Spectroscopic Visualization of Surface Electronic State in High Temperature Superconducting Oxide Thin Films

Okinawa Institute of Science and Technology Graduate UniversityOkinawa Institute of Science and Technology Graduate UniversityThe study of strongly correlated electron systems is one of the central topics in condensed matter physics. The phenomena occurring in these systems are so complicated that the unifying theory is far from complete, but their applications, such as high-temperature superconductors (HTSCs), have become an integral part of everyday life. The study of strongly correlated systems therefore bridges the gap between fundamental physics and the application of cutting-edge technology. Angle-resolved photoemission spectroscopy (ARPES) has proven to be a powerful tool to study surface electronic states. Thus, we study the HTSCs- YBa2Cu3O7−x (YBCO) and the spinel superconductor LiTi2O4 (LTO) in thin őlms using ARPES. Despite their potential importance, the non-cleavable nature of both materials hinders the study of ARPES, and the electronic states on the surface have not yet been fully revealed. For YBCO, we show that the doping of Y with Ca in YBCO exhibits anomalous carrier doping associated with the appearance of a folded chain-derived band dispersion, implying the possibility of controlling the surface state. On the other hand, the LTO has demonstrated its potential as an intriguing platform for exploring novel exotic states with geometrically frustrated pyrochlore lattices. Thus, our results open a rich playground for the future development of HTSC heterointerfaces with atomic design.doctoral thesi

Imaging Josephson Vortices on Curved Junctions

Understanding the nature of vortices in type-II superconductors has been vital for deepening the physics of exotic superconductors and applying superconducting materials to future electronic devices. A recent study has shown that the LiTi2O4(111) thin film offers a unique experimental platform to unveil the nature of the vortex along the curved Josephson junction. This study successfully visualized individual Josephson vortices along the curved Josephson junctions using in-situ spectroscopic scanning tunneling microscopy on LiTi2O4 (111) epitaxial thin films. Notably, the local curvature of the Josephson junction was discovered to control the position of Josephson vortices. Furthermore, the numerical simulation reproduces the critical role of the curvature of the Josephson junction. This study provides guidelines to control Josephson vortices through geometrical ways, such as mechanical controlling of superconducting materials and their devices

One-Step Growth of Core\u2013Shell (PtPd)@Pt and (PtPd)@Pd Nanoparticles in the Gas Phase

Pt–Pd nanoparticles are grown in the gas phase by a magnetron-sputtering source and characterized by electron microscopy techniques for both Pt-rich and Pd-rich compositions of the metallic vapor. It is shown that this growth procedure can produce different types of core–shell nanoparticles, in one step, with sizes in the range of 4–10 nm, according to the composition of the vapor being rich either in Pt or in Pd. In all cases, the nanoparticles present intermixed cores containing both Pt and Pd and shells made of the majority element, i.e., of (PtPd)@Pt structure for the Pt-rich vapor and (PtPd)@Pd structure for the Pd-rich vapor. Global searches of the optimal chemical ordering show that none of these structures correspond to equilibrium configurations. On the contrary, these core–shell structures are strongly out-of-equilibrium, being the result of kinetic trapping phenomena. This is verified by molecular dynamics growth simulations which are able to reproduce both the different types of chemical ordering and the variety of geometric shapes found in the experiments

Controlling inversion disorder in a stoichiometric spinel magnet

In the study of frustrated quantum magnets, it is essential to be able to control the nature and degree of site disorder during the growth process, as many measurement techniques are incapable of distinguishing between site disorder and frustration-induced spin disorder. Pyrochlore-structured spinel oxides can serve as model systems of geometrically frustrated three-dimensional quantum magnets; however, the nature of the magnetism in one well-studied spinel, ZnFe2O4, remains unclear. Here, we demonstrate simultaneous control of both stoichiometry and inversion disorder in the growth of ZnFe2O4 single crystals, directly yielding a revised understanding of both the collective spin behavior and lattice symmetry. Crystals grown in the stoichiometric limit with minimal site inversion disorder contravene all the previously suggested exotic spin phases in ZnFe2O4. Furthermore, the structure is confirmed on the F4¯3m space group with broken inversion symmetry that induces antiferroelectricity. The effective tuning of magnetic behavior by site disorder in the presence of robust antiferroelectricity makes ZnFe2O4 of special interest to multiferroic devices