which are separated by an amorphous metal electrode covered with thin helium film. These measurement regions measure the electron density with a Sommer-Tanner scheme in thick helium-filled channel reservoirs. By adjusting the timing of when electrons are emitted from a reservoir on one end of the channel and when they are collected into the reservoir on the other end, we perform time of flight measurements of the electrons across the thin film. We use these measurements to put a lower bound on the electron mobility.