A magnetization and $^{11}$B NMR study of Mg$_{1-x}$Al$_x$B$_2$ superconductors

TL;DR

This study uses $^{11}$B NMR to investigate the magnetic field distribution in lightly Al-doped MgB$_2$ superconductors, revealing doping-dependent anisotropy changes that support the $\sigma$-band hole-driven superconductivity model.

Contribution

First demonstration of magnetic field distribution in lightly doped Mg$_{1-x}$Al$_x$B$_2$ using $^{11}$B NMR, showing doping-dependent anisotropy reduction.

Findings

Magnetic field distribution varies with Al doping.

Anisotropy decreases as doping increases.

Results support $\sigma$-band hole-driven superconductivity.

Abstract

We demonstrate for the first time the magnetic field distribution of the pure vortex state in lightly doped Mg$_{1-x}$Al$_x$B$_2$ ($x\leq 0.025$) powder samples, by using $^{11}$B NMR in magnetic fields of 23.5 and 47 kOe. The magnetic field distribution at T=5 K is Al-doping dependent, revealing a considerable decrease of anisotropy in respect to pure MgB$_2$. This result correlates nicely with magnetization measurements and is consistent with $\sigma$-band hole driven superconductivity for MgB$_2$.