arXiv subjects

U. Schwingenschloegl

Publications and source records attributed to U. Schwingenschloegl.

18 recordsLinked to original sources

arXiv2013

Constrained-DFT method for accurate energy level alignment of metal/molecule interfaces

We present a computational scheme for extracting the energy level alignment of a metal/molecule interface, based on constrained density functional theory and local exchange and correlation functionals. The method, applied here to benzene on Li(100), allows us to evaluate charge transfer energies, as well as the spatial distribution of the image charge induced on the metal surface. We systematically study the energies for charge transfer from the molecule to the substrate as function of the molecule-substrate distance, and investigate the effects arising from image charge confinement and local charge neutrality violation. For benzene on Li(100) we find that the image charge plane is located at about 1.8 Åabove the Li surface, and that our calculated charge transfer energies compare perfectly with those obtained with a classical electrostatic model having the image plane located at the same position. The methodology outlined here can be applied to study any metal/organic interface in the weak coupling limit at the computational cost of a total energy calculation. Most importantly, as the scheme is based on total energies and not on correcting the Kohn-Sham quasiparticle spectrum, accurate results can be obtained with local/semi-local exchange and correlation functionals. This enables a systematic approach to convergence.

cond-mat.mtrl-sci↗

arXiv2009

Electronic transport calculations for rough interfaces in Al, Cu, Ag, and Au

We present results of electronic structure and transport calculations for metallic interfaces, based on density functional theory and the non-equilibrium Green's functions method. Starting from the electronic structure of smooth Al, Cu, Ag, and Au interfaces, we study the effects of different kinds of interface roughness on the transmission coefficient and the I-V characteristic. In particular, we compare prototypical interface distortions, including vacancies and metallic impurities.

cond-mat.mes-hall↗

arXiv2009

Electronic states and self-doping at a 45^o YBa2Cu3O7 grain boundary

The charge redistribution at grain boundaries determines the applicability of high-Tc superconductors in electronic devices, because the transport across the grains can be hindered considerably. We investigate the local charge transfer and the modification of the electronic states in the vicinity of the grain-grain interface by first principles calculations for a (normal-state) 45^o tilted [001] grain boundary in YBa2Cu3O7. Our results explain the suppressed interface transport and the influence of grain boundary doping in a quantitative manner, in accordance with the experimental situation. The charge redistribution is found to be strongly inhomogeneous, which has a substantial effect on transport properties since it gives rise to a self-doping of 0.10 (+/- 0.02) holes per Cu atom.

cond-mat.mtrl-sci↗

arXiv2009

Interface relaxation and electrostatic charge depletion in the oxide heterostructure LaAlO3/SrTiO3

Performing an analysis within density functional theory, we develop insight into the structural and electronic properties of the oxide heterostructure LaAlO3/SrTiO3. Electrostatic surface effects are decomposed from the internal lattice distortion in order to clarify their interplay. We first study the interface relaxation by a multi-layer system without surface, and the surface effects, separately, by a substrate-film system. While elongation of the TiO6 octahedra at the interface enhances the metallicity, reduction of the film thickness has the opposite effect due to a growing charge depletion. The interplay of these two effects, as reflected by the full lattice relaxation in the substrate-film system, however, strongly depends on the film thickness. An inversion of the TiO6 distortion pattern for films thinner than four LaAlO3 layers results in an insulating state.

cond-mat.mtrl-sci↗

arXiv2009

Quantitative calculations of charge carrier densities in the depletion layers at YBa2Cu3O7-x interfaces

Charge redistribution at high-Tc superconductor interfaces and grain boundaries on the one hand is problematic for technological application. On the other hand, it gives rise to a great perspective for tailoring the local electronic states. For prototypical (metallic) interfaces, we derive quantitative results for the intrinsic doping of the CuO2-planes, i.e. for the deviation of the charge carrier density from the bulk value. Our data are based on ab-initio supercell calculations within density functional theory. A remarkable hole-underdoping is inherent to the clean interface, almost independent of the interface geometry. On the contrary, cation substitution as well as incorporation of electronegative impurities can compensate the intrinsic charge transfer and provide access to an exact adjustment of the superconductor's doping. The effects of oxygen deficiency are discussed.

cond-mat.mtrl-sci↗

arXiv2009

Exponential decay of relaxation effects at LaAlO3/SrTiO3 heterointerfaces

We study the decay of interface induced structural and electronic relaxation effects in epitaxial LaAlO3/SrTiO3 heterostructures. The results are based on first-principles band structure calculations for a multilayer configuration with an ultrathin LaAlO3 layer sandwiched between bulk-like SrTiO3 layers. We carry out the structure optimization for the heterointerface and investigate the electronic states of the conducting interface layer, which is found to extend over two SrTiO3 unit cells. The decay of atomic displacements is analyzed as a function of the distance to the interface, and the resulting exponential law is evaluated quantitatively.

cond-mat.mtrl-sci↗

arXiv2007

Confined Ge-Pt states in self-organized Pt nanowire arrays on Ge(001)

By means of band structure calculations within the density functional theory and the generalized gradient approximation, we investigate the electronic structure of self-organized Pt nanowires on the Ge(001) surface. In particular, we deal with a novel one-dimensional surface state confined in the nanowire array and clarify its origin. Due to large Pt contributions, the novel state is rather a mixed Ge-Pt hybrid state than a confined Ge surface state. Moreover, we compare our results to data from scanning tunneling microscopy.

cond-mat.mes-hall↗

arXiv2007

Surface Effects on Oxide Heterostructures

We report on surface effects on the electronic properties of interfaces in epitaxial LaAlO$_3$/SrTiO$_3$ heterostructures. Our results are based on first-principles electronic structure calculations for well-relaxed multilayer configurations, terminated by an ultrathin LaAlO$_3$ surface layer. On varying the thickness of this layer, we find that the interface conduction states are subject to almost rigid band shifts due to a modified Fermi energy. Confirming experimental data, the electronic properties of heterointerfaces therefore can be tuned systematically by alterating the surface-interface distance. We expect that this mechanism is very general and applies to most oxide heterostructures.

cond-mat.mtrl-sci↗

arXiv2007

Magnetic ground state of coupled edge-sharing CuO_2 spin-chains

By means of density functional theory, we investigate the magnetic ground state of edge-sharing CuO_2 spin-chains, as found in the (La,Ca,Sr)_14Cu_24O_41system, for instance. Our data rely on spin-polarized electronic structure calculations including onsite interaction (LDA+U) and an effective model for the interchain coupling. Strong doping dependence of the magnetic order is characteristic for edge-sharing CuO_2 spin-chains. We determine the ground state magnetic structure as function of the spin-chain filling and quantify the competing exchange interactions.

cond-mat.str-el↗

arXiv2007

One-dimensional Hubbard model at quarter filling on periodic potentials

Using the Hubbard chain at quarter filling as a model system, we study the ground state properties of highly doped antiferromagnets. In particular, the Hubbard chain at quarter filling is unstable against 2k_F- and 4k_F-periodic potentials, leading to a large variety of charge and spin ordered ground states. Employing the density matrix renormalization group method, we compare the energy gain of the ground state induced by different periodic potentials. For interacting systems the lowest energy is found for a 2k_F-periodic magnetic field, resulting in a band insulator with spin gap. For strong interaction, the 4k_F-periodic potential leads to a half-filled Heisenberg chain and thus to a Mott insulating state without spin gap. This ground state is more stable than the band insulating state caused by any non-magnetic 2k_F-periodic potential. Adding more electrons, a cluster-like ordering is preferred.

cond-mat.str-el↗

arXiv2006

The antiferromagnetic insulator Ca3FeRhO6: characterization and electronic structure calculations

We investigate the antiferromagnetic insulating nature of Ca3FeRhO6 both experimentally and theoretically. Susceptibility measurements reveal a Neel temperature T_N = 20 K, and a magnetic moment of 5.3 muB/f. u., while Moessbauer spectroscopy strongly suggests that the Fe ions, located in trigonal prismatic sites, are in a 3+ high spin state. Transport measurements display a simple Arrhenius law, with an activation energy of 0.2 eV. The experimental results are interpreted with LSDA band structure calculations, which confirm the Fe 3+ state, the high-spin/low-spin scenario, the antiferromagnetic ordering, and the value for the activation energy.

cond-mat.str-el↗

arXiv2005

Magnetic ordering in trigonal chain compounds

We present electronic structure calculations for the one-dimensional magnetic chain compounds Ca_3CoRhO_6 and Ca_3FeRhO_6. The calculations are based on density functional theory and the local density approximation. We use the augmented spherical wave (ASW) method. The observed alternation of low- and high-spin states along the Co-Rh and Fe-Rh chains is related to differences in the oxygen coordination of the transition metal sites. Due to strong hybridization the O 2p states are polarized, giving rise to extended localized magnetic moments centered at the high-spin sites. Strong metal-metal overlap along the chains leads to a substantial contribution of the low-spin Rh 4d_{3z^2-r^2} orbitals to the exchange coupling of the extended moments. Interestingly, this mechanism holds for both compounds, even though the coupling is ferromagnetic for the cobalt and antiferromagnetic for the iron compound. However, our results allow to understand the different types of coupling from the filling dependence of the electronic properties.

cond-mat.str-el↗

arXiv2005

Covalent bonding and hybridization effects in the corundum-type transition-metal oxides V2O3 and Ti2O3

The electronic structure of the corundum-type transition-metal oxides V2O3 and Ti2O3 is studied by means of the augmented spherical wave method, based on density-functional theory and the local density approximation. Comparing the results for the vanadate and the titanate allows us to understand the peculiar shape of the metal 3d a_{1g} density of states, which is present in both compounds. The a_{1g} states are subject to pronounced bonding-antibonding splitting due to metal-metal overlap along the c-axis of the corundum structure. However, the corresponding partial density of states is strongly asymmetric with considerably more weight on the high energy branch. We argue that this asymmetry is due to an unexpected broadening of the bonding a_{1g} states, which is caused by hybridization with the e_g^{pi} bands. In contrast, the antibonding a_{1g} states display no such hybridization and form a sharp peak. Our results shed new light on the role of the a_{1g} orbitals for the metal-insulator transitions of V2O3. In particular, due to a_{1g} - e_g^{pi} hybridization, an interpretation in terms of molecular orbital singlet states on the metal-metal pairs along the c-axis is not an adequate description.

cond-mat.str-el↗

arXiv2004

Charge order, orbital order, and electron localization in the Magneli phase Ti4O7

The metal-insulator transition of the Magneli phase Ti4O7 is studied by means of augmented spherical wave (ASW) electronic structure calculations as based on density functional theory and the local density approximation. The results show that the metal-insulator transition arises from a complex interplay of charge order, orbital order, and singlet formation of those Ti 3d states which mediate metal-metal bonding inside the four-atom chains characteristic of the material. Ti4O7 thus combines important aspects of Fe3O4 and VO2. While the charge ordering closely resembles that observed at the Verwey transition, the orbital order and singlet formation appear to be identical to the mechanisms driving the metal-insulator transition of vanadium dioxide.

cond-mat.str-el↗

arXiv2003

The Metal-Insulator Transition of the Magneli phase V_4O_7: Implications for V_2O_3

The metal-insulator transition (MIT) of the Magneli phase V_4O_7 is studied by means of electronic structure calculations using the augmented spherical wave method. The calculations are based on density functional theory and the local density approximation. Changes of the electronic structure at the MIT are discussed in relation to the structural transformations occuring simultaneously. The analysis is based on a unified point of view of the crystal structures of all Magneli phase compounds V_nO_2n-1 (3 =< n =< 9) as well as of VO_2 and V_2O_3. This allows to group the electronic bands into states behaving similar to the dioxide or the sesquioxide. In addition, the relationship between the structural and electronic properties near the MIT of these oxides can be studied on an equal footing. For V_4O_7, a strong influence of metal-metal bonding across octahedral faces is found for states both parallel and perpendicular to the hexagonal c_hex axis of V_2O_3. Furthermore, the structural changes at the MIT cause localization of those states, which mediate in-plane metal-metal bonding via octahedral edges. This band narrowing opens the way to an increased influence of electronic correlations, which are regarded as playing a key role for the MIT of V_2O_3.

cond-mat↗

arXiv2003

Octahedral Tilting in ACu3Ru4O12 (A=Na,Ca,Sr,La,Nd)

The perovskite-like compounds ACu3Ru4O12 (A=Na,Ca,Sr,La,Nd) are studied by means of density functional theory based electronic structure calculations using the augmented spherical wave (ASW) method. The electronic properties are strongly influenced by covalent type bonding between transition metal d and oxygen p states. The characteristic tilting of the RuO6 octahedra arises mainly from the Cu--O bonding, allowing for optimal bond lengths between these two atoms. Our results provide a deeper understanding of octahedral tilting as a universal mechanism, applicable to a large variety of multinary compounds.

cond-mat.mtrl-sci↗

arXiv2002

From VO2 to V2O3: The Metal-Insulator Transition of the Magneli Phase V6O11

The metal-insulator transition (MIT) of V6O11 is studied by means of electronic structure calculations using the augmented spherical wave method. The calculations are based on density functional theory and the local density approximation. Changes of the electronic structure at the MIT are discussed in relation to the structural transformations occuring simultaneously. The analysis is based on a unified point of view of the crystal structures of V6O11, VO2, and V2O3. This allows to group the electronic bands into states behaving similar to the dioxide or the sesquioxide. While the sesquioxide-like V 3d_yz states show rather weak changes on entering the low-temperature structure, some of the dioxide-like V 3d_x^2-y^2 states display splittings and shifts similar to those known from VO2. The MIT of V6O11 arises as a combination of changes appearing in both of these compounds. Our results shed new light onto the role of particular electronic states for the MIT of V2O3.

cond-mat.str-el↗

arXiv2002

Spectroscopic signatures of spin-charge separation in the quasi-one-dimensional organic conductor TTF-TCNQ

The electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ is studied by angle-resolved photoelectron spectroscopy (ARPES). The experimental spectra reveal significant discrepancies to band theory. We demonstrate that the measured dispersions can be consistently mapped onto the one-dimensional Hubbard model at finite doping. This interpretation is further supported by a remarkable transfer of spectral weight as function of temperature. The ARPES data thus show spectroscopic signatures of spin-charge separation on an energy scale of the conduction band width.

cond-mat.str-el↗