Speaker: Dr Christoph Tegenkamp(TU Chemnitz, Germany)

Host: Michael Tringides

Title: Proximity coupling effects in epitaxial 2D graphene-based heterostructures

Bio:

 Professor Dr. Christoph Tegenkamp received his PhD in 2000 at the University of Hannover working on the growth and characterization of 1-d metal nanowires. In 2020-01 he was a postdoc at the University Maryland with Prof. E. Williams working on electromigration in 2-d nanostructures. He completed his 12/2006 Habilitation and until 07/2017 he was W2 Professor at the University of Hannover. Since 2017 he moved to the Technical University (TU) Chemnitz, where he is now W3 professor and currently dean of study. He is the spokeperson of the collaborative DFG Research Unit FOR5242 that involves 6 Universities and he is also spokeperson of the new TEM Microscopy Center at the TU Chemnitz. He has served in various scientific committees: the editorial board of Wiley and Institute of Physics UK , member of the MAX IV synchrotron advisory panel, et. He has published ~180 refereed publications and has given ~140 invited at conferences and institutions worldwide.

Abstract: 

Currently 2D materials and their stacking are used as building blocks to grow new quantum materials. Proximity coupling between the stacked layers is in general a very important research area to tune their properties. Epitaxial graphene (EG) grown on SiC(0001) resembles a truly 2D electron gas (2DEG) system and is known for its flexible functionalization at its interface. Different functionalization schemes are used to realize tunable doping profiles in graphene, to control the spin-orbit coupling, to engineer interface states and to introduce mini-bands by zone folding. The controlled transition from linear to flat bands in EG as well as the coupling of functionalized graphene to 2DEGs opens the possibilities for electronic correlation effects and mesoscopic phenomena in 2D materials, e.g. superconductivity, charge and spin density waves, Mott states, etc.

In this talk I will highlight three recent proximity experiments based on metal functionalization of graphene. 

(i)Epitaxially grown Bi islands on top. Upon adsorption, epitaxial Bi(110) islands form, which change the initial n-type doping of graphene [1]. The proximitized Bi islands induce a well-defined lateral doping profile, so that the electrons are not penetrating into the areas of the Bi-islands. The carrier concentration profile is confirmed from Shubnikov-de Haas oscillations. and signatures of weak localization are seen in the magnetoresistivity, which vanishes with increasing Bi coverage. 

(ii) Intercalated Pb layers below graphene. Intercalated Pb bilayers form nanostripes under graphene and show fingerprints of plumbene (a Pb-based monolayer similar to graphene)[2]. These Pb layers are rotated with respect to graphene which breaks the sublattice graphene symmetry. The Pb monolayer gives rise to charge-neutral monolayer graphene[2,3]. The conductivity decreases with decreasing temperature down to 30 K and reveals non-metallic behavior. We explain the origin of a small gap opening (1-5 meV) in graphene via the proximitized interaction with the Pb intercalated layer, e.g. spin orbit coupling in graphene.

 (iii) Sn submonolayer layers below graphene. The adsorption of 1/3 ML of Sn under graphene reveals a robust 2D Mott state. High resolution surface diffraction reveals new √3-reconstruction spots after the intercalation process.Using Electron Energy Loss Spectroscopy and Scanning Tunneling Spectroscopy we found strong evidence of hybridization between Sn-induced Mott states and the graphene 𝜋𝜋-bands. This leads to a gap opening of around 200 meV at the Dirac point. Moreover, a new state at 1.2 eV emerges which we assign to the upper Hubbard band. Combined DFT and dynamical mean field theory calculations support the existence of strong Mott Hubbard correlations in this heterostructure.