Electron transport through quantum wires and point contacts

We have studied quantum wires using the Green's function technique and the density-functional theory, calculating the electronic structure and the conductance. All the numerics are implemented using the finite-element method with a high-order polynomial basis. For short wires, i.e. quantum point contacts, the zero-bias conductance shows, as a function of the gate voltage and at a finite temperature, a plateau at around 0.7G_0. (G_0 = 2e^2/h is the quantum conductance). The behavior, which is caused in our mean-field model by spontaneous spin polarization in the constriction, is reminiscent of the so-called 0.7-anomaly observed in experiments. In our model the temperature and the wire length affect the conductance-gate voltage curves in the same way as in the measured data.Comment: 8 page

Large-scale surface reconstruction energetics of Pt(100) and Au(100) by all-electron DFT

The low-index surfaces of Au and Pt all tend to reconstruct, a fact that is of key importance in many nanostructure, catalytic, and electrochemical applications. Remarkably, some significant questions regarding their structural energies remain even today, in particular for the large-scale quasihexagonal reconstructed (100) surfaces: Rather dissimilar reconstruction energies for Au and Pt in available experiments, and experiment and theory do not match for Pt. We here show by all-electron density-functional theory that only large enough "(5 x N)" approximant supercells capture the qualitative reconstruction energy trend between Au(100) and Pt(100), in contrast to what is often done in the theoretical literature. Their magnitudes are then in fact similar, and closer to the measured value for Pt(100); our calculations achieve excellent agreement with known geometric characteristics and provide direct evidence for the electronic reconstruction driving force.Comment: updated version - also includes EPAPS information as auxiliary file; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

Spin-asymmetric graphene nanoribbons in graphane on silicon dioxide

Hydrogenated graphene, graphane, is studied on oxygen-terminated silicon dioxide substrate using ab initio calculations. A structure with hydrogenation only on one side of the graphene layer is found stable and its hydrogen configurations are presented. Additionally, we form zigzag graphene nanoribbons by selectively removing hydrogens from the epitaxial graphane layer. In these ribbons, the spin degeneracy of the freestanding antiferromagnetic zigzag ribbons is broken, and band gaps of different magnitude emerge for the opposite spin species. This degeneracy breaking is due to a charge imbalance in the substrate below the ribbon, introduced through the asymmetric alignment of the substrate atoms with respect to the edges of the graphene ribbon

Finite-element implementation for electron transport in nanostructures

We have modeled transport properties of nanostructures using Green’s-function method within the framework of the density-functional theory. The scheme is computationally demanding, so numerical methods have to be chosen carefully. A typical solution to the numerical burden is to use a special basis-function set, which is tailored to the problem in question, for example, the atomic-orbital basis. In this paper we present our solution to the problem. We have used the finite-element method with a hierarchical high-order polynomial basis, the so-called p elements. This method allows the discretation error to be controlled in a systematic way. The p elements work so efficiently that they can be used to solve interesting nanosystems described by nonlocal pseudopotentials. We demonstrate the potential of the implementation with two different systems. As a test system a simple Na-atom chain between two leads is modeled and the results are compared with several previous calculations. Secondly, we consider a thin hafnium dioxide (HfO2) layer on a silicon surface as a model for a gate structure of the next generation of microelectronics.Peer reviewe

Conductance oscillations in metallic nanocontacts

We examine the conductance properties of a chain of Na atoms between two metallic leads in the limit of low bias. Resonant states corresponding to the conductance channel and the local charge neutrality condition cause conductance oscillations as a function of the number of atoms in the chain. Moreover, the geometrical shape of the contact leads influences the conductivity by giving rise to additional oscillations as a function of the lead opening angle.Peer reviewe

Spin dependent electron transport through a magnetic resonant tunneling diode

Electron transport properties in nanostructures can be modeled, for example, by using the semiclassical Wigner formalism or the quantum mechanical Green's functions formalism. We compare the performance and the results of these methods in the case of magnetic resonant-tunneling diodes. We have implemented the two methods within the self-consistent spin-density-functional theory. Our numerical implementation of the Wigner formalism is based on the finite-difference scheme whereas for the Green's function formalism the finite-element method is used. As a specific application, we consider the device studied by Slobodskyy et all. [Phys. Rev. Lett. 90, 246601 (2003)] and analyze their experimental results. The Wigner and Green's functions formalisms give similar electron densities and potentials but, surprisingly, the former method requires much more computer resources in order to obtain numerically accurate results for currents. Both of the formalisms can successfully be used to model magnetic resonant tunneling diode structures.Comment: 13 pages and 12 figure

The impact of TMS and PNS frequencies on MEP potentiation in PAS with high-frequency peripheral component

Paired associative stimulation (PAS) combines transcranial magnetic stimulation (TMS) and peripheral nerve stimulation (PNS) to induce plastic changes in the corticospinal tract. PAS employing single 0.2-Hz TMS pulses synchronized with the first pulse of 50–100 Hz PNS trains potentiates motor-evoked potentials (MEPs) in a stable manner in healthy participants and enhances voluntary motor output in spinal cord injury (SCI) patients. We further investigated the impact of settings of this PAS variant on MEP potentiation in healthy subjects. In experiment 1, we compared 0.2-Hz vs 0.4-Hz PAS. In experiment 2, PNS frequencies of 100 Hz, 200 Hz, and 400 Hz were compared. In experiment 3, we added a second TMS pulse. When compared with 0.4-Hz PAS, 0.2-Hz PAS was significantly more effective after 30 minutes (p = 0.05) and 60 minutes (p = 0.014). MEP potentiation by PAS with 100-Hz and 200-Hz PNS did not differ. PAS with 400-Hz PNS was less effective than 100-Hz (p = 0.023) and 200-Hz (p = 0.013) PNS. Adding an extra TMS pulse rendered PAS strongly inhibitory. These negative findings demonstrate that the 0.2-Hz PAS with 100-Hz PNS previously used in clinical studies is optimal and the modifications employed here do not enhance its efficacy.Peer reviewe

Core level binding energies of functionalized and defective graphene

X-ray photoelectron spectroscopy (XPS) is a widely used tool for studying the chemical composition of materials and it is a standard technique in surface science and technology. XPS is particularly useful for characterizing nanostructures such as carbon nanomaterials due to their reduced dimensionality. In order to assign the measured binding energies to specific bonding environments, reference energy values need to be known. Experimental measurements of the core level signals of the elements present in novel materials such as graphene have often been compared to values measured for molecules, or calculated for finite clusters. Here we have calculated core level binding energies for variously functionalized or defected graphene by delta Kohn–Sham total energy differences in the real-space grid-based projector-augmented wave density functional theory code (GPAW). To accurately model extended systems, we applied periodic boundary conditions in large unit cells to avoid computational artifacts. In select cases, we compared the results to all-electron calculations using an ab initio molecular simulations (FHI-aims) code. We calculated the carbon and oxygen 1s core level binding energies for oxygen and hydrogen functionalities such as graphane-like hydrogenation, and epoxide, hydroxide and carboxylic functional groups. In all cases, we considered binding energy contributions arising from carbon atoms up to the third nearest neighbor from the functional group, and plotted C 1s line shapes by using experimentally realistic broadenings. Furthermore, we simulated the simplest atomic defects, namely single and double vacancies and the Stone–Thrower–Wales defect. Finally, we studied modifications of a reactive single vacancy with O and H functionalities, and compared the calculated values to data found in the literature.Peer reviewe