Generalized non-equilibrium vertex correction method in coherent medium theory for quantum transport simulation of disordered nanoelectronics

In realistic nanoelectronics, disordered impurities/defects are inevitable and play important roles in electron transport. However, due to the lack of effective quantum transport method to do disorder average, the important effects of disorders remain largely un-explored or poorly understood. Here, we report a generalized non-equilibrium vertex correction method with coherent potential approximation for the non-equilibrium quantum transport simulation of disordered nanoelectronics. In this method, the disorder average of various Green's functions are computed by a generalized coherent potential approximation. A generalized non-equilibrium vertex correction algorithm is then developed to calculate disorder average of the product of any two real time single-particle Green's functions. We obtain nine non-equilibrium vertex corrections and find they can be solved by a set of simple linear equations. As a result, the averaged non-equilibrium density matrix and various important transport properties, including averaged current, disordered induced current fluctuation and the averaged shot noise, can all be efficiently computed in a unified simple scheme. Moreover, the relationship between the non-equilibrium vertex correction method and the non-equilibrium coherent potential approximation theory is clarified, and we prove the non-equilibrium coherent potential equals the non-equilibrium vertex correction and this equivalence is guaranteed by the Keldysh's formulas. In addition, a generalized form of conditionally averaged non-equilibrium Green's function is derived to incorporate with density functional theory to enable first-principles quantum transport simulation. Our approach provides a unified, efficient and self-consistent method for simulating non-equilibrium quantum transport through disordered nanoelectronics

Nonlinear bias dependence of spin-transfer torque from atomic first principles

We report first-principles analysis on the bias dependence of spin-transfer torque (STT) in Fe/MgO/Fe magnetic tunnel junctions. The in-plane STT changes from linear to nonlinear dependence as the bias voltage is increased from zero. The angle dependence of STT is symmetric at low bias but asymmetric at high bias. The nonlinear behavior is marked by a threshold point in the STT versus bias curve. The high-bias nonlinear STT is found to be controlled by a resonant transmission channel in the anti-parallel configuration of the magnetic moments. Disorder scattering due to oxygen vacancies in MgO significantly changes the STT threshold bias.Comment: 6page,4figure

Poster Specular to Diffusive Scattering in Fe/MgO/Fe Magnetic Tunnel Junctions

Nanoelectronic device properties are usually influenced rather strongly by impurities and atomistic disorder. Examples are electron scattering by dopants in semiconductor nano-wires, spin scattering by disorder in magnetic tunnel junctions, and spin polarized current in dilute magnetic semiconductors. Theoretically, any calculated transport quantity must be averaged over the ensemble of possible impurity configurations. There has been no theoretical formalism and computational tool that can effectively carried out disorder averaging for nonequilibrium quantum transport problems. In this poster presentation, we report our recently developed formalism and numerical implementation of the non-equilibrium vertex correction (NVC) theory which is a first principles solution of the nonequilibrium impurity average problem for quantum transport. We apply this theory to investigate effects due to oxygen vacancy to spin polarized quantum transport in magnetic tunneling junction (MTJ) Fe/MgO/Fe. To treat nonequilibrium quantum transport properties of nanoelectronic devices having atomistic substitutional disorder under external bias potential, our first principle formalism is based on carrying out density functional theory (DFT) within the Keldysh non-equilibrium Green's function (NEGF) framework, where the configurational average over random disorde

Random Green's function method for large-scale electronic structure calculation

We report a linear-scaling random Green's function (rGF) method for large-scale electronic structure calculation. In this method, the rGF is defined on a set of random states to stochastically express the density matrix, and rGF is calculated with the linear-scaling computational cost. We show the rGF method is generally applicable to the nonorthogonal localized basis, and circumvent the large Chebyshev expansion for the density matrix. As a demonstration, we implement rGF with density-functional Tight-Binding method and apply it to self-consistently calculate water clusters up 9984 H2Os. We find the rGF method combining with a simple fragment correction can reach an error of ~1meV per H2O in total energy, compared to the deterministic calculations, due to the self-average. The development of rGF method advances the stochastic electronic structure theory to a new stage of the efficiency and applicability.Comment: 5 pages, 2 figure

Association Between Viral Infections and Glioma Risk: A Two-Sample Bidirectional Mendelian Randomization Analysis

Background: Glioma is one of the leading types of brain tumor, but few etiologic factors of primary glioma have been identified. Previous observational research has shown an association between viral infection and glioma risk. In this study, we used Mendelian randomization (MR) analysis to explore the direction and magnitude of the causal relationship between viral infection and glioma. Methods: We conducted a two-sample bidirectional MR analysis using genome-wide association study (GWAS) data. Summary statistics data of glioma were collected from the largest meta-analysis GWAS, involving 12,488 cases and 18,169 controls. Single-nucleotide polymorphisms (SNPs) associated with exposures were used as instrumental variables to estimate the causal relationship between glioma and twelve types of viral infections from corresponding GWAS data. In addition, sensitivity analyses were performed. Results: After correcting for multiple tests and sensitivity analysis, we detected that genetically predicted herpes zoster (caused by Varicella zoster virus (VZV) infection) significantly decreased risk of low-grade glioma (LGG) development (OR = 0.85, 95% CI: 0.76-0.96, P = 0.01, FDR = 0.04). No causal effects of the other eleven viral infections on glioma and reverse causality were detected. Conclusions: This is one of the first and largest studies in this field. We show robust evidence supporting that genetically predicted herpes zoster caused by VZV infection reduces risk of LGG. The findings of our research advance understanding of the etiology of glioma

Theory of non-equilibrium vertex correction

For realistic nanostructures, there are inevitably some degree ofdisorder such as impurity atoms, imperfect lattices, surfaceroughness, etc.. For situations where disorder locate randomly inthe nanostructure, any calculated quantum transport results shouldbe averaged over disorder distributions. A brute force approach isto generate many disorder configurations, calculate each of them,and then average the results. For atomistic first principlesmodeling, such a brute force averaging is computationallyprohibitive - if not impossible, to perform. It is therefore veryimportant and useful to develop a theoretical framework where thedisorder averaging is done analytically before atomic firstprinciples analysis is carried out.In this thesis, we have developed such a first principlesnon-equilibrium quantum transport theory and its associated modelingsoftware for predicting disorder scattering in nano-electronicdevices. Our theoretical formalism is based on carrying out densityfunctional theory (DFT) within the Keldysh non-equilibrium Green'sfunction (NEGF) framework, and a non-equilibrium vertex correction(NVC) theory for handling disorder configurational average at thenon-equilibrium density matrix level. In our theory, we use thecoherent potential approximation to calculate disorder averaging ofthe device Hamiltonian and one particle Green's functions, and useNVC to calculate correlated multiple impurity scattering at thenon-equilibrium density matrix level. After the NEGF-DFT-NVCself-consistent calculation is converged, we calculate thetransmission coefficients by a second, unavoidable, vertexcorrection. The NEGF-DFT-NVC theory allows us to predictnon-equilibrium quantum transport properties of nanoelectronicdevices with atomistic disorder from first principles without anyphenomenological parameters. The theory and implementation detailsare presented.We have applied the NEGF-DFT-NVC method to investigate severalimportant problems associated with disorder scattering innano-electronic device systems. These include interface roughnessscattering in Fe/vacuum/Fe magnetic tunnel junctions; the diffusivescattering of carriers due to oxygen vacancies in Fe/MgO/Fe magnetictunnel junctions; the surface roughness scattering that enhancesresistivity of copper interconnect wires; and effects of barrierlayer coating for Cu interconnects. Our investigations reveal veryimportant role played by the atomic level defects and impurities toboth equilibrium and nonequilibrium quantum transport properties,and results compare favorably with the corresponding experimentaldata.Dans le cas de nanostructures concrètes, un certain degré de désordre apparaît inévitable tel que la présence d'impuretés, de structures cristallines imparfaites, de surfaces rugueuses, etc. Dans les situations où le désordre se matérialise aléatoirement dans la nanostructure, tout calcul de transport quantique devrait être réalisé en tant que moyenne sur plusieurs distributions désordonnées. Une approche par force brute consiste à générer plusieurs configurations désordonnées, calculer les propriétés d'intérêt pour chacune d'entre elles, et ensuite effectuer la moyenne des résultats. Dans le cas de la modélisation atomique à partir des principes premiers, une telle moyenne par force brute est prohibitive en terme de temps de calcul - sinon impossible. Il est ainsi très important et utile de développer un cadre théorique où la moyenne de désordre est faite analytiquement avant que l'analyse par les principes premiers ne soit effectuée. Dans cette thèse, nous avons développé une telle théorie de transport quantique hors équilibre à partir des principes premiers et le logiciel de modélisation associé pour la prédiction de la diffusion par désordre dans des dispositifs nanoélectroniques. Notre formalisme théorique est basé sur l'utilisation de la théorie de la fonctionnelle de densité (DFT) dans le cadre de la fonction de Green hors équilibre de Keldysh(NEGF), et sur l'emploi d'une correction de sommet hors équilibre (NVC) pour le traitement des moyennes configurationnelles de désordre au niveau de la matrice de densité hors équilibre. Dans notre théorie, nous utilisons l'approximation du potentiel cohérent afin de calculer les moyennes de désordre de l'Hamiltonien du dispositif et les fonctions de Green à une particule, et nous utilisons la NVC pour calculer la diffusion par impuretés multiples corrélée au niveau de la matrice de densité hors équilibre. Après que le calcul auto-cohérent NEGF-DFT-NVC ait convergé, nous calculons les coefficients de transmission par le biais d'une seconde correction de sommet inévitable. La théorie NEGF-DFT-NVC nous permet de prédire les propriétés de transport quantique hors équilibre de dispositifs nanoélectroniques avec désordre au niveau atomique à partir des principes premiers sans aucun paramètre phénoménologique. La théorie et les détails d'implémentation sont présentés dans ce travail. Nous avons appliqué la méthode NEGF-DFT-NVC afin d'examiner plusieurs problèmes importants associés à la diffusion par désordre dans des systèmes de dispositif nanoélectronique. Cela inclut la diffusion par rugosité de surface dans des jonctions tunnel magnétiques Fe/vide/Fe; la diffusion due à des lacunes d'oxygène dans des jonctions tunnel magnétiques Fe/MgO/Fe; la diffusion par rugosité de surface qui décuple la resistivité de fils deconnexion en cuivre; et les effets des revêtements couche barrière pour des connexionsen Cu. Notre étude révèle le rôle très important joué par les défauts de niveau atomique et les impuretés vis-à-vis des propriétés de transport quantique à la fois en équilibre et hors équilibre, et les résultats se comparent favorablement aux données expérimentales correspondantes

Giant Influence of Clustering and Anti-Clustering of Disordered Surface Roughness on Electronic Tunneling

This work reveals the giant influence of spatial distribution of disordered surface roughness on electron tunneling, which is of immediate relevance to the magneto tunnel device and imaging technologies. We calculate the spin-dependent tunneling in Fe/vacuum/Fe junction with disordered surface roughness with the first-principles non-equilibrium dynamical cluster theory. It is found that, at high concentration of surface roughness, different spatial distributions, including the clustered, anti-clustered and completely random roughness characterized by Warren–Cowley parameters, present large deviations from each other in all spin channels. By changing from clustered to anti-clustered roughness, it is surprising that spin polarization of tunneling in parallel configuration (PC) can be drastically reversed from –0.52 to 0.93, while complete randomness almost eliminates the polarization. It is found that the anti-clustered roughness can dramatically quench the tunneling of minority spin in both PC and anti-PC by orders of magnitude, but significantly enhance the transmission of majority spin in PC (by as large as 40%) compared to the results of clustered roughness, presenting distinct influences of differently correlated surface roughness. The spatial correlation of disordered surface roughness can significantly modify the surface resonance of Fe minority spin.</jats:p