NSRRC Activity Report 2023

014 NSRRC ACTIVITY REPORT 2023 To tackle the true nature of this unexpected metallic property, Lin and his co-workers employed polarization- dependent X-ray absorption spectroscopy (XAS; TLS 11A1 and TLS 20A1 ) at both the Cu L-edge and the O K-edge, Laue X-ray nanodiffraction measurements at TPS 21A , hard X-ray core-level photoemission measurements (HAXPES) at SP 12U1 , and scanning transmission electron microscopy (STEM) techniques to study both the atomic structure and electronic structure of the SCO thin film. By carefully analyzing the lattice constants from STEM images in Fig. 2(a) and through the energy scan of X-ray Laue diffraction in Fig. 2(b) , Lin and his co-workers concluded that the first few layers of SCO at the interface display a chain-type structure, which is consistent with the theoretical calculation. 1 Based on the transport, structure, and soft X-ray spectroscopic results, Lin and his co-workers proposed a model for this emergent metallic state, illustrated in Fig. 3 . For SCO ultrathin films grown on STO, where the epitaxial strain plays an important role, it was predicted that a structural transformation in SCO from chain type to planar type would occur when the SCO thickness exceeds 5 u.c. Near the STO interface, the initial growth of SCO will have a chain-type structure. With increasing SCO thickness, there exist residual layers of chain-type SCO even when the planar-type structure has developed. Within the chain- type SCO, SrO layers alternate with CuO layers, while Sr layers exist with CuO 2 layers in the planar type. Moreover, there exist SrO x layers in the transient regime (highlighted by the red box at the middle of Fig. 3 ) in which excess oxygen is positioned on top of Cu in the neighboring CuO 2 plane(s), the excess oxygen in the SrO x layers effectively dopes the holes into the nearby CuO 2 planes. The observed conductivity hence arises from the few doped CuO 2 planes similar to that in cuprate superconductors. Lin and his co-workers state that the model, although plausible, might not be the ultimate solution for describing the nature of the emergent metallic state, but may be a promising first step to further explore the abundant physics in the SCO cuprate thin films. Whether this metallic state can be tuned into a superconducting state may be further explored. Lin and his co-workers are currently exploring a potential avenue for inducing superconductivity through variation of the doping level. (Reported by Chia-Hao Chen) This report features the work of Shih-Wen Huang, Ying-Hao Chu, Jiunn-Yuan Lin, and their collaborators published in Phys. Rev. B 107 , 075104 (2023). Fig. 3 : Schematic of the layered SCO/STO structure displaying the transition region from chain-type to planar-type SCO. The metallic region and doping mechanism are shown in the red box. [Reproduced from Ref. 2] TPS 21A X-ray Nanodiffraction TLS 11A1 (Dragon) MCD, XAS TLS 20A1 XAS SP 12U1 HAXPES • XAS, Laue XRD, HAXPES • Condensed Matter Physics References 1. Z. Zhong, G. Koster, P. J. Kelly, Phys. Rev. B 85 , 121411(R) (2012). 2. P.-C. Chiang, S. C. Lin, C.-Y. Chiang, C.-S. Ku, S. W. Huang, J. M. Lee, Y.-D. Chuang, H. J. Lin, Y. F. Liao, C.- M. Cheng, S. C. Haw, J. M. Chen, Y.-H. Chu, T. H. Do, C. W. Luo, J.-Y. Juang, K. H. Wu, Y.-W. Chang, J.-C. Yang, J.-Y. Lin, Phys. Rev. B 107 , 075104 (2023).