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Register a new userEffect 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 Zone measured by Inelastic X-ray Scattering (IXS), where its dispersion can be accounted for by standard harmonic phonon theory, adding only a moderate electron-phonon coupling. Here we show that the effects rapidly disappear when electron-phonon coupling is switched off by Al substitution on the Mg sites. Moreover, using IXS with very high wave-vector resolution in MgB$_2$, we can follow the dispersion connecting the Raman and the IXS signal, in agreement with a theory using only electron-phonon coupling but without strong anharmonic terms. The observation is important in order to understand the effects of electron-phonon coupling on zone center phonons modes in MgB$_2$, but also in all metals characterized by a small Fermi velocity in a particular direction, typical for layered compounds.
Polarization-dependent x-ray absorption spectroscopy at the B 1s edge of single-crystalline Mg(x)Al(1-x)B(2) reveals a strongly anisotropic electronic structure near the Fermi energy. Comparing spectra for superconducting compounds (x=0.9, 1.0) with 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.
Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to follow the change of superconductivity with systematic Al doping of MgB$_2$. By using x-ray diffraction and Vegards law to assess the actual Al content in the samples, changes in behavior were found to be much more in agreement with theoretical predictions than in earlier studies. EELS data show that $sigma$-band hole states disappear above 33% Al, approximately the composition at which the $sigma$ band Fermi surface is predicted to lose its cylindrical shape in reciprocal space and break apart into ellipsoidal pockets. At this composition, the $sigma$ gap obtained from the heat capacity data falls to the level of the $pi$ gap, implying that band filling results in the loss of strong superconductivity on the $sigma$ band. However, superconductivity is not quenched completely, but persists with $T_c < 7$ K up to about 55% Al, the Al concentration at which the entire $sigma$ band is predicted to fall below the Fermi surface. Since, in the region $0.33 alt x alt 0.55$, only the $pi$ band has appreciable density of states, it becomes the stronger of the 2 bands, thus inverting the 2-band hierarchy of MgB$_2$.
We have studied the effect of Al doping on the structural, magnetic and electrical properties of La$_{1-x}$Ba$_x$Mn$_{1-x}$Al$_x$O$_3$ ($0leq x leq 0.25$) manganite, annealed in two 750$^oC$ and 1350$^oC$ temperatures. The XRD analysis shows that the structures in all samples have single phase rhombohedral structure with R$bar{3}$c space group. The unit cell volume almost decrease with increasing the Al doping in all samples. The grain growth with increasing annealing temperature and Al doping also have been studied. We observed that, T$_c$ temperature decreases when the Al ion substitute in Mn ion site. The magnetic study of the samples via magnetic susceptibility results in Griffiths and spin-glass phase for samples doped with aluminium. Along the resistivity measurement results, the $T_{MIT}$ (metal-insulator) transition temperatures decrease and the system become an insulator. The insulator-metal transition occurs for L0 sample in near 165K, while this transition is weak for H0 sample due to oxygen non-stoichiometry.Using three models viz. 1. Adiabatic small polaron hoping, 2.Variable range hopping, and 3. Percolation model, the resistance have been studied.
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