Comment on "Superconducting anisotropy and evidence for intrinsic pinning in single crystalline MgB2"

TL;DR

This paper critiques a previous study on MgB2's anisotropy and pinning, showing that the earlier conclusions about field independence and intrinsic pinning are not fully supported by the data.

Contribution

It challenges prior claims by analyzing the validity of the theoretical model and presenting new torque measurements to clarify the nature of pinning in MgB2.

Findings

The theoretical expression used has limited validity.

The anisotropy is not field independent as previously claimed.

The observed peak in torque is extrinsic, not intrinsic.

Abstract

In a recent paper [Phys. Rev. B 66, 012501 (2002)], torque data measured on a MgB2 single crystal in fields from 10 to 60 kOe at 10 K were presented. The authors obtained the anisotropy gamma by fitting a theoretical expression to the data and concluded that the anisotropy is field independent gamma \approx 4.3. They also reported the observation of "intrinsic pinning", which they take as experimental evidence for the occurence of superconductivity in the boron layers. In this comment, we discuss the range of validity of the theoretical expression used by the authors and show that the conclusion of a field independent anisotropy does not hold. Furthermore, we present torque data measured on two crystals of MgB2, establishing the extrinsic nature of the peak in the irreversible torque observed in some crystals.