arXiv subjects

U. Schwingenschlögl

Publications and source records attributed to U. Schwingenschlögl.

26 records · Page 2Linked to original sources

arXiv2013

Quasi free-standing silicene in a superlattice with hexagonal boron nitride

We study a superlattice of silicene and hexagonal boron nitride by first principles calculations and demonstrate that the interaction between the layers of the superlattice is very small. As a consequence, quasi free-standing silicene is realized in this superlattice. In particular, the Dirac cone of silicene is preserved, which has not been possible in any other system so far. Due to the wide band gap of hexagonal boron nitride, the superlattice realizes the characteristic physical phenomena of free-standing silicene. In particular, we address by model calculations the combined effect of the intrinsic spin-orbit coupling and an external electric field, which induces a transition from a metal to a topological insulator and further to a band insulator.

cond-mat.mes-hall↗

arXiv2013

Metal-insulator transition at the LaAlO3/SrTiO3 interface revisited: A hybrid functional study

We investigate the electronic properties of the LaAlO3/SrTiO3 interface using density functional theory. In contrast to previous studies, which relied on (semi-)local functionals and the GGA+U method, we here use a recently developed hybrid functional to determine the electronic structure. This approach offers the distinct advantage of accessing both the metallic and insulating multilayers on a parameter-free equal footing. As compared to calculations based on semilocal GGA functionals, our hybrid functional calculations lead to a considerably increased band gap for the insulating systems. The details of the electronic structure show substantial deviations from those obtained by GGA calculations. This casts severe doubts on all previous results based on semilocal functionals. In particular, corrections using rigid band shifts ("scissors operator") cannot lead to valid results.

cond-mat.str-el↗

arXiv2013

Surface antiferromagnetism and incipient metal-insulator transition in strained manganite films

Using first-principles calculations, we show that the (001) surface of the ferromagnet La$_{1-x}$Sr$_{x}$MnO$_{3}$ under epitaxial compression favors antiferromagnetic ordering in the first few surface layers, coexisting with ferromagnetic bulk order. The change in magnetic order is accompanied by a marked surface-related spectral pseudogap, signaling a metal-insulator near-transition at the surface. All surfaces exhibit huge outward relaxations an rumpling of the MnO$_{2}$ and LaO surface planes, causing a distinct shift in work function. The AF phase is an extreme surface-assisted case of the combination of in-plane AF super-exchange and vertical FM double-exchange couplings that rules magnetism in manganites under in-plane compression.

cond-mat.mtrl-sci↗

arXiv2012

Induced magnetism in transition metal intercalated graphitic systems

We investigate the structure, chemical bonding, electronic properties, and magnetic behavior of a three-dimensional graphitic network in aba and aaa stacking with intercalated transition metal atoms (Mn, Fe, Co, Ni, and Cu). Using density functional theory, we find induced spin-polarization of the C atoms both when the graphene sheets are aba stacked (forming graphite) and aaa stacked (resembling bi-layer graphene). The magnetic moment induced by Mn, Fe, and Co turns out to vary from 1.38 μB to 4.10 μB, whereas intercalation of Ni and Cu does not lead to a magnetic state. The selective induction of spin-polarization can be utilized in spintronic and nanoelectronic applications.

cond-mat.mes-hall↗

arXiv2010

Finite-bias electronic transport of molecules in water solution

The effects of water wetting conditions on the transport properties of molecular nano-junctions are investigated theoretically by using a combination of classical molecular dynamics and first principles electronic transport calculations. These are at the level of the non-equilibrium Green's function method implemented for self-interaction corrected density functional theory. We find that water effectively produces electrostatic gating to the molecular junction, with a gating potential determined by the time-averaged water dipole field. Such a field is large for the polar benzene-dithiol molecule, resulting in a transmission spectrum shifted by about 0.6 eV with respect to that of the dry junction. The situation is drastically different for carbon nanotubes (CNTs). In fact, because of their hydro-phobic nature the gating is almost negligible, so that the average transmission spectrum of wet Au/CNT/Au junctions is essentially the same as that in dry conditions. This suggests that CNTs can be used as molecular interconnects also in water-wet situations, for instance as tips for scanning tunnel microscopy in solution or in biological sensors.

cond-mat.mes-hall↗

arXiv2005

Charge order and spin-singlet pairs formation in Ti4O7

Charge ordering in the low-temperature triclinic structure of titanium oxide (Ti4O7) is investigated using the local density approximation (LDA)+U method. Although the total 3d charge separation is rather small, an orbital order parameter defined as the difference between t2g occupancies of Ti$^{3+}$ and Ti$^{4+}$ cations is large and gives direct evidence for charge ordering. Ti 4s and 4p states make a large contribution to the static "screening" of the total 3d charge difference. This effective charge screening leads to complete loss of the disproportionation between the charges at 3+ and 4+ Ti sites. The occupied t2g states of Ti$^{3+}$ cations are predominantly of $d_{xy}$ character and form a spin-singlet molecular orbital via strong direct antiferromagnetic exchange coupling between neighboring Ti(1) and Ti(3) sites, whereas the role of superexchange is found to be negligible.

cond-mat.str-el↗

arXiv2004

One-Electron Singular Branch Lines of the Hubbard Chain

The momentum and energy dependence of the weight distribution in the vicinity of the one-electron spectral-function singular branch lines of the 1D Hubbard model is studied for all values of the electronic density and on-site repulsion $U$. To achieve this goal we use the recently introduced pseudofermion dynamical theory. Our predictions agree quantitatively for the whole momentum and energy bandwidth with the peak dispersions observed by angle-resolved photoelectron spectroscopy in the quasi-1D organic conductor TTF-TCNQ.

cond-mat.str-el↗

arXiv2003

Electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ

We study the electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ by means of density-functional band theory, Hubbard model calculations, and angle-resolved photoelectron spectroscopy (ARPES). The experimental spectra reveal significant quantitative and qualitative discrepancies to band theory. We demonstrate that the dispersive behavior as well as the temperature-dependence of the spectra can be consistently explained by the finite-energy physics of the one-dimensional Hubbard model at metallic doping. The model description can even be made quantitative, if one accounts for an enhanced hopping integral at the surface, most likely caused by a relaxation of the topmost molecular layer. Within this interpretation the ARPES data provide spectroscopic evidence for the existence of spin-charge separation on an energy scale of the conduction band width. The failure of the one-dimensional Hubbard model for the {\it low-energy} spectral behavior is attributed to interchain coupling and the additional effect of electron-phonon interaction.

cond-mat.str-el↗