Defect-induced magnetism and Yu-Shiba-Rusinov states in twisted bilayer graphene

Abstract

Atomic defects have a significant impact in the low-energy properties of graphene systems. By means of first-principles calculations and tight-binding models we provide evidence that chemical impurities modify both the normal and the superconducting states of twisted bilayer graphene. A single hydrogen atom attached to the bilayer surface yields a triple-point crossing, whereas selfdoping and three-fold symmetry-breaking are created by a vacant site. Both types of defects lead to time-reversal symmetry-breaking and the creation of local magnetic moments. Hydrogen-induced magnetism is found to exist also at the doping levels where superconductivity appears in magic angle graphene superlattices. As a result, the coexistence of superconducting order and defect-induced magnetism yields in-gap Yu-Shiba-Rusinov excitations in magic angle twisted bilayer graphene. I.arXiv:1906.02711v1 [cond-mat.mtrl-sci]

Electrical band flattening, valley flux, and superconductivity in twisted trilayer graphene

“…As a reference, for twisted graphene multilayers t ≈ 3 eV and t ⊥ ≈ 0.15t. 1 Similar real-space models were used to study a variety of twisted graphene multilayers [10,36,50,51], providing a simple formalism to study the effect of dopants and impurities [52,53]. However, in contrast to continuum models [9,42,54], measuring of valley-related quantities with a real-space-based formalism is nontrivial.…”

“…A similar effect can be induced with chemical doping, namely, adding one N atom in substitution of a C atom. The effects of chemical substitution have been extensively studied in graphene [72][73][74][75][76][77][78][79][80][81], including twisted bilayers [53,82]. Figure 5 shows the effect on the bands structure of one N atom in one of the surface layers.…”

Kondo-lattice-mediated interactions in flat band systems

“…The lattice version of the problem, known as the Kondo-lattice problem, represents the starting point for exotic physical phenomena found in heavy-fermion systems [34,[37][38][39][40][41][42][43][44]. Interestingly, the Kondo physics outlined above is usually addressed in systems with strong electronic dispersion, while the Kondo problem for flat bands has been much less explored [45][46][47][48][49]. In the dispersive limit, the interaction between a local magnetic impurity and the conduction bath is determined by the Kondo temperature, increasing with the density of states, and therefore divergent in the flat band regime.…”

Effect of Van Hove singularities on Shiba states in two-dimensional $s$-wave superconductors

“…In the present work, motivated by the recent discovery of superconductivity in TBG [48], the quest to find YSR states in graphene [49][50][51], and the promising pathway to VHS tunability in these materials, we explore the influence of conventional and high-order VHS on YSR states. We find that tuning the Fermi level to a Van Hove singularity is a powerful way to enhance the coupling of YSR states to the substrate, while at the same time, it provides a possibility to extract information from the DOS singularity itself.…”