CDW COMPOUNDS: A COMPARISON OF X-RAY DIFFRACTION AND SCANNING TUNNELING MICROSCOPY
Sander van Smaalen1, Jens Luedecke1, Jan L. de Boer2, A. Spijkerman2 and G.A. Wiegers2
1 Laboratory of Crystallography,
University of Bayreuth, D-95440 Bayreuth, Germany.
2 Chemical Physics, University of
Groningen, Nijenborgh 4, NL-9747 AG Groningen, The Netherlands
Keywords: aperiodic crystals, Charge-density-wave, incommensurately modulated structures
Charge-density wave (CDW) compounds are metals, in which the conduction bands have low-dimensional character. The energy of the conduction electrons can be lowered by a modulation with a period determined by the Fermi wavevector: q = 2kF. The result is an opening of a gap in the conduction band at the Fermi level, and it gives rise to a metal-insulator transition at TCDW between a normal high-temperature state and the CDW state. Because kF is not necessarily related to the lattice of the structure, the modulation is often incommensurate. Detailed knowledge of the incommensurately modulated structures provides essential information for an understanding of the properties of these compounds.
CDW compounds compounds exhibit a series of phenoma that are specific to the CDW state. They are interesting from the point of view of potential applications, as well as from the point of view of the fundamental physics of low dimensional systems. Best known property is the non-linear electrical conductivity: beyond a certain value of the electrical field (Threshold field) the CDW gives an independent contribution to the conductance, and a more than proportional increase of the current can be observed. Important questions are about the mechanisms of the CDW transition and the non-linear conductivity, and their dependence on the chemical composition.
Here we will present the results of determinations of the room temperature structures of the layered compounds 1T-TaS2 and 4Hb-TaSe2. It is shown that a detailed description of a complicated domain structure can be obtained from the analysis of the intensities of Bragg reflections. The superspace description is used to analyse the correlations between the displacements of the different atoms. The consequences are derived for the shapes of the electron bands, and for the understanding of the CDW transition. It will be shown that a complete knowledge of all structural aspects is essential to understand the observations made by Scanning Tunneling Microscopy on the CDW state.