Orth Peter P, Fernandes Rafael M, Walter Jeff, Leighton C, Shklovskii B I
School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA.
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, USA.
Phys Rev Lett. 2017 Mar 10;118(10):106801. doi: 10.1103/PhysRevLett.118.106801. Epub 2017 Mar 7.
Stimulated by experimental advances in electrolyte gating methods, we investigate theoretically percolation in thin films of inhomogeneous complex oxides, such as La_{1-x}Sr_{x}CoO_{3} (LSCO), induced by a combination of bulk chemical and surface electrostatic doping. Using numerical and analytical methods, we identify two mechanisms that describe how bulk dopants reduce the amount of electrostatic surface charge required to reach percolation: (i) bulk-assisted surface percolation and (ii) surface-assisted bulk percolation. We show that the critical surface charge strongly depends on the film thickness when the film is close to the chemical percolation threshold. In particular, thin films can be driven across the percolation transition by modest surface charge densities. If percolation is associated with the onset of ferromagnetism, as in LSCO, we further demonstrate that the presence of critical magnetic clusters extending from the film surface into the bulk results in considerable enhancement of the saturation magnetization, with pronounced experimental consequences. These results should significantly guide experimental work seeking to verify gate-induced percolation transitions in such materials.
