New Light on a Famous Insulator: Photoinduced Polaronic Conduction in Anatase

Stoichiometric anatase, one of the three polymorphs of TiO₂, is a band insulator with a 3.2-eV gap recently proposed for several applications, ranging from photovoltaic cells to memristors to transparent conducting panels. Working on both single crystals and epitaxial films grown in situ with the new pulsed-laser deposition facility available on Beamline 7.0.1 (now moved to Beamline 7.0.2), researchers have found that the conduction band of an undoped sample can also be populated with carriers simply by exposure to a beam of UV photons.

Using angle-resolved photoemission spectroscopy (ARPES), the group has shown that the number of conduction electrons in anatase, as well as their degree of correlation, can be patterned by exposure to UV light in a controllable and reversible way. The band structure generated by the photoinduced doping revealed that, at low densities, the electrons are dressed by lattice vibrations and form composite particles named polarons, which means that they behave as if they had a much larger mass. At higher densities, the coherence of the quasiparticles is lost and the system gradually turns into a weakly correlated Fermi liquid with nearly-free electrons. Therefore, the material can be controllably carried into different conduction regimes without using any extrinsic dopant.