Vienna University of Technology

About: Vienna University of Technology is a education organization based out in Vienna, Austria. It is known for research contribution in the topics: Laser & Context (language use). The organization has 16723 authors who have published 49341 publications receiving 1302168 citations.

Charge distribution and electric-field gradients in YBa 2 Cu 3 O 7 − x

Abstract: The electric-field gradients (EFG's) of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.5}$, and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$ are calculated on a first-principles basis using the full-potential linear augmented-plane-wave (LAPW) method in which exchange and correlation effects are treated by the local-density approximation (LDA). Good agreement with experimental EFG's and their anisotropies is found for the Cu(1) position in all three compounds. For ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ the same is true for all oxygen positions, while at Cu(2) the direction of the EFG is predicted correctly, but a transfer of 0.07 electrons from ${\mathit{d}}_{\mathit{x}}^{2}$-${\mathit{y}}^{2}$ to ${\mathit{d}}_{\mathit{z}}^{2}$ symmetry would be needed to bring the theoretical EFG into agreement with the experimental value. The EFG calculations on ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$ and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.5}$ [assuming an ordered structure in which Cu(1) is threefold coordinated by oxygens] confirm the experimental assignment and strengthen the confidence in our results. The origin of the EFG is discussed and the relation to the anisotropy of the electronic charge distribution is illustrated in connection with symmetry-decomposed partial charges and difference-electron-density maps. It is shown that the LDA calculations yield reliable charge distributions to which the EFG is so sensitive.

The multiradical character of one- and two-dimensional graphene nanoribbons.

Abstract: Since the first experimental realizations of graphene nanoribbons,1 graphene nanodevices2 have attracted enormous attention in the quest for future nanoscale technologies. Because of their small band gaps and high charge-carrier mobilities, n-acenes (Figure ​(Figure1)1) and functionalized acenes are being considered as highly interesting building units for organic electronic materials.3 Zigzag nanoribbons feature remarkable spin-polarization and half-metallic properties4 and chemically fascinating challenges have to be solved in the synthesis of such extended polyaromatic hydrocarbons (PAHs).5 Moreover, the chemical properties of graphene nanoribbons may be linked to the reactivity of black carbon surfaces and PAHs in soils, affecting the accumulation of persistent organic pollutants with environmental implications.6

Photopolymers with tunable mechanical properties processed by laser-based high-resolution stereolithography

Abstract: Stereolithography (SLA) is a widely used technique for the fabrication of prototypes and small series products. The main advantage of SLA and related solid freeform fabrication (SFF) techniques is their capability to fabricate parts with complex shapes with high resolution. Although the spectrum of available materials has been widened in recent years, there is still a lack of materials which can be processed with SLA on a routine basis. In this work, a micro-SLA (?SLA) system is presented which can shape a number of different photopolymers with resolutions down to 5 ?m in the xy-plane and 10 ?m in the z-direction. The system is capable of processing various specifically tailored photopolymers which are based on acrylate chemistry. The materials processed for this work range from hybrid sol?gel materials (ORMOCER) to photo-crosslinked elastomers and hydrogels. The elastic moduli of these materials can be tuned over several orders of magnitude and range from 0.1 MPa to 8000 MPa. The reactivity of these monomers is sufficient for achieving writing speeds up to 500 mm s?1 which is comparable to commercial SLA resins. Various test structures are presented which show the suitability of the process for fabricating parts required for applications in micro-mechanical systems as well as for applications in biomedical engineering. Using the presented system, internal channels with a diameter of 50 ?m and a length of 1500 ?m could be fabricated. It was also possible to manufacture a micro-mechanical system consisting of a fixed axe and a free spinning turbine wheel.

Convergence analysis for finite element discretizations of the Helmholtz equation with Dirichlet-to-Neumann boundary conditions

Abstract: A rigorous convergence theory for Galerkin methods for a model Helmholtz problem in $\Bbb R^d, d \in \{1,2,3\}$ is presented. General conditions on the approximation properties of the approximation space are stated that ensure quasi-optimality of the method. As an application of the general theory, a full error analysis of the classical $hp$-version of the finite element method is presented for the model problem where the dependence on the mesh width $h,$ the approximation order $p,$ and the wave number $k$ is given explicitly. In particular, it is shown that quasi-optimality is obtained under the conditions that $kh/p$ is sufficiently small and the polynomial degree $p$ is at least $O(\log k).$