arXiv subjects

Angel Rubio

Publications and source records attributed to Angel Rubio.

At least 343 records · Page 19Linked to original sources

arXiv2016

Atoms and Molecules in Cavities: From Weak to Strong Coupling in QED Chemistry

In this work, we provide an overview of how well-established concepts in the fields of quantum chemistry and material sciences have to be adapted when the quantum nature of light becomes important in correlated matter-photon problems. Therefore, we analyze model systems in optical cavities, where the matter-photon interaction is considered from the weak- to the strong coupling limit and for individual photon modes as well as for the multi-mode case. We identify fundamental changes in Born-Oppenheimer surfaces, spectroscopic quantities, conical intersections and efficiency for quantum control. We conclude by applying our novel recently developed quantum-electrodynamical density-functional theory to single-photon emission and show how a straightforward approximation accurately describes the correlated electron-photon dynamics. This paves the road to describe matter-photon interactions from first-principles and addresses the emergence of new states of matter in chemistry and material science.

quant-ph↗

arXiv2016

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$_2$, and hexagonal BN under different laser configurations.

physics.comp-ph↗

arXiv2016

Systematic construction of density functionals based on matrix product state computations

We propose a systematic procedure for the approximation of density functionals in density functional theory that consists of two parts. First, for the efficient approximation of a general density functional, we introduce an efficient ansatz whose non-locality can be increased systematically. Second, we present a fitting strategy that is based on systematically increasing a reasonably chosen set of training densities. We investigate our procedure in the context of strongly correlated fermions on a one-dimensional lattice in which we compute accurate training densities with the help of matrix product states. Focusing on the exchange-correlation energy, we demonstrate how an efficient approximation can be found that includes and systematically improves beyond the local density approximation. Importantly, this systematic improvement is shown for target densities that are quite different from the training densities.

cond-mat.str-el↗

arXiv2016

Efficient and accurate modeling of electron photoemission in nanostructures with TDDFT

We review different computational methods for the calculation of photoelectron spectra and angular distributions of atoms and molecules when excited by laser pulses using time-dependent density-functional theory (TDDFT) that are suitable for the description of electron emission in compact spatial regions. We derive and extend the time-dependent surface-flux method introduced in Reference [Tao L and Scrinzi A 2012 New Journal of Physics 14 013021] within a TDDFT formalism and compare its performance to other existing methods. We illustrate the performance of the new method by simulating strong-field ionization of C$_{60}$ fullerene and discuss final state effects in the orbital reconstruction of planar organic molecules.

physics.chem-ph↗

arXiv2016

Polyyne Electronic and Vibrational Properties under Environmental Interactions

Recently, the novel system of linear carbon chains inside of double-walled carbon nanotubes has extended the length of $sp^1$ hybridized carbon chains from 44 to thousands of atoms [L. Shi et al., Nat. Mater. 15, 634 (2016)]. The optoelectronic properties of these ultra-long chains are poorly described by current theoretical models, which are based on short chain experimental data and assume a constant environment. As such, a physical understanding of the system in terms of charge transfer and van der Waals interactions is widely missing. We provide a reference for the intrinsic Raman frequency of polyynes in vacuo and explicitly describe the interactions between polyynes and carbon nanotubes. We find that van der Waals interactions strongly shift the Raman frequency, which has been neither expected nor addressed before. As a consequence of charge transfer from the tube to the chain, the Raman response of long chains is qualitatively different from the known phonon dispersion of polymers close to the $Γ$-point. Based on these findings we show how to correctly interpret the Raman data, considering the nanotube's properties. This is essential for its use as an analytical tool to optimize the growth process for future applications.

cond-mat.mes-hall↗

arXiv2016

Disentangling Vacancy Oxidation on Metallicity-Sorted Carbon Nanotubes

Pristine single-walled carbon nanotubes (SWCNTs) are rather inert to O$_2$ and N$_2$, which for low doses chemisorb only on defect sites or vacancies of the SWCNTs at the ppm level. However, very low doping has a major effect on the electronic properties and conductivity of the SWCNTs. Already at low O$_2$ doses (80 L), the X-ray photoelectron spectroscopy (XPS) O 1s signal becomes saturated, indicating nearly all the SWCNT's vacancies have been oxidized. As a result, probing vacancy oxidation on SWCNTs via XPS yields spectra with rather low signal-to-noise ratios, even for metallicity-sorted SWCNTs. We show that, even under these conditions, the first principles density functional theory calculated Kohn-Sham O 1s binding energies may be used to assign the XPS O 1s spectra for oxidized vacancies on SWCNTs into its individual components. This allows one to determine the specific functional groups or bonding environments measured. We find the XPS O 1s signal is mostly due to three O-containing functional groups on SWCNT vacancies: epoxy (C$_2$$>$O), carbonyl (C$_2$$>$C$=$O), and ketene (C$=$C$=$O), as ordered by abundance. Upon oxidation of nearly all the SWCNT's vacancies, the central peak's intensity for the metallic SWCNT sample is 60\% greater than for the semiconducting SWCNT sample. This suggests a greater abundance of O-containing defect structures on the metallic SWCNT sample. For both metallic and semiconducting SWCNTs, we find O$_2$ does not contribute to the measured XPS O~1s spectra.

cond-mat.mes-hall↗

arXiv2016

Square selenene and tellurene: novel group VI elemental 2D semi-Dirac materials and topological insulators

With first principles calculations, we predict a novel stable 2D layered structure for group VI elements Se and Te that we call square selenene and square tellurene, respectively. They have chair-like buckled structures similar to other layered materials such as silicene and germanene but with a square unit cell rather than hexagonal. This special structure gives rise to anisotropic band dispersions near the Fermi level that can be described by a generalized semi-Dirac Hamiltonian. We show that the considerably large band gap ($\sim$0.1 eV) opened by spin-orbit coupling makes square selenene and tellurene topological insulators, hosting non-trivial edge states. Therefore, square selenene and tellurene are promising materials for novel electronic and spintronic applications. Finally, we show that this new type of 2D elemental material can potentially be grown on proper substrates, such as a Au(100) surface.

cond-mat.mes-hall↗

arXiv2016

Correlated Electron-Nuclear Dynamics with Conditional Wave Functions

The molecular Schrödinger equation is rewritten in terms of non-unitary equations of motion for the nuclei (or electrons) that depend parametrically on the configuration of an ensemble of generally defined electronic (or nuclear) trajectories. This scheme is exact and does not rely on the tracing-out of degrees of freedom. Hence, the use of trajectory-based statistical techniques can be exploited to circumvent the calculation of the computationally demanding Born-Oppenheimer potential-energy surfaces and non-adiabatic coupling elements. The concept of potential-energy surface is restored by establishing a formal connection with the exact factorization of the full wave function. This connection is used to gain insight from a simplified form of the exact propagation scheme.

cond-mat.mes-hall↗

arXiv2016

Photoinduced Absorption within Single-Walled Carbon Nanotube Systems

We study the photoabsorption properties of photoactive bulk polymer/ fullerene/nanotube heterojunctions in the near-infrared region. By combining pump-probe spectroscopy and linear response time-dependent density functional theory within the random phase approximation (TDDFT-RPA) we elucidate the excited state dynamics of the $E_{11}$ transition within (6,5) and (7,5) single-walled carbon nanotubes (SWNTs) and combined with poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM) in P3HT/PCBM/SWNT blended samples. We find the presence of a photoinduced absorption (PA) peak is related mainly to the width of the photobleach (PB) peak and the charge carrier density of the SWNT system. For mixed SWNT samples, the PB peak is too broad to observe the PA peak, whereas within P3HT/PCBM/SWNT blended samples P3HT acts as a hole acceptor, narrowing the PB peak by exciton delocalization, which reveals a PA peak. Our results suggest that the PA peak originates from a widening of the band gap in the presence of excited electrons and holes. These results have important implications for the development of new organic photovoltaic heterojunctions including SWNTs.

cond-mat.mtrl-sci↗

arXiv2016

Stable Dirac semi-metal in the allotrope of IV elements

Three dimensional topological Dirac semi-metals represent a novel state of quantum matter with exotic electronic properties, in which a pair of Dirac points with the linear dispersion along all momentum directions exist in the bulk. Herein, by using the first principles calculations, we discover a new metastable allotrope of Ge and Sn in the staggered layered dumbbell structure, named as germancite and stancite, to be Dirac semi-metals with a pair of Dirac points on its rotation axis. On the surface parallel to the rotation axis, a pair of topologically non-trivial Fermi arcs are observed and a Lifshitz transition is found by tuning the Fermi level. Furthermore, the quantum thin film of germancite is found to be an intrinsic quantum spin Hall insulator. These discoveries suggest novel physical properties and future applications of the new metastable allotrope of Ge and Sn.

cond-mat.mtrl-sci↗

arXiv2016

Tailored pump-probe transient spectroscopy with time-dependent density-functional theory: controlling absorption spectra

Recent advances in laser technology allow us to follow electronic motion at its natural time-scale with ultra-fast time resolution, leading the way towards attosecond physics experiments of extreme precision. In this work, we assess the use of tailored pumps in order to enhance (or reduce) some given features of the probe absorption (for example, absorption in the visible range of otherwise transparent samples). This type of manipulation of the system response could be helpful for its full characterization, since it would allow us to visualize transitions that are dark when using unshaped pulses. In order to investigate these possibilities, we perform first a theoretical analysis of the non-equilibrium response function in this context, aided by one simple numerical model of the Hydrogen atom. Then, we proceed to investigate the feasibility of using time-dependent density-functional theory as a means to implement, theoretically, this absorption-optimization idea, for more complex atoms or molecules. We conclude that the proposed idea could in principle be brought to the laboratory: tailored pump pulses can excite systems into light-absorbing states. However, we also highlight the severe numerical and theoretical difficulties posed by the problem: large-scale non-equilibrium quantum dynamics are cumbersome, even with TDDFT, and the shortcomings of state-of-the-art TDDFT functionals may still be serious for these out-of-equilibrium situations.

cond-mat.other↗

arXiv2016

Taming singularities of the diagrammatic many-body perturbation theory

In a typical scenario the diagrammatic many-body perturbation theory generates asymptotic series. Despite non-convergence, the asymptotic expansions are useful when truncated to a finite number of terms. This is the reason for popularity of leading-order methods such as $GW$ approximation in condensed matter, molecular and atomic physics. Emerging higher-order implementations suffer from the appearance of nonsimple poles in the frequency-dependent Green's functions and negative spectral densities making self-consistent determination of the electronic structure impossible. Here a method based on the Padé approximation for overcomming these difficulties is proposed and applied to the Hamiltonian describing a core electron coupled to a single plasmonic excitation. By solving the model purely diagrammatically, expressing the self-energy in terms of combinatorics of chord diagrams, and regularizing the diverging perturbative expansions using the Padé approximation the spectral function is determined self-consistently using 3111 diagrams up to the sixth order.

cond-mat.str-el↗

arXiv2015

Exact Maps in Density Functional Theory for Lattice Models

In the present work, we employ exact diagonalization for model systems on a real-space lattice to explicitly construct the exact density-to-potential and for the first time the exact density-to-wavefunction map that underly the Hohenberg-Kohn theorem in density functional theory. Having the explicit wavefunction-to- density map at hand, we are able to construct arbitrary observables as functionals of the ground-state density. We analyze the density-to-potential map as the distance between the fragments of a system increases and the correlation in the system grows. We observe a feature that gradually develops in the density-to-potential map as well as in the density-to-wavefunction map. This feature is inherited by arbitrary expectation values as functional of the ground-state density. We explicitly show the excited-state energies, the excited-state densities, and the correlation entropy as functionals of the ground-state density. All of them show this exact feature that sharpens as the coupling of the fragments decreases and the correlation grows. We denominate this feature as intra-system steepening. We show that for fully decoupled subsystems the intra-system steepening transforms into the well-known inter-system derivative discontinuity. An important conclusion is that for e.g. charge transfer processes between localized fragments within the same system it is not the usual inter-system derivative discontinuity that is missing in common ground-state functionals, but rather the differentiable intra-system steepening that we illustrate in the present work.

physics.chem-ph↗

arXiv2015

Kohn-Sham Approach to Quantum Electrodynamical Density Functional Theory: Exact Time-Dependent Effective Potentials in Real Space

The density-functional approach to quantum electrodynamics is extending traditional density-functional theory and opens the possibility to describe electron-photon interactions in terms of effective Kohn-Sham potentials. In this work, we numerically construct the exact electron-photon Kohn-Sham potentials for a prototype system which consists of a trapped electron coupled to a quantized electromagnetic mode in an optical high-Q cavity. While the effective current that acts on the photons is known explicitly, the exact effective potential that describes the forces exerted by the photons on the electrons is obtained from a fixed-point inversion scheme. This procedure allows us to uncover important beyond-mean-field features of the effective potential which mark the breakdown of classical light-matter interactions. We observe peak and step structures in the effective potentials, which can be attributed solely to the quantum nature of light, i.e., they are real-space signatures of the photons. Our findings show how the ubiquitous dipole interaction with a classical electromagnetic field has to be modified in real-space in order to take the quantum nature of the electromagnetic field fully into account.

quant-ph↗

arXiv2015

Orbitals from local RDMFT: Are they Kohn-Sham or Natural Orbitals?

Recently, an approximate theoretical framework was introduced, called local reduced density matrix functional theory (local-RDMFT), where functionals of the one-body reduced density matrix (1-RDM) are minimized under the additional condition that the optimal orbitals satisfy a single electron Schrödinger equation with a local potential. In the present work, we focus on the character of these optimal orbitals. In particular, we compare orbitals obtained by local-RDMFT with those obtained with the full minimization (without the extra condition) by contrasting them against the exact NOs and orbitals from a density functional calculation using the local density approximation (LDA). We find that the orbitals from local-RMDFT are very close to LDA orbitals, contrary to those of the full minimization that resemble the exact NOs. Since local RDMFT preserves the good quality of the description of strong static correlation, this finding opens the way to a mixed density/density matrix scheme, where Kohn-Sham orbitals obtain fractional occupations from a minimization of the occupation numbers using 1-RDM functionals. This will allow for a description of strong correlation at a cost only minimally higher than a density functional calculation.

physics.chem-ph↗

arXiv2015

Time-dependent thermal transport theory

Understanding thermal transport in nanoscale systems presents important challenges to both theory and experiment. In particular, the concept of local temperature at the nanoscale appears difficult to justify. Here, we propose a novel theoretical approach where we replace the temperature gradient with controllable external black-body radiations. The theory recovers known physical results, for example the linear relation between the thermal current and the temperatures difference of two black-bodies. Furthermore, our theory is not limited to the linear regime and goes beyond accounting for non linear effects and transient phenomena. In the strong coupling and large temperature gradients, we show that there is a maximum energy current that the system can sustain and we recover the Kramers' turnover. Since the present theory is general and can be adapted to describe both electron and phonon dynamics, it provides a first step towards a unified formalism for investigating thermal and electronic transport.

cond-mat.mes-hall↗

arXiv2015

Comparing quasiparticle H$_2$O level alignment on anatase and rutile TiO$_2$

Knowledge of the molecular frontier levels' alignment in the ground state can be used to predict the photocatalytic activity of an interface. The position of the adsorbate's highest occupied molecular orbital (HOMO) levels relative to the substrate's valence band maximum (VBM) in the interface describes the favorability of photogenerated hole transfer from the VBM to the adsorbed molecule. This is a key quantity for assessing and comparing H$_2$O photooxidation activities on two prototypical photocatalytic TiO$_2$ surfaces: anatase (A)-TiO$_2$(101) and rutile (R)-TiO$_2$(110). Using the projected density of states (DOS) from state-of-the-art quasiparticle (QP) $G_0W_0$ calculations, we assess the relative photocatalytic activity of intact and dissociated H$_2$O on coordinately unsaturated (Ti$_{\textit{cus}}$) sites of idealized stoichiometric A-TiO$_2$(101)/R-TiO$_2$(110) and bridging O vacancies (O$_{\textit{br}}^{\textit{vac}}$) of defective A-TiO$_{2-x}$(101)/R-TiO$_{2-x}$(110) surfaces ($x=\frac{1}{4},\frac{1}{8}$) for various coverages. Such a many-body treatment is necessary to correctly describe the anisotropic screening of electron-electron interactions at a photocatalytic interface, and hence obtain accurate interfacial level alignments. The more favorable ground state HOMO level alignment for A-TiO$_2$(101) may explain why the anatase polymorph shows higher photocatalytic activities than the rutile polymorph. Our results indicate that (1) hole trapping is more favored on A-TiO$_2$(101) than R-TiO$_2$(110) and (2) HO@Ti$_{\textit{cus}}$ is more photocatalytically active than intact H$_2$O@Ti$_{\textit{cus}}$.

cond-mat.mtrl-sci↗

arXiv2015

Static correlation and electron localization in molecular dimers from the self-consistent RPA and GW approximation

We investigate static correlation and delocalization errors in the self-consistent GW and random-phase approximation (RPA) by studying molecular dissociation of the H_2 and LiH molecules. Although both approximations contain topologically identical diagrams, the non-locality and frequency dependence of the GW self-energy crucially influence the different energy contributions to the total energy as compared to the use of a static local potential in the RPA. The latter leads to significantly larger correlation energies which allow for a better description of static correlation at intermediate bond distances. The substantial error found in GW is further analyzed by comparing spin-restricted and spin-unrestricted calculations. At large but finite nuclear separation their difference gives an estimate of the so-called fractional spin error normally determined only in the dissociation limit. Furthermore, a calculation of the dipole moment of the LiH molecule at dissociation reveals a large delocalization error in GW making the fractional charge error comparable to the RPA. The analyses are supplemented by explicit formulae for the GW Green's function and total energy of a simplified two-level model providing additional insights into the dissociation limit.

cond-mat.str-el↗