Connection Between Magnetism and Structure in Fe Double Chains on the Ir(100) Surface

The magnetic ground state of nanosized systems such as Fe double 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, the second 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 (5x1) 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.Comment: to appear on PR

Structural and electronic properties of hybrid graphene and boron nitride nanostructures on Cu

Recently, 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 behaviour. 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.Comment: 22 pages, 8 figure

Fractional charges in pyrochlore lattices

A pyrochlore lattice is considered where the average electron number of electrons per site is half--integer, concentrating on the case of exactly half an electron per site. Strong on-site repulsions are assumed, so that all sites are either empty or singly occupied. Where there are in addition strong nearest--neighbour repulsions, a tetrahedron rule comes into effect, as previously suggested for magnetite. We show that in this case, there exist excitations with fractional charge (+/-) e/2. These are intimately connected with the high degeneracy of the ground state in the absence of kinetic energy terms. When an additional electron is inserted into the system, it decays into two point like excitations with charge -e/2, connected by a Heisenberg spin chain which carries the electron's spin.Comment: 10 pages, 4 eps figures. To appear in Decemeber issue of Annalen der Physi

Electronic and magnetic properties of zigzag graphene nanoribbons on the (111) surface of Cu, Ag and Au

We have carried out an ab initio study of the structural, electronic and magnetic properties of zigzag graphene nanoribbons on Cu(111), Ag(111) and Au(111). Both, H-free and H-terminated nanoribbons are considered revealing that the nanoribbons invariably possess edge states when deposited on these surfaces. In spite of this, they do not exhibit a significant magnetization at the edge, with the exception of H-terminated nanoribbons on Au(111), whose zero-temperature magnetic properties are comparable to those of free-standing nanoribbons. These results are explained by the different hybridization between the graphene 2p orbitals and those of the substrates and, for some models, by the sizable charge transfer between the surface and the nanoribbon. Interestingly, H-free nanoribbons on Au(111) and Ag(111) exhibit two main peaks in the local density of states around the Fermi energy, which originate from different states and, thus, do not indicate edge magnetism.Comment: 5pages, 3figure

Absence of Partial Amorphization in GeSbTe Chalcogenide Superlattices

Phase-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

Vegetable gardens for educational purposes: a specific toolkit for didactic contexts

[EN] The paper reports on how urban agriculture, as a sharing system, is becoming a way to increase aggregation, grouping, relationships in a local context, which could turn into an educational and emotional resource within the urban context. This paper will examine the design of community gardens within semi-public spaces in didactic context (schools, associations, learning spaces). One of the research objectives is to improve the quality of urban landscapes by answering citizens’ need for social interaction and fostering the role that community plays in it. Through co-design sessions with different communities related to specific schools, the design output aims at the creation of a systemic space made by a vegetable garden and his convivial spaces. This would strengthen internal local connections, and trigger positivity and better learning performances among users. The expected result is a set of design tools and guidelines that allow these realities to deal with the creation of vegetable gardens by defining the layouts, the functions and the experiences.http://ocs.editorial.upv.es/index.php/HEAD/HEAD18Mastrantoni, C.; Mazzarello, M. (2018). Vegetable gardens for educational purposes: a specific toolkit for didactic contexts. Editorial Universitat Politècnica de València. 1297-1306. https://doi.org/10.4995/HEAD18.2018.8194OCS1297130

Computer simulation of quantum melting in hydrogen clusters

We introduce a new criterion--based on multipole dynamical correlations calculated within Reptation Quantum Monte Carlo--to discriminate between a melting vs. freezing behavior in quantum clusters. This criterion is applied to small clusters of para-hydrogen molecules (both pristine and doped with a CO cromophore), for cluster sizes around 12 molecules. This is a magic size at which para-hydrogen clusters display an icosahedral structure and a large stability. In spite of the similar geometric structure of CO@(pH2)_12 and (pH2)_13, the first system has a rigid, crystalline, behavior, while the second behaves more like a superfluid (or, possibly, a supersolid).Comment: 5 pages, 5 pdf. Chem Phys Chem, in press (2005

Onychoheterotopia in children

The ectopic nail (EN) is an additional nail located in an abnormal site. It belongs to the onycho-heterotopia, a rare condition whose pathogenesis is indeterminate. This article illustrates the clinical-morphological and dermoscopic points of view, the diagnostic criteria, the possible pathogenesis, and surgical treatment of this pediatric onycho-heterotopia

Changes of Structure and Bonding with Thickness in Chalcogenide Thin Films

Extreme miniaturization is known to be detrimental for certain properties, such as ferroelectricity in perovskite oxide films below a critical thickness. Remarkably, few-layer crystalline films of monochalcogenides display robust in-plane ferroelectricity with potential applications in nanoelectronics. These applications critically depend on the electronic properties and the nature of bonding in the 2D limit. A fundamental open question is thus to what extent bulk properties persist in thin films. Here, this question is addressed by a first-principles study of the structural, electronic, and ferroelectric properties of selected monochalcogenides (GeSe, GeTe, SnSe, and SnTe) as a function of film thickness up to 18 bilayers. While in selenides a few bilayers are sufficient to recover the bulk behavior, the Te-based compounds deviate strongly from the bulk, irrespective of the slab thickness. These results are explained in terms of depolarizing fields in Te-based slabs and the different nature of the chemical bond in selenides and tellurides. It is shown that GeTe and SnTe slabs inherit metavalent bonding of the bulk phase, despite structural and electronic properties being strongly modified in thin films. This understanding of the nature of bonding in few-layers structures offers a powerful tool to tune materials properties for applications in information technology