7 research outputs found
Anharmonicity and self-energy effects of the E2g phonon in MgB2
We present a Raman scattering study of the E2g phonon anharmonicity and of
superconductivity induced self-energy effects in MgB2 single crystals. We show
that anharmonic two phonon decay is mainly responsible for the unusually large
linewidth of the E2g mode. We observe ~ 2.5 % hardening of the E2g phonon
frequency upon cooling into the superconducting state and estimate the
electron-phonon coupling strength associated with this renormalization.Comment: 4 pages, 3 figures, accepted to PR
Raman scattering from the CaC6 superconductor in the presence of disorder
Polarized Raman scattering has been performed on CaC6 superconductor. We
identify two of the three Raman active Eg phonon modes at 440 and 1508 cm-1
expected for the R-3m space group of CaC6. These first order scattering modes
appear along with the D and G bands around 1300 cm-1 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 CaC6 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 approximately 35 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 Angstrom by comparing the intensity of the D band and the 1508
cm-1 Eg mode in CaC6 to calibrated intensity ratio of analogous bands in
disordered graphites. A sharp superconducting coherence peak at 24 cm-1 is
observed below Tc.Comment: 10 pages, 5 figure
Observation of Leggett's collective mode in a multi-band MgB2 superconductor
We report observation of Leggett's collective mode in a multi-band MgB2
superconductor with T_c=39K arising from the fluctuations in the relative phase
between two superconducting condensates. The novel mode is observed by Raman
spectroscopy at 9.4 meV in the fully symmetric scattering channel. The observed
mode frequency is consistent with theoretical considerations based on the first
principle computations.Comment: Accepted for PR
Multi-Gap Superconductivity in MgB2: Magneto-Raman Spectroscopy
Electronic Raman scattering studies on MgB2 single crystals as a function of
excitation and polarization have revealed three distinct superconducting
features: a clean gap below 37 cm-1 and two coherence peaks at 109 cm-1 and 78
cm-1 which we identify as the superconducting gaps in \pi- and \sigma-bands and
as the Leggett's collective mode arising from the fluctuation in the relative
phase between two superconducting condensates residing on corresponding bands.
The temperature and field dependencies of the superconducting features have
been established. A phononic Raman scattering study of the E2g boron stretching
mode anharmonicity and of superconductivity induced self-energy effects is
presented. We show that anharmonic two phonon decay is mainly responsible for
the unusually large linewidth of the E2g mode. We observe ~2.5% hardening of
the E2g phonon frequency upon cooling into the superconducting state and
estimate the electron-phonon coupling strength associated with this
renormalization.Comment: Invited review in a special issue "Superconductivity in MgB_2:
Physics and Applications