Slow-Motion Electron Camera Catches Graphene Electrons Leaping Energy Gaps With Up to 100% Odds
A Nature study from German teams films light-driven quantum jumps that skip the space in between, like an elevator going from floor 3 to floor 20.
Physicists in Germany have filmed electrons making a quantum leap that looks impossible: crossing a forbidden energy gap without passing through the states in between. The team used a new slow-motion electron camera and watched the jumps happen in graphene as intense light fields pushed the electrons around.
The researchers, from the University of Regensburg, Philipps-Universität Marburg and Forschungszentrum Jülich, report the work in the journal Nature. They compare the effect to an elevator whose floor indicator jumps from 1, 2 and 3 straight to 20. You never passed through the 17 floors in between, yet you arrive on the 20th. In the quantum world, that is allowed. The processes are called Landau-Zener-Majorana transitions, and they have long been suspected of mattering more as semiconductor electronics get faster.
In a semiconductor and related materials, electrons can only occupy certain energy ranges, called bands, which are separated by a forbidden gap. Normally electrons stay in their bands. During a Landau-Zener-Majorana transition they appear to overcome the gap with ease.
To watch that happen, the team combined laser technology with electron microscopy. They used a photoemission momentum microscope, which does not produce a magnified picture of an object. Instead it records the energies and states of motion of electrons and builds a map of their distribution. The hard part was pairing that with strong, extremely fast oscillating light fields that set the electrons moving on purpose.
"If you want to know who has won a race, you have to watch all participants," said Vincent Eggers, first author of the study. "Likewise, we must observe as many electrons as possible at one time to catch those that undergo a quantum mechanical transition while moving."
Rupert Huber said the team worked on the experiment for many years and can now follow electrons across the whole energy landscape of a solid in real time. Ulrich Höfer, a pioneer of ultrafast electron spectroscopy, said seeing such a fundamental process in the first measurement campaign was a very special moment.
The samples mattered. The measurements used tailor-made, atomically thin carbon layers of very high purity, produced at Forschungszentrum Jülich. Stefan Tautz of Jülich said the work needs material that is nearly perfect at the atomic level so impurities do not hide the delicate dynamics. The team also used "rotated" graphene, a Jülich specialty, which lets the quantum behavior show up especially clearly.
The slow-motion images showed that under strong light fields, electrons crossed the band gap with a tunneling probability of up to 100%. That means the transitions can happen with high probability even when the energy states involved are clearly separated.
The result gives researchers a direct view of a process that has long been suspected to matter for high-speed semiconductor electronics.
