量子材料大观园(1)

物理学进展 第39卷 第1期 2019年2月 PROGRESS IN PHYSICS

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刘俊明 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。

I. 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 名曰“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 lay-ers 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 con-ductivity around that temperature. The conductivi-ty measured at 255 K during warming is about 10% higher than upon cooling, a hysteretic behavior domi-nated by phonons. With a performance comparable to other phase-change materials, MoTe2is a very

promis-ing candidate for the implementation of all-phononic thermal memories, working at 255 K with the thermal

FIG. 1 (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 rep-resent 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

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information being “written/erased” by adjusting the temperature.

II. 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 fluc-tuations and a spin subsystem in theiron-based super-conductor iron selenide. Layered iron-based materials recently emerged as a new class of high temperature su-perconductor. The mechanism of superconductivity in these materials, however, is a contentious issue. Nemat-ic 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 sim-plest structure, has caused confusion over what drives the nematicity. An international team of researchers led by Chih-Wei Luo and Jenh-Yih Juang from Na-tional Chiao Tung University use polarized ultrafast spectroscopy tounveil a hidden spin subsystem in FeS-e, along with both nematic and magnetic fluctuations at relatively high temperatures, providing insights in-to the driving facin-tors of nematicity in this fascinating material.

FIG. 2 Phase diagram of FeSe by nematic ultrafast dynam-ics. Temperature dependence of the resistivity ρ shows clearly an anomaly at Ts 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 ar-rows 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 tem-perature range is depicted. The picture of FS for T < Ts

follows ref. 49. The dashed green line denoted the proposed FS fluctuations at the Γ point

III. 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

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changes and even concomitant electronic phase tran-sitions with electrolyte gating is under debate. An in-ternational team led by Ivan Boˇzovi´c at USA’s Brook haven National Laboratory and Yale University report a series of experiments based on WO3 films, which is

found to exhibit an insultator-to-metal transition under gating, with both ionic liquids and polymer electrolytes. The experimental results allow to rule out some mecha-nisms – 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 electrolyte gating in WO3 is

hydrogen intercalation. Hydrogenation leads to the for-mation of a dense polaronic gas that explains the con-ductive ground state. The doping mechanism behind electrolyte gating seems to be material dependent.

IV. 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 magnet-ic interactions on the unconventional superconductivity in strontium ruthenate. Strontium ruthenate is an un-conventional superconductor that used to be touted a potential three-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 mag-netic interactions. An international team of researchers led by Bongjae Kim and Sergii Khmelevskyi from the University of Vienna and Vienna University of Technol-ogy now present a framework that can incorporate the leading isotropic and anisotropic magnetic interaction-s in a different but complimentary way to the widely used Hubbard-model, providing analternative way of exploring the superconducting pairing symmetry.

FIG. 3 Lowest energy magnetic structures (q = (1, 1, 0)2π/3a) of RuO2 basal plane in Sr2RuO4. The (a)–(c) structures represent different types of spiral mag-netic order and (d)–(f) corresponds to the collinear up-up-down magnetic order with different moment directions

V. 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 identi-fied in a magnetic material by researchers in China and Germany. There sults from Huiqiu Yuan from Zhe-jiang University and co-workers indicate a newclass of

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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 mag-netic field was applied in a direction parallel to an elec-tric current passing through caesium antimonide at low temperature. This signature of a Weyl-fermion state was further supported by electronic-structure calcula-tions. While these evasive particles have been identi-fied in a number ofmaterials in the past, caesium anti-monide is different because it is ferromagnetic.

FIG. 4 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 resistiv-ity indicating a large positive magnetoresistance. (b) Up-per panel: Magnetoresistance as a function of applied field with B ⊥ I at various temperatures, demonstrating that the positive magnetoresistance becomes much more signif-icant 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 be-comes 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。