Strain engineering has been demonstrated to be an effective knob to tune the bandgap in perovskite oxides, which is highly desired for applications in optics, optoelectronics, and ferroelectric photovoltaics. Multiferroic BiFeO3 exhibits great potential in photovoltaic applications and its bandgap engineering is of great interest. However, the mechanism of strain induced bandgap engineering in BiFeO3 remains elusive to date. Here, we perform in situ ellipsometry measurements to investigate the bandgap evolution as a function of uniaxial strain on freestanding BiFeO3 films. Exotic anisotropic bandgap engineering has been observed, where the bandgap increases (decreases) by applying uniaxial tensile strain along the pseudocubic [100]p ([110]p) direction. First-principles calculations indicate that different O6 octahedral rotations under strain are responsible for this phenomenon. Our work demonstrates that the extreme freedom in tuning the strain and symmetry of freestanding films opens a new fertile playground for novel strain-driven phases in transition metal oxides.

No abstract

No abstract

Charge transfer in perovskite oxide heterostructures could break the delicate balance among charge, spin, orbital and lattice order at the interface, producing exotic phenomena that cannot be observed in bulk materials. Here, opposite interfacial charge transfer directions are observed in SrIrO3/NdNiO3 and SrIrO3/LaNiO3 3d/5d perovskite heterostructures. This is accompanied with an inverse change of Ni eg orbital polarization and Ni-O pd hybridization across the interface, by stretching/compressing the out-of-plane Ni-O bond in the opposite internal electrical field due to the opposite electron transfer direction. These interfacial reconstructions finally bring about a manipulation on the transport and magnetic characteristics. This work reveals that A site cation in perovskite heterostructures could be a knob to control the interfacial charge transfer direction, and the 3d/5d perovskite interfaces are excellent platform to study the complex interplay between various order parameters and stimulate novel interfacial effects.