Magnetism of iron: from the bulk to the monoatomic wire

The magnetic properties of iron (spin and orbital magnetic moments, magnetocrystalline anisotropy energy) in various geometries and dimensionalities are investigated by using a parametrized tight-binding model in an $s$, $p$ and $d$ atomic orbital basis set including spin polarization and the effect of spin-orbit coupling. The validity of this model is well established by comparing the results with those obtained by using an ab-initio code. This model is applied to the study of iron in bulk bcc and fcc phases, $(110)$ and $(001)$ surfaces and to the monatomic wire, at several interatomic distances. New results are derived. The variation of the component of the orbital magnetic moment on the spin quantization axis has been studied as a function of depth, revealing a significant enhancement in the first two layers, especially for the $(001)$ surface. It is found that the magnetic anisotropy energy is drastically increased in the wire and can reach several meV. This is also true for the orbital moment, which in addition is highly anisotropic. Furthermore it is shown that when the spin quantization axis is neither parallel nor perpendicular to the wire the average orbital moment is not aligned with the spin quantization axis. At equilibrium distance the easy magnetization axis is along the wire but switches to the perpendicular direction under compression. The success of this model opens up the possibility of obtaining accurate results on other elements and systems with much more complex geometries

Electromechanical Oscillations in Bilayer Graphene

Nanoelectromechanical systems (NEMS) constitute a class of devices lying at the interface between fundamental research and technological applications. Integrating novel materials such as graphene into NEMS allows studying their mechanical and electromechanical characteristics at the nanoscale and addressing fundamental questions such as electron-phonon interaction and bandgap engineering. In this work, we integrate single and bilayer graphene into NEMS and probe the interplay between their mechanical and electrical properties. We show that the deflection of monolayer graphene nanoribbons results in a linear increase in their electrical resistance. Surprisingly, we observe oscillations in the electromechanical response of bilayer graphene. The proposed theoretical model suggests that these oscillations arise from quantum mechanical interference taking place due to the lateral displacement of graphene layers with respect to each other. Our work shows that bilayer graphene conceals unexpectedly rich and novel physics with promising potential in NEMS-based applications.Comment: First three authors contributed equall

Two-Orbital Kondo Screening in a Self-Assembled Metal Organic Complex

Iron atoms adsorbed on a Cu(111) surface and buried under polyphenyl dicarbonitrile molecules exhibit strongly spatial anisotropic Kondo features with directionally dependent Kondo temperatures and line shapes, as evidenced by scanning tunneling spectroscopy. First-principles calculations find nearly full polarization for the half-filled Fe 3d(xz) and 3d(yz) orbitals, which therefore can give rise to Kondo screening with the experimentally observed directional dependence and distinct Kondo temperatures. X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements confirm that the spin in both channels is effectively Kondo-screened. At ideal Fe coverage, these two-orbital Kondo impurities are arranged in a self-assembled honeycomb superlattice

Electronic Transport in Graphene with Aggregated Hydrogen Adatoms

Hydrogen adatoms and other species covalently bound to graphene act as resonant scattering centers affecting the electronic transport properties and inducing Anderson localization. We show that attractive interactions between adatoms on graphene and their diffusion mobility strongly modify the spatial distribution, thus fully eliminating isolated adatoms and increasing the population of larger size adatom aggregates. Such spatial correlation is found to strongly influence the electronic transport properties of disordered graphene. Our scaling analysis shows that such aggregation of adatoms increases conductance by up to several orders of magnitude and results in significant extension of the Anderson localization length in the strong localization regime. We introduce a simple definition of the effective adatom concentration x., which describes the transport properties of both random and correlated distributions of hydrogen adatoms on graphene across a broad range of concentrations

Transport électronique polarisé en spin dans les contacts atomiques de fer

PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Controlling edge states in the Kane-Mele model via edge chirality

We investigate the dependence of band dispersion of the quantum spin Hall effect (QSHE) edge states in the Kane-Mele model on crystallographic orientation of the edges. Band structures of the one-dimensional honeycomb lattice ribbons show the presence of the QSHE edge states at all orientations of the edges given sufficiently strong spin-orbit interactions. We find that the Fermi velocities of the QSHE edge-state bands increase monotonically when the edge orientation changes from zigzag (chirality angle theta = 0 degrees) to armchair (theta = 30 degrees). We propose a simple analytical model to explain the numerical results. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Electronic transport in iron atomic contacts: from the infinite wire to realistic geometries.

We present a theoretical study of spin polarized transport in Fe atomic contacts using a self-consistent tight-binding Hamiltonian in a non-orthogonal $s$, $p$ and $d$ basis set, the spin-polarization being obtained from a non-collinear Stoner-like model and the transmission probability from the Fisher-Lee formula. The behaviour of an infinite perfect Fe wire is compared with that of an infinite chain presenting geometric defects or magnetic walls and with that of a finite chain connected to infinite one-dimensional or three-dimensional leads. In the presence of defects or contacts the transmission probability of $d$ electrons is much more affected than that of $s$ electrons, in particular, contact effects may suppress some transmission channels. It is shown that the behaviour of an infinite wire is never obtained even in the limit of long chains connected to electrodes. The introduction of the spin-orbit coupling term in the Hamiltonian enables us to calculate the anisotropy of the magneto-resistance. Finally whereas the variation of the magneto-resistance as a function of the magnetization direction is step-like for an infinite wire, it becomes smooth in the presence of defects or contacts

Topological Fermi-arc surface resonances in bcc iron

The topological classification of matter has been extended to include semimetallic phases characterized by the presence of topologically protected band degeneracies. In Weyl semimetals, the foundational gapless topological phase, chiral degeneracies are isolated near the Fermi level and give rise to the Fermi-arc surface states. However, it is now recognized that chiral degeneracies are ubiquitous in the band structures of systems with broken spatial inversion (P) or time-reversal (T) symmetry. This leads to a broadly defined notion of topological metals, which implies the presence of disconnected Fermi surface sheets characterized by nonzero Chern numbers inherited from the enclosed chiral degeneracies. Here, we address the possibility of experimentally observing surface-related signatures of chiral degeneracies in metals. As a representative system we choose bcc iron, a well-studied archetypal ferromagnetic metal with two nontrivial electron pockets. We find that the (110) surface presents arclike resonances attached to the topologically nontrivial electron pockets. These Fermi-arc resonances are due to two different chiral degeneracies, a type-I elementary Weyl point and a type-II composite (Chern numbers +/- 2) Weyl point, located at slightly different energies close to the Fermi level. We further show that these surface resonances can be controlled by changing the orientation of magnetization, eventually being eliminated following a topological phase transition. Our study thus shows that the intricate Fermi-arc features can be observed in materials as simple as ferromagnetic iron and are possibly very common in polar and magnetic materials broadly speaking. Our study also provides methodological guidelines to identifying Fermi-arc surface states and resonances, establishing their topological origin and designing control protocols

Z2Pack: Numerical implementation of hybrid Wannier centers for identifying topological materials

The intense theoretical and experimental interest in topological insulators and semimetals has established band structure topology as a fundamental material property. Consequently, identifying band topologies has become an important, but often challenging, problem, with no exhaustive solution at the present time. In this work we compile a series of techniques, some previously known, that allow for a solution to this problem for a large set of the possible band topologies. The method is based on tracking hybrid Wannier charge centers computed for relevant Bloch states, and it works at all levels of materials modeling: continuous k . p models, tight-binding models, and ab initio calculations. We apply the method to compute and identify Chern, Z(2), and crystalline topological insulators, as well as topological semimetal phases, using real material examples. Moreover, we provide a numerical implementation of this technique (the Z2Pack software package) that is ideally suited for high-throughput screening of materials databases for compounds with nontrivial topologies. We expect that our work will allow researchers to (a) identify topological materials optimal for experimental probes, (b) classify existing compounds, and (c) reveal materials that host novel, not yet described, topological states