Direct Observation of Global Elastic Intervalley Scattering Induced by Impurities on Graphene

Abstract

The scattering process induced by impurities in graphene plays a key role in transport properties. Especially, the disorder impurities can drive the ordered state with a hexagonal superlattice on graphene by electron-mediated interaction at a transition temperature. Using angle-resolved photoemission spectroscopy (ARPES), we reveal that the epitaxial monolayer and bilayer graphene with various impurities display global elastic intervalley scattering and quantum interference below the critical temperature (34 K), which leads to a set of new folded Dirac cones at the Brillouin-zone center by mixing two inequivalent Dirac cones. The Dirac electrons generated from intervalley scattering without chirality can be due to the breaking of the sublattice symmetry. In addition, the temperature-dependent ARPES measurements indicate the thermal damping of quantum interference patterns from Dirac electron scattering on impurities. Our results demonstrate that the electron scattering and interference induced by impurities can completely modulate the Dirac bands of graphene.

Emergence of Different Replica Dirac Cones and Intra- and Intervalley Scatterings in Short-Wavelength Graphene Superlattices Modulated by an Atomic-Scale Sharp Potential

“…This result differs from that of the smallest Kekulésuperlattice (G−√3 × √3−R30°), where the inequivalent replica cones emerge at the γ point on the K−B path, leading to an intervalley backscattering at the PDP and, thus, opening a significant energy gap. 14,15 In the case of the unrelaxed G−9 × 9/SiC−4√3 × 4√3−R30°model (Figure 1e), the unfolded graphene band structure (Figure 3e) exhibits two emerged replica cones centered at two γ points on the A−K−B path. These replicas are equivalent to each other and to the main cone, as confirmed by the anticrossing of the blue-colored cones (i.e., intravalley backscattering) located at the SBZ edge (κ′ − μ − κ path) in Figure 3e,f, and thus, leads to the generation of three SDPs at the three equivalent corners of the SBZ (κ points), as a result of the intravalley backscattering combined with Klein tunneling.…”

“…This result differs from that of the smallest Kekuleś uperlattice (G−√3 × √3−R30°). 14,15 Interestingly, in the case of a graphene superlattice with a rotational angle in the range of 0 < φ < 30°, the inequivalent intrinsic replica cones have comparable relative intensities. Therefore, the opening of an energy gap at the PDP by chiral symmetry breaking could be easier on this graphene superlattice than on the G−3N × 3N or G−N√3 × N√3−R30°superlattices.…”

“…Previously, several researchers made efforts to open a band gap at the primitive Dirac point (PDP) of graphene, via intervalley backscattering, by using graphene superlattices with the center of the superlattice Brillouin zone (SBZ) mapped into the GBZ corner, such as the unrotated graphene superlattices with periods of N3 × N3 (G–N3 × N3) (N: a positive integer) or a graphene superlattice with a rotational angle of φ = 30° (G–N√3 × N√3–R30°). Previous studies were solely focused on the smallest graphene superlattice ( N = 1), specifically, the G–√3 × √3–R30° superlattice with Kekulé distortions. ,− However, the chiral symmetry breaking and different scattering behaviors of the valleys on the graphene superlattice associated with more complications, such as a larger period ( N > 1), an arbitrary rotation angle (0 < φ < 30°), and complex substrate-induced potential, have not yet been explored.…”

Suppression of Intervalley Coupling in Graphene via Potassium Doping

“…32 Experimentally, ARPES measurements have been used to explore the replica Dirac band dispersion of (√3 × √3)R30°s uperlattice graphene in previous works. 6,15,33,34 Thus, ARPES measurements can provide important insights to the energyand momentum-dependent upon the Kekuléphase in graphene.…”

“…Consequently, the Dirac electrons at K and K′ valleys fold at the Γ point with the chiral symmetry breaking. 14,15,33 Without potassium doping, we present a series of constant energy cuts in the folded Dirac cone at the Γ point in MLG and BLG in panels a and b of Figure 1, respectively. The folded Dirac cones at the Γ point originate from a periodic (√3 × √3)R30°pattern with band folding.…”