91 research outputs found
A first principles TDDFT framework for spin and time-resolved ARPES in periodic systems
We present a novel theoretical approach to simulate spin, time and
angular-resolved photoelectron spectroscopy (ARPES) from first principles that
is applicable to surfaces, thin films, few layer systems, and low-dimensional
nanostructures. The method is based on a general formulation in the framework
of time-dependent density functional theory (TDDFT) to describe the real
time-evolution of electrons escaping from a surface under the effect of any
external (arbitrary) laser field. By extending the so called t-SURFF method to
periodic systems one can calculate the final photoelectron spectrum by
collecting the flux of the ionization current trough an analysing surface. The
resulting approach, that we named t-SURFFP, allows to describe a wide range of
irradiation conditions without any assumption on the dynamics of the ionization
process allowing for pump-probe simulations on an equal footing. To illustrate
the wide scope of applicability of the method we present applications to
graphene, mono- and bi-layer WSe, and hexagonal BN under different laser
configurations
Phonon driven Floquet matter
A resonantly excited coherent phonon leads to a periodic oscillation of the
atomic lattice in a crystal structure bringing the material into a
non-equilibrium electronic configuration. Periodically oscillating quantum
systems can be understood in terms of Floquet theory and we show these concepts
can be applied to coherent lattice vibrations reflecting the underlying
coupling mechanism between electrons and bosonic modes. This coupling leads to
dressed quasi-particles imprinting specific signatures in the spectrum of the
electronic structure. Taking graphene as a paradigmatic material we show how
the phonon-dressed states display an intricate sideband structure revealing
electron-phonon coupling and topological ordering. This work establishes that
the recently demonstrated concept of light-induced non-equilibrium Floquet
phases can also be applied when using coherent phonon modes for the dynamical
control of material properties. The present results are generic for bosonic
time-dependent perturbations and similar phenomena can be observed for plasmon,
magnon or exciton driven materials
Cavity control of Excitons in two dimensional Materials
We propose a robust and efficient way of controlling the optical spectra of
two-dimensional materials and van der Waals heterostructures by quantum cavity
embedding. The cavity light-matter coupling leads to the formation of
exciton-polaritons, a superposition of photons and excitons. Our first
principles study demonstrates a reordering and mixing of bright and dark
excitons spectral features and in the case of a type II van-der-Waals
heterostructure an inversion of intra and interlayer excitonic resonances. We
further show that the cavity light-matter coupling strongly depends on the
dielectric environment and can be controlled by encapsulating the active 2D
crystal in another dielectric material. Our theoretical calculations are based
on a newly developed non-perturbative many-body framework to solve the coupled
electron-photon Schr\"odinger equation in a quantum-electrodynamical extension
of the Bethe-Salpeter approach. This approach enables the ab-initio simulations
of exciton-polariton states and their dispersion from weak to strong cavity
light-matter coupling regimes. Our method is then extended to treat van der
Waals heterostructures and encapsulated 2D materials using a simplified
Mott-Wannier description of the excitons that can be applied to very large
systems beyond reach for fully ab-initio approaches.Comment: 32 pages. 10 figures, 2 tabl
New perspectives in time-resolved laser-induced electron diffraction
Imaging the microscopic world in real space and real time is a grand challenge of science. In the landscape of time-resolved imaging techniques, laser-induced electron diffraction (LIED) has recently shown to be a promising candidate to push the frontiers of ultrafast molecular imaging. In this work, we review the main achievements of LIED research in terms of experimental results and advanced modelling. We also envision interesting perspectives toward the future advancement of time-resolved LIED imaging
Stark Ionization of Atoms and Molecules within Density Functional Resonance Theory
We show that the energetics and lifetimes of resonances of finite systems
under an external electric field can be captured by Kohn--Sham density
functional theory (DFT) within the formalism of uniform complex scaling.
Properties of resonances are calculated self-consistently in terms of complex
densities, potentials and wavefunctions using adapted versions of the known
algorithms from DFT. We illustrate this new formalism by calculating ionization
rates using the complex-scaled local density approximation and exact exchange.
We consider a variety of atoms (H, He, Li and Be) as well as the hydrogen
molecule. Extensions are briefly discussed.Comment: 5 pages, 5 figures. This document is the unedited Author's version of
a Submitted Work that was subsequently accepted for publication in
J.Phys.Chem.Lett., copyright (c) American Chemical Society after peer review.
To access the final edited and published work see
http://pubs.acs.org/doi/abs/10.1021/jz401110
Modelling the effect of nuclear motion on the attosecond time-resolved photoelectron spectra of ethylene
Using time dependent density functional theory (TDDFT) we examine the energy,
angular and time-resolved photoelectron spectra (TRPES) of ethylene in a
pump-probe setup. To simulate TRPES we expose ethylene to an ultraviolet (UV)
femtosecond pump pulse, followed by a time delayed extreme ultraviolet (XUV)
probe pulse. Studying the photoemission spectra as a function of this delay
provides us direct access to the dynamic evolution of the molecule's electronic
levels. Further, by including the nuclei's motion, we provide direct chemical
insight into the chemical reactivity of ethylene. These results show how
angular and energy resolved TRPES could be used to directly probe electron and
nucleus dynamics in molecules.Comment: 9 pages, 6 figure
Creating stable Floquet-Weyl semimetals by laser-driving of 3D Dirac materials
Tuning and stabilising topological states, such as Weyl semimetals, Dirac
semimetals, or topological insulators, is emerging as one of the major topics
in materials science. Periodic driving of many-body systems offers a platform
to design Floquet states of matter with tunable electronic properties on
ultrafast time scales. Here we show by first principles calculations how
femtosecond laser pulses with circularly polarised light can be used to switch
between Weyl semimetal, Dirac semimetal, and topological insulator states in a
prototypical 3D Dirac material, NaBi. Our findings are general and apply to
any 3D Dirac semimetal. We discuss the concept of time-dependent bands and
steering of Floquet-Weyl points (Floquet-WPs), and demonstrate how light can
enhance topological protection against lattice perturbations. Our work has
potential practical implications for the ultrafast switching of materials
properties, like optical band gaps or anomalous magnetoresistance. Moreover, we
introduce Floquet time-dependent density functional theory (Floquet-TDDFT) as a
general and robust first principles method for predictive Floquet engineering
of topological states of matter.Comment: 21 pages, 4 figure
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