Percolation via Combined Electrostatic and Chemical Doping in Complex Oxide Films

Abstract

Stimulated by experimental advances in electrolyte gating methods, we investigate theoretically percolation in thin films of inhomogenous complex oxides, such as La1−xSrxCoO3 (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.

Giant electrostatic modification of magnetism via electrolyte-gate-induced cluster percolation in La1−xSrxCoWalter1, Charlton2, Ambaye3 et al. 2018Phys. Rev. MaterialsSelf Cite

“…This result further validates our recent percolation theory, occurring due to gatemediated connection of existing finite clusters that penetrate the film thickness, i.e., surfaceassisted bulk percolation (Fig. 1(a)) [40]. Importantly, this demonstrates that electrostatic modulation of magnetism need not be confined to extreme surface regions.…”

“…While transport evidence for gate-induced percolation to an FM state is strong, in operando PNR was also performed, seeking confirmation of long-range FM, as well as the depth-profile of the induced magnetization, M. The latter is important, as our recent theory predicts anomalously deep penetration of M, due to surface-gating-mediated connection of finite clusters in the film interior [40]. Fig.…”

“…1(a)) and then electrostatically gate across the transition, potentially generating anomalously large increases in TC, magnetization, and conductivity. This combined bulk chemical and surface electrostatic doping was considered in our recent percolation theory [40], resulting in Fig. 1(b).…”

Electrochemically Triggered Metal–Insulator Transition between VO2 and V2O5

“…In this paper, we report the electrochemical triggering of a reversible phase transition between the conductive dioxide VO 2 and the insulating pentoxide V 2 O 5 , accompanied by a reversible metal–insulator transition. This phase transition was obtained by adding/removing oxygen ions via the application of an electrochemical bias . This is an electrochemically driven phase transition, similar to our previous study of the SrCoO x system, that can be switched electrochemically between the layered SrCoO 2.5 brownmillerite and the SrCoO 3 perovskite phase, by application of appropriate electrochemical potentials …”

The essential role of magnetic frustration in the phase diagrams of doped cobaltites

“…2 this is reflected in a sharp upturn in magnetization (blue, right axis) and F volume fraction (black, right axis). This percolation transition can also be controlled with voltage in electrolyte-gated LSCO [32,33]. A final important composition on the phase diagram is at x ≈ 0.22 (vertical line, Fig.…”