Studying electrons on helium via surface acoustic wave techniques

This thesis describes novel methods and experiments for investigating electrons floating on the surface of liquid helium by utilizing surface acoustic wave (SAW) techniques. Electrons on helium are an ideal system for studying a non-degenerate two-dimensional electron system (2DES), Wigner cyrstallization, and the coupling of a 2DES to a bosonic field. The electrons in this system are also interesting with regard to quantum information science due to their predicted long coherence time. An evanescent coupling of electrons to high-frequency SAWs submerged beneath the liquid helium provides a probe of the high-frequency conductivity of the 2DES and can be used to create SAW-driven charge transport of the electron system and opens the door for resonant interaction between SAWs and collective modes of the electron system.Before employing SAW techniques, we first examined the low-frequency transport properties of electrons on helium at several tens of kHz using a lock-in method, which confirms the realization of a stable and reproducible 2D electron system on liquid helium. Furthermore, taking advantage of the capactive coupling of the electron layer and submerged electrodes, we are able to precisely control AC current flows in the 2DES and achieve the analog of a field-effect transistor with electrons on helium and demonstrate the applicability of long wavelength transport at high temperature and low electron density. By coupling SAWs to the system, the SAW-liquid helium interaction is investigated by measuring the loss of SAW energy into the liquid. We find that the attenuation of the SAW in bulk liquid helium is dominated by the radiation of compressional wave into the liquid, in a good agreement with theory and previous experiments. However, for sufficiently thin helium coverage (several tens of nm), the attenuation anomalously increases beyond that measured in a thick helium layer. This novel phenomenon is likely associated with the electrostrictive actuation of the helium surface. Furthermore, this study is a prerequisite for quantitatively analyzing the SAW measurement results obtained for the 2DES on the liquid helium surface. In last two chapters, we present a detailed study of the SAW coupling to surface electrons floating above liquid helium in both the weak and the strong coupling regimes. These experiments demonstrate find acoustoelectric charge transport and the SAW attenuation in this system of electrons on helium for the first time. These experiments open the door to a new class of studies with this system. These include quantized charge pumping, the possibility of electrical metrology, and ultimately single electron state transfer with electrons floating on the helium. Also, this coupled system can be utilized to study a commensurateincommensurate transition of the 2DES under a time-dependent periodic electric potential (i.e. a standing piezoelectric SAW). Furthermore, the high-sensitivity of the SAW to the spatial structure of the 2DES can provide a new method of studying the 2D phase transition in the electrons on helium system.