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 - 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 - 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 - 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 - 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 yield a dielectric function for the stoichiometric material 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 self-energy effects improve the agreement between experiment and theory. The intrinsic defects of congruent samples reduce the optical nonlinearities, in particular for the 21 and 31 tensor components, further improving the agreement with measured data. AU - Riefer, Arthur AU - Rohrmüller, Martin AU - Landmann, Marc AU - Sanna, Simone AU - Rauls, Eva AU - Vollmers, Nora Jenny AU - Hölscher, Rebecca AU - Witte, Matthias AU - Li, Yanlu AU - Gerstmann, Uwe AU - Schindlmayr, Arno AU - Schmidt, Wolf Gero ED - Nagel, Wolfgang E. ED - Kröner, Dietmar H. ED - Resch, Michael M. ID - 18475 SN - 978-3-319-02164-5 T2 - High Performance Computing in Science and Engineering ‘13 TI - Lithium niobate dielectric function and second-order polarizability tensor from massively parallel ab initio calculations ER - TY - JOUR AB - We investigate the band dispersion and relevant electronic properties of rubrene single crystals within the GW approximation. Due to the self-energy correction, the dispersion of the highest occupied molecular orbital (HOMO) band increases by 0.10 eV compared to the dispersion of the Kohn-Sham eigenvalues within the generalized gradient approximation, and the effective hole mass consequently decreases. The resulting value of 0.90 times the electron rest mass along the Γ-Y direction in the Brillouin zone is closer to experimental measurements than that obtained from density-functional theory. The enhanced bandwidth is explained in terms of the intermolecular hybridization of the HOMO(Y) wave function along the stacking direction of the molecules. Overall, our results support the bandlike interpretation of charge-carrier transport in rubrene. AU - Yanagisawa, Susumu AU - Morikawa, Yoshitada AU - Schindlmayr, Arno ID - 18476 IS - 11 JF - Physical Review B SN - 1098-0121 TI - HOMO band dispersion of crystalline rubrene: Effects of self-energy corrections within the GW approximation VL - 88 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 - TY - JOUR AB - We present recent advances in numerical implementations of hybrid functionals and the GW approximation within the full-potential linearized augmented-plane-wave (FLAPW) method. The former is an approximation for the exchange–correlation contribution to the total energy functional in density-functional theory, and the latter is an approximation for the electronic self-energy in the framework of many-body perturbation theory. All implementations employ the mixed product basis, which has evolved into a versatile basis for the products of wave functions, describing the incoming and outgoing states of an electron that is scattered by interacting with another electron. It can thus be used for representing the nonlocal potential in hybrid functionals as well as the screened interaction and related quantities in GW calculations. In particular, the six-dimensional space integrals of the Hamiltonian exchange matrix elements (and exchange self-energy) decompose into sums over vector–matrix–vector products, which can be evaluated easily. The correlation part of the GW self-energy, which contains a time or frequency dependence, is calculated on the imaginary frequency axis with a subsequent analytic continuation to the real axis or, alternatively, by a direct frequency convolution of the Green function G and the dynamically screened Coulomb interaction W along a contour integration path that avoids the poles of the Green function. Hybrid-functional and GW calculations are notoriously computationally expensive. We present a number of tricks that reduce the computational cost considerably, including the use of spatial and time-reversal symmetries, modifications of the mixed product basis with the aim to optimize it for the correlation self-energy and another modification that makes the Coulomb matrix sparse, analytic expansions of the interaction potentials around the point of divergence at k=0, and a nested density and density-matrix convergence scheme for hybrid-functional calculations. We show CPU timings for prototype semiconductors and illustrative results for GdN and ZnO. AU - Friedrich, Christoph AU - Betzinger, Markus AU - Schlipf, Martin AU - Blügel, Stefan AU - Schindlmayr, Arno ID - 18542 IS - 29 JF - Journal of Physics: Condensed Matter SN - 0953-8984 TI - Hybrid functionals and GW approximation in the FLAPW method VL - 24 ER - TY - JOUR AB - We present a nonequilibrium ab initio method for calculating nonlinear and nonlocal optical effects in metallic slabs with a thickness of several nanometers. The numerical analysis is based on the full solution of the time‐dependent Kohn–Sham equations for a jellium system and allows to study the optical response of metal electrons subject to arbitrarily shaped intense light pulses. We find a strong localization of the generated second‐harmonic current in the surface regions of the slabs. AU - Wand, Mathias AU - Schindlmayr, Arno AU - Meier, Torsten AU - Förstner, Jens ID - 4091 IS - 4 JF - Physica Status Solidi B KW - tet_topic_shg SN - 0370-1972 TI - Simulation of the ultrafast nonlinear optical response of metal slabs VL - 248 ER - TY - CONF AB - We present an ab-initio method for calculating nonlinear and nonlocal optical effects in metallic slabs with sub-wavelength thickness. We find a strong localization of the second-harmonic current at the metal-vacuum interface. AU - Wand, Mathias AU - Schindlmayr, Arno AU - Meier, Torsten AU - Förstner, Jens ID - 4048 KW - tet_topic_shg SN - 2160-8989 T2 - CLEO:2011 - Laser Applications to Photonic Applications TI - Theoretical approach to the ultrafast nonlinear optical response of metal slabs ER - TY - CHAP AB - We describe the software package SPEX, which allows first-principles calculations of quasiparticle and collective electronic excitations in solids using techniques from many-body perturbation theory. The implementation is based on the full-potential linearized augmented-plane-wave (FLAPW) method, which treats core and valence electrons on an equal footing and can be applied to a wide range of materials, including transition metals and rare earths. After a discussion of essential features that contribute to the high numerical efficiency of the code, we present illustrative results for quasiparticle band structures calculated within the GW approximation for the electronic self-energy, electron-energy-loss spectra with inter- and intraband transitions as well as local-field effects, and spin-wave spectra of itinerant ferromagnets. In all cases the inclusion of many-body correlation terms leads to very good quantitative agreement with experimental spectroscopies. AU - Schindlmayr, Arno AU - Friedrich, Christoph AU - Şaşıoğlu, Ersoy AU - Blügel, Stefan ED - Dolg, Franz Michael ID - 18549 SN - 978-3-486-59827-8 T2 - Modern and Universal First-Principles Methods for Many-Electron Systems in Chemistry and Physics TI - First-principles calculation of electronic excitations in solids with SPEX VL - 3 ER - TY - JOUR AB - The structural and electronic properties of strained silicon are investigated quantitatively with ab initio computational methods. For this purpose we combine densityfunctional theory within the local‐density approximation and the GW approximation for the electronic self‐energy. From the variation of the total energy as a function of applied strain we obtain the elastic constants, Poisson ratios and related structural parameters, taking a possible internal relaxation fully into account. For biaxial tensile strain in the (001) and (111) planes we then investigate the effects on the electronic band structure. These strain configurations occur in epitaxial silicon films grown on SiGe templates along different crystallographic directions. The tetragonal deformation resulting from (001) strain induces a valley splitting that removes the sixfold degeneracy of the conduction‐band minimum. Furthermore, strain in any direction causes the band structure to warp. We present quantitative results for the electron effective mass, derived from the curvature of the conduction band, as a function of strain and discuss the implications for the mobility of the charge carriers. The inclusion of proper self‐energy corrections within the GW approximation in our work not only yields band gaps in much better agreement with experimental measurements than the localdensity approximation, but also predicts slightly larger electron effective masses. AU - Bouhassoune, Mohammed AU - Schindlmayr, Arno ID - 18562 IS - 2 JF - Physica Status Solidi C SN - 1862-6351 TI - Electronic structure and effective masses in strained silicon VL - 7 ER - TY - JOUR AB - Given the vast range of lithium niobate (LiNbO3) applications, the knowledge about its electronic and optical properties is surprisingly limited. The direct band gap of 3.7 eV for the ferroelectric phase – frequently cited in the literature – is concluded from optical experiments. Recent theoretical investigations show that the electronic band‐structure and optical properties are very sensitive to quasiparticle and electron‐hole attraction effects, which were included using the GW approximation for the electron self‐energy and the Bethe‐Salpeter equation respectively, both based on a model screening function. The calculated fundamental gap was found to be at least 1 eV larger than the experimental value. To resolve this discrepancy we performed first‐principles GW calculations for lithium niobate using the full‐potential linearized augmented plane‐wave (FLAPW) method. Thereby we use the parameter‐free random phase approximation for a realistic description of the nonlocal and energydependent screening. This leads to a band gap of about 4.7 (4.2) eV for ferro(para)‐electric lithium niobate. AU - Thierfelder, Christian AU - Sanna, Simone AU - Schindlmayr, Arno AU - Schmidt, Wolf Gero ID - 13573 IS - 2 JF - Physica Status Solidi C SN - 1862-6351 TI - Do we know the band gap of lithium niobate? VL - 7 ER - TY - JOUR AB - We present a computational scheme to study spin excitations in magnetic materials from first principles. The central quantity is the transverse spin susceptibility, from which the complete excitation spectrum, including single-particle spin-flip Stoner excitations and collective spin-wave modes, can be obtained. The susceptibility is derived from many-body perturbation theory and includes dynamic correlation through a summation over ladder diagrams that describe the coupling of electrons and holes with opposite spins. In contrast to earlier studies, we do not use a model potential with adjustable parameters for the electron-hole interaction but employ the random-phase approximation. To reduce the numerical cost for the calculation of the four-point scattering matrix we perform a projection onto maximally localized Wannier functions, which allows us to truncate the matrix efficiently by exploiting the short spatial range of electronic correlation in the partially filled d or f orbitals. Our implementation is based on the full-potential linearized augmented-plane-wave method. Starting from a ground-state calculation within the local-spin-density approximation (LSDA), we first analyze the matrix elements of the screened Coulomb potential in the Wannier basis for the 3d transition-metal series. In particular, we discuss the differences between a constrained nonmagnetic and a proper spin-polarized treatment for the ferromagnets Fe, Co, and Ni. The spectrum of single-particle and collective spin excitations in fcc Ni is then studied in detail. The calculated spin-wave dispersion is in good overall agreement with experimental data and contains both an acoustic and an optical branch for intermediate wave vectors along the [100] direction. In addition, we find evidence for a similar double-peak structure in the spectral function along the [111] direction. To investigate the influence of static correlation we finally consider LSDA+U as an alternative starting point and show that, together with an improved description of the Fermi surface, it yields a more accurate quantitative value for the spin-wave stiffness constant, which is overestimated in the LSDA. AU - Şaşıoğlu, Ersoy AU - Schindlmayr, Arno AU - Friedrich, Christoph AU - Freimuth, Frank AU - Blügel, Stefan ID - 18560 IS - 5 JF - Physical Review B SN - 1098-0121 TI - Wannier-function approach to spin excitations in solids VL - 81 ER - TY - JOUR AB - We describe the software package SPEX, which allows first-principles calculations of quasiparticle and collective electronic excitations in solids using techniques from many-body perturbation theory. The implementation is based on the full-potential linearized augmented-plane-wave (FLAPW) method, which treats core and valence electrons on an equal footing and can be applied to a wide range of materials, including transition metals and rare earths. After a discussion of essential features that contribute to the high numerical efficiency of the code, we present illustrative results for quasiparticle band structures calculated within the GW approximation for the electronic self-energy, electron-energy-loss spectra with inter- and intraband transitions as well as local-field effects, and spin-wave spectra of itinerant ferromagnets. In all cases the inclusion of many-body correlation terms leads to very good quantitative agreement with experimental spectroscopies. AU - Schindlmayr, Arno AU - Friedrich, Christoph AU - Şaşıoğlu, Ersoy AU - Blügel, Stefan ID - 18557 IS - 3-4 JF - Zeitschrift für Physikalische Chemie SN - 0942-9352 TI - First-principles calculation of electronic excitations in solids with SPEX VL - 224 ER - TY - JOUR AB - We present an implementation of the GW approximation for the electronic self-energy within the full-potential linearized augmented-plane-wave (FLAPW) method. The algorithm uses an all-electron mixed product basis for the representation of response matrices and related quantities. This basis is derived from the FLAPW basis and is exact for wave-function products. The correlation part of the self-energy is calculated on the imaginary-frequency axis with a subsequent analytic continuation to the real axis. As an alternative we can perform the frequency convolution of the Green function G and the dynamically screened Coulomb interaction W explicitly by a contour integration. The singularity of the bare and screened interaction potentials gives rise to a numerically important self-energy contribution, which we treat analytically to achieve good convergence with respect to the k-point sampling. As numerical realizations of the GW approximation typically suffer from the high computational expense required for the evaluation of the nonlocal and frequency-dependent self-energy, we demonstrate how the algorithm can be made very efficient by exploiting spatial and time-reversal symmetry as well as by applying an optimization of the mixed product basis that retains only the numerically important contributions of the electron-electron interaction. This optimization step reduces the basis size without compromising the accuracy and accelerates the code considerably. Furthermore, we demonstrate that one can employ an extrapolar approximation for high-lying states to reduce the number of empty states that must be taken into account explicitly in the construction of the polarization function and the self-energy. We show convergence tests, CPU timings, and results for prototype semiconductors and insulators as well as ferromagnetic nickel. AU - Friedrich, Christoph AU - Blügel, Stefan AU - Schindlmayr, Arno ID - 18558 IS - 12 JF - Physical Review B SN - 1098-0121 TI - Efficient implementation of the GW approximation within the all-electron FLAPW method VL - 81 ER - TY - JOUR AB - We present measurements of the effective electron mass in biaxial tensile strained silicon on insulator (SSOI) material with 1.2 GPa stress and in unstrained SOI. Hall-bar metal oxide semiconductor field effect transistors on 60 nm SSOI and SOI were fabricated and Shubnikov–de Haas oscillations in the temperature range of T=0.4–4 K for magnetic fields of B=0–10 T were measured. The effective electron mass in SSOI and SOI samples was determined as mt=(0.20±0.01)m0. This result is in excellent agreement with first-principles calculations of the effective electron mass in the presence of strain. AU - Feste, Sebastian F. AU - Schäpers, Thomas AU - Buca, Dan AU - Zhao, Qing Tai AU - Knoch, Joachim AU - Bouhassoune, Mohammed AU - Schindlmayr, Arno AU - Mantl, Siegfried ID - 18632 IS - 18 JF - Applied Physics Letters SN - 0003-6951 TI - Measurement of effective electron mass in biaxial tensile strained silicon on insulator VL - 95 ER -