MgB2; Al and C doping, sigma-band filling and anisotropy reduction

Journal of Physics and Chemistry of Solids, 2004

We have investigated the effect of electron doping on the superconducting properties of MgB 2. For the purpose we have synthesized several samples along the Mg 12x Sc x B 2 section. The X-ray diffraction measurements reveal small changes in the lattice parameters suggesting that the Sc doping could be considered to simply fill the boron s bands. Radio frequency surface resistivity measurements has been used to obtain the variation of T c with Sc doping. Increasing the Sc content, the experimental T c diverges from the T c predicted by the BCS single band theory showing the key role of interchannel pairing near a shape resonance.

Journal of Superconductivity and Novel Magnetism, 2007

We present the results of point-contact spectroscopy (PCS) measurements in single-phase Mg1−x Alx B2 crystals and polycrystals with x up to 0.32, recently grown at ETH (Zurich) and University of Genova, respectively. The differential conductance curves of our “soft” point contacts were measured as a function of temperature from 4.2 K up to the Andreev critical temperature T c A , at which the normal-state conductance is recovered. The gap amplitudes, Δσ and Δπ , were obtained through a two-band BTK fit of the normalized conductance curves. The results extend previous data to the range of heavy Al doping and show that contrary to expectations, no gap merging is observed up to x=0.32 (i.e., down to T c A =9 K). The analysis of the gap trends within the two-band Eliashberg theory shows that band filling is the main effect of Al doping, even though a small increase in interband scattering might be necessary to account for the experimental data in single crystals.

Physical Review B, 2006

We report data on the thermal conductivity ͑T , H͒ along the basal plane of the hexagonal crystal structure of superconducting Mg 1−y Al y B 2 with y = 0.02 and 0.07 at temperatures between 0.5 and 50 K and in external magnetic fields between 0 and 70 kOe. The substitution of Al for Mg leads to a substantial reduction of the heat transport via electronic quasiparticles. The analysis of the ͑T , H͒ data implies that the Al impurities provoke an enhancement of the intraband scattering rate, almost equal in magnitude for both the and the bands of electronic excitations. This is in contrast with conclusions drawn from analogous data sets for material in which carbon replaces boron and where mainly the intraband scattering rate of the band is enhanced. Our complete data set, including additional results of measurements of the low-temperature thermal conductivity of pure MgB 2 , confirms the validity of the Wiedemann-Franz law for both pure and doped MgB 2 .

Journal of Physics and Chemistry of Solids, 2008

The thermoelectric power of C, Mn, C:Li, and Al:Li substituted MgB 2 single crystals has been investigated in the temperature range 10-300 K. Both the in-plane (S ab ) and the outof-plane (S c ) thermopowers are positive for the non-substituted crystal and both S ab and S c change a sign for crystals doped with electrons where C is substituted for B in the amount larger than 5 %at. When Li is substituted for Mg, the π band rather than the σ band is doped with holes and the doping effects are much more subtle. The anisotropy ratio of the nonsubstituted crystal S ab /S c ≈ 3 and this ratio is strongly reduced by the substitution of C.

Journal of Solid State Chemistry, 2002

By means of density functional theory the electronic structure of the MgB 2 superconductor was characterised and compared with that of the related iso-structural systems: AlB 2 , ZrB 2 , NbB 2 , and TaB 2 . Using the full-potential linearized augmented plane waves (FP-LAPW) method and the generalised gradient approximation, the electronic density distribution, density of states, and band structures were obtained for these compounds. The electrical conductivity, which cannot be easily measured in the c-direction, was calculated, in the relaxation time approximation using band structure results. It was found that the two-dimensional (2D) crystal structure character of these metallic diborides is also reflected in the electronic charge distribution. This 2D pattern is not completely seen in the electrical conductivity as it is, for instance, in the superconductor high Tc cuprates. Indeed, it was found that, by the electrical conductivity calculations, all these compounds have a bulk, yet anisotropic, conductivity.

IEEE Transactions on Applied Superconductivity, 2000

A comparative study of the electron transport properties of MgB 2 bulk samples was performed aimed at addressing the role of annealing as well as of SiC and Zn doping on sample anisotropy and band conduction parameters. Samples were produced by Reactive Mg Liquid Infiltration (RLI) technique in boron powder preforms. Both irreversibility ( ) and upper critical fields ( 2 ) are enhanced by doping procedure. In all samples the 2 anisotropy factor ( ), evaluated by analyzing the and 2 temperature dependence in the framework of the current percolation model proposed by Eisterer et al. [Phys. Rev. Lett. 90, 247002 (2003)], decreases at increasing temperature, and it is reduced by SiC and Zn doping-induced defects. Accordingly, in both annealed and doped samples the -band diffusivity, obtained by fitting the 2 dependence on temperature within the two-band model formulated by Gurevich [Phys. Rev. B 67, 184515 (2003)], is higher than the -band one. A decrease of the more anisotropic -band diffusivity was also found in the doped samples.

Physical Review B, 2005

In this Letter, the study of the effect of Al substitution on the upper critical field, Bc2, in AlxMg1-xB2 samples is presented. We find a straightforward correlation between Bc2 and the sigma-band gap, Delta_sigma, evaluated by point-contact measurements. Up to x=0.2 Bc2 can be well described within a clean limit model and its decrease with x is directly related to the suppression of Delta_sigma. For larger doping we observed the crossover to the dirty regime driven mostly by the strong decrease of Delta_sigma rather than by the increase of the sigma-band scattering rate

Physical Review B, 2003

We have examined polycrystalline MgB 2 by electron energy loss spectroscopy (EELS) and density of state calculations. In particular, we have studied two different crystal orientations, [110] and [001] with respect to the incident electron beam direction, and found significant changes in the near-edge fine-structure of the B K-edge. Density functional theory suggests that the pre-peak of the B K-edge core loss is composed of a mixture of p xy and p z hole states and we will show that these contributions can be distinguished only with an experimental energy resolution better than 0.5 eV. For conventional TEM/STEM instruments with an energy resolution of ~1.0 eV the pre-peak still contains valuable information about the local charge carrier concentration that can be probed by core-loss EELS. By considering the scattering momentum transfer for different crystal orientations, it is possible to analytically separate p xy and p z components from of the experimental spectra With careful experiments and analysis, EELS can be a unique tool measuring the superconducting properties of MgB 2 , doped with various elements for improved transport properties on a sub-nanometer scale. 74.70.Ad, 79.20 Uv, 31.15 Ew

Physica C: Superconductivity, 2007

We prepared a series of Mg 1-x (AlLi) x B 2 samples with 0≤x≤0.45 in order to compensate with Li the electron doping induced by Al. Structural characterization by means of neutron and X-ray diffraction confirms that Li enters the MgB 2 structure even though in an amount less than nominal one. We performed susceptibility, resistivity and specific heat measurements. Vibrational properties were also investigated by means of Raman spectroscopy. We compare these results with those obtained on a homologous series of Mg 1-x Al x B 2 samples. The systematic success of scaling the relevant properties with the Al content rather than with the electron doping suggests that lattice deformation plays an important role in tuning the superconducting properties.

Journal of Applied Physics, 2006

A series of polycrystalline bulk samples of Mg 1−2x ͑AgAl͒ x B 2 ͑0.0% ഛ x ഛ 1.0% ͒ has been synthesized by a solid state reaction method. The structure, Raman spectrum, and superconducting properties have been investigated by x-ray diffraction, Raman spectroscopy, and low-temperature resistivity measurements. It is found that the Ag, Al double doping causes the expansion of crystal lattice along the a-and c-axis orientations due to the substitution inducing ionic size variation. A redshift of peak position is observed in Raman spectra, which is ascribed to the crystal cell volume change inducing the variation of the phonon frequency. The superconducting transition temperature ͑T c ͒ is degressive with the doping level ͑x͒ increase. By the Ag and Al double doping, the hole concentration is kept to be unchanged in MgB 2 , which eliminates the effects of the charge carrier concentration change and band filling on T c . It is suggested that the reason of the T c suppression caused by the double doping is the co-operating results of the disorder effect and the chemical pressure effect induced by the chemical substitution.