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Item type:Publication, Access status: without files , Geometric approach to design of photonic crystals and metamaterials via optimal toric packingsDesign of photonic crystals having large band gaps above a prescribed band is a well-known physical problem with many applications. A connection to an interesting mathematical construction was pointed out some time ago: It had been conjectured that optimal structures for gaps between bands n and n + 1 correspond, in the case of transverse magnetic polarization, to rods located at the generators of centroidal Voronoi tessellation (CVT) and, in the case of transverse electric polarization, to the walls of this tessellation. We discover another mathematical recipe which produces even better solutions: optimal packing of disks in square and triangular tori. It provides solutions qualitatively different from CVT, sometimes increasing the resulting band gap size several times. We therefore introduce two classes of periodic structures with remarkable properties which may find applications in many other areas of modern solid state physics: arrays of particles located at the centers of optimally packed disks on tori and nets corresponding to the walls of their Voronoi tessellations.Publicationtype: Journal Article17 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Influence of disorder at insulator-metal interface on spin transport(American Physical Society (APS), 2025-02-10); ; The spin transport is carried by spin-polarized electrons in the metal and by magnons in the insulator. We compute the spin current in the presence of a spin accumulation in the metal, caused by the spin Hall effect, and a thermal gradient using Fermi's golden rule in the presence of interfacial disorder. For a perfectly clean interface, the in-plane momentum is conserved by the electron-magnon scattering events that govern the spin transport through the interface. We calculate how the disorder-induced broadening of scattering matrix elements with respect to the in-plane momentum influences the spin current. As a general result, we observe that, specifically for high temperatures, one should expect a rather small effect of interface disorder on the measured spin current, while for small temperatures there is a significant reduction of a spin current with increasing disorder.Publicationtype: Journal Article13 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Oxygen-vacancy driven electron localization and itinerancy in rutile-based TiO2(American Physical Society (APS), 2017-05-30); ; ; Oxygen-deficient TiO2 in the rutile structure as well as the Ti3O5 Magnéli phase is investigated within the charge self-consistent combination of density functional theory with dynamical mean-field theory. It is shown that an isolated oxygen vacancy (VO) in titanium dioxide is not sufficient to metallize the system at low temperatures. In a semiconducting phase, an in-gap state is identified at IG∼-0.75eV in excellent agreement with experimental data. Bandlike impurity levels, resulting from a threefold VO-Ti coordination as well as entangled (t2g,eg) states, become localized due to site-dependent electronic correlations. Charge localization and strong orbital polarization occur in the VO-near Ti ions, the details of which can be modified by a variation of the correlated subspace. At higher oxygen vacancy concentration, a correlated metal is stabilized in the Magnéli phase. A VO-defect rutile structure of identical stoichiometry shows key differences in the orbital-resolved character and the spectral properties. Charge disproportionation is vital in the oxygen-deficient compounds, but obvious metal-insulator transitions driven or sustained by charge order are not identified.Publicationtype: Journal Article139 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Quantum many-body intermetallics: Phase stability of Fe3Al and small-gap formation in Fe2VAl(American Institute of Physics (AIP), 2017-01-10); ; ; ; Various intermetallic compounds harbor subtle electronic correlation effects. To elucidate this fact for the Fe-Al system, we perform a realistic many-body investigation based on a combination of density functional theory with dynamical mean-field theory in a charge self-consistent manner. A better characterization and understanding of the phase stability of bcc-based D03-Fe3Al through an improved description of the correlated charge density and the magnetic energy is achieved. Upon replacement of one Fe sublattice with V, the Heusler compound Fe2VAl is realized, known to display bad-metal behavior and increased specific heat. Here we document a charge-gap opening at low temperatures in line with previous experimental work. The gap structure does not match conventional band theory and is reminiscent of (pseudo)gap characteristics in correlated oxides.Publicationtype: Journal Article152 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Hubbard band versus oxygen vacancy states in the correlated electron metal SrVO3(2016-12-19); ; ; ; We study the effect of oxygen vacancies on the electronic structure of the model strongly correlated metal SrVO3. By means of angle-resolved photoemission spectroscopy (ARPES) synchrotron experiments, we investigate the systematic effect of the UV dose on the measured spectra. We observe the onset of a spurious dose-dependent prominent peak at an energy range where the lower Hubbard band has been previously reported in this compound, raising questions on its previous interpretation. By a careful analysis of the dose-dependent effects we succeed in disentangling the contributions coming from the oxygen vacancy states and from the lower Hubbard band. We obtain the ARPES spectrum in the limit of a negligible concentration of vacancies, where a clear signal of a lower Hubbard band remains. We support our study by means of state-of-the-art ab initio calculations that include correlation effects and the presence of oxygen vacancies. Our results underscore the relevance of potential spurious states affecting ARPES experiments in correlated metals, which are associated with the ubiquitous oxygen vacancies as extensively reported in the context of a two-dimensional electron gas at the surface of insulating d0 transition metal oxides.Publicationtype: Journal Article95 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Configurationally exhaustive first-principles study of a quaternary superalloy with a vast configuration space(2016-07-25); ; Exploration of the vast configuration space encountered in a multicomponent alloy is impossible without a suitable engine like the cluster-expansion (CE) method. While a CE ansatz can be formulated for an arbitrary number of components n, the combinatorial explosion of configuration space with increasing n can still be prohibitive. In this paper, we present a configurationally exhaustive study of a four-component nickel-based superalloy. We obtain all ground-state compounds, temperature- and concentration-dependent configurational energies, and micrographs of the γ/γ′ microstructure of the γ′-strengthened superalloy Ni-Al-Ta-W. Several phenomena that cannot be studied from the binary building blocks Ni-Al, Ni-W, or Ni-Ta alone are discussed, e.g., the suppression of γ′′ formation in Al-Ni-Ta-W, the effect of Ta on the γ′ composition, and the tungsten partitioning ratio as a function of both temperature and bulk composition.Publicationtype: Journal Article123 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Structure and ordering of oxygen on unreconstructed Ir(100)(2016-06-03); ; ; ; The adsorption of oxygen on the unreconstructed Ir(100) surface is investigated by a combination of experimental and theoretical methods comprising low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density-functional theory (DFT). Apart from the well-known (2×1)-O phase, we find a new (3×1)-O phase for temperatures below 180 K. Our DFT calculations predict these two phases to be the only fundamental ground states of the system in the coverage range up to 0.5 monolayers. An analysis of the phase transitions as a function of coverage reveals extended coexistence ranges between the clean surface and the 3×1 phase, or between the 3×1 and 2×1 phases, respectively. As a function of temperature, both phases undergo order-disorder transitions at about 650 K for the 2×1 phase and 180 K for the 3×1 phase, the latter being only partially reversible. The complete ordering behavior can be consistently explained by the energetics of model defect structures calculated by DFT. The crystallographic structure of the phases is determined by full-dynamical LEED intensity analyses, yielding excellent agreement between experimental and calculated data sets (Pendry R-factors: RP≈0.1). Oxygen was found to assume bridge sites always inducing significant relaxations within the substrate. The derived structural parameters coincide with the respective predictions from DFT on the picometer scale. It is also shown that remnants and precursor stages of the clean surface's reconstruction can only be detected through the application of real-space methods such as STM. The overarching objective of the present study is to demonstrate how precisely and accurately such an adsorption system can be investigated nowadays by using a concerted experimental and theoretical approach.Publicationtype: Journal Article128 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Self-assembly of nanoscale lateral segregation profiles(2016-04-07); ; ; ; The surface segregation profile of an intermetallic compound becomes vertically and laterally modulated upon epitaxial growth of a single-layer hexagonal boron nitride (h-BN) nanomesh. h-BN on PtRh(111) forms an 11-on-10 superhoneycomb, such as that on Rh(111) [Corso, Science 303, 217 (2004)SCIEAS0036-807510.1126/science.1091979], though with a smaller lattice constant of 2.73 nm. X-ray photoelectron diffraction shows that the h-BN layer reduces the Pt enrichment of the first layer by promoting site swapping of about 10 Pt-Rh pairs within the 10×10 unit cell between the first and second layers. This segregation profile is confirmed by density-functional-theory-based cluster-expansion calculations. Generally, a strong modulation of the h-BN bonding strength and a higher affinity to one of the constituents leads to self-assembly of top layer patches underneath the nanomesh pores.Publicationtype: Journal Article73 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Electron dichotomy on the SrTiO3 defect surface augmented by many-body effects(2016-03-09); ; ; ; In a common paradigm, the electronic structure of condensed matter is divided into weakly and strongly correlated compounds. While conventional band theory usually works well for the former class, many-body effects are essential for the latter. Materials such as the familiar SrTiO3 (STO) compound that bridge or even abandon this characterization scheme are highly interesting. Here, it is shown, by means of combining density functional theory with dynamical mean-field theory, that oxygen vacancies on the STO (001) surface give rise to a dichotomy of weakly correlated t2g low-energy quasiparticles and localized "in-gap" states of dominant eg character with a subtle correlation signature. We furthermore touch base with recent experimental work and study the surface instability towards magnetic order.Publicationtype: Journal Article74 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Versatile Approach to the Spin Dynamics in Correlated Electron Systems(American Physical Society (APS), 2016); ; ; Time-dependent spin phenomena in condensed matter are most often either described in the weakly correlated limit of metallic Stoner/Slater-like magnetism via band theory or in the strongly correlated limit of Heisenberg-like interacting spins in an insulator. However many experimental studies, e.g. of (de)magnetization processes, focus on itinerant local-moment materials such as transition metals and various of their compounds. We here present a general theoretical framework that is capable of addressing correlated spin dynamics, also in the presence of a vanishing charge gap. A real-space implementation of the time-dependent rotational-invariant slave boson methodology allows to treat non-equilibrium spins numerically fast and efficiently beyond linear response as well as beyond the band-theoretical or Heisenberg limit.Publicationtype: Journal Article94