量子限制和结构各向异性导致少层黑磷中电调谐Dirac锥
Quantum-confinement and Structural Anisotropy result in Electrically-Tunable Dirac Cone in Few-layer Black Phosphorous
摘要：
众所周知，2D材料由于量子限制而表现出有趣的现象。在这里，我们表明，量子限制，加上结构各向异性，导致在二维黑磷电场可调狄拉克锥。利用密度泛函理论计算发现，垂直于二维黑磷薄膜施加的电场E_ext，可以减小少层黑磷的直接带隙，导致在临界电场E_c处发生绝缘体-金属转变。当黑磷膜足够薄时，增大E_ext，使之超过E_c，可以在系统中诱导出狄拉克锥。电场强度可以调节狄拉克锥的位置和狄拉克-费米速度，后者的大小与石墨烯中的相似。我们表明，狄拉克锥产生于各向异性之间的相互作用项的前沿轨道，空间上分离，由于所施加的字段，在不同的一半的2D板。当这种相互作用项变得非常小，较厚的膜，狄拉克锥不再可以诱导。自旋-轨道耦合可以在一定的电场下使狄拉克锥产生间隙;然而，场强的进一步增加会减小自旋-轨道诱导的间隙，最终导致拓扑绝缘体到狄拉克半金属的转变。
英文摘要：
2D materials are well-known to exhibit interesting phenomena due to quantum confinement. Here, we show that quantum confinement, together with structural anisotropy, result in an electric-field-tunable Dirac cone in 2D black phosphorus. Using density functional theory calculations, we find that an electric field, E_ext, applied normal to a 2D black phosphorus thin film, can reduce the direct band gap of few-layer black phosphorus, resulting in an insulator-to-metal transition at a critical field, E_c. Increasing E_ext beyond E_c can induce a Dirac cone in the system, provided the black phosphorus film is sufficiently thin. The electric field strength can tune the position of the Dirac cone and the Dirac-Fermi velocities, the latter being similar in magnitude to that in graphene. We show that the Dirac cone arises from an anisotropic interaction term between the frontier orbitals that are spatially separated due to the applied field, on different halves of the 2D slab. When this interaction term becomes vanishingly small for thicker films, the Dirac cone can no longer be induced. Spin-orbit coupling can gap out the Dirac cone at certain electric fields; however, a further increase in field strength reduces the spin-orbit-induced gap, eventually resulting in a topological-insulator-to-Dirac-semi-metal transition.
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