Caiyun Hong, Wenjun Zou, Pengxu Ran, K. Tanaka, M. Matzelle, Wei-Chi Chiu,   R. S. Markiewicz, B. Barbiellini, Changxi Zheng, Sheng Li, Arun Bansil, and Rui-Hua He

Nature 617, 493–498 (2023)

Photocathodes—materials that convert photons into electrons through a phenomenon known as the photoelectric effect—are important for many modern technologies that rely on light detection or electron-beam generation. However, current photocathodes are based on conventional metals and semiconductors that were mostly discovered six decades ago with sound theoretical underpinnings. Progress in this field has been limited to refinements in photocathode performance based on sophisticated materials engineering. Here we report unusual photoemission properties of the reconstructed surface of single crystals of the perovskite oxide SrTiO3(100), which were prepared by simple vacuum annealing. These properties are different from the existing theoretical descriptions. In contrast to other photocathodes with a positive electron affinity, our SrTiO3 surface produces, at room temperature, discrete secondary photoemission spectra, which are characteristic of efficient photocathode materials with a negative electron affinity. At low temperatures, the photoemission peak intensity is enhanced substantially and the electron beam obtained from non-threshold excitations shows longitudinal and transverse coherence that differs from previous results by at least an order of magnitude. The observed emergence of coherence in secondary photoemission points to the development of a previously undescribed underlying process in addition to those of the current theoretical photoemission framework. SrTiO3 is an example of a fundamentally new class of photocathode quantum materials that could be used for applications that require intense coherent electron beams, without the need for monochromatic excitations.