MgB_2 under pressure: phonon calculations, Raman spectroscopy, and optical reflectance

Details MgB_2 under pressure: phonon calculations, Raman spectroscopy, and optical reflectance MgB_2 under pressure: phonon calculations, Raman spectroscopy, and optical reflectance

2001-05-21

arxiv.org/abs/cond-mat/0105402v1

J. Phys.: Condens. Matter 13, 9945--9962 (2001)

Physics

Condensed Matter

Superconductivity

18 pages, 8 figures (EPS), revtex

Scientific paper

10.1088/0953-8984/13/44/310

The effect of pressure on optical phonon frequencies of MgB$_2$ has been calculated using the frozen-phonon approach based on a pseudopotential method. Gr\"uneisen parameters of the harmonic mode frequencies are reported for the high-frequency zone-center $E_{2g}$ and $B_{1g}$ and the zone-boundary $E_{2u}$ and $B_{2u}$ modes at $A$. Anharmonic effects of phonon frequencies and the implications of the calculated phonon frequency shifts for the pressure dependence of the superconducting transition temperature of MgB$_2$ are discussed. Also reported are Raman and optical reflectance spectra of MgB$_2$ measured at high pressures. The experimental observations in combination with calculated results indicate that broad spectral features we observed in the Raman spectra at frequencies between 500 and 900 cm$^{-1}$ cannot be attributed to first-order scattering by zone-center modes, but originate in part from a chemical species other than MgB$_2$ at the sample surface and in part from a maximum in the MgB$_2$ phonon density of states. Low-temperature Raman spectra taken at ambient pressure showed increased scattering intensity in the region below 300 cm$^{-1}$.

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