arXiv subjects

Angel Rubio

Publications and source records attributed to Angel Rubio.

At least 325 records · Page 18Linked to original sources

arXiv2018

First-principles simulations for attosecond photoelectron spectroscopy based on time-dependent density functional theory

We develop a first-principles simulation method for attosecond time-resolved photoelectron spectroscopy. This method enables us to directly simulate the whole experimental processes, including excitation, emission and detection on equal footing. To examine the performance of the method, we use it to compute the reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) experiments of gas-phase Argon. The computed RABBITT photoionization delay is in very good agreement with recent experimental results from [Klünder et al, Phys. Rev. Lett. 106 143002 (2011)] and [Guénot et al, Phys. Rev. A 85 053424 (2012)]. This indicates the significance of a fully-consistent theoretical treatment of the whole measurement process to properly describe experimental observables in attosecond photoelectron spectroscopy. The present framework opens the path to unravel the microscopic processes underlying RABBITT spectra in more complex materials and nanostructures.

physics.atom-ph↗

arXiv2018

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.

cond-mat.mtrl-sci↗

arXiv2017

Electronic band gaps of confined linear carbon chains ranging from polyyne to carbyne

Ultra long linear carbon chains of more than 6000 carbon atoms have recently been synthesized within double-walled carbon nanotubes, and they show a promising new route to one--atom--wide semiconductors with a direct band gap. Theoretical studies predicted that this band gap can be tuned by the length of the chains, the end groups, and their interactions with the environment. However, different density functionals lead to very different values of the band gap of infinitely long carbyne. In this work, we applied resonant Raman excitation spectroscopy with more than 50 laser wavelengths to determine for the first time the band gap of long carbon chains encapsulated inside DWCNTs. The experimentally determined band gaps ranging from 2.253 to 1.848 eV follow a linear relation with Raman frequency. This lower bound is the smallest band gap of linear carbon chains observed so far. The comparison with experimental data obtained for short chains in gas phase or in solution demonstrates the effect of the DWCNT encapsulation, leading to an essential downshift of the band gap. This is explained by the interaction between the carbon chain and the host tube, which greatly modifies the chain's bond length alternation.

cond-mat.mtrl-sci↗

arXiv2017

Ultrafast modification of Hubbard $U$ 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 $U$, 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 $U$ (TDDFT+U) method, we demonstrate the importance of a dynamically modulated $U$ 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.

cond-mat.str-el↗

arXiv2017

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.

cond-mat.str-el↗

arXiv2017

Ab-initio Optimized Effective Potentials for Real Molecules in Optical Cavities: Photon Contributions to the Molecular Ground state

We introduce a simple scheme to efficiently compute photon exchange-correlation contributions due to the coupling to transversal photons as formulated in the newly developed quantum-electrodynamical density functional theory (QEDFT). Our construction employs the optimized-effective potential (OEP) approach by means of the Sternheimer equation to avoid the explicit calculation of unoccupied states. We demonstrate the efficiency of the scheme by applying it to an exactly solvable GaAs quantum ring model system, a single azulene molecule, and chains of sodium dimers, all located in optical cavities and described in full real space. While the first example is a two-dimensional system and allows to benchmark the employed approximations, the latter two examples demonstrate that the correlated electron-photon interaction appreciably distorts the ground-state electronic structure of a real molecule. By using this scheme, we not only construct typical electronic observables, such as the electronic ground-state density, but also illustrate how photon observables, such as the photon number, and mixed electron-photon observables, e.g. electron-photon correlation functions, become accessible in a DFT framework. This work constitutes the first three-dimensional ab-initio calculation within the new QEDFT formalism and thus opens up a new computational route for the ab-initio study of correlated electron-photon systems in quantum cavities.

quant-ph↗

arXiv2017

Strongly bound excitons in anatase TiO2 single crystals and nanoparticles

Anatase TiO$_2$ is among the most studied materials for light-energy conversion applications, but the nature of its fundamental charge excitations is still unknown. Yet it is crucial to establish whether light absorption creates uncorrelated electron-hole pairs or bound excitons and, in the latter case, to determine their character. Here, by combining steady-state angle-resolved photoemission spectroscopy and spectroscopic ellipsometry with state-of-the-art ab initio calculations, we demonstrate that the direct optical gap of single crystals is dominated by a strongly bound exciton rising over the continuum of indirect interband transitions. This exciton possesses an intermediate character between the Wannier-Mott and Frenkel regimes and displays a peculiar two-dimensional wavefunction in the three-dimensional lattice. The nature of the higher-energy excitations is also identified. The universal validity of our results is confirmed up to room temperature by observing the same elementary excitations in defect-rich samples (doped single crystals and nanoparticles) via ultrafast two-dimensional deep-ultraviolet spectroscopy.

cond-mat.mtrl-sci↗

arXiv2017

Momentum-Resolved View of Electron-Phonon Coupling in Multilayer WSe$_2$

We investigate the interactions of photoexcited carriers with lattice vibrations in thin films of the layered transition metal dichalcogenide (TMDC) WSe$_2$. Employing femtosecond electron diffraction with monocrystalline samples and first principle density functional theory calculations, we obtain a momentum-resolved picture of the energy-transfer from excited electrons to phonons. The measured momentum-dependent phonon population dynamics are compared to first principle calculations of the phonon linewidth and can be rationalized in terms of electronic phase-space arguments. The relaxation of excited states in the conduction band is dominated by intervalley scattering between $Σ$ valleys and the emission of zone-boundary phonons. Transiently, the momentum-dependent electron-phonon coupling leads to a non-thermal phonon distribution, which, on longer timescales, relaxes to a thermal distribution via electron-phonon and phonon-phonon collisions. Our results constitute a basis for monitoring and predicting out of equilibrium electrical and thermal transport properties for nanoscale applications of TMDCs.

cond-mat.mes-hall↗

arXiv2017

Exact functionals for correlated electron-photon systems

For certain correlated electron-photon systems we construct the exact density-to-potential maps, which are the basic ingredients of a density-functional reformulation of coupled matter-photon problems. We do so for numerically exactly solvable models consisting of up to four fermionic sites coupled to a single photon mode. We show that the recently introduced concept of the intra-system steepening (T.Dimitrov et al., 18, 083004 NJP (2016)) can be generalized to coupled fermion-boson systems and that the intra-system steepening indicates strong exchange-correlation (xc) effects due to the coupling between electrons and photons. The reliability of the mean-field approximation to the electron-photon interaction is investigated and its failure in the strong coupling regime analyzed. We highlight how the intra-system steepening of the exact density-to-potential maps becomes apparent also in observables such as the photon number or the polarizability of the electronic subsystem. We finally show that a change in functional variables can make these observables behave more smoothly and exemplify that the density-to-potential maps can give us physical insights into the behavior of coupled electron-photon systems by identifying a very large polarizability due to ultra-strong electron-photon coupling.

quant-ph↗

arXiv2017

Ellipticity dependence of high-harmonic generation in solids: unraveling the interplay between intraband and interband dynamics

The strong ellipticity dependence of high-harmonic generation in gases enables numerous experimental techniques that are nowadays routinely used, for instance, to create isolated attosecond pulses. Extending such techniques to high-harmonic generation in solids requires a fundamental understanding of the microscopic mechanism of the high-harmonic generation. Here, using extensive first-principles simulations within a time-dependent density-functional framework, we show how intraband and interband mechanisms are strongly and differently affected by the ellipticity of the driving laser field. The complex interplay between intraband and interband effects can be used to tune and improve harmonic emission in solids. In particular, we show that the energy cutoff of the high-harmonic plateau can be increased by as much as 30\% using a finite ellipticity of the driving field, opening a new avenue for better understanding and control of HHG in solids based on ellipticity. Also, we demonstrate the possibility to generate, from a single circularly polarized driving field, circularly polarized harmonics with alternating helicity. Our work shows that ellipticity provides an additional knob to experimentally control high-order harmonic generation in solids.

physics.optics↗

arXiv2017

Non-locally sensing the magnetic states of nanoscale antiferromagnets with an atomic spin sensor

The ability to sense the magnetic state of individual magnetic nano-objects is a key capability for powerful applications ranging from readout of ultra-dense magnetic memory to the measurement of spins in complex structures with nanometer precision. Magnetic nano-objects require extremely sensitive sensors and detection methods. Here we create an atomic spin sensor consisting of three Fe atoms and show that it can detect nanoscale antiferromagnets through minute surface-mediated magnetic interaction. Coupling, even to an object with no net spin and having vanishing dipolar stray field, modifies the transition matrix element between two spin states of the Fe-atom-based spin sensor that changes the sensor's spin relaxation time. The sensor can detect nanoscale antiferromagnets at up to three nanometers distance and achieves an energy resolution of 10 micro-electronvolts surpassing the thermal limit of conventional scanning probe spectroscopy. This scheme permits simultaneous sensing of multiple antiferromagnets with a single spin sensor integrated onto the surface.

cond-mat.mes-hall↗

arXiv2017

Anomalous Anisotropic Exciton Temperature Dependence in Rutile TiO2

Elucidating the details of the electron-phonon coupling in semiconductors and insulators is a topic of pivotal interest, as it governs the transport mechanisms and is responsible for various phenomena such as spectral-weight transfers to phonon sidebands and self-trapping. Here, we investigate the influence of the electron-phonon interaction on the excitonic peaks of rutile TiO$_2$, revealing a strong anisotropic polarization dependence with increasing temperature, namely an anomalous blueshift for light polarized along the a-axis and a conventional redshift for light polarized along the c-axis. By employing many-body perturbation theory, we identify two terms in the electron-phonon interaction Hamiltonian that contribute to the anomalous blueshift of the a-axis exciton. Our approach paves the way to a complete ab initio treatment of the electron-phonon interaction and of its influence on the optical spectra of polar materials.

cond-mat.mtrl-sci↗

arXiv2017

Electron-energy-loss and time-dependent density functional theory study on the plasmon dispersion in 2H-NbS2

We examine the experimental and theoretical electron-energy loss spectra in 2$H$-Cu$_{0.2}$NbS$_2$ and find that the 1 eV plasmon in this material does not exhibit the regular positive quadratic plasmon dispersion that would be expected for a normal broad-parabolic-band system. Instead we find a nearly non-dispersing plasmon in the momentum-transfer range $q<0.35$ Å$^{-1}$. We argue that for a stoichiometric pure 2$H$-NbS$_2$ the dispersion relation is expected to have a negative slope as is the case for other transition-metal dichalcogenides. The presence of Cu impurities, required to stabilize the crystal growth, tends to shift the negative plasmon dispersion into a positive one, but the doping level in the current system is small enough to result in a nearly-non-dispersing plasmon. We conclude that a negative-slope plasmon dispersion is not connected with the existence of a charge-density-wave order in transition metal dichalcogenides.

cond-mat.str-el↗

arXiv2017

Generation and evolution of spin-, valley- and layer-polarized excited carriers in inversion-symmetric WSe2

Manipulation of spin and valley degrees of freedom is a key step towards realizing novel quantum technologies, for which atomically thin transition metal dichalcogenides (TMDCs) have been established as promising candidates. In monolayer TMDCs, the lack of inversion symmetry gives rise to a spin-valley correlation of the band structure allowing for valley-selective electronic excitation with circularly polarized light. Here we show that, even in centrosymmetric samples of 2H-WSe2, circularly polarized light can generate spin-, valley- and layer-polarized excited states in the conduction band. Employing time- and angle-resolved photoemission spectroscopy (trARPES) with spin-selective excitation, the dynamics of valley and layer pseudospins of the excited carriers are investigated. Complementary time-dependent density functional theory (TDDFT) calculations of the excited state populations reveal a strong circular dichroism of the spin-, valley- and layer-polarizations and a pronounced 2D character of the excited states in the K valleys. We observe scattering of carriers towards the global minimum of the conduction band on a sub-100 femtosecond timescale to states with three-dimensional character facilitating inter-layer charge transfer. Our results establish the optical control of coupled spin-, valley- and layer-polarized states in centrosymmetric materials and suggest the suitability of TMDC multilayer materials for valleytronic and spintronic device concepts.

cond-mat.mes-hall↗

arXiv2016

Cavity Born-Oppenheimer Approximation for Correlated Electron-Nuclear-Photon Systems

In this work, we illustrate the recently introduced concept of the cavity Born-Oppenheimer approximation for correlated electron-nuclear-photon problems in detail. We demonstrate how an expansion in terms of conditional electronic and photon-nuclear wave functions accurately describes eigenstates of strongly correlated light-matter systems. For a GaAs quantum ring model in resonance with a photon mode we highlight how the ground-state electronic potential-energy surface changes the usual harmonic potential of the free photon mode to a dressed mode with a double-well structure. This change is accompanied by a splitting of the electronic ground-state density. For a model where the photon mode is in resonance with a vibrational transition, we observe in the excited-state electronic potential-energy surface a splitting from a single minimum to a double minimum. Furthermore, for a time-dependent setup, we show how the dynamics in correlated light-matter systems can be understood in terms of population transfer between potential energy surfaces. This work at the interface of quantum chemistry and quantum optics paves the way for the full ab-initio description of matter-photon systems.

quant-ph↗

arXiv2016

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, Na$_3$Bi. 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.

cond-mat.mtrl-sci↗

arXiv2016

Impact of the electronic band structure in high-harmonic generation spectra of solids

An accurate analytic model describing 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.

physics.optics↗

arXiv2016

Monitoring in real time the photon-dressing and undressing of quasiparticles from first principles time-resolved photoelectron spectroscopy

Optical pumping of solids creates a non-equilibrium electronic structure where electrons and photons combine to form quasiparticles of dressed electronic states. The resulting shift of electronic levels is known as the optical Stark effect, visible as a red shift in the optical spectrum. Here we show that in a pump-probe setup we can uniquely define a non-equilibrium quasiparticle bandstructure that can be directly measurable with photo-electron spectroscopy. The dynamical photon-dressing (and undressing) of the many-body electronic states can be monitored by pump-probe time and angular resolved photoelectron spectroscopy (tr-ARPES) as the photon-dressed bandstructure evolves in time depending on the pump-probe pulse overlap. The computed tr-ARPES spectrum agrees perfectly with the quasi-energy spectrum of Floquet theory at maximum overlap and goes to the the equilibrium bandstructure as the pump-probe overlap goes to zero. Additionally, we show how this time-dependent non-equilibrium quasiparticle structure can be understood to be the bandstructure underlying the optical Stark effect. The extension to spin-resolved PES can be used to predict asymmetric dichroic response linked to the valley selective optical excitations in monolayer transition metal dichalcogenides (TMDs).

cond-mat.mtrl-sci↗