The first of its kind in the world! He Ruihua's team at Westlake University discovers photocathode quantum materials
Recently, Ruihua He's group in the School of Science at Westlake University, together with research collaborators, has discovered the world's first photo-cathode quantum material with intrinsic coherence, whose properties far exceed those of conventional photo-cathode materials and cannot be explained by existing theories, opening up new horizons for the development of photo-cathode research, applications and fundamental theories.
On March 8, the paper "Anomalous intense coherent secondary photoemission from a perovskite oxide" was published online in Nature ahead of schedule. Caiyun Hong, Wenjun Zou and Pengxu Ran, PhD students at Westlake University, are the co-first authors, and Ruihua He, Associate Professor of Science at Westlake University, is the corresponding author. All experimental and theoretical work was done at Westlake University.
Photographer's lens of the first photocathode quantum material with intrinsic coherence: strontium titanate.
In 1887, the German physicist Hertz accidentally discovered in his experiments that ultraviolet light would produce sparks when irradiated on metal surface electrodes. 1905, Einstein proposed a theoretical explanation of the phenomenon based on the conjecture of the quantization of light. This marked the official opening of the door to quantum mechanics, and for this contribution, Einstein was awarded the Nobel Prize in Physics in 1921. As a result, the "photoelectric effect", which converts "light" into "electricity", and the "photocathode", which can produce this effect "material, officially entered the human vision.
Along with the deepening understanding of the photoelectric effect, people later developed better theories that could explain the basic properties of all photocathode materials, and successfully predicted the then unknown photocathode materials. These photocathode materials are essentially conventional metallic and semiconductor materials, most of which were discovered 60 years ago. They have become the core components of contemporary particle gas pedals, free electron lasers, ultrafast electron microscopes, high-resolution electron spectrometers, and other cutting-edge scientific and technological devices. In addition to being commonly found in laboratories, such highly sophisticated devices are also used in popular life, such as particle gas pedals that have been used to treat cancer, kill bacteria, develop packaging materials, and improve fuel injection into vehicles. Simply put, whether a photocathode material is "good" or not is directly related to the performance of such devices.
However, these conventional photocathode materials have an inherent performance defect - they emit a poorly "coherent" electron beam, meaning that the emission angle of the beam is too large and the electrons therein do not move at a uniform rate. Such "initial" electron beams must rely on a range of materials processes and electrical engineering technologies to enhance their coherence if they are to meet the requirements of cutting-edge science and technology applications, and the introduction of these special processes and supporting technologies has greatly increased the The introduction of these special processes and supporting technologies has greatly increased the complexity, construction requirements, and cost of "electron gun" systems.
Although the photocathode based electron gun technology has developed significantly in recent decades, it has gradually failed to keep pace with the development of related technology applications. Many of the aforementioned cutting-edge technological upgrades call for an order of magnitude improvement in initial electron beam coherence, which can no longer be achieved by general photocathode performance optimization and can only be expected from source innovation at the material and theoretical levels.
He Ruihua's team in the School of Science at Westlake University, which has been working on the physical properties of materials for a long time, has unexpectedly achieved a breakthrough in strontium titanate, a "common" figure in similar physics labs.
Quantum materials, a new class of materials that has emerged in recent years, are known for their complex and versatile properties and rich and diverse functions. Strontium titanate (SrTiO3), which has a chalcogenide structure, is one of the important representatives of such materials. Professor K. A. Muller, the father of strontium titanate, the discoverer of high-temperature superconductivity and Nobel laureate in physics, calls strontium titanate "the fruit fly of solid state physics" because many important solid state physical phenomena were first discovered from this material. These include many phenomena that have not yet been understood.
However, the mainstream of research on oxide quantum materials, led by strontium titanate, has been to study these materials as potential alternatives to silicon-based semiconductors, focusing mainly on their unique electronics-related properties. However, Ruihua He's team found in their experiments that these familiar materials surprisingly carry the same ability to trigger novel photovoltaic effects - it has a key photocathode property that far exceeds that of existing photocathode materials: coherence (see Figure 1 for illustration), thus greatly compensating for the shortcomings of existing photocathode materials.
Figure 1. Comparison of the initial electron beam energy spectra of strontium titanate and other materials. The former has higher initial electron beam coherence, as evidenced by an electron emission kinetic energy dispersion of less than 0.01 eV (a) and a dispersion angle of less than 2° (b), which is an order of magnitude improvement over the approximately 0.5 eV and 20° of ordinary materials.
The most important property of the strontium titanate photocathode is the order of magnitude improvement in the coherence of the initial electron beam it emits compared to other existing photocathodes under similar experimental conditions," noted the anonymous reviewer of the Nature paper. This huge leap in performance allows (one) to obtain an electron beam with intrinsic coherence in its entirety without having to sacrifice the electron beam intensity in order to improve the coherence. This discovery could lead to a paradigm shift in photocathode technology, which has long been plagued by the paradox of (electron gun) electron beams that do not have both high coherence and high beam intensity, (a paradox) that is rooted in the intrinsic incoherence of the initial electron beam."
According to Changxi Zheng, an ultrafast electron microscope expert and co-author of the paper and a researcher in the School of Science at Westlake University, the importance of the team's discovery "lies not in adding a new property to the list of amazing properties of strontium titanate, but in the property itself, which could reopen an extremely important and generally considered mature field of photocathode technology and change many long-established rules of the game. rules of the game."
Photo Designer: Lin Chen
Scientific exploration often touches new sparks by accident. Why did Ruihua He's team make a new discovery on a "common" material? This is due to a powerful experimental tool that is rarely used in photocathode research: angle-resolved photoelectron spectroscopy.
Previously, because most conventional photocathode materials with high performance had polycrystalline or amorphous structures on their surfaces, the dominant research approach in the field of photocathodes relied primarily on photocurrent detection, an experimental tool that has been in use since 135 years ago. This has also rendered useless a large class of newly developed experimental tools for the study of single-crystal quantum materials, including angle-resolved photoelectron spectroscopy.
The essence of this technique is the photoelectron effect, which is the working principle of angle-resolved photoelectron spectroscopy. It is used to probe the electronic structure of a material, i.e., to understand how electrons move through the material. In the past few decades, angular-resolution photoelectron spectroscopy has been used to study the part of the electronic structure that is related to the optical, electrical and thermal properties of materials. Driven by this strong scientific interest, most of the existing experimental facilities have been configured and optimized for electronic structure measurements in the relevant energy region.
Who would have thought that this technique, which uses the principle of the photoelectric effect, could "attack the shield with the spear of the son" and uncover new physics in the photoelectric effect - in the experiment, He Ruihua's team at Westlake University used this quantum material research tool derived from the photoelectric effect In the experiment, He Ruihua's team used this quantum material research tool derived from the photoelectric effect to unexpectedly capture the unique photoelectric emission properties of single-crystal quantum materials.
By using an "unconventional" configuration of the angle-resolved photoelectron spectrometer to measure the electronic structure in the unconventional energy region associated with the photoelectric effect, they found that the superior photocathode properties of strontium titanate are due to its unique photoemission properties (Figure 2), which are clearly different from all known photocathode materials. It can be argued that they exceed the expectations of established photoelectron emission theories in almost every major respect.
Figure 2. The difference between the initial electron beams emitted by the ordinary photocathode material (a) and the photocathode quantum material strontium titanate (b).
The above findings of the WSU team were theoretically confirmed by Arun Bansil, an authority on angle-resolved photoelectron spectroscopy theory and co-author of the paper and professor at Northeastern University, who stated, "[This finding] suggests that something very fundamental is missing from our complete understanding of the physical processes associated with the photoelectric effect, and that this missing element could be the element that unlocks the entire door to a family of photocathode quantum materials (that) have unique photocathode properties not possessed by existing materials."
The discovery is often just the first step into the vast ocean of the unknown. After the exciting discovery, the He Ruihua lab immediately plunged into the next step of exploration.
According to Caiyun Hong, the first author of this achievement and a 2019 PhD student in the School of Science at Westlake University, they will next further develop their research work on strontium titanate materials in terms of theory and application.
On the theoretical side, since the existing theory fails, it means that a new theory needs to be established to explain the observed photocathode properties of strontium titanate. Ruihua He gave a very bold conjecture on this, working with the Bansil group to propose a completely new photoelectric emission mechanism. Following this new theory, they have predicted a large class of candidate photocathode quantum materials dominated by this new mechanism, and the experimental team is planning to verify each of these material predictions.
In terms of applications, since strontium titanate material performs better than even existing photocathode materials, the team also plans to collaborate with teams in related fields to explore the practical applications of this material.
On his personal profile page of Westlake University, Ruihua He wrote his wish for this school: "I hope that Westlake University will become an adventurer's paradise with a unique positioning that encourages cross-discipline and bold innovation". In fact, the discovery of strontium titanate, the first photocathode quantum material, blossomed out of a years-long immersive "adventure" that he led his team on.
Originally, one of the "small" research projects in the lab was to study the escape work of quantum materials (note: in the photoelectric effect, electrons leap off the surface of a material at a certain energy "cost", i.e., escape work). When they were measuring the fugitive work of each material in bulk with a "high throughput" approach by relying on the ultra-high vacuum interconnection system of the Material Science Platform, they stumbled upon something "different" about strontium titanate and caught the "accident". "This led to the subsequent discoveries.
Interestingly, the discovery of the "willow" in Ruihua He's lab seems to echo the starting point of mankind's accidental "encounter" with the photoelectric effect - 1887 In 1887, Hertz, in order to prove Maxwell's electromagnetic wave prediction, conducted a spark discharge experiment, and stumbled upon this amazing phenomenon.
Explore where no one has gone before. The scientists of Westlake, who love "adventure", will further explore more mysteries of photocathode materials.
https://www.nature.com/articles/s41586-023-05900-4
