Marta Pelc

This paper addresses the challenge of modeling relaxation dynamics in quantum many-body systems, specifically focusing on electrons in graphene nanoflakes. While quantum many-body techniques effectively describe dynamics up to a few particles, these approaches become computationally intractable for large systems. Larger systems may be tackled with a single-particle approach that, however, struggles to incorporate relaxation effects. Existing relaxation models encounter issues such as an inability to capture system complexity and violation of the Pauli principle. In this paper, we propose a single-particle model that accounts for various relaxation effects at the crossroads of quantum optics and solid-state photonics, that overcomes the limitations of previous models. Our approach is rooted in the quantum-optical Lindblad model, where relaxation rates are deactivated once the target levels saturate due to the Pauli principle. This approach is referred to as the saturated-Lindblad model. To validate the predictions of the saturated-Lindblad model, we confront them against phenomenological and many-body physics models in low-dimensional systems, including atomic chains and graphene nanoflakes. Remarkably, the saturated-Lindblad model exhibits excellent agreement with few-body calculations, distinguishing itself from other existing approaches. Moreover, by assigning different relaxation rates to different transitions, we successfully reproduce cascade deexcitation dynamics and predict emission spectra. The saturated-Lindblad model offers the ability to describe dynamics in systems of practical sizes, encompassing a wide range of structures that can be effectively captured within the single-particle description.

Our stability study on the stackings in trilayer graphene finds that the rhombohedral one turns out to be favorable. Furthermore, lattice distortions can change the relative stability betwen the Bernal and rhombohedral stacking.

Adsorbed atoms (adatoms) coupled to the matrix of solid state host materials as impurities can significantly modify their properties. Especially in low-dimensional materials, such as one-dimensional organic polymer chains or quasi-one-dimensional graphene nanoribbons, intriguing manipulation of the optical properties, such as the absorption cross section, is possible. The most widely used approach to couple quantum emitters to optical antennas is based on the Purcell effect. This formalism, however, does not comprise charge transfer from the emitter to the antenna, but only spontaneous emission of the quantum emitter into the tailored photonic environment, that is evoked by the antenna. To capture such effects, we present a tight-binding formalism to couple an adatom to a finite Su-Schrieffer-Heeger chain, where the former is treated as a two-level system and the latter acts as an optical antenna. We systematically analyze how the coupling strength and the position of the adatom influence the optical properties of the molecular chains in the model. We take into account charge transfer from the adatom to the chain and vice versa via an intersystem hopping parameter, and also include Coulomb interaction within the chain as well as between the adatom and the chain. We show that coupling the adatom to one of the bulk atoms of the linear chain results in a substantial change in optical properties already for comparatively small coupling strengths. We also find that the position of the adatom crucially determines if and how the optical properties of the chains are altered. Therefore, we identify this adatom-chain hybrid system as a tunable platform for light-matter interaction at the nanoscale.

Plasmonic nanostructures attract tremendous attention as they confine electromagnetic fields well below the diffraction limit while simultaneously sustaining extreme local field enhancements. To fully exploit these properties, the identification and classification of resonances in such nanostructures is crucial. Recently, a novel figure of merit for resonance classification has been proposed [Müller et al., J. Phys. Chem. C 124, 24331–24343 (2020)] and its applicability was demonstrated mostly to toy model systems. This novel measure, the energy-based plasmonicity index (EPI), characterizes the nature of resonances in molecular nanostructures. The EPI distinguishes between either a single-particle-like or a plasmonic nature of resonances based on the energy space coherence dynamics of the excitation. To advance the further development of this newly established measure, we present here its exemplary application to characterize the resonances of graphene nanoantennas. In particular, we focus on resonances in a doped nanoantenna. The structure is of interest, as a consideration of the electron dynamics in real space might suggest a plasmonic nature of selected resonances in the low doping limit but our analysis reveals the opposite. We find that in the undoped and moderately doped nanoantenna, the EPI classifies all emerging resonances as predominantly single-particle-like, and only after doping the structure heavily, the EPI observes plasmonic response.

Experiments in gated bilayer graphene with stacking domain walls present topological gapless states protected by no-valley mixing. Here we research these states under gate voltages using atomistic models, which allow us to elucidate their origin. We find that the gate potential controls the layer localization of the two states, which switches non-trivially between layers depending on the applied gate voltage magnitude. We also show how these bilayer gapless states arise from bands of single-layer graphene by analyzing the formation of carbon bonds between layers. Based on this analysis we provide a model Hamiltonian with analytical solutions, which explains the layer localization as a function of the ratio between the applied potential and interlayer hopping. Our results open a route for the manipulation of gapless states in electronic devices, analogous to the proposed writing and reading memories in topological insulators.

Recent experiments [L. Ju, et al., Nature, 2015, 520, 650] confirm the existence of gapless states at domain walls created in gated bilayer graphene, when the sublattice stacking is changed from AB to BA. These states are significant because they are topologically protected, valley-polarized and give rise to conductance along the domain wall. Current theoretical models predict the appearance of such states only at domain walls, which preserve the sublattice order. Here we show that the appearance of the topologically protected states in stacking domain walls can be much more common in bilayer graphene, since they can also emerge in unexpected geometries, e.g., at grain boundaries with atomic-scale topological defects. We focus on a bilayer system in which one of the layers contains a line of octagon–double pentagon defects that mix graphene sublattices. We demonstrate that gap states are preserved even with pentagonal defects. Remarkably, unlike previous predictions, the number of gap states changes by inverting the gate polarization, yielding an asymmetric conductance along the grain boundary under gate reversal. This effect, linked to defect states, should be detectable in transport measurements and could be exploited in electrical switches.

We investigate the electronic and transport properties of gated bilayer graphene with one corrugated layer, which results in a stacking AB/BA boundary. When a gate voltage is applied to one layer, topologically protected gap states appear at the corrugation, which reveal as robust transport channels along the stacking boundary. With increasing size of the corrugation, more localized, quantum-well-like states emerge. These finite-size states are likewise conductive along the fold, but in contrast to the stacking boundary states, which are gapless, they present a gap. Additionally, we have studied periodic corrugations in bilayer graphene; our findings show that such corrugations between AB- and BA-stacked regions also behave as conducting channels in the direction of the folds. These topological states could be easily identified due to the spatial shape of the corrugations.