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Matrix Isolation Spectroscopy in Helium Droplets
Published in Leonid Khriachtchev, Physics and Chemistry at Low Temperatures, 2019
Kirill Kuyanov-Prozument, Dmitry Skvortsov, Mikhail N. Slipchenko, Boris G. Sartakov, Andrey Vilesov
The long lifetime of the molecular rotation in liquid helium is in agreement with the sharp spectrum of the elementary excitations in superfluid 4He, which is shown in Fig. 3a. At low excitation energy it has a phonon branch. Coupling of molecular rotation with He environment originates from the short range van der Waals interaction between the molecules and He atoms of the droplet. Due to the long wavelength of the phonons they couple ineffectively with molecular rotation, which enables observation of the narrow rotational lines of heavy molecules in He droplets, as seen in Fig. 3b. In light rotors such as CH4 the rotational energy may exceed the energy required for creation of the roton excitations in liquid helium. Rotons are elementary excitations with energy of about 6 cm−1 and a short wavelength (λ ≈ 3.3 Å),9 which enables efficient coupling to molecular rotation. Therefore, the rotational relaxation of light molecules possibly involves creation of rotons. This conjecture is in agreement with observation of broadening of the rotational lines of light molecules such as HF,16 NH3,17 H2O,18 and CH4,10,19 which have rotational energy in excess of the roton gap (see Fig. 3b).
A Comparison of the Properties of Superconductors and Superfluid Helium
Published in R. D. Parks, Superconductivity, 2018
where S (k) is the liquid structure factor, which can be obtained experimentally from either X-ray or neutron scattering. As can be seen from Fig. 3, the form of Eq. (14) is qualitatively correct, although the roton minimum occurs at an energy that is too large by a factor of about 2. Thus the wave function of Eq. (13)is not quite correct for a roton; a roton cannot be a pure density fluctuation. A better wave function was proposed by Feynman and Cohen (16) on the basis of the following simple, but important, physical argument. Suppose that we wish to describe a roton that is localized in space. We can do so by a suitable superposition of the functions (13) with a range of wave numbers centered on a particular value k0. The momentum associated with this localized excitation will be ℏk0, and the momentum itself must be localized. Suppose, however, that k0 is situated at the minimum in the energy spectrum. The velocity of the wave packet, equal to ℏ ∂E/∂k, is then zero. Thus we have described a situation in which there is a localized mass current, equal to ℏk0, fixed in space in a fluid of practically constant density. Such a situation is impossible. To conserve mass, the localized current ℏk0 must be accompanied by a “backflow” of the fluid around the localized excitation. The wave function of Eq. (13) is defective in that it does not include this backflow. Feynman and Cohen therefore constructed a new wave function which included such a backflow. This wave function was then used in a variational calculation, the strength of the backflow being initially undetermined. The results for the energy are shown in Fig. 3, and it can be seen that there is quite good agreement with experiment. Since this work of Feynman and Cohen, it has become clear that a backflow of this kind must often be taken into account in considering excitations in a many- body system; as we shall see later, this is true of the excitations in a superconductor.
Helium II phase: superfluid, supersolid, liquid crystal or spin ice?
Published in Molecular Physics, 2022
It can be seen from Equation (58) that spin subsystem plays the role of an external force for phonon excitations, and when approaching some ‘resonant’ wavelength the phonon dispersion curve will be ‘subsided’ due to loading from spin degrees of freedom (roton minimum of dispersion curve). Mechanically, the translation–rotation interaction could be a justification for the phonon–‘roton’ coupling in He II (phonons are translation excitations in the medium, but spin degrees of freedom have the well-known transformation properties of angular momentum). In this connection the physical sense of term ‘roton’, introduced by I.E. Tamm [14] in 1941 could be clarified. The phenomenon of microwave resonant absorption is the manifestation of direct interaction between macro- and microscopic degrees of freedom in condensed helium phases, and concept of the spin–phonon interaction is quite realistic as the mechanism for a number of experimentally observed effects.