以多體理論研究氮化硼奈米管及單原子層之強激子增幅位移電流

Abstract

低於能隙的激子位移電流近期在非中心對稱半導體硫化鎘中被發現，而且激子位移電流的大小與自由電子-電洞對激發所驅動的常規位移電流相當。由於其光譜被電子-電洞相互作用強烈的重整化，氮化硼奈米管的激子吸收光譜與自由電子-電洞激發的連續譜明顯分離。而且，具有大結合能的強束縛激子可以抑制電子-空穴對的熱解離。基於以上兩個特點，我們預計氮化硼奈米管將成為低於能隙 的激子位移電流的理想材料。我們使用先進的第一原理多體微擾理論 GW 以及 Bethe-Salpeter 方程來研究單壁zigzag [(5,0),(6,0),(7,0),(8,0)]氮化硼奈米管中的激子位移電流。我們的結果顯示位移電流譜被電子-電洞相互作用強烈的重整化。激子效應使得位移電流的最大峰值提高了三倍以上。這是由於低於能隙的A激子態共 振的光學偶極子矩陣元素發生了顯著的放大。與早期的模型研究相反，位移電流的方向與氮化硼奈米管的手性指數無關。然而我們的研究表明，氮化硼奈米管中的激子位移光譜隨著手性指數的降低而發生紅移。而該紅移是由氮化硼奈米管曲率引起的軌道再雜化所導致的。我們的研究表明，奈米管曲率和多體效應在小直徑氮化硼奈米管的位移電流產生中起著重要作用。

Recent observations of the sub-bandgap exciton shift current in the noncentrosym- metric semiconductor CdS at 2 K using THz emission spectroscopy have been reported. The magnitude of the exciton shift current is comparable to that of the shift current driven by the excitation of free electron-hole pairs. In addition, we anticipate that BN nanotubes will be ideal for studying the subbandgap exciton shift current due to the following two features. Because of their strongly renormalized optical spectra and high exciton bind- ing energy, the exciton absorption spectra of BN nanotubes are clearly separated from the continuum of free electron-hole excitation. Second, strongly bound excitons with a large binding energy inhibit the thermal dissociation of electron-hole pairs. We investigate the exciton shift current in single-walled zigzag [(5,0),(6,0),(7,0),(8,0)] BN nanotubes by per- forming ab initio many-body calculations using the most recent GW plus Bethe-Salpeter equation approaches. We demonstrate that the excitonic effect substantially renormalizes the shift current spectrum. The maximal peak value of the shift current is enhanced by more than three times via the excitonic effect. Due to the large optical dipole matrix element of the sub-bandgap A exciton resonance, a significant amplification has occurred. Contrary to earlier model research, the direction of the shift current is independent of the chiral index. However, our research suggests that the exciton shift spectra in BN nan- otubes have a red shift with respect to the decreasing chiral index. The red shift is caused by the orbital rehybridization induced by curvature. Our research indicates that the cur- vature and many-electron effects play a significant role in the shift current generation in small diameter BN nanotubes.