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18th International Conference on High Pressure Science and technology

作者： Hasanudin t.kagayama N.Kuroda t.Sugimoto Department of Mechanical Engineering and Materials Science Faculty of EngineeringKumamoto UniversityKumamoto 860-0855Japan Department of Mechanical Engineering and Materials Science Faculty of EngineeringKumamoto UniversityKumamoto 860-0855Japan Department of Mechanical Engineering and Materials Science Faculty of EngineeringKumamoto UniversityKumamoto 860-0855 Japan CRESTJapan Science and Technology CorporationKawaguchi 332-0012Japan Research Institute for Advanced Science and technology Osaka Prefecture UniversitySakai 599-8570Japan

＜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].

关键词： tCNQ compound semiconductor -to- metal transition molecular vibration

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Effect of Pressure on Intermediate Valence States in Cerium Kondo Compounds

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Journal of Rare Earths 2004年第1期22卷 7-12页

作者： Oomi G kagayama t Smith J L Lacerda A H Kaji S Ohashi M Department of Physics Kyushu University Ropponmatsu Fukuoka 810-8560 Japan Department of Mechanical Engineering and Materials Sciences Kumamoto University Kumamoto 860-8555 Japan Materials Science and technology Los Alamos National Laboratory Los Alamos NM 87545 USA

Electrical resistance, thermal expansion, lattice constants of the intermediate valence materials, α-Ce and CeBe 13, were measured at high pressure. It is found that the Grüneisen parameters of Kondo temperature t K are 13 and 42 for α-Ce and CeBe 13, respectively, and it decreases with increasing pressure. the magnetoresistance of α-Ce shows H2 dependence and its coefficient increases with pressure below 1 GPa, but becomes nearly constant above 1 GPa. the results indicate that the intermediate valence states are enhanced at high pressure indicating the enhancement of hybridization between the 4f electron and conduction band.

关键词： condensel physics intermediate valence materials cerium rare earths

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