The Observation of Bipolarons on the Anatase TiO₂(001) surface

The polaron is a fermionic quasi-particle formed by a conduction electron repelling the negative ions and attracting the positive ions in an ionic crystal or a polar semiconductor. Among the polarizable examples of metal oxides, anatase TiO₂ and SrTiO₃ have been mostly studied. Here we epitaxially grow anatase TiO₂(001) thin films on the SrTiO₃(001) substrate with pulsed laser deposition method. The anatase TiO₂(001) surface terminal is demonstrated to be the (1×4) reconstruction by scanning tunneling microscopy and LEED pattern. Subsequent in-situ angle-resolved photoemission measurements demonstrate that the two-dimensional electron gas, polarons and bipolarons can be available on this surface at different charge carrier densities. The strong electron-phonon coupling is found on this surface. Our findings reveal the 2D nature of electron gas and polarons on this surface and provide the essential role of the phonon-related bipolarons in understanding the electronic structure of anatase TiO₂(001) surface.