This task was planned to be tackled after the first year. Nevertheless, being ahead of our schedule we could already start much earlier to work on several milestones.
The Graz group has applied its Raman code to two-dimensional systems, the vanadium ladder compounds [G8, G15] and could predict a pronounced resonant behaviour for several modes in different scattering geometries. There are experimental groups interested to verify these findings by measuring such samples with different incident laser light wavelengths.
The Uppsala group has studied the electronic properties and the optical absorption of lead iodide (PbI2), which has been recognized as a promising detector material with a large technological applicability. PbI2 has been investigated experimentally by means of optical absorption and spectroscopic ellipsometry, and theoretically by the full-potential linear muffin-tin-orbital (LMTO) method [U1]. Its band-gap energy as a function of temperature has also been measured by optical absorption. The temperature dependence has been fitted by two different relations (Milestone M20), where a discussion of these fittings has been given [U1].
Additionally, the optical properties of Si1-xGex have been investigated theoretically [U2] using the full-potential LMTO method. The calculated dielectric function is in good agreement with the spectroscopic ellipsometry measurements by J. Bahng et al. (J. Phys.: Condens. Matter 13, 777 (2001)), and a static dielectric constant of e0=12.19+2.45x in the Si rich regime (x≤0.5) was obtained [U2].
Also the Aarhus group is interested in semiconductors and semiconductor superlattices [A2-A5], where the most important parameter is the fundamental band gap and its nature (direct or indirect). This specifies the optical absorption threshold, and in the search for new materials for specific optoelectronic applications it is the most important design parameter. The gap can be modified in several ways, for example by alloying of different compounds, doping, or by producing new, artificial structures like superlattices, quantum wells and quantum dots. III-V nitrides and various alloys have been investigated. Several studies of defects in GaN have been performed in collaboration with experimentalists. At the present, the EX!TiNG@WIEN code is used to calculate optical spectra including excitonic effects for strained semiconductors. Milestone 9.)
The groups in Uppsala and Kaiserslautern are interested in the electronic and magnetic properties of nanostructures and transition metals (Ni, Co, Fe) and their oxides. First-principles calculations, based on the full-potential linearized augmented plane wave (FLAPW) method were carried out to obtain the nonlinear optical (NLO) response of the ferromagnetic multilayer [Ni/Cu]2 (with one monolayer of each material). Spin-orbit coupling (SOC) was included in a second variational treatment within a self-consistent procedure. The results for the NLO response of the multilayer was compared with that of the single fcc Ni/Cu(001) bilayer demonstrating pronounced differences in the NLO responses and confirming the sensitivity of nonlinear optics to surface and interface magnetism, the important role of magnetic properties in the NLO response and the enhanced potential of nonlinear spectroscopy in particular for in-plane magnetized thin magnetic films in the monolayer range [EX-6,K1]. The nonlinear magneto-optical response and the Kerr effect have been obtained in reflection geometry for both, longitudinal and polar magneto-optical configurations. The calculation have, for the first time, been converged to an accuracy where a correspondence between the nonlinear magneto-optical Kerr rotation and the different intensities in reflection from a surface could be established. This result was obtained for a Ni/Cu(001) bilayer [T. Andersen, R. Ahuja, and W. Hübner, Verhandlungen, DGP Frühjahrstagung, Vortrag 236402 (2004)].
The interface sensitivity of the NLO response was also shown by the Graz node for several semiconductor superlattices [G1]. Moreover, it was demonstrated that nonlinear optical spectroscopy can contribute to distinguish between different structural phases [G4]. A current work shared by three different nodes (UKL, UU, and KFUG) is a thorough investigation of NLO properties of surfaces. Their sensitivity to relaxation effects, the number of substrate layers, and the convergence with respect to various parameters, etc. is being studied in very detail. The surface magnetic anisotropy energy of thin magnetic films on semiconductor substrates are subject of a common investigation between Uppsala and Graz.
The group of Modena also investigated the impact of interchain interactions on the optical and transport properties of some semiconducting polymers that are relevant for device applications, i.e. poly-(para-phenylene-vinylene) [M3, M14] and polythiophene [M6, M9]. The real-space analysis of the excitonic states, their energy spectra, and the first-principles calculation of the interchain transfer integrals allowed to confirm that the solid-state packing can become a tunable parameter for the ab-initio design of efficient opto-electronic devices.
One controlled way to study these inter-chain interactions is the application of hydrostatic pressure. By combined theoretical and experimental investigations of the structural properties of anthracene under pressure [G2] the validity of utilizing LDA and GGA for describing exchange and correlation effects was proven, although these materials are commonly known as van-der-Waals-crystals. The optical properties as a function of pressure within the BSE formalism were studied recently [G7, G10]. It turned out that depending on the polarization direction both, strongly bound excitons as well as free electron-hole pairs can be found. Moreover, the dependence of the binding energy and the singlet-triplet splitting on the molecular size has been investigated [G16]. All these BSE investigations contribute to the solution of the long-standing problem, whether strongly bound excitons or free charge carriers are dominating the low energy excitations in organic semiconductors.
A collaboration between Louvain and Modena concerning GW for organic semiconductors has started recently.
The electronic properties of superconducting cuprates were calculated in the Modena group in collaboration with the Graz node [EX-5, EX-9]. As a next step, correlation effects have been taken into account applying a procedure as described in Milestone 12 [EX-11] as a joint work of Modena and Berlin. Matrix element effects in the photoemission process are a current common topic between Graz and Modena.
Furthermore, the Berlin group developed a density-functional-type method allowing for an ab-initio calculation of material-specific properties in the superconducting phase [B15]. The critical temperatures of elemental superconductors and the gap functions of MgB2 were successfully calculated. In a collaboration of the Berlin team with an experimental group in Würzburg, resonant inelastic soft X-ray scattering data of BeTe and BeSe were analysed on the basis of the Kramers-Heisenberg formalism. From the emission spectra above the second threshold, direct evidence for the presence of core excitons in these systems was found [B7].
In Kaiserslautern, the GW method has been implemented in a real-space code for finite systems. The approach has been used to calculate optical properties of transition-metal and of sodium clusters (up to Na25) and of Pt3 - the latter with 72 active electrons.
The Berlin team, in collaboration with Angel Rubio's group in San Sebastian (Nanophase network), has implemented the computation of non-linear optical properties in the TDDFT-code OCTOPUS. The generation of high harmonics in femto-second laser pulses and the dissociation dynamics of small molecules such as LiCN, He3, Na2 was studied with this code [B11].
A formalism for the calculation of near-field spectra has been developed, that allows in comparison with experiment the detection of excitonic states that are dipole-forbidden in far-field spectroscopy [EX-10].
Current interest on quantum dots (QDs) [EX-1, EX-3, EX-8] stems from their technological potentialities as optoelectronic devices as well as from proposals of QD-based quantum-information processing schemes. The control on the carrier localization, characteristic of nanometer-size devices, results in a wide tuneability of both the Coulomb and spin interactions, and in their possible enhancement compared to systems of higher dimensionality. Studies on neutral and charged excitonic states in quantum dots allowed proposal of the interband excitations of the dot as computational degrees of freedom (qubits) and their all-optical manipulation as a suitable strategy to implement the required set of quantum gates. As a further step, a novel implementation strategy, which aims at merging the most favourable features of the exciton-based schemes with the robustness of the electron-spin states was recently proposed. A density functional formalism to calculate the current-voltage characteristics of a single molecule trapped between metallic leads has been developed by the Berlin group. On the basis of this method, the possibility of using Chrysazin as a molecular optical switch has been explored [B14].