@article{60565, author = {{Bocchini, Adriana and Gerstmann, Uwe and Schmidt, Wolf Gero}}, issn = {{2469-9950}}, journal = {{Physical Review B}}, number = {{10}}, publisher = {{American Physical Society (APS)}}, title = {{{Microscopic origin of gray tracks in KTiOPO4}}}, doi = {{10.1103/physrevb.111.104103}}, volume = {{111}}, year = {{2025}}, } @article{61356, abstract = {{First-principles calculations reveal how topological defects in semiconducting carbon nanotubes trap triplet excitons and enable single-photon emission at telecom wavelengths, offering new insights into their potential for photonic devices.}}, author = {{Biktagirov, Timur and Gerstmann, Uwe and Schmidt, Wolf Gero}}, issn = {{2040-3364}}, journal = {{Nanoscale}}, number = {{11}}, pages = {{6884--6891}}, publisher = {{Royal Society of Chemistry (RSC)}}, title = {{{Topological defects in semiconducting carbon nanotubes as triplet exciton traps and single-photon emitters}}}, doi = {{10.1039/d4nr03904a}}, volume = {{17}}, year = {{2025}}, } @article{60568, author = {{Bocchini, Adriana and Kollmann, S. and Gerstmann, Uwe and Schmidt, Wolf Gero and Grundmeier, Guido}}, issn = {{0039-6028}}, journal = {{Surface Science}}, publisher = {{Elsevier BV}}, title = {{{Phosphonic acid adsorption on α-Bi2O3 surfaces}}}, doi = {{10.1016/j.susc.2025.122776}}, volume = {{760}}, year = {{2025}}, } @article{61353, abstract = {{Abstract Muonic hydrogen is an exotic atom where a muon instead of an electron is bound to a proton. The comparably high mass of the muon (≈ 207 · me ) has two important effects, (i) the reduced mass of the system becomes more important, and (ii) the muon is localized much closer to the nucleus. Thus, muonic hydrogen is not only excellently suitable for evaluating highly precise quantum electrodynamic (QED) calculations, but may also be used for assessing new approaches including finite nuclear size (FNS) effects to evaluate the proton structure and improve calculation schemes for the hyperfine splittings of many-particle systems, as e.g. to be implemented in density functional theory (DFT) software packages. Here, starting from Dirac’s equation we calculate the relativistic hyperfine splitting of the ground state and several excited states of muonic hydrogen analytically for different charge and magnetization models. The FNS related hyperfine shifts are compared with the differences between QED calculations and experimental measurements. This comparison also allows to unravel the role of the reduced mass, which is on one hand crucial in case of muonic atoms, but on the other hand is by no means well defined in relativistic quantum mechanics.}}, author = {{Franzke, Katharina L. and Schmidt, Wolf Gero and Gerstmann, Uwe}}, issn = {{1742-6588}}, journal = {{Journal of Physics: Conference Series}}, number = {{1}}, publisher = {{IOP Publishing}}, title = {{{Finite-size and relativistic effects onto hyperfine interaction of muonic hydrogen}}}, doi = {{10.1088/1742-6596/3027/1/012001}}, volume = {{3027}}, year = {{2025}}, } @article{60566, author = {{Bocchini, Adriana and Rüsing, Michael and Bollmers, Laura and Lengeling, Sebastian and Mues, Philipp and Padberg, Laura and Gerstmann, Uwe and Silberhorn, Christine and Eigner, Christof and Schmidt, Wolf Gero}}, issn = {{2475-9953}}, journal = {{Physical Review Materials}}, number = {{7}}, publisher = {{American Physical Society (APS)}}, title = {{{Mg dopants in lithium niobate: Defect models and impact on domain inversion}}}, doi = {{10.1103/5wz1-bjyr}}, volume = {{9}}, year = {{2025}}, } @article{61357, author = {{Krenz, Marvin and Sanna, Simone and Gerstmann, Uwe and Schmidt, Wolf Gero}}, issn = {{1932-7447}}, journal = {{The Journal of Physical Chemistry C}}, number = {{41}}, pages = {{17774--17778}}, publisher = {{American Chemical Society (ACS)}}, title = {{{Understanding and Improving Triplet Exciton Transfer in Sensitized Silicon Solar Cells}}}, doi = {{10.1021/acs.jpcc.4c05446}}, volume = {{128}}, year = {{2024}}, } @article{54868, abstract = {{AbstractMost properties of solid materials are defined by their internal electric field and charge density distributions which so far are difficult to measure with high spatial resolution. Especially for 2D materials, the atomic electric fields influence the optoelectronic properties. In this study, the atomic‐scale electric field and charge density distribution of WSe2 bi‐ and trilayers are revealed using an emerging microscopy technique, differential phase contrast (DPC) imaging in scanning transmission electron microscopy (STEM). For pristine material, a higher positive charge density located at the selenium atomic columns compared to the tungsten atomic columns is obtained and tentatively explained by a coherent scattering effect. Furthermore, the change in the electric field distribution induced by a missing selenium atomic column is investigated. A characteristic electric field distribution in the vicinity of the defect with locally reduced magnitudes compared to the pristine lattice is observed. This effect is accompanied by a considerable inward relaxation of the surrounding lattice, which according to first principles DFT calculation is fully compatible with a missing column of Se atoms. This shows that DPC imaging, as an electric field sensitive technique, provides additional and remarkable information to the otherwise only structural analysis obtained with conventional STEM imaging.}}, author = {{Groll, Maja and Bürger, Julius and Caltzidis, Ioannis and Jöns, Klaus D. and Schmidt, Wolf Gero and Gerstmann, Uwe and Lindner, Jörg K. N.}}, issn = {{1613-6810}}, journal = {{Small}}, publisher = {{Wiley}}, title = {{{DFT‐Assisted Investigation of the Electric Field and Charge Density Distribution of Pristine and Defective 2D WSe2 by Differential Phase Contrast Imaging}}}, doi = {{10.1002/smll.202311635}}, year = {{2024}}, } @article{54856, abstract = {{Abstract Theoretical spectroscopy based on double perturbation theory is typically challenged by systems with large orbital hyperfine splitting. Therefore, we here derive a rigorous, non-perturbative scheme starting from Dirac’s equation which allows to calculate the contribution of the orbital HFI for complex structures including heavy atoms with strong spin-orbit coupling (SOC). Using the PAW formalism, the method has been implemented in the software package Quantum ESPRESSO. We show that the ‘orbital part’ actually scales with SOC strength if orbital quenching is hindered by low local symmetry, i.e. in case of dimers or atoms at surfaces. This holds true in particular when the unpaired electron is localized in quasi-atomic p-like orbitals. Here, the orbital part is by far not negligible, but becomes dominant by surpassing the dipolar contribution by a factor of five.}}, author = {{Franzke, Katharina and Schmidt, Wolf Gero and Gerstmann, Uwe}}, issn = {{1742-6588}}, journal = {{Journal of Physics: Conference Series}}, number = {{1}}, publisher = {{IOP Publishing}}, title = {{{Relativistic calculation of the orbital hyperfine splitting in complex microscopic structures}}}, doi = {{10.1088/1742-6596/2701/1/012094}}, volume = {{2701}}, year = {{2024}}, } @article{54865, author = {{Krenz, Marvin and Gerstmann, Uwe and Schmidt, Wolf Gero}}, issn = {{0031-9007}}, journal = {{Physical Review Letters}}, number = {{7}}, publisher = {{American Physical Society (APS)}}, title = {{{Defect-Assisted Exciton Transfer across the Tetracene-Si(111):H Interface}}}, doi = {{10.1103/physrevlett.132.076201}}, volume = {{132}}, year = {{2024}}, } @article{54854, abstract = {{Batteries based on heavier alkali ions are considered promising candidates to substitute for current Li-based technologies. In this theoretical study, we characterize the structural properties of a novel material, i.e., F-doped RbTiOPO4 (RbTiPO4F, RTP:F), and discuss aspects of its electrochemical performance in Rb-ion batteries (RIBs) using density functional theory (DFT). According to our calculations, RTP:F is expected to retain the so-called KTiOPO4 (KTP)-type structure, with lattice parameters of 13.236 Å, 6.616 Å, and 10.945 Å. Due to the doping with F, the crystal features eight extra electrons per unit cell, whereby each of these electrons is trapped by one of the surrounding Ti atoms in the cell. Notably, the ground state of the system corresponds to a ferromagnetic spin configuration (i.e., S=4). The deintercalation of Rb leads to the oxidation of the Ti atoms in the cell (i.e., from Ti3+ to Ti4+) and to reduced magnetic moments. The material promises interesting electrochemical properties for the cathode: rather high average voltages above 2.8 V and modest volume shrinkages below 13% even in the fully deintercalated case are predicted.}}, author = {{Bocchini, Adriana and Xie, Yingjie and Schmidt, Wolf Gero and Gerstmann, Uwe}}, issn = {{2073-4352}}, journal = {{Crystals}}, number = {{1}}, publisher = {{MDPI AG}}, title = {{{Structural and Electrochemical Properties of F-Doped RbTiOPO4 (RTP:F) Predicted from First Principles}}}, doi = {{10.3390/cryst14010005}}, volume = {{14}}, year = {{2023}}, } @inproceedings{61362, abstract = {{We study the interaction of gray tracking and DC ionic conductivity in Potassium Titanyl Phosphate (KTiOPO4, KTP) and present a novel way to reduce conductivity via a potassium nitrate treatment improving the device quality.}}, author = {{Eigner, Christof and Padberg, Laura and Quiring, Viktor and Bocchini, Adriana and Santandrea, Matteo and Gerstmann, Uwe and Schmidt, Wolf Gero and Silberhorn, Christine}}, booktitle = {{CLEO 2023}}, publisher = {{Optica Publishing Group}}, title = {{{Potassium Titanyl Phosphate Material Engineering Boosting Integrated Optical Source Performance}}}, doi = {{10.1364/cleo_at.2023.jw2a.57}}, year = {{2023}}, } @article{37711, abstract = {{AbstractPolarons influence decisively the performance of lithium niobate for optical applications. In this work, the formation of (defect) bound polarons in lithium niobate is studied by ab initio molecular dynamics. The calculations show a broad scatter of polaron formation times. Rising temperature increases the share of trajectories with long formation times, which leads to an overall increase of the average formation time with temperature. However, even at elevated temperatures, the average formation time does not exceed the value of 100 femtoseconds, i.e., a value close to the time measured for free, i.e., self-trapped polarons. Analyzing individual trajectories, it is found that the time required for the structural relaxation of the polarons depends sensitively on the excitation of the lithium niobate high-frequency phonon modes and their phase relation.}}, author = {{Krenz, Marvin and Gerstmann, Uwe and Schmidt, Wolf Gero}}, issn = {{0947-8396}}, journal = {{Applied Physics A}}, keywords = {{General Materials Science, General Chemistry}}, pages = {{480}}, publisher = {{Springer Science and Business Media LLC}}, title = {{{Bound polaron formation in lithium niobate from ab initio molecular dynamics}}}, doi = {{10.1007/s00339-022-05577-y}}, volume = {{128}}, year = {{2022}}, } @article{33484, abstract = {{We study the DC conductivity in potassium titanyl phosphate (KTiOPO4, KTP) and its isomorphs KTiOAsO4 (KTA) and Rb1%K99%TiOPO4 (RKTP) and introduce a method by which to reduce the overall ionic conductivity in KTP by a potassium nitrate treatment. Furthermore, we create so-called gray tracking in KTP and investigate the ionic conductivity in theses areas. A local unintended reduction of the ionic conductivity is observed in the gray-tracked regions, which also induce additional optical absorption in the material. We show that a thermal treatment in an oxygen-rich atmosphere removes the gray tracking and brings the ionic conductivity as well as the optical transmission back to the original level. These studies can help to choose the best material and treatment for specific applications.}}, author = {{Padberg, Laura and Quiring, Viktor and Bocchini, Adriana and Santandrea, Matteo and Gerstmann, Uwe and Schmidt, Wolf Gero and Silberhorn, Christine and Eigner, Christof}}, issn = {{2073-4352}}, journal = {{Crystals}}, pages = {{1359}}, title = {{{DC Ionic Conductivity in KTP and Its Isomorphs: Properties, Methods for Suppression, and Its Connection to Gray Tracking}}}, doi = {{10.3390/cryst12101359}}, volume = {{12}}, year = {{2022}}, } @article{33965, author = {{Bocchini, Adriana and Gerstmann, Uwe and Bartley, Tim and Steinrück, Hans-Georg and Henkel, Gerald and Schmidt, Wolf Gero}}, journal = {{Phys. Rev. Materials}}, pages = {{105401}}, publisher = {{American Physical Society}}, title = {{{Electrochemical performance of KTiOAsO_4 (KTA) in potassium-ion batteries from density-functional theory}}}, doi = {{10.1103/PhysRevMaterials.6.105401}}, volume = {{6}}, year = {{2022}}, } @article{31254, author = {{Bocchini, Adriana and Gerstmann, Uwe and Schmidt, Wolf Gero}}, journal = {{Phys. Rev. B}}, pages = {{205118}}, publisher = {{American Physical Society}}, title = {{{Oxygen vacancies in KTiOPO_4: Optical absorption from hybrid DFT}}}, doi = {{10.1103/PhysRevB.105.205118}}, volume = {{105}}, year = {{2022}}, } @article{44088, abstract = {{Hole polarons and defect-bound exciton polarons in lithium niobate are investigated by means of density-functional theory, where the localization of the holes is achieved by applying the +U approach to the oxygen 2p orbitals. We find three principal configurations of hole polarons: (i) self-trapped holes localized at displaced regular oxygen atoms and (ii) two other configurations bound to a lithium vacancy either at a threefold coordinated oxygen atom above or at a two-fold coordinated oxygen atom below the defect. The latter is the most stable and is in excellent quantitative agreement with measured g factors from electron paramagnetic resonance. Due to the absence of mid-gap states, none of these hole polarons can explain the broad optical absorption centered between 2.5 and 2.8 eV that is observed in transient absorption spectroscopy, but such states appear if a free electron polaron is trapped at the same lithium vacancy as the bound hole polaron, resulting in an exciton polaron. The dielectric function calculated by solving the Bethe–Salpeter equation indeed yields an optical peak at 2.6 eV in agreement with the two-photon experiments. The coexistence of hole and exciton polarons, which are simultaneously created in optical excitations, thus satisfactorily explains the reported experimental data.}}, author = {{Schmidt, Falko and Kozub, Agnieszka L. and Gerstmann, Uwe and Schmidt, Wolf Gero and Schindlmayr, Arno}}, issn = {{2073-4352}}, journal = {{Crystals}}, number = {{11}}, publisher = {{MDPI AG}}, title = {{{A density-functional theory study of hole and defect-bound exciton polarons in lithium niobate}}}, doi = {{10.3390/cryst12111586}}, volume = {{12}}, year = {{2022}}, } @article{37713, author = {{Murzakhanov, Fadis F. and Mamin, Georgy Vladimirovich and Orlinskii, Sergei Borisovich and Gerstmann, Uwe and Schmidt, Wolf Gero and Biktagirov, Timur and Aharonovich, Igor and Gottscholl, Andreas and Sperlich, Andreas and Dyakonov, Vladimir and Soltamov, Victor A.}}, issn = {{1530-6984}}, journal = {{Nano Letters}}, keywords = {{Mechanical Engineering, Condensed Matter Physics, General Materials Science, General Chemistry, Bioengineering}}, number = {{7}}, pages = {{2718--2724}}, publisher = {{American Chemical Society (ACS)}}, title = {{{Electron–Nuclear Coherent Coupling and Nuclear Spin Readout through Optically Polarized VB– Spin States in hBN}}}, doi = {{10.1021/acs.nanolett.1c04610}}, volume = {{22}}, year = {{2022}}, } @inbook{30288, abstract = {{Lithium niobate (LiNbO3), a material frequently used in optical applications, hosts different kinds of polarons that significantly affect many of its physical properties. In this study, a variety of electron polarons, namely free, bound, and bipolarons, are analyzed using first-principles calculations. We perform a full structural optimization based on density-functional theory for selected intrinsic defects with special attention to the role of symmetry-breaking distortions that lower the total energy. The cations hosting the various polarons relax to a different degree, with a larger relaxation corresponding to a larger gap between the defect level and the conduction-band edge. The projected density of states reveals that the polaron states are formerly empty Nb 4d states lowered into the band gap. Optical absorption spectra are derived within the independent-particle approximation, corrected by the GW approximation that yields a wider band gap and by including excitonic effects within the Bethe-Salpeter equation. Comparing the calculated spectra with the density of states, we find that the defect peak observed in the optical absorption stems from transitions between the defect level and a continuum of empty Nb 4d states. Signatures of polarons are further analyzed in the reflectivity and other experimentally measurable optical coefficients.}}, author = {{Schmidt, Falko and Kozub, Agnieszka L. and Gerstmann, Uwe and Schmidt, Wolf Gero and Schindlmayr, Arno}}, booktitle = {{New Trends in Lithium Niobate: From Bulk to Nanocrystals}}, editor = {{Corradi, Gábor and Kovács, László}}, isbn = {{978-3-0365-3340-7}}, pages = {{231--248}}, publisher = {{MDPI}}, title = {{{Electron polarons in lithium niobate: Charge localization, lattice deformation, and optical response}}}, doi = {{10.3390/books978-3-0365-3339-1}}, year = {{2022}}, }