TY - JOUR
AB - 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.
AU - Schmidt, Falko
AU - Kozub, Agnieszka
AU - Biktagirov, Timur
AU - Eigner, Christof
AU - Silberhorn, Christine
AU - Schindlmayr, Arno
AU - Schmidt, Wolf Gero
AU - Gerstmann, Uwe
ID - 19190
IS - 4
JF - Physical Review Research
TI - Free and defect-bound (bi)polarons in LiNbO3: Atomic structure and spectroscopic signatures from ab initio calculations
VL - 2
ER -
TY - JOUR
AB - 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.
AU - Schmidt, Falko
AU - Riefer, Arthur
AU - Schmidt, Wolf Gero
AU - Schindlmayr, Arno
AU - Imlau, Mirco
AU - Dobener, Florian
AU - Mengel, Nils
AU - Chatterjee, Sangam
AU - Sanna, Simone
ID - 10014
IS - 5
JF - Physical Review Materials
TI - Quasiparticle and excitonic effects in the optical response of KNbO3
VL - 3
ER -
TY - JOUR
AB - 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.
AU - Neufeld, Sergej
AU - Bocchini, Adriana
AU - Gerstmann, Uwe
AU - Schindlmayr, Arno
AU - Schmidt, Wolf Gero
ID - 13365
IS - 4
JF - Journal of Physics: Materials
TI - Potassium titanyl phosphate (KTP) quasiparticle energies and optical response
VL - 2
ER -
TY - JOUR
AU - Friedrich, Michael
AU - Schmidt, Wolf Gero
AU - Schindlmayr, Arno
AU - Sanna, Simone
ID - 13410
IS - 1
JF - Physical Review Materials
TI - Erratum: Optical properties of titanium-doped lithium niobate from time-dependent density-functional theory [Phys. Rev. Materials 1, 034401 (2017)]
VL - 2
ER -
TY - JOUR
AB - The transverse dynamic spin susceptibility is a correlation function that yields exact information about spin excitations in systems with a collinear magnetic ground state, including collective spin-wave modes. In an ab initio context, it may be calculated within many-body perturbation theory or time-dependent density-functional theory, but the quantitative accuracy is currently limited by the available functionals for exchange and correlation in dynamically evolving systems. To circumvent this limitation, the spin susceptibility is here alternatively formulated as the solution of an initial-value problem. In this way, the challenge of accurately describing exchange and correlation in many-electron systems is shifted to the stationary initial state, which is much better understood. The proposed scheme further requires the choice of an auxiliary basis set, which determines the speed of convergence but always allows systematic convergence in practical implementations.
AU - Schindlmayr, Arno
ID - 18466
JF - Advances in Mathematical Physics
SN - 1687-9120
TI - Exact formulation of the transverse dynamic spin susceptibility as an initial-value problem
VL - 2018
ER -
TY - JOUR
AB - The optical properties of pristine and titanium-doped LiNbO3 are modeled from first principles. The dielectric functions are calculated within time-dependent density-functional theory, and a model long-range contribution is employed for the exchange-correlation kernel in order to account for the electron-hole binding. Our study focuses on the influence of substitutional titanium atoms on lithium sites. We show that an increasing titanium concentration enhances the values of the refractive indices and the reflectivity.
AU - Friedrich, Michael
AU - Schmidt, Wolf Gero
AU - Schindlmayr, Arno
AU - Sanna, Simone
ID - 10021
IS - 3
JF - Physical Review Materials
SN - 2475-9953
TI - Optical properties of titanium-doped lithium niobate from time-dependent density-functional theory
VL - 1
ER -
TY - JOUR
AB - The electronic band structures of hexagonal ZnO and cubic ZnS, ZnSe, and ZnTe compounds are determined within hybrid-density-functional theory and quasiparticle calculations. It is found that the band-edge energies calculated on the G0W0 (Zn chalcogenides) or GW (ZnO) level of theory agree well with experiment, while fully self-consistent QSGW calculations are required for the correct description of the Zn 3d bands. The quasiparticle band structures are used to calculate the linear response and second-harmonic-generation (SHG) spectra of the Zn–VI compounds. Excitonic effects in the optical absorption are accounted for within the Bethe–Salpeter approach. The calculated spectra are discussed in the context of previous experimental data and present SHG measurements for ZnO.
AU - Riefer, Arthur
AU - Weber, Nils
AU - Mund, Johannes
AU - Yakovlev, Dmitri R.
AU - Bayer, Manfred
AU - Schindlmayr, Arno
AU - Meier, Cedrik
AU - Schmidt, Wolf Gero
ID - 7481
IS - 21
JF - Journal of Physics: Condensed Matter
SN - 0953-8984
TI - Zn–VI quasiparticle gaps and optical spectra from many-body calculations
VL - 29
ER -
TY - JOUR
AB - We perform a comprehensive theoretical study of the structural and electronic properties of potassium niobate (KNbO3) in the cubic, tetragonal, orthorhombic, monoclinic, and rhombohedral phase, based on density-functional theory. The influence of different parametrizations of the exchange-correlation functional on the investigated properties is analyzed in detail, and the results are compared to available experimental data. We argue that the PBEsol and AM05 generalized gradient approximations as well as the RTPSS meta-generalized gradient approximation yield consistently accurate structural data for both the external and internal degrees of freedom and are overall superior to the local-density approximation or other conventional generalized gradient approximations for the structural characterization of KNbO3. Band-structure calculations using a HSE-type hybrid functional further indicate significant near degeneracies of band-edge states in all phases which are expected to be relevant for the optical response of the material.
AU - Schmidt, Falko
AU - Landmann, Marc
AU - Rauls, Eva
AU - Argiolas, Nicola
AU - Sanna, Simone
AU - Schmidt, Wolf Gero
AU - Schindlmayr, Arno
ID - 10023
JF - Advances in Materials Science and Engineering
SN - 1687-8434
TI - Consistent atomic geometries and electronic structure of five phases of potassium niobate from density-functional theory
VL - 2017
ER -
TY - JOUR
AB - The optical properties of congruent lithium niobate are analyzed from first principles. The dielectric function of the material is calculated within time-dependent density-functional theory. The effects of isolated intrinsic defects and defect pairs, including the NbLi4+ antisite and the NbLi4+−NbNb4+ pair, commonly addressed as a bound polaron and bipolaron, respectively, are discussed in detail. In addition, we present further possible realizations of polaronic and bipolaronic systems. The absorption feature around 1.64 eV, ascribed to small bound polarons [O. F. Schirmer et al., J. Phys.: Condens. Matter 21, 123201 (2009)], is nicely reproduced within these models. Among the investigated defects, we find that the presence of bipolarons at bound interstitial-vacancy pairs NbV−VLi can best explain the experimentally observed broad absorption band at 2.5 eV. Our results provide a microscopic model for the observed optical spectra and suggest that, besides NbLi antisites and Nb and Li vacancies, Nb interstitials are also formed in congruent lithium-niobate samples.
AU - Friedrich, Michael
AU - Schmidt, Wolf Gero
AU - Schindlmayr, Arno
AU - Sanna, Simone
ID - 13416
IS - 5
JF - Physical Review Materials
TI - Polaron optical absorption in congruent lithium niobate from time-dependent density-functional theory
VL - 1
ER -
TY - JOUR
AB - The influence of electronic many-body interactions, spin-orbit coupling, and thermal lattice vibrations on the electronic structure of lithium niobate is calculated from first principles. Self-energy calculations in the GW approximation show that the inclusion of self-consistency in the Green function G and the screened Coulomb potential W opens the band gap far stronger than found in previous G0W0 calculations but slightly overestimates its actual value due to the neglect of excitonic effects in W. A realistic frozen-lattice band gap of about 5.9 eV is obtained by combining hybrid density functional theory with the QSGW0 scheme. The renormalization of the band gap due to electron-phonon coupling, derived here using molecular dynamics as well as density functional perturbation theory, reduces this value by about 0.5 eV at room temperature. Spin-orbit coupling does not noticeably modify the fundamental gap but gives rise to a Rashba-like spin texture in the conduction band.
AU - Riefer, Arthur
AU - Friedrich, Michael
AU - Sanna, Simone
AU - Gerstmann, Uwe
AU - Schindlmayr, Arno
AU - Schmidt, Wolf Gero
ID - 10024
IS - 7
JF - Physical Review B
SN - 2469-9950
TI - LiNbO3 electronic structure: Many-body interactions, spin-orbit coupling, and thermal effects
VL - 93
ER -