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A copper wire hit by a laser is like a kitchen where the chef turns on the stove and watches the stopwatch at the same time. Copper atoms start to dance, losing the order of the crystal lattice, while electrons, like waiters in the dining room, instantly feel the change. But how much disorder do they need to notice? It turns out, just the very beginning of melting is enough.

Scientists fired single laser pulses of varying intensity at thin copper films. A special setup measured how terahertz radiation, something between a radio wave and infrared light, passed through the metal in the first picoseconds after the flash, which are trillionths of a second. From this data, they calculated conductivity: copper's ability to carry electric current. Before melting, electrons constantly bumped into boundaries between microscopic grains of the metal, like runners on a track with hurdles. But when copper began to melt precisely at these boundaries, the hurdles disappeared, and the current briefly became stronger.

Computer simulations confirmed that atoms and electrons here cannot act independently. Melting is not just the crystal thawing; it's an instant rewiring of the paths for electric current. The laser pulse that was supposed to destroy everything momentarily makes the metal more conductive. That same chef, by turning on the stove, inadvertently opens a shortcut through the kitchen for the waiters.

Now it's clear: ultrafast optical measurements can show the exact moment melting begins. This matters for laser material processing and for understanding how matter behaves under extreme conditions.

Original abstract

as written by the authors arXiv:2604.15491

Ultrafast melting is fundamentally a structural transition of the ionic lattice, but this rearrangement also reshapes the electronic properties by changing the energy landscape and scattering mechanisms. Although the electrons react almost instantaneously, it is not a priori clear how much lattice disorder is required for a significant response. Here, we show that the onset of melting already produces a clear electronic signature in polycrystalline copper. Using single-shot terahertz time-domain spectroscopy on thin films excited over a wide range of laser fluences, we infer the transient conductivity during the first picoseconds after excitation. The data, supported by two-temperature molecular-dynamics simulations, show that before melting, electron transport is substantially limited by grain-boundary scattering and that melting strongly suppresses this channel. As melting begins at these interfaces, we observe a transient increase in the conductivity that directly marks the onset of the phase transition. More broadly, these results show that ionic and electronic relaxation stages are closely coupled in nonequilibrium laser-driven matter and that optical measurements can resolve distinct stages of melting.