Abstract
The last decade has witnessed explosive growth of research on various oxide heterostructures, resulting in the discovery of exciting interfacial phenomena. However, it remains elusive hitherto to control and subsequently measure oxygen sub-lattice in oxide thin films/heterostructures that plays a pivotal role in the structure-property affairs. Specifically, the crystallographic mismatch, violation of local charge neutrality and the difference in chemical potentials between the constituents belonging to thin films/heterostrutures, strongly modify the atomic/electronic structure by – (i) electronic and/or atomic reconstruction (ii) formation of oxygen and/or cation vacancies as well as (iii) large atomic displacements. In this talk, I will present our recent results on ultrathin cuprate layers that undergo profound change in the oxygen sub-lattice structure giving rise to novel structural and electronic properties. In the first half, I will discuss about the infinite-layer SrCuO2, which in the bulk-form, has each Cu2+ ion coordinated by four planar O2- ions. However, for ultrathin layers, we find atransformation from bulk-planar to chain-type structure having apical oxygen via atomic rearrangement [1,2]. This phenomena turns out to be a direct consequence of electrostatic instability associated with the polar nature of SrCuO2. We illustrate that the geometry of CuO4 plaquette in polar cuprates can precisely be controlled from in-plane to out-of-plane upon reducing the film thickness at the sub-unit cell level and this opens possibilities to design structures that have specific functions, e.g. current-carrying layers, charge reservoirs and scaffolding layers. In the second half, I will discuss about CuO, which unlike other 3d transition metal monoxides is found in a low symmetry distorted monoclinic structure rather than rocksalt structure. We demonstrate direct evidence of strain-induced structural transformation from monoclinic to tetragonal phase for ultrathin CuO films grown on SrTiO3 [3]. The experimental confirmation of the high symmetry structure of CuO opens up new avenues to explore electronic and magnetic properties in the context of high Tcsuperconductivity.References:1. D. Samal, G. Koster, et al., Phys. Rev. Lett., 111, 096102 (2013)2. B. Kuiper, D. Samal, G. Koster, et al., Control of oxygen sub-lattice structure in ultrathinSrCuO2 films studied by X-ray photoelectron diffraction (APL materials, 2013 in press)3. D. Samal, G. Koster, et al., Direct structural and spectroscopic investigation of ultrathinfilms of tetragonal CuO: Six-fold co-ordinated copper (Under review)
