arXiv subjects

Thorsten Deilmann

Publications and source records attributed to Thorsten Deilmann.

At least 19 recordsLinked to original sources

arXiv2026

Extreme Polarization of the Optical Gap and High-Energy Exciton Landscape in CrSBr

We reveal a strongly anisotropic excitonic landscape in monolayer and bulk-like CrSBr using optical absorption spectroscopy and $GW$-Bethe-Salpeter equation $\textit{ab initio}$ calculations. The direct absorptive determination of the lowest bright optical onsets i.e. $X_0^a$ and $X_0^b$ excitons for the two in-plane polarization eigenaxes yield an in-plane optical gap anisotropy of $470 \pm 15$ meV. This is the highest observed value for any material in the near-infrared-to-visible spectral region to the best of our knowledge. Energetically above, we identify multiple strongly polarized excitons spanning $1.25$ eV to $3.1$ eV selectively aligned along the two orthogonal axes. A resonance $X^-$, located $24$ meV below the $X_0^b$ progressively transfers oscillator strength to $X_b^0$, a behavior consistent with a coupled trion (Fermi-polarion)/exciton pair. Our experiments also provide polarization-resolved broadband dielectric functions of CrSBr. These results establish CrSBr as a strongly polarization-selective excitonic system and highlight its potential for polarization-selective optoelectronics enabled with its large optical-gap anisotropy.

cond-mat.mes-hall↗

arXiv2026

Layer thickness dependent band gap of MBE grown single- to few-layer MoS$_{2}$

In light of the rise of transition metal dichalcogenides as 2D semiconductors for device applications, band engineering becomes very important from an application point of view. In many of these materials, such as the canonical example of MoS$_{2}$, the semiconductor band gap depends on the layer number. It changes from indirect to direct as it evolves from a bulk semiconductor to a monolayer. Interestingly, it was predicted and experimentally confirmed that, by thinning the material from bulk to a bilayer, the indirect transition shows a strong blue-shift. Here, we present the results of scanning tunnelling spectroscopy measurements on MoS$_{2}$ that has been grown \textit{in situ} via molecular beam epitaxy on graphene on Ir(111) at thicknesses ranging from 1 to 5 layers. We find a drastic decrease of the band gap with increasing layer number, to values even below the band gap in bulk. We also observe that the pinning of the conduction band vanishes above 4 layers. Comparing our experimental data with density functional theory and \textit{GW} calculations indicates that an additional screening is introduced by the sample growth conditions.

cond-mat.mtrl-sci↗

arXiv2026

Coulomb Interaction in Atomically Thin Semiconductors and Density-Independent Exciton-Scattering Processes

In quantum-kinetic approaches to the dynamics of Coulomb-bound many-body correlations such as excitons, trions, biexcitons or higher-order correlations, a detailed knowledge of the many-body Coulomb Hamiltonian serving as a starting point is important. In this manuscript, the second-quantized description of the Coulomb interaction between Bloch electrons in a Heisenberg-equation-of-motion approach in atomically thin semiconductors is derived and reviewed. Emphasis is put on a discussion of Umklapp processes and the dielectric screening including all local-field effects. A link between \textit{ab initio} methods of screening and few-band models in effective-mass approximations for the quantum kinetics is established and all important Coulomb scattering processes contributing to the exciton energy landscape and density-independent exciton scattering are discussed.

cond-mat.mes-hall↗

arXiv2025

Optical Readout of Reconfigurable Layered Magnetic Domain Structure in CrSBr

The van der Waals magnetic semiconductor CrSBr combines multistable magnetic order with strong light--matter coupling, enabling optical access to a rich and reconfigurable layered magnetic domain structure. A purely optical, non-destructive, and non-contact readout of layered magnetic configurations is realized here by magneto-reflectance measurements and interpreted using an optical multilayer model. The magnetic state is tunable by applied magnetic fields and by interfacing CrSBr with the antiferromagnet MnPS$_3$. Applying an external magnetic field along the easy axis drives the hysteretic antiferromagnetic--to--ferromagnetic transition, which is not universally binary but instead develops through a cascade of intermediate magnetic configurations whose multiplicity and stability scale systematically with layer thickness and can be tailored by magnetic interfaces. The intertwined optical and magnetic properties of CrSBr provide a readout mechanism for information encoded in and processed through its magnetic configuration that is compatible with modern on- and off-chip photonic and electronic technologies. These properties identify CrSBr as a promising platform for intelligent matter and for spin-optoelectronics, in particular for neuromorphic architectures that can learn and evolve in response to changing environments.

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arXiv2025

Precise Twist Angle Determination in twisted WSe2 via Optical Moir\'e Phonons

Twisted bilayers of transition metal dichalcogenides (TMDC) form moir\'e superlattices resulting in moir\'e minibands in momentum space and hosting localized excitons in real space. While moir\'e superlattices provide access to Mott-Hubbard physics, their energy potential landscape and electronic correlations are highly sensitive to fluctuations of the twist angle, disorder and lattice reconstructions. However, fast and non-invasive experimental access to local twist angle and its spatial variations is challenging. Here, we systematically correlate twist angle variations of twisted WSe2 bilayers across micrometer length scales using a combined lateral force microscopy (LFM) and a micro- Raman spectroscopy approach. These measurements uncover lateral variations in the twist angle by more than 1{\deg} across length scales relevant to optical and transport measurements. We demonstrate that twist angles in the range of 3{\deg} < $\alpha$ < 12{\deg} show distinct Raman response from scattering on optical moir\'e phonons allowing twist angle determination with high precision and sub-micrometer spatial resolution under ambient conditions. These modes are particularly sensitive in the low-angle twist regime, predicted to host emergent quantum phases. Our results establish micro-Raman spectroscopy of optical moir\'e phonons as a rapid, non-invasive probe to determine twist angle and to screen local twist angle variations with a precision better than $\pm$ 0.3{\deg} and a lateral resolution below one micrometer. This methodology is also applicable to fully hBN-encapsulated heterostructures.

cond-mat.mtrl-sci↗

arXiv2025

Roadmap for Photonics with 2D Materials

Triggered by the development of exfoliation and the identification of a wide range of extraordinary physical properties in self-standing films consisting of one or few atomic layers, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and other van der Waals (vdW) crystals currently constitute a wide research field protruding in multiple directions in combination with layer stacking and twisting, nanofabrication, surface-science methods, and integration into nanostructured environments. Photonics encompasses a multidisciplinary collection of those directions, where 2D materials contribute with polaritons of unique characteristics such as strong spatial confinement, large optical-field enhancement, long lifetimes, high sensitivity to external stimuli (e.g., electric and magnetic fields, heating, and strain), a broad spectral range from the far infrared to the ultraviolet, and hybridization with spin and momentum textures of electronic band structures. The explosion of photonics with 2D materials as a vibrant research area is producing breakthroughs, including the discovery and design of new materials and metasurfaces with unprecedented properties as well as applications in integrated photonics, light emission, optical sensing, and exciting prospects for applications in quantum information, and nanoscale thermal transport. This Roadmap summarizes the state of the art in the field, identifies challenges and opportunities, and discusses future goals and how to meet them through a wide collection of topical sections prepared by leading practitioners.

cond-mat.mtrl-sci↗

arXiv2024

Quadrupolar and dipolar excitons in symmetric trilayer heterostructures: Insights from first principles theory

Excitons in van der Waals heterostructures come in many different forms. In bilayer structures, the electron and hole may be localized on the same layer or they may be separated forming an interlayer exciton with a finite out-of-plane dipole moment. Using first principles calculations, we investigate the excitons in a symmetric WS$_2$/MoS$_2$/WS$_2$ heterostructure in the presence of a vertical electric field. The excitons exhibit a quadratic Stark shift for low field strengths and a linear Stark shift for stronger fields. This behaviour is traced to the coupling of interlayer excitons with opposite dipole moments, which lead to the formation of quadrupolar excitons at small fields. The formation of quadrupolar excitons is determined by the relative size of the electric field-induced splitting of the dipolar excitons and the coupling between them given by the hole tunneling across the MoS$_2$ layer. For the inverted structure, MoS$_2$/WS$_2$/MoS$_2$, the dipolar excitons are coupled by electron tunneling across the WS$_2$ layer. Because this effect is much weaker, the resulting quadrupolar excitons are more fragile and break at a weaker electric field.

cond-mat.mes-hall↗

arXiv2024

Quadratic optical response of CrSBr controlled by spin-selective interlayer coupling

The optical properties of the layered magnet CrSBr are dominated by intralayer excitons: the antiferromagnetic order between the layers makes layer-to-layer charge hopping, and therefore interlayer excitons, spin-forbidden. An external magnetic field, however, continuously drives the magnetic order towards layer-to-layer ferromagnetic, which opens spin-allowed charge-transfer channels between the layers. Here we elaborate how their admixture changes the composition and nature of the excitons, leading to an extension over many layers, and causes a quadratic red-shift with respect to the external magnetic field. We address these effects by ab-initio $GW$-BSE calculations as a function of magnetic field and cast the data into a minimal four-band model to elucidate the interplay between the various interaction and coupling mechanisms. Our findings should be generally valid for antiferromagnetic layered magnets with and without external magnetic fields, and moreover for any couple of layers with different spin directions. Our insights help to systematically address excitons and predict their optical signatures in such systems.

cond-mat.mes-hall↗

arXiv2023

Pressure dependence of intra- and interlayer excitons in 2H-MoS$_2$ bilayers

The optical and electronic properties of multilayer transition metal dichalcogenides differ significantly from their monolayer counterparts due to interlayer interactions. The separation of individual layers can be tuned in a controlled way by applying pressure. Here, we use a diamond anvil cell to compress bilayers of 2H-MoS$_2$ in the gigapascal range. By measuring optical transmission spectra, we find that increasing pressure leads to a decrease in the energy splitting between the A and interlayer exciton. Comparing our experimental findings with ab initio calculations, we conclude that the observed changes are not due to the commonly assumed hydrostatic compression. This effect is attributed to the MoS$_2$ bilayer adhering to the diamond, which reduces in-plane compression. Moreover, we demonstrate that the distinct real-space distributions and resulting contributions from the valence band account for the different pressure dependencies of the inter- and intralayer excitons in compressed MoS$_2$ bilayers.

cond-mat.mes-hall↗

arXiv2022

The bulk van der Waals layered magnet CrSBr is a quasi-1D material

Correlated quantum phenomena in one-dimensional (1D) systems that exhibit competing electronic and magnetic order are of strong interest for studying fundamental interactions and excitations, such as Tomonaga-Luttinger liquids and topological orders and defects with properties completely different from the quasiparticles expected in their higher-dimensional counterparts. However, clean 1D electronic systems are difficult to realize experimentally, particularly magnetically ordered systems. Here, we show that the van der Waals layered magnetic semiconductor CrSBr behaves like a quasi-1D material embedded in a magnetically ordered environment. The strong 1D electronic character originates from the Cr-S chains and the combination of weak interlayer hybridization and anisotropy in effective mass and dielectric screening with an effective electron mass ratio of $m^e_X/m^e_Y \sim 50$. This extreme anisotropy experimentally manifests in strong electron-phonon and exciton-phonon interactions, a Peierls-like structural instability and a Fano resonance from a van Hove singularity of similar strength of metallic carbon nanotubes. Moreover, due to the reduced dimensionality and interlayer coupling, CrSBr hosts spectrally narrow (1 meV) excitons of high binding energy and oscillator strength that inherit the 1D character. Overall, CrSBr is best understood as a stack of weakly hybridized monolayers and appears to be an experimentally attractive candidate for the study of exotic exciton and 1D correlated many-body physics in the presence of magnetic order.

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arXiv2020

Towards fully automatized GW band structure calculations: What we can learn from 60.000 self-energy evaluations

We analyze a data set comprising 370 GW band structures composed of 61716 quasiparticle (QP) energies of two-dimensional (2D) materials spanning 14 crystal structures and 52 elements. The data results from PAW plane wave based one-shot G$_0$W$_0$@PBE calculations with full frequency integration. We investigate the distribution of key quantities like the QP self-energy corrections and renormalization factor $Z$ and explore their dependence on chemical composition and magnetic state. The linear QP approximation is identified as a significant error source and propose schemes for controlling and drastically reducing this error at low computational cost. We analyze the reliability of the $1/N_\text{PW}$ basis set extrapolation and find that is well-founded with narrow distributions of $r^2$ peaked very close to 1. Finally, we explore the validity of the scissors operator approximation concluding that it is generally not valid for reasonable error tolerances. Our work represents a step towards the development of automatized workflows for high-throughput G$_0$W$_0$ band structure calculations for solids.

cond-mat.mtrl-sci↗

arXiv2020

Ab-initio studies of exciton $g$ factors: Monolayer transition metal dichalcogenides in magnetic fields

The effect of a magnetic field on the optical absorption in semiconductors has been measured experimentally and modeled theoretically for various systems in previous decades. We present a new first-principles approach to systematically determine the response of excitons to magnetic fields, i.e. exciton $g$ factors. By utilizing the $GW$-Bethe-Salpeter equation methodology we show that $g$ factors extracted from the Zeeman shift of electronic bands are strongly renormalized by many-body effects which we trace back to the extent of the excitons in reciprocal space. We apply our approach to monolayers of transition metal dichalcogenides (MoS$_2$, MoSe$_2$, MoTe$_2$, WS$_2$, and WSe$_2$) with strongly bound excitons for which $g$ factors are weakened by about 30%.

cond-mat.mes-hall↗

arXiv2020

Light-matter interaction in van der Waals heterostructures

Even if individual two-dimensional materials own various interesting and unexpected properties, the stacking of such layers leads to van der Waals solids which unite the characteristics of two dimensions with novel features originating from the interlayer interactions. In this topical review, we cover fabrication and characterization of van der Waals heterosructures with a focus on heterobilayers made of monolayers of semiconducting transition metal dichalcogenides. Experimental and theoretical techniques to investigate those heterobilayers are introduced. Most recent findings focusing on different transition metal dichalcogenides heterostructures are presented and possible optical transitions between different valleys, appearance of moire patterns and signatures of moire excitons are discussed. The fascinating and fast growing research on van der Waals hetero-bilayers provide promising insights required for their application as emerging quantum-nano materials.

cond-mat.mes-hall↗

arXiv2019

Dark trions govern the temperature-dependent optical absorption and emission of doped atomically thin semiconductors

We perform absorption and photoluminescence spectroscopy of trions in hBN-encapsulated WSe$_2$, WS$_2$, MoSe$_2$, and MoS$_2$ monolayers, depending on temperature. The different trends for W- and Mo-based materials are excellently reproduced considering a Fermi-Dirac distribution of bright and dark trions. We find a dark trion, $\rm{X_D^-}$ 19 meV $\textit{below}$ the lowest bright trion, $\rm{X}_1^-$ in WSe$_2$ and WS$_2$. In MoSe$_2$, $\rm{X_D^-}$ lies 6 meV $\textit{above}$ $\rm{X}_1^-$, while $\rm{X_D^-}$ and $\rm{X}_1^-$ almost coincide in MoS$_2$. Our results agree with GW-BSE $\textit{ab-initio}$ calculations and quantitatively explain the optical response of doped monolayers with temperature.

cond-mat.mes-hall↗

arXiv2019

Inelastic electron tunneling spectroscopy for probing strongly correlated many-body systems by scanning tunneling microscopy

We present an extension of the tunneling theory for scanning tunneling microcopy (STM) to include different types of vibrational-electronic couplings responsible for inelastic contributions to the tunnel current in the strong-coupling limit. It allows for a better understanding of more complex scanning tunneling spectra of molecules on a metallic substrate in separating elastic and inelastic contributions. The starting point is the exact solution of the spectral functions for the electronic active local orbitals in the absence of the STM tip. This includes electron-phonon coupling in the coupled system comprising the molecule and the substrate to arbitrary order including the anti-adiabatic strong coupling regime as well as the Kondo effect on a free electron spin of the molecule. The tunneling current is derived in second order of the tunneling matrix element which is expanded in powers of the relevant vibrational displacements. We use the results of an ab-initio calculation for the single-particle electronic properties as an adapted material-specific input for a numerical renormalization group approach for accurately determining the electronic properties of a NTCDA molecule on Ag(111) as a challenging sample system for our theory. Our analysis shows that the mismatch between the ab-initio many-body calculation of the tunnel current in the absence of any electron-phonon coupling to the experiment scanning tunneling spectra can be resolved by including two mechanisms: (i) a strong unconventional Holstein term on the local substrate orbital leads to reduction of the Kondo temperature and (ii) a different electron-vibrational coupling to the tunneling matrix element is responsible for inelastic steps in the $dI/dV$ curve at finite frequencies.

cond-mat.mes-hall↗

arXiv2018

Discovering new two-dimensional topological insulators from computational screening

We have performed a computational screening of topological two-dimensional (2D) materials from the Computational 2D Materials Database (C2DB) employing density functional theory. A full \textit{ab initio} scheme for calculating hybrid Wannier functions directly from the Kohn-Sham orbitals has been implemented and the method was used to extract $\mathbb{Z}_2$ indices, Chern numbers and Mirror Chern numbers of 3331 2D systems including both experimentally known and hypothetical 2D materials. We have found a total of 46 quantum spin Hall insulators, 7 quantum anomalous Hall insulators and 9 crystalline topological insulators that are all predicted to be dynamically stable. Roughly one third of these were known prior to the screening. The most interesting of the novel topological insulators are investigated in more detail. We show that the calculated topological indices of the quantum anomalous Hall insulators are highly sensitive to the approximation used for the exchange-correlation functional and reliable predictions of the topological properties of these materials thus require methods beyond density functional theory. We also performed $GW$ calculations, which yield a gap of 0.65 eV for the quantum spin Hall insulator PdSe$_2$ in the MoS$_2$ crystal structure. This is significantly higher than any known 2D topological insulator and three times larger than the Kohn-Sham gap.

cond-mat.mtrl-sci↗

arXiv2018

The Computational 2D Materials Database: High-Throughput Modeling and Discovery of Atomically Thin Crystals

We introduce the Computational 2D Materials Database (C2DB), which organises a variety of structural, thermodynamic, elastic, electronic, magnetic, and optical properties of around 1500 two-dimensional materials distributed over more than 30 different crystal structures. Material properties are systematically calculated by state-of-the art density functional theory and many-body perturbation theory (G$_0\!$W$\!_0$ and the Bethe-Salpeter Equation for $\sim$200 materials) following a semi-automated workflow for maximal consistency and transparency. The C2DB is fully open and can be browsed online or downloaded in its entirety. In this paper, we describe the workflow behind the database, present an overview of the properties and materials currently available, and explore trends and correlations in the data. Moreover, we identify a large number of new potentially synthesisable 2D materials with interesting properties targeting applications within spintronics, (opto-)electronics, and plasmonics. The C2DB offers a comprehensive and easily accessible overview of the rapidly expanding family of 2D materials and forms an ideal platform for computational modeling and design of new 2D materials and van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2016

Adsorption and STM imaging of tetracyanoethylene on Ag(001): An ab-initio study

We investigate the adsorption of a single tetracyanoethylene (TCNE) molecule on the silver (001) surface. Adsorption structures, electronic properties, and scanning tunneling microscopy (STM) images are calculated within density-functional theory. Adsorption occurs most favorably in on-top configuration, with the C=C double bond directly above a silver atom and the four N atoms bound to four neighboring Ag atoms. The lowest unoccupied molecular orbital of TCNE becomes occupied due to electron transfer from the substrate. This state dominates the electronic spectrum and the STM image at moderately negative bias. We discuss and employ a spatial extrapolation technique for the calculation of STM and scanning tunneling spectroscopy (STS) images. Our calculated images are in good agreement with experimental data.

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