CPM Seminar

Strain doping: a new avenue for understanding and controlling materials

Zac Ward

Thin films and Nanostructures Group
Oak Ridge National Laboratory

The strong electronic correlations arising from overlapping spin-charge-orbital-lattice order parameters in complex oxides are of fundamental importance to many desirable characteristics such as metal-insulator transitions, ferroicity, colossal magnetoresistance, and high TC superconductivity. Control over the orbital and lattice parameters in these systems is generally achieved through strain engineering imposed by heteroepitaxial film growth on non-lattice-matched substrates. This method involves perturbation in all three unit cell dimensions since the in-plane strain induced into a film's lattice is accommodated by an elastic reaction along the out-of-plane lattice direction driven by the Poisson effect. This talk will give an overview of our group's recent work in developing low energy, low dose He ion implantation as a means to strain dope thin film materials. Unlike conventional epitaxy-based strain tuning methods, strain doping is an effective means of continuously controlling lattice expansion along the out-of-plane axis while leaving the in-plane axes locked to the substrate [1]. Functionality is then controlled by modifying crystal symmetry [2] and the tuning of orbital polarizations [1,3].

We will focus on recent results of several functional materials and demonstrate how this technique delivers never before possible control over magnetoresistance, multiferroicity, magnetic anisotropy, catalytic selectivity, optical bandgaps, and magnetocaloric response. We will also discuss the implications of the ability to control a single order parameter in how we interrogate theoretical models of correlated materials.

[1]� H.W. Guo, S. Dong, P.D. Rack, J.D. Budai, C. Beekman, Z. Gai, W. Siemons, C.M. Gonzalez, R. Timilsina, A.T. Wong, A. Herklotz, P.C. Snijders, E. Dagotto, and T.Z. Ward, Strain Doping: Reversible Single-Axis Control of a Complex Oxide Lattice via Helium Implantation, Physical Review Letters 114, 256801 (2015).
[2] A. Herklotz, A.T. Wong, T. Meyer, M.D. Biegalski, H.N. Lee, T.Z. Ward, Controlling octahedral rotations in a perovskite via strain doping, Scientific Reports 6, 26491 (2016).
[3] A. Herklotz, S.R. Rus, T.Z. Ward, Continuously Controlled Optical Band Gap in Oxide Semiconductor Thin Films, Nano Letters 16, 1782 (2016).