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ICQM members publish an article in Physical Review Letters reporting "Magneto-infrared spectroscopy of Landau levels and Zeeman splitting of 3D massless Dirac Fermions in ZrTe5"
《物理评论快报》刊登量子材料科学中心对三维狄拉克费米子材料ZrTe5的红外磁光光谱研究

Three dimensional (3D) topological Dirac/Weyl semimetals are currently a new frontier and focus topic in condensed matter physics and materials science. Their low energy electronic excitations are located around a few Dirac/Weyl nodes in momentum space and resemble the relativistic Dirac/Weyl fermions in high energy physics.

Recently Prof. Nanlin Wang’s group in ICQM performed magneto infrared spectroscopy on ZrTe5 single crystals, which were identified to be a 3D Dirac semimetal earlier by transport and ARPES measurements. Their initial optical spectroscopy measurement without magnetic field revealed a linear rising of the optical conductivity with frequency over a relatively broad frequency range, which is a hallmark of 3D massless Dirac fermions. This result was published in Physical Review B 92, 075107 (2015). When magnetic field was applied, transitions between Landau levels were clearly observed in optical reflectance. The transition energies follow the sequence of 1:1+√2:√2+√3:… and are proportional to the square root of the magnetic field in the low field regime, which is a strong evidence of massless Dirac dispersion. It is also found that an exceptionally low magnetic field (less than 1 Tesla) was capable of driving the compound into its quantum limit, which demonstrates that ZrTe5 is an extremely clean system and ideal platform for studying 3D massless Dirac fermions. Most significantly, the splitting of Landau levels at relatively small magnetic field was explicitly identified in the reflectance data. Such Zeeman splitting has never been observed in 2D massless Dirac fermions, such as in graphene. Prof. Fa Wang’s theoretical group at the ICQM built an effective model for this material and analyzed the experimental data. They found that the ZrTe5 compound was transformed into a linenode semimetal under the current experimental configuration with Zeeman field along the crystalline b axis. According to this analysis the Dirac node of this material will be split to two Weyl nodes if the Zeeman field is applied along the c axis, leaving open the possibility of controlled realization of Weyl semimetals in this compound.

This is the first magneto-infrared measurement on 3D Dirac materials. This work has been published in Physical Review Letter 115, 176404 (2015) and selected as Editor’s Suggestion. Rongyan Chen (post doctor in Prof. Nanlin Wang’s group) and Zhiguo Chen (former student of Prof. Nanlin Wang) are leading authors and contribute equally to this work. The samples are provided by Prof. Genda Gu’s group from Brookhaven national laboratory in the US. Prof. Nanlin Wang and Prof. Fa wang are both corresponding authors of this paper.

The work was supported by the National Science Foundation of China, the 973 project of the Ministry of Science and Technology of China,and the Collaborative Innovation Center of Quantum Matter in Beijing, China.

        三维拓扑狄拉克/外尔半金属是近期凝聚态物理和材料科学领域的一个新的研究热点,并得到了广泛的关注。这些材料的低能激发集中在动量空间的一些狄拉克/外尔点附近, 与相对论高能物理中的狄拉克/外尔费米子非常相似。   

  最近,量子材料中心的王楠林研究组对ZrTe5单晶进行了红外磁光光谱研究。在此之前,针对ZrTe5的输运和角分辨光电子能谱测量表明该材料属于三维狄拉克半金属。在零场下,红外光谱的测量发现其光电导在较宽的能量区间都表现出随频率线性增加的行为,而这种现象恰恰是三维狄拉克费米子的标志性行为。这个研究结果已经被发表在《物理评论B》92, 075107 (2015)。当对样品加磁场时,从反射光谱上可以明确的观察到朗道能级之间的跃迁。这些跃迁的能量依次满足1:1+√2:√2+√3:…的比例,并且低场时与场强的平方根成正比,这些都是无质量狄拉克费米子的有力证据。另外,研究表明只需要加较低的磁场(低于1特斯拉)就足以驱动样品到达量子极限,说明ZrTe5是一个非常干净的体系,是研究三维狄拉克费米子的理想材料。最重要的是朗道能级的劈裂可以直接从反射谱上观察到,而这种劈裂在二维狄拉克费米子材料(例如石墨烯)中从来没有探测到过。量子材料科学中心的王垡理论小组建立了相关理论模型对实验数据进行了分析,他们发现在现有的磁光实验配置中,沿着晶轴b方向的塞曼场会将ZrTe5单晶转变为“线节点”半金属。而且理论计算表明,如果沿着晶轴c方向加磁场,那么该材料中的狄拉克点会劈裂成两个外尔点,这为用磁场操控和实现外尔半金属留下了足够的可能。   

  该研究是第一个针对三维狄拉克费米子材料的红外磁光测量,已发表在《物理评论快报》115, 176404 (2015),并被选为编辑推荐文章。王楠林教授的博士后陈荣艳和已经毕业的学生谌志国博士是该文章的并列第一作者。样品是由美国布鲁克海文国家实验室的顾根大教授提供的。王楠林教授和王垡教授是该文章的共同通讯作者。   

  上述研究得到国家自然科学基金、中国科技部973项目以及量子物质科学协同创新中心等项目经费的资助。

   

     

  磁场下朗道能级的形成以及塞曼效应引起的朗道能级劈裂示意图。