Real-space grids and the Octopus code as tools for the development of new simulation approaches for electronic systems

Andrade, Xavier; Strubbe, David A.; Giovannini, Umberto De; Larsen, Ask Hjorth; Oliveira, Micael J. T.; Alberdi-Rodríguez, Joseba; Varas, Alejandro; Θεοφίλου, Ίρις; Helbig, Nicole; Verstraete, Matthieu J.; Stella, Lorenzo; Nogueira, Fernando; Aspuru‐Guzik, Alán; Castro, Alberto; Marques, Miguel A. L.; Rubio, Ángel

doi:10.1039/c5cp00351b

Cited by 426 publications

(392 citation statements)

References 189 publications

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“…Atomic units are used throughout. The real-time propagation is done using real-space code octopus [48][49][50] with a time step of 0.005 a.u.. The interacting system is prepared in an excited eigenstate of the unperturbed Hamiltonian and then evolved in the presence of a weak monochromatic laser resonant with the photoexcited-to-CT transition frequency ω CT .…”

Section: A Charge Transfer From a Photoexcited Statementioning

confidence: 99%

Studies of spuriously shifting resonances in time-dependent density functional theory

Luo

Fuks

Maitra

2016

The Journal of Chemical Physics

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Adiabatic approximations in time-dependent density functional theory (TDDFT) will in general yield unphysical time-dependent shifts in the resonance positions of a system driven far from its ground-state. This spurious time-dependence is explained in [J. I. Fuks, K. Luo, E. D. Sandoval and N. T. Maitra, Phys. Rev. Lett. 114, 183002 (2015)] in terms of the violation of an exact condition by the non-equilibrium exchange-correlation kernel of TDDFT. Here we give details on the derivation and discuss reformulations of the exact condition that apply in special cases. In its most general form, the condition states that when a system is left in an arbitrary state, the TDDFT resonance position for a given transition in the absence of time-dependent external fields and ionic motion, is independent of the state. Special cases include the invariance of TDDFT resonances computed with respect to any reference interacting stationary state of a fixed potential, and with respect to any choice of appropriate stationary Kohn-Sham reference state. We then present several case studies, including one that utilizes the adiabatically-exact approximation, that illustrate the conditions and the impact of their violation on the accuracy of the ensuing dynamics. In particular, charge-transfer across a long-range molecule is hampered, and we show how adjusting the frequency of a driving field to match the time-dependent shift in the charge-transfer resonance frequency, results in a larger charge transfer over time.

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Section: A Charge Transfer From a Photoexcited Statementioning

confidence: 99%

Studies of spuriously shifting resonances in time-dependent density functional theory

Luo

Fuks

Maitra

2016

The Journal of Chemical Physics

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“…For such few-cycle driver pulses, the HHG spectra from solids have been shown to be quite insensitive to the carrier-envelope phase [9,11,44], which is therefore taken to be 0 here. The evolution of the wave functions and the evaluation of the time-dependent current is computed by propagating the Kohn-Sham equations within TDDFT, as provided by the Octopus package [45], in the local-density approximation (LDA).…”

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confidence: 99%

Impact of the Electronic Band Structure in High-Harmonic Generation Spectra of Solids

et al. 2017

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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. DOI: 10.1103/PhysRevLett.118.087403 Atoms and molecules interacting with strong laser pulses emit high-order harmonics of the fundamental driving laser field. The high-harmonic generation (HHG) in gases is routinely used nowadays to produce isolated attosecond pulses [1][2][3][4] and coherent radiation ranging from the visible to soft x rays [5]. Because of a higher electronic density, solids are one promising route towards compact, brighter HHG sources. The recent observation of nonperturbative HHG in solids without damage [6][7][8][9][10], extending even beyond the atomic limit [10], has opened the door to the observation and control of attosecond electron dynamics in solids [8,9,11], all-optical band-structure reconstruction [12], and solid-state sources of isolated extreme-ultraviolet pulses [9,11]. However, in contrast to HHG from gases, the microscopic mechanism underlying HHG from solids is still controversially debated in the attoscience community, in some cases casting doubts on the validity of the proposed microscopic model and resulting in confusion about the correct interpretation of experimental data. Various competing simplified models have been proposed but they often are based on strong approximations and a priori assumptions, often stating that there is a strong similarity with the processes underlying atomic-gas HHG emission. However, it is clear that many-body effects due to the crystalline structure of solids and the fermionic nature of interaction electrons play a decisive role that fundamentally distinguishes the solid from the gas case. It is the scope of the present work to unravel within an ab initio approach what the impact is of the underlying electronic band structure of the solids in the observed HHG emission.The process of HHG from gases is by now well understood in terms of the three-step model [13][14][15] in which electrons are first promoted from the ground state of the atom (or molecule) to the continuum, then accelerated by the electric field, and finally recombine with the parent ion. With this simple, intuitive model most of the observed effects are well described, in particular, the dependence of the harmonic cutoff energ...

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“…While standard DFT is a ground state theory, RT-TDDFT allows electron density to dynamically respond to laser irradiation 34,35 . Inter-and intra-site electron interactions, a manifold of energy levels, and temporally varying electronic orbitals with a complex spatial character are all captured within this computational paradigm which has been successfully applied to study excited-state electron dynamics [36][37][38] .…”

Section: A Tight-binding Modelmentioning

confidence: 99%

Engineering and manipulating exciton wave packets

et al. 2017

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When a semiconductor absorbs light, the resulting electron-hole superposition amounts to a uncontrolled quantum ripple that eventually degenerates into diffusion. If the conformation of these excitonic superpositions could be engineered, though, they would constitute a new means of transporting information and energy. We show that properly designed laser pulses can be used to create such excitonic wave packets. They can be formed with a prescribed speed, direction and spectral make-up that allows them to be selectively passed, rejected or even dissociated using superlattices. Their coherence also provides a handle for manipulation using active, external controls. Energy and information can be conveniently processed and subsequently removed at a distant site by reversing the original procedure to produce a stimulated emission. The ability to create, manage and remove structured excitons comprises the foundation for opto-excitonic circuits with application to a wide range of quantum information, energy and light-flow technologies. The paradigm is demonstrated using both Tight-Binding and Time-Domain Density Functional Theory simulations.

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Real-space grids and the Octopus code as tools for the development of new simulation approaches for electronic systems

Cited by 426 publications

References 189 publications

Studies of spuriously shifting resonances in time-dependent density functional theory

Studies of spuriously shifting resonances in time-dependent density functional theory

Impact of the Electronic Band Structure in High-Harmonic Generation Spectra of Solids

Engineering and manipulating exciton wave packets

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