https://backend.orbit.dtu.dk/ws/files/128737511/Untitled.pdf

Improvements on non-equilibrium and transport Green function techniques: The next-generation TRANSIESTA

Based on the nonequilibrium Green's function (NEGF) coupled with density function theory (DFT), namely, NEGF-DFT quantum transport theory, we propose an efficient formalism to calculate the transient current of molecular devices under a step-like pulse from first principles. By combining NEGF-DFT with the complex absorbing potential (CAP), the computational complexity of our formalism (NEGF-DFT-CAP) is proportional to $O(N)$ where $N$ is the number of time steps in the time-dependent transient current calculation. Compared with the state-of-the-art algorithm of first-principles time-dependent calculation that scales with at least ${N}^{2}$, this order $N$ technique drastically reduces the computational burden making it possible to tackle realistic molecular devices. We have presented a detailed discussion on how to implement this scheme numerically from first principles. To check the accuracy of our method, we carry out the benchmark calculation compared with NEGF-DFT formalism and they agree well with each other. As an application of this method, we investigate the transient current of a molecular device Al--C${}_{3}$--Al from first principles.

/pdf/first-principles-investigation-of-transient-current-in-3sh1zvmq9b.pdf

First-principles investigation of transient current in molecular devices by using complex absorbing potentials

Multiterminal molecular devices controlled with the quantum interference effect (QIE) are studied by using a nonequiblium Green’s function (NEGF) method with density functional theory (DFT). An antiresonance quantum interference connection of the source and drain leads, which gives almost zero transmission in the absence of the gate lead, is considered for benzenedithiol (BDT) and 18-annulene molecules. We investigate how the gate lead approaching the molecule affects the quantum interference effect. The results show that the gate lead strongly influences the transmission spectra, and the behaviors of the transmission spectra in the QIE-controlled molecular devices depend on the detailed molecular orbital structure and the coupling strength of the leads and molecules. In particular, the σ-type orbital between the gate lead and the molecule has a strong effect on the transmission spectra at the Fermi level, and it masks the QIE control of the gate lead in some cases.

First-Principles Study of Multiterminal Quantum Interference Controlled Molecular Devices

Nano-electronics industry has during the past decade decreased feature sizes to roughly 10nm. Such feature sizes are at the quantum limit, requiring a description at the quantum mechanical level. Parallel to the experimental work reside the theoretical tools used to investigate and understand such systems. These theoretical tools still have a high computational requirement. Thus more efficient algorithms are needed to perform studies on even larger systems. Although the gap between the theoretical tools and the experimental setups are reduced, there is still a gap, and the used theoretical methods require revised algorithms. Furthermore, the advent of 2D materials may prove prominent in future nanoelectronics for electronic and heat transport devices. Such materials include the Nobel Prize winning material, graphene which has unique properties. The main focus of the work presented in this thesis has been to introduce extensions to the non-equilibrium Green function code TranSIESTA which will help reduce the gap between experimental and theoretical studies. One main achievement in this work is a truly Ne ≥ 1 electrode implementation. Another contribution is a general Ne ≥ 1 tight-binding transport code which enables not only electronic transport but also phonon transport. This tight-binding code includes features such as, bond-currents, transmission projections and bias-interpolations. For both codes the inversion algorithm for calculating the Green function is revised. We implement a high performing block-tridiagonal algorithm for calculating the Green function and spectral density function — even for Ne > 2. This is accomplished by bandwidth reduction schemes that increase the quasi-1D interpretation. We also present a new gating method capable of calculating gating effects. A graphite example is used to highlight the importance of the quantum capacitance that is evident in low density of states systems. Additionally the gating method was used in nonequilibrium to study the gate-bias dependence on graphene nano-constrictions. This indicated a pinning effect arising due to differences in coupling strength between the device and the two electrodes. Two studies are presented using the non-equilibrium method with Ne = 3. First, graphene T-junctions are studied to uncover potential interconnects in future graphene based devices. This T-junction is studied under two non-equilibrium situations. Our second study is a graphene Scanning TunnellingMicroscopy setup where the inelastic current is calculated. We show, that the experimental inelastic signal may be fully recovered using density functional theory and non-equilibrium Green function techniques.

Computational Tools and Studies of Graphene Nanostructures

Correlated electron transport across atomic and molecular tunnel junctions

A method to calculate the transmission coefficient in multiterminal systems is presented. By adding a complex absorbing potential to the Hamiltonian of the semi-infinite leads, the problem of inverting an infinite dimensional matrix is transformed into a finite dimensional eigenvalue problem. Using this approach transmission coefficients are calculated for all energies at once. The accuracy of the approach is demonstrated with an analytically solvable model system. Numerical examples of a four-terminal graphene cross junction and six-terminal carbon nanotube junction are presented.

/pdf/calculation-of-electron-transport-in-multiterminal-systems-gpunqcg0y1.pdf

Calculation of electron transport in multiterminal systems using complex absorbing potentials

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Calculation of electron transport in multiterminal systems using complex absorbing potentials