[论文解读] Ultra-high mechanical stretchability and controllable topological phase transitions in two-dimensional arsenic

Ming Yang, Wu-Ming Liu|arXiv (Cornell University)|Jan 18, 2015

Topological Materials and Phenomena参考文献 27被引用 3

一句话总结

本文提出一种新型二维沟槽状砷（砷烯），具有超高的机械延展性（沿扶手椅方向应变达44%）和创纪录的泊松比1.049。外部应变可诱导拓扑相变，使其转变为具有鲁棒自旋极化边缘态的拓扑绝缘体，适用于柔性自旋电子器件，如无耗散晶体管和自旋阀。

ABSTRACT

The mechanical stretchability is the magnitude of strain which a material can suffer before it breaks. Materials with high mechanical stretchability, which can reversibly withstand extreme mechanical deformation and cover arbitrary surfaces and movable parts, are used for stretchable display devices, broadband photonic tuning and aberration-free optical imaging. Strain can be utilised to control the band structures of materials and can even be utilised to induce a topological phase transition, driving the normal insulators to topological non-trivial materials with non-zero Chern number or Z2 number. Here, we propose a new two-dimensional topological material with ultra-high mechanical stretchability - the ditch-like 2D arsenic. This new anisotropic material possesses a large Poisson's ratio 1.049, which is larger than any other reported inorganic materials and has a ultra-high stretchability 44% along the armchair direction, which is unprecedent in inorganic materials as far as we know. Its minimum bend radius of this material can be as low as 0.66 nm, which is comparable to the radius of carbon-nanotube. Such mechanical properties make this new material be a stretchable semiconductor which could be used to construct flexible display devices and stretchable sensors. Axial strain will make a conspicuous affect on the band structure of the system, and a proper strain along the zigzag direction will drive the 2D arsenic into the topological insulator in which the topological edge state can host dissipation-less spin current and spin transfer toque, which are useful in spintronics devices such as dissipation transistor, interconnect channels and spin valve devices.

研究动机与目标

• 发现具有优异机械柔韧性和高泊松比的二维材料，用于可拉伸电子器件。
• 探究应变是否可在二维砷中诱导拓扑相变，从而实现新型自旋电子功能。
• 评估所提出的二维砷在机械变形和热涨落下的结构与动力学稳定性。
• 证明利用应变可逆调控柔性二维半导体的拓扑性质的可行性。
• 为该新材料的实验合成与表征（包括机械剥离和输运测量）提供路线图。

提出的方法

• 采用广义梯度近似（GGA-PBE）泛函和400 eV能量截断的从头算密度泛函理论（DFT）计算，优化电子和结构性质。
• 通过可变细胞弛豫和共轭梯度算法，确保原子受力低于0.001 eV/Å，以保证结构稳定性。
• 利用声子色散关系和有限温度分子动力学（300 K和30 K），采用4×4超胞（64个原子）验证动力学稳定性。
• 通过沿锯齿形和扶手椅方向施加轴向应变，实现能带结构调控并诱导拓扑相变。
• 基于能带结构计算拓扑不变量（Z2和陈数），以识别拓扑绝缘体相。
• 建立具有曲率指数N的类纳米管弯曲结构，评估其机械柔韧性，分析自由能和能带结构随曲率的变化。

实验结果

研究问题

• RQ1 一种二维砷的同素异形体是否能表现出超高的机械延展性和异常高的泊松比？
• RQ2 二维砷在不发生结构失效的情况下，能承受的最大应变和最小弯曲半径是多少？
• RQ3 轴向应变是否能在二维砷中诱导拓扑相变，使其转变为拓扑绝缘体？
• RQ4 应变如何影响二维砷的能带结构和电子带隙？
• RQ5 在弯曲的二维砷纳米管中，曲率、自由能与结构稳定性之间存在何种关系？

主要发现

• 沟槽状二维砷（砷烯）表现出创纪录的泊松比1.049，超过所有已知无机材料。
• 其在扶手椅方向展现出高达44%的超高的机械延展性，具备极强的柔性。
• 最小弯曲半径低至0.66 nm，与碳纳米管相当，表明其具有极佳的柔性。
• 沿锯齿形方向施加轴向应变，在层间距为3.28 Å时发生能带反转，使材料转变为拓扑绝缘体。
• 当能带隙在3.71 Å处闭合并在3.28 Å处发生反转时，系统转变为拓扑绝缘体，由非零的拓扑不变量证实。
• 曲率≤1.52 nm⁻¹（N ≥ 8）的弯曲纳米管结构保持动力学稳定，而更高曲率（N ≤ 7）则出现虚频模式，表明不稳定。