Study of quantum phase transition of two-dimensional dilute Bose gas by helium surface electrons

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Two-dimensional dilute Bose gas is expected to undergo quantum phase transitions at low temperatures. The expected low temperature phase is quasi-condensate. The reduced density matrix of quasi-condensate vanishes as a power low. In that sense, quasi-condensate does not show a true off-diagonal-long range order but it has a quasi-long range order. It is pointed out that quasi-condensate exhibits superfluidity by Kosterlitz-Thouless mechanism, in which the system is stabilized by a formation of quantized vortex pair.With the aim of realize and observe superfluidity of two-dimensional dilute Bose gas, we cooled adsorbed two-dimensional atomic hydrogen gas on liquid helium surface. Hydrogen atoms are bounded on liquid helium surface and they lose the freedom of motion perpendicular to the surface while the motion along the surface is free. Since hydrogen atoms are Bosons, they form a two-dimensional Bose gas. Electrons, on the other hand, are also bounded on liquid helium surface and the … More system of helium surface state electrons (SSE) is a two-dimensional Coulomb gas. Since all the impurity is frozen below 1 K, the liquid helium surface is extraordinarily clean. By virtue of that, SSE exhibit very high mobility. We employed SSE as a probe to sense the state of adsorbed atomic hydrogen gas, expecting that the mobility of SSE would be ruled by collision with hydrogen atoms or excited quasiparticles.We embedded SSE in adsorbed hydrogen gas and measured its mobility. However, unfortunately, all the measured SSE mobility showed no difference between with or without adsorbed hydrogen gas. This may be because scattering cross section of electron-hydrogen collision is too small and SSE mobility is dominated by ripplon scattering regardless of the existence of hydrogen atoms.Although we have not yet observed two-dimensional superfluidity, we made good progresses in this research. We found that electron attachment to a hydrogen atom takes place when SSE is immersed in adsorbed atomic hydrogen and our precise measurement revealed the mechanism of the reaction. We developed pulse excited edgemagnetoplasmon technique which enabled fast SSE mobility measurement in strong magnetic fields. Recently, we found anomalous behaviors of edgemagntoplasmon signal at reduced SSE densities which cannot be explained by simple Drude model. The study of the anomalous behavior is in progress. Less