46 research outputs found
High-harmonic generation from few layer hexagonal boron nitride: evolution from the monolayer to the bulk response
Two-dimensional materials offer a versatile platform to study high-harmonic
generation (HHG), encompassing as limiting cases bulk-like and atomic-like
harmonic generation [Tancogne-Dejean and Rubio, Science Advance \textbf{4},
eaao5207 (2018)]. Understanding the high-harmonic response of few-layer
semiconducting systems is important, and might open up possible technological
applications. Using extensive first-principle calculations within a
time-dependent density functional theory framework, we show how the in-plane
and out-of-plane nonlinear non-perturbative response of two-dimensional
materials evolve from the monolayer to the bulk. We illustrate this phenomenon
for the case of multilayer hexagonal BN layered systems. Whereas the in-plane
HHG is found not to be strongly altered by the stacking of the layers, we found
that the out-of-plane response is strongly affected by the number of layers
considered. This is explained by the interplay between the induced electric
field by electron-electron interactions and the interlayer delocalization of
the wave-functions contributing most to the HHG signal. The gliding of a
bilayer is also found to affect the high-harmonic emission. Our results will
have important ramifications for the experimental study of monolayer and
few-layer two-dimensional materials beyond the case of hexagonal BN studied
here as the result we found arew generic and applicable to all 2D
semiconducting multilayer systems
Ultrafast modification of Hubbard in a strongly correlated material: ab initio high-harmonic generation in NiO
Engineering effective electronic parameters is a major focus in condensed
matter physics. Their dynamical modulation opens the possibility of creating
and controlling physical properties in systems driven out of equilibrium. In
this work, we demonstrate that the Hubbard , the on-site Coulomb repulsion
in strongly correlated materials, can be modified on femtosecond time scales by
a strong nonresonant laser excitation in the prototypical charge transfer
insulator NiO. Using our recently developed time-dependent density functional
theory plus self-consistent (TDDFT+U) method, we demonstrate the importance
of a dynamically modulated in the description of the high-harmonic
generation of NiO. Our study opens the door to novel ways of modifying
effective interactions in strongly correlated materials via laser driving,
which may lead to new control paradigms for field-induced phase transitions and
perhaps laser-induced Mott insulation in charge-transfer materials
Self-consistent DFT+U method for real-space time-dependent density functional theory calculations
We implemented various DFT+U schemes, including the ACBN0 self-consistent
density-functional version of the DFT+U method [Phys. Rev. X 5, 011006 (2015)]
within the massively parallel real-space time-dependent density functional
theory (TDDFT) code Octopus. We further extended the method to the case of the
calculation of response functions with real-time TDDFT+U and to the description
of non-collinear spin systems. The implementation is tested by investigating
the ground-state and optical properties of various transition metal oxides,
bulk topological insulators, and molecules. Our results are found to be in good
agreement with previously published results for both the electronic band
structure and structural properties. The self consistent calculated values of U
and J are also in good agreement with the values commonly used in the
literature. We found that the time-dependent extension of the self-consistent
DFT+U method yields improved optical properties when compared to the empirical
TDDFT+U scheme. This work thus opens a different theoretical framework to
address the non equilibrium properties of correlated systems
Self-interaction correction schemes for non-collinear spin-density-functional theory
We extend some of the well established self-interaction correction (SIC)
schemes of density-functional theory to the case of systems with noncollinear
magnetism. Our proposed SIC schemes are tested on a set of molecules and
metallic clusters in combination with the widely used local spin-density
approximation. As expected from the collinear SIC, we show that the
averaged-density SIC works well for improving ionization energies but fails to
improve more subtle quantities like the dipole moments of polar molecules. We
investigate the exchange-correlation magnetic field produced by our extension
of the Perdew-Zunger SIC, showing that it is not aligned with the local total
magnetization, thus producing an exchange-correlation torque
Effect of spin-orbit coupling on the high harmonics from the topological Dirac semimetal Na3Bi
In this work, we performed extensive first-principles simulations of
high-harmonic generation in the topological Diract semimetal Na3Bi using a
time-dependent density functional theory framework, focusing on the effect of
spin-orbit coupling (SOC) on the harmonic response. We also derived a general
analytical model describing the microscopic mechanism of strong-field dynamics
in presence of spin-orbit coupling, starting from a locally U(1)xSU(2)
gauge-invariant Hamiltonian. Our results reveal that SOC: (i) affects the
strong-field ionization by modifying the bandstructure of Na3Bi, (ii) modifies
the electron velocity, making each spin channel to react differently to the
pump laser field, (iii) changes the emission timing of the emitted harmonics.
Moreover, we show that the SOC affects the harmonic emission by directly
coupling the charge current to the spin currents, paving the way to the
high-harmonic spectroscopy of spin currents in solids
High-harmonics and isolated attosecond pulses from MgO
On the basis of real-time ab initio calculations, we study the
non-perturbative interaction of two-color laser pulses with MgO crystal in the
strong field regime to generate isolated attosecond pulse from high-harmonic
emissions from MgO crystal. In this regard, we examine the impact of incident
pulse characteristics such as its shape, intensity, and ellipticity as well as
the consequence of the crystal anisotropy on the emitted harmonics and their
corresponding isolated attosecond pulses. Our calculations predict the creation
of isolated attosecond pulses with a duration of ~ 300 attoseconds; in
addition, using elliptical driving pulses, the generation of elliptical
isolated attosecond pulses is shown. Our work prepares the path for all
solid-state compact optical devices offering perspectives beyond traditional
isolated attosecond pulse emitted from atoms
All-optical nonequilibrium pathway to stabilizing magnetic Weyl semimetals in pyrochlore iridates
Nonequilibrium many-body dynamics is becoming one of the central topics of
modern condensed matter physics. Floquet topological states were suggested to
emerge in photodressed band structures in the presence of periodic laser
driving. Here we propose a viable nonequilibrium route without requiring
coherent Floquet states to reach the elusive magnetic Weyl semimetallic phase
in pyrochlore iridates by ultrafast modification of the effective
electron-electron interaction with short laser pulses. Combining \textit{ab
initio} calculations for a time-dependent self-consistent reduced Hubbard
controlled by laser intensity and nonequilibrium magnetism simulations for
quantum quenches, we find dynamically modified magnetic order giving rise to
transiently emerging Weyl cones that are probed by time- and angle-resolved
photoemission spectroscopy. Our work offers a unique and realistic pathway for
nonequilibrium materials engineering beyond Floquet physics to create and
sustain Weyl semimetals. This may lead to ultrafast, tens-of-femtoseconds
switching protocols for light-engineered Berry curvature in combination with
ultrafast magnetism.Comment: 27 pages including methods and supplementary information, 4 figures,
4 supplementary figure
Impact of the Electronic Band Structure in High-Harmonic Generation Spectra of Solids
An accurate analytic model describing the microscopic mechanism of high-harmonic generation (HHG) in solids is derived. Extensive first-principles simulations within a time-dependent density-functional framework corroborate the conclusions of the model. Our results reveal that (i) the emitted HHG spectra are highly anisotropic and laser-polarization dependent even for cubic crystals; (ii) the harmonic emission is enhanced by the inhomogeneity of the electron-nuclei potential; the yield is increased for heavier atoms; and (iii) the cutoff photon energy is driver-wavelength independent. Moreover, we show that it is possible to predict the laser polarization for optimal HHG in bulk crystals solely from the knowledge of their electronic band structure. Our results pave the way to better control and optimize HHG in solids by engineering their band structure.European Research Council (Grant ERC-2015-AdG-694097)European Cooperation in the Field of Scientific and Technical Research (Organization) (Action Grant MP1306)German Science Foundation. Hamburg Centre for Ultrafast Imaging-Structure, Dynamics and Control of Matter at the Atromic ScaleGerman Science Foundation (Grant SPP1840 SOLSTICE