Ramp-Reversal Memory and Phase-Boundary Scarring in Transition Metal Oxides.

Transition metal oxides are complex electronic systems that exhibit a multitude of collective phenomena. Two archetypal examples are VO2 and NdNiO3 , which undergo a metal-insulator phase transition (MIT), the origin of which is still under debate. Here this study reports the discovery of a memory effect in both systems, manifested through an increase of resistance at a specific temperature, which is set by reversing the temperature ramp from heating to cooling during the MIT. The characteristics of this ramp-reversal memory effect do not coincide with any previously reported history or memory effects in manganites, electron-glass or magnetic systems. From a broad range of experimental features, supported by theoretical modelling, it is found that the main ingredients for the effect to arise are the spatial phase separation of metallic and insulating regions during the MIT and the coupling of lattice strain to the local transition temperature of the phase transition. We conclude that the emergent memory effect originates from phase boundaries at the reversal temperature leaving "scars" in the underlying lattice structure, giving rise to a local increase in the transition temperature. The universality and robustness of the effect shed new light on the MIT in complex oxides.

Visualizing short-range charge transfer at the interfaces between ferromagnetic and superconducting oxides.

The interplay between antagonistic superconductivity and ferromagnetism has been a interesting playground to explore the interaction between competing ground states. Although this effect in systems of conventional superconductors is better understood, the framework of the proximity effect at complex-oxide-based superconductor/ferromagnet interfaces is not so clear. The main difficulty originates from the lack of experimental tools capable of probing the interfaces directly with high spatial resolution. Here we harness cross-sectional scanning tunnelling microscopy and spectroscopy together with atomic-resolution electron microscopy to understand the buried interfaces between cuprate and manganite layers. The results show that the fundamental length scale of the electronic evolution between YBa2Cu3O(7-δ) (YBCO) and La2/3Ca1/3MnO3 (LCMO) is confined to the subnanometre range. Our findings provide a complete and direct microscopic picture of the electronic transition across the YBCO/LCMO interfaces, which is an important step towards understanding the competition between ferromagnetism and superconductivity in complex-oxide heterostructures.