Electronic devices based on organic semiconductors are a fast-growing field of technology. A detailed understanding of the interplay between optical and electronic properties of organic molecular thin films as well as the physical structure is highly beneficial for the optimization of such devices. This requires investigations by means of different yet complementary spectroscopic techniques, where the comparability between the different data sets can become confusing. Consequently, the core aspect of this work is the consistent unification of spectroscopic data obtained by a diversity of experimental techniques within the complementary pictures of molecular states and energy levels. The confusion is obvious in the example of epitaxial films of tetraphenyldibenzoperiflanthene (DBP) on graphite(0001) surfaces being discussed here. One puzzling issue is the widening of the transport gap by around 0.6 eV during film growth, whereas the optical gap remains virtually constant. Furthermore, two-photon photoemission spectroscopy (2PPE) yields several features related to the same unoccupied orbital, and it is a nontrivial task to correlate them with the results of other spectroscopies. These issues can be resolved when all spectroscopic data are consistently interpreted in the framework of a theoretical model which was recently introduced by us, taking the initial and final states of the underlying probe processes into account. Applying that model, all peak shifts are explained here consistently by means of polarization and charging energies and further physical effects in context with the microscopic structure of the DBP thin films.

No abstract

We study the effects of hydrostatic pressure (HP) compression on the superconducting transition of severely strained Nb samples, whose grain sizes are reduced to the submicrometer level. Engineered granularity by high-pressure torsion (HPT) treatment changes the strength of coupling between submicrometer-scale grains and introduces lattice strain. We attempt to utilize the initially accumulated shear strain in the starting material for increasing the superconducting transition temperature Tc under HP compression. The HP effects on non-strained Nb have already been investigated in the pressure regime over 100 GPa by Struzhkin et al. [Phys. Rev. Lett. 79, 4262 (1997)], and Tc reportedly exhibited an increase from 9.2 to 9.9 K at approximately 10 GPa. (1) Slightly strained Nb in the HPT treatment exhibits the increase in Tc under HP due to the strengthening of the intergrain coupling, so the pressure scale of the pressure response observed by Struzhkin et al. is reduced to approximately one-seventh at the maximum. (2) Prominently strained Nb in the HPT treatment exhibits the increase in Tc under HP due to a reduction in structural symmetry at the unit-cell level: In a Nb sample subjected to HPT (6 GPa, 10 revolutions), Tc exceeds 9.9 K at approximately 2 GPa. According to our first-principle calculations, the reduction in the structural symmetry affords an increase in the density of states at the Fermi energy, thereby yielding a prominent increase in Tc at low pressures.

No abstract