Nature子刊：鐵電材料對應力出現異常反應，顛覆傳統理論(雙語)

材料牛注：鐵電材料受力越大，電極極性就越強？當大部分人還沉浸在這樣的慣性思維里時，美國西北大學的科學家已經發現了部分鐵電材料不同尋常的特性：當鐵電氧化物所受應力增加到一定程度，其電極或可消失不見！

Up until recently, researchers thought they had the behaviors of ferroelectric materials mostly figured out.

就在不久之前，科學家們還以為有關鐵電材料的性能，他們已經研究得差不多了。

"The conventional wisdom is that you can put almost any material under mechanical stress, and provided the stress is coherently maintained, the material will become ferroelectric or exhibit an electrical polarization," said James Rondinelli, assistant professor of materials science and engineering at Northwestern University's McCormick School of Engineering. "If you apply similar stresses to a compound that's already ferroelectric, then its polarization increases."

“傳統觀點認為，對幾乎所有材料施加機械應力，且在持續加載的條件下，材料會擁有鐵電特性或表現出電極化現象，”美國西北大學助理教授James Rondinelli說道，“如果你向已經具有鐵電特性的化合物施加同樣的應力，其極化強度就會增加。”

Rondinelli and his team, however, have made a theoretical discovery that flips this widely accepted fact on its head. They found that when a unique class of ferroelectric oxides are stretched or compressed, the polarization does not simply increase as expected. Instead, it goes away completely. "Based on everything we have known for the past two decades," Rondinelli said, "this is completely unexpected."

然而，Rondinelli及其團隊卻發現了一項能夠顛覆這一傳統觀點的理論：當某一種特定的鐵電氧化物受拉或是受壓時，其極化強度并非如預期那樣簡單地增加，而是消失得無影無蹤。Rondinelli說：“回溯過去二十年里的科研發現，這種現象前所未有。”

Supported by the National Science Foundation, the research is described in the June 13 issue of Nature Materials. Xue-Zeng Lu, a PhD student in Rondinelli's laboratory, served as the paper's first author.

這項研究受美國國家科學基金會資助，并將其成果發表在6月13號的 Nature Materials 上。Rondinelli研究室的博士生Xue-Zeng Lu是該文的第一作者。

Ferroelectrics are found everywhere: in smart phones, watches, and computers. Because they are so technologically useful, researchers have long been interested in creating new or improved ferroelectric materials — especially in two-dimensional geometries as thin films where they are readily integrated into electronic devices. Ferroelectricity is a property that occurs when a material exhibits a spontaneous electric polarization, which arises from is a shift of positive and negative charges in opposite directions.

鐵電材料隨處可見，在智能手機、手表和電腦里都能覓其蹤影。鐵電材料的技術應用如此之廣，以至于一直以來，科學家們都致力于這種材料的研發和改進，尤其作為薄膜集成于電子設備中。鐵電特性即正負電荷突然轉向時，材料呈現出自發電極的特性。

When strain is applied to the class of oxides called layered perovskites grown as a thin film, they initially react the same way as other ferroelectrics. Their polarization increases. But if further strain is applied, the polarization completely turns off.

當我們對一種名為層狀鈣鈦礦的薄膜氧化物材料施以應力時，最初它們表現出與其他鐵電材料相同的特性，即極化程度增加，但當應力進一步增大，其極化強度竟會蕩然無存。

Layered perovskites have recently seen a resurgence of attention because they host functional physical properties like high-temperature super conductivity and support electrochemical or photocatalytic energy conversion processes. Their structures are also much more defect tolerant. Rondinelli's discovery adds a new level of interest to these popular materials.

層狀鈣鈦礦材料具備一定功能物理特性，例如高溫超導、能夠進行電化學或光催化能量轉換過程等，所以最近，這種材料又重新受到了關注。該材料的結構特點也使其允許存在更多的晶體缺陷。Rondinelli的科學發現大大提高了人們對這些材料的研究熱度。

"You can't strain the material too much because it might lose its functionality," Rondinelli said. "But if you operate near where the polarization turns on and off, you really have a switch. If you're monitoring the polarization for a logic device or memory element, you can apply a small electric field to traverse this boundary and simultaneously read and write the on-and-off state."

Rondinelli說：“如果對這種材料施加太大的拉力，它可能會喪失功能特性。但我們在恰能使其極性消長的范圍內對它施以應力，就能把它變為開關裝置。如果這一極性的有無能夠被邏輯裝置或是記憶元件測得，我們就可以用一微弱電場來改變其極性，同時獲知極性的有無狀態。”

Rondinelli's team made this discovery using a theoretical materials tools and quantum mechanical simulations and is now working with experimental collaborators to validate the finding in the laboratory. Another next step is to better understand how this new functionality could help or hinder ferroelectric applications.

Rondinelli團隊的科學發現也要歸功于量子力學模擬這一材料理論研究工具，這一成果現由其合作者進行實驗驗證。接下來，他們將對這一新功能進行進一步探索，研究其對鐵電裝置的助益或阻礙機理。

In the meantime, Rondinelli said researchers will now need to be careful when applying mechanical stress to layered perovskite ferroelectrics. Applying too much strain could have unintended consequences.

與此同時，Rondinelli表示，研究人員在對層狀鈣鈦礦材料施加機械應力時應當謹慎。應力過大會導致不可預測的后果。

"This finding motivates us to recalibrate our intuition regarding what interactions are expected between mechanical forces and dielectric properties," Rondinelli said. "It requires us to think more carefully, and I suspect there is much more to learn."

“這一發現重塑了我們關于機械應力和介電性能關系的認知” Rondinelli說，“它敦促我們謹慎思考，這里面一定還大有文章。”

新聞鏈接：Ferroelectric materials react unexpectedly to strain

文獻鏈接：Epitaxial-strain-induced polar-to-nonpolar transitions in layered oxides

本文由編輯部王宇提供素材，張文揚編譯，趙建偉審核