Controlled Manipulation of Intermediate State in a Type-I Superconductor

TL;DR

This study visualizes and actively controls flux structures in a type-I superconductor, revealing dynamic reorganization and hysteresis, and enabling potential flux-based device applications.

Contribution

We demonstrate direct imaging and manipulation of flux patterns in a type-I superconductor, including controlled reconfiguration and phase transitions driven by magnetic tips and AC currents.

Findings

Flux morphology evolves from tubes to stripes during penetration and expulsion.

Magnetic tips can drag, merge, and reconfigure flux structures.

A reversible stripe-grid-stripe transition occurs under AC excitation.

Abstract

The intermediate state of type-I superconductors presents a classic paradigm of modulated pattern formation, arising from the competition between short-range attractive and long-range repulsive vortex-vortex interactions. However, direct visualization and, more importantly, active control over the topology and dynamics of these flux structures have remained significant challenges, limiting our ability to manipulate them for fundamental studies and potential applications. Here, using low-temperature magnetic force microscopy, we achieve direct imaging and controllable manipulation of the flux structures in a high-purity tantalum single crystal. We systematically track the evolution of flux morphology - from tubes to stripes - during flux penetration and expulsion, revealing a pronounced topological hysteresis originating from the geometric barrier. Furthermore, we demonstrate precise local control by using the magnetic tip to drag and merge individual flux tubes and to reconfigure entire stripe domains. Under global alternating current (AC) excitation, we discover a reversible stripe-grid-stripe transition, a dynamic reorganization driven by current-induced flux penetration and pinning effects. The corresponding phase diagram shows that the threshold current decreases with magnetic field but increases with AC frequency. Our work establishes a pathway for active flux manipulation in type-I superconductors, revealing rich dynamics and paving the way for flux-based superconducting devices.