In this work, we have systematically studied structural, electronic and magnetic properties of atomic scale defects in 2D transition metal dichalcogenides MX 2 , (M = Mo and W; X = S, Se and Te) by density functional theory. Various types of defects, e.g., X vacancy, X interstitial, M vacancy, M interstitial, MX and XX double vacancies have been considered. It has been found that the X interstitial has the lowest formation energy (∼ 1 eV) for all the systems in the X-rich condition whereas for M-rich condition, X vacancy has the lowest formation energy except for MTe 2 systems. Both these defects have very high equilibrium defect concentrations at growth temperatures (1000K-1200K) reported in literature. A pair of defects, e.g., two X vacancies or one M and one X vacancies tend to occupy the nearest possible distance. No trace of magnetism has been found for any one of the defects considered. Apart from X interstitial, all other defects have defect states appearing in the band gap, which can greatly affect the electronic and optical properties of the pristine systems. Our calculated optical properties show that the defect states cause optical transitions at ∼ 1.0 eV, which can be beneficial for light emitting devices. The results of our systematic study are expected to guide the experimental nanoengineering of defects to achieve suitable properties related to band gap modifications and characterization of defect fingerprints via optical absorption measurements. arXiv:1509.01445v2 [cond-mat.mes-hall]

There exists an extensive literature on the electronic structure of transition-metal phthalocyanines (TMPcs), either as single molecules or adsorbed on surfaces, where explicit intra-atomic Coulomb interactions of the strongly correlated orbitals are included in the form of a Hubbard U term. The choice of U is, to a large extent, based solely on previous values reported in the literature for similar systems. Here, we provide a systematic analysis of the influence of electron correlation on the electronic structure and magnetism of several TMPcs (MnPc, FePc, CoPc, NiPc, and CuPc). By comparing calculated results to valence-band photoelectron spectroscopy measurements, and by determining the Hubbard term from linear response, we show that the choice of U is not as straightforward and can be different for each different TMPc. This, in turn, highlights the importance of individually estimating the value of U for each system before performing any further analysis and shows how this value can influence the final results.

While graphene is a semi-metal, a recently synthesized hydrogenated graphene called graphane is an insulator. We have probed the transformation of graphene upon hydrogenation to graphane within the framework of density functional theory. By analysing the electronic structure for 18 different hydrogen concentrations, we bring out some novel features of this transition. Our results show that the hydrogenation favours clustered configurations leading to the formation of compact islands. The analysis of the charge density and electron localization function (ELF) indicates that, as hydrogen coverage increases, the semi-metal turns into a metal, showing a delocalized charge density, then transforms into an insulator. The metallic phase is spatially inhomogeneous in the sense it contains islands of insulating regions formed by hydrogenated carbon atoms and metallic channels formed by contiguous bare carbon atoms. It turns out that it is possible to pattern the graphene sheet to tune the electronic structure. For example, removal of hydrogen atoms along the diagonal of the unit cell, yielding an armchair pattern at the edge, gives rise to a bandgap of 1.4 eV. We also show that a weak ferromagnetic state exists even for a large hydrogen coverage whenever there is a sublattice imbalance in the presence of an odd number of hydrogen atoms.

We show that thermal stability of magnetic skyrmions can be strongly affected by entropic effects. The lifetimes of isolated skyrmions in atomic Pd/Fe bilayers on Ir(111) and on Rh(111) are calculated in the framework of harmonic transition state theory based on an atomistic spin model parametrized from density functional theory. Depending on the system the attempt frequency for skyrmion collapse can change by up to nine orders of magnitude with the strength of the applied magnetic field. We demonstrate that this effect is due to a drastic change of entropy with skyrmion radius which opens a novel route towards stabilizing sub-10 nm skyrmions at room temperature.

Transition-metal interfaces and multilayers are a promising class of systems to realize nanometer-sized, stable magnetic skyrmions for future spintronic devices. For room temperature applications, it is crucial to understand the interactions which control the stability of isolated skyrmions. Typically, skyrmion properties are explained by the interplay of pair-wise exchange interactions, the Dzyaloshinskii-Moriya interaction and the magnetocrystalline anisotropy energy. Here, we demonstrate that higher-order exchange interactions – which have so far been neglected – can play a key role for the stability of skyrmions. We use an atomistic spin model parametrized from first-principles and compare three different ultrathin film systems. We consider all fourth-order exchange interactions and show that, in particular, the four-site four spin interaction has a large effect on the energy barrier preventing skyrmion and antiskyrmion collapse into the ferromagnetic state. Our work opens perspectives to stabilize topological spin structures even in the absence of Dzyaloshinskii-Moriya interaction.