The observation of nuclear quadrupole interactions in amorphous solids provides a unique possibility of obtaining information about the angular distribution of local ionic coordinations, complementary to the information about radial distributions deduced from x-ray and neutron diA'raction and from extended x-ray absorption fine structure measurements. In the present paper the relation between ionic coordinations and the distribution of electric field gradients {EFG)is investigated. It is shown that the distribution function P(V",g) of the splitting parameters V" (the electric field gradient) and y (the asymmetry parameter) in general yields zero probability both for V"=0 and for y = 0. For solids which are isotropic on the average, the distribution function of the components V, . I, of the EFG tensor depends only on two variables, the invariant functions of the tensor components [Det(V, ,) and XV, . '"].Expressions for these quantities in terms of the radial coordinates of the ions causing the EFG and of the bond angles between pairs of ions are given. For amorphous solids with random ionic coordination an analytic approximation for the distribution function P(V",y) is derived. This function is strongly dominated by the distribution of ions in the first coordination shell. The results are applied to the analysis of Mossbauer spectra of '"Gd in amorphous Gd-Ni alloys.

We have studied the molecular orientation of the commonly used organic semiconductor copper phthalocyanine (CuPC) grown as thin films on the technically relevant substrates indium tin oxide, oxidized Si, and polycrystalline gold using polarization-dependent x-ray absorption spectroscopy, and compare the results with those obtained from single crystalline substrates [Au(110) and GeS(001)]. Surprisingly, the 20–50 nm thick CuPC films on the technical substrates are as highly ordered as on the single crystals. Importantly, however, the molecular orientation in the two cases is radically different: the CuPC molecules stand on the technical substrates and lie on the single crystalline substrates. The reasons for this and its consequences for our understanding of the behavior of CuPC films in devices are discussed.

Interface between poly (9,9-dioctylfluorene) and alkali metals: cesium, potassium, sodium, and lithium J.Role of metal-molecule chemistry and interdiffusion on the electrical properties of an organic interface: The Al-F 16 CuPc case Chemical and electrical properties of interfaces between magnesium and aluminum and tris-(8-hydroxy quinoline) aluminum Organic semiconductor interfaces: Discrimination between charging and band bending related shifts in frontier orbital line-up measurements with photoemission spectroscopy

We compare the electronic structure of differently fluorinated copper phthalocyanines (CuPC, CuPCF4, and CuPCF16) using x-ray photoemission spectroscopy and valence-band ultraviolet photoemission spectroscopy. Whereas the ionization potential (IP) is increased by more than 1 eV as a function of the degree of fluorination, further electronic properties such as the optical gap or the composition of the highest occupied molecular orbital and lowest unoccupied molecular orbital remain nearly unchanged. This fact renders these compounds an ideal tool for the investigation of the influence of the IP on the interface properties. At the interface to gold, besides interface dipoles we observe both downward and upward band bending. These phenomena depend clearly on the IP of the phthalocyanines.

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We present a systematic study of the energy level alignment at the interfaces between gold and organic semiconductors. It is shown that there are at least two leading contributions to the potential drop (dipole) across the metal/organic interface: A modification of the metal work function due to the adsorption of the organic molecules and a potential change in the organic semiconductor.

From a combination of high resolution angle-resolved photoemission spectroscopy and density functional calculations, we show that BaFe2As2 possesses essentially two-dimensional electronic states, with a strong change of orbital character of two of the Γ-centered Fermi surfaces as a function of kz. Upon Co doping, the electronic states in the vicinity of the Fermi level take on increasingly three-dimensional character. Both the orbital variation with kz and the more three-dimensional nature of the doped compounds have important consequences for the nesting conditions and thus possibly also for the appearance of antiferromagnetic and superconducting phases. 74.25.Jb, Since the discovery of high T c superconductivity in Fepnictides [1], many experiments have been carried out to reveal the physical and electronic properties of these materials [2,3,4,5]. The parent compounds of Fepnictide superconductors are antiferromagnetic (AFM) metals. Both electron and hole doping suppresses the AFM order and leads to a superconducting phase. The AFM ordering is supposed to occur by nesting of hole pockets at the center of the Brillouin zone (BZ) and electron pockets at the zone corner. Nesting may be also important for the pairing mechanism in these compounds [6] although there are alternative scenarios based on the high polarizability of the As ions [7]. The nesting scenario could explain why in the SmFeAsO-based superconductors [8], predicted to have an almost twodimensional electronic structure [9, 10], higher superconducting transition temperatures T c are observed than in BaFe 2 As 2 -based systems [2] which are predicted to have a more three-dimensional electronic structure [11]. In general, reduction of the dimensionality increases the number of states that could be considered to be well nested. Furthermore, we point out that the orbital character of the states at the Fermi level E F is very important for the nesting conditions as the interband transitions which determine the electronic susceptibility, as calculated by the Lindhard function, are (in weak coupling scenarios) by far strongest when the two Fermi surfaces have the same orbital character [12]. The admixture of threedimensionality, arising from interlayer coupling, makes the materials potentially more useful in devices and other applications. Thus the dimensionality of the electronic structure, i.e., the k z dispersion of the electronic states is of great importance for the understanding and application of these new superconductors.Although angle-resolved photoemission spectroscopy (ARPES) is an ideal tool to study the dispersion of bands parallel and perpendicular to the FeAs layers there exist only a few experimental studies of these issues [13,14,15]. In this letter, we report a systematic study of the dimensionality of the electronic structure of BaFe 2−x Co x As 2 (x= 0 to 0.4) using polarization dependent ARPES, uncovering two new factors which are of great signi cance for the nesting of the Fermi surfaces of these systems. Firstly we show that the Co d...