钛酸锶上外延铁硒薄膜表面碱金属吸附及其超导性质研究

发布时间：2018-03-09 05:14

本文选题：界面超导　切入点：电子掺杂　出处：《清华大学》2016年博士论文　论文类型：学位论文

【摘要】：界面超导的概念起源于上世纪50年代物理学家对二维超导的研究。随着实验技术的发展,目前人们可以很容易地实现原子尺度可控的薄膜生长并制备出原子级平整的界面,这大大推动了界面超导的研究。2012年,我们研究组用分子束外延技术在SrTiO_3(001)表面上制备出了单原子层FeSe薄膜,并用扫描隧道显微镜观察到了~20 meV的超导能隙,这一工作立即引发了物理学家对界面超导现象的研究热情。为了研究FeSe与SrTiO_3的界面在其超导增强中起到的作用,我们利用分子束外延技术在SrTiO_3(001)衬底上制备了1-4原子层厚的FeSe薄膜,利用扫描隧道显微镜研究了FeSe薄膜表面的K原子吸附及其对电子结构和超导性质的影响。我们的实验揭示了电子掺杂和界面电声耦合作用对界面高温超导的重要作用。论文取得的主要成果如下:(1)我们发现第2-4层FeSe薄膜在表面吸附碱金属后,均出现大于11 meV的超导能隙,其最大能隙随FeSe厚度增加而降低。在SrTiO_3上外延1.5-UC KxFe2Se2薄膜上,我们也观察到了14.5 meV的超导能隙。而对单层FeSe来讲,K原子表面吸附总是抑制其超导特性。此项结果表明,电子掺杂是增强超导的关键因素之一,而FeSe/SrTiO_3界面除了对FeSe薄膜提供电子以外还有其他的增强效应。(2)我们通过超导隧穿谱研究了SrTiO_3(001)上1-4 UC FeSe超导薄膜中的电声耦合相互作用。在超导隧穿谱中,我们观察到了两支玻色子模式(bosonic-modes),它们的能量分别为~11.0 meV与~21.5 meV。玻色子模式的能量大小不随超导能隙变化,且与FeSe薄膜的层厚无关。通过与拉曼散射与中子散射实验数据对比,我们认为实验中观测到的两支玻色子模式是声子模式。这一结果显示Fe Se与SrTiO_3界面电声耦合增强作用也是导致这一体系具有高温超导特性的关键因素。综合以上结果,我们认为,界面电荷转移和界面电声耦合增强均是导致FeSe/SrTiO_3体系界面高温超导的关键因素:掺杂可以使FeSe具有~9 meV的超导能隙,而界面处电声耦合可以继续增大其超导能隙至~15-20 meV。我们的研究结果显示人工构建异质结构是提高超导转变温度的有效方法,为寻找新的高温超导体系开拓了新的方向。
[Abstract]:The concept of interface superconductivity originates from the research of two-dimensional superconductivity by physicists in -50s. With the development of experimental technology, it is easy to achieve the atom-scale controllable growth of thin films and to fabricate atomic-level flat interfaces. In 2012, monatomic layer FeSe thin films were deposited on SrTiO3C001 surface by molecular beam epitaxy (MBE) technique, and the superconducting gap of 20 meV was observed by scanning tunneling microscope (SEM). This work immediately aroused physicists' enthusiasm for the study of interfacial superconductivity. In order to study the role of the interface between FeSe and SrTiO_3 in superconducting enhancement, FeSe thin films with 1-4 atomic layer thickness were prepared on SrTiOS _ 3N _ (001) substrates by molecular beam epitaxy (MBE) technique. The adsorption of K atoms on the surface of FeSe thin films and their effects on the electronic structure and superconducting properties have been studied by using a scanning tunneling microscope. Our experiments have revealed the importance of electron doping and interface electro-acoustic coupling to the interfacial high temperature superconductivity. The main results obtained in this paper are as follows: 1) We found that layer 2-4 FeSe films adsorbed alkali metals on the surface, The maximum energy gap decreases with the increase of FeSe thickness, and the maximum gap decreases with the increase of FeSe thickness. The superconducting gap of 1.5-UC KxFe2Se2 thin films is grown on SrTiO_3, and the maximum gap decreases with the increase of FeSe thickness. We have also observed the superconducting gap of 14.5 meV. For monolayer FeSe, the adsorption of K atom always suppresses its superconducting properties. The results show that electron doping is one of the key factors in enhancing superconductivity. In addition to providing electrons to FeSe films, the FeSe/SrTiO_3 interface has other enhancement effects. (2) We have studied the electro-acoustic coupling interaction in 1-4UC FeSe superconducting films on SrTiO3PX _ (001) by means of superconducting tunneling spectroscopy, and in the superconducting tunneling spectra, Two bosonic-modesons have been observed. Their energies are 11.0 meV and 21.5meV, respectively. The energy of boson mode does not change with the superconducting gap and is independent of the thickness of the FeSe film. The results are compared with the experimental data of Raman scattering and neutron scattering. We consider that the two boson modes observed in the experiment are phonon modes. The results show that the electroacoustic coupling enhancement between Fe se and SrTiO_3 interface is also the key factor leading to the high temperature superconducting properties of the system. We consider that interface charge transfer and interface electro-acoustic coupling enhancement are the key factors leading to high temperature superconductivity at the interface of FeSe/SrTiO_3 system. Doping can make FeSe have superconducting gap of 9 meV. The electro-acoustic coupling at the interface can increase the superconducting gap to 15-20 MEV. Our results show that the artificial construction of heterostructure is an effective method to increase the transition temperature of superconducting, which opens up a new direction for finding new high temperature superconducting systems.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O511.3;O484

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