Yun-Peng Wang

Absence of a Dirac cone in silicene on Ag(111): First-principles density functional calculations with a modified effective band structure technique

We investigate the currently debated issue of the existence of the Dirac cone in silicene on an Ag(111) surface, using first-principles calculations based on density functional theory to obtain the band structure. By unfolding the band structure in the Brillouin zone of a supercell to that of a primitive cell, followed by projecting onto Ag and silicene subsystems, we demonstrate that the Dirac cone in silicene on Ag(111) is destroyed. Our results clearly indicate that the linear dispersions observed in both angular-resolved photoemission spectroscopy (ARPES) [P. Vogt et al., Phys. Rev. Lett. 108, 155501 (2012)] and scanning tunneling spectroscopy (STS) [L. Chen et al., Phys. Rev. Lett. 109, 056804 (2012)] come from the Ag substrate and not from silicene.

Resistance of Ag-silicene-Ag junctions: A combined nonequilibrium Green's function and Boltzmann transport study

For several years the electronic structure properties of the novel two-dimensional system silicene have been studied extensively. Electron transport across metal-silicence junctions, however, remains relatively unexplored. To address this issue, we developed and implemented a theoretical framework that utilizes the tight-binding Fisher-Lee relation to span non-equilibrium Green’s function (NEGF) techniques, the scattering method, and semiclassical Boltzmann transport theory. Within this hybrid quantum-classical, two-scale framework, we calculated transmission and reflection coefficients of monolayer and bilayer Ag-silicene-Ag junctions using the NEGF method in conjunction with density functional theory; derived and calculated the group velocities; and computed resistance using the semi-classical Boltzmann equation. We found that resistances of these junctions are ∼0.08 fΩm for monolayer silicene junctions and ∼0.3 fΩm for bilayer ones, factors of ∼8 and ∼2, respectively, smaller than Sharvin resistances estimated via the Landauer formalism.

Comparative investigation of electronic transport across three-dimensional nanojunctions

We carry out a layer-by-layer investigation to understand electron transport across metal-insulator-metal junctions. Interfacial structures of junctions were studied and characterized using first-principles density functional theory within the generalized gradient approximation. We found that as a function of the number of crystal layers the calculated transmission coefficients of multilayer silicene junctions decay much slower than for BN-based junctions We revisited the semiclassical Boltzmann theory of electronic transport and applied to multilayer silicene and BN-based junctions. The calculated resistance in the high-transmission regime is smaller than that provided by the Landauer formula. As the thickness of the barrier increases, results from the Boltzmann and the Landauer formulae converge. We provide a upper limit in the transmission coefficient below which, the Landauer method becomes valid. Quantitatively, when the transmission coefficient is lower than

Molecular analogue of the perovskite repeating unit and evidence for direct Mn III -Ce IV -Mn III exchange coupling pathway

\sim 0.05

Accurate projected augmented wave datasets for BaFe2As2

per channel, the error introduced by the Landauer formula for calculating the resistance is negligible. In addition, we found that the resistance of a junction is not entirely determined by the averaged transmission, but also by the distribution of the transmission over the first Brillouin zone.

Multicontrol Over Graphene–Molecule Hetereojunctions

The perovskite manganites AMnO3 and their doped analogues A1–xBxMnO3 (A and B = main group and lanthanide metals) are a fascinating family of magnetic oxides exhibiting a rich variety of properties. They are thus under intense investigation along multiple fronts, one of which is how their structural and physical properties are modified at the nanoscale. Here we show that the molecular compound [Ce3Mn8O8(O2CPh)18(HO2CPh)2] (CeIII2CeIVMnIII8; hereafter Ce3Mn8) bears a striking structural resemblance to the repeating unit seen in the perovskite manganites. Further, magnetic studies have established that Ce3Mn8 exhibits both the combination of pairwise MnIII2 ferromagnetic and antiferromagnetic exchange interactions, and the resultant spin vector alignments that are found within the 3-D C-type antiferromagnetic perovskites. First-principles theoretical calculations reveal not only the expected nearest-neighbor MnIII2 exchange couplings via superexchange pathways through bridging ligands but also an unusual, direct MnIII–CeIV–MnIII metal-to-metal channel involving the CeIVf orbitals.Perovskite manganites exhibit intriguing but poorly understood properties, including multiferroicity. Here, the authors synthesize a Ce3Mn8 cluster that structurally resembles a perovskite repeat unit, and use this molecular analogue to elucidate mechanisms driving bulk perovskite properties.

CONTROL OF CONDUCTANCE AND MAGNETORESISTANCE OF MOLECULAR JUNCTIONS

By carefully choosing parameters and including more semi-core orbitals as valence electrons, we have constructed a high-quality projected augmented wave dataset that yields results comparable to existing full-potential linearized augmented plane-wave calculations. The dataset was then applied to BaFe2As2 to study the effects of different levels of structure optimization, as well as different choices of exchange-correlation functionals. It was found that the local density approximation exchange-correlation functional fails to find the correct spin-density-wave anti-ferromagnetic (SDW-AFM) ground state under full optimization, while the Perdew?Burke?Ernzerhof (PBE) exchange-correlation functional obtains the correct state but significantly overestimates the magnetism. The electronic structure of the SDW-AFM state is not very sensitive to structure optimizations with the PBE exchange-correlation functional because the positions of the As atoms are preserved under optimizations. We further investigated the Ba atom diffusion process on the BaFe2As2 surface using the nudged elastic bands method. The Ba atom was found to be stable above the center of the squares formed by the surface As atoms, and a diffusion barrier of 1.2?eV was found. Our simulated scanning tunneling microscopy image suggests an ordered surface Ba atom structure, in agreement with Massee et al (2009 Phys. Rev. B 80 140507; van Heumen E et al 2010 arXiv:1009.3493v1).

Large extraordinary Hall effect in [Pt/Co](5)/Ru/[Co/Pt](5) multilayers

The vertical configuration is a powerful tool recently developed experimentally to investigate field effects in quasi two-dimensional systems. Prototype graphene-based vertical tunneling transistors can achieve an extraordinary control over current density utilizing gate voltages. In this work, we study theoretically vertical tunneling junctions that consist of a monolayer of photoswitchable aryl azobenzene molecules sandwiched between two sheets of graphene. Azobenzene molecules transform between trans and cis conformations upon photoexcitation, thus adding a second knob that enhances the control over physical properties of the junction. Using first-principles methods within the density functional framework, we perform simulations with the inclusion of field effects for both trans and cis configurations. We find that the interference of interface states resulting from molecule–graphene interactions at the Fermi energy introduces a dual-peak pattern in the transmission functions and dominates the transport properties of gate junctions, shedding new light on interfacial processes.

Adsorption of tris(8-hydroxyquinoline)aluminum molecules on cobalt surfaces

Spin-dependent transport through a family of biphenyl-dithiolate molecules was investigated theoretically. Torsion angles ϕ between the two phenyl rings in these molecules can be chemically tuned. Calculated conductances of molecular junctions are proportional to cos2ϕ, but their magnetoresistances are nearly independent on ϕ. Calculation results indicate that resistances and magnetoresistances of molecular junctions can be engineered separately.