量子材料大觀園(1)

劉俊明

浪淘沙·文心漲落

無欲睹芳容，經(jīng)典西東。

北南風景枉玲瓏。

正看時光回轉后，反演無窮。

有意步萍蹤，量子匆匆。

對角一幕解三重。

欲挽本征何處是，簡并寒冬。

I.讓聲子來存儲信息-熱導回線

在信息領域，熱很大程度上是個壞小子。不過，也有時候，熱導跟電導一樣，是可以利用來存儲信息的。與電輸運比較，熱輸運毫無疑問比較慢、比較粗，且熱量自由散漫慣了，難以駕馭，也是事實。不過，我們并不能由此就論斷這種莫名其妙的想法一定沒有科學的道理。事實上，傳熱對應的相與電荷輸運對應的態(tài)也有很多類似的地方，并不一定令人感覺到“悶熱”。

過去很多年，已經(jīng)有很多工作來調控熱導，且美其名曰“熱管理”，可見已經(jīng)到了相當嫻熟的程度。在一些特定環(huán)境領域，這種熱存儲、熱探測和熱處理有其獨特的優(yōu)勢。比如，可以利用升溫降溫循環(huán)對應的熱導回線來“儲存”材料的聲子狀態(tài)，也算是奇思妙想。這里，來自南京大學的一幫年輕人就對疊層的MoTe2開展了熱導測量與分析，堂而皇之地編撰了一個不失為美妙的故事。

原文鏈接：https://www.nature.com/articles/s41535-017-0031-x

Thermal conductivity:in a loop

The thermal conductivity across stacked MoTe2layers 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 wellknown 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,MoTe2is 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.

FIG.1(a)&(b)Hysteretic behaviors of c-axis thermal conductivity in the metastable phase MoTe2sample 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 MoTe2samples during the TDTR measurements

II.您有暗物質，我有隱藏序

鐵基超導和銅基超導一樣，物理豐富多彩但是缺乏核心靈魂信仰，比如鐵基超導的核心配對機制是啥？甚至目前對銅基高溫超導電子配對機制的爭論有“復辟”的勢頭，這可不是一件小事。

最近有一些學派開始信仰向列序在鐵基超導電子配對中所起的作用。問題是有些體系根本就不存在磁有序，哪里來的向列序呢？后來又有人勾畫出所謂的hidden orders，看起來好像跟宇宙學的暗物質和暗能量差不多，令人莫名其妙，卻也按捺不住內(nèi)心的波瀾激動?？傊?，物理學家總是希望給鐵基超導配對機制找一個新的誕生地：磁漲落！

事實上，物理學的研究很多情況下就是莫名其妙的，所以才有張力和驅動力！這個勾畫很了不起！這里是一個很好的例子。

原文鏈接：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 FeS-e,along with both nematic and magnetic fluctuations at relatively high temperatures,providing insights into the driving factors of nematicity in this fascinating material.

FIG.2 Phase diagram of FeSe by nematic ultrafast dynamics.Temperature dependence of the resistivity ρ shows clearly an anomaly at Tsand 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＜Ts follows ref.49.The dashed green line denoted the proposed FS fluctuations at the Γ point

III.電解質柵極就是王道

過去幾年，將液態(tài)和聚合物電解質作為柵極層來調控半導體及至各種各樣新功能已經(jīng)成為大熱門。只要將需要關注的材料做成溝道，電解質做成柵極（液態(tài)電解質通過滴一滴就OK了），就可以開始搗鼓了。因為液態(tài)電解質的離子電荷在柵極電場作用下很容易遷移聚集在界面處，形成很大的柵極電場，溝道層材料的一些難以企及的新效應就被脫掉包裝、露出原形。

當然，這里的問題是，電解質與溝道層之間在界面處的電化學反應和其它高場下的物理很復雜，說得好聽一點是“非常豐富”，說得不好聽就是“反復無常”！最近清華大學的于浦博士就干了一票，將這種復雜性和衍生出來的一些新效應挖掘出來。這里也是一個例子，點擊如下鏈接可閱讀原文。

原文鏈接：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 Booviat USA’s Brook haven National Laboratory and Yale University report a series of experiments based on WO3films,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 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 electrolyte gating in WO3is 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.

IV.鍶釕氧化合物中什么都有

鍶釕氧化合物是凝聚態(tài)研究4d物理的最好對象，也是研究超導母體物理的極好樣本，幾十年長盛不衰。最近對超導物理和材料的研究有一個趨勢，就是去看低維和表面，有很多出人意料的結果出來。比如，好像開始有人問銅基高溫超導氧化物到底有沒有d波??？之前所有的“反常”超導是不是都是雜質或者缺陷所致??？

如果是這樣，那就要逆天了。這里，回歸到經(jīng)典高溫超導體系的超導電性問題，看看磁關聯(lián)到底在干什么！

原文鏈接：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 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 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.

FIG.3 Lowestenergy magnetic structures(q =(1,1,0)2π/3a)of RuO2basal plane in Sr2RuO4. The(a)-(c)structures represent different types of spiral magnetic order and(d)-(f)corresponds to the collinear up-updown magnetic order with different moment directions

V.負磁阻是外爾費米子的尾巴

外爾費米子因為據(jù)說沒有有效質量，成為一類準粒子，這好像是公認知識。不過，如果一個凝聚態(tài)體系存在鐵磁性的話，這種沒有有效質量的準粒子會有什么新特性？至少從材料角度去尋找合適的對象來關注這個問題是有價值的。我們很榮幸發(fā)表了浙大袁輝球老師在CeSb體系中看到的負磁阻效應，并且與某種外爾費米子聯(lián)系起來。您看，看點在這里，其實一點就亮了

原文鏈接：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 testbed 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 of materials in the past,caesium antimonide 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 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 high field near 0°

注：文中英文簡介由Nature Publishing Group編輯隊伍專門為npj Quantum Materials刊物論文所撰寫，于每篇文章的鏈接里可以看到。