Anomalous intense coherent secondary photoemission from a perovskite oxide – Nature

Photocathodes—materials that convert photons into electrons using the photoelectric effect—are a critical foundation for many modern technologies that rely on light detection or electron-beam generation^1,2,3. Currently existing photocathodes, however, are based on conventional metals and semiconductors that were mostly discovered six decades ago with sound theoretical underpinnings^4,5. Progress in this mature field has been limited to refinements in photocathode performance based on sophisticated materials engineering^1,6. Here we report unusual photoemission properties of a reconstructed surface of SrTiO₃(100) single crystals prepared by simple vacuum annealing that go beyond the existing theoretical descriptions^4,8,7-10. Unlike other positive-electron-affinity (PEA) photocathodes, our PEA SrTiO₃ surface produces discrete secondary photoemission spectra at room temperature, characteristic of the efficient negative-electron-affinity photocathode materials^11,12. At low temperatures, the photoemission peak intensity is enhanced substantially, and the electron beam obtained upon non-threshold excitations displays longitudinal and transverse coherence that shatters known records by at least an order of magnitude^6,13,14. 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. SrTiO₃ thus presents the first example of a fundamentally new class of photocathode quantum materials, opening new prospects for applications that require intense coherent electron beams without the need for monochromatic excitations, electron filtering or beam acceleration.