arXiv subjects

J. P. Toennies

Publications and source records attributed to J. P. Toennies.

8 recordsLinked to original sources

arXiv2017

Electron-Phonon Coupling Constant of Metallic Overlayers from Specular He-Atom Scattering

He atom scattering has been shown to be a sensitive probe of electron-phonon interaction properties at surfaces. Here it is shown that measurements of the thermal attenuation of the specular He atom diffraction peak (the Debye-Waller effect) can determine the electron-phonon coupling constant $λ$ for ultrathin films of metal overlayers on various close-packed metal substrates. Values of $λ$ obtained for single and multiple monolayers of alkali metals, and for Pb layers on Cu(111), extrapolated to large thicknesses, agree favorably with known bulk values. This demonstrates that He atom scattering can measure the electron-phonon coupling strength as a function of film thickness on a layer-by-layer basis.

cond-mat.supr-con↗

arXiv2006

Theory of Small Para-Hydrogen Clusters: Magic Numbers and Superfluid Sizes

The interplay between magic number stabilities and superfluidity of small para-hydrogen clusters with sizes $N = 5$ to 40 and temperatures $0.5 K \leq T \leq 4.5 $K is explored with classical and quantum Path Integral Monte Carlo calculations. Clusters with $N < 26$ and T $\leq 1.5 K$ have large superfluid fractions even at the stable magic numbers 13, 19, and 23. In larger clusters, superfluidity is quenched especially at the magic numbers 23, 26, 29, 32, and 37 while below 1 K, superfluidity is recovered for the pairs $(27,28)$, $(30,31)$, and $(35,36)$. For all clusters superfluidity is localized at the surface and correlates with long exchange cycles involving loosely bound surface molecules.

physics.comp-ph↗

arXiv2004

Matter Wave Diffraction from an Inclined Transmission Grating: Searching for the Elusive He-4 Trimer Efimov State

The size of the helium trimer is determined by diffracting a beam of He-4 clusters from a 100 nm grating inclined by 21 degree. Due to the bar thickness the projected slit width is roughly halved to 27 nm, increasing the sensitivity to the trimer size. The peak intensities measured out to the 8th order are evaluated via a few-body scattering theory. The trimer pair distance is found to be 1.1+0.4/-0.5 nm in agreement with predictions for the ground state. No evidence for a significant amount of Efimov trimers is found. Their concentration is estimated to be less than 6%.

quant-ph↗

arXiv2004

Expansion of Solid Helium into Vacuum: the Geyser Effect

The particle flux through a two micron diameter orifice into vacuum from a source chamber filled with solid He exhibits a striking periodic behavior similar to that of a geyser. This phenomenon is attributed to a periodic collapse of the solid inside the source induced by the accumulation of excess vacancies injected at the orifice. The flux-time curves agree well with a kinetic model and provide direct information on the diffusivity of vacancies in solid He.

cond-mat.mtrl-sci↗

arXiv2000

Combined He-atom scattering and theoretical study of the low energy vibrations of physisorbed monolayers of Xe on Cu(111) and Cu(001)

The surface phonon dispersion curves of commensurate Xe monolayers on Cu(111) and incommensurate Xe monolayers on Cu(001) surfaces have been measured using He atom scattering (HAS) time of flight (TOF) spectroscopy. The TOF spectra are interpreted by combining quantum scattering calculations with the dynamical matrix description of the surface vibrations. Both a vertically polarized Einstein-like mode and another, acoustic-like mode of dominantly longitudinal character, are identified. The latter mode is characterized by the presence and absence of the zone center frequency gap in the commensurate and incommensurate adlayers, respectively. The microscopic description of the TOF spectral intensities is based on the extensive theoretical studies of the interplay of the phonon dynamics, projectile-surface potentials, multi-quantum interference and projectile recoil, and their effect on the HAS spectra. Both single and multi-quantum spectral features observed over a wide range of He atom incident energies and substrate temperatures are successfully explained by the theory.

cond-mat.mtrl-sci↗

arXiv1999

Study of energy transfer in helium atom scattering from surfaces

Recently developed quantum mechanical theory of inelastic He atom scattering (HAS) from solid surfaces is employed to analyze the energy transfer between projectile particles (thermal energy He-atoms) and vibrational degrees of freedom (phonons) characteristic of a variety of experimentally studied surfaces. We have first calculated the angular resolved energy transfer which can be directly compared with the values deducible from the HAS time-of-flight spectra and a good agreement with experimental data has been found. This enabled us to calculate the total or angular integrated energy transfer, which is of paramount importance in the studies of gas-surface scattering, but is neither accessible in HAS (which yields only the angular resolved quantities), nor in the wind tunnel measurements for surfaces whose atomic composition and cleanliness must be maintained during the experiment. Here we present the results for prototype collision systems of this kind, viz. He => Cu(001), He => Xe/Cu(111) and He => Xe(111) which are representative of the very different types of surface vibrational dynamics and thereby provide an insight into some common properties of energy transfer in gas-surface scattering.

cond-mat.stat-mech↗

arXiv1999

Recovery temperature for nonclassical energy transfer in atom-surface scattering

Nonperturbative expressions are derived for the angular resolved energy transfer spectra in the quantum regime of multiphonon scattering of inert gas atoms from surfaces. Application to He atom scattering from a prototype heatbath Xe/Cu(111) shows good agreement with experiments. This enables a full quantum calculation of the total energy transfer $μ$ and therefrom the much debated recovery or equilibrium temperature $T_{r}$ characteristic of zero energy transfer in gas-surface collisions in the free molecular flow regime. Classical universal character of $μ$ and $T_{r}$ is refuted.

cond-mat.stat-mech↗

arXiv1999

Determination of Atom-Surface van der Waals Potentials from Transmission-Grating Diffraction Intensities

Molecular beams of rare gas atoms and D_2 have been diffracted from 100 nm period SiN_x transmission gratings. The relative intensities of the diffraction peaks out to the 8th order depend on the diffracting particle and are interpreted in terms of effective slit widths. These differences have been analyzed by a new theory which accounts for the long-range van der Waals -C_3/l^3 interaction of the particles with the walls of the grating bars. The values of the C_3 constant for two different gratings are in good agreement and the results exhibit the expected linear dependence on the dipole polarizability.

quant-ph↗