@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{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}}, } @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}}, } @article{29748, author = {{Slawig, Diana and Gruschwitz, Markus and Gerstmann, Uwe and Rauls, Eva and Tegenkamp, Christoph}}, issn = {{1932-7447}}, journal = {{The Journal of Physical Chemistry C}}, keywords = {{Surfaces, Coatings and Films, Physical and Theoretical Chemistry, General Energy, Electronic, Optical and Magnetic Materials}}, number = {{36}}, pages = {{20087--20093}}, publisher = {{American Chemical Society (ACS)}}, title = {{{Adsorption and Reaction of PbPc on Hydrogenated Epitaxial Graphene}}}, doi = {{10.1021/acs.jpcc.1c06320}}, volume = {{125}}, year = {{2021}}, } @article{21946, 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}}, issn = {{2073-4352}}, journal = {{Crystals}}, pages = {{542}}, publisher = {{MDPI}}, title = {{{Electron polarons in lithium niobate: Charge localization, lattice deformation, and optical response}}}, doi = {{10.3390/cryst11050542}}, volume = {{11}}, year = {{2021}}, } @article{22881, author = {{Nguyen, T. T. Nhung and Sollfrank, T. and Tegenkamp, C. and Rauls, E. and Gerstmann, Uwe}}, issn = {{2469-9950}}, journal = {{Physical Review B}}, pages = {{L201408}}, title = {{{Impact of screening and relaxation on weakly coupled two-dimensional heterostructures}}}, doi = {{10.1103/physrevb.103.l201408}}, volume = {{103}}, year = {{2021}}, } @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{22960, abstract = {{We perform a theoretical analysis of the structural and electronic properties of sodium potassium niobate K1-xNaxNbO3 in the orthorhombic room-temperature phase, based on density-functional theory in combination with the supercell approach. Our results for x=0 and x=0.5 are in very good agreement with experimental measurements and establish that the lattice parameters decrease linearly with increasing Na contents, disproving earlier theoretical studies based on the virtual-crystal approximation that claimed a highly nonlinear behavior with a significant structural distortion and volume reduction in K0.5Na0.5NbO3 compared to both end members of the solid solution. Furthermore, we find that the electronic band gap varies very little between x=0 and x=0.5, reflecting the small changes in the lattice parameters.}}, author = {{Bidaraguppe Ramesh, Nithin and Schmidt, Falko and Schindlmayr, Arno}}, issn = {{1434-6036}}, journal = {{The European Physical Journal B}}, number = {{8}}, publisher = {{EDP Sciences, Società Italiana di Fisica and Springer}}, title = {{{Lattice parameters and electronic band gap of orthorhombic potassium sodium niobate K0.5Na0.5NbO3 from density-functional theory}}}, doi = {{10.1140/epjb/s10051-021-00179-8}}, volume = {{94}}, year = {{2021}}, } @article{23418, abstract = {{Density-functional theory within a Berry-phase formulation of the dynamical polarization is used to determine the second-order susceptibility χ(2) of lithium niobate (LiNbO3). Defect trapped polarons and bipolarons are found to strongly enhance the nonlinear susceptibility of the material, in particular if localized at NbV–VLi defect pairs. This is essentially a consequence of the polaronic excitation resulting in relaxation-induced gap states. The occupation of these levels leads to strongly enhanced χ(2) coefficients and allows for the spatial and transient modification of the second-harmonic generation of macroscopic samples.}}, author = {{Kozub, Agnieszka L. and Schindlmayr, Arno and Gerstmann, Uwe and Schmidt, Wolf Gero}}, issn = {{2469-9969}}, journal = {{Physical Review B}}, pages = {{174110}}, publisher = {{American Physical Society}}, title = {{{Polaronic enhancement of second-harmonic generation in lithium niobate}}}, doi = {{10.1103/PhysRevB.104.174110}}, volume = {{104}}, year = {{2021}}, } @article{29747, author = {{Jurgen von Bardeleben, Hans and Cantin, Jean-Louis and Gerstmann, Uwe and Schmidt, Wolf Gero and Biktagirov, Timur}}, issn = {{1530-6984}}, journal = {{Nano Letters}}, keywords = {{Mechanical Engineering, Condensed Matter Physics, General Materials Science, General Chemistry, Bioengineering}}, number = {{19}}, pages = {{8119--8125}}, publisher = {{American Chemical Society (ACS)}}, title = {{{Spin Polarization, Electron–Phonon Coupling, and Zero-Phonon Line of the NV Center in 3C-SiC}}}, doi = {{10.1021/acs.nanolett.1c02564}}, volume = {{21}}, year = {{2021}}, } @article{22010, author = {{Aldahhak, Hazem and Hogan, Conor and Lindner, Susi and Appelfeller, Stephan and Eisele, Holger and Schmidt, Wolf Gero and Dähne, Mario and Gerstmann, Uwe and Franz, Martin}}, issn = {{2469-9950}}, journal = {{Physical Review B}}, pages = {{035303}}, title = {{{Electronic structure of the Si(111)3×3R30∘−B surface from theory and photoemission spectroscopy}}}, doi = {{10.1103/physrevb.103.035303}}, volume = {{103}}, year = {{2021}}, } @article{17067, author = {{Speiser, Eugen and Esser, Norbert and Halbig, Benedikt and Geurts, Jean and Schmidt, Wolf Gero and Sanna, Simone}}, issn = {{0167-5729}}, journal = {{Surface Science Reports}}, number = {{1}}, title = {{{Vibrational Raman spectroscopy on adsorbate-induced low-dimensional surface structures}}}, doi = {{10.1016/j.surfrep.2020.100480}}, volume = {{75}}, year = {{2020}}, } @article{19190, abstract = {{Polarons in dielectric crystals play a crucial role for applications in integrated electronics and optoelectronics. In this work, we use density-functional theory and Green's function methods to explore the microscopic structure and spectroscopic signatures of electron polarons in lithium niobate (LiNbO3). Total-energy calculations and the comparison of calculated electron paramagnetic resonance data with available measurements reveal the formation of bound polarons at Nb_Li antisite defects with a quasi-Jahn-Teller distorted, tilted configuration. The defect-formation energies further indicate that (bi)polarons may form not only at Nb_Li antisites but also at structures where the antisite Nb atom moves into a neighboring empty oxygen octahedron. Based on these structure models, and on the calculated charge-transition levels and potential-energy barriers, we propose two mechanisms for the optical and thermal splitting of bipolarons, which provide a natural explanation for the reported two-path recombination of bipolarons. Optical-response calculations based on the Bethe-Salpeter equation, in combination with available experimental data and new measurements of the optical absorption spectrum, further corroborate the geometries proposed here for free and defect-bound (bi)polarons.}}, author = {{Schmidt, Falko and Kozub, Agnieszka L. and Biktagirov, Timur and Eigner, Christof and Silberhorn, Christine and Schindlmayr, Arno and Schmidt, Wolf Gero and Gerstmann, Uwe}}, issn = {{2643-1564}}, journal = {{Physical Review Research}}, number = {{4}}, publisher = {{American Physical Society}}, title = {{{Free and defect-bound (bi)polarons in LiNbO3: Atomic structure and spectroscopic signatures from ab initio calculations}}}, doi = {{10.1103/PhysRevResearch.2.043002}}, volume = {{2}}, year = {{2020}}, } @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{10014, abstract = {{The cubic, tetragonal, and orthorhombic phase of potassium niobate (KNbO3) are studied based on density-functional theory. Starting from the relaxed atomic geometries, we analyze the influence of self-energy corrections on the electronic band structure within the GW approximation. We find that quasiparticle shifts widen the direct (indirect) band gap by 1.21 (1.44), 1.58 (1.55), and 1.67 (1.64) eV for the cubic, tetragonal, and orthorhombic phase, respectively. By solving the Bethe-Salpeter equation, we obtain the linear dielectric function with excitonic and local-field effects, which turn out to be essential for good agreement with experimental data. From our results, we extract an exciton binding energy of 0.6, 0.5, and 0.5 eV for the cubic, tetragonal, and orthorhombic phase, respectively. Furthermore, we investigate the nonlinear second-harmonic generation (SHG) both theoretically and experimentally. The frequency-dependent second-order polarization tensor of orthorhombic KNbO3 is measured for incoming photon energies between 1.2 and 1.6 eV. In addition, calculations within the independent-(quasi)particle approximation are performed for the tetragonal and orthorhombic phase. The novel experimental data are in excellent agreement with the quasiparticle calculations and resolve persistent discrepancies between earlier experimental measurements and ab initio results reported in the literature.}}, author = {{Schmidt, Falko and Riefer, Arthur and Schmidt, Wolf Gero and Schindlmayr, Arno and Imlau, Mirco and Dobener, Florian and Mengel, Nils and Chatterjee, Sangam and Sanna, Simone}}, issn = {{2475-9953}}, journal = {{Physical Review Materials}}, number = {{5}}, publisher = {{American Physical Society}}, title = {{{Quasiparticle and excitonic effects in the optical response of KNbO3}}}, doi = {{10.1103/PhysRevMaterials.3.054401}}, volume = {{3}}, year = {{2019}}, } @article{29746, author = {{Nicholson, C. W. and Puppin, M. and Lücke, A. and Gerstmann, Uwe and Krenz, Marvin and Schmidt, Wolf Gero and Rettig, L. and Ernstorfer, R. and Wolf, M.}}, issn = {{2469-9950}}, journal = {{Physical Review B}}, number = {{15}}, publisher = {{American Physical Society (APS)}}, title = {{{Excited-state band mapping and momentum-resolved ultrafast population dynamics in In/Si(111) nanowires investigated with XUV-based time- and angle-resolved photoemission spectroscopy}}}, doi = {{10.1103/physrevb.99.155107}}, volume = {{99}}, year = {{2019}}, } @article{10015, author = {{Dues, Christof and Schmidt, Wolf Gero and Sanna, Simone}}, issn = {{2470-1343}}, journal = {{ACS Omega}}, pages = {{3850--3859}}, title = {{{Water Splitting Reaction at Polar Lithium Niobate Surfaces}}}, doi = {{10.1021/acsomega.8b03271}}, year = {{2019}}, } @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{10013, abstract = {{Ultrafast nonequilibrium dynamics offer a route to study the microscopic interactions that govern macroscopic behavior. In particular, photoinduced phase transitions (PIPTs) in solids provide a test case for how forces, and the resulting atomic motion along a reaction coordinate, originate from a nonequilibrium population of excited electronic states. Using femtosecond photoemission, we obtain access to the transient electronic structure during an ultrafast PIPT in a model system: indium nanowires on a silicon(111) surface. We uncover a detailed reaction pathway, allowing a direct comparison with the dynamics predicted by ab initio simulations. This further reveals the crucial role played by localized photoholes in shaping the potential energy landscape and enables a combined momentum- and real-space description of PIPTs, including the ultrafast formation of chemical bonds.}}, author = {{Nicholson, C. W. and Lücke, A. and Schmidt, Wolf Gero and Puppin, M. and Rettig, L. and Ernstorfer, R. and Wolf, M.}}, issn = {{0036-8075}}, journal = {{Science}}, pages = {{821--825}}, title = {{{Beyond the molecular movie: Dynamics of bands and bonds during a photoinduced phase transition}}}, doi = {{10.1126/science.aar4183}}, year = {{2018}}, }