Anomalous intense coherent secondary photoemission from a perovskite oxide.

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 SrTiO 3 (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 SrTiO 3 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. SrTiO 3 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.