Experimental Investigation of Electrons In and Above Liquid Helium

Abstract

Electrons on the surface of liquid helium form a nearly ideal 2-dimensional electron system (2DES). An electron density up to 2 × 10^9 cm-2, known as the critical electron density, can be achieved on the liquid helium surface, above which an electro-hydrodynamic (EHD) instability sets in, which results in the formation of MEBs. Due to this limitation in maximum possible density, only the classical liquid and solid phases of the 2DES can be accessed in this system. But at the same time, on the surface of thin liquid helium film and with the multi-electron bubbles (MEBs), it may be possible to achieve high electron density than that of the critical electron density. This can allow the observation of quantum melting, i.e., the phase transition between the quantum solid to the liquid phase of the 2DES. Although extensive theoretical and experimental studies have already been done, the quantum melting transition has not been achieved experimentally on these systems yet. In this thesis, we have used multiple new experimental approaches to obtain electron densities higher than what has been achieved before and to study the MEBs effectively.

First, we studied the temporal dynamics of the EHD instability and the effect of the applied electric field and charge density on the instability. The unstable wave vectors were determined experimentally, and their temporal growth was studied carefully. The determined unstable wave vectors were found to be a good match with the theoretically expected values obtained from the dispersion relation. At the same time, the analysis of the growth rate of unstable vectors were found to be limited by the kinematic viscosity of the liquid helium.

Next, we investigated the lifetime of MEBs trapped on a dielectric surface and compared the result with previous results on free bubbles in bulk liquid helium. The reduced lifetime of trapped bubbles suggested an impact of convective heat flow around the bubbles on their lifetimes. Then we performed an experimental investigation that confirmed the effect of convective heat flow direction inside the experimental cell on the lifetime of such trapped MEBs. Determination of the electronic phase inside an MEB is one of the biggest challenges of the time. Unfortunately, there is no direct way or technique for such investigation. We discussed how the MEB surface fluctuation with an external oscillating electric field could be observed, which may allow a possible way of studying the phase of the 2DES. We studied the surface fluctuations of electrically excited MEBs and observed different normal mechanical modes of the bubble wall. Then we extended our discussion on why liquid helium-4 is not a suitable medium to study the MEBs at low temperatures (below λ), where interesting phenomena occur, and how liquid helium-3, based on its physical property, can be a suitable replacement for this purpose. We generated MEBs inside liquid helium for the first time. The generated MEBs at 1.1 K were found to be stable with long lifetimes. This result opens the possibility of studying the MEBs at much lower temperatures where quantum properties dominate over classical for the 2DES.

Finally, we discussed the problem associated with achieving high electron density on the thin helium film and how integrating an NEA material as a substrate can help us overcome the problem. We fabricated NEA materials, i.e., cBN pellet, and optimized the rf sputtering deposition of cBN film. We performed a preliminary pick-up measurement on the charged thin helium with these materials as substrates, which showed some positive indications that need to be confirmed with further advanced experimental investigations.