T. O. Wehling

First-principles studies of water adsorption on graphene: The role of the substrate

We investigate the electronic properties of graphene upon water adsorption and study the influence of the SiO2 substrate in this context using density functional calculations. Perfect suspended graphene is rather insensitive to H2O adsorbates, as doping requires highly oriented H2O clusters. For graphene on a defective SiO2 substrate, we find a strongly different behavior: H2O adsorbates can shift the substrate’s impurity bands and change their hybridization with the graphene bands. In this way, H2O can lead to doping of graphene for much lower adsorbate concentrations than for free hanged graphene. The effect depends strongly on the microscopic substrate properties.

Strength of effective coulomb interactions in graphene and graphite

To obtain an effective many-body model of graphene and related materials from first principles we calculate the partially screened frequency dependent Coulomb interaction. In graphene, the effective on-site (Hubbard) interaction is U(00)=9.3  eV in close vicinity to the critical value separating conducting graphene from an insulating phase emphasizing the importance of nonlocal Coulomb terms. The nearest-neighbor Coulomb interaction strength is computed to U(01)=5.5  eV. In the long-wavelength limit, we find the effective background dielectric constant of graphite to be ϵ=2.5 in very good agreement with experiment.

Adhesion and electronic structure of graphene on hexagonal boron nitride substrates

We investigate the adsorption of graphene sheets on h-BN substrates by means of first-principles calculations in the framework of adiabatic connection fluctuation-dissipation theory in the random phase approximation. We obtain adhesion energies for different crystallographic stacking configurations and show that the interlayer bonding is due to long-range van der Waals forces. The interplay of elastic and adhesion energies is shown to lead to stacking disorder. Band structure calculations reveal substrate induced mass terms in graphene which change their sign with the stacking configuration. Our results are in agreement with recent STM experiments.

Impurities on graphene: Midgap states and migration barriers

Monovalent impurities on graphene can be divided into ionically and covalently bond impurities. The covalent impurities cause universal midgap states as the carbon atom next to the impurity is effectively decoupled from the graphene pi-bands. The electronic structure of graphene suppresses migration of these impurities and making the universal midgap very stable. This effect is strongest for neutral covalently bond impurities. The ionically bond impurities have migration barriers of typically less than 0.1eV. An asymmetry between anions and cations regarding their adsorption sites and topology of their potential energy landscape is predicted.

Resonant Scattering by Realistic Impurities in Graphene

We develop a first-principles theory of resonant impurities in graphene and show that a broad range of typical realistic impurities leads to the characteristic sublinear dependence of the conductivity on the carrier concentration. By means of density functional calculations various organic groups as well as adatoms such as H absorbed to graphene are shown to create midgap states within ±0.03  eV around the neutrality point. A low energy tight-binding description is mapped out. Boltzmann transport theory as well as a numerically exact Kubo formula approach yield the conductivity of graphene contaminated with these realistic impurities in accordance with recent experiments.

Current-Driven Spin Dynamics of Artificially Constructed Quantum Magnets

Atomic Spin-Transfer Torque Efficient electrical control of magnetism is a major goal of spin-based electronics. In many setups, spin-polarized current is used to switch the magnetization of a magnetic layer. This phenomenon, known as the spin-transfer torque (ST T), has mainly been studied on a larger scale. Working at the atomic scale, Khajetoorians et al. (p. 55) observed ST T in a structure of 5 to 7 magnetic atoms adsorbed on a metallic surface. The tip of a spinpolarized scanning tunneling microscope (STM) acted as the source of the spin-polarized current, and the reversal of the sign of the STM voltage resulted in the reversal of the preferred spin direction. By varying the temperature, the roles of different quantum processes were elucidated. These results will be of significance as spintronic components are further miniaturized. Spin-polarized scanning tunneling microscopy is used to exert the spin-transfer torque on a small atomic cluster. The future of nanoscale spin-based technologies hinges on a fundamental understanding and dynamic control of atomic-scale magnets. The role of the substrate conduction electrons on the dynamics of supported atomic magnets is still a question of interest lacking experimental insight. We characterized the temperature-dependent dynamical response of artificially constructed magnets, composed of a few exchange-coupled atomic spins adsorbed on a metallic substrate, to spin-polarized currents driven and read out by a magnetic scanning tunneling microscope tip. The dynamics, reflected by two-state spin noise, is quantified by a model that considers the interplay between quantum tunneling and sequential spin transitions driven by electron spin-flip processes and accounts for an observed spin-transfer torque effect.

Local electronic signatures of impurity states in graphene

Defects in graphene are of crucial importance for its electronic and magnetic properties. Here impurity effects on the electronic structure of surrounding carbon atoms are considered and the distribution of the local densities of states (LDOS) is calculated. As the full range from near field to the asymptotic regime is covered, our results are directly accessible by scanning tunnelling microscopy (STM). We also include exchange scattering at magnetic impurities and eludicate how strongly spin polarized impurity states arise.

Transition metal ad-atoms on graphene: Influence of local coulomb interactions on chemical bonding and magnetic moments

We address the interaction of graphene with 3d transition metal adatoms and the formation of localized magnetic moments by means of first-principles calculations. By comparing calculations within the generalized gradient approximation (GGA) to GGA+U we find that the electronic configuration and the adsorption geometries can be very sensitive to effects of local Coulomb interactions U in the transition metal d-orbitals. We find high-spin configurations being favorable for Cr and Mn adatoms independent of the functional. For Fe, Co and Ni different electronic configurations are realized depending on the value of the local Coulomb interaction strength U. Chemical control over the spin of the adatoms by hydrogenation is demonstrated: NiH and CoH adsorbed to graphene exhibit spin S=1/2 and S=1, respectively.

Ultrafast Transport of Laser-Excited Spin-Polarized Carriers in Au/Fe/MgO(001)

Hot carrier-induced spin dynamics is analyzed in epitaxial Au/Fe/MgO(001) by a time domain approach. We excite a spin current pulse in Fe by 35 fs laser pulses. The transient spin polarization, which is probed at the Au surface by optical second harmonic generation, changes its sign after a few hundred femtoseconds. This is explained by a competition of ballistic and diffusive propagation considering energy-dependent hot carrier relaxation rates. In addition, we observe the decay of the spin polarization within 1 ps, which is associated with the hot carrier spin relaxation time in Au.

Doping mechanisms in graphene-MoS2 hybrids

We present a joint theoretical and experimental investigation of charge doping and electronic potential landscapes in hybrid structures composed of graphene and semiconducting single layer molybdenum disulfide (MoS2). From first-principles simulations, we find electron doping of graphene due to the presence of rhenium impurities in MoS2. Furthermore, we show that MoS2 edges give rise to charge reordering and a potential shift in graphene, which can be controlled through external gate voltages. The interplay of edge and impurity effects allows the use of the graphene-MoS2 hybrid as a photodetector. Spatially resolved photocurrent signals can be used to resolve potential gradients and local doping levels in the sample.