arXiv subjects

U. Schwingenschlögl

Publications and source records attributed to U. Schwingenschlögl.

At least 19 recordsLinked to original sources

arXiv2018

S-wave elastic scattering of ${\it o}$-Ps from $\text{H}_2$ at low energy

The confined variational method is applied to investigate the low-energy elastic scattering of ortho-positronium from $\text{H}_2$ by first-principles quantum mechanics. Describing the correlation effect with explicitly correlated Gaussians, we obtain accurate $S$-wave phase shifts and pick-off annihilation parameters for different incident momenta. By a least-squares fit of the data to the effective-range theory, we determine the $S$-wave scattering length, $A_s=2.06a_0$, and the zero-energy value of the pick-off annihilation parameter, $^1\!\text{Z}_\text{eff}=0.1858$. The obtained $^1\!\text{Z}_\text{eff}$ agrees well with the precise experimental value of $0.186(1)$ (J.\ Phys.\ B \textbf{16}, 4065 (1983)) and the obtained $A_s$ agrees well with the value of $2.1(2)a_0$ estimated from the average experimental momentum-transfer cross section for Ps energy below 0.3 eV (J.\ Phys.\ B \textbf{36}, 4191 (2003)).

physics.atom-ph↗

arXiv2017

Spin-polarized ballistic conduction through correlated Au-NiMnSb-Au heterostructures

We examine the ballistic conduction through Au-NiMnSb-Au heterostructures consisting of up to four units of NiMnSb in the scattering region. We investigate the dependence of the transmission function computed within the local spin density approximation (LSDA) of the density functional theory (DFT) on the number of half-metallic units in the scattering region. For a single NiMnSb unit the transmission function displays a spin polarization of around 50 % in a window of 1 eV centered around the Fermi level. By increasing the number of layers an almost complete spin polarization of the transmission is obtained in the same energy window. Supplementing the DFT-LSDA calculations with local electronic interactions, of Hubbard-type on the Mn sites, leads to a hybridization between the interface and many-body states. The significant reduction of the spin polarization seen in the density of states is not apparent in the spin-polarization of the conduction electron transmission, which suggests the localized nature of the hybridized interface and many-body induced states.

cond-mat.str-el↗

arXiv2015

Current developments in silicene and germanene

Exploration of the unusual properties of the two-dimensional materials silicene and germanene is a very active research field in recent years. This article therefore reviews the latest developments, focusing both on the fundamental materials properties and on possible applications.

cond-mat.mtrl-sci↗

arXiv2015

Electronic transport properties of (fluorinated) metal phthalocyanine

The magnetic and transport properties of the metal phthalocyanine (MPc) and F$_{16}$MPc (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn and Ag) families of molecules in contact with S-Au wires are investigated by density functional theory within the local density approximation, including local electronic correlations on the central metal atom. The magnetic moments are found to be considerably modified under fluorination. In addition, they do not depend exclusively on the configuration of the outer electronic shell of the central metal atom (as in isolated MPc and F$_{16}$MPc) but also on the interaction with the leads. Good agreement between the calculated conductance and experimental results is obtained. For M = Ag, a high spin filter efficiency and conductance is observed, giving rise to a potentially high sensitivity for chemical sensor applications.

cond-mat.mes-hall↗

arXiv2014

Quantum spin Hall states in graphene interacting with WS$_2$ or WSe$_2$

In the framework of first-principles calculations, we investigate the structural and electronic properties of graphene in contact with as well as sandwiched between WS$_2$ and WSe$_2$ monolayers. We report the modification of the band characteristics due to the interaction at the interface and demonstrate that the presence of the dichalcogenides results in quantum spin Hall states in the absence of a magnetic field.

cond-mat.mes-hall↗

arXiv2014

Weak interaction between germanene and GaAs(0001) by H intercalation: A route to exfoliation

The epitaxial growth of germanene on semiconducting GaAs(0001) substrate is studied using $ab$-$initio$ calculations. The interaction of the germanene is found to be minimum with the substrate. Our obtained results strongly indicate that it is possible to take off germanene from the substrate to make it free standing. We demonstrate a methods to achieve this aim. We also address the electronic structure of germanene on GaAs(0001) substrate and found to be the Dirac cone shift above the Fermi level with the spltting of 160 meV. Forthermore, we calculate the band structure of the free standing germanene with and without substrate and found band gap of 24 meV due to the intrinsic spin orbit coupling.

cond-mat.mes-hall↗

arXiv2014

Solid argon as a possible substrate for quasi-freestanding silicene

We study the structural and electronic properties of silicene on solid Ar(111) substrate using ab-initio calculations. We demonstrate that due to weak interaction quasi-freestanding silicene is realized in this system. The small binding energy of only $-32$ meV per Si atom also indicates the possibility to separate silicene from the solid Ar(111) substrate. In addition, a band gap of $11$ meV and a significant splitting of the energy levels due to spin-orbit coupling are observed.

cond-mat.mes-hall↗

arXiv2014

Band gap tunning in BN-doped graphene systems with high carrier mobility

Using density functional theory, we present a comparative study of the electronic properties of BN-doped graphene monolayer, bilayer, trilayer, and multilayer systems. In addition, we address a superlattice of pristine and BN-doped graphene. Five doping concentrations between 12.5% and 75% are considered, for which we obtain band gaps from 0.02 eV to 2.43 eV. We show that the effective mass varies between 0.007 and 0.209 free electron masses, resembling a high mobility of the charge carriers.

cond-mat.mes-hall↗

arXiv2014

Prediction of a quantum anomalous Hall state in Co decorated silicene

Based on first-principles calculations, we demonstrate that Co decorated silicene can host a quantum anomalous Hall state. The exchange field induced by the Co atoms combined with the strong spin orbit coupling of the silicene opens a nontrivial band gap at the K-point. As compared to other transition metals, Co decorated silicene is unique in this respect, since usually hybridization and spin-polarization induced in the silicene suppress a quantum anomalous Hall state.

cond-mat.mes-hall↗

arXiv2013

Electronic properties of Mn decorated silicene on hexagonal boron nitride

We study silicene on hexagonal boron nitride, using first principles calculations. Since hexagonal boron nitride is semiconducting, the interaction with silicene is weaker than for metallic substrates. It therefore is possible to open a 50 meV band gap in the silicene. We further address the effect of Mn decoration by determining the onsite Hubbard interaction parameter, which turns out to differ significantly for decoration at the top and hollow sites. The induced magnetism in the system is analyzed in detail.

cond-mat.mes-hall↗

arXiv2013

Substrate enhanced superconductivity in Li-decorated graphene

We investigate the role of the substrate for the strength of the electon phonon coupling in Li-decorated graphene. We find that the interaction with a $h$-BN substrate leads to a significant enhancement from $λ_0=0.62$ to $λ_1=0.67$, which corresponds to a $25\%$ increase of the transition temperature from $T_{c0}=10.33$ K to $T_{c1}=12.98$ K. The superconducting gaps amount to 1.56 meV (suspended) and 1.98 meV (supported). These findings open up a new route to enhanced superconducting transition temperatures in graphene-based materials by substrate engineering.

cond-mat.supr-con↗

arXiv2013

Colossal Thermoelectric Power Factor in K$_{7/8}$RhO$_2$

We discuss the thermoelectric and optical properties of layered K$_{x}$RhO$_{2}$ (\emph{x} = 1/2 and 7/8) in terms of the electronic structure determined by first principles calculations as well as Boltzmann transport theory. Our optimized lattice constants differ significantly from the experiment, but result in optical and transport properties close to the experiment. The main contribution to the optical spectra are due to intra and inter-band transitions between the Rh 4\emph{d} and O 2\emph{p} states. We find a similar power factor for pristine K$_{x}$RhO$_{2}$ at low and high cation concentartions. Our transport results of hydrated K$_{x}$RhO$_{2}$ at room temperature show highest value of the power factor among the hole-type materials. Specially at 100 K, we obtain a value of 3$\times$10$^{-3}$ K$^{-1}$ for K$_{7/8}$RhO$_{2}$, which is larger than that of Na$_{0.88}$CoO$_{2}$ {[}M. Lee \emph{et al}., Nat. Mater. 5, 537 (2006){]}. In general, the electronic and optical properties of K$_{x}$RhO$_{2}$ are similar to Na$_{x}$CoO$_{2}$ with enhanced transport properties in the hydrated phase.

cond-mat.mtrl-sci↗

arXiv2013

Half-metallicity and giant magneto-optical Kerr effect in N-doped NaTaO$_3$

We employ density functional theory using the modified Becke-Johnson (mBJ) approach to investigate the electronic and magneto-optical properties of N-doped NaTaO$_3$. The mBJ results reveal a half metallic nature of NaTaO$_2$N, in contrast to results obtained by the generalized gradient approximation. We find a giant polar Kerr rotation of 2.16$^{\circ}$ at 725 nm wave length (visible region), which is high as compared to other half metallic perovskites as well as to the prototypical half metal PtMnSb.

cond-mat.mtrl-sci↗

arXiv2013

Influence of substitution on the optical properties of functionalized pentacene monomers and crystals: Experiment and theory

The influence of solubilizing substitutional groups on the electronic and optical properties of functionalized pentacene molecules and crystals have been investigated. Density functional theory is used to calculate the electronic and optical properties of pentacene, TIBS-CF$_{3}$-pentacene, and TIPS-pentacene. The results are compared with experimental absorption spectra of solutions and the complex dielectric function of thin films in the 1 eV to 3 eV energy range. In all cases, the band gaps of the isolated molecules are found to be smaller than those of the crystals. The absorption spectra and dielectric function are interpreted in terms of the transitions between the highest occupied molecular orbitals and lowest unoccupied molecular orbitals. The bands associated to C and Si atoms connecting the functional side group to the pentacene in the (6,13) positions are found to be the main contributors to the optical transitions. The calculated dielectric functions of thin films agree with the experimental results. A redshift is observed in crystals as compared to molecules in experiment and theory both, where the amplitude depends on the packing structure.

cond-mat.mtrl-sci↗

arXiv2013

Impact of lattice strain on the tunnel magneto-resistance in Fe/Insulator/Fe and Fe/Insulator/La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ magnetic tunnel junctions

The objective of this work is to describe the tunnel electron current in single barrier magnetic tunnel junctions within a new approach that goes beyond the single-band transport model. We propose a ballistic multi-channel electron transport model that can explain the influence of in-plane lattice strain on the tunnel magnetoresistance as well as the asymmetric voltage behavior. We consider as an example single crystal magnetic Fe(110) electrodes for Fe/Insulator/Fe and Fe/Insulator/La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ tunnel junctions, where the electronic band structures of Fe and La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ are derived by \it{ab-initio} calculations.

cond-mat.mtrl-sci↗

arXiv2013

Modelling magnetism of C at O and B monovacancies in graphene

The presence of defects can introduce important changes in the electronic structure of graphene, leading to phenomena such as C magnetism. In addition, vacancies are reactive and permit the incorporation of dopants. This paper discusses the electronic properties of defective graphene for O and B decoration. Phonon calculations allow us to address directly the stability of the systems under study. We show that it is possible to obtain magnetic solutions with and without dangling bonds, demonstrating that C magnetism can be achieved in the presence of B and O.

cond-mat.mes-hall↗

arXiv2013

Stability of germanene under tensile strain

The stability of germanene under biaxial tensile strain and the accompanying modifications of the electronic properties are studied by density functional theory. The phonon spectrum shows that up to $16\%$ strain the germanene lattice is stable, where the Dirac cone shifts towards higher energy and hole-doped Dirac states are achieved. The latter is due to weakening of the Ge-Ge bonds and reduction of the s-p hybridization. Our calculated Grüneisen parameter shows a similar dependence on the strain as reported for silicene (which is different from that of graphene).

cond-mat.mtrl-sci↗

arXiv2013

Hole doped Dirac states in silicene by biaxial tensile strain

The effects of biaxial tensile strain on the structure, electronic states, and mechanical properties of silicene are studied by ab-initio calculations. Our results show that up to 5% strain the Dirac cone remains essentially at the Fermi level, while higher strain induces hole doping because of weakening of the Si$-$Si bonds. We demonstrate that the silicene lattice is stable up to 17% strain. It is noted that the buckling first decreases with the strain (up to 10%) and then increases again, which is accompanied by a band gap variation. We also calculate the Grüneisen parameter and demonstrate a strain dependence similar to that of graphene.

cond-mat.mes-hall↗