Effect of doping on irreversible Campbell length in MgB$_2$ single crystals

TL;DR

This study investigates how doping MgB$_2$ single crystals with C, Li, and (Li+C) affects the irreversible Campbell length and peak effect signatures in rf penetration depth, revealing doping-dependent shifts in the H-T phase diagram.

Contribution

It provides the first detailed analysis of doping effects on the irreversible Campbell length and peak effect in MgB$_2$ single crystals, linking doping to changes in magnetic hysteresis behavior.

Findings

Carbon doping enhances the peak effect and shifts it to higher temperatures.

Li doping suppresses the peak effect, moving it to lower temperatures.

Hysteresis is attributed to macroscopic screening supercurrents during magnetic field changes.

Abstract

We have measured small-amplitude rf penetration depth $\lambda(H,T)$ in pure and C, Li and (Li+C) doped single crystals of MgB$_2$. The effect of doping on the critical temperature T$_c$ and on the upper critical field H$_{c2}$ was found to be in good agreement with previous results. We report the presence of clear signatures of irreversibility in $\lambda(H,T)$, associated with the peak effect. Carbon doping enhances the signature and shifts its position on the H-T phase diagram to higher temperatures. Contrary to this, Li substitution has the effect of suppressing the peak effect to lower temperatures. Analysis of both zero field (ZFC) and field cooled (FC) measurements suggests that the hysteresis associated with the peak effect is due to macroscopic screening supercurrents, $j$, generated during the magnetic field ramp.