Zhou Jianqiang Sky, Monacelli Lorenzo, Bianco Raffaello, Errea Ion, Mauri Francesco, Calandra Matteo
Sorbonne Université, CNRS, Institut des Nanosciences de Paris, UMR7588, F-75252, Paris, France.
Dipartimento di Fisica, Università di Roma Sapienza, Piazzale Aldo Moro 5, I-00185 Roma, Italy.
Nano Lett. 2020 Jul 8;20(7):4809-4815. doi: 10.1021/acs.nanolett.0c00597. Epub 2020 Jun 9.
Low-dimensional systems with a vanishing band gap and a large electron-hole interaction have been proposed to be unstable toward exciton formation. As the exciton binding energy increases in low dimension, conventional wisdom suggests that excitonic insulators should be more stable in 2D than in 3D. Here we study the effects of the electron-hole interaction and anharmonicity in single-layer TiSe. We find that, contrary to the bulk case and to the generally accepted picture, in single-layer TiSe, the electron-hole exchange interaction is much smaller in 2D than in 3D and it has weak effects on phonon spectra. By calculating anharmonic phonon spectra within the stochastic self-consistent harmonic approximation, we obtain ≈ 440 K for an isolated and undoped single layer and ≈ 364 K for an electron-doping = 4.6 × 10 cm, close to the experimental result of 200-280 K on supported samples. Our work demonstrates that anharmonicity and doping melt the charge density wave in single-layer TiSe.
具有消失带隙和大电子-空穴相互作用的低维系统已被认为对激子形成不稳定。随着低维中激子结合能增加，传统观点认为激子绝缘体在二维中应比在三维中更稳定。在此我们研究单层TiSe中电子-空穴相互作用和非谐性的影响。我们发现，与体材料情况和普遍接受的图像相反，在单层TiSe中，二维中的电子-空穴交换相互作用比三维中小得多，并且它对声子谱的影响较弱。通过在随机自洽谐波近似内计算非谐声子谱，对于孤立且未掺杂的单层我们得到≈440 K，对于电子掺杂浓度 = 4.6 × 10 cm的情况得到≈364 K，接近在支撑样品上200 - 280 K的实验结果。我们的工作表明，非谐性和掺杂使单层TiSe中的电荷密度波熔化。
