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Science magazine reports the discovery of quantum Griffiths singularity of superconductor-metal transition in Ga thin films
《科学》发表量子材料科学中心在超导领域的重要发现:二维超导的量子Griffiths相变

   More than 40 years ago, Robert B. Griffiths predicted that phase transitions can be dramatically changed by disorder effect and in particular the dynamical critical exponent can diverge. In the last 40 years, this theory has been applied to quantum phase transitions and developed into the theory of “quantum Griffiths singularity”. However, the major signature of the theory, the divergence of dynamical critical exponent, is very difficult to observe in experiments.   

   Recently, Prof. Jian Wang, in collaboration with Prof. Xincheng Xie, Prof. Xi Lin, and Prof. Fa Wang at Peking University, as well as Prof. Qi-Kun Xue and Prof. Xu-Cun Ma at Tsinghua University, observed for the first time the quantum Griffiths singularity in two dimensional (2D) superconducting system. They studied three monolayer thick Ga films in ultralow temperature regime, in which 2D superconductivity and superconductor to metal transition were detected. Furthermore, when approaching the zero temperature quantum critical point, they found the divergence of the dynamical critical exponent, which is consistent with the Griffiths singularity behavior. The superconductor-metal quantum phase transition in this 2D superconducting system with disorder could thus be explained by the theory of quantum Griffiths singularity, which is different with the previous understanding of quantum phase transition in 2D superconductors.   

  The paper was selected by Science Express and online published in Science on October 15, 2015 (DOI: 10.1126/science.aaa7154): http://www.sciencemag.org/content/early/2015/10/14/science.aaa7154.full.

   Prof. Jian Wang, Prof. Xi Lin at Peking University and Prof. Xu-Cun Ma at Tsinghua University are corresponding authors of this paper. Ying Xing, Hai-Long Fu, Dr. Haiwen Liu at Peking University and Hui-Min Zhang at Institute of Physics, Chinese Academy of Sciences contributed equally to this work.   

  The work was supported by National Basic Research Programs of China, National Natural Science Foundation of China, 1000 Talents Program for Young Scientists of China, the Research Fund for the Doctoral Program of Higher Education (RFDP) of China, and Collaborative Innovation Center of Quantum Matter, China.

  

  

   Figure: The superconductivity in 3 monolayer thick Ga film and the divergence of the critical exponent approaching the quantum critical point (zero temperature limit).

        物质的物相和相变是物理学领域最重要的科学问题之一,相关研究曾多次获得诺贝尔奖。四十多年前Robert B. Griffiths从理论上预测,无序效应会定性地改变物相和相变临界点的行为,特别是临界点的动力学临界指数将趋于无穷大,这种现象被称作Griffiths奇异性。随着时间推移,这一预测现已拓展到量子相变形成了量子Griffiths奇异性理论。所谓量子相变,是指在绝对零度下系统处于量子基态时随着参数变化而发生的相变。然而实验上要直接观测到动力学临界指数的发散行为,即量子Griffiths奇异性,非常困难。超导体作为一种重要的量子物质和物相,其量子相变与量子临界点现象已得到学术界的广泛关注,但直到最近仍未在超导中发现量子Griffiths奇异性行为。   

  二维超导体,因其中量子涨落或热力学涨落带来的诸多新奇现象,以及在无耗散或低耗散的电子学方面的潜在应用价值,已成为超导领域的重要研究方向。2015年美国凝聚态物理最高奖Buckley奖颁发给四位物理学家,以表彰他们在二维超导体系中发现超导-绝缘体相变现象,该相变被认为是量子相变的范例。超导-绝缘体相变早在二十多年前就被报道,随后研究者发现了性质类似的超导-金属相变,但如今仍有不少实验或理论相互间有所冲突和争议。对于二维超导中磁场调制的超导-绝缘体(或金属)相变,早期发现不同温度下电阻随磁场的响应曲线会交于一点,该点被称为量子临界点,对应的临界指数可以从不同温度下电阻-磁场曲线的标度行为得到,这个临界指数与样品的细节无关是一个确定的数值。然而近期有实验报道在不同温度区间观测到两个不同的量子临界点及不同的临界指数。   

  最近北京大学物理学院量子材料科学中心王健研究组,与中心谢心澄教授、林熙研究员、王垡研究员,以及清华大学的薛其坤院士和马旭村研究员等人合作,在三个原子层厚(小于1纳米厚)的Ga(镓)薄膜中发现了二维超导和超导-金属相变行为。实验中的Ga超薄膜是通过分子束外延生长在GaN(氮化镓)衬底上,并采用100纳米量级厚度的无定形硅作为覆盖层进行保护。研究人员通过最低到25mK的极低温精密测量发现,Ga超导薄膜中超导-金属相变对应的临界点既不是以前报道的一个临界点也不是近期报道的两个临界点,而随着温度变化形成了一条临界线。相应的临界指数在临界线上连续变化,在趋近零温量子临界点时会发散,而不是通常认知的固定值。因此,传统的超导量子相变理论无法解释该实验结果。分析表明该相变正是理论上预测已久的量子Griffiths奇异性。这是首次在低维体系以及超导体系中发现和证实量子Griffiths奇异性,并且有可能是对超导-金属相变的具有普适性的物理解释。这项工作不仅是发现了一种新的量子相变,而且对超导(包括高温超导)等量子材料体系中量子临界行为的理解提供了新的思路。相关文章于2015年10月15日提前在线发表在Science上(Science DOI: 10.1126/science.aaa7154):http://www.sciencemag.org/content/early/2015/10/14/science.aaa7154.full。北大谢心澄教授建议了在极低温下观测量子相变行为的实验,并与刘海文、王垡负责此项工作的理论部分。北大王健研究员、林熙研究员与清华马旭村研究员为文章并列通讯作者。北大博士生邢颖、付海龙、助理研究员刘海文与中科院物理所博士生张慧敏为文章并列第一作者。   

  上述研究得到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、高等学校博士学科点专项科研基金以及量子物质科学协同创新中心等项目经费的资助。       

  图:三个原子层厚的镓薄膜超导与临界指数在靠近量子相变点(绝对零度)时的发散行为。