量子材料大观园(1) | Ising专栏

海归学者发起的公益学术平台

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浪淘沙 · 文心涨落

无欲睹芳容，经典西东。北南风景枉玲珑。

正看时光回转后，反演无穷。

有意步萍踪，量子匆匆。对角一幕解三重。

欲挽本征何处是，简并寒冬。

一、让声子来存储信息——热导回线

在信息领域，热很大程度上是个坏小子。不过，也有时候，热导跟电导一样，是可以利用来存储信息的。与电输运比较，热输运毫无疑问比较慢、比较粗，且热量自由散漫惯了，难以驾驭，也是事实。不过，我们并不能由此就论断这种莫名其妙的想法一定没有科学的道理。事实上，传热对应的相与电荷输运对应的态也有很多类似的地方，并不一定令人感觉到“闷热”。

过去很多年，已经有很多工作来调控热导，且美其名阅“热管理”，可见已经到了相当娴熟的程度。在一些特定环境领域，这种热存储、热探测和热处理有其独特的优势。比如，可以利用升温降温循环对应的热导回线来“储存”材料的声子状态，也算是奇思妙想。这里，来自南京大学的一帮年轻人就对叠层的MoTe₂开展了热导测量与分析，堂而皇之地编撰了一个不失为美妙的故事。

原文链接：https://www.nature.com/articles/s41535-017-0031-x

Thermal conductivity: in a loop

The thermal conductivity across stacked MoTe₂ layers exhibits a hysteresis loop, as the temperature changes. Similar effects in other materials have been exploited for the implementation of thermal memories for the storage of phononic information. Now, a team from Nanjing University in China studies the thermal properties along the perpendicular axis of stacked layers of MoTe₂, a 2D material that undergoes a well-known structural phase transition around 250 K. The authors report an abrupt jump in the thermal conductivity around that temperature. The conductivity measured at 255 K during warming is about 10% higher than upon cooling, a hysteretic behavior dominated by phonons. With a performance comparable to other phase-change materials, MoTe₂ is a very promising candidate for the implementation of all-phononic thermal memories, working at 255 K with the thermal information being “written/erased” by adjusting the temperature.

link: https://www.nature.com/articles/s41535-017-0031-x

a & b Hysteretic behaviors of c-axis thermal conductivity in the metastable phase MoTe₂ sample 1 & 2. The triangular scatters represent the experimental data measured by ‘the static method’, while the solid lines represent the results continuously measured by ‘the dynamic method’. The red and blue arrows indicate the directions of warming up and cooling down routes, respectively. The inserted images show the locations of focused laser spots on the MoTe₂  samples during the TDTR measurements.

二、您有暗物质，我有隐藏序

铁基超导和铜基超导一样，物理丰富多彩但是缺乏核心灵魂信仰，比如铁基超导的核心配对机制是啥？甚至目前对铜基高温超导电子配对机制的争论有“复辟”的势头，这可不是一件小事。

最近有一些学派开始信仰向列序在铁基超导电子配对中所起的作用。问题是有些体系根本就不存在磁有序，哪里来的向列序呢？后来又有人勾画出所谓的hidden orders，看起来好像跟宇宙学的暗物质和暗能量差不多，令人莫名其妙，却也按捺不住内心的波澜激动。总之，物理学家总是希望给铁基超导配对机制找一个新的诞生地：磁涨落！

事实上，物理学的研究很多情况下就是莫名其妙的，所以才有张力和驱动力！这个勾画很了不起！这里是一个很好的例子。

原文链接： https://www.nature.com/articles/s41535-017-0036-5

Iron-based superconductors: Hidden nematic and magnetic fluctuations in iron selenide

Ultrafast spectroscopy unveils hidden nematic fluctuations and a spin subsystem in theiron-based superconductor iron selenide. Layered iron-based materials recently emerged as a new class of high temperature superconductor. The mechanism of superconductivity in these materials, however, is a contentious issue. Nematic ordering is thought to be a key ingredient, but the apparent absence of magnetic ordering in iron selenide, which is the iron-based superconductor with the simplest structure, has caused confusion over what drives the nematicity. An international team of researchers led by Chih-Wei Luo and Jenh-Yih Juang from National Chiao Tung University use polarized ultrafast spectroscopy tounveil a hidden spin subsystem in FeSe, along with both nematic and magnetic fluctuations at relatively high temperatures, providing insights into the driving factors of nematicity in this fascinating material.

link: https://www.nature.com/articles/s41535-017-0036-5

Phase diagram of FeSe by nematic ultrafast dynamics. Temperature dependence of the resistivity ρ shows clearly an anomaly at T s and indicates the high quality of FeSe together with a large residual-resistance ratio (RRR). T* denotes the temperature at which ρ(T) shows a rapid change of slope. Insets illustrate the nematic evolution of charge and spin subsystems in various phases. The thin arrows indicate sketchily the individual moment of Fe ions. The thick arrows indicate the “net” magnetic moments of FeSe in the stripe form. The simplified FS in each temperature range is depicted. The picture of FS for T < T_s follows ref. 49. The dashed green line denoted the proposed FS fluctuations at the Γ point.

三、电解质栅极就是王道

过去几年，将液态和聚合物电解质作为栅极层来调控半导体及至各种各样新功能已经成为大热门。只要将需要关注的材料做成沟道，电解质做成栅极(液态电解质通过滴一滴就OK了^_^)，就可以开始捣鼓了。因为液态电解质的离子电荷在栅极电场作用下很容易迁移聚集在界面处，形成很大的栅极电场，沟道层材料的一些难以企及的新效应就被脱掉包装、露出原形。

当然，这里的问题是，电解质与沟道层之间在界面处的电化学反应和其它高场下的物理很复杂，说得好听一点是“非常丰富”，说得不好听就是“反复无常”！最近清华大学的于浦博士就干了一票，将这种复杂性和衍生出来的一些新效应挖掘出来。这里也是一个例子，点击如下链接可阅读原文。

原文链接：https://www.nature.com/articles/s41535-017-0039-2

Electrolyte gating: Hydrogenation mechanism in WO₃

The mechanism leading to large carrier density changes and even concomitant electronic phase transitions with electrolyte gating is under debate. An international team led by Ivan Božović at USA’s Brook haven National Laboratoryand Yale University report a series of experiments based on WO₃ films, which is found to exhibit an insultator-to-metal transition undergating, with both ionic liquids and polymer electrolytes. The experimental results allow to rule out some mechanisms—such as charge accumulation near the interface or oxygen vacancy formation—previously suggested in other material systems. Instead, the authors propose that the primary effect of electrolytegating in WO₃ is hydrogen intercalation. Hydrogenation leads to the formation of a dense polaronic gas that explains the conductive ground state.The doping mechanism behind electrolyte gating seems to be material dependent.

link: https://www.nature.com/articles/s41535-017-0039-2

四、锶钌氧化合物中什么都有

锶钌氧化合物是凝聚态研究4d物理的最好对象，也是研究超导母体物理的极好样本，几十年长盛不衰。最近对超导物理和材料的研究有一个趋势，就是去看低维和表面，有很多出人意料的结果出来。比如，好像开始有人问铜基高温超导氧化物到底有没有d波啊？之前所有的“反常”超导是不是都是杂质或者缺陷所致啊？

如果是这样，那就要逆天了。这里，回归到经典高温超导体系的超导电性问题，看看磁关联到底在干什么！

原文链接：https://www.nature.com/articles/s41535-017-0041-8

Unconventional superconductivity: role of magnetic interactions in strontium ruthenate

A new framework for analysing the role of magnetic interactions on the unconventional superconductivity in strontium ruthenate. Strontium ruthenate is an unconventional superconductor that used to be touted a potentialthree-dimensional analogue of Helium-3, as it was thought to have the same type of chiral p-wave pairing. It is now widely accepted that this is not the case, but many questions remain over the exact nature of the pairing, particularly regarding the role of magnetic interactions. An international team of researchers led by Bongjae Kim and Sergii Khmelevskyi from the University of Vienna and Vienna University of Technology now present a framework that can incorporate the leading isotropic and anisotropic magnetic interactions in a different but complimentary way to the widely used Hubbard-model, providing analternative way of exploring the superconducting pairing symmetry.

link: https://www.nature.com/articles/s41535-017-0041-8

Lowest energy magnetic structures (q=(1,1,0)2π/3a) of RuO₂ basal plane in Sr₂RuO_4. The a–c structures represent different types of spiral magnetic order and d–f corresponds to the collinear up-up-down magnetic order with different moment directions.

五、负磁阻是威尔费米子的尾巴

威尔费米子因为据说没有有效质量，成为一类准粒子，这好像是公认知识。不过，如果一个凝聚态体系存在铁磁性的话，这种没有有效质量的准粒子会有什么新特性？至少从材料角度去寻找合适的对象来关注这个问题是有价值的。我们很荣幸发表了浙大袁辉球老师在CeSb体系中看到的负磁阻效应，并且与某种威尔费米子联系起来。您看，看点在这里，其实一点就亮了。

原文链接：https://www.nature.com/articles/s41535-017-0038-3

Condensed matter: Magnetic collectivity

A signature of an exotic state of matteris identified in a magnetic material by researchers in China and Germany. There sults from Huiqiu Yuan from Zhejiang University and co-workers indicate a newclass of topological materials. In the right circumstances, the electrons in amaterial work collectively to behave like particles known as Weyl fermions.Weyl fermions have effectively no mass, making them a fascinating test-bed for novel physics. Yuan and colleagues observed a pronounced negative magnetoresistance when a magnetic field was applied in a direction parallel to an electric current passing through caesium antimonide at low temperature. This signature of a Weyl-fermion state was further supported by electronic-structure calculations. While these evasive particles have been identified in a number ofmaterials in the past, caesium antimonide is different because it is ferromagnetic.

link: https://www.nature.com/articles/s41535-017-0038-3

Large positive magnetoresistance in CeSb for the magnetic field applied perpendicular to the current. a Temperature dependence of the electric alresistivity of CeSb in various applied fields with B ⊥ I. At low temperatures, the field leads to a significant enhancement of the resistivity indicating a large positive magnetoresistance. b Upper panel: Magnetoresistance as a function of applied field with B ⊥ I at various temperatures, demonstrating that the positive magnetoresistance becomes much more significant below 10 K. The arrows point to B FM, the field at which there is a transition from an AFF to the FM state with increasing field. b Lower panel: Magnetoresistance as afunction of applied field for different θ, where θ isthe angle between the applied field and the current. As θ is reduced from 90°, the magnetoresistance decreases and becomes negative at highfield near 0°

注：文中英文简介由Nature Publishing Group编辑队伍专门为npj Quantum Materials刊物论文所撰写，于每篇文章的链接里可以看到。

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