Groundbreaking discoveries in materials science challenge the current understanding of photoemission

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What exactly is light – and what is it made of? It’s an age-old question, dating back to ancient times, and one of the most important investigations scientists have undertaken to understand the nature of reality.

The question of what constitutes light – a form of energy that allows us to see the world as it bounces off objects – has sparked such lively debate and discussion in the scientific community that it has spawned a whole new field : Quantum Mechanics.

Underlying the debate about the nature of light is another mystery. That is, does light behave like a wave or like a particle? When Albert Einstein proposed in the early 20th century that light is both particulate (having small particles called photons) and wavy, many were pleased, if slightly disturbed, by his results.

Einstein supported his new theory with his work on the so-called photoelectric effect, which earned him the 1921 Nobel Prize in Physics. The photoelectric effect, first discovered by Heinrich Rudolf Hertz in 1887, describes the process by which light releases electrons from a material when it is illuminated.

Photoemission is now the leading experimental approach used by researchers to study the chemical and electronic properties of materials, and has led to practical applications for a range of technologies, particularly those that depend on light detection or electron beam generation, including medical imaging devices and semiconductor manufacturing others.

However, researchers from the Northeast have made a discovery that challenges our knowledge of how photoemission is supposed to work and lays the groundwork for a new understanding of how light interacts with materials.

In a publication in Nature On March 8, researchers observed what they termed “unusual photoemission properties” of a particular material, strontium titanate — an oxide of the chemical element pair that first became popular more than half a century ago, primarily as a diamond simulant.

Experimentally, the researchers used strontium titanate as a photocathode, or an engineered surface capable of converting light into electrons through the photoelectric effect.

Photocathodes are also used in photodetectors or sensory devices such as photomultipliers; They are also used in infrared viewers, streak cameras, image intensifiers – or image intensifiers – and image converters.

Strontium titanate has historically been overlooked as a potential photocathode candidate, says Arun Bansil, a distinguished physics professor at Northeastern University who co-authored the study.

“This material has many other uses and applications,” says Bansil.

By using multiple photon energies in the 10 eV (electron volt) range, the researchers were able to produce “a very intense coherent secondary photoemission” that is stronger than anything seen before, says Bansil.

“This is a big deal because there is no mechanism in our existing understanding of photoemission that can produce such an effect,” says Bansil. “In other words, we don’t currently have a theory for it, so it’s a miraculous breakthrough in that sense.”

Secondary electron emission describes a phenomenon in which the detached primary electrons have suffered energy loss due to collisions within the material before being ejected.

“If you excite electrons, some of those electrons will actually come out of the solid,” says Bansil. “Primary electrons refer to those that have not been scattered, while secondary electrons mean they have experienced collisions before getting out of the solid.”

The research team, which included scientists from Westlake University in China, Lappeenranta-Lahti University of Technology (LUT) in Finland and the Northeast, said such a result points to “underlying novel processes” that are still understood.

“The observed occurrence of coherence in secondary photoemission indicates the evolution of an underlying novel process that was developed in addition to the processes contained in the current theoretical photoemission framework,” the researchers wrote.

According to Bansil, the results turn on its head what scientists thought they knew about the photoemission process and open the door to new applications across industries that would harness the power of these advanced quantum materials.

“We all thought we understood the basic physics that we’re talking about here, to the point where the development of applications follows a certain paradigm of theory and thought,” says Bansil. “As nature often does, this paper throws a curveball at all of this.”

More information:
Caiyun Hong et al., Anomalous Intense Coherent Secondary Photoemission from a Perovskite Oxide, Nature (2023). DOI: 10.1038/s41586-023-05900-4

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