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 -