1,720,996 research outputs found
The local adsorption structure of benzene on Si(001)-(2 × 1): a photoelectron diffraction investigation
Full text linkScanned-energy mode C 1s photoelectron diffraction has been used to investigate the local adsorption geometry of benzene on Si(001) at saturation coverage and room temperature. The results show that two different local bonding geometries coexist, namely the 'standard butterfly' (SB) and 'tilted bridge' (TB) forms, with a composition of 58 ± 29% of the SB species. Detailed structural parameter values are presented for both species including Si–C bond lengths. On the basis of published measurements of the rate of conversion of the SB to the TB form on this surface, we estimate that the timescale of our experiment is sufficient for achieving equilibrium, and in this case our results indicate that the difference in the Gibbs free energy of adsorption, ΔG(TB)−ΔG(SB), is in the range −0.023 to +0.049 eV. We suggest, however, that the relative concentration of the two species may also be influenced by a combination of steric effects influencing the kinetics, and a sensitivity of the adsorption energies of the adsorbed SB and TB forms to the nature of the surrounding benzene molecules
Prediction for Two Spatially Modulated Superfluids: 4He on Fluorographene and on Hexagonal BN
No full textWe have derived the adsorption potential of 4He atoms on fluorographene (GF), on graphane and on hexagonal boron nitride (hBN) by a recently developed ab initio method that incorporates the van der Waals interaction. The stability of the commensurate R30 degrees phase of 4He on GF and on hBN is studied by state-of-the-art quantum simulations at T=0 K. With our adsorption potentials, we find that in both cases this commensurate state of 4He is unstable toward a fluid state in which the 4He atoms are delocalized, and not localized like in the case of 4He on graphite or on graphene. In the case of GF, the present result is in qualitative agreement with the superfluid phase that was obtained using an empirical adsorption potential (Nava et al. in Phys Rev B 86:174509, 2012). This fluid state of 4He on GF and on hBN is characterized by a very large density modulation. For instance, the local density changes by a factor of order 2 along the path connecting two adsorption sites. Recent experiments (Nyeki et al. in Nat Phys 13:455, 2017) have discovered a superfluid phase in the second layer 4He. This is a spatially modulated superfluid that turns out to have anomalous thermal properties. This gives a strong motivation for an experimental study of monolayer 4He on GF and on hBN that we predict to be a superfluid with a much stronger spatial modulation
Coexistence of vortex arrays and surface capillary waves in spinning prolate superfluid He 4 nanodroplets
No full textWithin density functional theory, we have studied the interplay between vortex arrays and capillary waves in spinning prolate He4 droplets made of several thousand helium atoms. Surface capillary waves are ubiquitous in prolate superfluid He4 droplets, and depending on the size and angular momentum of the droplet, they may coexist with vortex arrays. We have found that the equilibrium configuration of small prolate droplets is vortex free, evolving towards vortex hosting as the droplet size increases. This result is in agreement with a recent experiment [O'Connell, Phys. Rev. Lett. 124, 215301 (2020)PRLTAO0031-900710.1103/PhysRevLett.124.215301] that disclosed that vortex arrays and capillary waves coexist in the equilibrium configuration of very large drops. In contrast to viscous droplets executing rigid-body rotation, the stability phase diagram of spinning He4 droplets cannot be universally described in terms of dimensionless angular momentum and angular velocity variables: Instead, the rotational properties of superfluid helium droplets display a clear dependence on the droplet size and the number of vortices they host
Merging of superfluid helium nanodroplets with vortices
No full textWithin density functional theory, we have investigated the coalescence dynamics of two superfluid helium nanodroplets hosting vortex lines in different relative orientations, which are drawn towards each other by the Van der Waals mutual attraction. We have found a rich phenomenology depending on how the vortex lines are oriented. In particular, when a vortex and antivortex lines are present in the merging droplets, a dark soliton develops at the droplet contact region, which eventually decays into vortex rings. Reconnection events are observed between the vortex lines or rings, leading to the creation of more vortices. Our simulations show the interplay between vortex creation and reconnections, as well as the effect of the droplet surface which pins the vortex ends and, by reflecting short-wavelength excitations produced by the interactions between vortices, strongly affects the droplet final state. Additional vorticity is nucleated in the proximity of surface indentations produced in the course of the dynamics, which in turn interact with other vortices present in the droplets. These effects, obviously absent in the case of bulk liquid helium, show that the droplet surface may act as a multiplier of vortex reconnections. The analysis of the energy spectrum shows that vortex-antivortex ring annihilation, as well as vortex-antivortex reconnections, yields roton bursts of different intensity
Vortex properties in the extended supersolid phase of dipolar Bose-Einstein condensates
Full text linkWe study the properties of singly quantized linear vortices in the supersolid phase of a dipolar Bose-Einstein condensate at zero-temperature modeling Dy164 atoms. The system is extended in the x-y plane and confined by a harmonic trap in the polarization direction z. Our study is based on a generalized Gross-Pitaevskii equation. We characterize the ground state of the system in terms of spatial order and superfluid fraction and compare the properties of a single vortex and of a vortex dipole in the superfluid phase (SFP) and in the supersolid phase (SSP). At variance with a vortex in the SFP, which is free to move in the superfluid, a vortex in the SSP is localized at the interstitial sites and does not move freely. We have computed the energy barrier for motion from an equilibrium site to another. The fact that the vortex is submitted to a periodic potential has a dramatic effect on the dynamics of a vortex dipole made of two counter-rotating parallel vortices; instead of rigidly translating as in the SFP, the vortex and antivortex approach each other by a series of jumps from one site to another until they annihilate in a very short time and their energy is transferred to bulk excitations
SURFACE PHONONS AND DIPOLE ACTIVITY OF SI(111)2X1 FROM ABINITIO CALCULATIONS
No full textWe present a first-principles linear-response study of the phonon spectrum and dipole activity of the Si(111)2 x 1 surface. Our results are in good agreement with the experimental He-scattering results for the acoustic modes, as well as the electron-energy-loss data for the optical modes. We clarify a current controversy on the assignment of the electron-energy-loss peak at approximately 56 meV. We find that the dominant contribution to the loss peak is anisotropic and arises from a longitudinal-optical vibration of the surface chains. However, a small but non-negligible isotropic contribution comes from subsurface modes polarized perpendicular to the surface
Novel aspects of wedge filling by liquid helium
Full text linkWe discuss several features of the condensation of helium in linear wedges with Cs and Na walls, such as the existence of pre-filling jumps, and the relationship to prior calculations of the growth of 1D helium stripes in the outer grooves of carbon nanotube bundles. We also present preliminary results of condensation between parallel Cs plates.Fil: Mayol, Ricardo. Universidad de Barcelona; EspañaFil: Ancilotto, F.. Università di Padova; ItaliaFil: Barranco, M.. Universidad de Barcelona; EspañaFil: Hernandez, Ester Susana. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Pi, M.. Universidad de Barcelona; Españ
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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