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ICQM faculty member publishes an article in Physical Review Letters reporting ‘Nematic Crossover in BaFe2As2 under Uniaxial Stress’
《物理评论快报》刊登量子材料科学中心李源研究组及合作者对铁基超导材料BaFe2As2在单轴向压力下的向列序crossover的研究

    In the phase diagram of iron-based superconductors, superconductivity is commonly situated near the vanishing point of the phase boundary between tetragonal and orthorhombic crystal structures. This commonality has invoked major research efforts to elucidate the nature of the so-called nematic phase, in which the discrete C4 rotational symmetry is lowered into C2 but the lattice translational symmetry is intact. The exact onset temperature of spontaneous nematicity, Tnem and, in particular, whether Tnem is simply the orthorhombic structural transition temperature Ts or is even higher, has been a subject of intense research interest and debate.

   Recently, Prof. Yuan Li and his PhD student Xiao Ren, undergraduate students Lian Duan, Yuwen Hu and Jiarui Li published a paper titled ‘Nematic Crossover in BaFe2As2 under Uniaxial Stress’ in Physical Review Letters115, 197002 (2015). The work was done in collaboration with Prof. Pengcheng Dai’s group at Rice University, USA and the Institute of Physics, Chinese Academy of Sciences, where high-quality single crystals of BaFe2As2 were synthesized. Through systematic measurement of Eg phonon splitting as a function of temperature using high-resolution Raman spectroscopy, in both freestanding and uniaxially-stressed BaFe2As2 single crystals, it is revealed that (1) modest uniaxial stress on the order of a few MPa changes the phase transition at Ts into a crossover that spans a considerable temperature range above Ts, and (2) a hard-to-avoid distribution of residual stress of the same order of magnitude remains in nominally freestanding sample, causing similar crossover phenomena that can be easily mistaken as a separate phase transition above Ts. The results indicate a lack of genuine phase transition at T > Ts.

   Compared toprevious experimental attempts on resolving this issue, most of which required the use of single-domain samples produced under external stress, this is the first time to use Raman scattering, which can detect spontaneous point-group symmetry breaking without resorting to single-domain samples, to investigate the phase boundary of the nematic order in detail. The work provides strong evidence for a non-negligible influence of stress, either externally applied or residual in realistic samples, on the phenomena that are seemingly indicative of C4 symmetry breaking, hence establishing the existence of only one true nematic phase transition in the phase diagram.

    The work was supported by the NSF of China (Grant No. 11374024) and the NBRP of China (Grant No. 2013CB921903).


Figure: Raman spectroscopy can detect point-group symmetry breaking without resorting to single-domain samples. In the above figure, it is shown that upon application of a uniaxial stress on a BaFe2As2 crystal, phonon energy in Raman spectra obtained near the transition temperature Ts changes significantly, contrary to a commonly used assumption that the uniaxial stress only ‘detwins’ the sample without altering the physics. The observed behavior is typically found when a sharp phase transition becomes a broad crossover under a polarizing field.

    在铁基超导相图中,超导经常存在于晶格四方相和正交相的相界线消失点附近。由于这种共性的存在,一种只有四重到二重的旋转对称性破缺但无晶格平移对称性破缺的向列序相被广泛研究。在这些研究中,向列序自发发生温度Tnem 特别是Tnem 等于还是高于正交结构相变温度Ts是被关注的重点。

       最近,北京大学量子材料科学中心李源研究员及其直博生任霄,本科生段炼,胡毓文,李佳睿在《物理评论快报》 Physical Review Letters 115, 197002 (2015))中发表题为“Nematic Crossover in BaFe2As2 under Uniaxial Stress”的文章。该项工作与美国Rice大学以及中科院物理所戴鹏程研究组合作完成,并由其提供高质量的BaFe2As2单晶样品。李源研究组通过运用高分辨率拉曼光谱仪系统的测定加压以及不加压BaFe2As2单晶中Eg声子峰宽随温度的变化行为,得出以下结论:(1)在兆帕量级下,适度的轴向压会将在Ts 温度发生的结构相变转变为延伸到Ts温度以上很大范围的crossover现象;(2)在名义上不加压的样品中存在的难以避免的兆帕量级的残应力会引起和上述(1)中一样的crossover现象,并且这种现象会容易被误认为是在结构相变温度Ts以上产生的独立相变。以上结果表明,BaFe2As2单晶在结构相变温度Ts以上的温度范围内不存在一个真正的相变。

       与以往需要通过额外加压力的方式得到单筹晶体来研究铁基超导材料向列序发生温度的实验不同的是,李源研究组首次采用拉曼光谱的方法详细研究向列序边界点问题。这种方法可以在不依赖于单筹晶体的前提下直接探测自发的点群对称性破缺。此项工作有力的印证了来自于额外的或者是样品中自身残存的压力确实对结构相变温度以上发生的四重对称性破缺有着不可忽略的影响,同时也表明,在铁基超导相图中真正的向列序转变只有一个。

       上述研究得到国家自然科学基金以及国家重大科学研究计划的经费支持。

  

图注: 拉曼光谱仪可以在不依托单筹样品的基础上直接探测点群的对称性破缺。上图表明在轴向压作用下,BaFe2As2Eg声子在结构相变温度Ts附近有着显著的改变。这和通常所认为的压力只是对样品进行了简单的退筹而没有改变样品中物理性质这一假设是相违背的。图中观察到的现象在极化场作用下由一个窄的相变演变成一个宽的crossover的过程中经常发生。