Phase transition of thin films of organic conductors studied by scanning tunneling spectroscopy

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We have investigated the surface electronic structure of thin films of an organic conductor TTF-TCNQ by scanning tunneling microscopy(STM) and scanning tunneling spectroscopy(STS), in order to understand the mechanism of Peierls transition of thin films at low temperature. In addition, we clarified some problems (described below) to be overcome in the STM measurement of organic films.1. Experimental Set-upBecause of the higher vapor pressure, thin films of organic conductors were easily sublimated in ultra-high vacuum below 3x10^<-10> torr and eventually they vanished into vacuum after a long period. Then we used rather thick films as samples and performed STM/STS measurements in a short time. In addition, at low temperatures, the film was stripped away from the substrate and was unable to contact the electrode because of the difference in expansion coefficient between them. For this, organic thin films were prepared on the conductive Au film/mica.2. Experimental ResultsWe obtained STM images of a TTF-TCNQ film at a tip voltage of 200mV, where three dots from a TCNQ molecule and one dot from a TTF molecule were arranged two-dimensionally on the surface. The I-V spectra showed metallic behavior at almost all the sampling points. At 77K, which was close to the critical temperature of Peierls transition (53K), the measurement was unstable and the I-V spectra showed both metallic and non-metallic characters depending on the sampling points, indicating that the transition already occurred at this temperature in a part of the sample. At 10K, the non-metallic phase became dominant, in which the spectra had a wide band-gap. We obtained STM images both at 77K and 10K, which were different from the image obtained at room temperature. Detailed interpretation of the images in conjunction with the phase transition is underway at present.