arXiv subjects

Aleksander Bach Lorentzen

Publications and source records attributed to Aleksander Bach Lorentzen.

5 recordsLinked to original sources

arXiv2026

Zandpack: A General Tool for Time-dependent Transport Simulation of Nanoelectronics

The auxiliary mode approach to time-dependent open quantum system calculations is implemented and refined to yield a feasible computational approach to simulate nanostructures far from equilibrium. It is done by a careful diagonalization of the electrode level-width function, and provides an efficient approach which can simulate large, open systems at the level of time-dependent density functional theory. The approach, as given in this work, is implemented in the new open-source code Zandpack. The framework is applied to three systems perturbed by the same THz electromagnetic field pulse-form: 1) A Hubbard model for hydrogen on graphene is used to calculate spin-currents, mutual information, spin-transitions, and a pump-probe setup. 2) An armchair graphene nanoribbon (AGNR) probed by a metal tip showing electrons excited from the valence band of the AGNR into the tip via electron-electron interactions. 3) A gold break-junction is modeled with various gap distances, and displays behavior that is more different from the adiabatic case as the gap widens. In the examples 2 and 3, we develop and use a general linearization scheme for time-dependent open system calculations, which utilizes the DFTB+ or SIESTA codes.

cond-mat.mes-hall↗

arXiv2024

Twisted nanoporous graphene/graphene bilayers: electronic decoupling and chiral currents

We investigate bilayers of nanoporous graphene (NPG), laterally bonded carbon nanoribbons, and graphene. The electronic and transport properties are explored as a function of the interlayer twist angle using an atomistic tight-binding model combined with non-equilibrium Green's functions. At small twist angles ($θ\lesssim 10^\circ$), NPG and graphene are strongly coupled, as revealed by the hybridization of their electronic bands. As a result, when electrons are point-injected in NPG, a substantial interlayer transmission occurs and an electronic Talbot-like interference pattern appears in the current flow on both layers. Besides, the twist-induced mirror-symmetry-breaking leads to chiral features in the injected current. Upon increasing the twist angle, the coupling is weakened and the monolayer electronic properties are restored. Furthermore, we demonstrate the emergence of resonant peaks in the electronic density of states for small twist angles, allowing to probe the twist-dependent interlayer coupling via scanning tunneling microscopy.

cond-mat.mes-hall↗

arXiv2024

First Principles Study of Electronic Structure and Transport in Graphene Grain Boundaries

Grain boundaries play a major role for electron transport in graphene sheets grown by chemical vapor deposition. Here we investigate the electronic structure and transport properties of idealized graphene grain boundaries (GBs) in bi-crystals using first principles density functional theory (DFT) and non-equilibrium Greens functions (NEGF). We generated 150 different grain boundaries using an automated workflow where their geometry is relaxed with DFT. We find that the GBs generally show a quasi-1D bandstructure along the GB. We group the GBs in four classes based on their conductive properties: transparent, opaque, insulating, and spin-polarizing and show how this is related to angular mismatch, quantum mechanical interference, and out-of-plane buckling. Especially, we find that spin-polarization in the GB correlates with out-of-plane buckling. We further investigate the characteristics of these classes in simulated scanning tunnelling spectroscopy and diffusive transport along the GB which demonstrate how current can be guided along the GB.

cond-mat.mes-hall↗

arXiv2023

Tunable interfacial chemisorption with atomic-level precision in a graphene WSe2 heterostructure

It has long been an ultimate goal to introduce chemical doping at the atomic level to precisely tune properties of materials. Two-dimensional materials have natural advantage because of its highly-exposed surface atoms, however, it is still a grand challenge to achieve this goal experimentally. Here, we demonstrate the ability to introduce chemical doping in graphene with atomic-level precision by controlling chemical adsorption of individual Se atoms, which are extracted from the underneath WSe2, at the interface of graphene-WSe2 heterostructures. Our scanning tunneling microscopy (STM) measurements, combined with first-principles calculations, reveal that individual Se atoms can chemisorbed on three possible positions in graphene, which generate distinct pseudospin-mediated atomic-scale vortices in graphene. We demonstrate that the chemisorbed positions of individual Se atoms can be manipulated by STM tip, which enables us to achieve atomic-scale controlling quantum interference of the pseudospin-mediated vortices in graphene. This result offers the promise of controlling properties of materials through chemical doping with atomic-level precision.

cond-mat.mtrl-sci↗

arXiv2023

Manipulation of magnetization and spin transport in hydrogenated graphene with THz pulses

Terahertz (THz) field pulses can now be applied in Scanning Tunnelling Microscopy (THz-STM) junction experiments to study time resolved dynamics. The relatively slow pulse compared to the typical electronic time-scale calls for approximations based on a time-scale separation. Here, we contrast three methods based on non-equilibrium Green's functions (NEGF): (i) the steady-state, adiabatic results, (ii) the lowest order dynamic expansion in the time-variation (DE), and (iii) the auxiliary mode (AM) propagation method without approximations in the time-variation. We consider a concrete THz-STM junction setup involving a hydrogen adsorbate on graphene where the localized spin polarization can be manipulated on/off by a local field from the tip electrode and/or a back-gate affecting the in-plane transport. We use steady-state NEGF combined with Density Functional Theory (DFT-NEGF) to obtain a Hubbard model for the study of the junction dynamics. Solving the Hubbard model in a mean-field approximation, we find that the near-adiabatic first order dynamical expansion provides a good description for STM voltage pulses up to the 1 V range.

cond-mat.mes-hall↗