Formation of Nuclear-Polarized Phases of H Atoms Embedded in Solid H_{2} Films.

We report on an experimental observation of two phases of hydrogen atoms in solid H-2 films at temperatures of 0.1-0.8 K, characterized by a large enhancement of the nuclear spin polarization compared to that given by Boltzmann statistics (p = 0.15 at T = 0.15 K). The first phase with p = 0.35(5) is formed spontaneously during sample storage in a high magnetic field (B = 4.6 T). The second phase with an even higher nuclear polarization, p = 0.75(7), can be achieved at T <= 0.55 K by repeating sequences of dynamic nuclear polarization followed by a system relaxation. Upon warming through the range 0.55-0.65 K, the highly nuclear-polarized phase undergoes a phase transition to the spontaneously polarized phase which breaks down at T similar or equal to 0.8 K, and the nuclear polarization gradually converges to the Boltzmann distribution. We discuss possible scenarios for explaining the nature of the observed phenomena.