Claudia Ambrosch-Draxl

Linear optical properties of solids within the full-potential linearized augmented planewave method

Abstract We present a scheme for the calculation of linear optical properties by the all-electron full-potential linearized augmented planewave (LAPW) method. A summary of the theoretical background for the derivation of the dielectric tensor within the random-phase approximation is provided. The momentum matrix elements are evaluated in detail for the LAPW basis, and the interband as well as the intra-band contributions to the dielectric tensor are given. As an example the formalism is applied to Aluminum. The program is available as a module within the WIEN2k code.

Theoretical study of PTCDA adsorbed on the coinage metal surfaces, Ag(111), Au(111) and Cu(111)

A thorough understanding of the adsorption of molecules on metallic surfaces is a crucial prerequisite for the development and improvement of functionalized materials. A prominent representative within the class of π-conjugated molecules is 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) which, adsorbed on the Ag(111), Au(111) or Cu(111) surfaces, shows characteristic trends for work-function modification, alignment of molecular levels with the substrate Fermi energy and binding distances. We carried out density functional theory (DFT) calculations to investigate to what extent these trends can be rationalized on a theoretical basis. We used different density functionals (DF) including a fully non-local van der Waals (vdW) DF capable of describing dispersion interactions. We show that, rather independent of the DF, the calculations yield level alignments and work-function modifications consistent with ultra-violet photoelectron spectroscopy when the monolayer is placed onto the surfaces at the experimental distances (as determined from x-ray standing wave experiments). The lowest unoccupied molecular orbital is occupied on the Ag and Cu surfaces, whereas it remains unoccupied on the Au surface. Simultaneously, the work function increases for Ag but decreases for Cu and Au. Adsorption distances and energies, on the other hand, depend very sensitively on the choice of the DF. While calculations in the local density approximation bind the monolayer consistently with the experimental trends, the generalized gradient approximation in several flavors fails to reproduce realistic distances and energies. Calculations employing the vdW-DF reveal that substantial bonding contributions arise from dispersive interactions. They yield reasonable binding energies but larger binding distances than the experiments.

Characterization of Step-Edge Barriers in Organic Thin-Film Growth

Detailed understanding of growth mechanisms in organic thin-film deposition is crucial for tailoring growth morphologies, which in turn determine the physical properties of the resulting films. For growth of the rodlike molecule para-sexiphenyl, the evolution of terraced mounds is observed by atomic force microscopy. Using methods established in inorganic epitaxy, we demonstrate the existence of an additional barrier (0.67 electron volt) for step-edge crossing—the Ehrlich-Schwoebel barrier. This result was confirmed by transition state theory, which revealed a bending of the molecule at the step edge. A gradual reduction of this barrier in the first layers led to an almost layer-by-layer growth during early deposition stage. The reported phenomena are a direct consequence of the complexity of the molecular building blocks versus atomic systems.

Force calculation and atomic-structure optimization for the full-potential linearized augmented plane-wave code WIEN

Abstract Following the approach of Yu, Singh, and Krakauer [Phys. Rev. B 43 (1991) 641], we extended the linearized augmented plane wave code WIEN of Blaha, Schwarz, and coworkers by the evaluation of forces. In this paper we describe the approach, demonstrate the high accuracy of the force calculation, and use them for an efficient geometry optimization of poly-atomic systems.

Optical Constants and Inelastic Electron-Scattering Data for 17 Elemental Metals

Two new sets of optical data, i.e., values for the real (e1) and imaginary (e2) parts of the complex dielectric constant as well as the energy loss function (ELF) (Im{−1∕e}), are presented for 16 elemental metals (Ti, V, Fe, Co, Ni, Cu, Zn, Mo, Pd, Ag, Ta, W, Pt, Au, Pb, and Bi) and 1 semimetal (Te) and are compared to available data in the literature. One data set is obtained from density functional theory (DFT) calculations and gives e from the infrared to the soft x-ray range of wavelengths. The other set of optical constants, derived from experimental reflection electron energy-loss spectroscopy (REELS) spectra, provides reliable optical data from the near-ultraviolet to the soft x-ray regime. The two data sets exhibit very good mutual consistency and also, overall, compare well with optical data found in the literature, most of which were determined several decades ago. However, exceptions to this rule are also found in some instances, some of them systematic, where the DFT and REELS mutually agree s...

Linear and second-order optical response of III-V monolayer superlattices

We report the fully self-consistent calculations of the nonlinear optical properties of superlattices. The materials investigated are monolayer superlattices with GaP grown on the the top of InP, AlP, and GaAs (110) substrates. We use the full-potential linearized augmented plane-wave method within the generalized gradient approximation to obtain the frequency-dependent dielectric tensor and the second-harmonic-generation susceptibility. The effect of lattice relaxations on the linear optical properties is studied. Our calculations show that the major anisotropy in the optical properties is the result of strain in GaP. This anisotropy is maximum for the superlattice with the maximum lattice mismatch between the constituent materials. In order to differentiate the superlattice features from the bulklike transitions, an improvement over the existing effective-medium model is proposed. The superlattice features are found to be more pronounced for the second order than the linear optical response, indicating the need for full supercell calculations in determining the correct second-order response.

The role of polymorphism in organic thin films: oligoacenes investigated from first principles

We investigate the cohesive properties of oligoacenes within the framework of density functional theory including van der Waals interactions. In comparison, we evaluate the local density approximation as well as gradient corrections, but also a widely used semiempirical procedure accounting for the long-range dispersive forces, in terms of their performance for the energetics of such weakly bound systems. Besides the cohesive energies we discuss in detail the surface energies which, in turn, allow for obtaining the crystal shapes based on Wulffs construction for the oligomer series from naphthalene to pentacene. In particular, we focus on comparing two different pentacene polymorphs, i.e. the so-called bulk structure and the thin film phase, the latter being predominately found in thin film growth. We not only study the impact of molecular conformation on the details of these polyhedra, but also the influence of the number of considered index planes and the role of the underlying exchange correlation functional. Based on the relaxed crystal structures for the two polymorphic phases, we compute the electronic band structures as well as the optical spectra. To account for excitonic effects in the latter, we solve the Bethe–Salpeter equation for the electron–hole pairs, thereby considering the coupling between resonant and anti-resonant terms.

Electronic structure and ferroelectricity in SrBi 2 Ta 2 O 9

The electronic structure of SrBi2Ta2O9 is investigated from first-principles, within the local density approximation, using the full-potential linearized augmented plane wave (LAPW) method. The results show that, besides the large Ta(5d)-O(2p) hybridization which is a common feature of the ferroelectric perovskites, there is an important hybridization between bismuth and oxygen states. The underlying static potential for the ferroelectric distortion and the primary source for ferroelectricity is investigated by a lattice-dynamics study using the Frozen Phonon approach.

Optical absorbance of oriented thin films

We report that the optical absorbance of oriented thin films of para-hexaphenyl (PHP), produced by vacuum deposition, shows strong anisotropy effects. An explanation for the orientation dependence of optical absorption of PHP is proposed, using first principles dielectric tensor calculations. The lack of the π-π* absorption band in the normal UV-Vis absorption spectrum is consistent with theoretical predictions for an orthorhombic thin PHP film with oligomer chains oriented near normal to the substrate surface.

Second-Harmonic Optical Response from First Principles

We present a full formalism for the calculation of the linear and second-order optical response for semiconductors and insulators. The expressions for the optical susceptibilities are derived within perturbation theory. As a starting point a brief background of the single and many particle Hamiltonians and operators is provided. As an example we report calculations of the linear and nonlinear optical properties of the mono-layer InP/GaP (110) superlattice. The features in the linear optical spectra are identified to be coming from various band combinations. The main features in the second-order optical spectra are analyzed in terms of resonances of peaks in linear optical spectra. With the help of the strain corrected effective-medium-model the interface selectivity of the second-order optical properties is highlighted.