Confinement-Induced Isosymmetric Metal-Insulator Transition in Ultrathin Epitaxial V2O3 Films.

Dimensional confinement has shown to be an effective strategy to tune competing degrees of freedom in complex oxides. Here, we achieved atomic layered growth of trigonal vanadium sesquioxide (V2O3) by means of oxygen-assisted molecular beam epitaxy. This led to a series of high-quality epitaxial ultrathin V2O3 films down to unit cell thickness, enabling the study of the intrinsic electron correlations upon confinement. By electrical and optical measurements, we demonstrate a dimensional confinement-induced metal-insulator transition in these ultrathin films. We shed light on the Mott-Hubbard nature of this transition, revealing a vanishing quasiparticle weight as demonstrated by photoemission spectroscopy. Furthermore, we prove that dimensional confinement acts as an effective out-of-plane stress. This highlights the structural component of correlated oxides in a confined architecture, while opening an avenue to control both in-plane and out-of-plane lattice components by epitaxial strain and confinement, respectively.

Efficient Direct Band-Gap Transition in Germanium by Three-Dimensional Strain.

Complementary to the development of highly three-dimensional (3D) integrated circuits in the continuation of Moore's law, there has been a growing interest in new 3D deformation strategies to improve the device performance. To continue this search for new 3D deformation techniques, it is essential to explore beforehand, using computational predictive methods, which strain tensor leads to the desired properties. In this work, we study germanium (Ge) under an isotropic 3D strain on the basis of first-principles methods. The transport and optical properties are studied by a fully ab initio Boltzmann transport equation and many-body Bethe-Salpeter equation (BSE) approach, respectively. Our findings show that a direct band gap in Ge could be realized with only 0.70% triaxial tensile strain (negative pressure) and without the challenges associated with Sn doping. At the same time, a significant increase in the refractive index and carrier mobility, particularly for electrons, is observed. These results demonstrate that there is a huge potential in exploring the 3D deformation space for semiconductors, and potentially many other materials, to optimize their properties.