1,720,971 research outputs found
Electrical tuning of the magnetic properties of two-dimensional magnets: Cr2Ge2Te6
Full text linkMotivated by growing interest in atomically thin van der Waals magnetic materials, we present an ab initio theoretical study of the dependence of their magnetic properties on the electron / hole density rho induced via the electrical field effect. By focusing on the case of monolayer Cr2Ge2Te6 (a prototypical 2D Ising ferromagnet) and employing a hybrid functional, we first study the dependence of the gap and effective mass on the carrier concentration rho. We then investigate the robustness of magnetism by studying the dependencies of the exchange couplings and magneto crystalline anisotropy energy (MAE) on rho. In agreement with experimental results, we find that magnetism displays a bipolar electrically -tunable character, which is, however, much more robust for hole (rho > 0) rather than electron (rho < 0) doping. Indeed, the MAE vanishes for an electron density rho approximate to - 7.5 x 10(13) e x cm(-2) , signaling the failure of a localized description based on a Heisenberg -type anisotropic spin Hamiltonian. This is in agreement with the rapid increase of the coupling between fourth -neighbor atoms with increasing electron density
Thermoelectric efficiency of nanoscale devices in the linear regime
Full text linkWe study quantum transport through two-terminal nanoscale devices in contact with two particle reservoirs at different temperatures and chemical potentials. We discuss the general expressions controlling the electric charge current, heat currents, and the efficiency of energy transmutation in steady conditions in the linear regime. With focus in the parameter domain where the electron system acts as a power generator, we elaborate workable expressions for optimal efficiency and thermoelectric parameters of nanoscale devices. The general concepts are set at work in the paradigmatic cases of Lorentzian resonances and antiresonances, and the encompassing Fano transmission function: the treatments are fully analytic, in terms of the trigamma functions and Bernoulli numbers. From the general curves here reported describing transport through the above model transmission functions, useful guidelines for optimal efficiency and thermopower can be inferred for engineering nanoscale devices in energy regions where they show similar transmission functions
Hilbert transform evaluation for electron-phonon self-energies
No full textThe electron tunneling current through nanostructures is considered in the presence ofthe electron-phonon interactions. In the Keldysh nonequilibrium formalism, the lesser,greater, advanced and retarded self-energies components are expressed by means ofappropriate Langreth rules. We discuss the key role played by the entailed Hilberttransforms, and provide an analytic way for their evaluation. Particular attention isgiven to the current-conserving lowest-order-expansion for the treament of theelectron-phonon interaction; by means of an appropriate elaboration of the analyticproperties and pole structure of the Green’s functions and of the Fermi functions, wearrive at a surprising simple, elegant, fully analytic and easy-to-use expression of theHilbert transforms and involved integrals in the energy domain
Strain Modulation of Band Offsets at the PCBM/P3HT Heterointerface
Full text linkImproving the efficiency of organic solar cells requires atomic insight into the interface electronic band alignment of the donor and acceptor moieties composing the device. In this article, we use ab initio calculations, with inclusion of long-range (van der Waals) interactions, to address the solid-state properties of a bulk heterojunction heterointerface between a single ordered layer of PCBM molecules adsorbed on a clean P3HT crystalline polymer. The studied interface model allowed us to focus on the basic mechanisms responsible for charge polarization and migration at the interface and to refer the energies of both moieties to the same origin. After the accurate evaluation of the relative energy positions of the near-gap electronic levels in the PCBM/P3HT complex and of the optical spectra useful for determining the nature of the electronic states, we analyzed the effect of uniaxial stress on the band alignment, and we found that both the polymer band gap and the offset between the LUMO levels of the donor and the acceptor materials decrease for compressive stress. This suggests that the donor band gap can be reduced, thus increasing solar energy harvest, and that the open-circuit voltage of the system can be tuned to improve the efficiency of PCBM/P3HT-based solar cell devices
Plasmon-magnon interactions in two-dimensional honeycomb magnets
Full text linkTwo-dimensional honeycomb ferromagnets offer the unprecedented opportunity to study interactions between collective modes that in standard bulk ferromagnets do not cross paths. Indeed, they harbor an optical spin-wave branch, i.e. a spin wave which disperses weakly near the Brillouin zone center. When doped with free carriers, they also host the typical gapless plasmonic mode of 2D itinerant electron/hole systems. When the plasmon branch meets the optical spin-wave branch, energy and momentum matching occurs, paving the way for interactions between the charge and spin sector. In this Letter we present a microscopic theory of such plasmon-magnon interactions, which is based on a double random phase approximation. We comment on the possibility to unveil this physics in recently isolated 2D honeycomb magnets such as .Main text: 7 pages; Supplemental Material: 10 page
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Graphene on clean (0001) α-quartz: Numerical determination of a minimum energy path from metal to semiconductor
No full textBy means of DFT calculations, we have individuated a minimum-energy path connecting two energy minima of clean graphene on clean and relaxed oxygen-terminated (0001)’SiOinline image substrate in the inline image-quartz configuration: one characterized by mutual graphene–SiOinline image substrate distance of inline image2.8 Å and weak (van der Waals) bonds between them, the other by mutual distance of inline image1.4 Å, and presence of strong covalent C–O bonds. Our calculations show that the pathway connecting the two minima goes through a transition state and that the two minima are separated by a barrier of inline image2.25 eV. The covalent C–O bonds, which characterize the lower-energy configuration, induce significant corrugation of the graphene overlayer with consequent important modification of its electronic band structure and transport properties. In particular, we show that a small gap (inline imageeV) opens in the electronic band structure of the graphene/SiOinline image system, and the conical features around the Dirac points are lost. Correspondingly, at the graphene/SiOinline image interface, the diffuse inline image conjugation of the isolated graphene layer is modified by the appearance of near’inline image carbon atoms bound to the top oxygens of the SiOinline image. This fact also affects conductances and I–V characteristics which become different along different cell directions of the graphene overlayer. Our analysis suggests that the energy barrier between the van der Waals and the covalent minima could be overcome by applying a uniform pressure on the graphene overlayer due to the formation of chemical bonds which are important for the experimental integration of graphene on Si-compatible technology
Heterointerface Electronic States and Charge Transport of Crystalline N,N′-1H,1H′-Perfluorobutil Dicyanoperylene Diimide in Field-Effect Transistor Configuration
No full textWe present a theoretical study on the nature and origin of charge carriers, charge transfer, and currents at the interface of an organic field-effect transistor (OFET) structure composed of the N,N′-1H,1H′-perfluorobutil dicyanoperylene diimide (PDIF-CN2) organic crystal adsorbed on the (001)-oriented, clean, silicon surface. Starting from the knowledge of the structural and electronic bulk properties of the organic crystal and of the surface of the dielectric substrate, we have evaluated the band structure, density of states, charge transfer, and surface currents in a model crystal made of a reconstructed silicon surface on which a single layer of PDIF-CN2 molecules was deposited in the same packing arrangement as in the bulk PDIF-CN2 organic crystal. In this way alignment of energy bands of the separate PDIF-CN2 and silicon crystals are naturally considered in the model crystal and information on the states involved in charge transport is evidenced. In particular, the analysis of the charge transfer between the dielectric substrate and the adsorbed PDIF-CN2 molecules, of the interface densities of states, and of the currents at the interface allows us to confirm existing experimental results on the distribution and origin of the currents in the FET structure. Namely, that these are essentially determined by the conducting channel made of the π orbitals of the perylene cores of the face-to-face packed PDIF-CN2 molecules on the dielectric substrate. Our results support the description of coherent bandlike transport in OFETs based on the n-type PDIF-CN2 organic crystal
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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