Torsten Methfessel

Formation of an intermolecular charge-transfer compound in UHV codeposited tetramethoxypyrene and tetracyanoquinodimethane

Ultrahigh vacuum UHV-deposited films of the mixed phase of tetramethoxypyrene and tetracyanoquinodimethane TMP1-TCNQ1 on gold have been studied using ultraviolet photoelectron spectroscopy UPS, x-ray diffraction XRD, infrared IR spectroscopy, and scanning tunneling spectroscopy STS. The formation of an intermolecular charge-transfer CT compound is evident from the appearance of new reflexes in XRD d1 = 0.894 nm and d2 = 0.677 nm. A softening of the CN stretching vibration redshift by 7 cm �1 of TCNQ is visible in the IR spectra, being indicative of a CT on the order of 0.3e from TMP to TCNQ in the complex. Characteristic shifts in the electronic level positions occur in UPS and STS that are in reasonable agreement with the prediction of density-functional theory DFT calculations GAUSSIAN03 with hybrid functional B3LYP. STS reveals a highest occupied molecular orbital HOMO-lowest unoccupied molecular orbital LUMO gap of the CT complex of about 1.25 eV being much smaller than the gaps 3.0 eV of the pure moieties. The electron-injection and hole-injection barriers are 0.3 eV and 0.5 eV, respectively. Systematic differences in the positions of the HOMOs determined by UPS and STS are discussed in terms of the different information content of the two methods.

Disorder-induced gap in the normal density of states of the organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br

The local density of states (DOS) of the organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br, measured by scanning tunneling spectroscopy on in situ cleaved surfaces, reveals a logarithmic suppression near the Fermi edge persisting above the critical temperature T(c). The experimentally observed suppression of the DOS is in excellent agreement with a soft Hubbard gap as predicted by the Anderson-Hubbard model for systems with disorder. The electronic disorder also explains the diminished coherence peaks of the quasi-particle DOS below T(c).