Crystal structure effects on vortex dynamics in superconducting MgB$_2$ thin films

TL;DR

This study examines how microstructural defects in MgB$_2$ thin films influence vortex dynamics and resistive transitions, combining experimental measurements with simulations to reveal the roles of pinning and interface effects.

Contribution

It provides new insights into how microstructure and interface quality affect resistive transitions and vortex behavior in MgB$_2$ superconducting films.

Findings

Buffer-layer roughness increases pinning energies and raises the current for superconductivity breakdown.

Simulations suggest resistive transitions are mediated by normal domain growth, not flux-flow instabilities.

Microstructure and interface properties critically influence dissipation at high currents.

Abstract

The current-driven resistive transition is central to superconducting single-photon detectors, transition-edge sensors, and fluxonic devices. Depending on sample uniformity, dimensions, and heat removal, it can be driven by phase-slip events, flux-flow instabilities (FFI), or normal-domain formation. Here, we investigate the influence of two types of microstructural defects on vortex dynamics in MgB$_2$ films: columnar growth in textured films and buffer-layer roughness in single-crystal films. The current-voltage ($I$-$V$) curves measured at $T \approx 0.25 T_\mathrm{c}$ for both films exhibit multiple steps. Time-dependent Ginzburg-Landau simulations reproduce the major features of the experimental $I$-$V$ curves and suggest that the resistive transitions for both films are mediated by the formation and growth of normal domains rather than FFI. The single-crystal film with buffer-layer roughness exhibits superconductivity breakdown at higher currents and pinning activation energies approximately twice those of the textured film, along with more pronounced multi-step features in the $I$-$V$ curves. These features are attributed to the combination of stronger pinning induced by lateral variations of the superconducting order parameter along the MgO buffer layer and its lower thermal boundary resistance. Our results show that both the film microstructure and the film-buffer interface are critical for the resistive transition, offering insights for superconducting devices requiring controlled dissipation at high transport currents.