Magnetic field- and light-driven spin molecular logic gates: A first-principles study

Herein, a highly practical first‐principles approach for studying phonon transport across various materials, ranging from low‐dimensional systems to three‐dimensional bulk structures, as well as nanostructures, is presented. This method integrates the nonequilibrium Green's function (NEGF) formalism with ab initio calculations of dynamical matrices generated by Quantum Espresso. A detailed technical approach for extracting atomistic force constants and forming dynamical matrices that adhere to the tri‐diagonal technique within the NEGF framework is provided. Additionally, an efficient method for constructing dynamical matrices of conventional cells from those of primary cells is introduced, remarkably reducing the computational costs associated with density‐functional theory calculations. The robustness and applicability of this methodology are validated through several case studies.

Using the non-equilibrium Green's function formalism in combination with density functional theory, we calculated the spin-dependent electronic properties of molecular devices consisting of pristine and hydrogen-terminated zigzag gallium nitride nanoribbons (ZGaNNRs). Computational results show that the proposed ZGaNNR models display multiple functions with perfect spin filtering, rectification, and a spin negative differential resistance (sNDR) effect. Spin-dependent transport properties, spin density and transmission pathways with applied bias values were calculated to understand the spin filter and the sNDR effect. The spin filtering efficiency can be up to -100% or 100% within a large range of biases, and a dual spin filtering effect can also be found in these model devices. The highest rectification ratio reaches 4.9 × 109 in spin-down current of ZGaNNRs with only the passivated nitrogen edge, and only ZGaNNRs with the passivated gallium edge exhibit an obvious sNDR behavior with the largest peak to valley current ratio of 1.25 × 107. The proposed hydrogenated ZGaNNRs can be preferred materials for realizing oscillators, memory circuits and fast switching applications.

This paper describes the point defect treatment in nanowire MOSFETs. The 3D Poisson equation is self-consistently coupled to the Schrodinger equation. We adopt the mode-space representation in which the 3D Schrodinger equation is divided into a 2D equation, including the confinement of the cross-section, and an ID equation describing the ballistic transport based on the nonequilibrium Green's function formalism (NEGF). We use the simplest form of the tight-binding theory, with one orbital per atom, and considering only the first-neighbor-interactions. In this framework, equivalent to the ellipsoidal energy band approximation, influence of point defects on electronic transport is discussed.

By using the Non-equilibrium Green's function (NEGF) formalism to study charge and spin transport in a quantum ribbon governed by two contrasting mechanisms: magnetic flux-induced orbital quantization described by the 1D effective orbital model, and internal exchange interactions described by the long-range RKKY Hamiltonian. By examining the energy-dependent transport response, we compare how these mechanisms affect the overall transport behavior of the system. The orbital effect leads to flux-controlled energy gaps and modulated charge transport, whereas RKKY interactions induce exchange-driven band modifications. Additionally, introducing an inelastic scattering parameter breaks phase coherence, driving a crossover from ballistic to diffusive transport, thereby highlighting the role of dephasing in realistic quantum devices.

We discuss within the nonequilibrium Green's function formalism the self-consistent Born approximation of the electronic self-energy for concurrent coupling of charge carriers to photons and phonons. The expansion of the charge carrier Green's function is shown to provide a description for a range of physical processes with relevance for the photovoltaic device operation, such as indirect optical transitions, thermally assisted photocarrier escape from quantum wells or phonon-mediated interband transfer in tunnel junctions. The nonequilibrium Green's function picture of these processes is reviewed with focus on the evolution of the system towards the steady state in the convergence of the self-consistency iteration process and on the impact of inelastic scattering on device operation.

A combination of density functional theory and nonequilibrium Green's function formalism has been applied to the atomic scale calculation of the leakage current through SiO2 and SiOxNy dielectrics of MOSFETs. Samples with different dielectric thicknesses and nitrogen concentrations have been taken into account in order to study the dependence of the leakage current on these two parameters. It has been shown that the incorporation of nitrogen atoms into SiO2 increases the density of gap states, which leads to barrier lowering for both electrons and holes. The calculated leakage currents through different SiO2 polymorphs (e.g., s-cristobalite, ?-quartz, and s-quartz) show thickness dependences, which are in very good agreement with measured values for amorphous SiO2. The dependence of the leakage current on the concentration of nitrogen atom in oxynitride gate dielectrics has also been calculated and shown to be in good agreement with the values extracted from measured data.

We examine the transport properties of a tunnel-coupled asymmetric double-dot structure in a parallel arrangement between leads in the presence of Aharonov-Bohm magnetic flux and an additional bias voltage applied between the dots. We solve the Schrodinger equation for the energy eigenfunctions and eigenvalues of a biased asymmetric double-dot system. The magnetic flux-induced Peierls phase factors experienced by electrons in tunneling from/to the leads to/from double-dot levels are determined and incorporated. Employing the nonequilibrium Green's function formalism we calculate the lead-to-lead current, taking account of dot-lead tunnel coupling and Coulomb repulsion. We demonstrate that, for appropriate parameter values (such as lead-to-lead bias voltage and equilibrium chemical potential of the leads), the lead-to-lead current oscillates as a function of applied magnetic field, if the dot-to-dot bias compensates the structure asymmetry and only one of the double-dot levels is conductive. We also show that when both levels become conductive, their interference is destructive and the total lead-to-lead current decreases with increasing magnetic field. This decrease is larger for larger dots.

The molecular spin filter constructed from 1D organic chain

The effect of inelastic scattering on quantum electron transport through double-barrier resonant-tunneling structures with large cross-sectional areas is studied numerically using the approach based on the nonequilibrium Green's-function formalism of Keldysh, Kadanoff, and Baym. The Markov assumption is not made, and the energy coordinate is retained. This makes the inclusion of the phonon-energy spectrum straightforward both conceptually and in practice. The electron-phonon interaction is treated in the self-consistent first Born approximation (SCFBA). The Pauli exclusion principle is taken into account exactly within the SCFBA. The retention of the energy coordinate allows the calculation of a number of quantities that give insight into the effect of inelastic scattering on electron transport: The effect of inelastic scattering on the occupation of the energy levels, the density of states, the energy distribution of the current density, and the power density is calculated from a quantum kinetic equation for actual device structures under high bias.

Influence of disorder on the perpendicular magnetoresistance of magnetic multilayers

We demonstrate that a local moment can automatically form in the vicinity of the quantum point contact if we (1) include the spin-orbit interaction and (2) allow the electric field along the longitudinal direction to be both nonuniform and asymmetric. The former breaks the spin degeneracy by introducing an effective magnetic field while the latter ensures that the moment is formed near the point contact where the nonuniformity is the largest. The asymmetry induced by the source-drain bias leads to an imbalance of spin-up and spin-down electrons on different sides of the point contact, which introduces a spin polarization. The nonequilibrium Green's-function formalism is employed to study the transport properties and local spin density. The electron-electron interaction is found to enhance the magnitude of the localized moment, necessary for the Kondo-type scenario of the 0.7 anomaly.

The influence of impurity doping distribution in the channel region of p-type silicon nanowire FET has been studied using atomistic three-dimensional quantum simulator based on non-equilibrium Green's function formalism. The valence band has been modeled using a 6-band k.p Hamiltonian. Carrier transport has been treated in the coupled mode-space which considerably reduces the simulation time compare to real-space approach without losing the accuracy. We consider a FET with 10-nm gate length, 5×5 nm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> cross-section and different doping densities.

Schottky barrier field-effect transistors (SBFETs) based on few and mono layer phosphorene are simulated by the non-equilibrium Green's function formalism. It is shown that scaling down the gate oxide thickness results in pronounced ambipolar I-V characteristics and significant increase of the minimal leakage current. The problem of leakage is especially severe when the gate insulator is thin and the number of layer is large, but can be effectively suppressed by reducing phosphorene to mono or bilayer. Different from two-dimensional graphene and layered dichalcogenide materials, both the ON-current of the phosphorene SBFETs and the metal-semiconductor contact resistance between metal and phosphorene strongly depend on the transport crystalline direction.

A novel scheme for the steady state solution of the standard Redfield quantum master equation is developed which yields agreement with the exact result for the corresponding reduced density matrix up to second order in the system-bath coupling strength. We achieve this objective by use of an analytic continuation of the off-diagonal matrix elements of the Redfield solution towards its diagonal limit. Notably, our scheme does not require the provision of yet higher order relaxation tensors. Testing this modified method for a heat bath consisting of a collection of harmonic oscillators we assess that the system relaxes towards its correct coupling-dependent, generalized quantum Gibbs state in second order. We numerically compare our formulation for a damped quantum harmonic system with the nonequilibrium Green's function formalism: we find good agreement at low temperatures for coupling strengths that are even larger than expected from the very regime of validity of the second-order Redfield quantum master equation. Yet another advantage of our method is that it markedly reduces the numerical complexity of the problem; thus, allowing to study efficiently large-sized system Hilbert spaces.

The outstanding transport properties expected at the edge of two-dimensional\ntime-reversal invariant topological insulators have proven to be challenging to\nrealize experimentally, and have so far only been demonstrated in very short\ndevices. In search for an explanation to this puzzling observation, we here\nreport a full first-principles calculation of topologically protected transport\nat the edge of novel quantum spin Hall insulators - specifically, Bismuth and\nAntimony halides - based on the non-equilibrium Green's functions formalism.\nOur calculations unravel two different scattering mechanisms that may affect\ntwo-dimensional topological insulators, namely time-reversal symmetry breaking\nat vacancy defects and inter-edge scattering mediated by multiple co-operating\nimpurities, possibly non-magnetic. We discuss their drastic consequences for\ntypical non-local transport measurements as well as strategies to mitigate\ntheir negative impact. Finally, we provide an instructive comparison of the\ntransport properties of topologically protected edge states to those of the\ntrivial edge states in MoS$_2$ ribbons. Although we focus on a few specific\ncases (in terms of materials and defect types) our results should be\nrepresentative for the general case and thus have significance beyond the\nsystems studied here.\n