arXiv subjects

Myrta Grüning

Publications and source records attributed to Myrta Grüning.

At least 19 recordsLinked to original sources

arXiv2026

Temperature-Dependent nonlinear optics from first-principles: Second-Harmonic Generation in few-layers MoS$_2$

We present a first-principles real-time approach to study non-linear response of solids at finite temperature. Finite temperature effects are included as renormalization of the quasiparticle energies and a dephasing term proportional to the quasiparticle lifetimes. We evaluate electron-phonon matrix elements from Density-Functional Perturbation Theory for lattice dynamics and then calculate quasiparticles renormalization and lifetime from the Fan and Debye-Waller terms of the electron self-energy. Electron excitations are treated at the independent particle level of approximation. We apply the approach to the second-harmonic generation (SHG) in monolayer and trilayer MoS$_2$. We observe a nontrivial temperature-dependence of the SHG due to a strong crystal-momentum dependent quasiparticle renormalization. From the phonon-mode analysis we find that the coupling with acoustic and shear modes determines the overall crystal-momentum dependence respectively in monolayer and trilayer MoS$_2$. The nontrivial temperature-dependence of the SHG can help rationalize the increase of SHG intensity with increasing temperature observed in monolayer MoS$_2$ at a given laser energy [Adv. Optical Mater. 8, 2000441 (2020)].

cond-mat.mtrl-sci↗

arXiv2026

β-Ga2O3-Based Heterojunctions: Exploring Growth Orientations and Alloying on Electronic Properties

We investigate the effects of alloying and growth orientation on the electronic properties of the ultra-wide bandgap semiconductor β-Ga2O3 and pseudomorphic (AlxGa1-x)2O3 alloy heterojunctions. Band offsets are computed from first principles using density functional theory (DFT) with the Heyd-Scuseria-Ernzerhof hybrid functional for different Al concentrations and four growth orientations, namely (100)B, (010), (001)B, and ($\bar{2}$01). Significant variations are found and ascribed to the strained pseudomorphic alloys. The values of the band offsets are fed into technology computer-aided design (TCAD) models of Schottky barrier diodes (SBD). I-V and C-V characteristics from the TCAD models show reasonable agreement with recent experimental measurements in the forward bias region. Discrepancies in the negative bias region are expected due to the ideality of the Schottky junctions considered in this study. Our findings underscore the critical role of growth orientation and strain in the accurate modelling of β-Ga2O3-based SBD.

cond-mat.mtrl-sci↗

arXiv2026

Universal Stability of Ga Split Vacancies across α-, β-, and κ-Ga2O3 Polymorphs: A Machine-Learning Accelerated Study

Split Ga vacancies are the dominant native acceptor in $β$-$Ga_2O_3$; however, their role in $α$ and $κ$ phases has been largely overlooked or assumed to be unfavorable. A detailed understanding of these defects is critical for tailoring the electrical conductivity and optical properties and optimising $Ga_2O_3$-based devices. In this work, we used machine learning interatomic potentials (MLIPs) to accelerate the discovery of non-local defect reconstructions, followed by HSE06 hybrid DFT to accurately quantify defect properties of single vacancy $V_{\text{Ga}}$, split vacancy $V_{\text{Ga}}^{\text{i}}$ and substitutional donors ($\mathrm{Hf_{Ga}}$ and $\mathrm{Si_{Ga}}$) across a wide range of experimentally relevant conditions for the oxygen chemical potential. We find that split vacancies are the ground-state vacancy for all studied polymorphs ($β$, $α$, and $κ$). Split vacancies are more stable than simple vacancies by ~0.75 eV ($β$), ~0.41 eV ($α$), and ~0.14 eV ($κ$). Notably, MLIPs correctly identified the specific split-vacancy ground states and yielded an energetic ordering of symmetry-inequivalent defect configurations in excellent agreement with HSE06 results. While Hf and Si show low formation energy and act as shallow donors, especially under oxygen-poor conditions, their efficiency is limited by split-vacancy compensation. The growth under oxygen-poor conditions is a universal requirement to suppress these defects and achieve high n-type conductivity across the $Ga_2O_3$ polymorph.

cond-mat.mtrl-sci↗

arXiv2025

Shift current in 2D Janus Transition-Metal Dichalcogenides: the role of excitons

We investigate the shift current in two-dimensional (2D) Janus transition-metal dichalcogenides (TMDs). The shift current is evaluated using a real-time approach, where the coupling with an external field is described in terms of a dynamical Berry phase. This methodology incorporates electron-hole interactions and quasiparticle band structure renormalization through an effective Hamiltonian derived from many-body perturbation theory. We find that the shift current is strongly enhanced in correspondence with C excitons. An analysis in terms of the electron-hole pairs reveals that electron and hole are localized on different atoms, and thus, following an optical excitation, the center of the electron charge is displaced, giving rise to a significant photocurrent. Janus TMDs, with their intrinsic out-of-plane asymmetry and tunable electronic properties, are particularly appealing for next-generation optoelectronic and energy-harvesting technologies. These results highlight the role of excitons in the shift-current response of Janus TMDs and demonstrate their potential as promising building blocks for future photovoltaic devices.

cond-mat.mes-hall↗

arXiv2025

A real-time approach to frequency-mixing spectroscopies: application to sum and difference frequency generation in two-dimensional crystals

We propose a computational framework to extract non-linear response functions from real-time simulations in the presence of more than one external field. We apply this approach to the calculation of sum frequency generation (SFG) and difference frequency generation (DFG). SFG and DFG are second-order nonlinear processes where two lasers with frequencies $ω_1$ and $ω_2$ combine to produce a response at frequency $ω= ω_1 \pm ω_2$. Compared with other nonlinear responses such as second-harmonic generation, SFG and DFG allow for tunability over a larger range. Moreover, the optical response can be enhanced by selecting the two laser frequencies in order to match specific electron-hole transitions. To assess the approach, we calculate the SFG and DFG of two-dimensional crystals, hBN and MoS2 monolayers, from real-time solution of an effective Schrödinger equation. Within the effective Schrödinger equation, one can select from various levels of theory for the effective one-particle Hamiltonian to account for local-field effects and electron-hole interactions. We compare results obtained within the independent-particle picture and including many-body effects. Such comparison allows us to identify and characterize excitonic features in the obtained spectra. Additionally, we demonstrate that our approach can also extract higher-order response functions, such as field-induced second-harmonic generation. We provide an example using the hBN bilayer.

cond-mat.mtrl-sci↗

arXiv2025

Tailoring the Electronic Properties of Monoclinic (InxAl1-x)2O3 Alloys via Substitutional Donors and Acceptors

Ultra-wide bandgap semiconductors such as \b{eta}-Ga2O3 are ideal materials for next-generation power electronic devices. Electronic and mechanical properties of \b{eta}-Ga2O3 can be tuned by alloying with other sesquioxides, notably Al2O3 and In2O3. Moreover, by tuning the In content of a (InxAl1-x)2O3 alloy, its lattice constants can be matched to those of Ga2O3, while preserving a large conduction-band offset. In view of potential applications to \b{eta}-Ga2O3-based heterostructure, we performed atomistic modelling of (InxAl1-x)2O3 alloys using density functional theory to investigate thermodynamic and electrical properties of conventional group IV dopants (Si, Sn, C, Ge), alternative metal donors (Ta, Zr, Hf), and acceptors (Mg, Zn, Cu). The hybrid Heyd-Scuseria-Ernzerhof functional (HSE06) is used to accurately quantify the defect formation energies, ionization levels, and concentrations over a wide range of experimentally relevant conditions for the oxygen chemical potential and temperature. In our atomistic models, Hf and Zr show favourable properties as alternative donors to Si and other group IV impurities, especially under oxygen-poor conditions. Our findings also suggest that acceptors Mg, Zn, and Cu, while they cannot promote p-doping, can be still beneficial for the compensation of unintentionally n-doped materials, e.g., to generate semi-insulating layers and improve rectification.

cond-mat.mtrl-sci↗

arXiv2025

Effect of pressure, doping and magnetism on electronic structure and phonon dispersion of FeSe

We present a Density Functional Theory (DFT) based first-principles study on iron chalcogenides superconductor FeSe, systematically investigating pressure and doping induced modifications to its electronic, magnetic, and lattice dynamical properties. Our constrained-DFT calculations in striped antiferromagnetic and staggered dimer phase reveal a non-trivial dependence of the electronic structure on the local magnetic moment at all pressures. Sulpher (S) and tellurium (Te) doping exert opposing effects on the electronic structure, attributable to their contrasting chemical pressure effects (negative for S, positive for Te). Lattice dynamics calculations divulge distinct dependence of different phonon modes on local magnetic moment in different magnetic phases. We identify pressure and magnetic moment-dependent dynamical instability in certain magnetic phases, underscoring the intricate interplay of structural, electronic, and magnetic properties in this system. Investigation of spin-phonon coupling for different phonon modes shows the presence of strong magneto-elastic coupling in FeSe with pressure distinctly affecting prominent optical phonon modes -- pure iron derived $B_{1g}$ and pure selenium derived $A_{1g}$. A clear indication of the change in spin-phonon coupling with pressure is visible, especially for the $B_{1g}$ mode.

cond-mat.mtrl-sci↗

arXiv2025

The 2025 Roadmap to Ultrafast Dynamics: Frontiers of Theoretical and Computational Modelling

The exploration of ultrafast phenomena is a frontier of condensed matter research, where the interplay of theory, computation, and experiment is unveiling new opportunities for understanding and engineering quantum materials. With the advent of advanced experimental techniques and computational tools, it has become possible to probe and manipulate nonequilibrium processes at unprecedented temporal and spatial resolutions, providing insights into the dynamical behavior of matter under extreme conditions. These capabilities have the potential to revolutionize fields ranging from optoelectronics and quantum information to catalysis and energy storage. This Roadmap captures the collective progress and vision of leading researchers, addressing challenges and opportunities across key areas of ultrafast science. Contributions in this Roadmap span the development of ab initio methods for time-resolved spectroscopy, the dynamics of driven correlated systems, the engineering of materials in optical cavities, and the adoption of FAIR principles for data sharing and analysis. Together, these efforts highlight the interdisciplinary nature of ultrafast research and its reliance on cutting-edge methodologies, including quantum electrodynamical density-functional theory, correlated electronic structure methods, nonequilibrium Green's function approaches, quantum and ab initio simulations.

cond-mat.mtrl-sci↗

arXiv2024

Strain-Induced Decoupling Drives Gold-Assisted Exfoliation of Large-Area Monolayer 2D Crystals

Gold assisted exfoliation (GAE) is a groundbreaking mechanical exfoliation technique, producing centimeter scale single crystal monolayers of 2D materials. Such large, high quality films offer unparalleled advantages over the micron sized flakes typically produced by conventional exfoliation techniques, significantly accelerating the research and technological advancements in the field of 2D materials. Despite its wide applications, the fundamental mechanism of GAE remains poorly understood. In this study, using MoS2 on Au as a model system, we employ ultralow frequency Raman spectroscopy to elucidate how the interlayer interactions within MoS2 crystals are impacted by the gold substrate. The results reveal that the coupling at the first interface between the adhered layer on the gold substrate and the adjacent layer, is substantially weakened, with the binding force being reduced to nearly zero. This renders the first interface the weakest point in the system, thereby the crystal preferentially cleaves at this junction, generating large area monolayers with sizes comparable to the parent crystal. Biaxial strain in the adhered layer, induced by the gold substrate, is identified as the driving factor for the decoupling effect. We establish the strain-induced decoupling effect as the primary mechanism of GAE, which could also play a significant role in general mechanical exfoliations.

cond-mat.mtrl-sci↗

arXiv2024

A first-principles study and mesoscopic modeling of two-dimensional spin and orbital fluctuations in FeSe

We calculated the structural, electronic and magnetic properties of FeSe within density-functional theory at the generalized gradient approximation level. First, we studied how the bandwidth of the d-bands at the Fermi energy are renormalized by adding simple corrections: Hubbard U, Hunds J and by introducing long-range magnetic orders. We found that introducing either a striped or a staggered dimer antiferromagnetic order brings the bandwidths -- which are starkly overestimated at the generalized gradient approximation level -- closer to those experimentally observed. Second, for the ferromagnetic, the striped, checkerboard and the staggered dimer antiferromagnetic order, we investigate the change in magnetic formation energy with local magnetic moment of Fe at a pressure up to 6 GPa. The bilinear and biquadratic exchange energies are derived from the Heisenberg model and noncollinear first-principles calculations, respectively. We found a non-trivial behavior of the spin-exchange parameters on the magnetization, and we put forward a field-theory model that rationalizes these results in terms of two-dimensional spin and orbital fluctuations. The character of these fluctuations can be either that of a standard density wave or a topological vortex. Topological vortexes can result in mesoscopic magnetization structures.

cond-mat.mtrl-sci↗

arXiv2023

Floquet formulation of the dynamical Berry-phase approach to non-linear optics in extended systems

We present a Floquet scheme for the ab-initio calculation of nonlinear optical properties in extended systems. This entails a reformulation of the real-time approach based on the dynamical Berry-phase polarisation [Attaccalite & Grüning, PRB 88, 1-9 (2013)] and retains the advantage of being non-perturbative in the electric field. The proposed method applies to periodically-driven Hamiltonians and makes use of this symmetry to turn a time-dependent problem into a self-consistent time-independent eigenvalue problem. We implemented this Floquet scheme at the independent particle level and compared it with the real-time approach. Our reformulation reproduces real-time-calculated $2^{nd}$ and $3^{rd}$ order susceptibilities for a number of bulk and two-dimensional materials, while reducing the associated computational cost by one or two orders of magnitude.

cond-mat.mtrl-sci↗

arXiv2023

QS$G\hat{W}$: Quasiparticle Self consistent $GW$ with ladder diagrams in $W$

We present an extension of the quasiparticle self-consistent $GW$ approximation (QS$GW$) [Phys. Rev. B, 76 165106 (2007)] to include vertex corrections in the screened Coulomb interaction $W$. This is achieved by solving the Bethe-Salpeter equation for the polarization matrix at all $k$-points in the Brillouin zone. We refer to this method as QS$G\hat{W}$. QS$GW$ yields a reasonable and consistent description of the electronic structure and optical response, but systematic errors in several properties appear, notably a tendency to overestimate insulating bandgaps, blue-shift plasmon peaks in the imaginary part of the dielectric function, and underestimate the dielectric constant $ε_{\infty}$. A primary objective of this paper is to assess to what extent including ladder diagrams in $W$ ameliorates systematic errors for insulators in the QS$GW$ approximation. For benchmarking we consider about 40 well understood semiconductors, and also examine a variety of less well characterized nonmagnetic systems, six antiferromagnetic oxides, and the ferrimagnet Fe$_3$O$_4$. We find ladders ameliorate shortcomings in QS$GW$ to a remarkable degree in both the one-body Green's function and the dielectric function for a wide range of insulators. New discrepancies with experiment appear, and a key aim of this paper is to establish to what extent the errors are systematic and can be traced to diagrams missing from the theory. One key finding of this work is to establish a relation between the bandgap and the dielectric constant $ε_{\infty}$. Good description of both properties together provides a much more robust benchmark than either alone. We show how this information can be used to improve our understanding of the one-particle spectral properties in materials systems such as SrTiO$_3$ and FeO.

cond-mat.mtrl-sci↗

arXiv2021

Double $\mathbf{k}$-Grid Method for Solving the Bethe-Salpeter Equation via Lanczos Approaches

Convergence with respect to the size of the k-points sampling-grid of the Brillouin zone is the main bottleneck in the calculation of optical spectra of periodic crystals via the Bethe-Salpeter equation (BSE). We tackle this challenge by proposing a double grid approach to k-sampling compatible with the effective Lanczos-based Haydock iterative solution. Our method relies on a coarse k-grid that drives the computational cost, while a dense k-grid is responsible for capturing excitonic effects, albeit in an approximated way. Importantly, the fine k-grid requires minimal extra computation due to the simplicity of our approach, which also makes the latter straightforward to implement. We performed tests on bulk Si, bulk GaAs and monolayer MoS2, all of which produced spectra in good agreement with data reported elsewhere. This framework has the potential of enabling the calculation of optical spectra in semiconducting systems where the efficiency of the Haydock scheme alone is not enough to achieve a computationally tractable solution of the BSE, e.g., large-scale systems with very stringent k-sampling requirements for achieving convergence.

cond-mat.mtrl-sci↗

arXiv2021

Optical response and band structure of LiCoO2 including electron-hole interaction effects

The optical response functions and band structures of LiCoO$_2$ are studied at different levels of approximation, from density functional theory (DFT) in the generalized gradient approximation (GGA) to quasiparticle self-consistent QS$GW$ (with $G$ for Green's function and $W$ for screened Coulomb interaction) without and with ladder diagrams (QS$G\hat W$) and the Bethe Salpeter Equation (BSE) approach. The QS$GW$ method is found to strongly overestimate the band gap and electron-hole or excitonic effects are found to be important. They lower the quasiparticle gap by only about 11~\% but the lowest energy peaks in absorption are found to be excitonic in nature. The contributions from different band to band transitions and the relation of excitons to band-to-band transitions are analyzed. The excitons are found to be strongly localized. A comparison to experimental data is presented.

cond-mat.mtrl-sci↗

arXiv2021

Electronic Structure of Chromium Trihalides beyond Density Functional Theory

We explore the electronic band structure of free standing monolayers of chromium trihalides, CrX\textsubscript{3}{, X= Cl, Br, I}, within an advanced \emph{ab-initio} theoretical approach based in the use of Green's function functionals. We compare the local density approximation with the quasi-particle self-consistent \emph{GW} approximation (QS\emph{GW}) and its self-consistent extension (QS$G\widehat{W}$) by solving the particle-hole ladder Bethe-Salpeter equations to improve the effective interaction \emph{W}. We show that at all levels of theory, the valence band consistently changes shape in the sequence Cl{\textrightarrow}Br{\textrightarrow}I, and the valence band maximum shifts from the M point to the $Γ$ point. However, the details of the transition, the one-particle bandgap, and the eigenfunctions change considerably going up the ladder to higher levels of theory. The eigenfunctions become more directional, and at the M point there is a strong anisotropy in the effective mass. Also the dynamic and momentum dependent self energy shows that QS$G\widehat{W}$ adds to the localization of the systems in comparison to the QS\emph{GW} thereby leading to a narrower band and reduced amount of halogens in the valence band manifold.

cond-mat.str-el↗

arXiv2020

Thermal conductivity of porous polycrystalline PbTe

PbTe is a leading thermoelectric material at intermediate temperatures, largely thanks to its low lattice thermal conductivity. However, its efficiency is too low to compete with other forms of power generation. This efficiency can be effectively enhanced by designing nanostructures capable of scattering phonons over a wide range of length scales to reduce the lattice thermal conductivity. The presence of grain boundaries can reduce the thermal conductivity to $\sim 0.5$ Wm$^{-1}$K$^{-1}$ for small vacancy concentrations and grain sizes. However, grains anneal at finite temperature, and equilibrium and metastable grain size distributions determine the extent of the reduction in thermal conductivity. In the present work, we propose a phase-field model informed by molecular dynamics simulations to study the annealing process in PbTe and how it is affected by the presence of grain boundaries and voids. We find that the thermal conductivity of PbTe is reduced by up to 35\% in the porous material at low temperatures. We observe that a phase transition at a finite density of voids governs the kinetics of impeding grain growth by Zener pinning.

cond-mat.mes-hall↗

arXiv2020

Towards temperature-induced topological phase transition in SnTe: A first principles study

The temperature renormalization of the bulk band structure of a topological crystalline insulator, SnTe, is calculated using first principles methods. We explicitly include the effect of thermal-expansion-induced modification of electronic states and their band inversion on electron-phonon interaction. We show that the direct gap decreases with temperature, as both thermal expansion and electron-phonon interaction drive SnTe towards the phase transition to a topologically trivial phase as temperature increases. The band gap renormalization due to electron-phonon interaction exhibits a non-linear dependence on temperature as the material approaches the phase transition, while the lifetimes of the conduction band states near the band edge show a non-monotonic behavior with temperature. These effects should have important implications on bulk electronic and thermoelectric transport in SnTe and other topological insulators.

cond-mat.mtrl-sci↗

arXiv2020

$GW$ study of pressure-induced topological insulator transition in group IV-tellurides

We calculate the electronic structure of the narrow gap semiconductors PbTe, SnTe and GeTe in the cubic phase using density functional theory (DFT) and the $G_0W_0$ method. Within DFT, we show that the band ordering obtained with a conventional semilocal exchange-correlation approximation is correct for SnTe and GeTe but wrong for PbTe. The correct band ordering at the high-symmetry point L is recovered adding $G_0W_0$ quasiparticle corrections. However, one-shot $G_0W_0$ produces artifacts in the band structure due to the wrong orbital character of the DFT single-particle states at the band edges close to L. We show that in order to correct these artifacts it is enough to consider the off-diagonal elements of the $G_0W_0$ self-energy corresponding to these states. We also investigate the pressure dependence of the band gap for these materials and the possibility of a transition from a trivial to a non-trivial topology of the band structure. For PbTe, we predict the band crossover and topological transition to occur at around 4.8 GPa. For GeTe, we estimate the topological transition to occur at 1.9 GPa in the constrained cubic phase, a pressure lower than the one of the structural phase transition from rombohedral to cubic. SnTe is a crystalline topological insulator at ambient pressure, and the transition into a trivial topology would take place under a volume expansion of approximately $10\%$.

cond-mat.mes-hall↗