Breakthrough discovery challenges current understanding of photoemission

It is a miraculous breakthrough.

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Another mystery underlies the discussion on the nature of light. Namely, is light a particle or a wave? 

In the early 20th century, Albert Einstein proposed that light is both particulate in nature and wave-like. Many were satisfied, if slightly uneasy, about his findings.

Einstein supported his novel idea through his research on the so-called photoelectric effect, for which he was awarded the Nobel Prize in Physics in 1921. The photoelectric effect, first described by Heinrich Rudolf Hertz in 1887, is the mechanism by which light causes electrons to be expelled from a material when shone on it.

Photoemission is now the most widely used experimental technique for studying materials’ chemical and electronic properties. It has produced useful applications for various technologies, particularly those that rely on light detection or electron-beam generation, such as semiconductor manufacturing and medical imaging devices.

But Northeastern researchers have made a discovery that challenges what we know about how photoemission is supposed to work. In a new study, they observed the “unusual photoemission properties” of a material called strontium titanate. Strontium titanate is an oxide of a pair of chemical elements that first came into widespread use more than half a century ago, primarily as a diamond simulant.

Strontium titanate was employed experimentally by scientists as a photocathode or an engineered surface that can transform light into electrons via the photoelectric effect. They then used several photon energies in the 10 eV (electron-volt) range to produce a “very intense coherent secondary photoemission.” The photoemission was stronger than anything seen before.

Arun Bansil, distinguished professor of physics at Northeastern, who co-authored the study, said, “This is a big deal because there is no mechanism within our existing understanding of photoemission that can produce such an effect. In other words, currently, we don’t have any theory for this, so it is a miraculous breakthrough in that sense.”

A secondary electron emission is a phenomenon in which the ejected primary electrons have lost energy due to collisions with other particles inside the substance.

Bansil says, “When you excite electrons, some of these electrons will come out of the solid. Primary electrons refer to those which have not scattered, whereas secondary electrons mean they have undergone collisions before they’ve come out of the solid.”

Scientists noted, “such a result points to “underlying novel processes” yet understood.”

“The observed emergence of coherence in secondary photoemission points to the development of an underlying novel process on top of those encompassed in the current theoretical photoemission framework.”

Bansil says“the results upend what scientists thought they knew about the photoemission process, opening the door for new applications across industries that would harness the power of these sophisticated quantum materials.” 

“We all thought we understood the basic physics involved here, to the point where the development of applications is pursuant to a certain paradigm of theory and thought. As nature often does, this is where this paper throws a curveball at all of this.”

Journal Reference:

  1. Hong, C., Zou, W., Ran, P. et al. Anomalous intense coherent secondary photoemission from a perovskite oxide. Nature (2023). DOI: 10.1038/s41586-023-05900-4
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