Thermal compression of atomic hydrogen on helium surface

TL;DR

This study explores thermal compression of atomic hydrogen on helium surfaces, achieving high surface densities near quantum degeneracy, and investigates thermal contact and potential superfluid transition in 2D hydrogen gas.

Contribution

It presents experimental techniques to increase hydrogen surface density and analyzes thermal contact and quantum degeneracy effects in 2D hydrogen on helium.

Findings

Achieved surface density of 5.5×10^{12} cm^{-2} at 110 mK.

Detected quantum degeneracy onset with de-Broglie wavelength exceeding interatomic spacing.

Estimated thermal contact between hydrogen gas and helium phonons.

Abstract

We describe experiments with spin-polarized atomic hydrogen gas adsorbed on liquid $^{4}$He surface. The surface gas density is increased locally by thermal compression up to $5.5\times10^{12}$ cm$^{-2}$ at 110 mK. This corresponds to the onset of quantum degeneracy with the thermal de-Broglie wavelength being 1.5 times larger than the mean interatomic spacing. The atoms were detected directly with a 129 GHz electron-spin resonance spectrometer probing both the surface and the bulk gas. This, and the simultaneous measurement of the recombination power, allowed us to make accurate studies of the adsorption isotherm and the heat removal from the adsorbed hydrogen gas. From the data, we estimate the thermal contact between 2D hydrogen gas and phonons of the helium film. We analyze the limitations of the thermal compression method and the possibility to reach the superfluid transition in 2D hydrogen gas.