Photoemission study of organic and inorganic CDW-systems

Abstract
We investigate by means of high resolution angular resolved photoemission spectroscopy the classical one-dimensional organic conductor TTF-TCNQ and the inorganic one-dimensional conductor (TaSe₄)₂I. We measure dispersive features (ID bands) in accordance with band structure calculations. We observe typical bulk specific properties like the band periodicities (i.e. the Fermi wavevector) and the CDW-gap, which are in agreement with other experimental techniques. Peculiar for one-dimensional systems, we do not observe a real Fermi level crossing of a quasi-particle state. Instead the spectral weight near the chemical potential is strongly suppressed, and we observe a pseudogap in the normal state in both systems. For the organic system our data suggest that electronic correlations are important. Our temperature dependent data further reveal that the pseudogap persists even above the mean field temperature, indicating that CDW-fluctuations cannot account for the pseudogap. For the inorganic compound CDW-fluctuations cannot be neglected in the measured temperature range. Nevertheless the observed energy scale of the pseudogap is much larger than expected from these fluctuations.