A light pulse redirects electrons in an ultrathin layered material, creating a stable new state without heat or damage and suggesting a low energy route to faster electronics.
(Nanowerk Spotlight) Most attempts to control the electronic behavior of materials depend on relatively blunt tools: raising the temperature, applying pressure, or changing the chemical makeup. These methods can work, but they operate on slow timescales and come with trade-offs in stability, efficiency, and energy cost.
Researchers are exploring an alternative that works on a different principle. Instead of pushing on the atoms that make up a material, they are asking whether it is possible to change the material’s state by redirecting the flow of its electrons. If the internal movement of charge can be guided with precision, then switching could happen far faster and with far less waste.
That idea has been difficult to test. In most materials, when electrons are jolted out of place, they quickly settle back to their original positions, leaving no lasting change behind. Only a small class of layered quantum materials hints at something different.
These systems contain separate regions where electrons behave in distinct ways. When a sudden disturbance forces electrons across the boundary between those regions, the structure of the material’s electronic state may reorganize in a way that does not immediately vanish. The question is whether that reorganization can be made predictable, stable, and measurable in real time.
By delivering an ultrafast pulse of light and then tracking how charge moves between those regions, the researchers show that the direction of electron flow alone is enough to trigger a new electronic phase. Even after the initial pulse fades, this phase remains in place, revealing a route to phase control driven only by electrons and not by heat, pressure, or chemistry.
A brief pulse of light forces electrons to flow from the surface of a thin bismuth selenide film into its interior, then reverses that flow, creating a new long-lasting electronic state. The chart shows how the film’s conductivity changes over time as this shift takes place. (Image: Reproduced from DOI:10.1002/advs.202507289, CC BY) (click on image to enlarge)
The material under study, bismuth selenide, is a topological insulator. A topological insulator conducts electricity along its surface but not through its bulk, giving it two channels through which electrons can move. The surface channel is known as a topological surface state. The interior contains a thin layer of electrons called a confined bulk state. These channels are normally distinct but are close enough to exchange charge when pushed out of equilibrium. In this system, the researchers see an opportunity to observe how electrons behave when they cross the boundary between the interior and the surface.
To probe this behavior, the team uses a two-step experimental setup. First, they trigger the material with a short pulse of light lasting about sixty femtoseconds, which excites electrons and drives them out of their equilibrium arrangement. Second, they use terahertz radiation to monitor changes in conductivity in real time.
Terahertz radiation is sensitive to how freely electrons are moving, and it can distinguish between contributions from the surface and the interior. By varying the timing between the light pulse and the terahertz probe, the researchers capture a sequence of measurements that reveal how the system evolves from the initial disturbance toward a new configuration.
Before the pulse, the terahertz signal shows two clear components. The surface channel has a sharp response with long scattering times, indicating that electrons can move there with relatively few collisions. The bulk channel has a broader response with shorter scattering times, showing that electrons there encounter more obstacles. These baseline signals allow the researchers to calculate two key quantities. One is carrier density, which measures how many electrons are free to move. The other is chemical potential, which tells where the electrons sit in energy.
After the light pulse, the signals change. In the first one to three picoseconds, the conductivity of the surface channel decreases, while the bulk channel increases. The calculations show that electrons have flowed from the surface into the interior. During this stage, the surface chemical potential falls, and the bulk chemical potential rises, both moving toward a new balance. Because the system is still close to equilibrium, the researchers use the same model as before to track these changes.
A second stage appears after about ten picoseconds. The surface channel begins to recover and then grows stronger than before. At the same time, the bulk channel falls below its initial value. The chemical potentials move in opposite directions. The surface climbs to a higher energy level than at the start, while the bulk falls below its original position. The direction of electron flow has reversed.
The authors attribute this behavior to the formation of excitons at the interface. An exciton is a pair formed by an electron and a hole bound together by attraction. In this case, the hole sits in the surface state, and the electron sits in the bulk, creating a spatially indirect exciton that changes how charge flows across the interface.
This new phase is notable for how long it lasts. The altered surface and bulk signals persist for more than six hundred picoseconds, far longer than the few picoseconds that usually describe electron relaxation in similar materials. The persistence shows that the system has entered a distinct state driven by electron flow and not just a transient response to excitation.
The researchers also identify a threshold for the effect. When the energy of the light pulse is below about 18 microjoules per square centimeter, the system returns to equilibrium without forming a second phase. Above that threshold, the effect appears reliably, and the changes grow stronger with increasing pulse energy.
Sample thickness also matters. In films thinner than five atomic layers, the surface states on the top and bottom merge and lose their separate character. In these samples, no second phase is observed, even when the pulse energy is high. This absence confirms that a distinct surface state and a separate bulk state are required to form the new excitonic phase. Without a clear interface, electron flow cannot build up enough contrast to create the new arrangement.
The authors check their results against possible artifacts. They show that the signals measured in the terahertz range come from free electrons and not from vibrations or changes in temperature. The timescales involved match known limits for electron movement and do not align with slower processes like heating. The behavior of the chemical potentials and the consistency across different measurements show that the phase change is real.
This study provides a concrete example of how electron flow across an internal boundary can drive a material into a new state that remains even after the driving pulse is gone. The transition depends not on the number of electrons added, but on the path they take when pushed out of equilibrium. This approach points toward forms of phase control that use small amounts of energy and produce changes on ultrafast timescales, which could be useful in future electronic or optical systems.
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Byung Cheol Park (Sungkyunkwan University)
, 0000-0001-5309-0685 corresponding author, first author
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