1,536 research outputs found
Exciton band topology in spontaneous quantum anomalous Hall insulators: applications to twisted bilayer graphene
We uncover topological features of neutral particle-hole pair excitations of
correlated quantum anomalous Hall (QAH) insulators whose approximately flat
conduction and valence bands have equal and opposite non-zero Chern number.
Using an exactly solvable model we show that the underlying band topology
affects both the center-of-mass and relative motion of particle-hole bound
states. This leads to the formation of topological exciton bands whose features
are robust to nonuniformity of both the dispersion and the Berry curvature. We
apply these ideas to recently-reported broken-symmetry spontaneous QAH
insulators in substrate aligned magic-angle twisted bilayer graphene.Comment: 5+10 pages, 2+2 figures; improved treatment of interaction effects,
leading to |C|=1 exciton
Supersymmetry on the honeycomb lattice: resonating charge stripes, superfrustration, and domain walls
We study a model of spinless fermions on the honeycomb lattice with
nearest-neighbor exclusion and extended repulsive interactions that exhibits
`lattice supersymmetry' [P. Fendley, K. Schoutens, and J. de Boer, Phys. Rev.
Lett. 90, 120402 (2003)]. Using a combination of exact diagonalization of large
( site) systems, mean-field numerics, and symmetry analysis, we
establish a rich phase structure as a function of fermion density, that
includes non-Fermi liquid behavior, resonating charge stripes, domain-wall and
bubble physics, and identify a finite range of fillings with extensive ground
state degeneracy and both gapped and gapless spectra. We comment on the
stability of our results to relaxing the stringent requirements for
supersymmetry, and on their possible broader relevance to systems of
strongly-correlated electrons with extended repulsive interactions.Comment: 15 pages, 12 figures, 1 tabl
Coulomb-driven band unflattening suppresses -phonon pairing in moir\'e graphene
It is a matter of current debate whether the gate-tunable superconductivity
in twisted bilayer graphene is phonon-mediated or arises from electron-electron
interactions. The recent observation of the strong coupling of electrons to
so-called -phonon modes in angle-resolved photoemission spectroscopy
experiments has resuscitated early proposals that -phonons drive
superconductivity. We show that the bandwidth-enhancing effect of interactions
drastically weakens both the intrinsic susceptibility towards pairing as well
as the screening of Coulomb repulsion that is essential for the phonon
attraction to dominate at low temperature. This rules out purely
-phonon-mediated superconductivity with the observed transition temperature
of K. We conclude that the unflattening of bands by Coulomb
interactions challenges any purely phonon-driven pairing mechanism, and must be
addressed by a successful theory of superconductivity in moir\'e graphene.Comment: 8 pages, 2 figures (+16 pages, 8 figures supplement
Skyrmions in Twisted Bilayer Graphene: Stability, Pairing, and Crystallization
We study the excitations that emerge upon doping the translationally invariant correlated insulating states in magic-angle twisted bilayer graphene at various integer filling factors ν. We identify parameter regimes where these are excitations associated with skyrmion textures in the spin or pseudospin degrees of freedom, and explore both short-distance pairing effects and the formation of long-range ordered skyrmion crystals. We perform a comprehensive analysis of the pseudospin skyrmions that emerge upon doping insulators at even ν, delineating the regime in parameter space where these are the lowest-energy charged excitations by means of self-consistent Hartree-Fock calculations on the interacting Bistritzer-MacDonald model. We explicitly demonstrate the purely electron-mediated pairing of skyrmions, a key ingredient behind a recent proposal of skyrmion superconductivity. Building upon this, we construct hopping models to extract the effective masses of paired skyrmions, and discuss our findings and their implications for skyrmion superconductivity in relation to experiments, focusing on the dome-shaped dependence of the transition temperature on the twist angle. We also investigate the properties of spin skyrmions about the quantized anomalous Hall insulator at ν=+3. In both cases, we demonstrate the formation of robust spin or pseudospin skyrmion crystals upon doping to a finite density away from integer filling
Coulomb-driven band unflattening suppresses K-phonon pairing in moire graphene
It is a matter of current debate whether the gate-tunable superconductivity in twisted bilayer graphene is phonon-mediated or arises from electron-electron interactions. The recent observation of the strong coupling of electrons to so-called K-phonon modes in angle-resolved photoemission spectroscopy experiments has resuscitated early proposals that K-phonons drive superconductivity. We show that the bandwidth-enhancing effect of interactions drastically weakens both the intrinsic susceptibility towards pairing as well as the screening of Coulomb repulsion that is essential for the phonon attraction to dominate at low temperature. This rules out purely K-phonon-mediated superconductivity with the observed transition temperature of ∼1 K. We conclude that the unflattening of bands by Coulomb interactions challenges any purely phonon-driven pairing mechanism, and must be addressed by a successful theory of superconductivity in moiré graphen
Electron-phonon coupling and competing Kekul\'e orders in twisted bilayer graphene
Recent scanning tunneling microscopy experiments [K.P. Nuckolls et al.,
arXiv:2303.00024] have revealed the ubiquity of Kekul\'e charge-density wave
order in magic-angle twisted bilayer graphene. Most samples are moderately
strained and show `incommensurate Kekul\'e spiral' (IKS) order involving a
graphene-scale charge density distortion uniaxially modulated on the scale of
the moir\'e superlattice, in accord with theoretical predictions. However,
ultra-low strain samples instead show graphene-scale Kekul\'e charge order that
is uniform on the moir\'e scale. This order, especially prominent near filling
factor , is unanticipated by theory which predicts a time-reversal
breaking Kekul\'e current order at low strain. We show that including the
coupling of moir\'e electrons to graphene-scale optical zone-corner (ZC)
phonons stabilizes a uniform Kekul\'e charge ordered state at with a
quantized topological (spin or anomalous Hall) response. Our work clarifies how
this phonon-driven selection of electronic order emerges in the strong-coupling
regime of moir\'e graphene.Comment: 5+4 page
Spin skyrmion gaps as signatures of strong-coupling insulators in magic-angle twisted bilayer graphene
The flat electronic bands in magic-angle twisted bilayer graphene (MATBG)
host a variety of correlated insulating ground states, many of which are
predicted to support charged excitations with topologically non-trivial spin
and/or valley skyrmion textures. However, it has remained challenging to
experimentally address their ground state order and excitations, both because
some of the proposed states do not couple directly to experimental probes, and
because they are highly sensitive to spatial inhomogeneities in real samples.
Here, using a scanning single-electron transistor, we observe thermodynamic
gaps at even integer moir\'e filling factors at low magnetic fields. We find
evidence of a field-tuned crossover from charged spin skyrmions to bare
particle-like excitations, suggesting that the underlying ground state belongs
to the manifold of strong-coupling insulators. From the spatial dependence of
these states and the chemical potential variation within the flat bands, we
infer a link between the stability of the correlated ground states and local
twist angle and strain. Our work advances the microscopic understanding of the
correlated insulators in MATBG and their unconventional excitations.Comment: Supplementary information available at
https://www.nature.com/articles/s41467-023-42275-
Helical trilayer graphene: a moir\'e platform for strongly-interacting topological bands
Quantum geometry of electronic wavefunctions results in fascinating
topological phenomena. A prominent example is the intrinsic anomalous Hall
effect (AHE) in which a Hall voltage arises in the absence of an applied
magnetic field. The AHE requires a coexistence of Berry curvature and
spontaneous time-reversal symmetry breaking. These conditions can be realized
in two-dimensional moir\'e systems with broken -inversion symmetry
() that host flat electronic bands. Here, we explore helical trilayer
graphene (HTG), three graphene layers twisted sequentially by the same angle
forming two misoriented moir\'e patterns. Although HTG is globally
-symmetric, surprisingly we observe clear signatures of topological
bands. At a magic angle , we uncover a
robust phase diagram of correlated and magnetic states using magnetotransport
measurements. Lattice relaxation leads to large periodic domains in which
is broken on the moir\'e scale. Each domain harbors flat topological
bands with valley-contrasting Chern numbers . We find correlated
states at integer electron fillings per moir\'e unit cell and
fractional fillings with the AHE arising at and .
At , a time-reversal symmetric phase appears beyond a critical electric
displacement field, indicating a topological phase transition. Finally,
hysteresis upon sweeping points to first-order phase transitions across a
spatial mosaic of Chern domains separated by a network of topological gapless
edge states. We establish HTG as an important platform that realizes ideal
conditions for exploring strongly interacting topological phases and, due to
its emergent moir\'e-scale symmetries, demonstrates a novel way to engineer
topology
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