Infrared study on pressure induced charge delocalization in Cs2TCNQ3
作者单位：Department of Mechanical Engineering and Materials ScienceFaculty of EngineeringKumamoto UniversityKumamoto 860-0855Japan Department of Mechanical Engineering and Materials ScienceFaculty of EngineeringKumamoto UniversityKumamoto 860-0855Japan Department of Mechanical Engineering and Materials ScienceFaculty of EngineeringKumamoto UniversityKumamoto 860-0855 Japan CRESTJapan Science and Technology CorporationKawaguchi 332-0012Japan Research Institute for Advanced Science and TechnologyOsaka Prefecture UniversitySakai 599-8570Japan
会议名称：《18th International Conference on High Pressure Science and Technology》
会议日期：1000年
学科分类：080903[工学-微电子学与固体电子学] 0809[工学-电子科学与技术（可授工学、理学学位）] 08[工学] 080501[工学-材料物理与化学] 0805[工学-材料科学与工程（可授工学、理学学位）] 080502[工学-材料学]
关 键 词：TCNQ compound semiconductor -to- metal transition molecular vibration
摘 要：正Cs2TCNQ3 crystallizes into a columnar structure consisting of a periodic stack of TCNQ0/TCNQ/TCNCT, resulting in a semiconductor with the lowest optical energy gap of 0.3 eV [1]. The radicals TCNQ are dimerized, so that the uppermost valence band is formed by the π* orbit of spin singlet. Since the on-site repulsion energy U ≈ 1.2 eV of the π* electrons is higher than the optical energy gap, Cs2TCNQ3 may be regarded as a charge-transfer semiconductor of the 1/3-filled Hubbard system. It has been reported that this material undergoes the phase transition to a metallic state at around 3 GPa [2], although the optical energy gap remains unclosed [1]. In the present work, to obtain information about the mechanism of this semiconductor-to-metal transition, we measure the pressure dependence of the infrared molecular vibration. Figure 1 shows the absorption spectra in the C-CN stretching region at several pressures. The features at 1205 and 1211 cm-1 at the ambient pressure arise from neutral and radical molecules, respectively, whereas the one at 1183 cm-1 is the electron-molecular-vibration （e-mv） coupled mode. The former two features show a blue-shift with increasing pressure, while increasing their energy spacing. At around 3.6 GPa they abruptly displace toward each other about half the way of the energy spacing, showing that the π* electrons delocalize significantly from radical to neutral molecules. Evidently this is related to the high electric conductivity at high pressures. It is noteworthy that in the high-pressure phase radical-like and neutral-like molecules still coexist spectroscopically, and thus the π* electrons are not completely delocalized. The frequency, on the other hand, of the e-mv mode remains almost unchanged at low pressures. The e-mv mode disappears above 3.6 GPa, in coincidence with the disappearance of the electronic band S1, which is the counterpart of the e-mv mode [1].