Ultrafast Unbalanced Electron Distributions in Quasicrystalline 30° Twisted Bilayer Graphene

Abstract

Layers of twisted bilayer graphene exhibit varieties of exotic quantum phenomena [1][2][3][4][5] . Today, the twist angle Θ has become an important degree of freedom for exploring novel states of matters, i.e. two-dimensional superconductivity ( Θ = 1.1°) 6, 7 and a two-dimensional quasicrystal (Θ = 30°) 8, 9 . We report herein experimental observation on the photo-induced ultrafast dynamics of Dirac fermions in the quasicrystalline 30° twisted bilayer graphene (QCTBG). We discover that hot carriers are asymmetrically distributed between the two graphene layers, followed by the opposing femtosecond relaxations, by using time-and angleresolved photoemission spectroscopy. The key mechanism involves the differing carrier transport between layers and the transient doping from the substrate interface. The ultrafast dynamics scheme continues after the Umklapp scattering, which is induced by the incommensurate interlayer stacking of the quasi-crystallinity. The dynamics in the atomic layer opens the possibility of new applications and creates interdisciplinary links in the optoelectronics of van der Waals crystals.Carrier dynamics in graphene is determined by Fermions in a linearly dispersing band structure, the Dirac cones, which have successfully offered unique electronic properties and achieved many applications 10 . Non-equilibrium electronic states in a matter, generated by optical pumping, are characterized by the transient temperature. In a case of the n-type graphene layer (Fig.1a), typically produced on a SiC substrate in a large area, the temporal chemical potential reduces with the temperature, as shown in Fig.1b,c. This dynamical phenomenon is specific to the massless Dirac I. Umklapp scattering and replica bands Supplementary Figure 1 | Umklapp scattering and replica bands. Red and blue hexagonal lines show the boundaries of the first Brillouin zone for the upper-layer (UL) and lower-layer (LL) graphene, respectively. The large (small) solid red circle represents the original (replica) band for the UL Dirac cone. and are reciprocal-lattice vectors of the crystal for the UL and LL, respectively.

Hot carriers in graphene – fundamentals and applications

“…Another highly interesting avenue is that of graphene interfaced with graphene itself. For example, we note that exciting hot-carrier dynamics have been observed for 30°-twisted bilayer graphene, 351 and have been predicted for graphene Moiré superlattices. 352 .…”

High-order nonlinear optical response of a twisted bilayer graphene

“…Qualitative results might be different for smaller angles and lowerfrequency lasers since there occur some emergent symmetries [80,81]. In addition, θ = 30 • is a particularly important twist angle, at which the TBG becomes a quasicrystal and can accommodate symmetries prohibited in ordinary crystals [82][83][84]. Another direction is to go into the deep nonperturbative regime with even stronger fields.…”

Fabrication Strategies of Twisted Bilayer Graphenes and Their Unique Properties

“…Consequently, the researches on the electronic properties of tBLG adopted this method to prepare samples. [ 91 ] Excluding the sacrificial layer of h ‐BN, the control over twist angle will be weakened. The tBLG with other twist angle of 4.5° can be obtained, and the uniformity in twist angle also decreased to some extent.…”