7,129 research outputs found
Quantum transport simulation scheme including strong correlations and its application to organic radicals adsorbed on gold
No full textWe present a computational method to quantitatively describe the linear-response conductance of nanoscale devices in the Kondo regime. This method relies on a projection scheme to extract an Anderson impurity model from the results of density functional theory and nonequilibrium Green's functions calculations. The Anderson impurity model is then solved by continuous-time quantum Monte Carlo. The developed formalism allows us to separate the different contributions to the transport, including coherent or noncoherent transport channels, and also the quantum interference between impurity and background transmission. We apply the method to a scanning tunneling microscope setup for the 1,3,5-triphenyl-6-oxoverdazyl (TOV) stable radical molecule adsorbed on gold. The TOV molecule has one unpaired electron, which when brought in contact with metal electrodes behaves like a prototypical single Anderson impurity. We evaluate the Kondo temperature, the finite-temperature spectral function, and transport properties, finding good agreement with published experimental results
Quantum Transport Simulations of Nano-systems: An Introduction to the Green’s Function Approach
No full textIn this chapter we present a detailed treatment of coherent charge transport at the nano-scale. To begin with, we explain the Landauer-Buttiker theory and its underlaying assumptions. Then we introduce the non-equilibrium Green’s function (NEGF) technique, which provides a powerful framework to implement the theory for real device simulations and to extend it beyond linear response. The goal of the first part of the chapter is to provide the reader with the basic knowledge required to understand and implement a NEGF-based computational scheme for non-interacting tight-binding models. The second part of the chapter illustrates how NEGF can be combined with density functional theory (DFT) to carry out first-principles simulations of nanoscale devices. We provide guidelines that will allow the reader to perform practical DFT+NEGF simulations by using the most common implementations available in electronic structure software
Non-equilibrium Green’s Function Methods for Spin Transport and Dynamics
No full textThe modeling of spintronic devices is a theoretical challenge, since one has to describe accurately both the electronic structure of the constituent materials and their charge- and spin-transport properties. In this chapter we present the state-of-the-art quantum transport theory appropriate for this task. The theory is based on the so-called non-equilibrium Green’s function formalism, which is combined with density functional theory in order to provide a first principles description of materials properties. This allows for the evaluation of the steady-state charge and spin current through a quantum system at a finite applied bias voltage between the electrodes. It also describes the spin-transfer torque that flowing spins exert on localized magnetic moments, which is able to switch the magnetization of a magnetic system. In this chapter the detailed discussion about the principal methodological aspects is accompanied by the review of a number of technologically relevant applications
Ab-initio transport across bismuth selenide surface barriers
No full textWe investigate the effect of potential barriers in the form of step edges on the scattering properties of Bi2Se3(111) topological surface states by means of large-scale ab initio transport simulations. Our results demonstrate the suppression of perfect backscattering, while all other scattering processes, which do not entail a complete spin and momentum reversal, are allowed. Furthermore, we find that the spin of the surface state develops an out-of-plane component as it traverses the barrier. Our calculations reveal the existence of quasibound states in the vicinity of the surface barriers, which appear in the form of an enhanced density of states in the energy window corresponding to the topological state. For double barriers we demonstrate the formation of quantum well states. To complement our first-principles results we construct a two-dimensional low-energy effective model and illustrate its shortcomings. Our findings are discussed in the context of a number of recent experimental works
Effect of dynamical electron correlations on the tunnelling magnetoresistance of Fe/MgO/Fe(001) junctions
No full textWe present a computational framework that combines dynamical mean-field theory (DMFT) with density functional theory (DFT) and the nonequilibrium Green's function (NEGF) technique to study the steady-state transport properties of magnetic tunnel junctions (MTJs). The objective of our calculations is then to understand the impact of dynamical electron correlations on the Fe 3d orbitals of an Fe/MgO/Fe system. By applying the rigid shift approximation, we study both the zero- and finite-bias properties in a simple and computationally efficient manner, obtaining the bias-dependent electronic structure, the current-versus-voltage characteristic curve in both the parallel and antiparallel configurations, and consequently, the tunneling magnetoresistance (TMR) ratio. We find that dynamical electron correlation manifests as a reduction in the spin splitting of the Fe 3dz2 state compared to DFT predictions and introduces a finite relaxation time. The impact of these effects on transport, however, varies significantly between magnetic configurations and applied bias voltages. In the parallel configuration, the characteristic curves obtained with DFT and DMFT are similar up to large biases, as the transport is mostly due to the coherent transmission of spin-up electrons through the MgO barrier. Conversely, in the antiparallel configuration, correlation effects become more significant as the bias increases, with DMFT predicting a sharp current increase due to bias-driven inelastic electron-electron scattering. At zero bias, both DMFT and DFT similarly overestimate the TMR ratio compared to experiments. However, at finite bias, DMFT predicts a lower bias threshold for the suppression of the TMR relative to DFT, improving the agreement with experimental data and underscoring the importance of dynamical correlation effects on finite-bias behavior. While these conclusions are specific to Fe/MgO/Fe MTJs, our computational approach can be applied to other MTJs as well, thereby advancing the use of DMFT in spintronics
Enhanced thermopower in covalent graphite–molecule contacts
No full textThe Seebeck effect is very attractive for technological applications as it leads to the direct conversion of heat into electricity. One of the key quantities determining the efficiency of such conversion is the thermopower S. In this paper we explore theoretically what electronic properties are responsible for the Seebeck effect in molecular junctions with graphite or graphene electrodes. We propose that S can be enhanced because of the combined effect of the dip in the density of states at the Fermi energy of these materials and the molecular resonance. Then to understand the impact of the covalent vs. non-covalent molecule–carbon bonding we calculate from first principles the electronic and transport properties of graphite/molecule/Au junctions, where both types of bonding have been reported experimentally. We ultimately predict that S is about 120 μV K−1 at room temperature for a 3,5-dimethyl-4-aminobenzene (DMAB) molecule covalently attached to the graphite electrode. This value is one order of magnitude larger than the typical values measured to date for molecular junctions and it is a signature of the direct C–C molecule–graphite bond. Finally we also demonstrate how one can control not just the absolute magnitude of S, but also its sign by designing the graphite–molecule contact. Our results lead the way towards the use of junctions with molecules covalently attached to a C-based substrate as possible new improved platforms for molecular thermoelectric devices.A. D. was supported by the “Ministerio de Economia, Industria y Competitividad - Agencia Estatal de Investigación” of Spain through the project “Transporte Electrónico, Térmico, y de Espin con la Teoría de Funcionales de Densidad” (FIS2016-79464-P), by the Basque Government through the project “Grupos Consolidados UPV/EHU” (IT1249-19) and by the European Regional Development Fund (FEDER). The initial idea of this work was conceived by the authors during the project ACMOL (Project No. 618082) funded by the European Commission under the FP7 framework.Peer reviewe
Dynamical Mean-Field Theory for spin-dependent electron transport in spin-valve devices
No full textWe present the combination of Density Functional Theory (DFT) and Dynamical
Mean Field Theory (DMFT) for computing the electron transmission through
two-terminals nanoscale devices. The method is then applied to metallic
junctions presenting alternating Cu and Co layers, which exhibit spin-dependent
charge transport and giant magnetoresistance (GMR) effect. The calculations
show that the coherent transmission through the states is greatly
suppressed by electron correlations. This is mainly due to the finite lifetime
induced by the electron-electron interaction and is directly related to the
imaginary part of the computed many-body DMFT self-energy. At the Fermi energy,
where in accordance with the Fermi-liquid behavior the imaginary part of the
self-energy vanishes, the suppression of the transmission is entirely due to
the shifts of the energy spectrum induced by electron correlations. Based our
results, we finally suggest that the GMR measured in Cu/Co heterostructures for
electrons with energies about 1 eV above the Fermi energy is a clear
manifestation of dynamical correlation effects
Yuriy Hryvniak as a researcher of the biography of Ivan Puluj
No full textМатеріал тез присвячено дослідженню сторінок біографії
Юрія Гривняка - автора монографії про Івана Пулюя, яку було
опубліковано 1971 року у Лондоні завдяки фінансовій підтримці Союзу
українців у Великій Британії. Автором було використано ряд фото та
архівних матеріалів, отриманих від О. Пулюя. На жаль, творчість і
життєвий шлях Ю. Гривняка є маловідомими в Україні, а доля зібраних
ним джерел потребує фахового дослідженняThese theses are devoted to the study of the pages of Yuriy
Hryvniak’s biography. He was the author of a monograph dedicated to the study
of the life and activities of the famous Ukrainian scientist and public figure Ivan
Puluj. The author used a number of photos and archival materials received from
O. Puluj. Unfortunately, the creativity and life path of Y. Hryvniak is little
known in Ukraine, and the fate of the sources collected by him requires
professional researc
Note from David Kirchman to Ivan Valiela at the Ecosystems Center
No full textTitle page of the book "Processes in Microbal Ecology". The author, David L. Kirchman, has written a note to Ecosystems Center scientist Ivan Valiela.Note reads: "Ivan- You & the MBL Marine Ecology course got me going in this field. Thanks. David Kirchman"photograph
DFT+Σ2 method for electron correlation effects at transition metal surfaces
No full textWe present a computational approach for electronically correlated metallic surfaces and interfaces, which combines density functional and dynamical mean-field theory using a multiorbital perturbative solver for the many-body problem. Our implementation is designed to describe ferromagnetic metallic thin films on a substrate. The performances are assessed in detail for a Fe monolayer on a W(110) substrate, a prototypical nanoscale magnetic system. Comparing our results to photoemission data, we find qualitative and quantitative improvements in the calculated spectral function with respect to the results of density functional theory within the local spin density approximation. In particular, the spin splitting of the d states is drastically reduced and, at the same time, their spectral width becomes narrower. The method is, therefore, able to account for the main correlation effects in the system
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