Energy-Dependent Chirality Effects in Quasifree-Standing Graphene Daniela Dombrowski, 1,2,* Wouter Jolie, 2 Marin Petrović, 3,† Sven Runte, 2 Fabian Craes, 2 Jürgen Klinkhammer, 2 Marko Kralj, 3 Predrag Lazić, 4 Eran Sela, 5 and Carsten Busse 1,2 1 Institut für Materialphysik, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany 2 II. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany 3 Center of Excellence for Advanced Materials and Sensing Devices, Institute of Physics, Bijenička 46, 10000 Zagreb, Croatia 4 Institut Ruđ er Bošković, Bijenička 54, 10000 Zagreb, Croatia 5 Raymond and Beverly Sackler School of Physics and Astronomy, Tel-Aviv University, Haim Levanon St 55, 6997801 Tel Aviv, Israel (Received 22 December 2016; published 13 March 2017) We present direct experimental evidence of broken chirality in graphene by analyzing electron scattering processes at energies ranging from the linear (Dirac-like) to the strongly trigonally warped region. Furthermore, we are able to measure the energy of the van Hove singularity at the M point of the conduction band. Our data show a very good agreement with theoretical calculations for free-standing graphene. We identify a new intravalley scattering channel activated in case of a strongly trigonally warped constant energy contour, which is not suppressed by chirality. Finally, we compare our experimental findings with T -matrix simulations with and without the presence of a pseudomagnetic field and suggest that higher order electron hopping effects are a key factor in breaking the chirality near to the van Hove singularity. DOI: 10.1103/PhysRevLett.118.116401 As a consequence of its lattice structure (two equivalent hexagonal sublattices A and B forming a honeycomb mesh), the low-energy quasiparticles of graphene behave as massless, chiral Dirac fermions, obeying the Dirac equation. They can be described by a two component wave function, where the components give the complex amplitudes on sublattices A and B, respectively. This sublattice degree of freedom is referred to as pseudospin, since the wave function behaves like a two component spinor. The projection of the pseudospin onto the wave vector ~ k defines the chirality or helicity of the quasiparticles, analogous to the general Dirac theory where the helicity is defined by the projection of the spin onto the direction of momentum [1,2]. The chiral nature plays a role in many interesting phenomena and properties of graphene such as Klein tunneling [3], and chiral quantum Hall effects [4], and could even affect graphene based devices [5]. Effects of chirality and pseudospin conservation can be investigated by looking at electron scattering processes using scanning tunneling microscopy (STM) and spectroscopy (STS) [6–8]. In general, two different scattering processes are possible: Intervalley scattering between two neighboring pockets K and K 0 , and intravalley scattering within one K or K 0 pocket [6]. It was shown that the conservation of pseudospin during the scattering process results in a strong suppression of zero order intravalley scattering [8], thereby modeling topological insulators, where the conservation of the real spin leads to similar effects [9,10]. For both systems, this simple picture has to change with the crossover from the linear to the warped region. However, to the best of our knowledge no experimental studies on the corresponding modifications of the scattering processes in graphene exist. Here we provide such a study, using Cs intercalated graphene (gr) on Ir(111). The intercalation of Cs decouples graphene electronically from the substrate, and additionally strongly n dopes it, shifting the Fermi level into the region of strong trigonal warping [11]. We probe the oscillations in the density of states originating from scattering at defects with low temperature STM and STS, allowing the direct energy- resolved observation of electron scattering and chirality effects in graphene [12]. The strong doping makes Cs intercalated graphene a perfect system to investigate the strong trigonal warping region, since tunneling spectroscopy works best near the Fermi energy. Samples are prepared in an ultrahigh vacuum (UHV) chamber with a base pressure ≤ 1.5 × 10 -10 mbar. The surface of the iridium crystal is cleaned by cycles of 1.5 keVAr þ sputtering at 300 K, oxygen firing at 1070 K and annealing to 1520 K. Graphene is grown by a combi- nation of temperature-programmed growth and chemical vapor deposition as described in Ref. [13], leading to large area, high quality epitaxial graphene. Cs is intercalated at 300 K, using commercially available alkali metal dispensers. STM and STS are carried out at 5 K in a separate chamber (sample transfer under UHV) with a background pressure lower than 10 -11 mbar. The dI=dV point spectra and constant energy maps are recorded using the lock-in tech- nique with a modulation frequency of 833.1 Hz and a modulation amplitude of 8 mV, providing an energy reso- lution of 14 meV [14]. An etched tungsten tip is used for all measurements, which is prepared in situ by applying positive PRL 118, 116401 (2017) PHYSICAL REVIEW LETTERS week ending 17 MARCH 2017 0031-9007=17=118(11)=116401(6) 116401-1 © 2017 American Physical Society