Researchers have directly tracked electrons leaping across forbidden energy gaps in graphene, a quantum phenomenon known as Landau-Zener-Majorana transitions, using a technique that combines laser technology with electron microscopy. The findings, published in Nature, provide a detailed look at this process and could open new avenues for light-controlled electronics.
In semiconductors and similar materials, electrons are restricted to particular energy bands separated by forbidden energy gaps, where they cannot ordinarily occupy stable states. Under normal conditions, electrons tend to remain within their existing bands. When graphene was exposed to intense pulses of light, the electric field accelerated some electrons across the gap through a quantum mechanical transition.
The team from the University of Regensburg, Philipps-Universität Marburg, and Forschungszentrum Jülich used a photoemission momentum microscope to map electron energies and how they were moving as intense light pulses accelerated electrons in atom-thin graphene layers.
"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: “Our team worked on this experiment for many years. Now, we can track electrons moving across the entire energy landscape of a solid in real time.”
Ulrich Hoefer, a collaborator and pioneer in ultrafast electron spectroscopy, described the excitement of witnessing the effect firsthand. "Being able to observe such a fundamental quantum mechanical process during our first measurement campaign was a very special moment for us - it was also made possible by the samples we used."
The experiment depended on exceptionally pure graphene samples manufactured at Forschungszentrum Juelich. Stefan Tautz from the centre said: "These precision experiments require material that is nearly perfect at the atomic level. This ensures that impurities do not obscure the delicate quantum dynamics of the electrons. Another important factor for the study was the use of 'rotated' graphene, a specialty from Juelich, which allows the quantum dynamics to emerge particularly clearly."
When exposed to sufficiently strong light fields, electrons crossed the forbidden energy gap with a transition probability reaching 100 percent. The crucial factor is how quickly the electrons approach the gap under the electric field, making the process fundamentally different from conventional quantum tunneling in physical space.
Beyond revealing a fundamental quantum phenomenon, the new imaging technique could help scientists understand what prevents electrons from moving in a controlled and predictable way. Giacomo Inzani from the University of Regensburg explained: "Our experiment opens a panoramic window into the quantum world. Now, we can follow in real time how electrons move through the energy landscape of a material. And this is just the beginning. Our method can be applied to many other quantum materials, opening up completely new possibilities for controlling their electronic properties selectively with light."
Analysis
Why This Matters
- The ability to observe electrons crossing energy gaps in real time could lead to faster electronic devices that use light to control electron flow rather than voltage.
- The technique can be applied to other quantum materials, potentially accelerating the development of new technologies.
- Understanding electron dynamics at these scales could help overcome limitations in current semiconductor electronics.
Background
Electrons in solids occupy discrete energy bands separated by forbidden gaps. Landau-Zener-Majorana transitions describe how particles can tunnel between bands when driven by a strong field. This phenomenon has been studied theoretically but direct observation was elusive until now. Graphene, a single layer of carbon atoms, provided an ideal platform due to its purity and unique electronic properties.
What to Watch
- Further experiments applying the technique to other materials like transition metal dichalcogenides or topological insulators.
- Potential integration of this imaging method into semiconductor research and development for ultra-fast electronics.
- Development of light-controlled electronic switches based on the observed transition.
Checked
12 claims in this story checked against 1 source: 12 supported.
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- SupportedPhysicists observed electrons making seemingly impossible quantum leaps across energy gaps in graphene.Latest Science News -- ScienceDaily: “Physicists have captured an extraordinary quantum phenomenon in which electrons appear to leap across energy gaps that would normally be off limits.”
- SupportedA new slow-motion electron camera captured Landau-Zener-Majorana transitions in real time.Latest Science News -- ScienceDaily: “Using a new type of slow-motion electron camera, researchers were able to watch these incredibly fast movements in graphene”
- SupportedThe experimental technique combines laser technology with electron microscopy.Latest Science News -- ScienceDaily: “the scientists developed an experimental technique that combines advanced laser technology with electron microscopy.”
- SupportedThe researchers were from the University of Regensburg, Philipps-Universität Marburg, and Forschungszentrum Jülich.Latest Science News -- ScienceDaily: “researchers from the University of Regensburg, Philipps-Universität Marburg, and Forschungszentrum Jülich have directly tracked these transitions in graphene”
- SupportedThe findings were published in Nature.Latest Science News -- ScienceDaily: “Their findings, published in Nature , provide a detailed look at how electrons respond to intense electric fields generated by light.”
- SupportedVincent Eggers said, “If you want to know who has won a race, you have to watch all participants,” and that many electrons must be observed at once to catch those undergoing a quantum mechanical transition while moving.Latest Science News -- ScienceDaily: “"If you want to know who has won a race, you have to watch all participants," explains 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."”
- SupportedRupert Huber said the team had worked on the experiment for many years and could now track electrons across a solid’s energy landscape in real time.Latest Science News -- ScienceDaily: “"Our team worked on this experiment for many years. Now, we can track electrons moving across the entire energy landscape of a solid in real time," says Rupert Huber.”
- SupportedUlrich Hoefer described observing the process during the first measurement campaign as a special moment, also made possible by the samples.Latest Science News -- ScienceDaily: “"Being able to observe such a fundamental quantum mechanical process during our first measurement campaign was a very special moment for us - it was also made possible by the samples we used."”
- SupportedStefan Tautz said the material needed to be nearly perfect at the atomic level so impurities would not obscure electron dynamics.Latest Science News -- ScienceDaily: “"These precision experiments require material that is nearly perfect at the atomic level. This ensures that impurities do not obscure the delicate quantum dynamics of the electrons.”
- SupportedWith sufficiently strong light fields, the transition probability for electrons crossing the forbidden energy gap reached 100 percent.Latest Science News -- ScienceDaily: “When exposed to sufficiently strong light fields, electrons crossed the forbidden energy gap with a transition probability reaching 100 percent.”
- SupportedThe process differs fundamentally from conventional quantum tunneling in physical space.Latest Science News -- ScienceDaily: “This makes the process fundamentally different from conventional quantum tunneling, in which particles pass through a barrier in physical space.”
- SupportedGiacomo Inzani said the experiment opens a panoramic window into the quantum world and that the method can be applied to other quantum materials.Latest Science News -- ScienceDaily: “"Our experiment opens a panoramic window into the quantum world. Now, we can follow in real time how electrons move through the energy landscape of a material. And this is just the beginning. Our method can be applied to many other quantum materials”