量子材料大觀園(2)

劉俊明

漁家傲·風月文章

焉入十年風月里，低吟秋水萬千思。

秀色文章難數已，重拾起，鉛華不過平常記。

尚憶當時青澀事，一橫一豎與君寄。

麾汗換來凌雪意，如今是，梅枝一夜紅黃系。

VI.鈦酸鍶是個神奇的世界

SrTiO3(STO)是一種神奇的材料，她是絕佳的絕緣體、氧化物單晶襯底、漂亮的裝飾物。不過，她神奇之處在于她的泰國人妖之面貌，經過簡單化妝，她可以是金屬、超導體、量子順電體、鐵電體、熱電材料、光催化材料···天知道還會有什么人能夠再搗鼓出來什么，比如說拓撲導電絕緣體！

STO的這些神奇中最不可思議的就是超導態(tài)和金屬態(tài)了。一個研究組，只要您膽子夠大、力量夠壯，都可以試試這個人妖千面美女。好吧，法國國立物理與材料實驗室的一組學者專門研究STO金屬態(tài)中電阻與載流子（準粒子）密度的關系，突出了載流子在STO中是實實在在的，主要結果是這一關系超越著名的 Mott-Ioffe-Regel極限！好吧，超越···

事實上，在大學物理層次，經典的凝聚態(tài)物理框架一向自祤以能帶理論為核心劃分世界：金屬、半導體、絕緣體、磁學、鐵電···這些物理形態(tài)在電動力學中都有所涉及。不過，我們很快就有了高溫超導、龐磁電阻等關聯量子系統(tǒng)，其中你我不分、非此非彼的物理形態(tài)比比皆是。如果要說有例外，極端情況下，還真有那么幾個體系能夠藐視一切：硅Si算一個吧？ZnO算一個吧？這里的STO也算一個吧。她們通過簡單的調控即可以在所有屬性中各態(tài)歷經(ergodicity)。從這個意義上，現在我們膽大妄為地去看十九世紀二十世紀的那些物理學“大師”們。他們受后輩恭維供奉了一兩百年，現在可以說：他們不過是看到那些簡單的、冰山一角的景觀。

妄議到此，我們不需要妄自菲薄，且來看看這篇文章吧：

原文鏈接：https://www.nature.com/articles/s41535-017-0044-5

Correlated matter:When metals go bad

An expected metallic behavior that defies existing theory is observed by researchers in France.Xiao Lin from the Laboratoire Physique et Etude de Matériaux and co-workers show that room temperature strontium titanate behaves like a so-called ‘bad’metal.Particles with an electric charge can flow through a metal with little resistance.But this resistivity increases with particle density.In conventional metals,this increase eventually stops at what is known as the Mott-Ioffe-Regel limit.Bad or strange metals,however,defy this rule and the resistivity continues to increase.This effect is thought to arise when the when the electrons work col-lectively,creating quasi-particles.Lin and colleagues show that this bad metal behavior can occur even in a material without quasi-particles.Instead,they argue the metallicity is caused by so-called Landauer transmission between individual dopants.

FIG.2(Resistivity and mobility in doped SrTiO3.(a)Temperature dependence of resistivity in SrTiO3-δ as the carrier concentrations is tuned from 1017to 1019cm-3.Roomtemperature resistivity is several hundred times higher than low-temperature resistivity.(b)Hall mobility as a function of temperature.Above 100 K,the mobility does not depend on carrier concentration and is roughly cubic in temperature.(c)Assuming that in elastic resistivity follows a power law,i.e.,ρ = ρ0+ATα,one can extract the exponent,α by taking a logarithmic derivative:α=dln(ρ-ρ0)/dlnT,as in the case of cuprates.At low temperature,α≈2.Above 50 K,it starts a significant shift upward and exceeds three around 150 K,before steadily decreasing afterwards.The antiferrodistortive transitionis the source of the small anomaly at 105 K

VII.電荷密度波中的隱含序

量子凝聚態(tài)系統(tǒng)中神秘莫測的序很多，有些我們認為已經搞得很清楚了但實際上沒搞清楚，有些我們認為沒有搞清楚的現在知道是因為有隱含的有序相。例如，在URu2Si2中，過去 30多年的研究都看到這是一個很典型的四方晶格，面內物理形態(tài)呈現四重對稱是順理成章的。但是，這一體系有一些電子態(tài)又呈現出典型的兩重對稱性，與凝聚態(tài)物理的對稱性規(guī)律不符。這一謎團直到最近才被弄清楚：這一體系的電子態(tài)實際上會沿著面內對角線方向發(fā)生小的形變，展現出正交對稱結構。我們將這一新形態(tài)稱之為hidden order，即隱含序。

當然，量子關聯系統(tǒng)的研究最近揭示出很多所謂的hidden orders，就好像量子凝聚態(tài)系統(tǒng)不讓宇宙學獨美，也不知道是不是受到暗物質或者暗能量概念的激勵，開始了一波又一波對量子隱含相的研究。要命的是，暗物質和暗能量的概念主要基于明物質和明能量的測量結果與大統(tǒng)一理論計算之間有很大出入而提出，是不是屬實還未有答案。而這里的隱含序卻并非如此，足夠精細和靈敏的探測手段還是能夠“看到”它們在那里。

這里是一個極好的例子，看看韓國、英國、法國和美國的噴子們是如何在1T-TaS2中找到隱含磁有序的。當然，既然隱含，就要搞清楚其面貌；但是隱含自然有隱含的道理，是不是有顯性的應用，我們拭目以待。

原文鏈接：https://www.nature.com/articles/s41535-017-0048-1

Charge-density waves:Alone no longer

Evidence for a hidden form of quantum correlation is uncovered by researchers in Korea,the UK,France and the USA.Marie Kratochvilova et al.from the Institute of Basic Science identify magnetic ordering in a material that is already well-studied for its chargeordering properties.Charge-density waves occur when the electrons in a crystalline material create a standingwave pattern.For example,an electron in each tantalum atom in tantalum disul fide(1T-TaS2)can strongly interact with its neighbor to form a Star of David pattern:one electron on each of the twelve vertices,and one in the middle.Kratochvilova et al.now find evidence that these thirteenth‘orphan’electrons exhibit their own type of ordering.The team measured the low-temperature magnetic properties of 1T-TaS2using multiple techniques and identified short-range magnetic ordering in agreement with theoretical predictions.

FIG.3 CDW structure and interplane stacking in 1T-TaS2.The lattice parameter a=3.36 is depicted in the top view.In the case of the repetitive stacking with the nearestneighbor distancesr r1=5.90 and r2=8.29 only double layers one and two alternate while the screw stacking with the nearest-neighbor distancesr r1and is formed by double layers one to four

VIII.多鐵性材料的老大也拓撲了

自然界有很多材料不但很神奇，而且是超級明星，且能長盛不衰。這在我們的很多文獻中都能看到類似描述。例如，半導體中的Si、超導中的YBCO、介電中的STO、鐵電中的PZT、永磁中的釹鐵硼、有機無機雜化的鈣鈦礦等等。一方面，每一種材料都有某個性能很難被超越。即便是某個單項被超越，也很難找到綜合性能亦非常優(yōu)異的表現者。我們習以為常的是太多文獻都宣稱某個材料的某個性能當屬當前的吉尼斯紀錄，但依然是曇花一現居多。

事實上，這些明星材料因為長期被關注，其里外材品都被研究得透徹，所以通過包裝來展現全才功能并不足為奇。當然，絕大多數情況下，一種明星材料基本上就一個性能被廣泛使用，更多的全才不是說不存在，但要成為多面手材料其實很難。

不過，例外總是有的，這里展示其中一個例外就是鐵電材料中的BiFeO3(BFO)。經過十五年包裝，BFO已經有鐵電性、磁性、半導體阻變、光伏、金屬導電性等等，真的是萬能材料，雖然還沒有一種特性真的被實際應用了。也許Ramesh是有點郁悶。

BFO豐富的疇結構已經獨領風騷十多年，主要由71°、109°和180°這樣三種疇和疇壁唱戲。不過，在這里，南京大學的呂笑梅說BFO中疇結構有類拓撲的vortex-antivortex組態(tài)，且這一結構是可擦寫的，稍稍跌了不少人的眼球。因為實空間的拓撲疇用于信息讀寫還是一個少人問津的方向，核心的科學問題可能是這些拓撲疇如何被探測和利用。如果您有一臺好的PFM儀器，到朱英豪那里要來一塊薄膜樣品，然后足夠細心和耐心，您就可以學習呂笑梅老師了。

原文鏈接：https://www.nature.com/articles/s41535-017-0047-2

Ferroelectrics:Electrically rewritable vortex pairs in bismuth ferrite

A demonstration of electrically-rewritable vortex pairs in a ferroelectric could provide a route to realizing vortex memory devices.Swirling vortex structures of electrical polarization that are relatively insensitive to external disturbances can form in ferroelectric materials.As they can exist indifferent polarization states,vortices could be exploited for information storage applications,but practical methods to manufacture and manipulate them are required first.Using scanning probe microscopy-based methods,an international team of researchers led by Xiaomei Lu from Nanjing University demonstrate that ferroelectric vortex-antivortex pairs can be created and erased in bismuth ferrite films using local electric fields.They also show that large-scale vortex networks can be created,which could bring the use of vortices inelectronic devices a step closer.

FIG.4 Ferroelectric domains in BiFeO3 films.(a)Schematic of vortex and antivortex structures.(b)Schematic of PFM detection of(001)BFO/SRO/STO.(c)Possible directions of the polarization vector P(solidlines)and switching paths in a(001)-oriented BFO thin film.(d)x-Lateral Piezoelectric Force Microscope(x-LPFM)(d1),y-LPFM(d2),Vertical Piezoelectric Force Microscope(VPFM)(d3)images,and 3D domain pattern(d4)of the same region.x and y indicate slow scan axis and fast scan axis,respectively.The dark blue ring represents the poling position.White,light gray,dark gray,and black regions represent for domains with in-plane polarization along[1-1],[-1-1],[11],and[-11]direction,respectively

IX.量子臨界性處的風景

量子臨界現象類比于經典相變中的臨界現象，是collective物理的重要方面，在我們目前所關注的幾乎所有量子物理行為中都能找到蹤影。那么到底什么是量子相變？針對某個材料對象，如果相圖中展示出一個連續(xù)相變的臨界溫度隨某個物理調控參數連續(xù)下降到絕對零度，這個相圖點一般稱之為量子相變。事實上，一個連續(xù)相變如果能發(fā)生，她總是會在某個有限溫度，而海森堡測不準原理蘊含的量子漲落會導致這一相變點走向絕對零度。

最著名的就是高溫超導了，相圖中反鐵磁與超導競爭的那幅架勢“全球”聞名，人人皆知。經典臨近相變行為因為平均場理論、普適標度理論和重整化理論的輝煌，使得量子物理研究者垂涎三尺。也因為如此，這個世界上有那么些個研究組不遺余力持之以恒地對量子臨界現象給予關注，試圖得出其中與經典臨界行為類似的漂亮規(guī)律。目前，尚不是很清楚量子臨界性有哪些潛在的出口可以吐納新的應用價值。

當然，這個方向還只是開始，應該說還在積累數據的初級或者中級階段，這里展示了美國人、瑞士人和德國人一起搗鼓的一個獨特而新穎的實例。

原文鏈接：https://www.nature.com/articles/s41535-017-0049-0

Quantum criticality:Iron aids restoration

An exotic quantum state of matter is identified in a new material by researchers in the USA,Switzerland and Germany.Yasutomo Uemura from Columbia U-niversity and co-workers observe quantum criticality in a material in which it was previously unseen just by adding iron.As pressure is increasingly applied topure manganese silicon at absolute zero temperature,it can suddenly change from an ordered magnetic state to an unordered one.Uemuraet al.now show that a slower continuous,or second-order,transition is observed instead when fifteen percent of the manganese atoms are replaced with iron atoms.This is indicative that a quantum critical point exists at a pressure of between 21 and 23 kilobar.They believe that criticality is restored because the iron atoms introduce disorder into the system.

FIG.5 Temperature-pressure phase diagrams of MnSi and(Mn,Fe)Si.(a)Temperature versus hydrostatic pressure phase diagram in pure MnSi and(Mn,Fe)Si in zero magnetic field. Open triangles represent the magnetic transition temperature determined by resistivity and susceptibility,gray-colored triangles from MuSR,and closed triangles represent the onset temperature of diffuse scattering“partial order” in quasi-elastic neutron scattering(QNS).The yellow-colored region(S)represents the helically ordered phase,the green colored region(P)indicates phase separation between ordered and paramagnetic phases,and the blue colored region(D)indicates slow,dynamic,diffusive helical spin fluctuations.The small pink region(HF)above TChashelical spin fluctuations leading to a weakly first-order thermal transition.Red circles indicate the transition temperature for(Mn0.85Fe0.15)Si,determined by the present MuSR measurements.(b)-(d)Expected phase diagrams of itinerant-electron magnets according to theories by Belitz,Kirkpatrick and co-workers with the effect of disorder.TCP denotes the tri-critical point and QCP denotes the quantum critical point.The blue line indicates a secondorder phase transition,while the red line shows a first-order phase transition.The first-order line in(b)intersects thepaxis vertically at T=0.The MuSR results for pure MnSi can be viewed as a manifestation of case(c),and the present results for(Mn,Fe)Si as case(d).The recent MuSR results on V2O3may correspond toc,while the second-order phase transition in a highly random percolation network may berelated to(d)

X.金屬間化合物是個寶？

金屬間化合物是個寶，這個我知道。以前一說金屬間化合物，我只知道是一些高溫合金材料，高溫力學和機械服役性能好。其實，金屬間化合物是以離子鍵和共價鍵為主的氧化物、硫化物和各種鹵素化合物并駕齊驅的大材料體系，其中蘊含的未被發(fā)現的新功能和新物理“罄竹難書”。

這里，我們的鐵基超導化合物發(fā)現者 Hideo Hosono（細野秀雄）與Yaoqing Zhang博士一起，報道了電子鹽化合物Nb5Ir3具有超導電性，讓我們嚇了一跳！

這里，所謂電子鹽，就是說化合物的“陰離子”是電子，而不是傳統(tǒng)意義上的帶負電的離子。這些陰離子電子被局域化在金屬間化合物的晶格間隙處！這個圖像應該是很不量子力學的，我還處在懵懵懂懂的，感覺也很是莫名其妙。很顯然，離子外層的電子被當作載流子輸運，這是常態(tài)，可以看作是離子“參與”導電。但是，將電子這個量子“粒子”或者“波”作為陰離子參與晶體結構的構建，這應該還是新鮮事，出來很多槽點并不奇怪，難的是誰能想到這個“量子”能夠作為一個“離子”呢？

稍微熟悉一點凝聚態(tài)和材料物理的人都會問：一個“量子”鑲嵌在晶格中，怎么個局域化？安德森先生處理過嗎？唉！

原文鏈接：https://www.nature.com/articles/s41535-017-0053-4

Intermetallics:Superconductivity coexists with electride behavior

Coexistence of electride behavior and superconductivity is observed in a hexagonal phase of a Nb5Ir3intermetallic with tunable electronic properties by introducing foreign atoms.A team led by Yaoqing Zhang and Hideo Hosono from Japan Science and Technology Agency and Tokyo Institute of Technology report a new hexagonal phase in the phase diagram of Nb-Ir binary intermetallics with interesting interplay of superconductivity and electride state.The electride state is formed by electrons detaching from the atoms but localizing in a one-dimensional channel space.This electride becomes a superconductor below a transition temperature of about 9.4 K,which could be enhanced to 10.5 K upon filling interstitial cavities with foreign oxygens.The results suggest a general rule governing the formation of electrides and form the basis for novel stable functional electrides.

FIG.6 Electronic structures of different materials.Calculated band structures and density of states(DOS)for(a)[Nb5Ir3]2+:2e-with corresponding electron density for the channel bands(ChBs)and electron localization function(ELF)map on the(1-100)plane parallel to the c-axis in(c)and(d),respectively,and f Nb5Ir3O.(b,e)Band structures of one-dimensional(1D)electrides(b)[La8Sr2(SiO4)6]4+:4e-and(e)Y5Si3.The characteristic ChBs are circled in red for all electrides

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。