TY - JOUR
AB - 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.
AU - Schmidt, Falko
AU - Kozub, Agnieszka
AU - Gerstmann, Uwe
AU - Schmidt, Wolf Gero
AU - Schindlmayr, Arno
ID - 21946
IS - 5
JF - Crystals
TI - Electron polarons in lithium niobate: Charge localization, lattice deformation, and optical response
VL - 11
ER -
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 -
TY - JOUR
AB - The phonon dispersions of the ferro‐ and paraelectric phase of LiTaO3 are calculated within density‐functional perturbation theory. The longitudinal optical phonon modes are theoretically derived and compared with available experimental data. Our results confirm the recent phonon assignment proposed by Margueron et al. [J. Appl. Phys. 111, 104105 (2012)] on the basis of spectroscopical studies. A comparison with the phonon band structure of the related material LiNbO3 shows minor differences that can be traced to the atomic‐mass difference between Ta and Nb. The presence of phonons with imaginary frequencies for the paraelectric phase suggests that it does not correspond to a minimum energy structure, and is compatible with an order‐disorder type phase transition.
AU - Friedrich, Michael
AU - Schindlmayr, Arno
AU - Schmidt, Wolf Gero
AU - Sanna, Simone
ID - 10025
IS - 4
JF - Physica Status Solidi B
SN - 0370-1972
TI - LiTaO3 phonon dispersion and ferroelectric transition calculated from first principles
VL - 253
ER -
TY - JOUR
AB - Using ab initio computational methods, we study the structural and electronic properties of strained silicon, which has emerged as a promising technology to improve the performance of silicon-based metal-oxide-semiconductor field-effect transistors. In particular, higher electron mobilities are observed in n-doped samples with monoclinic strain along the [110] direction, and experimental evidence relates this to changes in the effective mass as well as the scattering rates. To assess the relative importance of these two factors, we combine density-functional theory in the local-density approximation with the GW approximation for the electronic self-energy and investigate the effect of uniaxial and biaxial strains along the [110] direction on the structural and electronic properties of Si. Longitudinal and transverse components of the electron effective mass as a function of the strain are derived from fits to the quasiparticle band structure and a diagonalization of the full effective-mass tensor. The changes in the effective masses and the energy splitting of the conduction-band valleys for uniaxial and biaxial strains as well as their impact on the electron mobility are analyzed. The self-energy corrections within GW lead to band gaps in excellent agreement with experimental measurements and slightly larger effective masses than in the local-density approximation.
AU - Bouhassoune, Mohammed
AU - Schindlmayr, Arno
ID - 18470
JF - Advances in Condensed Matter Physics
SN - 1687-8108
TI - Ab initio study of strain effects on the quasiparticle bands and effective masses in silicon
VL - 2015
ER -
TY - JOUR
AB - The vibrational properties of stoichiometric LiNbO3 are analyzed within density-functional perturbation theory in order to obtain the complete phonon dispersion of the material. The phonon density of states of the ferroelectric (paraelectric) phase shows two (one) distinct band gaps separating the high-frequency (~800 cm−1) optical branches from the continuum of acoustic and lower optical phonon states. This result leads to specific heat capacites in close agreement with experimental measurements in the range 0–350 K and a Debye temperature of 574 K. The calculated zero-point renormalization of the electronic Kohn–Sham eigenvalues reveals a strong dependence on the phonon wave vectors, especially near Γ. Integrated over all phonon modes, our results indicate a vibrational correction of the electronic band gap of 0.41 eV at 0 K, which is in excellent agreement with the extrapolated temperature-dependent measurements.
AU - Friedrich, Michael
AU - Riefer, Arthur
AU - Sanna, Simone
AU - Schmidt, Wolf Gero
AU - Schindlmayr, Arno
ID - 10030
IS - 38
JF - Journal of Physics: Condensed Matter
SN - 0953-8984
TI - Phonon dispersion and zero-point renormalization of LiNbO3 from density-functional perturbation theory
VL - 27
ER -
TY - CHAP
AU - Friedrich, Christoph
AU - Schindlmayr, Arno
ED - Blügel, Stefan
ED - Helbig, Nicole
ED - Meden, Volker
ED - Wortmann, Daniel
ID - 18474
SN - 1866-1807
T2 - Computing Solids: Models, ab initio Methods and Supercomputing
TI - Many-body perturbation theory: The GW approximation
VL - 74
ER -
TY - CHAP
AB - Collective spin excitations form a fundamental class of excitations in magnetic materials. As their energy reaches down to only a few meV, they are present at all temperatures and substantially influence the properties of magnetic systems. To study the spin excitations in solids from first principles, we have developed a computational scheme based on many-body perturbation theory within the full-potential linearized augmented plane-wave (FLAPW) method. The main quantity of interest is the dynamical transverse spin susceptibility or magnetic response function, from which magnetic excitations, including single-particle spin-flip Stoner excitations and collective spin-wave modes as well as their lifetimes, can be obtained. In order to describe spin waves we include appropriate vertex corrections in the form of a multiple-scattering T matrix, which describes the coupling of electrons and holes with different spins. The electron–hole interaction incorporates the screening of the many-body system within the random-phase approximation. To reduce the numerical cost in evaluating the four-point T matrix, we exploit a transformation to maximally localized Wannier functions that takes advantage of the short spatial range of electronic correlation in the partially filled d or f orbitals of magnetic materials. The theory and the implementation are discussed in detail. In particular, we show how the magnetic response function can be evaluated for arbitrary k points. This enables the calculation of smooth dispersion curves, allowing one to study fine details in the k dependence of the spin-wave spectra. We also demonstrate how spatial and time-reversal symmetry can be exploited to accelerate substantially the computation of the four-point quantities. As an illustration, we present spin-wave spectra and dispersions for the elementary ferromagnet bcc Fe, B2-type tetragonal FeCo, and CrO2 calculated with our scheme. The results are in good agreement with available experimental data.
AU - Friedrich, Christoph
AU - Şaşıoğlu, Ersoy
AU - Müller, Mathias
AU - Schindlmayr, Arno
AU - Blügel, Stefan
ED - Di Valentin, Cristiana
ED - Botti, Silvana
ED - Cococcioni, Matteo
ID - 18471
SN - 0340-1022
T2 - First Principles Approaches to Spectroscopic Properties of Complex Materials
TI - Spin excitations in solids from many-body perturbation theory
VL - 347
ER -
TY - CHAP
AB - Many-body perturbation theory is a well-established ab initio electronic-structure method based on Green functions. Although computationally more demanding than density functional theory, it has the distinct advantage that the exact expressions for all relevant observables, including the ground-state total energy, in terms of the Green function are known explicitly. The most important application, however, lies in the calculation of excited states, whose energies correspond directly to the poles of the Green function in the complex frequency plane. The accuracy of results obtained within this framework is only limited by the choice of the exchange-correlation self-energy, which must still be approximated in actual implementations. In this respect, the GW approximation has proved highly successful for systems governed by the Coulomb interaction. It yields band structures of solids, including the band gaps of semiconductors, as well as atomic and molecular ionization energies in very good quantitative agreement with experimental photoemission data.
AU - Schindlmayr, Arno
ED - Bach, Volker
ED - Delle Site, Luigi
ID - 18472
SN - 0921-3767
T2 - Many-Electron Approaches in Physics, Chemistry and Mathematics
TI - The GW approximation for the electronic self-energy
VL - 29
ER -
TY - JOUR
AB - We investigate the band dispersion and related electronic properties of picene single crystals within the GW approximation for the electronic self-energy. The width of the upper highest occupied molecular orbital (HOMOu) band along the Γ–Y direction, corresponding to the b crystal axis in real space along which the molecules are stacked, is determined to be 0.60 eV and thus 0.11 eV larger than the value obtained from density-functional theory. As in our recent study of rubrene using the same methodology [S. Yanagisawa, Y. Morikawa, and A. Schindlmayr, Phys. Rev. B 88, 115438 (2013)], this increase in the bandwidth is due to the strong variation of the GW self-energy correction across the Brillouin zone, which in turn reflects the increasing hybridization of the HOMOu states of neighboring picene molecules from Γ to Y. In contrast, the width of the lower HOMO (HOMOl) band along Γ–Y remains almost unchanged, consistent with the fact that the HOMOl(Γ) and HOMOl(Y) states exhibit the same degree of hybridization, so that the nodal structure of the wave functions and the matrix elements of the self-energy correction are very similar.
AU - Yanagisawa, Susumu
AU - Morikawa, Yoshitada
AU - Schindlmayr, Arno
ID - 18473
IS - 5S1
JF - Japanese Journal of Applied Physics
SN - 0021-4922
TI - Theoretical investigation of the band structure of picene single crystals within the GW approximation
VL - 53
ER -
TY - JOUR
AB - The frequency-dependent dielectric function and the second-order polarizability tensor of ferroelectric LiNbO3 are calculated from first principles. The calculations are based on the electronic structure obtained from density-functional theory. The subsequent application of the GW approximation to account for quasiparticle effects and the solution of the Bethe-Salpeter equation for the stoichiometric material yield a dielectric function that slightly overestimates the absorption onset and the oscillator strength in comparison with experimental measurements. Calculations at the level of the independent-particle approximation indicate that these deficiencies are, at least, partially related to the neglect of intrinsic defects typical for the congruent material. The second-order polarizability calculated within the independent-particle approximation predicts strong nonlinear coefficients for photon energies above 1.5 eV. The comparison with measured data suggests that the inclusion of self-energy effects in the nonlinear optical response leads to a better agreement with experiments. The intrinsic defects of congruent samples reduce the optical nonlinearities, in particular, for the 21 and 31 tensor components, further improving the agreement between experiments and theory.
AU - Riefer, Arthur
AU - Sanna, Simone
AU - Schindlmayr, Arno
AU - Schmidt, Wolf Gero
ID - 13525
IS - 19
JF - Physical Review B
SN - 1098-0121
TI - Optical response of stoichiometric and congruent lithium niobate from first-principles calculations
VL - 87
ER -
TY - JOUR
AB - The GW approximation for the electronic self-energy is an important tool for the quantitative prediction of excited states in solids, but its mathematical exploration is hampered by the fact that it must, in general, be evaluated numerically even for very simple systems. In this paper I describe a nontrivial model consisting of two electrons on the surface of a sphere, interacting with the normal long-range Coulomb potential, and show that the GW self-energy, in the absence of self-consistency, can in fact be derived completely analytically in this case. The resulting expression is subsequently used to analyze the convergence of the energy gap between the highest occupied and the lowest unoccupied quasiparticle orbital with respect to the total number of states included in the spectral summations. The asymptotic formula for the truncation error obtained in this way, whose dominant contribution is proportional to the cutoff energy to the power −3/2, may be adapted to extrapolate energy gaps in other systems.
AU - Schindlmayr, Arno
ID - 18479
IS - 7
JF - Physical Review B
SN - 1098-0121
TI - Analytic evaluation of the electronic self-energy in the GW approximation for two electrons on a sphere
VL - 87
ER -