Mean-field theory for interacting rotons in superfluid helium

It is shown within the framework of mean-field theory that the gas of roton excitations in superfluid helium becomes unstable at a critical temperatureT _c if the roton-roton interaction is attractive. The instability is characterized by a singularity of the specific heat and signals the transition into the normal fluid state. The roton energy gap remains finite atT _c .     

Size dependent ultrasonic absorption in superfluid helium

Ultrasonic waves in liquid He II at temperatures below 0.6°K can be absorbed by thermal phonons in 3- or 4-phonon processes. For the case that the mean free path of thermal phonons is sufficiently long 3-phonon processes are generally excluded because energy and momentum cannot be conserved. The conservation requirements cannot be fullfilled because the group velocity of thermal phonons is lower than the ultrasonic velocity due to dispersion. — The purpose of this note is to point out that insmall samples of liquid the uncertainty in phonon energy prevents a violation of the conservation laws for 3-phonon processes. In small volumes of liquid, ultrasonic waves are therefore strongly absorbed in 3-phonon processes, while in larger volumes of liquid this contribution to the absorption is absent. — A similiar size dependent absorption is to be expected for longitudinal waves at low temperatures in ideal crystals.     

Detection of 4He atoms separately evaporated by phonons and rotons at a free superfluid surface

Using heat pulses, we detect ⁴He atoms separately by phonons and rotons from a superfluid surface.     

Calculation of the sound absorption in superfluid helium below 0.6 K

The sound absorption in superfluid helium is calculated using the renormalized perturbation expansion of the phonon self-energy. The second-orderωT ⁴-result is derived without the assumption of a Lorentzian form of the phonon spectral function. The fourth-order contribution leads to better agreement with experimental data. These results are compared with simple perturbation theory. A discussion of the Boltzmann-equation approach is also given.     

Trace detection of metastable helium molecules in superfluid helium by laser-induced fluorescence

We describe an approach to detecting ionizing radiation that combines the special properties of superfluid helium with the sensitivity of quantum optics techniques. Ionization in liquid helium results in the copious production of metastable He2 molecules, which can be detected by laser-induced fluorescence. Each molecule can be probed many times using a cycling transition, resulting in the detection of individual molecules with high signal to noise. This technique could be used to detect neutrinos, weakly interacting massive particles, and ultracold neutrons, and to image superfluid flow in liquid 4He.     

Atoms and ions in superfluid helium

Optical spectra, level population and nonradiative deexcitation processes of atomic impurities in liquid helium have been investigated. The recombination of electrons and positive metal ions are described by the tunneling model. This model explains the population gap of 1.8 eV below the ionisation limit which was observed for several neutral defect atoms in liquid helium. In the framework of a pseudopotential theory excited singlet levels of Ba, Ca and Na atoms are recalculated and compared to experimental data. Non-spherical atomic defects for excited p-states are treated also. Quadrupole vibrations of these distorted defect structures are assumed to be responsible for inducing multiphonon transitions between excited atomic states.     

Atoms and ions in superfluid helium

Optical spectra of atomic impurities in liquid helium have been investigated. Comparison is made between the wavelength of the free atomic and ionic lines and those in the liquid helium matrix. Simultaneously, the line width and a possible asymmetry is recorded. Presence and absence of radiative transitions depend on the species of the atom implanted in the quantum fluid. The absence of any optical transitions from states lying as low as 1,8 eV below the ionization limit will be explained in this paper.     

Rogue waves in superfluid helium

Rogue waves have been observed in superfluid helium. The experimental system consists of high intensity second sound (temperature-entropy) waves within a resonant cavity. Under steady state conditions, with a constant oscillatory driving force at the resonant frequency, the waves are turbulent and there are fluxes of energy towards both high and low frequencies. Rogue waves appear under the nonequilibrium conditions that prevail shortly after the drive has been switched on, prior to establishment of the steady state. The experiment is described briefly, relevant results are presented and discussed theoretically in terms of nonlinear wave interactions, and possible connections to rogue waves on the ocean are considered.     

Vortex wheels in superfluid helium

It is suggested that the continuous reversal of apparent circulation around a vibrating wire in liquid He II is due to the motion of a vortex-line array called a “vortex wheel”.     

Topological defects in superfluid helium

Some aspects of classical field-theoretic phenomenology of superfluid helium are presented.     

UCN production in superfluid helium

Ultra-cold neutrons (UCN) are produced in superfluid helium by single- and multi-phonon excitation. The UCN production rate density R _II via multiphonons can be larger than that by one-phonon excitation R _I being due to the dependence of the incident neutron spectral flux density dφ/dλ on the wavelength λ. Received: 28 March 2002 / Accepted: 19 December 2002 / Published online: 11 March 2003 RID="a" ID="a"e-mail: wschott@e18.physik.tu-muenchen.de RID="b" ID="b"On leave of absence from PNPI, Gatchina, Russia Communicated by T. Walcher     

Surface tension of superfluid helium

Summary We present here the calculation of the expression for the surface tension depression by taking into consideration the finite size of the molecular superfluid helium. The agreement between the experimental results (J. R. Eckardt, D. O. Edwards, S. Y. Shen and F. M. Gasparini:Phys. Rev. B,16, 1944 (1977)) and the theoretical results is much better than previously published (K. R. Atkins:Can. J. Phys.,31, 1165 (1953)).     

Thermodynamic inequalities of superfluid helium

We have obtained the thermodynamic inequalities warranting the stability of the equilibrium states of superfluid helium.     

The Fulde-Ferrell state in superfluid helium

The Fulde-Ferrell state in a superfluid³He-⁴HeII solution is investigated. Starting from its broken symmetry, a relative translation-gauge symmetry, the linearized hydrodynamic equations to lowest order in wave number are presented. To this order, the superfluid current perpendicular to the preferred direction vanishes identically; at the same time, supercurrents generated by a uniform change in temperature or density become possible. The Goldstone mode of the Fulde-Ferrell state is shown to be a propagating spin-temperature-shear wave.     

Filament formation by impurities embedding into superfluid helium

The hydrogen molecules embedded in superfluid helium as a gas jet are shown to form long thin filaments. These filaments survived under a helium transition to anormal phase demonstrating their conjugated entity. The concentration of an impurity in the core of the vortex may be the mechanism of the impurity coalescence providing a cotton-like structure of the condensate obtained by introducing an impurity-containing gas helium jet to He-II. The text was submitted by the authors in English.