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Nano Letters publishes Jian Wang research group and collaborators’ work on the coexistence of Ising superconductivity and quantum Griffiths singularity in macro-size monolayer NbSe2
王健研究组与合作者在大面积单层二硒化铌晶体中观测到伊辛超导和量子格里菲思奇异性的共存

Two-dimensional (2D) transition metal dichalcogenides (TMDs), regarded as the materials beyond graphene, have become a hot research topic due to a range of unique physical properties and potential applications of electronics, photonics, spintronics, and quantum computing devices. In previous reports, the mechanical exfoliation technique of micro-size TMD flakes has attracted particular interest on account of its simplicity and cost effectiveness. However, for most applications, high quality larger area crystalline films are preferred. Therefore, the preparation of high quality macroscopic area ultra-thin TMDs is highly desired in this field.

A recent review paper points out three major topics in 2D superconductors filed (Nat. Rev. Mater. 2, 16094 (2016)): quantum metal, quantum Griffiths singularity (QGS) and Ising superconductivity. In 2015, Jian Wang group and collaborators firstly reported the QGS in 2D and superconducting system (3 atomic layer Ga films, Science 350, 542(2015), Science 350, 509 (2015)), with the divergence of the dynamical critical exponent when approaching the zero temperature quantum critical point. Then they confirmed QGS in superconducting LaAlO3/SrTiO3(110) interface (PRB 94, 144517 (2016)). Theoretically, when the spins of the superconducting pairing electrons are strongly pinned by an effective Zeeman field, the superconductivity can survive in an ultrahigh external in-plane magnetic field far beyond the Pauli paramagnetic limit, which is named as Ising superconductivity. Experimentally, in both gating MoS2 flakes and NbSe2 flakes, Ising superconductivity was reported. Verifying QGS in Ising superconductors would not onlydemonstrate the universal property of QGS but also help tounderstand the underlying mechanism of the 2D superconductors where Ising superconductivity and QGS coexist.

Recently, Prof. Jian Wang at Peking University and Prof. Shuaihua Ji at Tsinghua University, in collaboration with Prof. Xi Lin, Prof. Ji Feng at Peking University, Prof. Qi-Kun Xue, Prof. Xi Chen at Tsinghua University, Prof. Mingliang Tian, Dr. Chuanying Xi at Heifei High Magnetic Field Laboratory of the Chinese Academy of Sciences, as well as Prof. Haiwen Liu at Beijing Normal University systematically studied macro-size (on the order of mm2) atomically flat monolayer NbSe2 films (only 0.6 nm thick) with Se protection layer grown on bilayer graphene terminated surface of 6H-SiC(0001) substrates by molecular beam epitaxy(MBE) method. The films exhibit an onset superconducting critical transition temperature above 6 K and the zero resistance superconducting critical transition temperature up to 2.40 K. The superconductivity is better than that of mechanical exfoliated monolayer NbSe2 flakes and MBE grown monolayer NbSe2 films in previous reports. Simultaneously, ultrahigh magnetic field measurements show that the extremely high parallel upper critical field Bc2// (T = 0) significantly exceeds the Pauli paramagnetic limiting field, consistent with Zeeman-protected Ising superconductivity. In this work, Bc2//(0) > 5Bp as T/Tc down to 0.13 is firstly demonstrated by high magnetic field up to 35 T and low temperature down to 0.35 K, other than data fitting extrapolation. Besides, by ultralow temperature and perpendicular magnetic field electrical transport measurements, the monolayer NbSe2 film shows the signature of QGS when approaching the zero-temperature quantum critical point. This is the first report of coexistence of two important quantum behaviors, Ising superconductivity and quantum Griffiths singularity, in superconductors at 2D limit.

The paper was published online on Nano Letters on October 10, 2017. (DOI: 10.1021/acs.nanolett.7b03026, http://pubs.acs.org/doi/10.1021/acs.nanolett.7b03026)

Prof. Jian Wang at Peking University and Prof. Shuaihua Ji at Tsinghua University are corresponding authors of this paper. Dr. Ying Xing at China University of Petroleum-Beijing and Dr. Kun Zhao at Tsinghua 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 Research Fund for the Doctoral Program of Higher Education of China, the Open Research Fund Program of the State Key Laboratory of Low Dimensional Quantum Physics, Tsinghua University, the Open Project Program of the Pulsed High Magnetic Field Facility, Huazhong University of Science and Technology and the Science Foundation of China University of Petroleum, Beijing.

Figure  Left: The characteristic magnetic field Bc for both perpendicular and parallel fields. The large parallel characteristic field Bc// provides the direct evidence of Ising superconductivity. The inset depicts the schematic diagram for heterostructure and standard four-probe configuration for electrical measurements. Right: superconductor-metal transition exponent as a function of magnetic field, diverges at critical point, provides the direct signature of quantum Griffiths singularity. The inset is atomic-resolved STM image on monolayer NbSe2 film.

二维层状过渡族金属硫化物(TMDs)在纳米电子学、光电子学和自旋电子学等领域有着潜在的应用价值,因此被誉为超越石墨烯(beyond graphene)的材料,已成为国际前沿领域的研究热点。在前期报道中,TMD薄片(横向尺寸为微米量级)主要由机械剥离技术获得,因其工艺简单和经济性而备受关注。然而实际应用倾向于高质量且更大面积的薄膜单晶材料。因此,制备高质量宏观面积的超薄过渡族金属硫化物对未来器件应用具有重要意义。

对于二维超导领域(Nat. Rev. Mater. 2, 16094(2016)),量子金属、量子格里菲斯奇异性(QGS)和以伊辛超导命名的超高平行临界磁场行为成为三个重要主题。王健课题组及合作者于2015年首次在二维超导体系中(3个原子层厚的Ga薄膜)观测到一种受无序影响的量子相变,其动力学临界指数在相变点发散,这种行为被称为量子格里菲斯奇异性(Science 350, 542(2015); Science 350, 509(2015))。随后课题组在LaAlO3/SrTiO3(110)界面超导中进一步证实了量子格里菲斯奇异性(PRB 94,144517(2016))。伊辛超导是指超导库珀对的自旋被有效的塞曼磁场固定住,由此表现出极强的的面内临界磁场(远超其泡利顺磁极限)。伊辛超导先后在栅极调控的过渡族金属硫化物MoS2薄片和NbSe2薄片中被发现和报道。

近日,王健教授和清华大学季帅华副教授、薛其坤院士、陈曦教授,北京大学林熙研究员、冯济研究员、中科院合肥强磁场中心的田明亮研究员、郗传英博士以及北京师范大学刘海文副研究员等人展开合作,通过分子束外延法在双层石墨烯终止的6H-SiC(0001)衬底上成功制备出大面积(毫米以上)原子级平整的高质量单层过渡族金属硫化物NbSe2薄膜(仅0.6 nm厚),在此基础上对其覆盖非晶态Se保护层,进而对非原位的电输运物性展开了系统研究。研究发现:单层NbSe2薄膜表现出超过6 K的起始超导临界转变温度和高达2.40 K的零电阻温度,超过了早期机械剥离获得的单层NbSe2以及分子束外延生长的单层NbSe2的超导转变温度。同时,强磁场和极低温下的输运测量结果直接证实了平行特征临界场Bc//(T = 0)是顺磁极限场的5倍以上,符合Zeeman保护的伊辛超导机制(前期NbSe2薄片中的伊辛超导证据需要实验数据的理论拟合在更低温更高磁场下的外推)。此外,极低温垂直磁场下的电输运测量表明,单层NbSe2薄膜在接近绝对零度时的量子临界点表现出量子格里菲斯奇异性。这是首次在同一体系中同时观测到伊辛超导和量子格里菲斯奇异性。

文章于2017年10月在Nano Letters上在线发表(DOI:10.1021/acs.nanolett.7b03026, http://pubs.acs.org/doi/10.1021/acs.nanolett.7b03026)

北京大学的王健教授和清华大学的季帅华副教授是本文的通讯作者。中国石油大学(北京)的邢颖博士和清华大学的赵琨博士为共同第一作者。

这项工作得到了国家重大科学研究计划、国家自然科学基金、中国高等教育博士研究基金、清华大学低维量子物理国家重点实验室开放研究基金、华中科技大学脉冲强磁场开放项目以及中国石油大学(北京)科学基金的资助。

图:左图为垂直和平行磁场下的特征临界磁场BC,插图为标准四电极法测量结构图。超大的平行特征临界磁场给出了伊辛超导的直接证据。右图为垂直磁场下超导-金属相变指数zv随磁场的变化趋势,在相变临界点发散,给出了量子格里菲思奇异性的证据。插图是单层NbSe2薄膜的原子尺度分辨率的STM图像。