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Physical Review X reports Jian Wang group and collaborators’ work on the observation of interface-induced Zeeman-protected superconductivity in ultrathin crystalline lead films
《Physical Review X》报道王健研究组关于超薄单晶铅膜界面超导研究新进展

Superconductivity, known as a quantum state in which current can flow without resistance, has attracted great attention in both academic and industrial fields. With the development of growing high-quality ultrathin superconducting films, research on two-dimensional crystalline superconductors has become a new frontier in condensed matter physics and materials science. Due to the reduced dimensionality and nanoscale effect, novel quantum phenomena and physics emerge in two-dimensional superconducting systems. For instance, superconducting transition in 2D system was predicted to be a topological KT transition, and related work has been awarded the 2016 Nobel prize in Physics. Moreover, interface can play an important role in the superconducting properties. Jian Wang group at Peking University and collaborators have made a series of progresses in 2D superconductivity research. Representative works include the discovery of quantum Griffiths singularity (Science 350, 542(2015) with a perspective paper Science 350, 509 (2015)), the discovery of a new 2D superconducting phase in Ga films (Physical Review Letters 114, 107003 (2015)) and the demonstration of high-temperature superconductivity in 1-UC FeSe/STO (Chinese Physics Letters 31, 017401 (2014) with a Editors’ Choice paper Science 343, 230 (2014)).

Recently, 2D transition metal dichalcogenides (TMDs), regarded as the materials beyond graphene, have become a highlightaround the world. The investigations on such crystalline 2D superconductors with strong spin-orbit interaction (SOI) have opened up a new direction to explore exotic quantum phenomena. For instance, the broken in-plane inversion symmetry in single layer NbSe2 and gated MoS2 gives rise to Zeeman-type SOI protected superconductivity (Zeeman-protected superconductivity). Its key signature is the large in-plane critical field up to several times of Pauli limit. Prof. Jian Wang and collaborators systematically studied macro-size atomically flat monolayer NbSe2 films grown by molecular beam epitaxy (MBE) method. Ultrahigh magnetic field and low temperature measurements show that the extremely high parallel upper critical field Bc2// (T = 0) of monolayer NbSe2 films significantly exceeds the Pauli paramagnetic limiting field, consistent with Zeeman-protected Ising superconductivity. (Nano Letters 17, 6802 (2017)) Nevertheless, most 2D superconductors naturally preserve the in-plane inversion symmetry and thus the Zeeman-protected superconductivity cannot exist, restricting the scope of related investigations and potential applications.

Recently, Prof. Jian Wang, Prof. Xincheng Xie and Prof. Ji Feng at Peking University, in collaboration with Prof. Haiwen Liu at Beijing Normal University, Prof. Junfeng Wang at Wuhan National High Magnetic Field Center, as well as Prof. Mingliang Tian, Dr. Chuanying Xi at Heifei High Magnetic Field Laboratory of the Chinese Academy of Sciences demonstrated that a new kind of Zeeman-protected superconducting system can be artificially created in the macroscopic scale by inserting striped incommensurate (SIC) phase at the interface between ultrathin crystalline lead film and Si(111) substrate through molecular beam epitaxy (MBE) techhique. By performing systematic transport measurements at low temperatures and ultrahigh magnetic fields, it is found that the superconductivity can survive under a large parallel magnetic field up to 40 T far beyond the Pauli limit, providing clear evidence of Zeeman-protected superconductivity in the Pb/SIC/Si heterostructure. The ab initio calculations demonstrate that the lattice distortion in SIC phase can extend to the neighboring lead film and induce considerable Zeeman-type SOI in the system. Furthermore, a general microscopic theory for the in-plane critical field of two-dimensional dirty superconductivity with various kinds of SOI and spin-orbit scattering is developed to quantitatively illuminate the underlying physical mechanism for the Zeeman-protected superconductivity. The work shows that by interface engineering Zeeman-protected superconductivity can be artificially induced in crystalline 2D superconducting heterostructures. The findings not only provide a new platform to create in-plane inversion symmetry breaking superconducting system, but may also stimulate further studies on the interface modulation of unconventional superconductivity in 2D heterostructures.

The paper was published in Physical Review X on April 2, 2018 (Physical Review X 8, 021002 (2018) DOI: 10.1103/PhysRevX.8.021002): https://journals.aps.org/prx/abstract/10.1103/PhysRevX.8.021002.

Prof. Jian Wang at Peking University and Prof. Haiwen Liu at Beijing Normal University are corresponding authors of this paper. Yi Liu and Ziqiao Wang at Peking University contributed equally to this work.

This work was supported by the National Basic Research Program of China, the National Natural Science Foundation of China, the Open Project Program of the Pulsed High Magnetic Field Facility at Huazhong University of Science and Technology, Collaborative Innovation Center of Quantum Matter, the Key Research Program of the Chinese Academy of Sciences, CAS Center for Excellence in Topological Quantum Computation and the Fundamental Research Funds for the Central Universities. Jian Wang would like to thank Xincheng Xie, Fa Wang, Limei Xue, Zefeng Ren and Collaborative Innovation Center of Quantum Matter for the lab space and patial funding support on the ultrahigh vacuum molecular beam epitaxy and low temperature scanning tunneling microscopy (UHV-MBE-LT STM) system.

Figure (a) Superconductivity in 6 monolayer (ML) Pb film survives at high parallel magnetic field up to 40 T at 1.7 K. (b) Experiments and analysis demonstrate the Zeeman-protected superconductivity in ultrathin Pb films. (c) Schematic for magnetoresistance measurements in epitaxial Pb film on silicon substrate with SIC interface.

  超导体自百年前发现以来,因其具有无耗散的零电阻和完全抗磁特性,在很多方面展现出巨大的应用前景,从而引起了学术界和工业界的持续广泛关注。近年来,随着超薄单晶薄膜或器件制备工艺的提高,二维超导晶体的研究逐渐成为国际研究的前沿热点之一。北京大学王健研究组与合作者在前期二维超导的相关研究中取得了一系列具有重要学术价值的创新性成果,如量子格里菲斯奇异性的发现(Science 350, 542 (2015), 同期perspective评论文章Science 350, 509 (2015)专题报道),被Iwasa研究组综述文章Nature Reviews Materials 2, 16094 (2016)誉为二维晶体超导中三个最重要的主题之一;通过界面调控实现新的不同于体材料的二维超导相(Physical Review Letters 114, 107003 (2015)编辑推荐)以及二维界面高温超导的证实(Chinese Physics Letters 31, 017401 (2014), 被Science编辑选择文章Science 343, 230 (2014)报道)等。

  近来,通过机械剥离成薄层的过渡族金属硫化物,作为超越石墨烯的候选材料,已成为国际研究的热点,其中强自旋轨道耦合的二维晶体超导体,为人们探索新奇量子现象提供了一个广阔的平台,如拓扑超导态的探索等。有报道指出,在单层NbSe2薄片和栅极调制的MoS2中,面内中心反演对称性的破缺产生了塞曼自旋轨道耦合保护的超导电性(Zeeman-protected superconductivity)。塞曼保护超导体系的重要特征是具有非常大的面内临界场,常常可达到数倍的泡利极限(Pauli limit),往往会对应几十特斯拉或更高的磁场。北京大学王健研究组与合作者在前期工作中首次报道了超高真空分子束外延制备的宏观面积的单层NbSe2薄片在强磁场和极低温下的直接输运测量结果,证实了平行特征临界场Bc//(T = 0)是顺磁极限场的5倍以上(Nano Letters 17, 6802 (2017))。然而,大部分二维超导体系都具有面内中心反演对称性,无法自发产生塞曼保护超导电性,大大限制了这一前沿领域的研究范围和潜在应用。

  近日,北京大学物理学院量子材料科学中心的王健教授与谢心澄院士、冯济教授,和北京师范大学的刘海文研究员、武汉国家强磁场科学中心王俊峰研究员以及中科院合肥强磁场科学中心的田明亮研究员、郗传英博士等人合作,通过使用铅的条状非公度相作为铅膜和硅衬底的界面,用超高真空分子束外延技术成功制备出一种宏观面积的、塞曼保护的新型二维超导体。系统的低温强磁场实验表明,该体系的超导电性可存在于超过40特斯拉的平行强磁场中,这一数值远超过体系的泡利极限,是塞曼保护超导电性的直接证据。第一性原理计算结果也表明,条状非公度相中特殊的晶格畸变会延伸至铅膜中,从而在该体系中引入很强的塞曼自旋轨道耦合。同时,新的微观理论也给出了强杂质情形下各种自旋轨道耦合及散射效应对二维超导临界场的影响并定量地解释了塞曼保护超导电性的物理机制。该工作表明,可以通过界面工程在中心反演对称性保护的二维超导中引入面内中心反演对称性破缺,也即在二维晶体超导体系中人工引入塞曼保护的超导电性机制。这一结果预示出人们有望在二维超导体系中,通过界面调制发现新的非常规超导特性。这种宏观尺度强自旋轨道耦合下的二维超导,也为拓扑超导的探索提供了新的平台,并为未来无耗散或低耗散量子器件的设计与集成奠定了基础。

  该工作于2018年4月2日发表于物理著名学术期刊Physical Review X。(Physical Review X 8, 021002 (2018) DOI: 10.1103/PhysRevX.8.021002): https://journals.aps.org/prx/abstract/10.1103/PhysRevX.8.021002.

  北京大学的刘易和王子乔为共同第一作者,北京大学的王健教授和北京师范大学刘海文研究员是本文的共同通讯作者。

  该工作得到了国家重大科学研究计划、国家自然科学基金、华中科技大学脉冲强磁场开放项目、量子物质科学协同创新中心、中国科学院先导培育项目、中科院拓扑量子计算卓越创新中心、中央高校基本科研基金的支持。王健特别感谢谢心澄、王垡、徐莉梅、任泽峰以及量子物质科学协同创新中心在北大超高真空分子束外延与低温扫描隧道显微镜实验室搭建过程中给予的实验室空间与部分经费的支持。

图(a) 脉冲强磁场实验表明6个原子层厚铅膜的超导电性在高达40 T的水平强场下仍不被破坏。(b) 临界场随温度的关系与理论高度重合,有力地证明了超薄铅膜中的塞曼自旋轨道耦合保护的超导电性。 (c) 对外延生长于条状非公度相(SIC)界面上的超薄铅膜进行磁阻测量的示意图。