We report the first observation of coherent surface states on cubic perovskite oxide SrVO3(001) thin films through spectroscopic imaging scanning tunneling microscopy. A direct link between the observed atomic-scale interference patterns and the formation of a dxy-derived surface state is supported by first-principles calculations. Furthermore, we show that the apical oxygens on the topmost VO2 plane play a critical role in controlling the spectral weight of the observed coherent surface state.
One of the simplest but universal challenges at the frontier of materials physics is controlling band structure, both for realizing novel phenomena and for practical functionalities. In this talk, I will describe our atomic scale investigations of novel quantum materials called topological crystalline insulators (TCIs). In TCIs, topology and crystal symmetry intertwine to create massless Dirac electrons, which can be described by the same equations used for relativistic particles traveling close to the speed of light. Using Landau level spectroscopy and atomic resolution imaging in TCIs, we have discovered massive Dirac electrons coexisting with massless Dirac electrons. Our findings experimentally demonstrate the unique and extraordinary tunability of Dirac electrons in TCIs, which provides a significant step for realizing fundamentally and practically important quantum states via strain engineering. As the final part of this talk, I will also introduce our recent attempt of combing visualization techniques with epitaxial thin film based quantum materials design.
The charge density wave (CDW) state is a widespread phenomenon in low-dimensional metals/semimetals. The spectral weight of the associated folded bands (shadow bands) can be an intriguing trigger leading to additional Fermi surface instability and unexplored phase transitions. The rare earth tri-telluride CeTe3 exhibits a single CDW stabilized below ~400 K and antiferromagnetism below ~3 K. The distinct periodicities between the Te-square net, the CeTe block layer, and the CDW give rise to rich shadow band formations. In this work, we reveal the predominant scattering between the original and shadow bands at 4 K, with the scattering within the original bands being relatively suppressed at Fermi energy. This unconventional quasi-particle scattering collectively underscores the vital role of the shadow bands' spectral weight and the hidden matrix element effect, which are crucial for controlling electronic properties in this system. Furthermore, our finding points to the existence of rich and unexplored Fermi surface instabilities, which potentially play a role in controlling the nature of long-range antiferromagnetism at lower temperatures in the presence of finite charge-spin interaction.
Using low temperature scanning tunneling microscopy (STM), we observed a single molecule magnet, Tb3+ double decker molecule TbPc2 (Pc = phthalocyaninato), adsorbed on a perovskite type transition metal oxide SrVO3 ultrathin film. TbPc2 molecules adsorbed intact and flat on two surface reconstructions of SrVO3, (\(\sqrt{2} \times \sqrt{2} \))-R45° and (\(\sqrt{5} \times \sqrt{5} \))-R26.6°, with different configurations. High resolution STM images revealed that the adsorption configurations were governed by the adatom structure of each surface reconstruction according to the proposed adsorption configurations. Using low-temperature scanning tunneling microscopy, we observed a single molecule magnet, terbium(III) bis-phthalocyaninato (TbPc2), adsorbed on a perovskite type SrVO3 ultrathin film. TbPc2 adsorbed intact and flat differently on two surface reconstructions of SrVO3, (\(\sqrt{2} \times \sqrt{2} \))-R45° and (\(\sqrt{5} \times \sqrt{5} \))-R26.6°. High-resolution images revealed that the adsorption configurations were governed by the adatom structure of each surface reconstruction according to the proposed adsorption configurations.
Interest in many strongly spin-orbit coupled 5d-transition metal oxide insulators stems from mapping their electronic structures to a J=1/2 Mott phase. One of the hopes is to establish their Mott parent states and explore these systems' potential of realizing novel electronic states upon carrier doping. However, once doped, little is understood regarding the role of their reduced Coulomb interaction U relative to their strongly correlated 3d-electron cousins. Here we show that, upon hole-doping a candidate J=1/2 Mott insulator, carriers remain localized within a nanoscale phase separated ground state. A percolative metal-insulator transition occurs with interplay between localized and itinerant regions, stabilizing an antiferromagnetic metallic phase beyond the critical region. Our results demonstrate a surprising parallel between doped 5d- and 3d-electron Mott systems and suggest either through the near degeneracy of nearby electronic phases or direct carrier localization that U is essential to the carrier response of this doped spin-orbit Mott insulator.
