Properties of surface state electrons on thin helium films

Abstract

Surface-state electrons (SSE) on liquid helium form a nearly ideal two-dimensional Coulomb system (2-DES). In our work we focus on the investigation of the properties of electrons on the surface of 4He with different thickness between two segments. Various substrates (gold, silicon, and polymer) and several sensitive techniques for determining the SSE density and the transport behavior of the SSE system in confined and non-confined geometry have been used. In all techniques the helium film surface was charged by a small filament.A first technique is the optical technique, which is surface plasmon resonance (SPR). The sensitivity of the SPR to the change of the dielectric constant of the medium, allowed us to determine the electron density on the helium film surface. In this technique we used metal samples; the main substrate of the sample is a glass prism in Kretschmann configuration, on the surface of the prism a gold layer was evaporated with thickness of about 43 nm. Although this technique is very sensitive and accurate, in principle due to problems with our system (high temperature, mechanical-instability, quality of the samples) we did not manage to get concrete results. Nevertheless in previous work -from our group- with this technique had been investigated the properties of electrons on thin films with very high accuracy.Second technique is the electronic or recharging technique. The samples which were used in this technique were two electrode samples with gap of 0.1mm in between. Originally the two electrodes were metal (gold or copper), but one of them was covered by a dielectric film (PMMA, kapton or tesa). In this technique the system is a capacitor with capacitance of both helium film and the dielectric film. Through this technique we calculated the amount of charges that can be put on the surface film and compared with the experimental values. In addition we observed electron transport between the two electrodes when we applied a potential difference between them. Due to the same problem mentioned with SPR technique, we did not have great results with this technique.Third is the field effect transistor (He-FET. By using this technique we investigated characteristics of the SSE’s transport through a micro-fabricated channel, using direct current (DC) measurement. For this purpose we used suitable samples micro-fabricated on a silicon wafer. All the samples have source and drain regions separated by two gold gate electrodes with a narrow gap (channel) through which the electron transport takes place, but they had different shapes of the source area.First the electrons are pulsed on the source area, and then they cross the channel of the split gate towards the drain, and measured via pick-up electrode. We determined the transport of the SSE through channels of various dimensions and wide range of helium film thickness. The number of the transported charges was represented as a function of the gate voltage. In this technique we used two different methods:1. He-FET with continuous charging measurement. In this method the helium surface was continuously charged, while the gate was gradually opened (by applying a positive voltage), or pulsed for short time. By using this method we observed that the electrons transported through the channels in lanes or steps, the number of these lanes depends on the applied gate potential.2. He-FET with stored electrons. In this method the experiments of pulsed measurements were run. Through these experiments we determined the maximum electron density on the helium film. Beside that we did time resolved measurements and obtained the electron mobility for different samples.Beside these techniques we performed a simulation of colloidal particles transport through a micro channel. These particles were super Para-magnetic and interacted via a dipole induced by an external magnetic field. Our molecular dynamic simulation described the movement of the SSEs through the channel in the He-FET measurements. In the SSEs system, the electrons are governed by a Coulomb interaction, but also included the force caused by the electron’s image charge on the substrate. Thus the SSE has also a dipolar contribution. So the simulation showed a great similarity between the SSE and colloidal systems. From this simulation we obtained a qualitative picture for the electron density distribution in front of the barrier and their velocity distribution along the channel.