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A copper wire under current is a familiar sight, but inside it something unexpected is happening. A strong electric current doesn't just heat the metal: it knocks atoms out of their rightful places, leaving behind tiny voids, as if a few balls were suddenly removed from a tightly packed arrangement. These voids exist only while the current flows, and after it's switched off, they vanish within minutes.

Scientists measured this using a beam of positrons — antiparticles of electrons that are especially sensitive to empty spots in the crystal lattice. When a positron enters such a void, it lives longer than usual, and by measuring its lifetime, one can infer the size and number of defects. It turns out that at currents above a certain threshold, a huge number of vacancies, empty lattice sites where atoms once stood, appear in copper. Their number is a million times greater than what would be expected from simple heating to 352 degrees Celsius.

This behavior explains a long-standing mystery of the so-called 'flash state' in materials: when a short, powerful current pulse is passed through ceramics or metals, the material's properties change abruptly, which cannot be explained by heating alone. Now it's clear that the current creates defects that accelerate sintering and densification processes. Understanding this mechanism opens the way to more precise control over material properties — from electronics to ceramics manufacturing.

Original abstract

as written by the authors arXiv:2604.21283

Whether the flash state in electrically driven solids involves non-equilibrium defect production or is accounted for by Joule heating alone has been debated since 2010. Using positron annihilation spectroscopy on copper, we observe a fully reversible, electrically switchable vacancy population: the DBS S-parameter rises above baseline whenever applied current exceeds a critical density and returns on current removal. Positron lifetime spectroscopy independently confirms open-volume defect formation and reveals a void to cluster relaxation hierarchy. The current-induced vacancy concentration exceeds the thermal-equilibrium value at 352C by > 106x, is present only while current is applied, and vanishes within minutes. The nucleation rate scales steeply with the applied current, connecting the minute-scale kinetics resolved here to the sub-second flash events observed in ceramic sintering. These results demonstrate current-induced Frenkel-pair production in a metal and identify a defect-mediated, non-equilibrium contribution to the flash state.