Density Functional Theory of the Hubbard-Holstein Model

We present a density functional theory (DFT) for lattice models with local electron-electron (e-e) and electron-phonon (e-ph) interactions. Exchange-correlation potentials are derived via dynamical mean field theory for the infinite-dimensional Bethe lattice, and analytically for an isolated Hubbard-Holstein site. These potentials exhibit discontinuities as a function of the density, which depend on the relative strength of the e-e and e-ph interactions. By comparing to exact benchmarks, we show that the DFT formalism gives a good description of the linear conductance and real-time dynamics.Comment: 5 pages, 3 figures, supplemental material provided as pd

Nonequilibrium Green's functions and atom-surface dynamics: Simple views from a simple model system

We employ Non-equilibrium Green's functions (NEGF) to describe the real-time dynamics of an adsorbate-surface model system exposed to ultrafast laser pulses. For a finite number of electronic orbitals, the system is solved exactly and within different levels of approximation. Specifically i) the full exact quantum mechanical solution for electron and nuclear degrees of freedom is used to benchmark ii) the Ehrenfest approximation (EA) for the nuclei, with the electron dynamics still treated exactly. Then, using the EA, electronic correlations are treated with NEGF within iii) 2nd Born and with iv) a recently introduced hybrid scheme, which mixes 2nd Born self-energies with non-perturbative, local exchange-correlation potentials of Density Functional Theory (DFT). Finally, the effect of a semi-infinite substrate is considered: we observe that a macroscopic number of de-excitation channels can hinder desorption. While very preliminary in character and based on a simple and rather specific model system, our results clearly illustrate the large potential of NEGF to investigate atomic desorption, and more generally, the non equilibrium dynamics of material surfaces subject to ultrafast laser fields.Comment: 10 pages, 5 figure

Non-Perturbative Theory for Dispersion Self-Energy of Atoms

We go beyond the approximate series-expansions used in the dispersion theory of finite size atoms. We demonstrate that a correct, and non-perturbative, theory dramatically alters the dispersion selfenergies of atoms. The non-perturbed theory gives as much as 100% corrections compared to the traditional series expanded theory for the smaller noble gas atoms.Comment: 3 pages, no figures, 1 tabl

Casimir attractive-repulsive transition in MEMS

Unwanted stiction in micro- and nanomechanical (NEMS/MEMS) systems due to dispersion (van der Waals, or Casimir) forces is a significant hurdle in the fabrication of systems with moving parts on these length scales. Introducing a suitably dielectric liquid in the interspace between bodies has previously been demonstrated to render dispersion forces repulsive, or even to switch sign as a function of separation. Making use of recently available permittivity data calculated by us we show that such a remarkable non-monotonic Casimir force, changing from attractive to repulsive as separation increases, can in fact be observed in systems where constituent materials are in standard NEMS/MEMS use requiring no special or exotic materials. No such nonmonotonic behaviour has been measured to date. We calculate the force between a silica sphere and a flat surface of either zinc oxide or hafnia, two materials which are among the most prominent for practical microelectrical and microoptical devices. Our results explicate the need for highly accurate permittivity functions of the materials involved for frequencies from optical to far-infrared frequencies. A careful analysis of the Casimir interaction is presented, and we show how the change in the sign of the interaction can be understood as a result of multiple crossings of the dielectric functions of the three media involved in a given set-up.Comment: 6 pages, 4 figure

Non-Perturbative Theory of Dispersion Interactions

Some open questions exist with fluctuation-induced forces between extended dipoles. Conventional intuition derives from large-separation perturbative approximations to dispersion force theory. Here we present a full non-perturbative theory. In addition we discuss how one can take into account finite dipole size corrections. It is of fundamental value to investigate the limits of validity of the perturbative dispersion force theory.Comment: 9 pages, no figure

Simultaneous Bedside Assessment of Global Cerebral Blood Flow and Effective Cerebral Perfusion Pressure in Patients with Intracranial Hypertension

Background: We examined a bedside technique transcerebral double-indicator dilution (TCID) for global cerebral blood flow (CBF) as well as the concept of effective cerebral perfusion pressure (CPPeff) during different treatment options for intracranial hypertension, and compared global CBF and CPPeff with simultaneously obtained conventional parameters. Methods: Twenty-six patients developing intracranial hypertension in the course of traumatic brain injury or subarachnoid hemorrhage were prospectively analyzed using a combined assessment during elevated ventilation (n=15) or osmotherapy (hypertonic saline or mannitol). For calculation of global CBF, injections of ice-cold indocyanine green boluses were performed and temperature and dye concentration changes were monitored in the thoracic aorta and the jugular bulb. CBF was then calculated according to the mean transit time principle. Estimation of CCP, the arterial pressure at which cerebral blood flow becomes zero, was performed by synchronized registration of corresponding values of blood flow velocity in the middle cerebral artery and arterial pressure and extrapolation to zero-flow velocity. CPPeff was calculated as mean arterial pressure minus critical closing pressure (CPPeff=MAPc−CCP). Results: Elevated ventilation causes a decrease in both ICP (P<0.001) and CBF (P<0.001). While CPPconv increased (P<0.001), CPPeff decreased during this observation (P=0.002). Administration of osmotherapeutic agents resulted in a decrease of ICP (P<0.001) and a temporary increase of CBF (P=0.052). CPPconv and CPPeff showed no striking difference under osmotherapy. Conclusion: TCID allows repeated measurements of global CBF at the bedside. Elevated ventilation lowered and osmotherapy temporarily raised global CBF. In situations of increased vasotonus, CPPeff is a better indicator of blood flow changes than conventional CP

Effective dynamic properties of 3D composite materials containing rigid penny-shaped inclusions

The propagation of time-harmonic plane elastic waves in infinite elastic composite materials consisting of linear elastic matrix and rigid penny-shaped inclusions is investigated in this paper. The inclusions are allowed to translate and rotate in the matrix. First, the three-dimensional (3D) wave scattering problem by a single inclusion is reduced to a system of boundary integral equations for the stress jumps across the inclusion surfaces. A boundary element method (BEM) is developed for solving the boundary integral equations numerically. Far-field scattering amplitudes and complex wavenumbers are computed by using the stress jumps. Then the solution of the single scattering problem is applied to estimate the effective dynamic parameters of the composite materials containing randomly distributed inclusions of dilute concentration. Numerical results for the attenuation coefficient and the effective velocity of longitudinal and transverse waves in infinite elastic composites containing parallel and randomly oriented rigid penny-shaped inclusions of equal size and equal mass are presented and discussed. The effects of the wave frequency, the inclusion mass, the inclusion density, and the inclusion orientation or the direction of the wave incidence on the attenuation coefficient and the effective wave velocities are analysed. The results presented in this paper are compared with the available analytical results in the low-frequency range

Marine diversity shift linked to interactions among grazers, nutrients and propagule banks

Diverse coastal seaweed communities dominated by perennial fucoids become replaced by species-poor turfs of annual algae throughout the Baltic Sea. A large-scale field survey and factorial field experiments indicated that grazers maintain the fucoid community through selective consumption of annual algae. Interactive effects between grazers and dormant propagules of annual algae, stored in a 'marine seed bank', determine the response of this system to anthropogenic nutrient loading. Nutrients override grazer control and accelerate the loss of algal diversity in the presence but not in the absence of a propagule bank. This implies a novel role of propagule banks for community regulation and ecosystem response to marine eutrophication

Charge separation in donor-C60 complexes with real-time Green's functions: The importance of nonlocal correlations

We use the Nonequilibrium Green's Function (NEGF) method to perform real-time simulations of the ultrafast electron dynamics of photoexcited donor-C60 complexes modeled by a Pariser-Parr-Pople Hamiltonian. The NEGF results are compared to mean-field Hartree-Fock (HF) calculations to disentangle the role of correlations. Initial benchmarking against numerically highly accurate time-dependent Density Matrix Renormalization Group calculations verifies the accuracy of NEGF. We then find that charge-transfer (CT) excitons partially decay into charge separated (CS) states if dynamical non-local correlation corrections are included. This CS process occurs in ~10 fs after photoexcitation. In contrast, the probability of exciton recombination is almost 100% in HF simulations. These results are largely unaffected by nuclear vibrations; the latter become however essential whenever level misalignment hinders the CT process. The robust nature of our findings indicate that ultrafast CS driven by correlation-induced decoherence may occur in many organic nanoscale systems, but it will only be correctly predicted by theoretical treatments that include time-nonlocal correlations.Comment: 9 pages, 6 figures + supplemental information (4 pages)