We present a systematic Raman study of unconventionally-stacked double-layer graphene, and find that the spectrum strongly depends on the relative rotation angle between layers. Rotation-dependent trends in the position, width and intensity of graphene 2D and G peaks are experimentally established and accounted for theoretically. Our theoretical analysis reveals that changes in electronic band structure due to the interlayer interaction, such as rotational-angle dependent Van Hove singularities, are responsible for the observed spectral features. Our combined experimental and theoretical study provides a deeper understanding of the electronic band structure of rotated double-layer graphene, and leads to a practical way to identify and analyze rotation angles of misoriented double-layer graphene.Recently there has been growing interest in double-layer graphene in which the two graphene layers are not conventionally stacked but relatively rotated by an arbitrary angle [1][2][3][4][5][6][7][8][9][10][11][12].Such graphene double layers are expected to display characteristics distinct from both monolayer 2 graphene as well as the extensively studied AB-stacked bilayer graphene [13][14][15][16]. Previous theoretical investigations suggest that electronic and optical properties of double layer graphene will strongly depend on this rotational angle [1][2][3][4][5]. Because the entire range of rotational angles is in principle experimentally accessible via artificial stacking, the properties of rotated double-layer graphene might be tuned to suit the application at hand, making this material a useful component in future nano-electronic devices. Limited low-energy electrical transport measurements have suggested that rotated graphene layers maintain the linear dispersion relation as in single-layer graphene [6,7].Furthermore, angle-resolved photoemission spectroscopy measurement has shown that rotated layers in multilayer epitaxial graphene exhibit weak interlayer interactions [8]. On the other hand, scanning tunnelling microscopy studies in the low rotation angle regime have demonstrated strong interlayer interactions such as carrier velocity renormalization and the occurrence of Van Hove singularities away from the Dirac point energy [9,10]. Despite these suggestive findings and the applications potential, there have unfortunately been no comprehensive experimental studies of the influence of rotation angle on the electronic properties of double-layer graphene.In this Letter we present a systematic experimental and theoretical study of rotated doublelayer graphene. We employ Raman spectroscopy, a powerful tool for investigating the electronic and vibrational properties of carbon-based materials [17][18][19][20], together with theoretical calculations of the electronic-structure-dependent Raman response. We experimentally sample a range of misorientation angles from 0 to 30 degrees in steps of ~1 degree, and we focus on the intensity, peak position, and peak width of the 2D and G Raman modes. Previous limited Raman stud...

Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low-frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in the cubic phase of both hybrid (CH_{3}NH_{3}PbBr_{3}) and all-inorganic (CsPbBr_{3}) lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. MD simulations indicate that head-to-head Cs motion coupled to Br face expansion, occurring on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr_{3}.

Organometal halide perovskites are promising solar-cell materials for next-generation photovoltaic applications. The long carrier lifetime and diffusion length of these materials make them very attractive for use in light absorbers and carrier transporters. While these aspects of organometal halide perovskites have attracted the most attention, the consequences of the Rashba effect, driven by strong spin-orbit coupling, on the photovoltaic properties of these materials are largely unexplored. In this work, taking the electronic structure of CH3NH3PbI3 (methylammonium lead iodide) as an example, we propose an intrinsic mechanism for enhanced carrier lifetime in three-dimensional (3D) Rashba materials. On the basis of first-principles calculations and a Rashba spin-orbit model, we demonstrate that the recombination rate is reduced due to the spin-forbidden transition. These results are important for understanding the fundamental physics of organometal halide perovskites and for optimizing and designing the materials with better performance. The proposed mechanism including spin degrees of freedom offers a new paradigm of using 3D Rashba materials for photovoltaic applications.