arXiv subjects

Cosima Schuster

Publications and source records attributed to Cosima Schuster.

16 recordsLinked to original sources

arXiv2012

Modified Li chains as atomic switches

We present electronic structure and transport calculations for hydrogen and lithium chains, using density functional theory and scattering theory on the Green's function level, to systematically study impurity effects on the transmission coefficient. To this end we address various impurity configurations. Tight-binding results allow us to interpret our the findings. We analyze under which circumstances impurities lead to level splitting and/or can be used to switch between metallic and insulating states. We also address the effects of strongly electronegative impurities.

cond-mat.mes-hall↗

arXiv2012

On the ferromagnetic character of (LaVO$_3$)$_m$/SrVO$_3$ superlattices

The experimental observation that vanadate superlattices (LaVO$_3$)$_m$/SrVO$_3$ show ferromagnetism up to room temperature [U.\ Lüders {\it et al.}, Phys.\ Rev.\ B {\bf 80}, 241102R (2009)] is investigated by means of density functional theory. First, the influence of the density functional on the electronic and magnetic structure of bulk ${\rm LaVO_3}$ is discussed. Second, the band structure of a (LaVO$_3$)$_m$/SrVO$_3$ slab for $m=5$ and 6 is calculated. Very different behaviors for odd and even values of $m$ are found: In the odd case lattice relaxation results into a buckling of the interface VO$_2$ layers that leads to spin-polarized interfaces. In the even case a decoupling of the interface VO$_2$ layers from the LaO layers is obtained, confining the interface electrons into a two-dimensional electron gas. The orbital reconstruction at the interface due to the lattice relaxation is discussed.

cond-mat.str-el↗

arXiv2012

Electronic Transport Through EuO Spin Filter Tunnel Junctions

Epitaxial spin filter tunnel junctions based on the ferromagnetic semiconductor europium monoxide, EuO, are investigated by means of density functional theory. In particular, we focus on the spin transport properties of Cu(100)/EuO(100)/Cu(100) junctions. The dependence of the transmission coefficient and the current-voltage curves on the interface spacing and on the EuO thickness is explained in terms of the EuO density of states and the complex band structure. Furthermore we also discuss the relation between the spin transport properties and the Cu-EuO interface geometry. The level alignment of the junction is sensitively affected by the interface spacing, since this determines the charge transfer between EuO and the Cu electrodes. Our calculations indicate that EuO epitaxially grown on Cu can act as a perfect spin filter, with a spin polarization of the current close to 100%, and with both the Eu-5d conduction band and the Eu-4f valence band states contributing to the coherent transport. For epitaxial EuO on Cu a symmetry filtering is observed, with the Δ_1 states dominating the transmission. This leads to a transport gap larger than the fundamental EuO band gap. Importantly the high spin polarization of the current is preserved up to large bias voltages.

cond-mat.mes-hall↗

arXiv2009

Stripe segregation and magnetic coupling in the nickelate La_5/3 Sr_1/3 NiO_4

We investigate the consequences of the stripe formation in the nickelate La5/3Sr1/3NiO4 for the details of its crystal structure and electronic states. Our data are based on numerical simulations within density functional theory (DFT) and the generalized gradient approximation (GGA). The on-site Coulomb interaction is included in terms of the LDA+U scheme. Structure optimization of preliminary experimental data indicates a strong interaction between the structural and electronic degrees of freedom. In particular, we find a segregation of the diagonal filled stripes induced by a delicate interplay with the magnetic coupling. Beyond the cooperative effect of stripe segregation and spin order, distinct octahedral distortions are essential for the formation of an insulating state.

cond-mat.str-el↗

arXiv2007

Self-assembled Pt nanowires on Ge(001): Relaxation effects

Absorption of Pt on the Ge(001) surface results in stable self-organized Pt nanowires, extending over some hundred nanometers. Based on band structure calculations within density functional theory and the generalized gradient approximation, the structural relaxation of the Ge--Pt surface is investigated. The surface reconstruction pattern obtained agrees well with findings from scanning tunneling microscopy. In particular, strong Pt--Pt dimerization is characteristical for the nanowires. The surface electronic structure is significantly perturbed due to Ge--Pt interaction, which induces remarkable shifts of Ge states towards the Fermi energy. As a consequence, the topmost Ge layers are subject to a metal-insulator transition.

cond-mat.mes-hall↗

arXiv2007

Geometry dependence of the charge transfer at YBa2Cu3O7-metal interfaces

Transport in electronic devices based on high-Tc superconductors depends critically on the charge redistribution at interfaces, since the band structure is modified on a local scale. Using the density functional theory approach for relaxed YBa2Cu3O7-metal contacts, the charge transfer into the superconductor has been studied in Appl. Phys. Lett. 90, 192502 (2007). In the present paper we discuss the systematics inherent in the local electronic structure of the near-contact YBa2Cu3O7 sites, in particular the dependence on the contact geometry.

cond-mat.supr-con↗

arXiv2007

Electronic structure of the c(4 x 2) reconstructed Ge(001) surface

We investigate the electronic structure of the c(4 x 2) reconstructed Ge(001) surface using band structure calculations based on density functional theory and the generalized gradient approximation. In particular, we take into account the details of surface reconstruction by means of well relaxed crystal structures. The surface electronic states are identified and the local density of states is compared to recent data from scanning tunneling spectroscopy. We obtain almost perfect agreement between theory and experiment for both the occupied and unoccupied states, which allows us to clarify the interpretation of the experimental data.

cond-mat.mtrl-sci↗

arXiv2007

Charge redistribution at YBa2Cu3O7-metal interfaces

Charge redistribution at interfaces is crucial for electronic applications of high-Tc superconductors, since the band structure is modified on a local scale. We address the normal-state electronic structure of YBa2Cu3O7 (YBCO) at an YBCO-metal contact by first principles calculations for prototypical interface configurations. We derive quantitative results for the intrinsic doping of the superconducting CuO2 planes due to the metal contact. Our findings can be explained in terms of a band-bending mechanism, complemented by local screening effects. We determine a net charge transfer of 0.09 to 0.13 electrons in favour of the intraplane Cu sites, depending on the interface orientation.

cond-mat.supr-con↗

arXiv2007

Geometry Effects at Atomic-Size Aluminium Contacts

We present electronic structure calculations for aluminium nanocontacts. Addressing the neck of the contact, we compare characteristic geometries to investigate the effects of the local aluminium coordination on the electronic states. We find that the Al 3pz states are very sensitive against modifications of the orbital overlap, which has serious consequences for the transport properties. Stretching of the contact shifts states towards the Fermi energy, leaving the system instable against ferromagnetic ordering. By spacial restriction, hybridization is locally suppressed at nanocontacts and the charge neutrality is violated. We discuss the influence of mechanical stress by means of quantitative results for the charge transfer.

cond-mat.mes-hall↗

arXiv2007

Intrinsic Doping at YBCO-metal Interfaces: Quantitative Results

Charge redistribution in high-Tc superconductors due to structural defects or interfaces is known to be crucial for electronic applications as the band structure is modified on a local scale. In order to investigate these effects in more detail, we address the normal-state properties of YBa2Cu3O7 (YBCO) in the vicinity of YBCO-metal interfaces by electronic structure calculations for well relaxed interface configurations. Our findings can be interpreted in terms of a band-bending mechanism complemented by local screening effects. We derive quantitative results for the intrinsic doping of the superconducting CuO2 planes due to the metal interface. In particular, the net charge transfer amounts to 0.13 electrons in favour of each intraplane copper site, which appears to be a typical value for interfaces of high-Tc superconductors, thus opening great possibilities for a systematic optimization of wires and tapes from high-Tc materials.

cond-mat.supr-con↗

arXiv2007

Electronic Structure of Spin-Chain Compounds: Common Features

The incommensurate composite systems M14Cu24O41 (M=Ca,Sr,La) are based on two fundamental structural units: CuO2 chains and Cu2O3 ladders. We present electronic structure calculations within density functional theory in order to address the interrelations between chains and ladders. The calculations account for the details of the crystal structure by means of a unit cell comprising 10 chain and 7 ladder units. It turns out that chains and ladders can be treated independently, which allows us to introduce a model system based on a reduced unit cell. For the CuO2 chains, we find two characteristic bands at the Fermi energy. Tight binding fits yield nearest and next-nearest neighbour interactions of the same order of magnitude.

cond-mat.str-el↗

arXiv2006

Electronic structure of the Au/benzene-1,4-dithiol/Au transport interface: Effects of chemical bonding

We present results of electronic structure calculations for well-relaxed Au/benzene-1,4-dithiol/Au molecular contacts, based on density functional theory and the generalized gradient approximation. Electronic states in the vicinity of the Fermi energy are mainly of Au 5d and S 3p symmetry, whereas contributions of C 2p states are very small. Hybridization between C 2p orbitals within the benzene substructure is strongly suppressed due to S-C bonding. In agreement with experimental findings, this corresponds to a significantly reduced conductance of the molecular contact.

cond-mat.mes-hall↗

arXiv2006

Chemical Bonding and Charge Distribution at Metallic Nanocontacts

We present results of electronic structure calculations for aluminium contacts of atomic size, based on density functional theory and the local density approximation. Addressing the atomic orbitals at the neck of the nanocontact, we find that the local band structure deviates strongly from bulk fcc aluminium. In particular, hybridization between Al 3s and 3p states is fully suppressed due to directed bonds at the contact. Moreover, a charge transfer of 0.6 electrons off the contact aluminium site is found. Both the suppressed hybridization and the violated charge neutrality are characteristic features of metallic nanocontacts. This fact has serious consequences for models aiming at a microscopic description of transport properties.

cond-mat.mes-hall↗

arXiv2005

Optical conductivity of a Hubbard ring with an impurity

We investigate the optical conductivity of a Hubbard ring in presence of an impurity by means of exact diagonalization using the Lanczos algorithm. We concentrate thereby on the first excited, open shell state, i.e. on twisted boundary conditions. In the metallic phase a substantial part of the spectral weight lies in the Drude peak, $σ(ω) = Dδ(ω)+ σ_{\rm reg}$. In the non-interacting system, the Drude peak can be visualized in our calculations at $ω= 0$ even for finite chain lengths. Adding the impurity, the main peak is shifted to finite frequencies proportional to the impurity strength. The shift indicates the energy gap of the disturbed finite size system, also in the interacting system. Thus, we can pursue in the optical conductivity for finite metallic systems the energy gap. However, due to level crossing, the impurity-induced peak arises in the interacting system first when a certain impurity strength is exceeded. In the Mott insulating phase, the impurity leads to an impurity state within the gap.

cond-mat.str-el↗

arXiv2002

Interacting particles at a metal-insulator transition

We study the influence of many-particle interaction in a system which, in the single particle case, exhibits a metal-insulator transition induced by a finite amount of onsite pontential fluctuations. Thereby, we consider the problem of interacting particles in the one-dimensional quasiperiodic Aubry-Andre chain. We employ the density-matrix renormalization scheme to investigate the finite particle density situation. In the case of incommensurate densities, the expected transition from the single-particle analysis is reproduced. Generally speaking, interaction does not alter the incommensurate transition. For commensurate densities, we map out the entire phase diagram and find that the transition into a metallic state occurs for attractive interactions and infinite small fluctuations -- in contrast to the case of incommensurate densities. Our results for commensurate densities also show agreement with a recent analytic renormalization group approach.

cond-mat.dis-nn↗

arXiv1998

Interaction versus dimerization in one-dimensional Fermi systems

In order to study the effect of interaction and lattice distortion on quantum coherence in one-dimensional Fermi systems, we calculate the ground state energy and the phase sensitivity of a ring of interacting spinless fermions on a dimerized lattice. Our numerical DMRG (Density Matrix Renormalization Group) studies, in which we keep up to 1000 states for systems of about 100 sites, are supplemented by analytical considerations using bosonization techniques. We find a delocalized phase for an attractive interaction, which differs from that obtained for random lattice distortions. The extension of this delocalized phase depends strongly on the dimerization induced modification of the interaction. Taking into account the harmonic lattice energy, we find a dimerized ground state for a repulsive interaction only. The dimerization is suppressed at half filling, when the correlation gap becomes large.

cond-mat.str-el↗