Another mystery underpins the debate about the nature of light. Is light a particle or a wave?

In the early 20th century, Albert Einstein proposed that light is both particle and wave-shaped. Many were pleased, if somewhat uneasy, with his findings.

Einstein supported his new 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.

Photoemission is now the most widely used experimental technique to study the chemical and electronic properties of materials. It has provided useful applications for various technologies, especially those that rely on light sensing or electron beam generation, such as semiconductor manufacturing and medical imaging equipment.

But Northeast researchers have made a discovery that challenges our understanding of 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 few chemical elements that first came into widespread use more than half a century ago, primarily as a diamond simulant.

Strontium titanate was used experimentally by scientists as a photocathode or engineered surface that can convert light into electrons via the photoelectric effect. They then used different photon energies in the 10 eV (electron volt) range to produce a “highly intense coherent secondary photoemission.” The photo emission was stronger than ever before.

Arun Bansil, distinguished professor of physics at Northeastern, who co-authored the study, said: “This is a big problem because within our existing understanding of photoemission there is no mechanism that can cause such an effect. In other words, we currently have no theory on this, so in that sense it is a miraculous breakthrough.”

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

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

Scientists noted, “such a result points to “underlying new processes” that are not yet understood.”

“The observed emergence of coherence in secondary photoemission points to the development of an underlying new process superimposed on those included in the current theoretical photoemission framework.”

Bansil says, “the results undermine what scientists thought they knew about the photoemission process, opening the door to new applications in various industries that would harness the power of these advanced quantum materials.”

“We all thought we understood the basic physics involved, to the point where application development follows a particular paradigm of theory and thinking. As nature often does, this paper throws all this into confusion.”

Magazine 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