We investigate the new type of excitations on the surface of liquid helium. These excitations, called surfons, appear because helium atoms have discrete energy level at the liquid surface, being attracted to the surface by the van der Waals force and repulsed at a hard-core interatomic distance. The concentration of the surfons increases with temperature. The surfons propagate along the surface and form a two-dimensional gas. Basing on the simple model of the surfon microscopic structure, we estimate the surfon activation energy and effective mass for both helium isotopes. We also calculate the contribution of the surfons to the temperature dependence of the surface tension. This contribution explains the great and long-standing discrepancy between theory and experiment on this temperature dependence in both helium isotopes. The achieved agreement between our theory and experiment is extremely high. The comparison with experiment allows to extract the surfon activation energy and effective mass. The values of these surfon microscopic parameters are in a reasonable agreement with the calculated from the proposed simple model of surfon structure.

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We investigate the possibility of ultracold neutron (UCN) storage in quantum states defined by the combined potentials of the Earth's gravity and the neutron optical repulsion by a horizontal surface of liquid helium. We analyse the stability of the lowest quantum state, which is most susceptible to perturbations due to surface excitations, against scattering by helium atoms in the vapor and by excitations of the liquid, comprised of ripplons, phonons and surfons. This is an unusual scattering problem since the kinetic energy of the neutron parallel to the surface may be much greater than the binding energies perpendicular. The total scattering time constant of these UCNs at 0.7 K is found to exceed one hour, and rapidly increasing with decreasing temperature. Such low scattering rates should enable high-precision measurements of the scheme of discrete energy levels, thus providing improved access to short-range gravity. The system might also be useful for neutron β-decay experiments. We also sketch new experimental concepts for level population and trapping of UCNs above a flat horizontal mirror.

We investigate the monotonic growth of longitudinal interlayer magnetoresistanceRzz (Bz), analytically and numerically in the self-consistent Born approximation. We show that in a weak magnetic field the monotonic part ofRzz (Bz) is almost constant and starts to grow only above the crossover field Bc, when the Landau levels (LL) become isolated, i.e. when the LL separation becomes greater than the LL broadening. In higher field Bz ≫ Bc,Rzz (Bz) ∝ B 1/2 z in agreement with previous works.

Recently a new type of surface excitations, surfons, has been proposed. Phenomenologically, these excitations can be interpreted as atoms on a surface quantum level. They explain the strong temperature dependence of surface tension coefficient of liquid helium. In the present work we studied theoretically the interaction of these excitations with ripplons, the quanta of surface waves. The lifetime of surfons due to the absorption of a ripplon and subsequent evaporation from the surface was estimated. The obtained surfon lifetime considerably exceeds the inverse surfon energy, substantiating the treatment of surfons as quasiparticles.