Influence of Al doping on the critical fields and gap values in magnesium diboride single crystals

Abstract

The lower ($H_{c1}$) and upper ($H_{c2}$) critical fields of Mg${}_{1-x}$Al${}_x$B${}_2$ single crystals (for $x=0$, 0.1 and $\gtrsim 0.2$) have been deduced from specific heat and local magnetization measurements, respectively. We show that $H_{c1}$ and $H_{c2}$ are both decreasing with increasing doping content. The corresponding anisotropy parameter ${\Gamma }_{H_{c2}}(0)=H_{c2}^{ab}(0)/H_{c2}^c(0)$ value also decreases from $\sim 5$ in pure MgB${}_2$ samples down to $\sim 1.5$ for $x\gtrsim 0.2$ whereas ${\Gamma }_{H_{c1}}(0)=H_{c1}^c(0)/H_{c1}^{ab}(0)$ remains on the order of 1 in all samples. The small and large gap values have been obtained by fitting the temperature dependence of the zero field electronic contribution to the specific heat to the two gap model for the three Al concentrations. Very similar values have also been obtained by point contact spectroscopy measurements. The evolution of those gaps with Al concentration suggests that both band filling and interband scattering effects are present.

References in corpus (2)

• Ginzburg-Landau theory of vortices in a multi-gap superconductor
• Band filling and interband scattering effects in MgB${}_2$: C vs Al doping

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• de Haas-van Alphen effect investigations of the electronic structure of pure and aluminum-doped MgB_2
• Theory of vortices in hybridized ballistic/diffusive-band superconductors
• Features of Excess Conductivity Behavior in a Magnetic Superconductor Dy${}_{0.6}$Y${}_{0.4}$Rh${}_{3.85}$Ru${}_{0.15}$B${}_4$
• Abrikosov flux-lines in two-band superconductors with mixed dimensionality
• Two-bands superconductivity with intra- and interband pairing for synthetic superlattices