NSRRC Activity Report 2023

Physics and Materials Science 013 T o understand the mechanism of high-temperature superconductors is considered a Holy Grail in condensed matter physics. Especially after the LK-99 incident happened in the summer of 2023, the society suddenly realized that the advances in fundamental physics might have the chance to influence the stock market. In this regard, the condensed matter research community has recently turned its attention to emergent low-dimensional electronic systems, with a particular focus on materials such as graphene, two-dimensional (2D) materials, and complex oxides. These materials have displayed unexpected phenomena such as Mott physics and superconductivity, especially in twisted bilayer forms and at interfaces. For example, Jiunn-Yuan Lin (National Yang Ming Chiao Tung University) and his co-workers found an unexpected metallic state in a bulk cuprate Mott insulator, which defied Landau’s Fermi liquid paradigm. By combining several experimental techniques, Lin and his co-workers proposed a scenario that has not been expected before. They concluded that a two-dimensional electron gas formed at the transient layer where the crystalline structure transformed naturally. The sample system under surveillance was an ultra-thin SrCuO 2 (SCO) film grown on a TiO 2 -terminated SrTiO 3 (STO) substrate. As is well-known, the bulk tetragonal (planar-type) SCO consisting of 2D CuO 2 planes alternating with Sr layers is the simplest structure among high-Tc cuprates. First principal calculations indicated a structural transformation from chain type to planar type with increasing SCO film thickness [> 5 unit cells (u.c.)]. 1 Why a Mott Insulator Turns into a Conductor? A deep lying transient doped layer can engineer the conductance of a Mott insulator. Fig. 1 : Schematic plots of the planar-type (left) and the chain-type (right) tetragonal SCO on TiO 2 -terminated STO substrates. [Reproduced from Ref. 2] Fig. 2 : (a) The STEM image clearly indicates that SCO close to the STO interface forms a chain-type structure similar to STO. The similarity is highlighted by the square black boxes of the same size at the center of this cross-section view. On the other hand, SCO away from the interface forms a planar-type structure with a smaller c-axis than those of SCO and STO. This is marked by a rectangular black box on the top part of image. (b) Energy scan of X-ray Laue diffraction. The existence of the chain-type SCO is evidenced by a shoulder next to the strong STO peak. [Reproduced from Ref. 2] Both structures are shown in Fig. 1 . However, traditionally, the SCO can only be synthesized under high pressure, making it difficult to study the thin-film properties and structural transformation. In the past decade, high-quality SCO thin films have been successfully fabricated using a pulsed-laser deposition method, which has greatly advanced research on SCO thin films and their structural transformation. Conducting transport measurements on these high-quality SCO thin films with 15 u.c. revealed a distinct metallic behavior, despite bulk SCO being known as a Mott insulator.