YBCO superconductors have mobile oxygen atoms in their Cu-O chain layers. Apply a current, and the oxygen migrates — electromigration selectively rearranges the oxygen sublattice, locally tuning the superconducting properties. This is the basis for memristive devices: the resistance state depends on the oxygen configuration, which depends on the current history.
The paper maps the threshold current for oxygen migration as a function of pulse duration, from 200 nanoseconds to 1 millisecond. The finding: below approximately 10 microseconds, the threshold current rises sharply. Shorter pulses require disproportionately more current to initiate migration.
The mechanism separates into thermal and non-thermal components. Long pulses heat the film — the current raises the temperature, the temperature activates the oxygen mobility, and the migration is thermally assisted. Temperature modeling shows that shorter pulses generate proportionally less heat, because the thermal diffusion time exceeds the pulse duration. Below 10 microseconds, the pulse ends before the heat has time to build up.
In this short-pulse regime, electromigration becomes increasingly athermal. The driving force is the electric field itself — the “electron wind” force — rather than thermally activated hopping. The threshold increases because the athermal mechanism is less efficient than the thermal one; you need a stronger push when you can't wait for temperature to help.
The practical implication: pulse engineering controls not just whether migration occurs but which mechanism drives it. Long pulses are thermal, short pulses are ballistic. Same physical process, different physics, selected by timing.