The diverse steady-state spectroscopic properties of poly(di-n-octylfluorene) are addressed from a molecular-level perspective. Modeling of representative oligomers support the experimental observation of at least three distinguishable classes of conformational isomers with differing chain torsion angles. One class appears to be populated by a single compact structural isomer, and this conforms to the so-called beta phase. A rigorous Franck-Condon analysis of the photoluminescence in conjunction with Frenkel-type exciton band structure calculations is performed. These results accurately reproduce all major spectral features of the photoabsorption and those of singlet exciton emission.

X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy have been used to study the well-known order-disorder transition (ODT) to the β phase in poly(9,9-(di-n,noctyl)fluorene)) (PF8) thin film samples through combination of time-dependent and temperaturedependent measurements. The ODT is well described by a simple Avrami picture of one-dimensional nucleation and growth but crystallization, on cooling, proceeds only after molecular-level conformational relaxation to the so called β phase. Rapid thermal quenching is employed for PF8 studies of pure α phase samples while extended low-temperature annealing is used for improved β phase formation. Low temperature PL studies reveal sharp Franck-Condon type emission bands and, in the β phase, two distinguishable vibronic sub-bands with energies of approximately 199 and 158 meV at 25 K. This improved molecular level structural order leads to a more complete analysis of the higher-order vibronic bands. A net Huang-Rhys coupling parameter of just under 0.7 is typically observed but the relative contributions by the two distinguishable vibronic sub-bands exhibit an anomalous temperature dependence. The PL studies also identify strongly correlated behavior between the relative β phase 0-0 PL peak position and peak width. This relationship is modeled under the assumption that emission represents excitons in thermodynamic equilibrium from states at the bottom of a quasi-one-dimensional exciton band. The crystalline phase, as observed in annealed thin-film samples, has scattering peaks which are incompatible with a simple hexagonal packing of the PF8 chains.

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The electron-phonon (e-p) interaction in Pentacene (Pn) films grown on Bi (001) was investigated using photoemission spectroscopy. The spectra reveal thermal broadening from which we determine an e-p mass enhancement factor of λ = 0.36 ± 0.05 and an effective Einstein energy of ω E = 11 ± 4 meV. From ω E it is inferred that dominant contributions to the e-p effects observed in ARPES comes from intermolecular vibrations. Based on the experimental data for λ we extract an effective Peierls coupling value of g eff = 0.55. The e-p coupling narrows the HOMO bandwidth by 15 ± 8% between 75 K and 300 K. 79.60.Fr, 71.20.Rv, 72.80.Le, 73.50.Gr Published in Phys. Rev. Lett. 104, 047601 (2010) For decades theorists have been working to explain the surprisingly high carrier mobilities and temperature dependence of organic semiconductors (OSCs). Pioneering work toward understanding the carrier transport mechanism in OSCs was done by

A quantum mean field theory of the pyrochlore lattice is presented. The starting point is not the individual magnetic ions, as in the usual Curie-Weiss mean field theory, but a set of interacting corner-sharing tetrahedra. We check the consistency of the model against magnetic susceptibility data and find good agreement between the theoretical predictions and the experimental data. Implications of the model and future extensions are also discussed.

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Electron spin resonance ͑ESR͒ experiments at different fields or frequencies ͑4.1Յ Յ 34.4 GHz͒ in the Kondo lattice ͑T K Ӎ 25 K͒ YbRh 2 Si 2 single-crystal compounds confirmed the observation of a single anisotropic Dysonian resonance with g Ќc Х 3.55 and no hyperfine components for 4.2Շ T Շ 20 K. However, our studies differently reveal that ͑i͒ the ESR spectra for H Ќc show strong-field-dependent spin-lattice relaxation, ͑ii͒ a weak-field and temperature-dependent effective g value, ͑iii͒ a dramatic suppression of the ESR intensity beyond 15% of Lu doping, and ͑iv͒ a strong sample and Lu-doping ͑Յ15%͒ dependence of the ESR data. These results suggest a different scenario where the ESR signal may be associated to a coupled Yb 3+ -conduction electron resonant collective mode with a strong bottleneck and dynamiclike behavior.

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