A thin film of neodymium nickelate, fifteen nanometers thick, sits on a neodymium gallate substrate. Below one hundred eighty kelvin it behaves as an insulator: electrons are locked inside their atomic cells and cannot move freely. But hit it with a laser pulse lasting thirty femtoseconds, that is, thirty millionths of a billionth of a second, and the film starts conducting current like a metal. Meanwhile the crystal lattice, meaning the order in which the atoms are arranged in space, does not have time to rearrange.

Usually the insulator-to-metal transition in such materials goes hand in hand with a change in crystal symmetry. At low temperature the atoms are packed in a pattern called monoclinic: it resembles a brick wall where the bricks are slightly slanted. When heated above one hundred eighty kelvin, the lattice takes on a more regular orthorhombic form, as if the bricks had straightened out to right angles. It was thought that conductivity and symmetry change together, like two wheels of the same cart.

To see this, the scientists measured two parameters at the same time. The first was the reflection of light from the film: it drops when the material becomes a metal. The second was second-harmonic generation, that is, the doubling of the light frequency upon reflection. This signal is very sensitive to how symmetrically the atoms are arranged on the surface. It turned out that the reflection changes within 45 femtoseconds, while the symmetry changes much more slowly. Even after 225 femtoseconds the lattice does not manage to fully rearrange.

With a weak laser pulse at an energy density of 0.64 millijoules per square centimeter, the reflection reaches the level of a metal, while the second-harmonic signal almost completely recovers. This means that the film conducts current, but its crystal lattice has remained monoclinic. Such a state had not been observed before: a metal with the "wrong" symmetry of an insulator.

To fully restore the high-temperature orthorhombic symmetry, a far more powerful pulse is needed: 2.3 millijoules per square centimeter. Only then does the lattice "straighten out." It follows that a small nudge is enough for the transition to a metal, while rearranging the atomic pattern takes several times more.

This discovery changes our picture of how conductivity and symmetry are related in complex oxides. It used to be thought that broken symmetry was a lock keeping electrons imprisoned. Now it is clear that the lock can be opened without changing the shape of the door. A metallic state can exist in a monoclinic lattice, which means that symmetry is not a mandatory condition for an insulator.

There is practical value too. If a material can be switched into a conducting state by light without waiting for the lattice to rearrange, then such switching happens faster and requires less energy. This could be useful in devices that need ultrafast control of current, for example, in the optical switches of the future.