We have performed magnetic compression experiments with the two-dimensional gas of hydrogen atoms on liquid ${}^4\mathrm{He}$ at $T=120\mathrm{}–200\mathrm{mK}$. For 2D phase-space densities higher than 3, the probability of three-body dipole recombination is observed to decrease gradually by an order of magnitude in comparison to its low-density value $K_{\mathrm{bbb}}=8.4\mathrm{}\left(3.5\right)\times {10}^{-25\mathrm{}}{\mathrm{cm}}^4{\mathrm{s}}^{-1\mathrm{}}$. This is attributed to local coherence developing in a 2D quasicondensate. We have also determined the value $U/k=5.1\mathrm{}\left(5\right)\times {10}^{-15\mathrm{}}\mathrm{K}{\mathrm{cm}}^2$ for the mean-field energy of the gas.

• Received 6 February 1998

DOI:https://doi.org/10.1103/PhysRevLett.81.4545