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

利用扫描隧道显微镜（STM）和扫描隧道光谱（STS）研究了有机导体TTF-TCNQ薄膜的表面电子结构，以了解薄膜在低温下的peerls转变机理。此外，我们还澄清了有机薄膜的STM测量中需要克服的一些问题（如下所述）。由于较高的蒸气压，有机导体薄膜在低于3 × 10^<-10> torr的超高真空中容易升华，并最终在长时间后消失在真空中。然后，我们使用较厚的薄膜作为样品，并在短时间内进行STM/STS测量。此外，在低温下，由于薄膜之间的膨胀系数不同，薄膜从衬底上剥离而无法接触电极。为此，在导电Au膜/云母上制备了有机薄膜。实验结果获得了TTF-TCNQ薄膜在尖端电压为200mV时的STM图像，其中来自TCNQ分子的3个点和来自TTF分子的1个点在表面上二维排列。I-V谱在几乎所有取样点均表现出金属行为。在77K，即接近peerls转变的临界温度（53K）时，测量结果不稳定，根据采样点的不同，I-V光谱显示出金属和非金属的特征，这表明部分样品已经在该温度下发生了转变。在10K时，非金属相占主导地位，其光谱具有较宽的带隙。我们获得了77K和10K的STM图像，与室温下获得的图像不同。结合相变对图像的详细解释目前正在进行中。

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.