本文選題：界面超導(dǎo)　切入點(diǎn)：電子摻雜　出處：《清華大學(xué)》2016年博士論文　論文類型：學(xué)位論文

【摘要】：界面超導(dǎo)的概念起源于上世紀(jì)50年代物理學(xué)家對(duì)二維超導(dǎo)的研究。隨著實(shí)驗(yàn)技術(shù)的發(fā)展,目前人們可以很容易地實(shí)現(xiàn)原子尺度可控的薄膜生長(zhǎng)并制備出原子級(jí)平整的界面,這大大推動(dòng)了界面超導(dǎo)的研究。2012年,我們研究組用分子束外延技術(shù)在SrTiO_3(001)表面上制備出了單原子層FeSe薄膜,并用掃描隧道顯微鏡觀察到了~20 meV的超導(dǎo)能隙,這一工作立即引發(fā)了物理學(xué)家對(duì)界面超導(dǎo)現(xiàn)象的研究熱情。為了研究FeSe與SrTiO_3的界面在其超導(dǎo)增強(qiáng)中起到的作用,我們利用分子束外延技術(shù)在SrTiO_3(001)襯底上制備了1-4原子層厚的FeSe薄膜,利用掃描隧道顯微鏡研究了FeSe薄膜表面的K原子吸附及其對(duì)電子結(jié)構(gòu)和超導(dǎo)性質(zhì)的影響。我們的實(shí)驗(yàn)揭示了電子摻雜和界面電聲耦合作用對(duì)界面高溫超導(dǎo)的重要作用。論文取得的主要成果如下:(1)我們發(fā)現(xiàn)第2-4層FeSe薄膜在表面吸附堿金屬后,均出現(xiàn)大于11 meV的超導(dǎo)能隙,其最大能隙隨FeSe厚度增加而降低。在SrTiO_3上外延1.5-UC KxFe2Se2薄膜上,我們也觀察到了14.5 meV的超導(dǎo)能隙。而對(duì)單層FeSe來講,K原子表面吸附總是抑制其超導(dǎo)特性。此項(xiàng)結(jié)果表明,電子摻雜是增強(qiáng)超導(dǎo)的關(guān)鍵因素之一,而FeSe/SrTiO_3界面除了對(duì)FeSe薄膜提供電子以外還有其他的增強(qiáng)效應(yīng)。(2)我們通過超導(dǎo)隧穿譜研究了SrTiO_3(001)上1-4 UC FeSe超導(dǎo)薄膜中的電聲耦合相互作用。在超導(dǎo)隧穿譜中,我們觀察到了兩支玻色子模式(bosonic-modes),它們的能量分別為~11.0 meV與~21.5 meV。玻色子模式的能量大小不隨超導(dǎo)能隙變化,且與FeSe薄膜的層厚無關(guān)。通過與拉曼散射與中子散射實(shí)驗(yàn)數(shù)據(jù)對(duì)比,我們認(rèn)為實(shí)驗(yàn)中觀測(cè)到的兩支玻色子模式是聲子模式。這一結(jié)果顯示Fe Se與SrTiO_3界面電聲耦合增強(qiáng)作用也是導(dǎo)致這一體系具有高溫超導(dǎo)特性的關(guān)鍵因素。綜合以上結(jié)果,我們認(rèn)為,界面電荷轉(zhuǎn)移和界面電聲耦合增強(qiáng)均是導(dǎo)致FeSe/SrTiO_3體系界面高溫超導(dǎo)的關(guān)鍵因素:摻雜可以使FeSe具有~9 meV的超導(dǎo)能隙,而界面處電聲耦合可以繼續(xù)增大其超導(dǎo)能隙至~15-20 meV。我們的研究結(jié)果顯示人工構(gòu)建異質(zhì)結(jié)構(gòu)是提高超導(dǎo)轉(zhuǎn)變溫度的有效方法,為尋找新的高溫超導(dǎo)體系開拓了新的方向。
[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.
【學(xué)位授予單位】：清華大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2016
【分類號(hào)】：O511.3;O484

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