Josephson effect in graphene bilayers with adjustable relative displacement

The Josephson current is investigated in a superconducting graphene bilayer where the pristine graphene sheets can make in-plane or out-of-plane displacements with respect to each other. The superconductivity can be of intrinsic nature, or due to a proximity effect. The results demonstrate that the supercurrent responds qualitatively differently to relative displacement if the superconductivity is due to either intralayer or interlayer spin-singlet electron-electron pairing, thus providing a tool to distinguish between the two mechanisms. Specifically, both the AA and AB stacking orders are studied with antiferromagnetic spin alignment. For the AA stacking order with intralayer and on-site pairing no current reversal is found. In contrast, the supercurrent may switch its direction as a function of the in-plane displacement and out-of-plane interlayer coupling for the cases of AA ordering with interlayer pairing and AB ordering with either intralayer or interlayer pairing. In addition to sign reversal, the Josephson signal displays many characteristic fingerprints which derive directly from the pairing mechanism. Thus, measurements of the Josephson current as a function of the graphene bilayer displacement open up means for achieving a deeper insight of the superconducting pairing mechanism

Hydrogen evolution descriptors of 55-atom PtNi nanoclusters and interaction with graphite

Density functional simulations have been performed for Ptn Ni 55 − n clusters ( n = 0 , 12 , 20 , 28 , 42 , 55 ) to investigate their catalytic properties for the hydrogen evolution reaction (HER). Starting from the icosahedral Pt 12 Ni 43 , hydrogen adsorption energetics and electronic d-band descriptors indicate HER activity comparable to that of pure Pt 55 (distorted reduced core structure). The PtNi clusters accommodate a large number of adsorbed hydrogen before reaching a saturated coverage, corresponding to 3-4 H atoms per icosahedron facet (in total ∼70-80). The differential adsorption free energies are well within the window of | Δ G H | < 0.1 eV which is considered optimal for HER. The electronic descriptors show similarities with the platinum d-band, although the uncovered PtNi clusters are magnetic. Increasing hydrogen coverage suppresses magnetism and depletes electron density, resulting in expansion of the PtNi clusters. For a single H atom, the adsorption free energy varies between −0.32 ( Pt 12 Ni 43 ) and −0.59 eV ( Pt 55 ). The most stable adsorption site is Pt-Pt bridge for Pt-rich compositions and a hollow site surrounded by three Ni for Pt-poor compositions. A hydrogen molecule dissociates spontaneously on the Pt-rich clusters. The above HER activity predictions can be extended to PtNi on carbon support as the interaction with a graphite model structure (w/o vacancy defect) results in minor changes in the cluster properties only. The cluster-surface interaction is the strongest for Pt 55 due to its large facing facet and associated van der Waals forces.Peer reviewe

Strain-Engineered Widely-Tunable Perfect Absorption Angle in Black Phosphorus from First-Principles

Using the density functional theory of electronic structure, we compute the anisotropic dielectric response of bulk black phosphorus subject to strain. Employing the obtained permittivity tensor, we solve Maxwell's equations and study the electromagnetic response of a layered structure comprising a film of black phosphorus stacked on a metallic substrate. Our results reveal that a small compressive or tensile strain, $\sim 4\%$, exerted either perpendicular or in the plane to the black phosphorus growth direction, efficiently controls the epsilon-near-zero response, and allows a perfect absorption tuning from low-angle of the incident beam $\theta=0^\circ$ to high values $\theta\approx 90^\circ$ while switching the energy flow direction. Incorporating a spatially inhomogeneous strain model, we also find that for certain thicknesses of the black phosphorus, near-perfect absorption can be achieved through controlled variations of the in-plane strain. These findings can serve as guidelines for designing largely tunable perfect electromagnetic wave absorber devices.Comment: 15 pages, 12 figure

Electron localization in recrystallized models of the Ge2Sb2Te5 phase-change memory material

Understanding the relation between the structural disorder in the atomic geometry of the recrystallized state of phase-change memory materials and the localized states in the electronic structure is necessary not only for technological advances, but also essential to achieve a fundamental understanding of these materials. In this computational study, hybrid density-functional theory simulations are employed to ascertain the impact of antisite defects on the spatial localization of the electronic states in the bottom of the conduction band in recrystallized models of the prototypical phase-change material Ge2Sb2Te5. Te-Te homopolar bonds are the local defective atomic environments mainly responsible for the electron localization of the conduction-band-edge states in the simulated structures, while Sb-Te chains can also induce spatial localization. Unoccupied defect-related electronic states can emerge in the band gap during a crystallization event, while Sb-Sb homopolar bonds have been identified in the defect environment of a deep localized state.publishedVersionPeer reviewe

Sulfur-deficient edges as active sites for hydrogen evolution on MoS2

A grand-canonical approach is employed to calculate the voltage-dependent activation energy and estimate the kinetics of the hydrogen evolution reaction (HER) on intrinsic sites of MoS2, including edges of varying S-coverage as well as S-vacancies on the basal plane. Certain edge configurations are found to be vastly more active than others, namely S-deficient edges on the Mo-termination where, in the fully S-depleted case, HER can proceed with activation energy below 0.5 eV at an electrode potential of 0 V vs. SHE. There is a clear distinction between the performance of Mo-rich and S-rich adsorption sites, as HER at the latter sites is characterized by large (generally above 1.5 eV) Heyrovsky and Tafel energy barriers despite near-thermoneutral hydrogen adsorption energy. Thus, exposing Mo-atoms on the edges to which hydrogen can directly bind is crucial for efficient hydrogen evolution. While S-vacancies on the basal plane do expose Mo-rich sites, the energy barriers are still significant due to high coordination of the Mo atoms. Kinetic modelling based on the voltage-dependent reaction energetics gives a theoretical overpotential of 0.25 V and 1.09 V for the Mo-edge with no S atoms and the weakly sulfur-deficient (2% S-vacancies) basal plane, respectively, with Volmer-Heyrovsky being the dominant pathway. These values coincide well with reported experimentally measured values of the overpotential for the edges and basal plane. For the partly Mo-exposed edges, the calculated overpotential is 0.6-0.7 V while edges with only S-sites give overpotential exceeding that of the basal plane. These results show that the overpotential systematically decreases with increased sulfur-deficiency and reduced Mo-coordination. The fundamental difference between Mo- and S-rich sites suggests that catalyst design of transition metal dichalcogenides should be focused on facilitating and modifying the metal sites, rather than activating the chalcogen sites.Peer reviewe

Inherent electron and hole trapping in amorphous phase-change memory materials : Ge2Sb2Te5

While the amorphous state of a chalcogenide phase-change material is formed inside an electronic-memory device via Joule heating, caused by an applied voltage pulse, it is in the presence of excess field-induced electrons and holes. Here, hybrid density-functional-theory calculations for glassy Ge2Sb2Te5 demonstrate that extra electrons are trapped spontaneously, creating deep traps in the band gap. Hole self-trapping is also energetically favourable, producing states around midgap. The traps have a relatively low ionization energy, indicating that they can easily be thermally released. Near-linear triatomic Te-Ge/Sb-Te/Ge/Sb environments are the structural motifs where the extra electrons/holes are trapped inside the glass network, highlighting that the intrinsic axial bonds of octahedral-like sites in amorphous Ge2Sb2Te5 can serve as charge-trapping centres. Trapping of two electrons in a chain-like structure of connected triads results in breaking of some of these highly polarizable long bonds. These results establish the foundations of the origin of charge trapping in amorphous phase-change materials, and they may have important implications for our understanding of resistance drift in electronic-memory devices and of electronic-excitation-induced athermal melting.publishedVersionPeer reviewe

Graphite nucleation on (Al, Si, Mg)-nitrides : Elucidating the chemical interactions and turbostratic structures in spheroidal graphite cast irons

The ubiquitous (Al,Si,Mg)-nitride has been the focus of recent investigations of spheroidal graphite irons. In particular, because they have been systematically found in the nucleus of graphite spheroids. Despite having a similar crystal structure as graphite, their lattice parameter is vastly different. Since the crystallographic match is mainly used to justify the potential of nucleation sites, challenges have been encountered to explain the mechanism of graphite nucleation in this type of inclusion (microparticle). The present work reports the structure, composition, and interactions of these (Al,Si,Mg)-nitrides with graphite and other compounds, such as (Zr,Ti,Nb)-carbonitrides. The latter were the only inclusions with Zr that could be found, while the former inclusion could also be found in the core of graphite. The results confirm that the graphite layers close to the surface of the (Al,Si,Mg)-nitrides have a turbostratic structure. Organized graphite layers are only observed far away from the nitride nucleus. Density functional theory simulations of this interface showed that the interaction between the first graphene layers and the (Al,Si,Mg)-nitrides has a covalent nature, which could explain the turbostratic structure of the inner part of the graphite nodule. Therefore, nucleation of graphite on nuclei with a large lattice mismatch (low planar misfit) may be facilitated by the covalent bonding of carbon atoms on this substrate. These results explain the observed disorder at the interface as well as the deformation of the graphene layers.Peer reviewe

Photoelectron spectra of aluminum cluster anions: Temperature effects and ab initio simulations

Photoelectron (PES) spectra from aluminum cluster anions (from 12 to 15 atoms) at various temperature regimes, were studied using ab-initio molecular dynamics simulations and experimentally. The calculated PES spectra, obtained via shifting of the simulated electronic densities of states by the self-consistently determined values of the asymptotic exchange-correlation potential, agree well with the measured ones, allowing reliable structural assignments and theoretical estimation of the clusters' temperatures.Comment: RevTex, 3 gif figures. Scheduled for Oct 15, 1999, issue of Phys. Rev. B as Rapid Communicatio