The correlation of lattice phonon frequencies with the superconducting transition temperatures of some (BEDT-TTF) salts

Abstract

The far-infrared powder absorption spectra, between 10 cm-' and 350 cm-', of seven BEDT-TTF salts that exhibit superconductivity at or near ambient pressure, have been measured as a function of temperature, between 5 K and 300 K. The temperature dependences of the peak frequencies, along with the measured deuterium isotopic frequency shifts, separated the lattice from the internal modes. The frequencies of the observed lattice modes were found to increase with T, in the three K-phase salts in agreement with the reduced volume of the unit cell, and to have a mixed response in the three B-phase compounds. Furthermore, the genera1 range and magnitude of the frequencies in the higher-T, K-phase compounds were found to be larger than in the lower T, pphase compounds. Consequently, no evidence was found for a correlation between higher values of T, and a softer lattice in these organic superconductors.

Raman spectroscopy study ofNaxCoO2and superconductingNaxShi1, Liu2, Yang3 et al. 2004Phys. Rev. B538312View full textAdd to dashboardCite

“…On the other hand, it is also known that the oxyhydrated superconducting phases have a much longer c-axis than the parent material, the Raman peaks are found to shift evidently towards lower frequency with the intercalation of H 2 O. However, it is believed that the modifications of the phonons in these two cases might have evidently different underlying structural and physical origins.It is worthy to point out that our results can be used to complete certain conclusions drawn by M. Dressel on the correlation of phonon frequencies with the superconducting transition temperatures of some (BEDT-TTF) salts, which suggests that, in high T c materials, the higher phonon frequencies possibly contribute to the increase of T c in certain systems, however, the issues on the connection of phonon and superconductivity in general is much more complex than that described by a BCS expression [18]. Therefore, in present case, it is also possible that a higher T c value doesn't have to in connection with soften lattice, but it is still possible the 'volume effect ' [18], showing up as changes in the c-axis parameter, maybe operate in the Na x CoO 2 ·1.3H 2 O system.…”

“…In each spectrum five clear modes can be evidently recognized, the data from the Na x CoO 2 (x=0.7) material shows up noticeable similarities in comparison with results shown in Fig. 3 It is worthy to point out that our results can be used to complete certain conclusions drawn by M. Dressel on the correlation of phonon frequencies with the superconducting transition temperatures of some (BEDT-TTF) salts, which suggests that, in high T c materials, the higher phonon frequencies possibly contribute to the increase of T c in certain systems, however, the issues on the connection of phonon and superconductivity in general is much more complex than that described by a BCS expression [18]. Therefore, in present case, it is also possible that a higher T c value doesn't have to in connection with soften lattice, but it is still possible the 'volume effect' [18], showing up as changes in the c-axis parameter, maybe operate in the Na Raman spectra from a Na x CoO 2 single crystal sample with Na 2 CO 3 on the surface, inset…”

Lattice dynamics and electron-phonon coupling in theβ−(BEDT−TTF)2I3Girlando1, Masino2, Visentini3 et al. 2000Phys. Rev. B351380View full textAdd to dashboardCite

“…Since for both I − 3 and BEDT-TTF several investigations 13,14,32 suggest that there are low frequency intra-molecular modes, the validity of RMA for (BEDT-TTF) 2 I 3 appears questionable. Therefore, we have decided to relax the RMA and to investigate the effects of the intra-molecular degrees of freedom.…”

“…In the RMA classification, below 220 cm −1 we expect Raman activity for 8 lattice modes, 9 BEDT-TTF intra-molecular modes and one stretching of I − 3 ; in IR we expect 6 lattice modes, 9 BEDT-TTF intra-molecular modes and three I − 3 modes. The modes calculated at the minimum G structure at 120 K are compared with experimental ones, 6,32,43 in Tables III and IV for A u and A g modes, respectively. We have chosen the 120 K temperature since in this way we can compare the normal state phonon frequencies and eigenvectors for the minimum G and the experimental 27 structure.…”

Quasi-two-dimensional organic superconductors: A review

“…Indeed, it is very probable that the only reliable estimates of the intralayer resistivity are those derived from skin-depth (see Reference [29] and references therein) and optical conductivity studies [129].…”