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Science Advances reports ICQM Faculty members Han and Xie’s experimental breakthrough towards spin superconductor
Science Advances报道韩伟、谢心澄研究组在自旋超导领域的重要进展

Spin superconductor is a novel emerging quantum matter arising from the Bose–Einstein condensate of spin-1 bosons. Similar to charge superconductor, it is expected to exhibit zero spin resistance and the electric Meissner effect. The zero spin resistance means the dissipationless spin current in the spin superconductor state, despite it is an insulator for charge current. The electrical Meissner effect means the expulsion of the electrical gradient in the spin superconductor states. Besides, spin Josephson effect is predicted to exist within the weak link between two superconductors with different phases. However, there is a distinct difference between a spin superconductor and a charge superconductor. The charge superconductor is formed by the Cooper pairs, while the spin superconductor arises from the Bose–Einstein condensate of spin-1 bosons with zero charge, such as electron-hole pair or magnon with spin-1. Spin superconductor has attracted a lot of attention due to its unique quantum properties. Recently, the spin transport in spin superfluid has been proposed in theory for ferromagnetic graphene and the ν = 0 quantum Hall state of graphene, ferromagnetic materials and antiferromagnetic insulators. However, the experimental observation has not been reported yet. The key challenge is the experimental observation of spin superconductor ground state and the spin transport in such state.

Recently, a major experimental breakthrough has been achieved by the ICQM team led by Prof. Wei Han and Prof. Xincheng Xie; the observation of experimental signatures of spin superfluid ground state. Using pulsed laser deposition technique, they grew single crystalline and atomically flat Cr2O3 thin films, which are antiferromagnetic and charge-insulating materials. The spin transport is performed using the nonlocal geometry; First, the spins are injected into the antiferromagnetic Cr2O3 thin films via thermal means. Then the spin information transports in the spin superfluid state, which ensures the nondissipative propogation towards the right Pt electrode. Subsequently, spins could be detected by voltage measurement across the Pt strip via the inverse spin Hall effect using standard low frequency lock-in technique (Fig. A). A large enhancement of the nonlocal spin signal is observed below ~ 20 K, and it saturates from ~ 5 K down to 2 K, which is a signature of the zero spin resistance effect, one of the main properties of the spin superfluid ground states (Fig. B). Furthermore, their experimental results show that the spins can propagate over very long distances (~ 20 µm) in such spin superfluid ground state, and the nonlocal spin signal decreases very slowly as the spacing increases, presenting an inverse relationship, which is consistent with theoretical prediction (Fig. C). This work is a major experimental breakthrough in the field of spin superconductor. The experimental demonstration of the spin superfluid ground state in canted antiferromagnet will be extremely important for the fundamental physics on the BEC of spin-1 bosons and paves the way for future spin supercurrent devices, such as spin-Josephson junctions.

 This work has been published in Science Advances on April 13th, 2018 (Science Advances,4, eaat1098 (2018). DOI: 10.1126/sciadv.aat1098). The link to the paper is: http://advances.sciencemag.org/content/4/4/eaat1098.

 This work is proposed and supervised by Prof. Wei Han and Prof. Xincheng Xie. The work benefits a lot from the discussion and help from Prof. Ryuichi Shindou, Prof. Jing Shi and Prof. Xi Lin. The first author is Mr. Wei Yuan, a fifth year graduate student of the International Center for Quantum Materials. Prof. Wei Han, Prof. Jing Shi, and Prof. Xincheng Xie are corresponding authors.The research is financially supported by National Basic Research Programs of China (973 program), National Natural Science Foundation of China, the 1000 Talents Program for Young Scientists of China, and the Collaborative Innovation Center of Quantum Matter.

Figure: Experimental signatures for spin superfluid ground states in canted antiferromagnet Cr2O3 via nonlocal spin transport. (A) Schematic of the nonlocal spin transport geometry for the spin transport measurement in the spin superfluid ground state. The canted magnetization direction is controlled by the external magnetic field along the x direction. In such canted antiferromagnetic configuration, the spin component (Sy + iSz) that is perpendicular to the magnetic field direction becomes coherent in the spin superfluid state. (B) The temperature dependence of the nonlocal spin signal. (C) The spacing dependence of nonlocal spin signal in the spin superfluid ground state. The red dashed line is the fitting curve based on theoretical model of spin transport in spin superfluid.

  自旋超导态是一种与电荷超导态对应的新型量子态,指电荷为零自旋非零的波色子在低温时凝聚成的超流态。自旋超导态具有零自旋阻现象和电迈斯纳效应:零自旋阻指的是自旋流能够无耗散地流过自旋超导态,虽然它是电荷绝缘体;电迈斯纳效应指自旋超导态的电场梯度屏蔽作用。此外,自旋约瑟夫森效应也被理论预言存在于二个具有相位差的自旋超导体构成的弱耦合结中。自旋超导态也与电荷超导态存在着基本不同,电荷超导是由电子或空穴配成库珀对,但是自旋超导态是由电荷为零自旋非零玻色子凝聚产生的,比如电荷-空穴对或者磁子等电荷为0,自旋为1的玻色子等。由于自旋超导态具有的独特量子性质,近年来受到了广泛关注。理论上预言铁磁性石墨烯、磁性绝缘体和反铁磁绝缘体等多种材料体系可能是自旋超导态的载体,但是在实际实验体系中自旋超导态还没有被观测到。如何在实验上观测到自旋超导基态和其中的自旋输运是自旋超导研究领域的核心问题。

  最近,北京大学量子材料中心韩伟、谢心澄等组成的研究团队在国际上首次成功从实验上观测到了自旋超流基态的重要实验证据,是自旋超导领域的一项重大突破性进展。研究小组首先利用激光分子束外延技术生长了具有原子级别平整度的反铁磁Cr2O3薄膜,是电荷的绝缘体。采用非局域自旋输运的技术,用热方法在铂电极和Cr2O3薄膜界面注入自旋流、产生自旋压,在另外一个铂电极处利用铂的自旋霍尔效应测量自旋流的输运(图A)。实验数据显示在低温下自旋输信号出现饱和现象,对应着自旋导的饱和,也就是零自旋阻效应;即自旋超流基态的最重要基本性质之一(图B)。在此基础上,该研究小组又系统研究了不同自旋输运距离下自旋超流的输运现象,证明了自旋在该自旋超流基态可以进行长距离的输运,并且其随输运距离的关系与自旋超流态输运理论预言一致(图C)。该工作是是自旋超导态领域研究的一项重大突破,势必推动自旋超导态的快速发展,为研究基于自旋玻色子的玻色爱因斯坦凝聚的基础物理研究提供了实验平台,并为新型量子自旋器件,如自旋流约瑟夫森结等,奠定了实验基础。

  该工作于2018年4月13日发表于物理著名学术期刊Science Advances上(Science Advances,4,eaat1098(2018).DOI: 10.1126/sciadv.aat1098)。论文链接是:http://advances.sciencemag.org/content/4/4/eaat1098.

  该项工作由韩伟研究员、谢心澄院士的合作设计和指导完成的,课题进展工程中得到了Ryuichi Shindou研究员、施靖教授和林熙研究员的帮助。北京大学量子材料科学中心2013级博士生袁伟为文章第一作者,韩伟研究员、施靖教授和谢心澄院士为文章共同通讯作者。该项工作得到了国家重大科学研究计划、国家自然科学基金、中组部青年千人计划和量子物质科学协同创新中心的支持。

  图:自旋超流基态的重要实验证据。(A)非局域自旋输运测量示意图。用热方法在左边铂电极和Cr2O3薄膜界面注入自旋流,在右边铂电极处利用铂的自旋霍尔效应测量自旋流的输运。(B)自低温下自旋输信号出现饱和现象,反映出自旋超流基态的零自旋阻效应。(C)自旋信号随其随输运距离的关系与自旋超流态输运理论预言一致。