arXiv subjects

Peter Blöchl

Publications and source records attributed to Peter Blöchl.

3 recordsLinked to original sources

arXiv2021

Hydrogen related defects in titanium dioxide at the interface to palladium

A metal oxide support and a catalytically active metal are the two main ingredients for complex catalysts used in heterogeneous catalysis. The gas environment can change the catalyst during the reaction, modifying its structural and electronic properties. Here, we use monochromated electron energy loss spectroscopy (EELS) to reveal hydrogen-pressure-dependent changes of the electronic structure at the Pd/rutile-TiO$_2$ interface in an environmental transmission electron microscope (ETEM). Hydrogen-induced changes are observed in rutile-TiO$_2$ within $2$~nm from the interface at $10$~Pa of hydrogen pressure, in the Ti $L_{3,2}$ EEL spectra. Lower pressures such as $1$~Pa show no changes in the EEL spectra. We attribute the observed changes in the EEL spectra to hydrogen-induced defects accumulating in the vicinity of the interface. Based on DFT calculations, we developed a thermodynamic multistate defect (TMD) model of the interface and the bulk of the rutile-TiO$_2$. This TMD model predicts high concentrations of positively charged defects accumulating at the interface. The presence of the Schottky barrier stabilizes these defects by significantly lowering their formation energy. Our findings provide important new insights into catalytic processes taking place at metal/metal oxide interfaces in hydrogen gas environments.

cond-mat.mtrl-sci↗

arXiv2017

Electronic structure of Pr$_{1-x}$Ca$_x$MnO$_3$

The electronic structure of Pr$_{1-x}$Ca$_x$MnO$_3$ has been investigated using a combination of first-principles calculations, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), electron-energy loss spectroscopy (EELS), and optical absorption. The full range of compositions, $x=0,1/2,1$, and a variety of magnetic orders have been covered. Jahn-Teller as well as Zener polaron orders are considered. The free parameters of the local hybrid density functionals used in this study has been determined by comparison with measured XPS spectra. A model Hamiltonian, valid for the entire doping range, has been extracted. A simple local-orbital picture of the electronic structure for the interpretation of experimental spectra is provided. The comparison of theoretical calculations and different experimental sprectra provide a detailed and consistent picture of the electronic structure. The large variations of measured optical absorption spectra are traced back to the coexistence of magnetic orders respectively to the occupation of local orbitals. A consistent treatment of the Coulomb interaction indicate a partial cancellation of Coulomb parameters and support the dominance of the electron-phonon coupling.

cond-mat.mtrl-sci↗

arXiv2015

Beam matching: a method to study phonon transport through interfaces and multilayer structures

Structuring materials is one mechanism to influence the thermal conductivity and thus thermoelectric efficiency. In order to investigate the scattering of phonons in multilayer structures we developed a beam matching technique, which is based on the concept of individual phonons and their scattering at interfaces. One of the major goals is to efficiently determine the complex band structure of the bulk materials. The complex band structure is determined using selected k-points on a triangulated grid in the complex plane of wave vectors perpendicular to the interface. Matching the phonon modes at an interface is translated to a singular value problem. Its null-vectors provide the coupling coefficients of the phonon modes across the interface. Besides giving explicit access to the modes as they scatter at the interface, the technique provides the transfer matrices, that provide the transmission coefficient of any multilayer structure. The transmission coefficient, in turn, yields the phononic thermal conductance for coherent transport. The knowledge of the matched phonons forms the basis of investigating incoherent transport under the influence of phonon-phonon or impurity scattering.

cond-mat.mes-hall↗