University of Vienna

About: University of Vienna is a education organization based out in Vienna, Austria. It is known for research contribution in the topics: Population & Context (language use). The organization has 44686 authors who have published 95840 publications receiving 2907492 citations.

Towards an exact description of electronic wavefunctions in real solids

Abstract: The properties of all materials arise largely from the quantum mechanics of their constituent electrons under the influence of the electric field of the nuclei. The solution of the underlying many-electron Schrodinger equation is a ‘non-polynomial hard’ problem, owing to the complex interplay of kinetic energy, electron–electron repulsion and the Pauli exclusion principle. The dominant computational method for describing such systems has been density functional theory. Quantum-chemical methods—based on an explicit ansatz for the many-electron wavefunctions and, hence, potentially more accurate—have not been fully explored in the solid state owing to their computational complexity, which ranges from strongly exponential to high-order polynomial in system size. Here we report the application of an exact technique, full configuration interaction quantum Monte Carlo to a variety of real solids, providing reference many-electron energies that are used to rigorously benchmark the standard hierarchy of quantum-chemical techniques, up to the ‘gold standard’ coupled-cluster ansatz, including single, double and perturbative triple particle–hole excitation operators. We show the errors in cohesive energies predicted by this method to be small, indicating the potential of this computationally polynomial scaling technique to tackle current solid-state problems. Recent developments that reduce the computational cost and scaling of wavefunction-based quantum-chemical techniques open the way to the successful application of such techniques to a variety of real-world solids. Computational descriptions of solid-state materials are currently dominated by methods based on density functional theory. An attractive and potentially more accurate approach would be to adopt the wavefunction-based methods of quantum chemistry, although these have not received as much attention because of the computational complexities involved. Now George Booth and colleagues show how recent developments that serve to reduce the computational cost and scaling of such quantum-chemical techniques open the way to their successful application to a variety of real-world solids.

Renormalisation of ϕ4-Theory on Noncommutative ℝ4 in the Matrix Base

Abstract: We prove that the real four-dimensional Euclidean noncommutative ϕ4-model is renormalisable to all orders in perturbation theory. Compared with the commutative case, the bare action of relevant and marginal couplings contains necessarily an additional term: an harmonic oscillator potential for the free scalar field action. This entails a modified dispersion relation for the free theory, which becomes important at large distances (UV/IR-entanglement). The renormalisation proof relies on flow equations for the expansion coefficients of the effective action with respect to scalar fields written in the matrix base of the noncommutative ℝ4. The renormalisation flow depends on the topology of ribbon graphs and on the asymptotic and local behaviour of the propagator governed by orthogonal Meixner polynomials.

Nitrate and nitrite in biology, nutrition and therapeutics

Abstract: Inorganic nitrate and nitrite from endogenous or dietary sources are metabolized in vivo to nitric oxide (NO) and other bioactive nitrogen oxides. The nitrate-nitrite-NO pathway is emerging as an important mediator of blood flow regulation, cell signaling, energetics and tissue responses to hypoxia. The latest advances in our understanding of the biochemistry, physiology and therapeutics of nitrate, nitrite and NO were discussed during a recent 2-day meeting at the Nobel Forum, Karolinska Institutet in Stockholm.

Real-time assessment of retinal blood flow with ultrafast acquisition by color Doppler Fourier domain optical coherence tomography.

Abstract: We interfaced color Doppler Fourier domain optical coherence tomography (CD-FDOCT) with a commercial OCT system to perform in vivo studies of human retinal blood flow in real time. FDOCT does not need reference arm scanning and records one full depth and Doppler profile in parallel. The system operates with an equivalent A-scan rate of 25 kHz and allows real time imaging of the color encoded Doppler information together with the tissue morphology at a rate of 2-4 tomograms (40 x 512 pixel) per second. The recording time of a single tomogram (160 x 512 data points) is only 6,4ms. Despite the high detection speed we achieve a system sensitivity of 86dB using a beam power of 500microW at the cornea. The fundus camera allows simultaneous view for selection of the region of interest. We observe bi-directional blood flow and pulsatility of blood velocity in retinal vessels with a Doppler detection bandwidth of 12.5 kHz and a longitudinal velocity sensitivity in tissue of 200microm/s.