Search

In contrast to two-level atoms, the interstate coherence of both three-level bosons and fermions contributes to the collisional frequency shift of internal transitions in a cold homogeneous gas. In particular, the measured contact shift of the b−c (a−d) hyperfine transition in 3D atomic hydrogen in the presence of incoherently populated state a (b) yields the difference at−as=30(5) pm between the triplet and singlet s-wave scattering lengths, which is a factor of 2 smaller than the previous result. The dynamic contact shift of the b−c resonance due to the Rabi oscillations between the coherently populated states a and b under the simultaneous continuous excitation of the a−b transition can explain the low-temperature electron-nuclear double resonance spectra of 2D hydrogen. The role of field inhomogeneity, spin diffusion, relaxation and recombination is discussed.

Seeking for manifestations of superfluidity in 2D spin-aligned atomic hydrogen (H↓) adsorbed on the surface of liquid helium at T∼0.1 K we consider possible acoustic modes in this 2D Bose gas depending on frequency and on characteristic times of energy and momentum transfer. At high frequencies, the analogues of ordinary and second sound are realized in 2D H↓. At low frequencies, the 2D analogue of the fourth sound is realized: the normal component of hydrogen and ripplons are immobile and only the superfluid component of hydrogen participates the oscillations. We also estimate the rate of momentum relaxation between superfluid hydrogen and ripplons, τHR−1∝T13/3. The most promising for observing the superfluidity in 2DH↓ is the fourth sound, which could be excited, e.g., by ESR.

We apply a quantum hydrodynamics of the surface of 3He-4He solutions to account for the in-plane transport of the two-dimensional (2D) spin-aligned atomic hydrogen (H↓) adsorbed on superfluid helium film. We discuss how the surface flow of 2D H↓ may be traced in experiment thus allowing to study the interaction of the 2D hydrogen with ripplons and surface-bound 3He as well as to observe the superfluidity of the 2D Bose gas of H↓. As an example we consider the formation of the ESR spectrum of the 2D H↓ and find that in spatially non-uniform case the surface flow contributes significantly to the conditions of the ESR spectrum instability observable at high microwave power. We also analyze the conditions at which the surface flow of the 2D hydrogen plays an important role in thermal compression experiments.

Heat transport by saturated 3 He- 4 He films has been studied at temperatures 50.,350 mK and the bulk concentration of 3 He ranging from 0.1 ppm to 5%. The cooling of the film, when locally heated above 160 mK, is mainly via 2D flow of surface 3He from colder area followed by evaporation of 3 He. At certain heating power the 2D flow becomes a bottleneck, the heated spot runs out of 3 He and its temperature abruptly increases. The critical power is nearly proportional to the surface density of 3 He. For higher 3 He concentrations another distinct step in temperature has been observed at a lower heating power. It is attributed to the existence of an excited 3 He surface state whose population starts at 3 He surface density of 3.5 x 10 14 cm -2 . The second state is located about 1.2 K higher in energy than the ground state and provides an additional channel for the 2D flow of 3 He.