1,721,054 research outputs found
Metadynamics Simulations of Nucleation
No full textThis chapter offers an overview of recent applications of the metadynamics method to the study of nucleation and related phenomena. In the first section, the classical nucleation theory and the metadynamics method are introduced. The second section is devoted to applications, including computational studies of the surface tension, which affects the size and energy of critical nuclei, and investigation of crystal nucleation from the amorphous and supercooled liquid state
Magnetic Contrast in Phase-Change Materials Doped with Fe Impurities
No full textAb initio simulation of the transition from the crystalline to the amorphous phase of a phase-change material doped with Fe impurities. The structural and magnetic properties of both phases are studied and a link is provided between the magnetic contrast recently observed in these materials and the drastic change in local order which occurs during amorphization
Point defects in disordered and stable GeSbTe phase-change materials
No full textPhase-change materials (PCMs) are promising candidates for efficient storage-class memory exploiting the pronounced resistivity contrast between their amorphous and crystalline phase. GeSbTe compounds, the prototypical class of PCMs, are theoretically predicted to be small-gap semiconductors in their disordered, cubic and ordered, hexagonal phase, but in experiment they show p-type conduction. While this is attributed to self-doping, the very defect types responsible have not been entirely identified. Here, we present an ab-initio study of point defects and their formation energies in GeSb2Te4 and Ge2Sb2Te5 in their disordered and ordered crystalline phases. Our simulations indicate that GeSb, rather than VGe or SbTe, is the most important defect to explain p-doping in the hexagonal structure. In the disordered phase, on the other hand, standard gradient-corrected functionals yield low-formation energies for defects associated with n-type conduction, in apparent contradiction with experimental data. We discuss possible sources for this discrepancy and argue that it stems from the underestimation of the band gap
Solidification of hydrogen clusters
No full textThe solidification of small parahydrogen clusters is studied. In such aggregates, where according to some Authors superfluid properties may be present, the principal antagonist of superfluidity is solidification. In this paper we investigate under what conditions solidification. In this paper we investigate under what conditions solidification either cannot occur at all, or would occur only in times much longer than the lifetime cluster. Due to surface melting effects (enhanced by the va der Waals forces) the exterior layers of the cluster do not solidify. As far as the inner core is concerned, the solidification times depend notoriously strongly on the exact values on the parameters; they also depend strongly on the thickness of the molten layer. Nucleation implies a barrier, which in principle can be overcome either by thermal fluctuations (at relatively high temperatures) or by tunneling (at very low temperatures). Although a better knowledge of the physical properties of parahydrogen (especially the liquid interface energy and the chemical potential of the two phases) is required, we confirm that in both cases the nucleation times can be exceedingly long
Structural and electronic properties of hybrid graphene and boron nitride nanostructures on Cu
No full textRecently, two-dimensional nanostructures consisting of alternating graphene and boron nitride (BN) domains have been synthesized. These systems possess interesting electronic and mechanical properties, with potential applications in electronics and optical devices. Here, we perform a first-principles investigation of models of BN-C hybrid monolayers and nanoribbons deposited on the Cu(111) surface, a substrate used for their growth in said experiments. For the sake of comparison, we also consider BN and BC2N nanostructures. We show that BN and BC2N monolayers bind weakly to Cu(111), whereas monolayers with alternating domains interact strongly with the substrate at the B-C interface, due to the presence of localized interface states. This binding leads to a deformation of the monolayers and sizable n doping. Nanoribbons exhibit a similar behavior. Furthermore, they also interact significantly with the substrate at the edge, even in the case of passivated edges. These findings suggest a route to tune the band gap and doping level of BN-C hybrid models based on the interplay between nanostructuring and substrate-induced effects
Defect-Induced Magnetism in Graphene: An Ab Initio Study
No full textGraphene is an amazing two-dimensional system with exceptional physical and chemical properties. Potential applications in quantum information processing have been proposed for C-based materials, in particular for graphene system, where electron spin is a promising candidate for a solid-state qubit. The preservation of long spin coherence time is the fundamental feature to get for efficient working spin-qubit system. Despite graphene environment seems to suit the goal, defects in the structure, interactions with impurities and edge states can be a source of alteration of quantum information, since they could enhance the decoherence effects. The present work is a computational analysis of defective systems. It focuses on the investigations of various prototypical defect states (vacancies) and impurities interacting with graphene surface (hydrogen, boron, nitrogen, and oxygen) by means of density functional theory (DFT). We provide a preliminary study about the effects of these interactions. Vacancy-type defects give rise to a breaking of graphene symmetry, promoting a localized state with a magnetic moment whose magnitude is concentration-dependent. Hydrogen promotes a locally hybridization of the structure providing a localized magnetic moment and giving rise to an enhancement of spin-orbit interaction of about three orders of magnitude, showing the impact of hydrogen on spin-relaxation time. Among boron, nitrogen, and oxygen, the work has shown that the only one which returns a magnetic ground state is nitrogen. Boron
provides an n-doping of defective-graphene. Oxygen leads to a hybridization of carbon atoms bonding, but its electronic structure does not allow a magnetic system. In the particular case of a bridge-like adsorption site. Among the different configurations for the adsorption sites, the bridge-site is energetically the most stable one, showing as in the other configurations for nitrogen, a magnetic system. Nitrogen adatoms develop a magnetic order (at zero temperature) which is always ferromagnetic independently from the distance between two adjacent nitrogen atoms
Connection between magnetism and structure in Fe double chains on the Ir(100) surface
No full textThe magnetic ground state of nanosized systems such as Fe double chains, chains recently shown to form in the early stages of Fe deposition on Ir(100), is generally nontrivial. Using ab initio density functional theory we find that the straight ferromagnetic (FM) state typical of bulk Fe as well as of isolated Fe chains and double chains is disfavored after deposition on Ir(100) for all the experimentally relevant double chain structures considered. So long as spin-orbit coupling (SOC) is neglected, the double chain lowest energy state is generally antiferromagnetic (AFM), a state which appears to prevail over the FM state due to Fe-Ir hybridization. Successive inclusion of SOC adds two further elements, namely, a magnetocrystalline anisotropy and a Dzyaloshinskii-Moriya (DM) spin-spin interaction; the former stabilizing the collinear AFM state and the latter favoring a long-period spin modulation. We find that anisotropy is most important when the double chain is adsorbed on the partially deconstructed Ir(100)-a state which we find to be substantially lower in energy than any reconstructed structure-so that in this case the Fe double chain should remain collinear AFM. Alternatively, when the same Fe double chain is adsorbed in a metastable state onto the (5 x 1) fully reconstructed Ir(100) surface, the FM-AFM energy difference is very much reduced and the DM interaction is expected to prevail, probably yielding a helical spin structure
Absence of Partial Amorphization in GeSbTe Chalcogenide Superlattices
Full text linkPhase-change materials (PCMs) are widely used for optical data storage due to their fast and reversible transitions between a crystalline and an amorphous phase that exhibit reflectivity contrast. In the last decade, PCMs have been found to be promising candidates for the development of nonvolatile electronic memories, as well. In this context, superlattices of thin layers of GeTe and Sb2Te3 show an unprecedented performance gain in terms of switching speed and power consumption with respect to bulk GeSbTe compounds. Models of crystalline–crystalline transitions, proposed to explain the improved properties, however, are challenged by recent experiments in which GeTe–Sb2Te3 superlattices are observed to reconfigure toward a van der Waals heterostructure of rhombohedral GeSbTe and Sb2Te3. Herein, ab initio molecular dynamics simulations are used to explore an alternative switching mechanism that comprises amorphous–crystalline transitions of ultrathin GeSbTe layers between crystalline Sb2Te3. Despite some positive results obtained by tailoring the quenching protocol, overall the extensive simulations do not yield clear evidence for this mechanism. Therefore, they suggest that the switching process probably involves a transition between two crystalline states
Spin-Orbit Modifications and Splittings of Deep Surface States on Clean Au(111)
No full textWe compare the electronic structure of the unreconstructed
Au(111) surface calculated within density functional theory by means of
plane waves and fully-relativistic ultrasoft pseudo-potentials (US-PPs)
on one hand, and scalar relativistic US-PPs on the other hand, where
spin-orbit is not included.
Several surface states are identified and discussed, in particular
their character and spin polarization caused by the spin-orbit
interaction. Besides the well known splitting of the L-gap shallow
surface state near the Fermi energy, the spin-orbit interaction
generates, modifies, and splits many other deep surface states
in the range 1-10 eV below . The existence and spin-orbit
splitting of these deeper states should be experimentally
detectable by angle-resolved photoelectron spectroscopy
Spin-orbit modifications and splittings of deep surface states on clean Au(111)
No full textWe compare the electronic structure of the unreconstructed Au(111) surface calculated within density functional theory by means of plane waves and fully-relativistic ultrasoft pseudo-potentials (US-PPs) on one hand, and scalar relativistic US-PPs oil the other hand, where spin-orbit is not included. Several surface states are identified and discussed, in particular their character and spin polarization caused by the spin-orbit interaction. Besides the well known splitting of the L-gap shallow surface state near the Fermi energy, the spin-orbit interaction generates, modifies, and splits many other deep surface states in the range 1-10 eV below E-F. The existence and spin-orbit splitting of these deeper states should be experimentally detectable by angle-resolved photoelectron spectroscopy. (C) 2007 Elsevier B.V. All rights reserved
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