Nonadiabatic effects in adsorbate-surface interactions from first principles

224 p.El objetivo principal de esta tesis es el estudio de la dinámica de adsorbatos sobre superficies metálicas. Estos procesos son importes para varias aplicaciones industriales y tecnológicas, como la síntesis de materiales, la catálisis heterogénea y la nanoelectrónica. El proceso que subyace en la interacción adsorbato-superficie es el acoplamiento entre los electrones de la superficie con el adsorbato en movimiento, lo que se conoce como acoplamiento no adiabático. En esta tesis nos concentramos en dos aspectos particulares de dicho acoplamiento: la dinámica de relajación de adsorbatos en superficies debida a excitaciones electrónicas y la inducción de reacciones por corrientes de electrones inelásticos. Como ejemplos del primer tipo proceso estudiamos (i) la termalización de H, N, y N2 sobre Pd(100), Ag(111) y Fe(110), respectivamente, empleando dinámica molecular ab initio con fricción electrónica (AIMDEF), y (ii) la relajación vibracional de una capa ordenada de CO sobre Cu(100) mediante teoría de perturbaciones de muchos cuerpos. En lo referente al segundo tipo de proceso, proponemos un modelo teórico para describir la tautomerización de porphyceno sobre Cu(111) inducida por una punta de microscopio de efecto túnel

Optical absorption and conductivity in quasi-two-dimensional crystals from first principles: Application to graphene

This paper gives a theoretical formulation of the electromagnetic response of the quasi-two-dimensional (Q2D) crystals suitable for investigation of optical activity and polariton modes. The response to external electromagnetic field is described by current-current response tensor $\Pi_{\mu\nu}$ calculated by solving the Dyson equation in the random phase approximation (RPA), where current-current interaction is mediated by the photon propagator $D_{\mu\nu}$. The irreducible current-current response tensor $\Pi^0_{\mu\nu}$ is calculated from the {\em ab initio} Kohn-Sham (KS) orbitals. The accuracy of $\Pi^0_{\mu\nu}$ is tested in the long wavelength limit where it gives correct Drude dielectric function and conductivity. The theory is applied to the calculation of optical absorption and conductivity in pristine and doped single layer graphene and successfully compared with previous calculations and measurements

Changing character of electronic transitions in graphene: From single particle excitations to plasmons

In this paper we clarify the nature of $\pi$ and $\pi+\sigma$ electron excitations in pristine graphene. We clearly demonstrate the continuous transition from single particle to collective character of such excitations and how screening modifies their dispersion relations. We prove that $\pi$ and $\pi+\sigma$ plasmons do exist in graphene, though occurring only for a particular range of wavevectors and with finite damping rate. The particular attention is paid to compare the theoretical results with available EELS measurements in optical ($\mathrm{Q\approx 0}$) and other ($\mathrm{Q\neq 0}$) limits. The conclusions, based on microscopic numerical results, are confirmed in an approximate analytical approach

Plasmon excitations across the charge-density-wave transition in single layer TiSe2_2

$1T$-TiSe$_2$ is believed to posses a soft electronic mode, i.e., plasmon or exciton, that might be responsible for the exciton condensation and charge-density-wave (CDW) transition. Here, we explore collective electronic excitations in single-layer $1T$-TiSe$_2$ by using the ab-initio electromagnetic linear response and unveil intricate scattering pathways of two-dimensional (2D) plasmon mode. We found the dominant role of plasmon-phonon scattering, which in combination with the CDW gap excitations leads to the anomalous temperature dependence of the plasmon linewidth across the CDW transition. Below the transition temperature $T_{\rm CDW}$ a strong hybridization between 2D plasmon and CDW excitations is obtained. These optical features are highly tunable due to temperature-dependent CDW gap modifications and are argued to be universal for the CDW-bearing 2D materials.Comment: 6 pages, 3 figure

Dynamical Phonons Following Electron Relaxation Stages in Photo-excited Graphene

Ultrafast electron-phonon relaxation dynamics in graphene hides many distinct phenomena, such as hot phonon generation, dynamical Kohn anomalies, and phonon decoupling, yet still remains largely unexplored. Here, we unravel intricate mechanisms governing the vibrational relaxation and phonon dressing in graphene at a highly non-equilibrium state by means of first-principles techniques. We calculate dynamical phonon spectral functions and momentum-resolved linewidths for various stages of electron relaxation and find photo-induced phonon hardening, overall increase of relaxation rate and nonadiabaticity as well as phonon gain. Namely, the initial stage of photo-excitation is found to be governed by strong phonon anomalies of finite-momentum optical modes along with incoherent phonon production. Population inversion state, on the other hand, allows production of coherent and strongly-coupled phonon modes. Our research provides vital insights into the electron-phonon coupling phenomena in graphene, and serves as a foundation for exploring non-equilibrium phonon dressing in materials where ordered states and phase transitions can be induced by photo-excitation.Comment: 12 pages, 5 figure