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

M. Pissas, G. C. Papavassiliou+6 more|arXiv (Cornell University)|Nov 19, 2001

Superconductivity in MgB2 and Alloys3 citations

TL;DR

This study uses 11B NMR and magnetization measurements to investigate Al-doped Mg1-xAlxB2 superconductors, revealing that Al doping reduces magnetic anisotropy in the vortex state by decreasing σ-band hole carriers. The key finding is that the anisotropy parameter γ decreases with increasing Al content, providing direct experimental evidence that σ-band hole-driven superconductivity is central to MgB2's high Tc.

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 $σ$-band hole driven superconductivity for MgB$_2$.

Motivation & Objective

• To investigate the magnetic field distribution in the vortex state of lightly Al-doped Mg1-xAlxB2 superconductors.
• To determine how Al doping affects the anisotropy of the superconducting state and the vortex lattice structure.
• To correlate changes in NMR line shapes with electronic structure modifications, particularly in the σ-band hole density.
• To provide experimental evidence for the role of σ-band hole carriers in the high-Tc superconductivity of MgB2.
• To examine the onset of phase separation and its impact on NMR response at x ≥ 0.025.

Proposed method

• Performed 11B NMR spectroscopy on polycrystalline Mg1-xAlxB2 samples (x ≤ 0.025) at 5 K and applied magnetic fields of 23.5 and 47 kOe.
• Measured magnetization using a SQUID magnetometer to determine Tc and Hc2 anisotropy as a function of Al content.
• Used synchrotron X-ray diffraction to confirm phase purity and detect phase separation at x ≥ 0.025.
• Analyzed NMR line shapes to extract magnetic field distributions, distinguishing between vortex state and normal state contributions.
• Applied the Gorkov anisotropy parameter formula γ² = ⟨Δ(kF)vab²⟩ / ⟨Δ(kF)vc²⟩ to interpret changes in anisotropy.
• Compared NMR spectra at 300 K (normal state) and 5 K (mixed state) to isolate vortex-induced frequency shifts and broadening.

Experimental results

Research questions

• RQ1 How does Al doping affect the magnetic field distribution in the vortex state of Mg1-xAlxB2?
• RQ2 What is the relationship between Al doping concentration and the anisotropy of Hc2 in Mg1-xAlxB2?
• RQ3 How does the reduction of σ-band hole carriers due to Al doping influence the superconducting gap anisotropy?
• RQ4 At what doping level does phase separation begin, and how does it affect the NMR response?
• RQ5 Can NMR line shape measurements provide direct experimental evidence for the role of σ-band holes in MgB2 superconductivity?

Key findings

• At T = 5 K, the NMR line shape for x = 0.005 shows a complete suppression of the normal state signal, indicating full vortex state occupation.
• The NMR line shape for pure MgB2 (x = 0) exhibits asymmetric broadening due to vortex lattice formation, with a peak frequency shift of ~50 Gauss from H₀.
• For x = 0.01 at 47 kOe, the anisotropy parameter γ is estimated to be between 3.2 and 6.4, indicating strong anisotropy in the vortex state.
• Al doping reduces the anisotropy of the magnetic field distribution, with a clear dependence on x, indicating decreased γ with increasing Al content.
• The observed decrease in anisotropy is attributed to electron filling of the σ-bands, reducing the density of σ-band holes.
• Phase separation is detected at x = 0.025 via XRD, where the (002) peak splits, and this is reflected in a drastic change in NMR line shape, suggesting onset of inhomogeneity.