Raman scattering from theCaC6superconductor in the presence of disorder

Abstract

Polarized Raman scattering has been performed on CaC 6 superconductor. We identify two of the three Raman-active E g phonon modes at 440 and 1508 cm −1 expected for the R3m space group of CaC 6 . These first-order scattering modes appear along with the D and G bands around 1300 and 1600 cm −1 that are similar in origin to the corresponding bands in plain graphite. The intensities of the D and G bands in CaC 6 correlate with degree of disorder. The D band arises from the double resonant Raman-scattering process; its frequency shifts as a function of excitation energy with ϳ35 cm −1 / eV. The double resonant Raman scattering probes phonon excitations with finite wave vector q. We estimate the characteristic spacing of structural defects to be on the scale of about 100 Å by comparing the intensity of the D band and the 1508 cm −1 E g mode in CaC 6 to calibrated intensity ratio of analogous bands in disordered graphites. A sharp superconducting coherence peak at 24 cm −1 is observed below T c .

Defect modulated Raman response of KC8 single crystals

“…Additionally, at this point it is worth mentioning that spectra like 2 and 3 in Fig. 1, are in turn similar to data reported for CaC 6 by Dean et al [11], and Mialitsin et al [13]. These two middle spectra exhibit a G peak, which splits into two peaks: at around 1602 cm À1 (upshifted) and 1571 cm À1 (down-shifted).…”

“…In analogy to the defect modulated double resonance K point phonons, it is tempting to assign it to the D-line [12]. This assignment D mode is similar to the one in CaC 6 [5,13], and XC 6 [11] and in doped graphene [10]. However, in our case there is no dispersion of this mode and no peaks were observed in the region of the 2D line for any of our samples.…”

“…Previously, the presence of a fine structure and correspondingly more than one component has been reported only for CaC 6 by Dean at al. [11], and Mialitsin et al [13]. They attributed the different spectral features observed, to surface de-intercalation and to defect modulation within the different positions on their samples.…”

Raman spectroscopy of graphite intercalation compounds: Charge transfer, strain, and electron–phonon coupling in graphene layers

“…These Raman reports have been performed below the T c of CaC 6 , and at 240K for Li GICs. The results showed reversible transition phases and the appearance of peaks not present at room temperature (RT) related to in-plane vibrations around 40 cm −1 [79]. In the case of LiC 17 , it shows a doublet G-line assigned to the presence of LiC 17 and LiC 18 phases.…”

“…Raman spectroscopy analyses on CaC 6 [79], and LiC 12 [80] had been made at low-T, but much less is know about them. These Raman reports have been performed below the T c of CaC 6 , and at 240K for Li GICs.…”