Al substitution inMgB2crystals: Influence on superconducting and structural properties

Abstract

Single crystals of Mg 1-x Al x B 2 have been grown at a pressure of 30 kbar using the cubic anvil technique. Precipitation free crystals with x < 0.1 were obtained as a result of optimization of already developed MgB 2 crystal growth procedure. Systematic decrease of the c-axis lattice constant with increasing Al content, when the a-axis lattice constant is practically unchanged, was observed. Variation of the critical temperature on Al content in Mg 1-x Al x B 2 crystals was found to be slightly different than that one observed for polycrystalline samples since, even a very small substitution of 1-2% of Al leads to the decrease of T c by about 2-3 K. X-ray and high resolution transmission electron microscopy investigations indicate on the appearance of second precipitation phase in the crystals with x > 0.1. This is in a form of non-superconducting MgAlB 4 domains in the structure of superconducting Mg 1-x Al x B 2 matrix. Resistivity and magnetic investigations show the slight increase of the upper critical field, H c2 , for H//c for the samples with small x, significant reduction of the H c2 anisotropy at lower temperatures, and decrease of the residual resistance ratio value for Al substituted samples as compared to those of unsubstituted crystals. Superconducting gaps variation as a function of Al content, investigated with point contact spectroscopy for the series of the crystals with T c in the range from 20 to 37 K, does not indicate on the merging of the gaps with decreasing T c down to 20 K. It may be related to an appearance of the precipitation phase in the Mg 1-x Al x B 2 structure. 74.70.Ad, 74.62.Dh, 81.10.-h, 74.25.Ha

Systematic study of the superconducting and normal-state properties of neutron-irradiatedMgB2

“…When aluminum substitutes for magnesium, H ab c2 decreases while H ⊥ab c2 remains constant or slightly increases, resulting in an decrease in the anisotropy ratio, γ H =H ab c2 /H ⊥ab c2 . 7,8,9 . These changes can be understood as the result of changes in the Fermi surface topology of the σ band 10 .…”

Strong competition between the δl and δTc flux pinning mechanisms in MgB2 doped with carbon containing compounds

“…19 such as silicone oil and other oxide or nonoxide nanomaterials. [5][6][7][8][9][10][11]19,20 Improvement of the j c ͑B͒ was quite successful for most of chemical doped MgB 2 . However, the origin of the pinning mechanisms are still not well understood except the silicon oil doped case.…”

Influence of carbon on intraband scattering in Mg(B 1-x C x ) 2

“…One of vigorously discussed issues is a role of defects, which can be systematically introduced into the structure of MgB 2 by irradiation [2,3] or partial chemical substitutions (often Al for Mg or C for B) [4,5,6,7]. While some consequences of the substitution or irradiation, such as modification of the upper critical field (H c2 ) or changes in the width of superconducting gaps are rather indubitable, there is still no agreement on microscopic mechanisms leading to these effects.…”