arXiv subjects

Peter Kratzer

Publications and source records attributed to Peter Kratzer.

At least 19 recordsLinked to original sources

arXiv2026

Facet-Dependent Electronic Properties and Interfacial Point Defect Interactions in WS$_2$/ZnO Heterostructures

Aiming at two-dimensional materials for high-efficiency optoelectronics, WS$2$/ZnO heterostructures are computationally screened for their facet-dependent electronic properties and interfacial defect thermodynamics using first-principles hybrid functional calculations. Interface comparison identifies the non-polar ($10\overline{1}0$) $m$-plane as the optimal substrate facet, maintaining a direct 2.42~eV bandgap and a robust type-I band alignment. Isolated sulfur ($\mathrm{V_S}$) and interfacial oxygen ($\mathrm{V_O}$) vacancies introduce deep non-radiative recombination centers. Conversely, zinc vacancies ($\mathrm{V{Zn}}$) act as shallow acceptors near the valence band edge, contributing to unintentional $p$-type behavior. Analysis of defect pairs reveals that neutral vacancies cluster across the van der Waals gap due to favorable binding energies. Under $n$-type conditions, defects stabilize as charged species. Although inter-layer Coulomb repulsion weakens the binding energy of $(\mathrm{V_S} - \mathrm{V_{Zn}})''''$ pairs, their formation energy drops to 2.61~eV under anion-poor conditions, making the $-4$ cluster the most thermodynamically abundant defect pair at the interface. Furthermore, native $\mathrm{V_{Zn}}$ prevents the Fermi level rise typically induced by interstitial hydrogen ($\mathrm{H_i}$), distributing donated electrons into shallow acceptor states and preserving host band edge rigidity. These findings establish a microscopic framework for substrate selection and defect engineering in 2D/3D hybrid light-emitting diodes.

cond-mat.mtrl-sci↗

arXiv2026

Quasiparticle level alignment in anthracene-MoS2 heterostructures

Heterostructures composed of transition metal dichalcogenides (TMDCs) and organic molecules have been extensively explored for optoelectronic devices. To maximize their application potential, it is essential to investigate the electronic band structures, which govern the charge response of the interfaces to external perturbations. Based on $GW$ calculations, we present a study of organic-inorganic heterostructures with anthracene molecules adsorbed on monolayer MoS2. Building on previous investigations of organic molecule self-assembly at surfaces, we systematically analyze anthracene configurations with various molecular orientations and surface coverages. Partially self-consistent $GW_0$ provides qualitatively different level alignments from those in DFT. Whereas the systems with sparse, horizontally adsorbed anthracenes exhibit type-I alignment, densely packed anthracenes in the head-on position lead to type-II alignment, which indicates the strong dependence of quasiparticle corrections on the interfacial configuration. These findings highlight the importance of level-alignment predictions for both interpreting experiments and guiding the design of organic-inorganic heterostructures.

cond-mat.mtrl-sci↗

arXiv2026

Enhanced Electron Reflectionat Mott-Insulator Interfaces

The Klein paradox describes an incoming electron being scattered at a supercritical barrier to create electron-positron pairs, a phenomenon widely discussed in textbooks. While demonstrating this phenomenon experimentally with the fundamental particles remains challenging, condensed matter analogs are more accessible to experimental realization. For spinless quasi-particles, theoretical works show an enhancement of the pair production rate, and analogs of this effect in condensed matter systems have been studied theoretically. Here, we present another condensed matter system, a heterostructure comprised of two materials with strongly and weakly interacting electrons, that allows for constructing analytical solutions using the hierarchy-of-correlations method. The results show enhanced electron reflection related with the production of doublon-holon pairs, as known from the Klein paradox.

cond-mat.str-el↗

arXiv2025

Signatures of Chiral Phonons in MnPS$_3$ from first principles

Two-dimensional (2D) materials may host circular phonons, considered as chiral if the presence of a substrate breaks mirror symmetry. In 2D transition metal dichalcogenide (TMDC) monolayers lacking inversion symmetry, phonons with a given chirality can be observed in the non-equilibrium state triggered by optical excitations using circularly polarized light. Backed by first-principles calculations, we present the antiferromagnetic semiconductor MnPS$_3$ with a hexagonal crystal structure and bandstructure similar to TMDCs, but a larger unit cell, as a novel candidate material that may allow for excitation of circular phonons. Using DFT+U and the finite displacement method we obtain in-plane chiral phonon modes at the valley points of a monolayer MnPS$_3$. These modes can be classified according to the Mn or S atoms performing circular motions about their equilibrium positions. In each case, the quantized angular momentum of the phonons is calculated. Moreover, we point out ways to populate the chiral phonons selectively via optical excitation with circularly polarized light.

cond-mat.mtrl-sci↗

arXiv2025

Bound States at Semiconductor -- Mott Insulator Interfaces

Utilizing the hierarchy of correlations in the context of a Fermi-Hubbard model, we deduce the presence of quasi-particle bound states at the interface between a Mott insulator and a semiconductor, as well as within a semiconductor-Mott-semiconductor heterostructure forming a quantum well. In the case of the solitary interface, the existence of bound states necessitates the presence of an additional perturbation with a minimal strength depending on the spin background of the Mott insulator. Conversely, within the quantum well, this additional perturbation is still required to have bound states while standing-wave solutions even exist in its absence.

cond-mat.str-el↗

arXiv2025

Roadmap on Advancements of the FHI-aims Software Package

Electronic-structure theory is the foundation of the description of materials including multiscale modeling of their properties and functions. Obviously, without sufficient accuracy at the base, reliable predictions are unlikely at any level that follows. The software package FHI-aims has proven to be a game changer for accurate free-energy calculations because of its scalability, numerical precision, and its efficient handling of density functional theory (DFT) with hybrid functionals and van der Waals interactions. It treats molecules, clusters, and extended systems (solids and liquids) on an equal footing. Besides DFT, FHI-aims also includes quantum-chemistry methods, descriptions for excited states and vibrations, and calculations of various types of transport. Recent advancements address the integration of FHI-aims into an increasing number of workflows and various artificial intelligence (AI) methods. This Roadmap describes the state-of-the-art of FHI-aims and advancements that are currently ongoing or planned.

cond-mat.mtrl-sci↗

arXiv2025

Transmission through multiple Mott insulator - semiconductor wells

Weakly and strongly interacting quantum many-body systems, namely semiconductors and Mott insulators, are combined into a layered heterostructure. Via the hierarchy of correlations, we derive and match the propagating quasi-particle solutions in the different regions and calculate the transmission coefficients through these layered structures. As a proof of principle, we find the well known transmission bands of a semiconductor heterostructure. Extending this idea to semiconductor and Mott insulator structures we calculate the transmittance and the resonance energies. Within a phase accumulation model we find analytical expressions for the scattering phase shift. Lastly, we find transmission curves with skewness for structures with applied voltage.

cond-mat.str-el↗

arXiv2025

Mechanism of Oleic Acid-Mediated Sulfur Vacancy Healing in monolayer WS$_2$

We uncover the mechanism behind the enhancement of photoluminescence yield in monolayer WS$_2$ through oleic acid treatment, a promising scalable strategy for defect healing. By inducing sulfur vacancies through thermal treatment and monitoring the changes in photoluminescence yield and emission spectra, we demonstrate that oleic acid heals the sulfur vacancy by providing substitutional oxygen. Using density functional theory calculations, we provide insight into the underlying mechanism governing the oleic acid-mediated sulfur vacancy healing process. Our findings suggest that effective defect passivation by oxygen doping can be achieved through chemical treatment, opening a pathway for oxygen doping in transition metal dichalcogenides. However, we also highlight the limitations of chemical treatment, which may only lead to small increases in photoluminescence yield beyond a certain point.

cond-mat.mtrl-sci↗

arXiv2024

Ultrafast Charge Transfer Dynamics at the MoS$_2$/Au Interface Observed via Optical Spectroscopy under Ambient Conditions

To take advantage of the exceptional properties of atomically thin transition metal dichalcogenides (TMDC) for advanced devices and catalysts, integration with metallic surfaces is an efficacious approach for facilitating charge carrier injection and extraction from TMDC monolayers. Light-matter interactions predominantly occur at the K point in TMDC monolayers, making the charge carrier dynamics at this point essential for their optimal performance. However, direct access to and comprehensive understanding of the charge carrier dynamics at the K point of TMDC monolayer on a metal substrate remains challenging. In this study, we employed azimuth- and polarization-dependent final-state sum frequency generation (FS-SFG) spectroscopy to investigate the ultrafast dynamics of charge transfer at the K point of a MoS$_2$ monolayer interfaced with an Au substrate. We observed an ultrafast injection (sub-20 fs) of photoexcited hot electrons from the Au substrate to the conduction band minimum (CBM) of the MoS$_2$ monolayer. Subsequently, driven by an internal electric field induced by charge redistribution, injected hot electrons in MoS$_2$ experience a relaxation and fast return ($\sim2$ ps) from the CBM and a trap state mediated slow return ($\sim60$ ps) process. The direct optical observation of the full electron dynamics at the K point of MoS$_2$ monolayer in ambient conditions provides valuable insights into the mechanisms of charge carrier transfer across the TMDC-metal interface, informing the design of advanced TMDC-based devices with enhanced charge transfer rates.

cond-mat.mes-hall↗

arXiv2024

Identifying band structure changes of FePS3 across the antiferromagnetic phase transition

Magnetic 2D materials enable novel tuning options of magnetism. As an example, the van der Waals material FePS3, a zigzag-type intralayer antiferromagnet, exhibits very strong magnetoelastic coupling due to the different bond lengths along different ferromagnetic and antiferromagnetic coupling directions enabling elastic tuning of magnetic properties. The likely cause of the length change is the intricate competition between direct exchange of the Fe atoms and superexchange via the S and P atoms. To elucidate this interplay, we study the band structure of exfoliated FePS3 by mu m scale ARPES (Angular Resolved Photoelectron Spectroscopy), both, above and, for the first time, below the Neel temperature TN. We find three characteristic changes across TN. They involve S 3p-type bands, Fe 3d-type bands and P 3p-type bands, respectively, as attributed by comparison with density functional theory calculations (DFT+U). This highlights the involvement of all the atoms in the magnetic phase transition providing independent evidence for the intricate exchange paths.

cond-mat.mtrl-sci↗

arXiv2024

Effect of biquadratic magnetic exchange interaction in the 2D antiferromagnets MPS_3 (M = Mn, Fe, Co, Ni)

The two-dimensional van der Waals (vdW) materials MPS_3(M = Mn, Fe, Co, Ni) display antiferromagnetic ordering of the magnetic moments at the transition metal ions. The possibility to exfoliate thin layers that preserve the magnetic order makes these materials interesting for numerous applications in devices that require integration of flexible patches of magnetic materials, e.g. in antiferromagnetic spintronics. Hence, an improved understanding of their magnetic properties is desirable. Here, we parameterize spin Hamiltonians for a monolayer of all four materials of this class using density functional theory plus Hubbard U calculations. We provide a step-by-step guide for calculating the magnetic exchange interactions and magnetic anisotropy energy using the (non-)collinear DFT+U(+ SOC) approach with a suitably chosen U for each material. It is found that the biquadratic interactions gain in importance while moving through the 3d series. Retaining the leading terms of a Holstein-Primakoff-transformed spin Hamiltonian, the magnon spectra are calculated. While MnPS_3 is found to be an almost isotropic antiferromagnet with a tiny gap, the biquadratic interaction opens an increasingly wider gap for FePS_3, CoPS_3 and NiPS_3. In line with this observation, Monte Carlo simulations demonstrate that the biquadratic interactions contribute to a systematic rise in the Neel temperature from FePS_3 to NiPS_3.

cond-mat.mtrl-sci↗

arXiv2023

Femtosecond spin-state switching dynamics of spin-crossover molecules condensed in thin films

The photoinduced switching of Fe(II)-based spin-crossover complexes from singlet to quintet takes place at ultrafast time scales. This a priori spin-forbidden transition triggered numerous time-resolved experiments of solvated samples to elucidate the mechanism at play. The involved intermediate states remain uncertain. We apply ultrafast x-ray spectroscopy in molecular films as a method sensitive to spin, electronic, and nuclear degrees of freedom. Combining the progress in molecule synthesis and film growth with the opportunities at x-ray free-electron lasers, we analyze the transient evolution of the Fe L3 fine structure at room temperature. Our measurements and calculations indicate the involvement of an Fe triplet intermediate state. The high-spin state saturates at half of the available molecules, limited by molecule-molecule interaction within the film.

cond-mat.mes-hall↗

arXiv2023

Isolating the Nonlinear Optical Response of a MoS$_2$ Monolayer under Extreme Screening of a Metal Substrate

Transition metal dichalcogenides (TMDCs) monolayers, as two-dimensional (2D) direct bandgap semiconductors, hold promise for advanced optoelectronic and photocatalytic devices. Interaction with three-dimensional (3D) metals, like Au, profoundly affects their optical properties, posing challenges in characterizing the monolayer's optical responses within the semiconductor-metal junction. In this study, using precise polarization-controlled final-state sum frequency generation (FS-SFG), we successfully isolated the optical responses of a MoS$_2$ monolayer from a MoS$_2$/Au junction. The resulting SFG spectra exhibit a linear lineshape, devoid of A or B exciton features, attributed to the strong dielectric screening and substrate induced doping. The linear lineshape illustrates the expected constant density of states (DOS) at the band edge of the 2D semiconductor, a feature often obscured by excitonic interactions in week-screening conditions such as in a free-standing monolayer. Extrapolation yields the onset of a direct quasiparticle bandgap of about $1.65\pm0.20$ eV, indicating a strong bandgap renormalization. This study not only enriches our understanding of the optical responses of a 2D semiconductor in extreme screening conditions but also provides a critical reference for advancing 2D semiconductor-based photocatalytic applications.

cond-mat.mes-hall↗

arXiv2023

Kibble-Zurek dynamics in the anisotropic Ising model of the Si(001) surface

As a simplified description of the non-equilibrium dynamics of buckled dimers on the Si(001) surface, we consider the anisotropic 2D Ising model and study the freezing of spatial correlations during a cooling quench across the critical point. Depending on the cooling rate, we observe a crossover from 1D to 2D behavior. For rapid cooling, we find effectively 1D behavior in the strongly coupled direction, for which we provide an exact analytic solution of the non-equilibrium dynamics. For slower cooling rates, we start to see 2D behavior where our numerical simulations show an approach to the usual Kibble-Zurek scaling in 2D.

cond-mat.stat-mech↗

arXiv2023

Critical behavior of the dimerized Si(001) surface: Continuous order-disorder phase transition in the two-dimensional Ising universality class

The critical behavior of the order-disorder phase transition in the buckled dimer structure of the Si(001) surface is investigated both theoretically by means of first-principles calculations and experimentally by spot profile analysis low-energy electron diffraction (SPA-LEED). We use density functional theory (DFT) with three different functionals commonly used for Si to determine the coupling constants of an effective lattice Hamiltonian describing the dimer interactions. Experimentally, the phase transition from the low-temperature $c(4 {\times} 2)$- to the high-temperature $p(2 {\times} 1)$-reconstructed surface is followed through the intensity and width of the superstructure spots within the temperature range 78-400 K. Near the critical temperature $T_c = 190.6\,\mathrm{K}$, we observe universal critical behavior of spot intensities and correlation lengths which falls into the universality class of the two-dimensional (2D) Ising model. From the ratio of correlation lengths along and across the dimer rows we determine effective nearest-neighbor couplings of an anisotropic 2D Ising model, $J_\parallel = (-24.9 \pm 0.9_\mathrm{stat} \pm 1.3_\mathrm{sys})\,\mathrm{meV}$ and $J_\perp = (-0.8 \pm 0.1_\mathrm{stat})\,\mathrm{meV}$. We find that the experimentally determined coupling constants of the Ising model can be reconciled with those of the more complex lattice Hamiltonian from DFT when the critical behavior is of primary interest. The anisotropy of the interactions derived from the experimental data via the 2D Ising model is best matched by DFT calculations using the PBEsol functional. The trends in the calculated anisotropy are consistent with the surface stress anisotropy predicted by the DFT functionals, pointing towards the role of surface stress reduction as a driving force for establishing the $c(4 {\times} 2)$-reconstructed ground state.

cond-mat.mes-hall↗

arXiv2023

First-principles Calculations of MoSeTe/WSeTe Bilayers: Stability, Phonons, Electronic Band Offsets, and Rashba Splitting

Janus materials have attracted much interest due to their intrinsic electric dipole moment and Rashba band splitting. We show that, by building bilayers of MoSeTe and WSeTe with different chalcogen atom sequences and different stacking patterns, one can modulate the net dipole moment strength and the Rashba effect, as well as the band alignment of the MoSeTe/WSeTe bilayer. Type-II band alignment is found which can be exploited to create long-lived interlayer excitons. Moreover, it is shown that the atomic sequence and stacking play pivotal roles in the interlayer distance of MoSeTe/WSeTe and thus its electronic structure and vibrational, especially low-frequency, characteristics. The long-range dispersion forces between atoms are treated with a conventional additive pairwise, as well as a many-body-dispersion method. It is shown that under the many-body dispersion method, more clear and rational thermodynamic trends of bilayer stacking are realized and interface distances are estimated more accurately. Vibrational spectra of the bilayers are calculated using first-principles phonon calculations and the fingerprints of monolayer attraction and repulsion are identified. An anti-correlation between distance and the shearing mode frequency of the rigid monolayers is demonstrated which agrees well with experimental findings. The results suggest that the judicious selection of the atomic sequence and stacking helps to widen the scope of the low-dimensional materials by adding or enhancing properties for specific applications, e.g. for spintronics or valleytronics devices.

cond-mat.mtrl-sci↗

arXiv2023

Quasi-particle propagation across semiconductor-Mott insulator interfaces

As a prototypical example for a heterostructure combining a weakly and a strongly interacting quantum many-body system, we study the interface between a semiconductor and a Mott insulator. Via the hierarchy of correlations, we derive and match the propagating or evanescent (quasi) particle solutions on both sides and assume that the interactions among the electrons in the semiconducting regions can be absorbed by an effective potential. While the propagation is described by a band-like dispersion in both the weakly and the strongly interacting case, the inverse decay length across the interface follows a different dependence on the band gap in the Mott insulator and the semiconductor. As one consequence, tunnelling through a Mott insulating layer behaves quite differently from a semiconducting (or band insulating) layer. For example, we find a strong suppression of tunnelling for energies in the middle between the upper and lower Hubbard band of the Mott insulator.

cond-mat.str-el↗

arXiv2023

Dimer Coupling Energies of the Si(001) Surface

The coupling energies between the buckled dimers of the Si(001) surface were determined through analysis of the anisotropic critical behavior of its order-disorder phase transition. Spot profiles in high-resolution low-energy electron diffraction as a function of temperature were analyzed within the framework of the anisotropic two-dimensional Ising model. The validity of this approach is justified by the large ratio of correlation lengths, $\xi_\parallel^+/\xi_\perp^+ = 5.2$ of the fluctuating $c(4 {\times} 2)$ domains above the critical temperature $T_\mathrm{c} = (190.6 \pm 10)$ K. We obtain effective couplings $J_\parallel = (-24.9 \pm 1.3)$ meV along the dimer rows and $J_\perp = (-0.8 \pm 0.1)$ meV across the dimer rows, i.e., antiferromagnetic-like coupling of the dimers with $c(4 {\times} 2)$ symmetry.

cond-mat.mes-hall↗