Experimental Evidence for a New State in 2D Bose Gas: Quasi-Condensation in Atomic Hydrogen

We have obtained two-dimensional (2D) Bose gas with the degeneracy parameter (phase-space density) up to 9 compressing magnetically atomic hydrogen adsorbed on liquid ⁴ He at about 0.2 K. We observed this 2D gas to go over to a state where the rate constant K _bbb of three-body recombination of hydrogen atoms decreases with respect to its normal value. A steep decline in K _bbb begins when the degeneracy parameter exceeds 3, i.e., before the criterion for 2D superfluidity is satisfied. The maximum observed suppresion of K _bbb is essentially larger than 3! predicted for a three-body process in a fully (quasi)condensed sample.