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Science highlighted Jian Wang and Qi-Kun Xue’s research in Editors' Choice: Direct evidence of the thinnest high temperature superconductors
《科学》编辑推荐报道 北大王健与清华薛其坤等人在界面高温超导领域的重要进展:最薄的高温超导体被实验证实

One unit-cell (1-UC, 0.55 nm) FeSe films grown on SrTiO3 (STO) substrate by molecular beam epitaxy were reported to show a possibility of high TC superconductivity by detecting the superconducting-like gap utilizing in situ scanning tunneling microscopy/spectroscopy and angle resolved photoemission spectroscopy(Chin. Phys. Lett. 29, 037402 (2012); Nat. Commun. 3, 931 (2012); Nat. Mater. 12, 605 (2013); Nat. Mater. 12, 634 (2013)). However, direct evidence of the superconductivity in 1-UC FeSe films, such as zero resistance and Meissner effect, has not been reported.

After almost two years’ great effort, Prof. Jian Wang and Prof. Qi-Kun Xue’s collaboration team firstly demonstrated that the 1-UC thick FeSe films grown on STO substrates are superconducting by direct transport and magnetic measurements, with an onset transition temperature TC above 40 K (Maximum value is 54.5 K) and a critical current density JC ~ 1.7×106 A/cm2 at 2 K, which are much higher than TC ~ 8 K and JC ~ 104 A/cm2 for bulk FeSe. The enhanced superconductivity with high TC, HC2 and JC makes the 1-UC FeSe thin films on high-dielectric STO substrate attractive for potential applications, such as superconducting interconnects, superconducting quantum interference device (SQUID), and field-effect transistor (FET) devices. Since the superconducting layer is only 0.55 nm thick, Wang and Xue’s research demonstrated that the 1-UC FeSe on STO is the thinnest high TC superconductor at present. This work not only paves the way to searching for new high temperature superconductors and enhancing superconductivity by interface engineering, but also offers an ideal platform for understanding the fundamental nature of unconventional superconductivity.

The paper was published in Chinese Physics Letters (CHIN. PHYS. LETT. 31, 017401 (2014)) with a title of “Direct observation of high-temperature superconductivity in one-unit-cell FeSe films” http://cpl.iphy.ac.cn/EN/Y2014/V31/I1/017401 and selected for the Editors’ Choice of Science with a title of “A very thin superconductor” (Science 343, 230 (2014)) http://www.sciencemag.org/content/343/6168/twil.full. This work was supported by the National Basic Research Program of Chinathe National Natural Science Foundation of China and the Ministry of Education of China .

 

Figure: Transport and diamagnetic measurements of 1-UC FeSe films grown on SrTiO3 (001). a. The temperature dependence of resistance under zero field. Inset: A schematic structure for the transport measurements in the heterostructure of 30 nm amorphous Si /10-UC FeTe/1-UC FeSe/STO. b. The diamagnetic response measured by a homebuilt two-coil mutual inductance system. 

2012年,薛其坤院士与马旭村研究员团队利用超高真空分子束外延技术在钛酸锶衬底上成功制备出单个原胞厚 (0.55nm) FeSe超导薄膜。不同于传统理论对超导薄膜的预言(薄膜越薄,Tc越低,在二维极限下因量子涨落超导消失),原位的扫描隧道谱实验在单层FeSe薄膜中发现20meV能隙,如果这一能隙是超导能隙,可以推断其TC可能会突破液氮温度(77K(Chin. Phys. Lett. 29, 037402 (2012))。要知道,FeSe块材的超导转变温度只有8 K左右,而除了铜氧化物超导体还没有第二种超导体可以突破液氮温度。因次,这一工作很快得到了国内外学术界的广泛关注,中国科学院物理所、复旦大学、中国人民大学、美国斯坦福大学、加州大学伯克利分校、休斯顿大学和日本国家材料研究所等单位的科学家都随后开始了这方面的研究。例如中科院物理所周兴江组和复旦大学封东来组的变温角分辨光电子谱实验研究进一步证实了单层FeSe薄膜中可能存在远大于体相的超导能隙,在65 K左右能隙消失 (Nat. Commun. 3, 931 (2012)Nat. Mater. 12, 605 (2013)Nat. Mater. 12, 634 (2013))。上述一系列重要的实验结果表明单层FeSe很可能是一种新型界面高温超导体。事实上,超导最重要的两个现象是零电阻和迈斯纳效应,相应的实验结果被看作是超导的直接证据。由于单层FeSe很难在大气环境中存活,相应的物性测量极其困难,因此单层FeSe超导的直接证据一直未被报道。

2012年,北京大学-清华大学-中国科学院物理研究所量子物质科学协同创新中心在教育部支持下开始筹备和运转,创新中心成员北京大学量子材料科学中心王健研究组与清华大学薛其坤院士研究组(包括马旭村、王立莉、王亚愚、陈曦、季帅华等)组成联合团队对单层FeSe薄膜的超导电性展开项目攻关。经过一年多的辛勤努力,发现可以利用FeTe膜做保护层,成功实现单层FeSe薄膜电输运和迈斯纳效应的测量。物性研究结果表明,盖有保护膜的单层FeSe薄膜具有最高到54.5K的超导转变(高温超导证据),在23.5K时显示出完全零电阻(当前实验仪器精度以内),并且拥有极高的临界磁场(大于52特斯拉)和临界电流(大于一百万安培每平方厘米)。单层FeSe薄膜的超导临界电流比体材料时大了两个数量级,并可以通过对FeSe薄膜的钛酸锶衬底加背电极进一步调制FeSe薄膜的物性,这就为单层FeSe在超导纳米器件方面的应用奠定了基础(比如FETSQUID等)。进一步的实验证实了单层FeSe的超导层只有0.55纳米厚,因此外延于钛酸锶衬底上的单层FeSe薄膜已成为目前发现的最薄的高温超导体。上述结果表明二维界面调制对于超导特性起着关键性作用,这不仅对高温超导和二维超导机理的研究有着重要科学意义,并且为探索新的高温超导体、甚至超导发现一百年来人类的梦想——室温超导体提供了新的平台和途径。单层FeSe超导物性的证明工作还得到了武汉强磁场中心李亮教授、王俊峰副教授和夏正才教授在强场测量方面以及日本东北大学陈明伟教授在高分辨电镜表征方面的鼎力协助。

这一成果Direct observation of high-temperature superconductivity in one-unit-cell FeSe films为题,作为 Express Letter 发表在《中国物理快报》上 (CHIN. PHYS. LETT. 31, 017401 (2014))http://cpl.iphy.ac.cn/EN/Y2014/V31/I1/017401,清华大学博士研究生张文号和北京大学博士后孙祎为并列第一作者,北京大学王健研究员、清华大学王立莉副研究员和薛其坤院士为文章通讯作者。文章发表后迅速得到国际学术界的重点关注,《科学》杂志(ScienceA very thin superconductor为题以编辑推荐(Editors’Choice)的形式对这一成果进行了专门报道 (Science 343, 230 (2014))http://www.sciencemag.org/content/343/6168/twil.full

 

上述研究工作得到了国家重大科学研究计划、自然科学基金以及教育部“2011计划等项目的资助

 

图:单原胞层厚度FeSe薄膜的输运测量。a.电阻随温度变化曲线;(插图:输运测量结构示意图)b.抗磁性测量曲线。