Strain Doping: Reversible Single-Axis Control of a Complex Oxide Lattice via Helium Implantation

Abstract

We report on the use of helium ion implantation to independently control the out-of-plane lattice constant in epitaxial La 0.7 Sr 0.3 MnO 3 thin films without changing the in-plane lattice constants. The process is reversible by a vacuum anneal. Resistance and magnetization measurements show that even a small increase in the out-of-plane lattice constant of less than 1% can shift the metal-insulator transition and Curie temperatures by more than 100°C. Unlike conventional epitaxy-based strain tuning methods which are constrained not only by the Poisson effect but by the limited set of available substrates, the present study shows that strain can be independently and continuously controlled along a single axis. This permits novel control over orbital populations through Jahn-Teller effects, as shown by Monte Carlo simulations on a double-exchange model. The ability to reversibly control a single lattice parameter substantially broadens the phase space for experimental exploration of predictive models and leads to new possibilities for control over materials' functional properties. The crystal lattice is one of the most accessible degrees of freedom in materials. In complex oxides, effective control over lattice parameters not only facilitates the understanding of multiple interactions in strongly correlated systems, but also creates new phases and emergent functionalities [1][2][3][4]. Lattice engineering has played an extremely important role in attempts to design strongly correlated systems and has led to many important discoveries [1,[5][6][7][8]. Control over lattice strain in films using different substrates [9] has revealed enhanced ferroelectricity [10] and superconductivity [11], as well as induced superconductivity in otherwise nonsuperconducting compounds [12]. However, the basic nature of the broken translational symmetry in the crystal lattice also entails a rigidity against arbitrary control [13]. There is so far no experimental technique that allows one to alter the lattice parameter solely along a single crystal axis, i.e., with an effective Poisson's ratio of zero. For strain engineering in systems with a nonzero Poisson ratio, the lattice constant, and hence the electronic structure, necessarily change in all three directions, clouding the cause-effect relations between single degrees of freedom and order parameters.We demonstrate an approach using helium implantation to effectively "strain dope" the lattice along a single axis of a La 0.7 Sr 0.3 MnO 3 (LSMO) film that is epitaxially latticelocked to a substrate. The out-of-plane (c-axis) lattice constant can be modified independently of the in-plane lattice constants. The c-axis strain can be continuously manipulated, and is thus not restricted by the limited collection of substrates that dictate conventional epitaxial strain engineering. The change in materials' properties, while reversible via a high temperature anneal, is persistent even well above room temperature. No continuous external actuation is required as with transient pressure-induc...

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