Growth and transport properties of topological insulator Bi2Se3 thin film on a ferromagnetic insulating substrate

Exchange coupling between topological insulator and ferromagnetic insulator through proximity effect is strongly attractive for both fundamental physics and technological applications. Here we report a comprehensive investigation on the growth behaviors of prototype topological insulator Bi2Se3 thin film on a single-crystalline LaCoO3 thin film on SrTiO3 substrate, which is a strain-induced ferromagnetic insulator. Different from the growth on other substrates, the Bi2Se3 films with highest quality on LaCoO3 favor a relatively low substrate temperature during growth. As a result, an inverse dependence of carrier mobility with the substrate temperature is found. Moreover, the magnetoresistance and coherence length of weak antilocalization also have a similar inverse dependence with the substrate temperature, as revealed by the magnetotransport measurements. Our experiments elucidate the special behaviors in Bi2Se3/LaCoO3 heterostructures, which provide a good platform for exploring related novel quantum phenomena, and are inspiring for device applications.

Simultaneous control of ferromagnetism and ferroelasticity by oxygen octahedral backbone stretching

Coexistence of ferromagnetism and ferroelasticity in a single material is an intriguing phenomenon,but has been rarely found.Here we studied both the ferromagnetism and ferroelasticity in a group of LaCoO3 films with systematically tuned atomic structures.We found that all films exhibit ferroelastic domains with four-fold symmetry and the larger domain size(higher elasticity)is always accompanied by stronger ferromagnetism.We performed synchrotron x-ray diffraction studies to investigate the backbone structure of the CoO6 octahedra,and found that both the ferromagnetism and the elasticity are simultaneously enhanced when the in-plane Co–O–Co bond angles are straightened.Therefore the study demonstrates the inextricable correlation between the ferromagnetism and ferroelasticity mediated through the octahedral backbone structure,which may open up new possibilities to develop multifunctional materials.

In-situ quantification of the surface roughness for facile fabrications of atomically smooth thin films

This work presents an in-situ technique to quantify the layer-by-layer roughness of thin films and heterostructures by measuring the spectral profile of the reflection high-energy electron diffraction(RHEED).The characteristic features of the diffraction spot,including the vertical to lateral size ratio c/b and the asymmetrical ratio c_(1)/c_(2) along the vertical direction,are found to be quantitatively dependent on the surface roughness.The quantitative relationships between them are established and discussed for different incident angles of high-energy electrons.As an example,the surface roughnesses of LaCoO_(3) films grown at different temperatures are obtained using such an in-situ technique,which are confirmed by the ex-situ atomic force microscopy.Moreover,the in-situ measured layer-by-layer roughness oscillations of two LaCoO_(3) films are demonstrated,revealing drastically different information from the intensity oscillations.The experiments assisted with the in-situ technique demonstrate an outstanding high resolution down to-0.1 A.Therefore,the new quantitative RHEED technique with real-time feedbacks significantly escalates the thin film synthesis efficiency,especially for achieving atomically smooth surfaces and interfaces.It opens up new prospects for future generations of thin film growth,such as the artificial intelligence-assisted thin film growth.