Oxygen Displacement in Cuprates under Ionic Liquid Field-Effect Gating

Guy Dubuis,Anthony T Bollinger,Yizhak Yacoby,Hua Zhou,Ivan Božović,Xi He,Davor Pavuna,Ron Pindak

doi:10.1038/srep32378

Abstract

We studied structural changes in a 5 unit cell thick La1.96Sr0.04CuO4 film, epitaxially grown on a LaSrAlO4 substrate with a single unit cell buffer layer, when ultra-high electric fields were induced in the film by applying a gate voltage between the film (ground) and an ionic liquid in contact with it. Measuring the diffraction intensity along the substrate-defined Bragg rods and analyzing the results using a phase retrieval method we obtained the three-dimensional electron density in the film, buffer layer, and topmost atomic layers of the substrate under different applied gate voltages. The main structural observations were: (i) there were no structural changes when the voltage was negative, holes were injected into the film making it more metallic and screening the electric field; (ii) when the voltage was positive, the film was depleted of holes becoming more insulating, the electric field extended throughout the film, the partial surface monolayer became disordered, and equatorial oxygen atoms were displaced towards the surface; (iii) the changes in surface disorder and the oxygen displacements were both reversed when a negative voltage was applied; and (iv) the c-axis lattice constant of the film did not change in spite of the displacement of equatorial oxygen atoms.

Highlights

Soon after the discovery of High-Temperature Superconductivity (HTS) in cuprates[1], it was proposed[2] that electrostatic field could be used to control HTS at the sample surface
If this is not the case, the response of the buffer layer will dominate, and hide any induced change in the top HTS layer. This calls for sample engineering at an atomic-layer level that can be achieved by Atomic-Layer-by-Layer Molecular Beam Epitaxy (ALL-MBE)[5], which provides HTS films with atomically sharp surfaces and interfaces[6,7,8]
The specific LSCO film studied in this experiment was an ALL-MBE grown film with its active part comprised of a 5 UC thick layer of La1.96Sr0.04CuO4, heavily underdoped so that it was neither metallic nor superconducting

Summary

Introduction

Soon after the discovery of High-Temperature Superconductivity (HTS) in cuprates[1], it was proposed[2] that electrostatic field could be used to control HTS at the sample surface. The screening length is extremely short, just a few Ångstroms, meaning that the field-induced modulation of carrier density will largely be confined to the surface layer just one unit cell (1 UC) thick This imposes very stringent requirements on the HTS samples. If this is not the case, the response of the buffer layer will dominate, and hide any induced change in the top HTS layer This calls for sample engineering at an atomic-layer level that can be achieved by Atomic-Layer-by-Layer Molecular Beam Epitaxy (ALL-MBE)[5], which provides HTS films with atomically sharp surfaces and interfaces[6,7,8]. Okuyama et al disputed this, rather ascribing the observations to a deformation of the crystal lattice of VO2 throughout the entire film volume[26] This scientific disagreement shows that much still needs to be understood about the effects of electrolyte gating on complex oxides

Methods

Results

Conclusion