arXiv subjects

Gabriel Bester

Publications and source records attributed to Gabriel Bester.

At least 19 recordsLinked to original sources

arXiv2026

Effective long-range attraction of moir\'e excitons under the influence of atomic reconstructions and anisotropic screening

The moir\'e pattern, which emerges due to a relative rotation between two monolayers of transition metal dichalcogenides, features a long lattice period for small twist angles. The resulting band structure modulation acts as an effective potential for interlayer excitons (IXs), which can realize correlated many-body phenomena. Here, we aim for a material-realistic modelling of the exciton-exciton interaction, taking into account lattice reconstructions and an exciton-exciton potential that incorporates the highly anisotropic screening imposed by the two-dimensional bilayer and the dielectric background. We find strong modifications of the on-site interaction induced by the change of the moir\'e potential during lattice reconstructions, while for long-range interactions on the length scale of the moir\'e period, anisotropic dielectric screening leads to a crossover from a repulsive to an attractive interaction. The interaction potential and hopping amplitudes serve as parameters for a Bose-Hubbard model on the moir\'e lattice, which we use to explain correlated behavior of interlayer excitons.

cond-mat.mtrl-sci↗

arXiv2026

Exciton fine structure in CdSe nanoplatelets using a quasi-2D screened configuration-interaction framework

We compute exciton binding energies and fine-structure splittings in CdSe nanoplatelets with two zincblende geometries and one wurtzite geometry, finding that the wurtzite structure exhibits the largest bright-bright splitting due to its intrinsic in-plane anisotropy, while the zincblende structures show smaller but finite splittings arising from atomistic symmetry breaking at edges and corners. These results are obtained using a theoretical framework that we developed, which combines DFT single-particle states with screened configuration interaction, a quasi-2D dielectric screening model, and an efficient Coulomb-cutoff scheme that eliminates periodic-image interactions and enables accurate Coulomb and exchange integrals at low computational cost. This methodology provides a transferable and practical route for studying excitons in CdSe nanoplatelets and other quasi-two-dimensional nanomaterials.

cond-mat.mtrl-sci↗

arXiv2026

Microscopic origin of Rashba coupling from first principles: Layer-resolved orbital asymmetry in transition metal dichalcogenides

Spin-orbit coupling in two-dimensional materials gives rise to a Rashba spin splitting when inversion and mirror symmetries are broken, yet its microscopic origin and quantitative characterization in transition metal dichalcogenides remains incomplete. Both symmetries are broken in certain bilayer structures, enabling Rashba splittings in the absence of external electric fields. We determine this zero-field offset and the Rashba parameters that dictate the spin splitting in the linear regime. Surprisingly, the splitting is substantially smaller in bilayers than in monolayers at typical fields. This is clarified within a perturbative microscopic model, revealing that the spin splitting results from a competition between internal polarization and interlayer hybridization. We further introduce the orbital polarization imbalance as an order parameter that captures the asymmetry of the valence bands and determines the spin ordering of the Rashba-split states. Our results are both quantitative and qualitative, as they clarify the nature and origin of Rashba coupling in transition metal dichalcogenides.

cond-mat.mes-hall↗

arXiv2025

Unconventional bright ground-state excitons in monolayer TiI$_2$ from first-principles calculations

Based on \textit{ab initio} screened configuration interaction calculations we find that TiI$_2$ has a bright exciton ground state and identify two key mechanisms that lead to this unprecedented feature among transition metal dichalcogenides. First, the spin-orbit induced conduction band splitting results in optically allowed spin-alignment for electrons and holes across a significant portion of the Brillouin zone around the $\mathbf{K}$-valley, avoiding band crossings seen in materials like monolayer MoSe$_2$. Second, a sufficiently weak exchange interaction ensures that the bright exciton remains energetically below the dark exciton state. We further show that the bright exciton ground state is stable under various mechanical strains and that trion states (charged excitons) inherit this bright ground state. Our findings are expected to spark further investigation into related materials that bring along the two key features mentioned, as bright ground-state excitons are crucial for applications requiring fast radiative recombination.

cond-mat.mes-hall↗

arXiv2024

GW effects on the topology of type-II Dirac cones in NiTe$_2$, PtSe$_2$ and PtTe$_2$

Many-body correlations are known to be responsible for a broad range of fascinating physical phenomena, introducing corrections that appear elusive at the mean-field level. An example of this is the Lifshitz transition that occurs as the Fermi surface topology changes when {\it e.g.} Coulomb interaction effects break into the picture. In particular, the Fermi velocity renormalization can lead a type-II Weyl semimetal at mean-field level to become a trivial or a type-I Dirac material when correlations are accounted for, which is far from being obvious. In this work we scrutinize the band structure of NiTe$_2$, a material that features a type-II Dirac point near the Fermi level within the mean-field approach. Including GW-level correlations, our findings showcase anisotropic corrections on the Dirac carrier velocity exceeding $100 \, \%$ enhancements, underscoring the nuanced influence of electronic interactions in the band structure. We also consider type-II Dirac crossings in PtSe$_2$ and PtTe$_2$ and observe that including many-body effects via GW the band topology changes, featuring trivial topology and type-I Dirac crossings, respectively. Our findings highlights the necessity to evaluate the many-body effects on non-trivial bands, contributing essential insights to the broader exploration of many-body correlation effects in type-II Dirac points of condensed-matter systems.

cond-mat.mes-hall↗

arXiv2024

Optical Properties of Gated Bilayer Graphene Quantum Dots with Trigonal Warping

We determine the optical properties of gated bilayer graphene quantum dots with trigonal warping (TW) of single-particle energy spectra. The lateral structure of metallic gates confines electrons and holes in a quantum dot (QD) electrostatically. The gated bilayer graphene energy spectrum is characterized by two K-valleys surrounded by three minivalleys with energies depending on the applied vertical electric field. Employing an atomistic tight-binding model, we compute the single-particle QD states and analyze the influence of TW on the energy spectrum as the lateral confining potential depth varies. We find a regime where the QD levels are dominated by the presence of three minivalleys around each K-valley. Next, we compute dipole matrix elements and analyze the oscillator strengths and optical selection rules for optical valence to conduction band transitions. We then include electron-electron interactions by first computing the microscopic Coulomb matrix elements, electron self-energy, and solving the Bethe-Salpeter equation to obtain the excitonic spectrum. Finally, we obtain the absorption spectrum for a shallow confining potential depth, which further amplifies the effects of TW on the optical properties. Our results predict the existence of two degenerate bright exciton states, each built of the three minivalley states that do not exist in the deep confinement regime, where the effects of TW are negligible.

cond-mat.mes-hall↗

arXiv2024

Exciton-Phonon Coupling in Single Band-Gap Engineered ZnCdSe-Dot/CdS-Rod Nanocrystals

Exciton-phonon coupling limits the homogeneous emission linewidth of nanocrystals. Hence, a full understanding of it is crucial. In this work, we statistically investigate exciton-phonon coupling by performing single-particle spectroscopy on Zn$_{1-x}$Cd$_{x}$Se/CdS dot-in-rod nanocrystals at cryogenic temperatures ($T\approx 10~\rm{K}$). In situ cation exchange enables us to analyze different band alignments and thereby different charge-carrier distributions. We find that the relative intensities of the longitudinal optical S- and Se-type phonon replicas correlate with the charge-carrier distribution. Our experimental findings are complemented with quantum mechanical calculations within the effective mass approximation that hint at the relevance of surface charges.

cond-mat.mes-hall↗

arXiv2024

Empirical Band-Gap Correction for LDA-Derived Atomic Effective Pseudopotentials

Atomic effective pseudopotentials enable atomistic calculations at the level of accuracy of density functional theory for semiconductor nanostructures with up to fifty thousand atoms. Since they are directly derived from ab-initio calculations performed in the local density approximation (LDA), they inherit the typical underestimated band gaps and effective masses. We propose an empirical correction based on the modification of the non-local part of the pseudopotential and demonstrate good performance for bulk binary materials (InP, ZnS, HgTe, GaAs) and quantum dots (InP, CdSe, GaAs) with diameters ranging from 1.0 nm to 4.45 nm. Additionally, we provide a simple analytic expression to obtain accurate quasiparticle and optical band gaps for InP, CdSe, and GaAs QDs, from standard LDA calculation.

cond-mat.mtrl-sci↗

arXiv2024

Superexchange Mechanism in Coupled Triangulenes Forming Spin-1 Chains

We show that the origin of the antiferromagnetic coupling in spin-1 triangulene chains, which were recently synthesized and measured by Mishra et al. Nature 598, 287-292 (2021) originates from a superexchange mechanism. This process, mediated by inter-triangulene states, opens the possibility to control parameters in the effective bilinear-biquadratic spin model. We start from the derivation of an effective tight-binding model for triangulene chains using a combination of tight-binding and Hartree-Fock methods fitted to hybrid density functional theory results. Next, correlation effects are investigated within the configuration interaction method. Our low-energy many-body spectrum for $N_{\rm Tr}=2$ and $N_{\rm Tr}=4$ triangulene chains agree well with the bilinear-biquadratic spin-1 chain antiferromagnetic model when indirect coupling processes, and superexchange coupling between triangulene spins are taken into account.

cond-mat.mtrl-sci↗

arXiv2023

Electrically Tunable Fine Structure of Negatively Charged Excitons in Gated Bilayer Graphene Quantum Dots

We predict here the fine structure of an electrically tunable negatively charged exciton (trion) composed of two electrons and a hole confined in a gated bilayer graphene quantum dot (QD). We start with an atomistic approach, allowing us to compute confined electron and confined hole QD states for a structure containing over one million atoms. Using atomistic wavefunctions we compute Coulomb matrix elements and self-energies. In the next step, by solving the Bethe-Salpeter-like equation for trions, we describe a negatively charged exciton, built as a strongly interacting interlayer complex of two electrons in the conduction band and one hole in the valence band. Unlike in conventional semiconducting QDs, we show that the trion contains a fine structure composed of ten states arising from the valley and spin degrees of freedom. Finally, we obtain absorption into and emission from the trion states. We predict the existence of bright low-energy states and propose to extract the fine structure of the trion using the temperature dependence of emission spectra.

cond-mat.mes-hall↗

arXiv2023

Determination of the phonon sidebands in the photoluminescence spectrum of semiconductor nanoclusters from ab initio calculations

We propose a theoretical approach based on (constrained) density functional theory and the Franck-Condon approximation for the calculation of the temperature dependent photoluminescence of nanostructures. The method is computationally advantageous and only slightly more demanding than a standard density functional theory calculation and includes transitions into multiphonon final states (higher class transitions). We use the approach for Si and CdSe colloidal nanoclusters (NCs) with up to 693 atoms and obtain very good agreement with experiment which allows us to identify specific peaks and explain their origin. Generally, breathing type modes are shown to dominate the phonon replicas, while optical modes have significant contributions for CdSe NCs and play a lesser role in Si NCs. We obtain significant anti-Stokes peak starting at 140K for Si NC explaining the broadening observed in the corresponding experiment. We also apply the method to small molecular-like carbon structures (diamondoids), where electron-phonon coupling is typically large, and find that multiphonon processes (up to class 4) are very relevant and necessary to compare favorably with experiment. While it is crucial to include these multiphonon states in the small diamondoids with few tens of atoms, neglecting them in only marginally larger Si$_{87}$H$_{76}$ and Cd$_{43}$Se$_{44}$H$^*_{76}$ (and larger) quantum dots represents a good approximation.

cond-mat.mes-hall↗

arXiv2023

Accurate force-field methodology capturing atomic reconstructions in transition metal dichalcogenide moir\'e systems

In this work, a generalized force-field methodology for the relaxation of large moir\'e heterostructures is proposed. The force-field parameters are optimized to accurately reproduce the structural degrees of freedom of some computationally manageable cells relaxed using density functional theory. The parameters can then be used to handle large moir\'e systems. We specialize to the case of 2H-phased twisted transition-metal dichalcogenide homo- and heterobilayers using a combination of the Stillinger-Weber intralayer- and the Kolmogorov-Crespi interlayer-potential. Force-field parameters are developed for all combinations of MX$_2$ for $\text{M}\in\{\text{Mo},\text{W}\}$ and $\text{X}\in\{\text{S},\text{Se},\text{Te}\}$. The results show agreement within 20 meV in terms of band structure between density functional theory and force-field relaxation. Using the relaxed structures, a simplified and systematic scheme for the extraction of the interlayer moir\'e potential is presented for both R- and H-stacked systems. We show that in-plane and out-of-plane relaxation effects on the moir\'e potential, which is made both deeper and wider after relaxation, are essential. An interpolation based methodology for the calculation of the interlayer binding energy is also proposed. Finally, we show that atomic reconstruction, which is captured by the force-field method, becomes especially prominent for angles below 4-5$^\circ$, when there is no mismatch in lattice constant between layers.

cond-mat.mtrl-sci↗

arXiv2022

Role of Magnetic Coupling in Photoluminescence Kinetics of Mn2+-doped ZnS Nanoplatelets

Mn2+-doped semiconductor nanocrystals with tuned location and concentration of Mn2+ ions can yield diverse coupling regimes, which can highly influence their optical properties such as emission wavelength and photoluminescence (PL) lifetime. However, investigation on the relationship between the Mn2+ concentration and the optical properties is still challenging because of the complex interactions of Mn2+ ions and the host and between the Mn2+ ions. Here, atomically flat ZnS nanoplatelets (NPLs) with uniform thickness were chosen as matrixes for Mn2+ doping. Using time-resolved (TR) PL spectroscopy and density functional theory (DFT) calculations, a connection between coupling and PL kinetics of Mn2+ ions was established. Moreover, it was found that the Mn2+ ions residing on the surface of a nanostructure produce emissive states and interfere with the change of properties by Mn2+/Mn2+ coupling. In a configuration with suppressed surface contribution to the optical response we show the underlying physical reasons for double and triple exponential decay by DFT methods. We believe that the presented doping strategy and simulation methodology of the Mn2+-doped ZnS system is a universal platform to study dopant location- and concentration-dependent properties also in other semiconductors.

cond-mat.mtrl-sci↗

arXiv2022

Fine structure splitting analysis of cavity-enhanced telecom-wavelength InAs quantum dots grown on a GaAs(111)A vicinal substrate

The effcient generation of entangled photons at telecom wavelength is crucial for the success of many quantum communication protocols and the development of fiber-based quantum networks. Entangled light can be generated by solid state quantum emitters with naturally low fine structure splitting, such as highly symmetric InAs quantum dots (QDs) grown on (111)-oriented surfaces. Incorporating this kind of QDs into optical cavities is critical to achieve sufficient signal intensitiesfor applications, but has so far shown major complications. In this work we present droplet epitaxy of telecom-wavelength InAs QDs within an optical cavity on a vicinal (2{\deg} miscut) GaAs(111)A substrate. We show a remarkable enhancement of the photon extraction efficiency compared to previous reports together with a reduction of the density that facilitates the isolation of single spectral lines. Moreover, we characterise the exciton fine structure splitting and employ numerical simulations under the framework of the empirical pseudopotential and configuration interaction methods to study the impact of the miscut on the optical properties of the QDs. We demonstrate that the presence of miscut steps influences the polarisation of the excitonic states and introduces a preferential orientation in the $C_{3v}$ symmetry of the surface.

quant-ph↗

arXiv2020

Single Crystalline Colloidal Quasi-Two-Dimensional Tin Telluride

Tin telluride is a narrow gap semiconductor with promising properties for IR optical applications and topological insulators. We report a convenient colloidal synthesis of quasi-two-dimensional SnTe nanocrystals through the hot-injection method in a non-polar solvent. By introducing the halide alkane 1-bromotetradecane as well as oleic acid and trioctylphosphine, the thickness of two dimensional SnTe nanostripes can be tuned down to 30 nm, while the lateral dimensional can reach 6 microns. The obtained SnTe nanostripes are single-crystalline with a rock-salt crystal structure. The absorption spectra demonstrate pronounced absorption features in the IR range revealing the effect of quantum confinement in such structures.

cond-mat.mtrl-sci↗

arXiv2019

From wurtzite nanoplatelets to zinc blende nanorods: Simultaneous control of shape and phase in ultrathin ZnS nanocrystals

Ultrathin semiconductor nanocrystals (NCs) with at least one dimension below their exciton Bohr radius receive a rapidly increasing attention due to their unique physicochemical properties such as strong quantum confinement, large surface-to-volume ratio, and giant oscillator strength. These superior properties highly depend on the shape and crystal phase of semiconductor NCs. Slight changes in the shape and phase of NCs can cause significant changes in their properties. Therefore, it is crucial to controllably synthesize semiconductor NCs. Here, we demonstrate not only the synthesis of robust well-defined ultrathin ZnS nanoplatelets (NPLs) with excitonic absorption and emission, but also the precise shape and phase control of ZnS NCs based on a soft template strategy. The key feature of our approach is the tuning of the sulfur precursor amount, resulting in a simultaneous shape/phase transformation between wurtzite (WZ) ZnS NPLs and zinc blende (ZB) ZnS nanorods (NRs) at moderate temperatures (150 degree). UV-vis absorption and photoluminescence (PL) spectra reveal very distinct optical properties between WZ-ZnS NPLs and ZB-ZnS NRs. UV-vis absorption spectra of WZ-ZnS NPLs clearly exhibit a sharp excitonic peak that is not observed in ZB-ZnS NRs. Besides, the PL characterization shows that WZ-ZnS NPLs have a narrow excitonic emission peak (292 nm), while the ZB-ZnS NRs exhibit a broad collective emission band consisting of four emission peaks (335, 359, 395, and 468 nm). The appearance of excitonic features in the absorption spectra of ZnS NPLs is explained by interband electronic transitions, which is simulated in the framework of density functional theory (DFT). The presented simple and effective synthetic strategy opens a new path to synthesize further NCs with shape and phase control for advanced applications in electronics and photonics.

cond-mat.mtrl-sci↗

arXiv2019

Frozen-phonon method for state anticrossing situations and its application to zero-point motion effects in diamondoids

The frozen-phonon method, used to calculate electron-phonon coupling effects, requires calculations of the investigated structure using atomic coordinates displaced according to a certain phonon eigenmode. The process of "freezing-in" the specific phonon can bring electronic eigenstates that are energetically close in energy into an anticrossing. This electronic anticrossing effect is, however, unrelated to the wanted electron-phonon coupling, and needs to be removed. We present a procedure how to deal with these problematic anticrossing situations and apply it to the band gap zero-point motion renormalization of sixteen diamondoids and urotropine using different exchange correlation functionals. We find gap renormalizations of diamondoids in the range of 150 - 400 meV and only 62 meV for urotropine due to the lone-pair character of the highest occupied molecular orbital of the latter.

physics.chem-ph↗

arXiv2018

Phonon-assisted Auger enables ultrafast charge transfer in CdSe Quantum Dot/Organic Molecule

Charge transfer between photoexcited quantum dots and molecular acceptors is one of the key limiting processes in most applications of colloidal nanostructures, most prominently in photovoltaics. An atomistic detailed description of this process would open new ways to optimize existing and create new structures with targeted properties. We achieve a one-to-one comparison between ab-initio non-adiabatic molecular dynamics calculations and transient absorption spectroscopy experiments, which allows us to draw a comprehensive atomistic picture of the charge transfer process, following the time evolution of the charge carrier across the electronic landscape and identifying the thereby induced vibrations. For two quantum dot sizes we find two qualitatively different processes. For the larger structure we find a relatively slow (\tau = 516 fs) transfer process that we explain by the existence of a large energy detuning and weak vibronic coupling. For the smaller structure the process is ultrafast (\tau = 20 fs) due to an efficient, phonon-assisted Auger process triggered by a strong electron-hole coupling.

cond-mat.mtrl-sci↗