Ataei S Samaneh, Varsano Daniele, Molinari Elisa, Rontani Massimo
Consiglio Nazionale delle Ricerche - Istituto Nanoscienze, 41125 Modena, Italy.
Dipartimento di Scienze Fisiche, Informatiche e Matematiche, Università degli Studi di Modena e Reggio Emilia, 41125 Modena, Italy.
Proc Natl Acad Sci U S A. 2021 Mar 30;118(13). doi: 10.1073/pnas.2010110118.
Spontaneous condensation of excitons is a long-sought phenomenon analogous to the condensation of Cooper pairs in a superconductor. It is expected to occur in a semiconductor at thermodynamic equilibrium if the binding energy of the excitons-electron (e) and hole (h) pairs interacting by Coulomb force-overcomes the band gap, giving rise to a new phase: the "excitonic insulator" (EI). Transition metal dichalcogenides are excellent candidates for the EI realization because of reduced Coulomb screening, and indeed a structural phase transition was observed in few-layer systems. However, previous work could not disentangle to which extent the origin of the transition was in the formation of bound excitons or in the softening of a phonon. Here we focus on bulk [Formula: see text] and demonstrate theoretically that at high pressure it is prone to the condensation of genuine excitons of finite momentum, whereas the phonon dispersion remains regular. Starting from first-principles many-body perturbation theory, we also predict that the self-consistent electronic charge density of the EI sustains an out-of-plane permanent electric dipole moment with an antiferroelectric texture in the layer plane: At the onset of the EI phase, those optical phonons that share the exciton momentum provide a unique Raman fingerprint for the EI formation. Finally, we identify such fingerprint in a Raman feature that was previously observed experimentally, thus providing direct spectroscopic confirmation of an ideal excitonic insulator phase in bulk [Formula: see text] above 30 GPa.
激子的自发凝聚是一种长期以来备受追寻的现象，类似于超导体中库珀对的凝聚。如果通过库仑力相互作用的激子（电子（e）和空穴（h）对）的结合能超过带隙，那么在热力学平衡状态下的半导体中预计会发生这种现象，从而产生一个新的相：“激子绝缘体”（EI）。由于库仑屏蔽的减弱，过渡金属二硫属化物是实现激子绝缘体的理想候选材料，并且在少层体系中确实观察到了结构相变。然而，先前的研究无法确定该相变的起源在多大程度上是由于束缚激子的形成还是声子的软化。在这里，我们聚焦于体相[化学式：见原文]，并从理论上证明在高压下它易于形成具有有限动量的真实激子的凝聚，而声子色散保持正常。从第一性原理多体微扰理论出发，我们还预测激子绝缘体的自洽电子电荷密度在层平面内维持一个具有反铁电纹理的面外永久电偶极矩：在激子绝缘体相开始时，那些与激子动量相同的光学声子为激子绝缘体的形成提供了独特的拉曼指纹。最后，我们在先前实验观测到的一个拉曼特征中识别出了这种指纹，从而为30吉帕以上体相[化学式：见原文]中的理想激子绝缘体相提供了直接的光谱学证实。
