Interactions and thermoelectric effects in a parallel-coupled double quantum dot

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.

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.

Charge transport accompanied by heat transfer through a single-level quantum dot coupled to ferromagnetic leads with noncollinear magnetic moments is studied theoretically in the linear and nonlinear transport regimes. Calculations performed in the framework of nonequilibrium Green's function formalism and the equation of motion method reveal a significant influence of Coulomb blockade on thermal transport processes. The thermopower $S$ and thermal efficiency described by the figure of merit $ZT$ depend on magnetic configuration of the system. Two physically different situations are considered; one appears when spin accumulation is excluded and the second one when spin accumulation is relevant. In the latter case we also calculate the corresponding spin thermopower. Apart from this, magnetothermopower is introduced and discussed.

Recent experimental advances probing coherent phonon and electron transport in nanoscale devices at contact have motivated theoretical channel-based analyses of conduction based on the nonequilibrium Green's function formalism. The transmission through each channel has been known to be bounded above by unity, yet actual transmissions in typical systems often fall far below these limits. Building upon recently derived radiative heat transfer limits and a unified formalism characterizing heat transport for arbitrary bosonic systems in the linear regime, we propose new bounds on conductive heat transfer. In particular, we demonstrate that our limits are typically far tighter than the Landauer limits per channel and are close to actual transmission eigenvalues by examining a model of phonon conduction in a 1-dimensional chain. Our limits have ramifications for designing molecular junctions to optimize conduction.

Shockley surface states (SS) have attracted much attention due to their role in various physical phenomena occurring at surfaces. It is also clear from experiments that they can play an important role in electron transport. However, accurate incorporation of surface states in $\textit{ab initio}$ quantum transport simulations remains still an unresolved problem. Here we go beyond the state-of-the-art non-equilibrium Green's function formalism through the evaluation of the self-energy in real-space, enabling electron transport without using artificial periodic in-plane conditions. We demonstrate the method on three representative examples based on Au(111): a clean surface, a metallic nanocontact, and a single-molecule junction. We show that SS can contribute more than 30\% of the electron transport near the Fermi energy. A significant and robust transmission drop is observed at the SS band edge due to quantum interference in both metallic and molecular junctions, in good agreement with experimental measurements. The origin of this interference phenomenon is attributed to the coupling between bulk and SS transport channels and it is reproduced and understood by tight-binding model. Furthermore, our method predicts much better quantized conductance for metallic nanocontacts.

Self assembled quantum dots have shown a great promise as a leading candidate for infrared detection at room temperature. In this paper, a theoretical model of the absorption coefficient of quantum dot devices is presented. Both of bound to bound absorption and bound to continuum absorption are taken into consideration in this model. This model is based on the effective mass theory and the Non Equilibrium Greens Function (NEGF) formalism. NEGF formalism is used to calculate the bound to continuum absorption coefficient. The results of the model have been compared with a published experimental work and a good agreement is obtained. Based on the presented model, the bound to bound absorption coefficient component is compared to the bound to continuum absorption coefficient component. In addition, the effects of the dot dimensions and electron filling on the bound to continuum absorption coefficient are also investigated. In general, increasing the dot filling increases the absorption and decreasing the dots dimensions will increase the absorption and move the absorption peak towards longer wavelengths.

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.

The ac response of a coupled double quantum dot system involving a phase-breaking effect is studied. We calculate the ac conductance based on the nonequilibrium Green’s-function formalism. Some parasitic and gate capacitances are included in our model, thus the displacement current is considered, and the overall charge and current conservation are fulfilled. In our results the double resonant structure of the conductance is observed. We find that the electron-phonon interaction has a significant effect on the ac conductance both for low and high temperatures. Due to the phase-breaking effect of electron-phonon scattering, the resonant conductance peak is suppressed very seriously, and the second peak of the ac conductance may disappear completely, However, for the nonresonant situation, the conductance is enhanced for small frequencies. Furthermore, we study the effect of the capacitances on the ac conductance, and find that, for small frequencies, the capacitances have a small effect on the real part of the admittance. On the imaginary part of the admittance, all the capacitances except for the interdot capacitance have a considerable effect. For high frequencies all the capacitances have a considerable effect on the ac conductance.

A two-dimensional domain decomposition technique for the simulation of quantum-scale devices

Influence of disorder on the perpendicular magnetoresistance of magnetic multilayers

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.

We propose a robust strategy for transforming the Hamiltonian of a metallic structure expressed in a nonorthogonal density-functional-theory (DFT) basis into a low-dimensional space compatible with electron-transport simulations. This mode-space approximation is applied locally to inhomogeneous material stacks including amorphous phases and interfaces. The contacts and periodic parts of the device are transformed into the subspace created, while the active inhomogeneous regions remain represented in real space. The various regions are connected to each other through mode-space--real-space hybrid blocks of the Hamiltonian and overlap matrices. This approach allows harnessing of the full flexibility of DFT combined with the power of the nonequilibrium Green's function formalism at a moderate cost. In particular, for a realistic resistive random-access memory cell composed of 3390 atoms, a performance improvement by a factor of 136 compared with a pure real-space treatment is demonstrated, with an error of less than 2%.

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 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.

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.