探討鈦酸鍶基板表面重構對單層鐵硒薄膜之載子濃度和超導溫度

Abstract

單層鐵硒（FeSe）薄膜成長在鈦酸鍶（SrTiO3,STO）基板上形成高於100K超導溫度的突破發現，近期引起科學界極大的重視。最近的研究文獻顯示，單層鐵硒薄膜超導溫度（Tc）與鈦酸鍶基板表面重構具有相關性，亦即不同鈦酸鍶基板表面結構可導致單層鐵硒超導薄膜改變表面電子濃度，間而影響單層鐵硒薄膜的超導溫度之表現。然而，對於發生在基板表面結構影響超導表現的研究議題，現今的論文都是從單層鐵硒表面進行量測和探討。至今，直接在單層鐵硒薄膜和鈦酸鍶基板之間的介面處，直接量測兩物質介面間的電性結構仍是缺乏的。 在本研究工作中，我們利用剖面掃描穿隧式顯微鏡和掃描穿隧式能譜於材料異質介面處，直接量測單層鐵硒薄膜和鈦酸鍶基板之間介面處的電子結構。樣品基板有兩種不同表面結構：一是鈦酸鍶表面上的√13 × √13重構，另一種是鈦酸鍶表面上的√20 × √20重構。我們的實驗結果闡明，鈦酸鍶基板√13 × √13 表面結構的表面載子密度是√20 × √20 表面結構的1.2倍。同時，成長在鈦酸鍶√13 × √13 表面結構上的鐵硒薄膜超導臨界溫度相較於成長在鈦酸鍶√20 × √20 表面結構上的鐵硒薄膜提高3K

Superconductivity of the monolayer Iron Selenide (FeSe) on Strontium titanate (SrTiO3, STO) substrate has drawn a great deal of interest due to the unusual superconducting phenomena. Recent studies speculated that the critical temperature (Tc) is dependent of the substrate-surface reconstruction. Different substrate-induced reconstruction resulted in the variation of the electron accumulation in the FeSe monolayer. However, the argument was only supported by the measurements detecting on the top surface of the FeSe monolayer. Direct evidence of the evolution of the electronic structure across the FeSe/STO interface with atomic-resolution is still lacking. In this work, we utilize cross-sectional scanning tunneling microscopy and spectroscopy to study the electronic structure directly at the interface between FeSe and STO. Two substrate-induced reconstructions are investigated: one is the √13 × √13 reconstruction terminated on the STO surface, and the other one is the √20 × √20 reconstruction terminated on the STO surface. Our experimental results clearly demonstrate the amount of surface carriers on the STO surface’s √13 × √13 reconstruction is 1.2 times to that on the √20 × √20 reconstruction, and the increased carrier density from different reconstruction enhances the critical temperature of the FeSe monolayer by 3K.