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Raman studies of low-dimensional conductors and superconductors Lin, Yuankun

Abstract

Using a Fourier Raman spectrometer equipped with an infrared laser, together with cryogenics, three types of materials have been investigated as a function of temperature in this thesis. The first is the investigation of organic materials including K-(BEDT-TTF)2Cu(NCS)2 (Tc=10.4 K), K-(BEDT-TTF)2Cu(N(CN)2)Br (Tc=11.6 K), αt-(BEDT-TTF)2I3 (Tc=8 K) and β-(BEDT-TTF)2AuI2 (Tc=5 K) which become superconductors at low temperature. The second is the study of the first organic conductor TTF-TCNQ which behaves in exactly the opposite way by becoming an insulator at low temperature. The third is the study of the strontium-doped lanthanum copper oxide superconductors with higher transition temperature. For BEDT-TTF based organic superconductors, the electron-phonon coupling is very strong. The frequencies and intensities of three strongest features ( v3 (Ag), vg (A9 ) and v60 (B3g) modes) in the Raman spectra have been analyzed as a function of temperature. The frequencies of some modes are observed to soften in the temperature range where antiferromagnetic spin fluctuations have been observed, providing evidence of interactions between the phonons and the magnetism. The v60 (B3g) mode is observed to be very unusual in many ways, such as having an inverse isotope frequency shift. Below Tc, this mode exhibits an increase of 2.2 cm-1 in K-(BEDT-TTF)2Cu(N(CN)2)Br (Tc=11.6 K) and a decrease of 1.7 cm-1 in αt-(BEDT-TTF)2l3 (Tc=8 K). This is the highest frequency phonon in any material to be affected by superconductivity. For TTF-TCNQ, many new lines are observed at temperatures below 150 K as the fluctuating charge-density-wave occurs. The intensity of these lines increases with decreasing temperature. These new lines are assigned according to the deuterium-isotope frequency shifts. In the fluctuating charge-density-wave phase the Frohlich electron-phonon interaction is the probable cause of the appearance of Raman-forbidden scattering originating from the infrared-active-only modes. The strong out-of-plane vibrational Raman modes correspond to the large out-of-plane distortion of the TCNQ molecule, which is in agreement with the x-ray results. For lanthanum copper oxide materials, firstly we observe seven Raman-forbidden longitudinal optical phonons, which appear to be activated by the Frohlich mechanism, in the single-phonon Raman scattering of La2CuO4. Good agreement is obtained between the peak frequencies and those of the longitudinal optical modes measured by infrared reflectivity and inelastic neutron scattering. Secondly, strong magnetic Raman scattering is observed in one crystal of La1.9Sr0.1CuO4, which has a suppressed Tc of 12 K, due to an ordered spin phase below 40 K. A weak second peak indicates the possible existence of phase separation in the sample. In agreement with neutron scattering results, the Raman intensity of the intense peak increases with decreasing temperature below 40 K. The line shape and the temperature dependence of the magnetic scattering intensity are totally different from those observed in the parent compound La2CuO4. The temperature dependences of the peak frequency and damping, however, are similar to those of other two-dimensional antiferromagnets. The line shape is fitted within the traditional Loudon-Fleury Raman theory of two-magnon scattering. The resulting super-exchange constant is found to be 1519 K, in accordance with EPR-measurements on the same compound. [Scientific formulae used in this abstract could not be reproduced.]

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