Scientists in Germany have observed electrons crossing an energy gap in graphene with up to 100% probability, showing how strong electric fields can drive quantum transitions in real time.
Researchers from the University of Regensburg, Philipps University of Marburg and Forschungszentrum Jülich used a new imaging technique that combines ultrafast lasers with electron microscopy to track the movement of electrons in the atomically thin material.
The experiments captured a process known as Landau-Zener-Majorana transitions, in which electrons move between energy bands without occupying the forbidden energy states that separate them.
The results could help scientists understand the behavior of electrons under strong electric fields and develop new ways to control electronic materials using light.
Track electrons in real time
In solids, electrons occupy specific energy ranges, so-called bands. These bands can be separated by energy gaps where electrons cannot normally exist.
However, quantum mechanics allows electrons to move between bands through a process called tunneling. The probability of such a transition depends on how quickly an electron approaches the energy gap.
To observe the process, the researchers combined extremely short laser pulses with a photoemission pulse microscope.
Unlike traditional microscopes, which produce magnified images of physical objects, the instrument measures the energies and movements of electrons to map their distribution within a material.
The laser pulses created strong, rapidly changing electric fields that accelerated electrons through graphene’s electronic energy structure.
“If you want to know who won a race, you have to watch all the participants,” said Vincent Eggers, the study’s lead author.
“Similarly, we need to observe as many electrons as possible at the same time in order to catch those that undergo a quantum mechanical transition during their movement.”
The measurements showed that under the experimental conditions, electrons were able to overcome the energy gap with a tunneling probability of 100%.
The result shows that transitions between separate energy states can become very likely if electrons are driven fast enough.
Light-driven quantum control
The team used specially prepared graphene samples produced at the Jülich Research Center.
Graphene consists of a single layer of carbon atoms arranged in a hexagonal structure. Its electronic properties make it useful for studying how electrons respond to external electric fields.
For the experiments, the researchers used exceptionally pure samples, including twisted graphene structures, which made it easier to observe quantum dynamics.
“These precision experiments require material that is almost perfect at the atomic level. This ensures that impurities do not affect the sensitive quantum dynamics of the electrons,” says Stefan Tautz from Forschungszentrum Jülich.
The technique also allowed researchers to study how interactions between electrons and the crystal lattice disrupt their originally coordinated motion.
Understanding these effects could help determine the physical limits of controlling electrons with rapidly oscillating light fields.
Such control is being explored as a possible path toward electronics that operate faster than devices based on traditional voltage switching.
The researchers said their approach could also be applied to other quantum materials, allowing scientists to study electronic behavior that has previously been difficult to observe directly.
“Our experiment opens a panoramic window into the quantum world. Now we can track in real time how electrons move through the energy landscape of a material. And that’s just the beginning,” said Giacomo Inzani from the University of Regensburg.
The study appears in Nature.
