arXiv subjects

Thomas Garm Pedersen

Publications and source records attributed to Thomas Garm Pedersen.

At least 19 recordsLinked to original sources

arXiv2026

One-Dimensional Frenkel and Wannier Excitons in Electric Fields: Stark Effect, Ionization, Polarizability and Electroabsorption

One-dimensional semiconductors are characterized by strongly bound excitons. Therefore, the Frenkel regime of excitons localized within a few unit cells is readily reached and traditional Wannier exciton models become inadequate. In the presence of strong electric fields, excitons are polarized and, in extreme cases, ionized. Such strong-field effects have previously been described analytically for Wannier excitons. In the present work, we show that analytical results can be extended to the more involved Frenkel case as well. Hence, by analytically solving the difference equation describing Frenkel excitons in electric fields, we derive close-form expressions for resonances providing Stark shifts and ionization rates. Moreover, closed-form results for exciton electroabsorption spectra and dynamic polarizability are obtained.

cond-mat.mes-hall↗

arXiv2026

1Q: First-Generation Wireless Systems Integrating Classical and Quantum Communication

We introduce the concept of 1Q, the first wireless generation of integrated classical and quantum communication. 1Q features quantum base stations (QBSs) that support entanglement distribution via free-space optical links alongside traditional radio communications. Key new components include quantum cells, quantum user equipment (QUEs), and hybrid resource allocation spanning classical time-frequency and quantum entanglement domains. Several application scenarios are discussed and illustrated through system design requirements for quantum key distribution, blind quantum computing, and distributed quantum sensing. A range of unique quantum constraints are identified, including decoherence timing, fidelity requirements, and the interplay between quantum and classical error probabilities. Protocol adaptations extend cellular connection management to incorporate entanglement generation, distribution, and handover procedures, expanding the Quantum Internet to the cellular wireless.

cs.NI↗

arXiv2025

Electrical Control of Excitons in Bare-MoSe2 and MoSe2/NbSe2 Heterostructure

Monolayer transition metal dichalcogenides (TMDCs) are promising materials for next-generation optoelectronic devices, owing to their strong excitonic responses and atomic thickness. Controlling their light emission electrically is a crucial step towards realizing practical nanoscale optoelectronic devices such as light-emitting diodes and optical modulators. However, photoluminescence (PL) quenching in van der Waals TMDC/metal heterostructures, caused by ultrafast interlayer charge or energy transfer, impedes such electrical modulation. Here, we investigate monolayer-MoSe2/bulk-NbSe2 heterostructures and demonstrate that a vertical electric field can effectively recover the PL intensity up to ~ 80% of bare-MoSe2. Furthermore, our analysis reveals that the room temperature PL intensity can be tuned by nearly three orders of magnitude in bare-MoSe2 and by about one order of magnitude in MoSe2/NbSe2 heterostructures. First-principles calculations incorporating spin-orbit coupling reveal that the perpendicular electric fields drive a transition from a direct to an indirect bandgap, fundamentally altering the optical response in the heterostructure. Unlike bare-MoSe2, the heterostructure exhibits a pronounced thermal dependence of the enhancement factor, implying that exciton lifetime dominates over interfacial transfer processes. Our findings demonstrate reversible, electric-field-driven PL control at a TMDC/metal interface, providing a pathway to electrically tunable light emission and improved contact engineering in two-dimensional optoelectronic devices.

physics.app-ph↗

arXiv2021

Dynamic polarizability of low-dimensional excitons

Excitons in low-dimensional materials behave mathematically as confined hydrogen atoms. An appealing unified description of confinement in quantum wells or wires, etc., is found by restricting space to a fractional dimension 1 < D <= 3 serving as an adjustable parameter. We compute the dynamic polarizability of D-dimensional excitons in terms of discrete and continuum oscillator strengths. Analyzing exact sum rules, we show that continuum contributions are increasingly important in low dimensions. The dynamical responses of excitons in various dimensions are compared. Finally, an exact and compact closed-form expression for the dynamic polarizability is found. This completely general formula takes D as input and provides exact results for arbitrary frequency.

cond-mat.mes-hall↗

arXiv2021

Exciton absorption, band structure, and optical emission in biased bilayer graphene

Biased bilayer graphene (BBG) is a variable band gap semiconductor, with a strongly field-dependent band gap of up to $300 \, \text{meV}$, making it of particular interest for graphene-based nano-electronic and -photonic devices. The optical properties of BBG are dominated by strongly bound excitons. We perform ab initio density-functional-theory+Bethe-Salpeter-equation modelling of excitons in BBG and calculate the exciton band structures and optical matrix elements for field strengths in the range $30-300 \, \text{mV}/{\unicode{x212B}}$. The exciton properties prove to have a strong field dependence, with both energy ordering and dipole alignment varying significantly between the low and high field regions. Namely, at low fields we find a mostly dark ground state exciton, as opposed to high fields, where the lowest exciton is bright. Also, excitons preferentially align with a dipole moment opposite the field, due to the field-induced charge transfer in the ground state of BBG. However, in stronger fields, this alignment becomes energetically less favorable. Additionally, the bright excitons show particle- and light-like bands similar to monolayer transition metal dichalchogenides. Finally, we model the radiative lifetimes and emission properties of BBG, which prove to be strongly dependent on temperature in addition to field strength.

cond-mat.mes-hall↗

arXiv2021

Optical Emission from Light-like and Particle-like Excitons in Monolayer Transition Metal Dichalcogenides

Several monolayer transition metal dichalcogenides (TMDs) are direct band gap semiconductors and potentially efficient emitters in light emitting devices. Photons are emitted when strongly bound excitons decay radiatively, and accurate models of such excitons are important for a full understanding of the emission. Importantly, photons are emitted in directions uniquely determined by the exciton center of mass momentum and with lifetimes determined by the exciton transition matrix element. The exciton band structures of two-dimensional hexagonal materials, including TMDs, are highly unusual with coexisting particle- and light-like bands. The latter is non-analytic with emission selection rules essentially opposite to the particle-like states, but has been ignored in analyses of TMD light emission so far. In the present work, we analyse the temperature and angular dependence of light emission from both exciton species and point out several important consequences of the unique exciton band structure. Within a first-principles Density-Functional-Theory+Bethe-Salpeter-Equation framework, we compute exciton band structures and optical matrix elements for the important TMDs MoS2, MoSe2, WS2, and WSe2. At low temperature, only the particle-like band is populated and our results agree with previous work. However, at slightly elevated temperatures, a significant population of the light-like band leads to modified angular emission patterns and lifetimes. Clear experimental fingerprints are predicted and explained by a simple four-state model incorporating spin-orbit as well as intervalley exchange coupling.

cond-mat.mtrl-sci↗

arXiv2020

On the two-dimensional quantum confined Stark effect in strong electric fields

We consider a Stark Hamiltonian on a two-dimensional bounded domain with Dirichlet boundary conditions. In the strong electric field limit we derive, under certain local convexity conditions, a three-term asymptotic expansion of the low-lying eigenvalues. This shows that the excitation frequencies are proportional to the square root of the boundary curvature at a certain point determined by the direction of the electric field.

math-ph↗

arXiv2017

Electron trajectories and magnetotransport in nanopatterned graphene under commensurability conditions

Commensurability oscillations in the magnetotransport of periodically patterned systems, emerging from the interplay of cyclotron orbit and the pattern periodicity, are a benchmark of mesoscopic physics in electron gas systems. Exploiting similar effects in 2D materials would allow exceptional control of electron behaviour, but is hindered by the requirement to maintain ballistic transport over large length scales. Recent experiments have overcome this obstacle and observed distinct magnetoresistance commensurability peaks for perforated graphene sheets (antidot lattices). Interpreting the exact mechanisms behind these peaks is of key importance, particularly in graphene where a range of regimes are accessible by varying the electron density. In this work a fully atomistic, device-based simulation of magnetoresistance experiments allows us to analyse both the resistance peaks and the current flow at commensurability conditions. Magnetoresistance spectra are found in excellent agreement with experiment, but we show that a semi-classical analysis, in terms of simple skipping or pinned orbits, is insufficient to fully describe the corresponding electron trajectories. Instead, a generalised mechanism in terms of states bound to individual antidots, or to groups of antidots, is required. Commensurability features are shown to arise when scattering between such states is enhanced. The emergence and suppression of commensurability peaks is explored for different antidot sizes, magnetic field strengths and electron densities. The insights gained from our study will guide the design and optimization of future experiments with nanostructured graphene.

cond-mat.mes-hall↗

arXiv2016

High-order harmonic generation from gapped graphene: perturbative response and transition to non-perturbative regime

We consider the interaction of gapped graphene in the two-band approximation using an explicit time-dependent approach. In addition to the full high-order harmonic generation (HHG) spectrum, we also obtain the perturbative harmonic response using the time-dependent method at photon energies covering all the significant features in the responses. The transition from the perturbative to the fully non-perturbative regime of HHG at these photon energies is studied in detail.

cond-mat.mtrl-sci↗

arXiv2016

Magnetic edge states and magnetotransport in graphene antidot barriers

Magnetic fields are often used for characterizing transport in nanoscale materials. Recent magnetotransport experiments have demonstrated that ballistic transport is possible in graphene antidot lattices (GALs). These experiments have inspired the present theoretical study of GALs in a perpendicular magnetic field. We calculate magnetotransport through graphene antidot barriers (GABs), which are finite rows of antidots arranged periodically in a pristine graphene sheet, using a tight-binding model and the Landauer-Büttiker formula. We show that GABs behave as ideal Dirac mass barriers for antidots smaller than the magnetic length, and demonstrate the presence of magnetic edge states, which are localized states on the periphery of the antidots due to successive reflections on the antidot edge in the presence of a magnetic field. We show that these states are robust against variations in lattice configuration and antidot edge chirality. Moreover, we calculate the transmittance of disordered GABs and find that magnetic edge states survive a moderate degree of disorder. Due to the long phase-coherence length in graphene and the robustness of these states, we expect magnetic edge states to be observable in experiments as well.

cond-mat.mes-hall↗

arXiv2015

Hypergeometric resummation of self-consistent sunset diagrams for electron-boson quantum many-body systems out of equilibrium

A newly developed hypergeometric resummation technique [H. Mera et al., Phys. Rev. Lett. 115, 143001 (2015)] provides an easy-to-use recipe to obtain conserving approximations within the self-consistent nonequilibrium many-body perturbation theory. We demonstrate the usefulness of this technique by calculating the phonon-limited electronic current in a model of a single-molecule junction within the self-consistent Born approximation for the electron-phonon interacting system, where the perturbation expansion for the nonequilibrium Green function in powers of the free bosonic propagator typically consists of a series of non-crossing \sunset" diagrams. Hypergeometric resummation preserves conservation laws and it is shown to provide substantial convergence acceleration relative to more standard approaches to self-consistency. This result strongly suggests that the convergence of the self-consistent \sunset" series is limited by a branch-cut singularity, which is accurately described by Gauss hypergeometric functions. Our results showcase an alternative approach to conservation laws and self-consistency where expectation values obtained from conserving perturbation expansions are \summed" to their self-consistent value by analytic continuation functions able to mimic the convergence-limiting singularity structure.

cond-mat.mes-hall↗

arXiv2015

Stark effect in low-dimension hydrogen

Studies of atomic systems in electric fields are challenging because of the diverging perturbation series. However, physically meaningful Stark shifts and ionization rates can be found by analytical continuation of the series using appropriate branch cut functions. We apply this approach to low-dimensional hydrogen atoms in order to study the effects of reduced dimensionality. We find that modifications by the electric field are strongly suppressed in reduced dimensions. This finding is explained from a Landau-type analysis of the ionization process.

quant-ph↗

arXiv2015

Self-consistent model of edge doping in graphene

Dopants positioned near edges in nanostructured graphene behave differently from bulk dopants. Most notable, the amount of charge transferred to delocalized states (i.e. doping efficiency) depends on position as well as edge chirality. We apply a self-consistent tight-binding model to analyze this problem focusing on substitutional nitrogen and boron doping. Using a Greens function technique, very large structures can be studied and artificial interactions between dopants in periodically repeated simulations cells are avoided. We find pronounced signatures of edges in the local impurity density of states. Importantly, the doping efficiency is found to oscillate with sublattice position, in particular, for dopants near zigzag edges. Finally, to assess the effect of electron-electron interactions, we compute the self-energy corrected Greens function.

cond-mat.mes-hall↗

arXiv2014

Stability and magnetization of free-standing and graphene-embedded iron membranes

Inspired by recent experimental realizations of monolayer Fe membranes in graphene perforations, we perform ab initio calculations of Fe monolayers and membranes embedded in graphene in order to assess their structural stability and magnetization. We demonstrate that monolayer Fe has a larger spin magnetization per atom than bulk Fe and that Fe membranes embedded in graphene exhibit spin magnetization comparable to monolayer Fe. We find that free-standing monolayer Fe is structurally more stable in a triangular lattice compared to both square and honeycomb lattices. This is contradictory to the experimental observation that the embedded Fe membranes form a square lattice. However, we find that embedded Fe membranes in graphene perforations can be more stable in the square lattice configuration compared to the triangular. In addition, we find that the square lattice has a lower edge formation energy, which means that the square Fe lattice may be favored during formation of the membrane.

cond-mat.mes-hall↗

arXiv2013

Plasmon-phonon coupling in large-area graphene dot and antidot arrays

Nanostructured graphene on SiO2 substrates pave the way for enhanced light-matter interactions and explorations of strong plasmon-phonon hybridization in the mid-infrared regime. Unprecedented large-area graphene nanodot and antidot optical arrays are fabricated by nanosphere lithography, with structural control down to the sub-100 nanometer regime. The interaction between graphene plasmon modes and the substrate phonons is experimentally demonstrated and structural control is used to map out the hybridization of plasmons and phonons, showing coupling energies of the order 20 meV. Our findings are further supported by theoretical calculations and numerical simulations.

cond-mat.mes-hall↗

arXiv2013

Theory of second harmonic generation in few-layered MoS2

Recent experimental results have demonstrated the ability of monolayer MoS$_2$ to efficiently generate second harmonic fields with susceptibilities between 0.1 and 100 nm/V. However, no theoretical calculations exist with which to interpret these findings. In particular, it is of interest to theoretically estimate the modulus of the second harmonic response, since experimental reports on this differ by almost three orders of magnitude. Here, we present single-particle calculations of the second harmonic response based on a tight-binding band structure. We compare directly with recent experimental findings and include in the discussion also spectral features and the effects of multiple layers.

cond-mat.mes-hall↗

arXiv2013

Hofstadter butterflies and magnetically induced band gap quenching in graphene antidot lattices

We study graphene antidot lattices (GALs) in magnetic fields. Using a tight-binding model and a recursive Green's function technique that we extend to deal with periodic structures, we calculate Hofstadter butterflies of GALs. We compare the results to those obtained in a simpler gapped graphene model. A crucial difference emerges in the behaviour of the lowest Landau level, which in a gapped graphene model is independent of magnetic field. In stark contrast to this picture, we find that in GALs the band gap can be completely closed by applying a magnetic field. While our numerical simulations can only be performed on structures much smaller than can be experimentally realized, we find that the critical magnetic field for which the gap closes can be directly related to the ratio between the cyclotron radius and the neck width of the GAL. In this way, we obtain a simple scaling law for extrapolation of our results to more realistically sized structures and find resulting quenching magnetic fields that should be well within reach of experiments.

cond-mat.mes-hall↗

arXiv2012

Optical Hall conductivity in bulk and nanostructured graphene beyond the Dirac approximation

We present a perturbative method for calculating the optical Hall conductivity in a tight-binding framework based on the Kubo formalism. The method involves diagonalization only of the Hamiltonian in absence of the magnetic field, and thus avoids the computational problems usually arising due to the huge magnetic unit cells required to maintain translational invariance in presence of a Peierls phase. A recipe for applying the method to numerical calculations of the magneto-optical response is presented. We apply the formalism to the case of ordinary and gapped graphene in a next-nearest neighbour tight-binding model as well as graphene antidot lattices. In both case, we find unique signatures in the Hall response, that are not captured in continuum (Dirac) approximations. These include a non-zero optical Hall conductivity even when the chemical potential is at the Dirac point energy. Numerical results suggest that this effect should be measurable in experiments.

cond-mat.mes-hall↗