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Peña, O.; Aguayo, A.; Coss, Romeo de

doi:10.1103/physrevb.66.012511

Cited by 111 publications

(92 citation statements)

References 20 publications

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“…These changes are fully consistent with firstprinciples studies using the virtual crystal approximation where a charge transfer from the B-B σ bonds to the π bonds in the inter-planar region was observed. 23,38 Our data also agree with the detailed supercell calculations in Ref. 38 where it was shown that covalent bond clusters composed of Al and the adjacent four B-B bonds emerge upon Al substitution.…”

Section: Resultssupporting

confidence: 89%

“…23 Up to now, several mechanisms like band filling due to substitution of Mg 2+ with Al 3+ , merging of the superconducting gaps, increased interband scattering, and a decrease in the electron-phonon coupling (Refs. 18,19,[23][24][25][26][27] have been suggested to explain the very rapidly reduced superconductivity at only modest substitution levels. 28 So far, however, a consistent picture is elusive and a direct investigation of the electronic structure and the bonding characteristics of the sp 2 and p z states in Mg x Al 1−x B 2 might help in an understanding of the suppression of superconductivity in Mg x Al 1−x B 2 for x 0.6 and of the changes of the electronic structure especially in the less investigated doping range 0 ≤ x < 0.6.…”

Section: Introductionmentioning

confidence: 99%

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Electronic Structure of Single-Crystalline Mg_xAl₁₋_xB₂ Probed by X-ray Diffraction Multipole Refinements and Polarization-Dependent X-ray Absorption Spectroscopy

et al. 2014

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X-ray diffraction multipole refinements of single-crystalline MgxAl1−xB2 and polarizationdependent near-edge x-ray absorption fine structure at the B 1s edge reveal a strongly anisotropic electronic structure. Comparing the data for superconducting compounds (x = 0.8, 1.0) with those for the non-superconductor (x = 0) gives direct evidence for a rearrangement of the hybridizations of the boron pz bonds and underline the importance of holes in the σ-bonded covalent sp 2 states for the superconducting properties of the diborides. The data indicate that Mg is approximately divalent in MgB2 and suggest predominantly ionic bonds between the Mg ions and the two-dimensional B rings. For AlB2 (x = 0), on the other hand, about 1.5 electrons per Al atom are transferred to the B sheets while the residual 1.5 electrons remain at the Al site which suggests significant covalent bonding between the Al ions and the B sheets. This finding together with the static electron deformation density points to almost equivalent electron counts on B sheets of MgB2 and AlB2, yet with a completely different electron/hole distribution between the σ and π bonds.

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Section: Resultssupporting

confidence: 89%

Section: Introductionmentioning

confidence: 99%

Electronic Structure of Single-Crystalline Mg_xAl₁₋_xB₂ Probed by X-ray Diffraction Multipole Refinements and Polarization-Dependent X-ray Absorption Spectroscopy

et al. 2014

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“…16,17,18,19 Interband scattering is weak in MgB 2 , 20 but this is not necessarily the case in samples in which Mg has been replaced by Al or B by C (Ref. 21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37) or which have been exposed to neutron irradiation. 38 Such samples exhibit considerably reduced T c , while the two gaps persist even at very low critical temperatures.…”

Section: Introductionmentioning

confidence: 99%

Thermodynamics of two-band superconductors: The case ofMgB2

et al. 2005

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Thermodynamic properties of the multiband superconductor MgB 2 have often been described using a simple sum of the standard BCS expressions corresponding to σ-and π-bands. Although, it is a priori not clear if this approach is working always adequately, in particular in cases of strong interband scattering. Here we compare the often used approach of a sum of two independent bands using BCS-like α-model expressions for the specific heat, entropy and free energy to the solution of the full Eliashberg equations. The superconducting energy gaps, the free energy, the entropy and the heat capacity for varying interband scattering rates are calculated within the framework of two-band Eliashberg theory. We obtain good agreement between the phenomenological two-band α-model with the Eliashberg results, which delivers for the first time the theoretical verification to use the α-model as a useful tool for a reliable analysis of heat capacity data. For the thermodynamic potential and the entropy we demonstrate that only the sum over the contributions of the two bands has physical meaning.

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“…By doping with Al the topology of σ Fermi surface changes: at x=0.33 (close to the maximum Al concentration we considered) the complete filling of the σ bands at the Γ point occurs and a crossover happens from a two-dimensional to a three-dimensional dispersion regime. 10,11 The Fermi velocities of the σ bands, defined by Eq.s (2a) and (2b) show a quadratic dependence on ∆ σ , thus, the σ gap determination as a function of doping becomes essential. Here, the gap energies have been evaluated by point-contact measurements.…”

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confidence: 99%

Critical field of Al-dopedMgB2samples: Correlation with the suppression of theσ-band gap

et al. 2005

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In this Letter, the study of the effect of Al substitution on the upper critical field, B c2 , in Al x Mg 1-x B 2 samples is presented. We find a straightforward correlation between B c2 and the σ−band gap, ∆ σ ., evaluated by point-contact measurements. Up to x=0.2 B c2 can be well described within a clean limit model and its decrease with x is directly related to the suppression of ∆ σ . For larger doping we observed the crossover to the dirty regime driven mostly by the strong decrease of ∆ σ rather than by the increase of the σ-band scattering rate.Superconductivity at 40K in magnesium diboride has been extensively studied since its discovery 1 to these days. The presence of two-bands crossing the Fermi level and having strongly different character 2,3 is well established: two π bands are formed by p z orbitals of boron and are threedimensional, electron-type and weakly coupled with phonons; two σ bands are formed by sp 2 -hybrids orbitals stretched along boron-boron bonds and are two-dimensional, hole-type and strongly coupled with the optical E 2g phonon mode. This peculiar band structure, joined to the fact that impurity interband scattering is inhibited by the different parity of π and σ orbitals, 4 is at the ground of the two-gap superconductivity, evidenced unambiguously for the first time in this compound. The lack of interband scattering yields important consequences in the transport behavior because it prevents the mixing of the σ and π carriers, which maintain their own characteristics. This gives a unique chance to selectively disorder each channel independently and offers many opportunities to tune superconducting and normal properties. Many attempts of selective doping have been carried out. For example, substitution of C in the B site significantly increases upper critical fields, as reported by several groups [5][6][7] suggesting that, in these substituted compounds, the dirty regime is well stabilized. On the other hand, Al substitution of Mg does not give univocal results. In Al doped single crystals, the critical field perpendicular to the ab-planes, , increases with increasing Al concentration, while the one parallel to the ab-planes, , decreases, as observed also in polycrystalline samples. Therefore both the energy gaps, and the scattering rates, vary with the doping determining the regime of conduction (clean or dirty) of each band. Thus the upper critical field must depend on Al doping in a not trivial way and the study of its behavior gives an unique opportunity to investigate the peculiar role of disorder in a two-gap superconductor. This Letter tackles the multifaced subject of transport regimes in Al doped samples. We correlated the upper critical fields, the energy gaps and the scattering rates in a complete set of poly-crystalline Al x Mg 1-x B 2 samples in order to investigate the effect of Al substitution on the conduction regimes of σ and π bands. Dense, clean and hard cylinder shaped samples have been prepared by direct synthesis of pure elements. 16 In order to improve the ...

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Effects of Al doping on the structural and electronic properties ofMg1−xAlx

Cited by 111 publications

References 20 publications

Electronic Structure of Single-Crystalline Mg_xAl₁₋_xB₂ Probed by X-ray Diffraction Multipole Refinements and Polarization-Dependent X-ray Absorption Spectroscopy

Electronic Structure of Single-Crystalline Mg_xAl₁₋_xB₂ Probed by X-ray Diffraction Multipole Refinements and Polarization-Dependent X-ray Absorption Spectroscopy

Thermodynamics of two-band superconductors: The case ofMgB2

Critical field of Al-dopedMgB2samples: Correlation with the suppression of theσ-band gap

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Effects of Al doping on the structural and electronic properties ofMg1−xAlx

Cited by 111 publications

References 20 publications

Electronic Structure of Single-Crystalline MgxAl1−xB2 Probed by X-ray Diffraction Multipole Refinements and Polarization-Dependent X-ray Absorption Spectroscopy

Electronic Structure of Single-Crystalline MgxAl1−xB2 Probed by X-ray Diffraction Multipole Refinements and Polarization-Dependent X-ray Absorption Spectroscopy

Thermodynamics of two-band superconductors: The case ofMgB2

Critical field of Al-dopedMgB2samples: Correlation with the suppression of theσ-band gap

Contact Info

Product

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Electronic Structure of Single-Crystalline Mg_xAl₁₋_xB₂ Probed by X-ray Diffraction Multipole Refinements and Polarization-Dependent X-ray Absorption Spectroscopy

Electronic Structure of Single-Crystalline Mg_xAl₁₋_xB₂ Probed by X-ray Diffraction Multipole Refinements and Polarization-Dependent X-ray Absorption Spectroscopy