6 research outputs found
Insulators at Fractional Fillings in Twisted Bilayer Graphene Partially Aligned to Hexagonal Boron Nitride
At partial fillings of its flat electronic bands, magic-angle twisted bilayer
graphene (MATBG) hosts a rich variety of competing correlated phases that show
sample to sample variations. Divergent phase diagrams in MATBG are often
attributed to the sublattice polarization energy scale, tuned by the degree of
alignment of the hexagonal boron nitride (hBN) substrates typically used in van
der Waals devices. Unaligned MATBG exhibits unconventional superconductivity
and correlated insulating phases, while nearly perfectly aligned MATBG/hBN
exhibits zero-field Chern insulating phases and lacks superconductivity. Here
we use scanning tunneling microscopy and spectroscopy (STM/STS) to observe
gapped phases at partial fillings of the flat bands of MATBG in a new
intermediate regime of sublattice polarization, observed when MATBG is only
partially aligned ( 1.65) to the underlying
hBN substrate. Under this condition, MATBG hosts not only phenomena that
naturally interpolate between the two sublattice potential limits, but also
unexpected gapped phases absent in either of these limits. At charge
neutrality, we observe an insulating phase with a small energy gap ( <
5 meV) likely related to weak sublattice symmetry breaking from the hBN
substrate. In addition, we observe new gapped phases near fractional fillings
= and = , which have not been previously
observed in MATBG. Importantly, energy-resolved STS unambiguously identifies
these fractional filling states to be of single-particle origin, possibly a
result of the super-superlattice formed by two moir\'e superlattices. Our
observations emphasize the power of STS in distinguishing single-particle
gapped phases from many-body gapped phases in situations that could be easily
confused in electrical transport measurements.Comment: 4 figure
Quantum textures of the many-body wavefunctions in magic-angle graphene
Interactions among electrons create novel many-body quantum phases of matter
with wavefunctions that often reflect electronic correlation effects, broken
symmetries, and novel collective excitations. A wide range of quantum phases
has been discovered in MATBG, including correlated insulating, unconventional
superconducting, and magnetic topological phases. The lack of microscopic
information, including precise knowledge of possible broken symmetries, has
thus far hampered our understanding of MATBG's correlated phases. Here we use
high-resolution scanning tunneling microscopy to directly probe the
wavefunctions of the correlated phases in MATBG. The squares of the
wavefunctions of gapped phases, including those of the correlated insulators,
pseudogap, and superconducting phases, show distinct patterns of broken
symmetry with a x super-periodicity on the graphene
atomic lattice that has a complex spatial dependence on the moir\'e
superlattice scale. We introduce a symmetry-based analysis to describe our
measurements of the wavefunctions of MATBG's correlated phases with a set of
complex-valued local order parameters. For the correlated insulators in MATBG,
at fillings of = 2 electrons per moir\'e unit cell relative to
charge neutrality, we compare the observed quantum textures to those expected
for proposed theoretical ground states. In typical MATBG devices, the textures
of correlated insulators' wavefunctions closely match those of the
theoretically proposed IKS order, while in ultra-low-strain samples our data
has local symmetries like those of a T-IVC phase. We also study the
wavefunction of MATBG's superconducting state, revealing strong signatures of
intervalley coherence that can only be distinguished from those of the
insulator with our phase-sensitive measurements.Comment: 5 figure
Strong Inter-valley Electron-Phonon Coupling in Magic-Angle Twisted Bilayer Graphene
The unusual properties of superconductivity in magic-angle twisted bilayer
graphene (MATBG) have sparked enormous research interest. However, despite the
dedication of intensive experimental efforts and the proposal of several
possible pairing mechanisms, the origin of its superconductivity remains
elusive. Here, using angle-resolved photoemission spectroscopy with micrometer
spatial resolution, we discover replicas of the flat bands in superconducting
MATBG unaligned with its hexagonal boron nitride (hBN) substrate, which are
absent in non-superconducting MATBG aligned with the hBN substrate. Crucially,
the replicas are evenly spaced in energy, separated by 150 +- 15 meV,
signalling the strong coupling of electrons in MATBG to a bosonic mode of this
energy. By comparing our observations to simulations, the formation of replicas
is attributed to the presence of strong inter-valley electron-phonon coupling
to a K-point phonon mode. In total, the observation of these replica flat bands
and the corresponding phonon mode in MATBG could provide important information
for understanding the origin and the unusual properties of its superconducting
phase.Comment: 17 pages, 4 figure
Spectroscopy of Twisted Bilayer Graphene Correlated Insulators
We analytically compute the scanning tunneling microscopy (STM) signatures of integer-filled correlated ground states of the magic angle twisted bilayer graphene (TBG) narrow bands. After experimentally validating the strong-coupling approach at ±4 electrons/moiré unit cell, we consider the spatial features of the STM signal for 14 different many-body correlated states and assess the possibility of Kekulé distortion (KD) emerging at the graphene lattice scale. Remarkably, we find that coupling the two opposite graphene valleys in the intervalley-coherent (IVC) TBG insulators does not always result in KD. As an example, we show that the Kramers IVC state and its nonchiral U (4) rotations do not exhibit any KD, while the time-reversal-symmetric IVC state does. Our results, obtained over a large range of energies and model parameters, show that the STM signal and Chern number of a state can be used to uniquely determine the nature of the TBG ground state