@article{26627, abstract = {{Many-body perturbation theory based on density-functional theory calculations is used to determine the quasiparticle band structures and the dielectric functions of the isomorphic ferroelectrics rubidium titanyl phosphate (RbTiOPO4) and potassium titanyl arsenide (KTiOAsO4). Self-energy corrections of more than 2 eV are found to widen the transport band gaps of both materials considerably to 5.3 and 5.2 eV, respectively. At the same time, both materials are characterized by strong exciton binding energies of 1.4 and 1.5 eV, respectively. The solution of the Bethe-Salpeter equation based on the quasiparticle energies results in onsets of the optical absorption within the range of the measured data.}}, author = {{Neufeld, Sergej and Schindlmayr, Arno and Schmidt, Wolf Gero}}, issn = {{2515-7639}}, journal = {{Journal of Physics: Materials}}, number = {{1}}, publisher = {{IOP Publishing}}, title = {{{Quasiparticle energies and optical response of RbTiOPO4 and KTiOAsO4}}}, doi = {{10.1088/2515-7639/ac3384}}, volume = {{5}}, year = {{2022}}, } @article{22310, author = {{Neufeld, Sergej and Bocchini, Adriana and Schmidt, Wolf Gero}}, issn = {{2475-9953}}, journal = {{Physical Review Materials}}, title = {{{Potassium titanyl phosphate Z- and Y-cut surfaces from density-functional theory}}}, doi = {{10.1103/physrevmaterials.5.064407}}, year = {{2021}}, } @article{17068, author = {{Braun, Christian and Neufeld, Sergej and Gerstmann, Uwe and Sanna, S. and Plaickner, J. and Speiser, E. and Esser, N. and Schmidt, Wolf Gero}}, issn = {{0031-9007}}, journal = {{Physical Review Letters}}, number = {{14}}, title = {{{Vibration-Driven Self-Doping of Dangling-Bond Wires on Si(553)-Au Surfaces}}}, doi = {{10.1103/physrevlett.124.146802}}, volume = {{124}}, year = {{2020}}, } @article{13365, abstract = {{The KTiOPO4 (KTP) band structure and dielectric function are calculated on various levels of theory starting from density-functional calculations. Within the independent-particle approximation an electronic transport gap of 2.97 eV is obtained that widens to about 5.23 eV when quasiparticle effects are included using the GW approximation. The optical response is shown to be strongly anisotropic due to (i) the slight asymmetry of the TiO6 octahedra in the (001) plane and (ii) their anisotropic distribution along the [001] and [100] directions. In addition, excitonic effects are very important: The solution of the Bethe–Salpeter equation indicates exciton binding energies of the order of 1.5 eV. Calculations that include both quasiparticle and excitonic effects are in good agreement with the measured reflectivity.}}, author = {{Neufeld, Sergej and Bocchini, Adriana and Gerstmann, Uwe and Schindlmayr, Arno and Schmidt, Wolf Gero}}, issn = {{2515-7639}}, journal = {{Journal of Physics: Materials}}, pages = {{045003}}, publisher = {{IOP Publishing}}, title = {{{Potassium titanyl phosphate (KTP) quasiparticle energies and optical response}}}, doi = {{10.1088/2515-7639/ab29ba}}, volume = {{2}}, year = {{2019}}, } @article{13429, author = {{Bocchini, Adriana and Neufeld, Sergej and Gerstmann, Uwe and Schmidt, Wolf Gero}}, issn = {{0953-8984}}, journal = {{Journal of Physics: Condensed Matter}}, pages = {{385401}}, title = {{{Oxygen and potassium vacancies in KTP calculated from first principles}}}, doi = {{10.1088/1361-648x/ab295c}}, volume = {{31}}, year = {{2019}}, } @article{13411, author = {{Halbig, B. and Liebhaber, M. and Bass, U. and Geurts, J. and Speiser, E. and Räthel, J. and Chandola, S. and Esser, N. and Krenz, Marvin and Neufeld, Sergej and Schmidt, Wolf Gero and Sanna, S.}}, issn = {{2469-9950}}, journal = {{Physical Review B}}, number = {{3}}, title = {{{Vibrational properties of the Au-(3×3)/Si(111) surface reconstruction}}}, doi = {{10.1103/physrevb.97.035412}}, volume = {{97}}, year = {{2018}}, } @article{4769, abstract = {{In recent years, Raman spectroscopy has been used to visualize and analyze ferroelectric domain structures. The technique makes use of the fact that the intensity or frequency of certain phonons is strongly influenced by the presence of domain walls. Although the method is used frequently, the underlying mechanism responsible for the changes in the spectra is not fully understood. This inhibits deeper analysis of domain structures based on this method. Two different models have been proposed. However, neither model completely explains all observations. In this work, we have systematically investigated domain walls in different scattering geometries with Raman spectroscopy in the common ferroelectric materials used in integrated optics, i.e., KTiOPO4, LiNbO3, and LiTaO3. Based on the two models, we can demonstrate that the observed contrast for domain walls is in fact based on two different effects. We can identify on the one hand microscopic changes at the domain wall, e.g., strain and electric fields, and on the other hand a macroscopic change of selection rules at the domain wall. While the macroscopic relaxation of selection rules can be explained by the directional dispersion of the phonons in agreement with previous propositions, the microscopic changes can be explained qualitatively in terms of a simplified atomistic model.}}, author = {{Rüsing, Michael and Neufeld, Sergej and Brockmeier, Julian and Eigner, Christof and Mackwitz, P. and Spychala, K. and Silberhorn, Christine and Schmidt, Wolf Gero and Berth, Gerhard and Zrenner, Artur and Sanna, S.}}, issn = {{2475-9953}}, journal = {{Physical Review Materials}}, number = {{10}}, publisher = {{American Physical Society (APS)}}, title = {{{Imaging of 180∘ ferroelectric domain walls in uniaxial ferroelectrics by confocal Raman spectroscopy: Unraveling the contrast mechanism}}}, doi = {{10.1103/physrevmaterials.2.103801}}, volume = {{2}}, year = {{2018}}, } @article{13426, author = {{Edler, F. and Miccoli, I. and Stöckmann, J. P. and Pfnür, H. and Braun, Christian and Neufeld, Sergej and Sanna, S. and Schmidt, Wolf Gero and Tegenkamp, C.}}, issn = {{2469-9950}}, journal = {{Physical Review B}}, number = {{12}}, title = {{{Tuning the conductivity along atomic chains by selective chemisorption}}}, doi = {{10.1103/physrevb.95.125409}}, volume = {{95}}, year = {{2017}}, } @article{13458, author = {{Liebhaber, M. and Halbig, B. and Bass, U. and Geurts, J. and Neufeld, Sergej and Sanna, S. and Schmidt, Wolf Gero and Speiser, E. and Räthel, J. and Chandola, S. and Esser, N.}}, issn = {{2469-9950}}, journal = {{Physical Review B}}, number = {{23}}, title = {{{Vibration eigenmodes of the Au-(5×2)/Si(111) surface studied by Raman spectroscopy and first-principles calculations}}}, doi = {{10.1103/physrevb.94.235304}}, volume = {{94}}, year = {{2016}}, } @article{10026, abstract = {{Congruent lithium niobate and lithium tantalate mixed crystals have been grown over the complete compositional range with the Czochralski method. The structural and vibrational properties of the mixed crystals are studied extensively by x-ray diffraction measurements, Raman spectroscopy, and density functional theory. The measured lattice parameters and vibrational frequencies are in good agreement with our theoretical predictions. The observed dependence of the Raman frequencies on the crystal composition is discussed on the basis of the calculated phonon displacement patterns. The phononic contribution to the static dielectric tensor is calculated by means of the generalized Lyddane-Sachs-Teller relation. Due to the pronounced dependence of the optical response on the Ta concentration, lithium niobate tantalate mixed crystals represent a perfect model system to study the properties of uniaxial mixed ferroelectric materials for application in integrated optics.}}, author = {{Rüsing, Michael and Sanna, Simone and Neufeld, Sergej and Berth, Gerhard and Schmidt, Wolf Gero and Zrenner, Artur and Yu, H. and Wang, Y. and Zhang, H.}}, issn = {{2469-9950}}, journal = {{Physical Review B}}, title = {{{Vibrational properties of LiNb1−xTaxO3 mixed crystals}}}, doi = {{10.1103/physrevb.93.184305}}, year = {{2016}}, } @article{4332, abstract = {{LiTaO3 and LiNbO3 crystals are investigated here in a combined experimental and theoretical study that uses Raman spectroscopy in a complete set of scattering geometries and corresponding density-functional theory calculations to provide microscopic information on their vibrational properties. The Raman scattering efficiency is computed from first principles in order to univocally assign the measured Raman peaks to the calculated eigenvectors. Measured and calculated Raman spectra are shown to be in qualitative agreement and confirm the mode assignment by Margueron et al. [J. Appl. Phys. 111, 104105 (2012)], thus finally settling a long debate. While the two crystals show rather similar vibrational properties overall, the E-TO9 mode is markedly different in the two oxides. The deviations are explained by a different anion-cation bond type in LiTaO3 and LiNbO3 crystals.}}, author = {{Sanna, Simone and Neufeld, Sergej and Rüsing, Michael and Berth, Gerhard and Zrenner, Artur and Schmidt, Wolf Gero}}, issn = {{1098-0121}}, journal = {{Physical Review B}}, number = {{22}}, publisher = {{American Physical Society (APS)}}, title = {{{Raman scattering efficiency in LiTaO3 and LiNbO3 crystals}}}, doi = {{10.1103/physrevb.91.224302}}, volume = {{91}}, year = {{2015}}, } @article{13520, abstract = {{Atomistic simulations in the framework of the density functional theory have been used to model morphologic and vibrational properties of lithium niobate–lithium tantalate mixed crystals as a function of the [Nb]/[Ta] ratio. Structural parameters such as the crystal volume and the lattice parameters a and c vary roughly linearly from LiTaO3 to LiNbO3, showing only minor deviations from the Vegard behavior. Our ab initio calculations demonstrate that the TO1, TO2 and TO4 vibrational modes become harder with increasing Nb concentration. TO3 becomes softer with increasing Nb content, instead. Furthermore, the investigated zone center A1 -TO phonon modes are characterized by a pronounced stoichiometry dependence. Frequency shifts as large as 30 cm−1 are expected as the [Nb]/[Ta] ratio grows from 0 to 1. Therefore, spectroscopic techniques sensitive to the A1 modes (such as Raman spectroscopy), can be employed for a direct and non-destructive determination of the crystal composition.}}, author = {{Sanna, Simone and Riefer, A. and Neufeld, Sergej and Schmidt, Wolf Gero and Berth, Gerhard and Rüsing, Michael and Widhalm, A. and Zrenner, Artur}}, issn = {{0015-0193}}, journal = {{Ferroelectrics}}, keywords = {{Ferroelectrics, vibrational properties, LiNbO3, LiTaO3, mixed crystals}}, number = {{1}}, pages = {{63--68}}, title = {{{Vibrational Fingerprints of LiNbO3-LiTaO3Mixed Crystals}}}, doi = {{10.1080/00150193.2013.821893}}, volume = {{447}}, year = {{2013}}, }