15 research outputs found
Imaging Electronic Correlations in Twisted Bilayer Graphene near the Magic Angle
Twisted bilayer graphene with a twist angle of around 1.1{\deg} features a
pair of isolated flat electronic bands and forms a strongly correlated
electronic platform. Here, we use scanning tunneling microscopy to probe local
properties of highly tunable twisted bilayer graphene devices and show that the
flat bands strongly deform when aligned with the Fermi level. At half filling
of the bands, we observe the development of gaps originating from correlated
insulating states. Near charge neutrality, we find a previously unidentified
correlated regime featuring a substantially enhanced flat band splitting that
we describe within a microscopic model predicting a strong tendency towards
nematic ordering. Our results provide insights into symmetry breaking
correlation effects and highlight the importance of electronic interactions for
all filling factors in twisted bilayer graphene.Comment: Main text 9 pages, 4 figures; Supplementary Information 25 page
Correlated insulator behaviour at half-filling in magic-angle graphene superlattices
Van der Waals (vdW) heterostructures are an emergent class of metamaterials comprised of vertically stacked two-dimensional (2D) building blocks, which provide us with a vast tool set to engineer their properties on top of the already rich tunability of 2D materials. 1 One of the knobs, the twist angle between different layers, plays a crucial role in the ultimate electronic properties of a vdW heterostructure and does not have a direct analog in other systems such as MBE-grown semiconductor heterostructures. For small twist angles, the moiré pattern produced by the lattice misorientation creates a long-range modulation. So far, the study of the effect of twist angles in vdW heterostructures has been mostly concentrated in graphene/hex a gonal boron nitride (h-BN) twisted structures, which exhibit relatively weak interlayer interaction due to the presence of a large bandgap in h-BN. 2-5 Here we show that when two graphene sheets are twisted by an angle close to the theoretically predicted ‘magic angle’, the resulting flat band structure near charge neutrality gives rise to a strongly-correlated electronic system . 6 These flat bands exhibit half-filling insulating phases at zero magnetic field, which we show to be a Mott-like insulator arising from electrons localized in the moiré superlattice. These unique properties of magic-angle twisted bilayer graphene (TwBLG) open up a new playground for exotic many-body quantum phases in a 2D platform made of pure carbon and without mag netic field. The easy accessibility of the flat bands, the electrical tunability, and the bandwidth tunability though twist angle may pave the way towards more exotic correlated systems, such as unconventional superconductors or quantum spin liquids
Jahn\u2013Teller coupling to moir\ue9 phonons in the continuum model formalism for small-angle twisted bilayer graphene
We show how to include the Jahn\u2013Teller coupling of moir\ue9 phonons to the electrons in the continuum model formalism which describes small-angle twisted bilayer graphene. These phonons, which strongly couple to the valley degree of freedom, are able to open gaps at most integer fillings of the four flat bands around the charge neutrality point. Moreover, we derive the full quantum mechanical expression of the electron\u2013phonon Hamiltonian, which may allow accessing phenomena such as the phonon-mediated superconductivity and the dynamical Jahn\u2013Teller effect
Unconventional superconductivity in magic-angle graphene superlattices
The understanding of strongly-correlated materials, and in particular
unconventional superconductors, has puzzled physicists for decades. Such
difficulties have stimulated new research paradigms, such as ultra-cold atom
lattices for simulating quantum materials. Here we report on the realization of
intrinsic unconventional superconductivity in a 2D superlattice created by
stacking two graphene sheets with a small twist angle. For angles near
, the first `magic' angle, twisted bilayer graphene (TBG) exhibits
ultra-flat bands near charge neutrality, which lead to correlated insulating
states at half-filling. Upon electrostatic doping away from these correlated
insulating states, we observe tunable zero-resistance states with a critical
temperature up to 1.7 K. The temperature-density phase diagram shows
similarities with that of the cuprates, including superconducting domes.
Moreover, quantum oscillations indicate small Fermi surfaces near the
correlated insulating phase, in analogy with under-doped cuprates. The relative
high , given such small Fermi surface (corresponding to a record-low 2D
carrier density of , renders TBG among the strongest
coupling superconductors, in a regime close to the BCS-BEC crossover. These
novel results establish TBG as the first purely carbon-based 2D superconductor
and as a highly tunable platform to investigate strongly-correlated phenomena,
which could lead to insights into the physics of high- superconductors and
quantum spin liquids.Comment: 18 pages, 9 figures (with Methods). A few typos correcte