Physicists have recorded an unusual quantum phenomenon for the first time, in which electrons appear to cross forbidden energy gaps instantaneously. Using a new type of slow-motion imaging technique, scientists tracked these extremely rapid movements in graphene and discovered how strong electric fields move electrons from one energy state to another.
The phenomenon can be imagined using an elevator as an example: a person enters the elevator, and the floor indicator jumps directly from 1, 2 and 3 to 20, seemingly skipping all the floors in between. This would appear impossible in everyday life, but electrons can make similar jumps in the quantum world.
The phenomenon is known as Landau-Zener-Majorana transition. Physicists have long suspected that it could play an important role in high-speed semiconductor electronics, but directly observing the process has remained difficult. Scientists from the University of Regensburg, Philipps University of Marburg and Forschungszentrum Jülich have now managed to do so using graphene. The study was published in the journal Nature.
In semiconductors, electrons usually occupy certain energy ranges known as energy bands. Between these bands are forbidden gaps, where electrons cannot normally exist in stable states. However, under the influence of intense light pulses, the electric field accelerates electrons, causing some of them to move from one energy band to another across these gaps.
To record the process, the scientists combined advanced laser technology with electron microscopy. They used photoemission momentum microscopy, which measures the energy and direction of electron movement, and then used the resulting data to create detailed images of their distribution.
The significance of this experiment goes beyond simply recording a new quantum phenomenon. Scientists can now study how electrons change their energy states while they are moving, rather than drawing conclusions based only on measurements taken before and after an experiment. This provides an additional way to understand the processes that limit the controlled movement of electrons in materials.
One of the next major research directions will be controlling electrons with light. If scientists can better understand how a material's crystal lattice interferes with the initially ordered movement, they may be able to control the process more precisely. This could have implications for future electronic technologies in which extremely rapid oscillations of light, rather than conventional electrical voltage, could be used to direct the movement of electrons.
The method may not be limited to graphene. Researchers plan to use it to study other quantum materials as well, making it possible to observe processes that have previously been inaccessible. One of the most intriguing questions is how precisely scientists will be able not only to observe electron behavior but also to control it deliberately.
