Grazing incidence fast atom diffraction from metal surfaces

C.A. RÍOS RUBIANO; G.A. BOCAN; J.I. JUARISTI; M.S. GRAVIELLE; N. BUNDALESKI; H. KHEMLICHE; P. RONCIN

San Sebastián

Workshop; Brandt-Ritchie Workshop; 2013

BR-Workshop

Since the unexpected observation of grazing incidence diffraction of fast atoms (GIFAD) on crystal surfaces [1, 2], extensive experimental and theoretical research has been devoted to the subject [3]. The first experimental evidences of this phenomenon were reported at insulator materials [1, 2], where the presence of a wide band-gap helps to suppress inelastic processes, thus preventing quantum decoherence. Soon afterwards the effect was observed at semi-conductor [4] and metallic surfaces [5,6] even though, for the latter case, energy loss values were found to be significant [7]. The aim of this work is to investigate the diffraction patterns produced by fast He atoms grazingly impinging on a Ag(110) surface. This collision system corresponds to the first and simplest metallic case for which GIFAD effects were experimentally observed [5], and it provides a useful prototype to examine both the performance of the theoretical model and the contribution of inelastic process. To describe the diffraction process we employ a distorted wave theory ? the surface eikonal (SE) approximation ? that makes use of the eikonal wave function to represent the elastic collision with the surface, while the projectile motion is classically described using different initial conditions [8]. The SE approach has been applied to evaluate GIFAD distributions from insulator surfaces, providing results in good agreement with the experimental data [9]. Concerning the projectile-surface interaction, we use a precise description of the potential energy surface (PES) obtained from an accurate density functional theory calculation, where the PBE functional is used to approximate the exchange-correlation energy. This PES takes into account the projectile's three degrees of freedom and it is built from a dense grid of ab-initio energies by means of a sophisticated interpolation technique. Projectile momentum distributions obtained with the SE approach are compared with the experimental spectra for helium incidence along three directions of the Ag(110) surface - [1-10], [001], and [1-12] ? considering different energies for the normal motion. The contribution coming from inelastic processes, originated by electron-hole pair excitations, is evaluated by introducing a friction force in the calculation of the classical projectile trajectories. This dissipative force is here expressed in terms of the transport cross section at the Fermi level by considering the scattering with an electron gas, whose density is evaluated from density functional theory calculations [10]. In this way, the model includes nonlinear effects both in the medium response to the projectile interaction and in the inelastic collision process. Results for incidence along the [1-10] channel are displayed in Fig. 2. [1] A. Schüller, S. Wethekam, and H. Winter, Phys. Rev. Lett. 98, 016103 (2007). [2] P. Rousseau, H. Khemliche, A.G. Borisov, and P. Roncin, Phys. Rev. Lett. 98, 016104 (2007). [3] H. Winter and A. Schüller, Prog. Surf. Sci. 86, 169 (2011) and references therein. [4] H. Khemliche et al., Appl. Phys. Lett. 95, 151901 (2009). [5] N. Bundaleski, H. Khemliche, P. Soulisse, and P. Roncin, Phys. Rev. Lett. 101, 177601 (2008). [6] M. Busch, A. Schüller, S. Wethekam, and H. Winter, Surf. Sci. 603, L23 (2009). [7] H. Khemliche et al., Nucl. Instrum. Meth. Phys. Res. B 267, 620 (2009). [8] M.S. Gravielle et al., Nucl. Instrum. Meth. Phys. Res. B 267, 610 (2009); ibidem 269, 1208 (2011). [9] A. Schüller et al., Phys Rev. A 80, 062903 (2009). [10] J. I. Juaristi et al., Phys. Rev. Lett. 100, 116102 (2008).