npj：負泊松比—源於軌道雜化

對於普通材料，沿一個方向的晶格分別隨著其他正交方向的壓縮或拉伸而擴展或收縮。泊松比（ν）是用於量化物理屬性的參數，在大多數情況下為正值。但對於某些特殊材料（所謂「膨脹材料」），ν可以為負。負泊松比（NPR）引起人們極大的興趣，因為具有NPR的材料類型通常具有更強的韌性、抗剪切性和有效的降噪能力，從而可以實現許多新穎的應用，如航空航天和國防等。從歷史上看，對NPR的研究大多是在體膨脹結構上進行的，進一步擴展到納米材料後，發現了更多有趣的現象。如，近來在碳納米管和金屬納米板中發現了NPR。另外，石墨烯等二維（2D）材料有望應用於多個領域，而其也已通過特殊工程實現了NPR，可切成納米帶、引入空位缺陷、在極高溫度下產生周期性多孔和波紋彎曲等。但2D材料的NPR機制如何尚不清楚。

來自湖南大學的秦光照和鄭州大學的秦真真基於第一性原理方法，合作研究了石墨烯、矽烯、h-BN、h-GaN、h-SiC和h-BAs等新型二維蜂窩狀材料在單軸應變下的機械響應，預測上述材料在沿扶手椅方向時均存在負泊松比現象。儘管它們元素成分不同，但其NPR均由鍵角的反常增加導致。這種鍵角的反常增加無法通過傳統層面的幾何結構和力學響應的觀點來解釋（如基於經驗勢函數的經典分子動力學模擬研究）。該工作通過對應力調控下的關鍵幾何參數及軌道雜化作用的變化趨勢進行分析，從電子結構層面闡明了鍵角反常增加及NPR的底層物理機制。該機制同樣可適用於其他具有NPR現象的納米結構。

該文近期發表於npj Computational Materials 6: 51 (2020)，英文標題與摘要如下，點擊https://www.nature.com/articles/s41524-020-0313-x可以自由獲取論文PDF。

Negative Poisson’s ratio in two-dimensional honeycomb structures

Guangzhao Qin and Zhenzhen Qin

Negative Poisson’s ratio (NPR) in auxetic materials is of great interest due to the typically enhanced mechanical properties, which enables plenty of novel applications. In this paper, by employing first-principles calculations, we report the emergence of NPR in a class of two-dimensional honeycomb structures (graphene, silicene, h-BN, h-GaN, h-SiC, and h-BAs), which are distinct from all other known auxetic materials. They share the same mechanism for the emerged NPR despite the different chemical composition, which lies in the increased bond angle (θ). However, the increase of θ is quite intriguing and anomalous, which cannot be explained in the traditional point of view of the geometry structure and mechanical response, for example, in the framework of classical molecular dynamics simulations based on empirical potential. We attribute the counterintuitive increase of θ and the emerged NPR fundamentally to the strain-modulated electronic orbital coupling and hybridization. It is proposed that the NPR phenomenon can also emerge in other nanostructures or nanomaterials with similar honeycomb structure. The physical origin as revealed in our study deepens the understanding on the NPR and would shed light on future design of modern nanoscale electromechanical devices with special functions based on auxetic nanomaterials and nanostructures.