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

TL;DR

This study investigates how aluminum doping affects the critical magnetic fields and superconducting gap values in magnesium diboride single crystals, revealing decreases in critical fields and anisotropy with increased doping.

Contribution

It provides the first detailed analysis of the doping dependence of critical fields and superconducting gaps in MgB$_2$ using specific heat and magnetization measurements.

Findings

Both $H_{c1}$ and $H_{c2}$ decrease with Al doping.

Anisotropy parameter $ ightarrow$ decreases from ~5 to ~1.5 with doping.

Superconducting gaps evolve due to band filling and interband scattering.

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 $\Gamma_{H_{c2}}(0) = H^{ab}_{c2}(0)/H^c_{c2}(0)$ value also decreases from $\sim 5$ in pure MgB$_2$ samples down to $\sim 1.5$ for $x \gtrsim 0.2$ whereas $\Gamma_{H_{c1}}(0)=H^c_{c1}(0)/H^{ab}_{c1}(0)$ 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.