Surface acoustic wave (SAW) based techniques are a promising complementary probe for investigating these high-frequency collective phenomena. Strong repulsive forces between electrons in this system conspire to produce electronic fluid 3 or crystallized electronic solid states 4 exhibiting exotic high-frequency dynamical response 5, 6, 7, 8, 9, 10, which is typically investigated via free-space coupling between the electrons and radio-frequency or microwave fields. Electrons on helium are quantum non-degenerate, but not classical, with quantum effects influencing the many-electron transport in high magnetic field 2 as well as the single-electron degrees of freedom. Electrons placed near this superfluid substrate are attracted to it and float ~10 nm above the surface, forming a unique two-dimensional electron system (2DES) with the highest electron mobility in condensed matter 1. The surface of superfluid helium at low temperatures is a fantastically pristine substrate without the defects that are unavoidable in almost all other material systems. We also show SAWs are a route to investigating the high-frequency dynamical response, and relaxational processes, of collective excitations of the electronic liquid and solid phases of electrons on helium. We demonstrate precision acoustoelectric transport of as little as ~0.01% of the electrons, opening the door to future quantized charge pumping experiments.
Here we report on the coupling of electrons on helium to an evanescent piezoelectric SAW. In contrast to semiconductors, electrons trapped above the surface of superfluid helium form an ultra-high mobility, two-dimensional electron system home to strongly-interacting Coulomb liquid and solid states, which exhibit non-trivial spatial structure and temporal dynamics prime for SAW-based experiments. In semiconductor two-dimensional electron systems SAWs have been used to reveal the spatial and temporal structure of electronic states, produce quantized charge pumping, and transfer quantum information. Piezoelectric surface acoustic waves (SAWs) are powerful for investigating and controlling elementary and collective excitations in condensed matter.
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