Showing papers in "European Physical Journal B in 2018"

Strong disorder RG approach – a short review of recent developments

Abstract: The strong disorder RG approach for random systems has been extended in many new directions since our previous review of 2005 [F. Igloi, C. Monthus, Phys. Rep. 412, 277 (2005)]. The aim of the present colloquium paper is thus to give an overview of these various recent developments. In the field of quantum disordered models, recent progress concern infinite disorder fixed points for short-ranged models in higher dimensions d > 1, strong disorder fixed points for long-ranged models, scaling of the entanglement entropy in critical ground-states and after quantum quenches, the RSRG-X procedure to construct the whole set excited stated and the RSRG-t procedure for the unitary dynamics in many-body-localized phases, the Floquet dynamics of periodically driven chains, the dissipative effects induced by the coupling to external baths, and Anderson Localization models. In the field of classical disordered models, new applications include the contact process for epidemic spreading, the strong disorder renormalization procedure for general master equations, the localization properties of random elastic networks, and the synchronization of interacting non-linear dissipative oscillators. Application of the method for aperiodic (or deterministic) disorder is also mentioned.

Influence of bolstering network reciprocity in the evolutionary spatial Prisoner’s Dilemma game: a perspective

Abstract: Many recent studies on evolutionary spatial Prisoner’s Dilemma (SPD) games have provided insights into the mechanisms and frameworks that bolster the effect of network reciprocity. In this article, we provide a concise and comprehensive review of previous studies on evolutionary games and network reciprocity. Subsequently, we evaluate and compare the results from such studies in a unified manner to answer an open question in evolutionary SPD games: What are the factors underlying network reciprocity and what effect do these factors have on the emergence and promotion of cooperation? As a first step, we introduce a novel indicator to quantitatively evaluate the effectiveness (contribution) of a final fraction of cooperators via the introduction of the associated mechanisms into a simple evolutionary SPD game. In this game, the players are located on a two-dimensional square lattice with the Moore neighborhood and update their strategies by imitating the strategy of the best performing player among their neighbors, and the dynamics are separated into two periods: the enduring (END) period and the expanding (EXP) period. The initial fraction of cooperators is decreased transiently via the invasion and exploitation of defectors in the END period, and over the period, the fraction of cooperators is increased to expand cooperative clusters in the EXP period. Moreover, we also evaluate the statistical validity of our indicator by performing regression analyses. Our results indicate that two factors bolster the effect of network reciprocity: (1) the shape of the cooperative cluster (C-cluster) formed in the END period and (2) the ability to expand a single “perfect C-cluster,” which is the smallest patch, to increase the opportunity for interactions between cooperators and defectors and reduce exploitation by defectors in the EXP period.

Capturing intensive and extensive DFT/TDDFT molecular properties with machine learning

Abstract: Machine learning has been successfully applied to the prediction of chemical properties of small organic molecules such as energies or polarizabilities. Compared to these properties, the electronic excitation energies pose a much more challenging learning problem. Here, we examine the applicability of two existing machine learning methodologies to the prediction of excitation energies from time-dependent density functional theory. To this end, we systematically study the performance of various 2- and 3-body descriptors as well as the deep neural network SchNet to predict extensive as well as intensive properties such as the transition energies from the ground state to the first and second excited state. As perhaps expected current state-of-the-art machine learning techniques are more suited to predict extensive as opposed to intensive quantities. We speculate on the need to develop global descriptors that can describe both extensive and intensive properties on equal footing.

Topological plasmons in dimerized chains of nanoparticles: robustness against long-range quasistatic interactions and retardation effects

Abstract: We present a simple model of collective plasmons in a dimerized chain of spherical metallic nanoparticles, an elementary example of a topologically nontrivial nanoplasmonic system. Taking into account long-range quasistatic dipolar interactions throughout the chain, we provide an exact analytical expression for the full quasistatic bandstructure of the collective plasmons. An explicit calculation of the Zak phase proves the robustness of the topological physics of the system against the inclusion of long-range Coulomb interactions, despite the broken chiral symmetry. Using an open quantum systems approach, which includes retardation through the plasmon–photon coupling, we go on to analytically evaluate the resulting radiative frequency shifts of the plasmonic spectrum. The bright plasmonic bands experience size-dependent radiative shifts, while the dark bands are essentially unaffected by the light-matter coupling. Notably, the upper transverse-polarized band presents a logarithmic singularity where the quasistatic spectrum intersects the light cone. At wavevectors away from this intersection and for subwavelength nanoparticles, the plasmon–photon coupling only leads to a quantitative reconstruction of the bandstructure and the topologically-protected states at the edge of the first Brillouin zone are essentially unaffected.

An efficient implementation of two-component relativistic density functional theory with torque-free auxiliary variables

Abstract: An integration and assembly strategy for efficient evaluation of the exchange correlation term in relativistic density functional theory within two-component Kohn–Sham framework is presented. Working equations that both take into account all the components of the spin magnetization and can exploit parallelism, optimized cache utilization, and micro-architecture specific-floating point operations are discussed in detail in this work. The presented assembly of the exchange correlation potential, suitable for both open and closed shell systems, uses spinor density and a set of auxiliary variables, ensuring easy retrofitting of existing density functionals designed for collinear density. The used auxiliary variables in this paper, based on the scalar and non-collinear density, can preserve non-zero exchange correlation magnetic field local torque, without violating the required overall zero torque, even for GGA functionals. This is mandatory to obtain accurate spin dynamics and proper time evolution of the magnetization. Spin frustrated hydrogen rings are used to validate the current implementation and phenoxy radicals of different sizes are used to monitor the performance. This approach is a step towards extending the applicability of relativistic two-component DFT to systems of large size (>100 atoms).

Time-dependent generalized Kohn–Sham theory

Abstract: Generalized Kohn–Sham (GKS) theory extends the realm of density functional theory (DFT) by providing a rigorous basis for non-multiplicative potentials, the use of which is outside original Kohn–Sham theory. GKS theory is of increasing importance as it underlies commonly used approximations, notably (conventional or range-separated) hybrid functionals and meta-generalized-gradient-approximation (meta-GGA) functionals. While this approach is often extended in practice to time-dependent DFT (TDDFT), the theoretical foundation for this extension has been lacking, because the Runge–Gross theorem and the van Leeuwen theorem that serve as the basis of TDDFT have not been generalized to non-multiplicative potentials. Here, we provide the necessary generalization. Specifically, we show that with one simple but non-trivial additional caveat – upholding the continuity equation in the GKS electron gas – the Runge–Gross and van Leeuwen theorems apply to time-dependent GKS theory. We also discuss how this is manifested in common GKS-based approximations.

Light-matter interactions via the exact factorization approach

Abstract: The exact factorization approach, originally developed for electron-nuclear dynamics, is extended to light-matter interactions within the dipole approximation. This allows for a Schrodinger equation for the photonic wavefunction, in which the potential contains exactly the effects on the photon field of its coupling to matter. We illustrate the formalism and potential for a two-level system representing the matter, coupled to an infinite number of photon modes in the Wigner-Weisskopf approximation, as well as to a single mode with various coupling strengths. Significant differences are found with the potential used in conventional approaches, especially for strong couplings. We discuss how our exact factorization approach for light-matter interactions can be used as a guideline to develop semiclassical trajectory methods for efficient simulations of light-matter dynamics.

CT-MQC – a coupled-trajectory mixed quantum/classical method including nonadiabatic quantum coherence effects

Abstract: Upon photoexcitation by a short light pulse, molecules can reach regions of the configuration space characterized by strong nonadiabaticity, where the motion of the nuclei is strongly coupled to the motion of the electrons. The subtle interplay between the nuclear and electronic degrees of freedom in such situations is rather challenging to capture by state-of-the-art nonadiabatic dynamics approaches, limiting therefore their predictive power. The Exact Factorization of the molecular wavefunction, though, offers new perspectives in the solution of this longstanding issue. Here, we investigate the performance of a mixed quantum/classical (MQC) limit of this theory, named Coupled Trajectory-MQC, which was shown to reproduce the excited-state dynamics of small systems accurately. The method is applied to the study of the photoinduced ring opening of oxirane and the results are compared with two other nonadiabatic approaches based on different Ansatze for the molecular wavefunction, namely Ehrenfest dynamics and Ab Initio Multiple Spawning (AIMS). All simulations were performed using linear-response time-dependent density functional theory. We show that the CT-MQC method can capture the (de)coherence effects resulting from the dynamics through conical intersections, in good agreement with the results obtained with AIMS and in contrast with ensemble Ehrenfest dynamics.

Extreme values of elastic strain and energy in sine-Gordon multi-kink collisions

Abstract: In our recent study the maximal values of kinetic and potential energy densities that can be achieved in the collisions of N slow kinks in the sine-Gordon model were calculated analytically (for N = 1, 2, and 3) and numerically (for 4 ≤ N ≤ 7). However, for many physical applications it is important to know not only the total potential energy density but also its two components (the on-site potential energy density and the elastic strain energy density) as well as the extreme values of the elastic strain, tensile (positive) and compressive (negative). In the present study we give (i) the two components of the potential energy density and (ii) the extreme values of elastic strain. Our results suggest that in multi-soliton collisions the main contribution to the potential energy density comes from the elastic strain, but not from the on-site potential. It is also found that tensile strain is usually larger than compressive strain in the core of multi-soliton collision.

Linear response time-dependent density functional theory of the Hubbard dimer

Abstract: The asymmetric Hubbard dimer is used to study the density-dependence of the exact frequency-dependent kernel of linear-response time-dependent density functional theory. The exact form of the kernel is given, and the limitations of the adiabatic approximation utilizing the exact ground-state functional are shown. The oscillator strength sum rule is proven for lattice Hamiltonians, and relative oscillator strengths are defined appropriately. The method of Casida for extracting oscillator strengths from a frequency-dependent kernel is demonstrated to yield the exact result with this kernel. An unambiguous way of labelling the nature of excitations is given. The fluctuation-dissipation theorem is proven for the ground-state exchange-correlation energy. The distinction between weak and strong correlation is shown to depend on the ratio of interaction to asymmetry. A simple interpolation between carefully defined weak-correlation and strong-correlation regimes yields a density-functional approximation for the kernel that gives accurate transition frequencies for both the single and double excitations, including charge-transfer excitations. Many exact results, limits, and expansions about those limits are given in the Appendices.

Nuclear quantum effects in electronic (non)adiabatic dynamics

Abstract: Trajectory-based approaches to excited-state, nonadiabatic dynamics are promising simulation techniques to describe the response of complex molecular systems upon photo-excitation. They provide an approximate description of the coupled quantum dynamics of electrons and nuclei trying to access systems of growing complexity. The central question in the design of those approximations is a proper accounting of the coupling electron-nuclei and of the quantum features of the problem. In this paper, we approach the problem in the framework of the exact factorization of the electron-nuclear wavefunction, re-deriving and improving the coupled-trajectory mixed quantum-classical (CT-MQC) algorithm recently developed to solve the exact-factorization equations. In particular, a procedure to include quantum nuclear effects in CT-MQC is derived, and tested on a model system in different regimes.

Anomalous Nernst effect in type-II Weyl semimetals

Abstract: Topological Weyl semimetals (WSM), a new state of quantum matter with gapless nodal bulk spectrum and open Fermi arc surface states, have recently sparked enormous interest in condensed matter physics. Based on the symmetry and fermiology, it has been proposed that WSMs can be broadly classified into two types, type-I and type-II Weyl semimetals. While the undoped, conventional, type-I WSMs have point like Fermi surface and vanishing density of states (DOS) at the Fermi energy, the type-II Weyl semimetals break Lorentz symmetry explicitly and have tilted conical spectra with electron and hole pockets producing finite DOS at the Fermi level. The tilted conical spectrum and finite DOS at Fermi level in type-II WSMs have recently been shown to produce interesting effects such as a chiral anomaly induced longitudinal magnetoresistance that is strongly anisotropic in direction and a novel anomalous Hall effect. In this work, we consider the anomalous Nernst effect in type-II WSMs in the absence of an external magnetic field using the framework of semi-classical Boltzmann theory. Based on both a linearized model of time-reversal breaking WSM with a higher energy cut-off and a more realistic lattice model, we show that the anomalous Nernst response in these systems is strongly anisotropic in space, and can serve as a reliable signature of type-II Weyl semimetals in a host of magnetic systems with spontaneously broken time reversal symmetry.

Kinetic models for optimal control of wealth inequalities

Abstract: We introduce and discuss optimal control strategies for kinetic models for wealth distribution in a simple market economy, acting to minimize the variance of the wealth density among the population. Our analysis is based on a finite time horizon approximation, or model predictive control, of the corresponding control problem for the microscopic agents’ dynamic and results in an alternative theoretical approach to the taxation and redistribution policy at a global level. It is shown that in general the control is able to modify the Pareto index of the stationary solution of the corresponding Boltzmann kinetic equation, and that this modification can be exactly quantified. Connections between previous Fokker–Planck based models for taxation-redistribution policies and the present approach are also discussed.

Process interpretation of current entropic bounds

Abstract: We show for Markov diffusion processes that the quadratic entropic bound, recently derived for the rate functions of nonequilibrium currents, can be seen as being produced by an effective process that creates current fluctuations in a sub-optimal way by modifying only the non-reversible part of the drift or force of the process considered while keeping its reversible part constant. This provides a clear interpretation of the bound in terms of a physical process, which explains, among other things, its relation to the fluctuation relation, linear response, and reversible limits. The existence of more general quadratic bounds, and related uncertainty relations, for physical quantities other than currents is also discussed.

Structural, optical and magnetic properties of nanophase NiWO4 for potential applications

Abstract: Nanocrystalline NiWO4 powder samples were synthesized by direct-chemical precipitation method. Thermal stability of the sample was studied by thermo gravimetric and differential thermal analysis. Structural characterization of NiWO4 nanoparticles was done with X-ray diffraction and field emission scanning electron microscopy. Elemental analysis of the samples were done with energy dispersive X-ray spectroscopy. Vibration modes of as prepared samples were analysed using Fourier transform infrared spectroscopy and Raman spectroscopy. Optical properties of the samples were explored using UV-vis spectroscopy and photoluminescence (PL) spectroscopy. Magnetic properties of NiWO4 nanoparticles were analysed using vibrating sample magnetometer (VSM). Effect of calcination temperature on structural, vibrational, optical and magnetic properties of the NiWO4 samples were also investigated. The results obtained from various characterization techniques found that NiWO4 nanoparticle have the potential use in light emitting diodes (LEDs), biomedical and sensing applications.

When the exact factorization meets conical intersections...

Abstract: Capturing nuclear dynamics through conical intersections is pivotal to understand the fate of photoexcited molecules. The concept of a conical intersection, however, belongs to a specific definition of the electronic states, within a Born–Huang representation of the molecular wavefunction. How would these ultrafast funneling processes be translated if an exact factorization of the molecular wavefunction were to be used? In this article, we build upon our recent analysis [B.F.E. Curchod, F. Agostini, J. Phys. Chem. Lett. 8, 831 (2017)] and address this question in a broader perspective by studying the dynamics of a nuclear wavepacket through two types of conical intersections, differing by the strength of their underlying diabatic coupling. Our results generalize our previous findings by (i) showing that the time-dependent potential energy surface smoothly varies, both in time and in position, between the corresponding diabatic and adiabatic potentials, with sometimes more complex features if interferences are observed, (ii) highlighting the non-trivial behavior of the time-dependent vector potential and the fact that it cannot be gauged away in general, and (iii) justifying some approximations employed in the derivation of a mixed quantum/classical scheme based on the exact factorization.

Spherical quantum dot in Kratzer confining potential: study of linear and nonlinear optical absorption coefficients and refractive index changes

Abstract: Linear and nonlinear optical absorption coefficients (ACs) and refractive index changes (RICs) between the ground and the excited states of the GaAs spherical quantum dot (QD) under the effect of Kratzer confining potential and laser field have been investigated theoretically. The electronic energy levels and their corresponding wave functions are obtained by solving Schrodinger equation using finite difference method within the effective mass approximation. The dependency of energies, probability densities, dipole matrix elements on the Kratzer potential parameters and on the size of QD are investigated. The use of density matrix formalism is made to study the variations in linear, nonlinear ACs and RICs with the energy and intensity of the laser field. Also the effect of variation of the QD size and Kratzer potential parameters on linear, nonlinear ACs and RICs are studied.

One-dimensional phononic crystals that incorporate a defective piezoelectric/piezomagnetic as a new sensor

Abstract: In this paper, we have studied the propagation and localization of acoustic waves in a one-dimensional (1D) phononic crystal at the effects of both electric and magnetic fields. We focused on the sensing and measuring the electric and magnetic fields by adjusting the position of the transmitted peak (defect mode) inside the two piezoelectric/piezomagnetic defect layers (0.7 PMN-0.3PT/Terfenol-D). The defect mode is shifted towards the lower frequencies by applying electric and magnetic fields and towards the higher frequencies by opposing the field’s direction. The bigger the value of the applied field is the greater movement in the peak position is. In addition, our results revealed that the magnetic field has a bigger influence than the electric field on the position of the defect mode. Moreover, we studied such movement of the defect mode position by applying the electric and magnetic field simultaneously. These results can be used to improve and enhance piezoelectric and piezomagnetic energy harvesters.

An exact-factorization perspective on quantum-classical approaches to excited-state dynamics

Abstract: Trajectory-based quantum-classical schemes to excited-state dynamics are the most promising approaches to study photochemical and photophysical phenomena occurring in complex molecular systems. Theoretical developments in this field are mainly directed towards proposing generally-applicable approximations to the full quantum-mechanical problem, able to capture the key features of the mechanisms of interest. The task is indeed hard. Therefore, comparisons among approximate methods, and comparisons with exact results for typical model examples provide guidelines for theoreticians who decide to embark on this path. In this work, Ehrenfest dynamics and surface hopping will be analyzed adopting the exact-factorization perspective. A theoretical investigation will be supported by numerical results on a two-electronic-state one-dimensional model system. Numerical results based on the quantum-classical algorithm derived from the exact factorization will be presented as well, allowing to point out strengths and deficiencies of Ehrenfest dynamics and surface hopping. The combined analysis of the potential that drives classical trajectories and of quantum decoherence is essential to highlight similarities or differences among these quantum-classical approaches.

The family of topological Hall effects for electrons in skyrmion crystals

Abstract: Hall effects of electrons can be produced by an external magnetic field, spin–orbit coupling or a topologically non-trivial spin texture. The topological Hall effect (THE) – caused by the latter – is commonly observed in magnetic skyrmion crystals. Here, we show analogies of the THE to the conventional Hall effect (HE), the anomalous Hall effect (AHE), and the spin Hall effect (SHE). In the limit of strong coupling between conduction electron spins and the local magnetic texture the THE can be described by means of a fictitious, “emergent” magnetic field. In this sense the THE can be mapped onto the HE caused by an external magnetic field. Due to complete alignment of electron spin and magnetic texture, the transverse charge conductivity is linked to a transverse spin conductivity. They are disconnected for weak coupling of electron spin and magnetic texture; the THE is then related to the AHE. The topological equivalent to the SHE can be found in antiferromagnetic skyrmion crystals. We substantiate our claims by calculations of the edge states for a finite sample. These states reveal in which situation the topological analogue to a quantized HE, quantized AHE, and quantized SHE can be found.

Hot-electron transport and ultrafast magnetization dynamics in magnetic multilayers and nanostructures following femtosecond laser pulse excitation

Abstract: Understanding and controlling the magnetization dynamics on the femtosecond timescale is becoming indispensable both at the fundamental level and to develop future technological applications. While direct laser excitation of a ferromagnetic layer was commonly used during the past twenty years, laser induced hot-electrons femtosecond pulses and subsequent transport in magnetic multilayers has attracted a lot of attention. Indeed, replacing photons by hot-electrons offers complementary information to improve our understanding of ultrafast magnetization dynamics and to provide new possibilities for manipulating the magnetization in a thin layer on the femtosecond timescale. In this review, we report on experiments of hot-electrons induced ultrafast magnetic phenomena. We discuss the role of hot-electrons transport in the ultrafast loss of magnetization in magnetic single and multilayers and how it is exploited to trigger magnetization dynamics in magnetic multilayers.

Evidence for criticality in financial data

Abstract: We provide evidence that cumulative distributions of absolute normalized returns for the 100 American companies with the highest market capitalization, uncover a critical behavior for different time scales Δt. Such cumulative distributions, in accordance with a variety of complex – and financial – systems, can be modeled by the cumulative distribution functions of q-Gaussians, the distribution function that, in the context of nonextensive statistical mechanics, maximizes a non-Boltzmannian entropy. These q-Gaussians are characterized by two parameters, namely (q, β), that are uniquely defined by Δt. From these dependencies, we find a monotonic relationship between q and β, which can be seen as evidence of criticality. We numerically determine the various exponents which characterize this criticality.

A screened independent atom model for the description of ion collisions from atomic and molecular clusters

Abstract: We apply a recently introduced model for an independent-atom-like calculation of ion-impact electron transfer and ionization cross sections to proton collisions from water, neon, and carbon clusters. The model is based on a geometrical interpretation of the cluster cross section as an effective area composed of overlapping circular disks that are representative of the atomic contributions. The latter are calculated using a time-dependent density-functional-theory-based single-particle description with accurate exchange-only ground-state potentials. We find that the net capture and ionization cross sections in p-X n collisions are proportional to n α with 2∕3 ≤ α ≤ 1. For capture from water clusters at 100 keV impact energy α is close to one, which is substantially different from the value α = 2∕3 predicted by a previous theoretical work based on the simplest-level electron nuclear dynamics method. For ionization at 100 keV and for capture at lower energies we find smaller α values than for capture at 100 keV. This can be understood by considering the magnitude of the atomic cross sections and the resulting overlaps of the circular disks that make up the cluster cross section in our model. Results for neon and carbon clusters confirm these trends. Simple parametrizations are found which fit the cross sections remarkably well and suggest that they depend on the relevant bond lengths.

Amplitude death induced by mixed attractive and repulsive coupling in the relay system

Abstract: The amplitude death (AD) phenomenon is found in the relay system in the presence of the mixed couplings composed of attractive coupling and repulsive coupling. The generation mechanism of AD is revealed and shows that the middle oscillator achieving AD is a prerequisite to further suppress oscillation of the outermost oscillators for the paradigmatic Stuart-Landau and Rossler models. Moreover, regarding the Stuart-Landau relay system as a small motif of star network, we also observe that the mixed couplings can facilitate AD state of the whole network system. Particularly, the threshold of coupling strength is invariable with the change of network size. Our findings may shed a new insight to explore the effects of hybrid coupling on complex systems, also provide a new strategy to control dynamic behaviors in engineering science and neuroscience fields.

Physical properties of niobium-based intermetallics (Nb 3 B ; B = Os, Pt, Au): a DFT-based ab-initio study

Abstract: Structural, elastic and electronic band structure properties of A-15 type Nb-based intermetallic compounds Nb3B (B = Os, Pt, Au) have been revisited using first principles calculations based on the density functional theory (DFT). All these show excellent agreement with previous reports. More importantly, electronic bonding, charge density distribution and Fermi surface features have been studied in detail for the first time. Vickers hardness of these compounds is also calculated. The Fermi surfaces of Nb3B contain both hole- and electron-like sheets, the features of which change systematically as one move from Os to Au. The electronic charge density distribution implies that Nb3Os, Nb3Pt and Nb3Au have a mixture of ionic and covalent bondings with a substantial metallic contribution. The charge transfer between the atomic species in these compounds has been explained via the Mulliken bond population analysis and the Hirshfeld population analysis. The bonding properties show a good correspondence to the electronic band structure derived electronic density of states (DOS) near the Fermi level. Debye temperature of Nb3B (B = Os, Pt, Au) has been estimated from the elastic constants and shows a systematic behavior as a function of the B atomic species. A good correspondence among the elastic, electronic and charge density distribution properties are found. The superconducting transition temperature is found to be dominated by the electronic density of states at the Fermi level. We have discussed possible implications of the results obtained in this study in details in this paper.

Identifying an influential spreader from a single seed in complex networks via a message-passing approach

Abstract: Identifying the most influential spreaders is one of outstanding problems in physics of complex systems. So far, many approaches have attempted to rank the influence of nodes but there is still the lack of accuracy to single out influential spreaders. Here, we directly tackle the problem of finding important spreaders by solving analytically the expected size of epidemic outbreaks when spreading originates from a single seed. We derive and validate a theory for calculating the size of epidemic outbreaks with a single seed using a message-passing approach. In addition, we find that the probability to occur epidemic outbreaks is highly dependent on the location of the seed but the size of epidemic outbreaks once it occurs is insensitive to the seed. We also show that our approach can be successfully adapted into weighted networks.

Shedding light on correlated electron–photon states using the exact factorization

Abstract: The exact factorization framework is extended and utilized to introduce the electronic-states of correlated electron–photon systems. The formal definitions of an exact scalar potential and an exact vector potential that account for the electron–photon correlation are given. Inclusion of these potentials to the Hamiltonian of the uncoupled electronic system leads to a purely electronic Schrodinger equation that uniquely determines the electronic states of the complete electron–photon system. For a one-dimensional asymmetric double-well potential coupled to a single photon mode with resonance frequency, we investigate the features of the exact scalar potential. In particular, we discuss the significance of the step-and-peak structure of the exact scalar potential in describing the phenomena of photon-assisted delocalization and polaritonic squeezing of the electronic excited-states. In addition, we develop an analytical approximation for the scalar potential and demonstrate how the step-and-peak features of the exact scalar potential are captured by the proposed analytical expression.

The effect of size and composition on structural transitions in monometallic nanoparticles

Abstract: Predicting the morphological stability of nanoparticles is an essential step towards the accurate modelling of their chemophysical properties. Here we investigate solid–solid transitions in monometallic clusters of 0.5–2.0 nm diameter at finite temperatures and we report the complex dependence of the rearrangement mechanism on the nanoparticle’s composition and size. The concerted Lipscomb’s Diamond-Square-Diamond mechanisms which connects the decahedral or the cuboctahedral to the icosahedral basins, take place only below a material dependent critical size above which surface diffusion prevails and leads to low-symmetry and defected shapes still belonging to the initial basin.

Negative differential conductivity in liquid aluminum from real-time quantum simulations

Abstract: The conduction of electricity in materials is usually described by Ohm’s law, which is a first order approximation to a more complex and non-linear behavior. It is well known that in some semiconductors, the conductivity, the constant that relates voltage and current, changes for high enough currents. In this work we predict for the first time that a metal, liquid aluminum, exhibits negative-differential conductivity, a non-linear effect where the current decreases as the applied voltage is increased. We observe this change in the conductivity for very high current densities of the order of 1012−1013 A∕cm2. Our predictions are based on a computational approach that can atomistically model, for the first time, non-linear effects in the conductivity from first principles by following in real-time the quantum dynamics of the electrons. From our simulations, we find that the change in the non-linear conductivity emerges from a competition between the current-induced accumulation of charge around the nuclei, which increases the scattering of the conduction electrons, and a decreasing ion-scattering cross-section at high currents. Our results illustrate how normal matter behaves under extreme fields that will become available from free electron lasers and other future experiments.

An ab-initio approach to describe coherent and non-coherent exciton dynamics

Abstract: The use of ultra-short laser pulses to pump and probe materials activates a wealth of processes which involve the coherent and non coherent dynamics of interacting electrons out of equilibrium. Non equilibrium (NEQ) many body perturbation theory (MBPT) offers an equation of motion for the density–matrix of the system which well describes both coherent and non coherent processes. In the non correlated case there is a clear relation between these two regimes and the matrix elements of the density–matrix. The same is not true for the correlated case, where the potential binding of electrons and holes in excitonic states need to be considered. In the present work we discuss how NEQ-MBPT can be used to describe the dynamics of both coherent and non-coherent excitons in the low density regime. The approach presented is well suited for an ab initio implementation.