Effects of magnetic and non-magnetic doping on the vortex lattice in MgB$_2$

TL;DR

This study investigates how magnetic and non-magnetic doping affect the vortex lattice in MgB$_2$, revealing that carbon doping suppresses lattice rotation transitions while manganese doping preserves them, with implications for vortex behavior and electronic structure.

Contribution

It provides the first detailed comparison of vortex lattice responses in MgB$_2$ doped with magnetic versus non-magnetic elements, highlighting the impact on vortex transitions and disorder.

Findings

C-doping suppresses vortex lattice rotation transition

Mn-doping preserves vortex lattice phase diagram

Vortex lattice disordering is not linked to increased pinning

Abstract

Using small-angle neutron scattering we have studied the vortex lattice in superconducting MgB$_2$ with the magnetic field applied along the $c$-axis, doped with either manganese or carbon to achieve a similar suppression of the critical temperature. For Mn-doping, the vortex lattice phase diagram remains qualitatively similar to that of pure MgB$_2$, undergoing a field-and temperature-driven $30^{\circ}$ rotation transition, indicating only a modest effect on the vortex-vortex interaction. In contrast, the vortex lattice rotation transition is completely suppressed in the C-doped case, likely due to a change in the electronic structure which affects the two-band/two-gap nature of superconductivity in MgB2. The vortex lattice longitudinal correlation length shows the opposite behavior, remaining roughly unchanged between pure and C-doped MgB$_2$ while it is significantly reduced in the Mn-doped case. However, the extensive vortex lattice metastability and related activated behavior, observed in conjunction with the vortex lattice transition in pure MgB$_2$, is also seen in the Mn doped sample. This shows that the vortex lattice disordering is not associated with a substantially increased vortex pinning.