I. V. Tokatly

Dielectric screening in two-dimensional insulators: Implications for excitonic and impurity states in graphane

For atomic thin layer insulating materials we provide an exact analytic form of the two-dimensional (2D) screened potential. In contrast to three-dimensional systems where the macroscopic screening can be described by a static dielectric constant, in 2D systems the macroscopic screening is nonlocal (

Strong Charge-Transfer Excitonic Effects and the Bose-Einstein Exciton Condensate in Graphane

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Nonlinear phenomena in time-dependent density-functional theory: What Rabi oscillations can teach us

dependent) showing a logarithmic divergence for small distances and reaching the unscreened Coulomb potential for large distances. The crossover of these two regimes is dictated by 2D layer polarizability that can be easily computed by standard first-principles techniques. The present results have strong implications for describing gap-impurity levels and also exciton binding energies. The simple model derived here captures the main physical effects and reproduces well, for the case of graphane, the full many-body

Sodium: A Charge-Transfer Insulator at High Pressures

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Quantum-electrodynamical density-functional theory: Bridging quantum optics and electronic-structure theory

plus Bethe-Salpeter calculations. As an additional outcome we show that the impurity hole-doping in graphane leads to strongly localized states, which hampers applications in electronic devices. In spite of the inefficient and nonlocal two-dimensional macroscopic screening we demonstrate that a simple

Time-dependent density functional theory on a lattice

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Time-dependent exchange-correlation functional for a Hubbard dimer: Quantifying nonadiabatic effects

approach is capable to describe the electronic and transport properties of confined 2D systems.

Continuum mechanics for quantum many-body systems: Linear response regime

Using first principles many-body theory methods (GW+Bethe-Salpeter equation) we demonstrate that the optical properties of graphane are dominated by localized charge-transfer excitations governed by enhanced electron correlations in a two-dimensional dielectric medium. Strong electron-hole interaction leads to the appearance of small radius bound excitons with spatially separated electron and hole, which are localized out of plane and in plane, respectively. The presence of such bound excitons opens the path towards an excitonic Bose-Einstein condensate in graphane that can be observed experimentally.

Physical meaning of the natural orbitals: Analysis of exactly solvable models

Through the exact solution of a two-electron singlet system interacting with a monochromatic laser we prove that all adiabatic density functionals within time-dependent density-functional theory are not able to discern between resonant and nonresonant (detuned) Rabi oscillations. This is rationalized in terms of a fictitious dynamical exchange-correlation (xc) detuning of the resonance while the laser is acting. The nonlinear dynamics of the Kohn-Sham system shows the characteristic features of detuned Rabi oscillations even if the exact resonant frequency is used. We identify the source of this error in a contribution from the xc functional to the set of nonlinear equations that describes the electron dynamics in an effective two-level system. The constraint of preventing the detuning introduces a new strong condition to be satisfied by approximate xc functionals.

Dyakonov-Perel spin relaxation for degenerate electrons in the electron-hole liquid

By first-principles methods we analyze the optical response of transparent dense sodium as a function of applied pressure. We discover an unusual kind of charge-transfer exciton that proceeds from the interstitial distribution of valence electrons. The absorption spectrum is strongly anisotropic, which, just at pressures above the metal-insulator transition, manifests as sodium being optically transparent in one direction but reflective in the other. This result provides key information about the crystal structure of transparent sodium, a new unconventional inorganic electride.