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تسجيل مستخدم جديدEffect of Al doping on the optical phonon spectrum in Mg(1-x)Al(x)B(2)
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P. Postorino
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Raman and infrared absorption spectra of Mg(1-x)Al(x)B(2) have been collected for 0<x<0.5 in the spectral range of optical phonons. The x-dependence of the peak frequency, the width and the intensity of the observed Raman lines has been carefully analized. A peculiar x-dependence of the optical modes is pointed out for two different Al doping ranges. In particular the onset of the high-doping structural phase previously observed in diffraction measurements is marked by the appearence of new spectral components at high frequencies. A connection between the whole of our results and the observed suppression of superconductivity in the high doping region is established.
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Superconducting MgB$_2$ shows an E$_{2g}$ zone center phonon, as measured by Raman spectroscopy, that is very broad in energy and temperature dependent. The Raman shift and lifetime show large differences with the values elsewhere in the Brillouin Zo
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those for the non-superconductor x=0.0 gives direct evidence on the importance of an in-plane spectral feature crossing E_F for the superconducting properties of the diborides. Good agreement is found with the projected B 2p density of states from LDA band structure calculations.
The reflectivity $R (omega)$ of $ab$-oriented Mg$_{1-x}$Al$_x$(B$_{1-y }$C$_y$)$_2$ single crystals has been measured by means of infrared microspectroscopy for $1300<omega<17000$ cm$^{-1}$. An increase with doping of the scattering rates in the $pi$
and $sigma$ bands is observed, being more pronounced in the C doped crystals. The $sigma$-band plasma frequency also changes with doping due to the electron doping, while the $pi$-band one is almost unchanged. Moreover, a $sigmatosigma$ interband excitation, predicted by theory, is observed at $omega_{IB} simeq 0.47$ eV in the undoped sample, and shifts to lower energies with doping. By performing theoretical calculation of the doping dependence $omega_{IB}$, the experimental observations can be explained with the increase with electron doping of the Fermi energy of the holes in the $sigma$-band. On the other hand, the $sigma$ band density of states seems not to change substantially. This points towards a $T_c$ reduction driven mainly by disorder, at least for the doping level studied here. The superconducting state has been also probed by infrared synchrotron radiation for $30<omega<150$ cm$^{-1}$ in one pure and one C-doped sample. In the undoped sample ($T_c$ = 38.5 K) a signature of the $pi$-gap only is observed. At $y$ = 0.08 ($T_c$ = 31.9 K), the presence of the contribution of the $sigma$-gap indicates dirty-limit superconductivity in both bands.
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