P. Knoll

Optical response of high temperature superconductors by full potential LAPW band structure calculations

Abstract Starting from selfconsistent full potential LAPW results we have investigated optical matrix elements for interband transitions and the e 2 tensor. These calculations are performed for YBa 2 Cu 3 O 7 and YBa 2 Cu 4 O 8 . The spectra are analyzed and differences are discussed with respect to the crystal structures of the two systems. Our results are in good agreement with the ellipsometric determination of the dielectric function and previous LMTO calculations.

Effective conjugation and Raman intensities in oligo(para-phenylene)s: A microscopic view from first-principles calculations

Electron-phonon coupling in oligo(para-phenylene)s is addressed in terms of the off-resonance Raman intensities of two distinct modes at 1220 and 1280 cm(-1). On the basis of Albrechts theory, vibrational coupling and Raman intensities are calculated from first-principles quantum-chemical methods. A few-state model is used to evaluate the dependence of the mode intensities on oligomer length, planarity, and excitation wavelength. The link between electron delocalizationconjugation and Raman intensities is highlighted. Extending on prior studies, the present work focuses on providing an in-depth understanding of the origin of this correlation in addition to reproducing experimental findings. The model applied here allows us to interpret the results on a microscopic, quantum-mechanical basis and to relate the observed trends to the molecular orbital structure and nature of the excited states in this class of materials. We find quantitative agreement between the results of the calculations and those of measurements performed on oligo(para-phenylene)s of various chain lengths in the solid state and in solution.

Ab initio study of vibrational anharmonic coupling effects in oligo(para-phenylenes)

In this work, we clarify the nature of a previously not precisely identified Fermi dyad in the frequency range around 1600 cm−1 in oligo( para-phenylenes). To this end, we deploy a novel method to calculate third order anharmonic coupling effects in molecules. This Fermi dyad is shown to yield important information on the structural properties of the investigated materials. The nature of all vibrations contributing to this quantum mechanical resonance phenomenon is explained on the basis of a detailed normal coordinate analysis. The anharmonic coupling is then closely investigated by applying our theoretical model. In particular, we discuss the intensity redistribution among the two components of the Fermi doublet as well as their energetic separation. Subsequently, we establish a relation between these features and the structural conformation of the molecules. We show, how oligomer length and planarity of these systems can be determined from experimental Raman spectra by extracting the positions and rela...

Self-absorption effects in a LEC with low Stokes shift

Abstract The electroluminescence spectra both in forward and reverse directions of a light-emitting electrochemical cell prepared from a methyl-substituted ladder-type poly(paraphenylene) (mLPPP), blended with a crown ether, Dicyclohexano18crown6 (DCH18C6), and a Li salt, lithium trifluoromethanesulfonate (litriflate, LiTf for short) are compared. Different from the forward direction, in reverse direction the spectrum is strongly influenced by self-absorption effects. We discuss this behavior by different degrees of doping of the p- and n-type doped zones. This effect can be applied for a new method to realize bias-dependent color emission from single-layer devices at low voltages that can be prepared by only one simple and easy spin-coating process.

MAGNETIC RAMAN SCATTERING IN UNDOPED AND DOPED ANTIFERROMAGNETS

Magnetic Raman scattering of doped and undoped high-Tc (HTc) cuprates is presented in comparison with the conventional antiferromagnet NiO. In both classes of materials the strong magnetic Raman response is caused by a Me–O superexchange mechanism. The difference in the absolute Raman cross-sections of 90×10−5 sr−1 cm−1 (YBa2Cu3O6.3) and 1.6×10−5 sr−1 cm−1 (NiO) is explained by the different strengths of the exchange energies. The magnetic scattering signal of NiO is explainable by the pure Heisenberg model, whereas the zone edge magnons of the HTc cuprates show additional damping due to magnon phonon interaction. On doping, the damping of the zone-edge magnons increases by a factor of 4 more than the decrease of magnon energy. This is characteristic of the interaction of the spin system with the charge carriers. The absolute Raman cross-section is reduced owing to the interruption of antiferromagnetic spin correlations on doping.

The influence of spatial disorder of the ion distribution on the surface morphology in thin films of blend based organic mixed ionic-electronic conductors

Abstract We report on investigations of the morphology of spin-coated thin films of an organic mixed ionic-electronic conductor consisting of the conjugated polymer methyl substituted ladder type poly(para- phenylene) (mLPPP) blended with a solid state electrolyte (Dicyclohexano18crown6 (DCH18C6), complexed with lithium trifluoromethanesulfonate (LiCF 3 SO 3 )). This blend system was successfully applied as active layer in light-emitting electrochemical cells (LECs). While thin films blends of the conjugated polymer and the pristine crown ether show a very smooth surface, the addition of LiCF 3 SO 3 causes a pronounced surface roughening. Since such a distinct surface roughness can be the reason for a device failure mechanism that limits the device lifetime, this attitude is investigated by tapping mode atomic force microscopy (AFM). These studies are complemented with X-ray analysis by means of energy dispersive X-ray spectrometry (EDXS), in the scanning electron microscope (SEM), in order to get a better insight into the ion distribution within the blend layer and its influence on the surface roughness formed.

The Raman spectra of methyl substituted ladder type poly(p-phenylene): Theoretical and experimental investigations

We report on the Raman behavior of methyl substituted ladder type poly(p-phenylene) (mLPPP), a conjugated polymer with a high application potential for optoelectronic devices. A funded explanation of the experimentally observed Raman modes based on ab initio calculations is given, which takes also the influence of the side chains into account. Experimental investigations were performed with different laser excitation wavelengths on mLPPP in solution and films drop-casted on silicon substrates. From the studies of the polymer in solution the differential Raman scattering cross-section of mLPPP were determined on the basis of the known values of different solvents.

The Raman spectra of different ladder type poly(p‐phenylenes) and ladder type oligo(p‐phenylenes)

We report on Raman spectra of ladder type oligo(p-phenylenes) of different chain lengths (5LOPP and 7LOPP) and make a comparison to the Raman spectrum of the corresponding polymer, ladder type poly(p-phenylene) (LPPP). For a better understanding of the vibrational behavior of LPPP, several LPPPs with different substituents (methyl, allyl, n-butyl, 1-adamantyl) were investigated. The recorded Raman spectra of these LPPPs show significant changes in the interring CC stretching regions. An experimental analysis of the frequencies, intensities and line-shapes of significant modes is given in dependence on the substituents and chain lengths.

Multiple Fireballs in a Reactive

Inverted fireballs (FBs) are plasma phenomena, which develop inside a highly transparent, positively biased grid electrode through electron impact ionization. The ionizing electrons are accelerated into the grid by the high potential on the wires. This leads to an enhanced plasma density inside the hollow anode as well as a strong light emission. FBs show a wide variety of properties. This behavior may manifest itself in different shapes of FBs, which are very sensitive to the physical parameters of the discharge.

{\rm H}_{2}/{\rm CH}_{4}

We report on atomic force microscopy (AFM) and scanning electron microscopy (SEM) studies in order to investigate the morphological aspects of a blend of a conjugated polymer (methyl substituted laddertype poly(p-phenylene) (mLPPP)) with a solid state electrolyte (a crown ether (Dicycolhexano18crown6 (DCH18C6) complexed with a lithium salt (Lithiumtrifluoromethanesulfonte, LiCF 3 SO 3 , LiTf for short)), ratio 20:20:4 by weight, as it was successfully applied in light-emitting electrochemical cells (LECs). While a blend of the polymer with the pristine crown ether (ratio 20:20 by weight) has a smooth surface morphology, the addition of ions causes a distinct increase of the surface roughness.