PHYSICAL REVIEW B 103, 035119 (2021) Electron-phonon coupling origin of the graphene π * -band kink via isotope effect F. Bisti , 1, 2, * F. Priante , 1, † A. V. Fedorov , 3, 4 M. Donarelli , 1 M. Fantasia , 1 L. Petaccia , 5 O. Frank , 6 M. Kalbac , 6 G. Profeta , 1, 7 A. Grüneis , 8 and L. Ottaviano 1, 7 1 Dipartimento di Scienze Fisiche e Chimiche, Università dell’Aquila, Via Vetoio 10, 67100 L’Aquila, Italy 2 ALBA Synchrotron Light Source, 08290 Cerdanyola del Vallès, Spain 3 IFW Dresden, Leibniz Institute for Solid State and Materials Research, D-01171 Dresden, Germany 4 Helmholtz-Zentrum Berlin für Materialien und Energie, 14109 Berlin, Germany 5 Elettra Sincrotrone Trieste, Strada Statale 14 km 163.5, 34149 Trieste, Italy 6 J. Heyrovský Institute of Physical Chemistry, Czech Academy of Sciences, Dolejškova 3, CZ-18223 Prague 8, Czech Republic 7 CNR-SPIN L’Aquila, Via Vetoio 10, 67100 L’Aquila, Italy 8 II Institute of Physics, University of Cologne, Zülpicher Strasse 77, 50937 Cologne, Germany (Received 19 June 2020; revised 6 November 2020; accepted 15 December 2020; published 14 January 2021) The π ∗ -band renormalization of Li-doped quasifreestanding graphene has been investigated by means of isotope ( 13 C) substitution and angle-resolved photoemission spectroscopy. The well documented sudden slope change (known as “kink”) located at 169 meV from the Fermi level in the graphene made of 12 C atoms shifts to 162 meV once the carbon monolayer is composed by 13 C isotope. Such an energy shift is in excellent agreement with the expected softening of the phonon energy distribution due to the isotope substitution and provides, therefore, an indisputable experimental proof of the electron-phonon coupling origin of this well known many- body feature in the electronic structure of graphene. DOI: 10.1103/PhysRevB.103.035119 I. INTRODUCTION Many-body interactions within charge carriers are of great interest in condensed matter physics due to their capability of destabilizing conventional metallic states leading them to new exotic ground states. Among them, the most notable example is the intimate connection of the electron-phonon coupling (EPC) with superconductivity. In angle-resolved photoemis- sion spectroscopy (ARPES) such interactions are detected as strong renormalization or abrupt changes (typically referred to as “kinks”) of the band dispersion in the proximity of the Fermi level. Being not reproducible in a single electron band theory, these sudden deviations are then generically ascribed to many-body effects. The ARPES investigation of kinks has been fundamental for the study of high-T c cuprate supercon- ductors [1,2]. To this end, an interesting strategy has been the isotopic substitution of oxygen, offering a controlled investi- gation of the presence of EPC (or lack thereof): For the nodal kink, the observation of few meV energy shift in agreement with the isotopic shift of phonon frequency has been used to directly demonstrate its EPC nature [3]. In graphene, the coupling within quasiparticles has been extensively investigated and the kink at around 169 meV below the Fermi energy in a strongly doped scenario has been generally attributed to EPC [4–9]. The plausibility of this assignment is justified by a likely coupling between the conduction electrons and the E 2g and A ′ 1 in-plane optical phonons [5,10], since both the Eliashberg function and the * fbisti@cells.es † fabio.priante@aalto.fi phonons density of states of the system present peaks around the energies of E 2g and A ′ 1 [7,11]. This assignment is of high interest because of the suggested theoretical strategy of in- ducing a superconducting phase through the classical phonon mediated process where EPC is the pairing mechanism [12]. A very recent theoretical study has shown that dopant adatoms could give an important contribution to the quasiparticle band renormalization, with a possible abrupt change in the same en- ergy region of the observed kink in the graphene π ∗ band [13]. In this scenario, the adatom-induced kink might be obscuring the EPC contribution. Spin fluctuation has been recently sug- gested to be the mechanism behind the strong renormalization of the band in the proximity of the van Hove singularity [14]. Even though the theoretical calculations were not suggesting abrupt changes [14] in the band renormalization, this possi- bility cannot be completely excluded. Moreover, the origin of superconductivity in twisted bilayer graphene [15] is still debated and theoretically interpreted either within a noncon- ventional [15,16] or within a conventional electron-phonon origin [17]. The possibility of a clear experimental demonstra- tion of an electron-phonon coupling for the electrons at the Fermi energy by means of ARPES experiments [18] would give important hints along this line. However, measurements of ARPES isotope effect in graphene is challenging and needs still to be demonstrated. In graphene, isotope substitution is experimentally viable with 13 C. More marked isotopic substitutions are impossible due to the 14 C instability. “Heavy” graphene ( 13 C) has been already successfully grown via chemical vapor deposition using methane as precursor gas, and the expected softening of graphene phonons by the √ 12/13 (about 0.96) factor has been detected in the Raman spectra [19,20]. To the best of 2469-9950/2021/103(3)/035119(5) 035119-1 ©2021 American Physical Society