Over a century after Albert Einstein received the Nobel Prize for his discovery of the photoelectric effect, a team of researchers has made a significant breakthrough. Scientists from China, Japan, and the United States have published a paper in the peer-reviewed journal Nature, which could lead to a new quantum revolution in physics.
The photoelectric effect was discovered in 1905 by Einstein, who posited that light comprises discrete packets, or "energy quanta," known as photons. Einstein predicted that photons above a certain threshold frequency could eject electrons from a specific material's surface. This discovery contradicted the widely accepted wave theory of light and resulted in the 20th-century quantum revolution in physics, leading to Einstein receiving the 1921 Nobel Prize in Physics.The discovery of the photoelectric effect has laid the foundation for modern-day technologies that depend on light detection or electron-beam generation. High-energy electron beams are used extensively to analyze crystal structures, treat cancer, kill bacteria, and machine alloys.However, most photocathodes known today were discovered around 60 years ago and have a defect. The electrons these photocathodes generate are dispersed in angle and speed.
This is where the recent breakthrough comes in. The team of researchers led by He Ruihua of Westlake University in China's eastern Zhejiang province has used a new material called strontium titanate (SrTiO3) to acquire a concentrated beam of electrons with a level of energy enhanced by at least an order of magnitude. Strontium titanate (SrTiO3) is a quantum material with a diverse set of interesting properties. According to He's team, electron beams obtained after exciting SrTiO3 are coherent. "Coherence is important to the beam; it concentrates the flow like a pipe on the tap. Without the pipe, water will spray everywhere when the tap is wide open. Without coherence, electrons will scatter," said Hong Caiyun, an author of the paper."With the coherence we acquired, we can increase the beam intensity while the beam could maintain its direction," Hong further said. Also, the intensity of photoemission from SrTiO3 is greatly enhanced, according to the team."This exceptional performance suggests novel physics beyond the well-established theoretical framework for photoemission," Hong said.
The discovery has driven the team to find a new theory to explain unparalleled coherence. "We came up with an explanation as a supplement to Einstein's original theoretical framework. It's in another paper which is under review right now," Professor He said.
Co-author of the paper Arun Bansil of Northeastern University in the US has hailed the discovery. "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, we don't have any theory for this, currently, so it is a miraculous breakthrough in that sense," Bansil said.According to Hong, the new theory predicts the existence of an entire class of materials with the same photoemissive properties as SrTiO3.
"SrTiO3 presents the first example of a fundamentally new class of photocathode quantum materials. It opens new prospects for applications that require intense electron beams," she said.
Professor He said the discovery came from their focus on a traditional technology called angle-resolved photoemission spectroscopy (ARPES). ARPES is widely used to study electron structures in solid materials, usually crystalline solids. It measures the kinetic energy and emission angle distributions of the emitted photoelectrons.“In the past few decades, physics and material scientists mainly used ARPES to study the electronic structures related to the optical, electrical, and thermal properties. Our team adapted an unconventional configuration of ARPES and measured another part that’s more related to the photoelectric effect,” He said. “During the test, we found the unusual photoemission properties of SrTiO3. Previously, quantum oxide materials represented by strontium titanate were mainly studied as substitutes for semiconductors and are currently used in the fields of electronics and photocatalysis. “The material will definitely be promising in the field of photocathode in the future.”