14 research outputs found
Visualizing and manipulating chiral interface states in a moir\'e quantum anomalous Hall insulator
Moir\'e systems made from stacked two-dimensional materials host novel
correlated and topological states that can be electrically controlled via
applied gate voltages. We have used this technique to manipulate Chern domains
in an interaction-driven quantum anomalous Hall insulator made from twisted
monolayer-bilayer graphene (tMBLG). This has allowed the wavefunction of chiral
interface states to be directly imaged using a scanning tunneling microscope
(STM). To accomplish this tMBLG carrier concentration was tuned to stabilize
neighboring domains of opposite Chern number, thus providing topological
interfaces completely devoid of any structural boundaries. STM tip
pulse-induced quantum dots were utilized to induce new Chern domains and
thereby create new chiral interface states with tunable chirality at
predetermined locations. Theoretical analysis confirms the chiral nature of
observed interface states and enables the determination of the characteristic
length scale of valley polarization reversal across neighboring tMBLG Chern
domains. tMBLG is shown to be a useful platform for imaging the exotic
topological properties of correlated moir\'e systems.Comment: 30 pages, 13 figures, 1 tabl
Large-gap insulating dimer ground state in monolayer IrTe2
Monolayers of two-dimensional van der Waals materials exhibit novel
electronic phases distinct from their bulk due to the symmetry breaking and
reduced screening in the absence of the interlayer coupling. In this work, we
combine angle-resolved photoemission spectroscopy and scanning tunneling
microscopy/spectroscopy to demonstrate the emergence of a unique insulating 2 x
1 dimer ground state in monolayer 1T-IrTe2 that has a large band gap in
contrast to the metallic bilayer-to-bulk forms of this material.
First-principles calculations reveal that phonon and charge instabilities as
well as local bond formation collectively enhance and stabilize a
charge-ordered ground state. Our findings provide important insights into the
subtle balance of interactions having similar energy scales that occurs in the
absence of strong interlayer coupling, which offers new opportunities to
engineer the properties of 2D monolayers
Visualizing delocalized correlated electronic states in twisted double bilayer graphene
The discovery of interaction-driven insulating and superconducting phases in
moir\'e van der Waals heterostructures has sparked considerable interest in
understanding the novel correlated physics of these systems. While a
significant number of studies have focused on twisted bilayer graphene,
correlated insulating states and a superconductivity-like transition up to 12 K
have been reported in recent transport measurements of twisted double bilayer
graphene. Here we present a scanning tunneling microscopy and spectroscopy
study of gate-tunable twisted double bilayer graphene devices. We observe
splitting of the van Hove singularity peak by ~20 meV at half-filling of the
conduction flat band, with a corresponding reduction of the local density of
states at the Fermi level. By mapping the tunneling differential conductance we
show that this correlated system exhibits energetically split states that are
spatially delocalized throughout the different regions in the moir\'e unit
cell, inconsistent with order originating solely from onsite Coulomb repulsion
within strongly-localized orbitals. We have performed self-consistent
Hartree-Fock calculations that suggest exchange-driven spontaneous symmetry
breaking in the degenerate conduction flat band is the origin of the observed
correlated state. Our results provide new insight into the nature of
electron-electron interactions in twisted double bilayer graphene and related
moir\'e systems.Comment: 24 pages, 5 figure
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Local spectroscopy of a gate-switchable moiré quantum anomalous Hall insulator.
In recent years, correlated insulating states, unconventional superconductivity, and topologically non-trivial phases have all been observed in several moiré heterostructures. However, understanding of the physical mechanisms behind these phenomena is hampered by the lack of local electronic structure data. Here, we use scanning tunnelling microscopy and spectroscopy to demonstrate how the interplay between correlation, topology, and local atomic structure determines the behaviour of electron-doped twisted monolayer-bilayer graphene. Through gate- and magnetic field-dependent measurements, we observe local spectroscopic signatures indicating a quantum anomalous Hall insulating state with a total Chern number of ±2 at a doping level of three electrons per moiré unit cell. We show that the sign of the Chern number and associated magnetism can be electrostatically switched only over a limited range of twist angle and sample hetero-strain values. This results from a competition between the orbital magnetization of filled bulk bands and chiral edge states, which is sensitive to strain-induced distortions in the moiré superlattice
Local spectroscopy of a gate-switchable moir\'e quantum anomalous Hall insulator
In recent years, correlated insulating states, unconventional
superconductivity, and topologically non-trivial phases have all been observed
in several moir\'e heterostructures. However, understanding of the physical
mechanisms behind these phenomena is hampered by the lack of local electronic
structure data. Here, we use scanning tunnelling microscopy and spectroscopy to
demonstrate how the interplay between correlation, topology, and local atomic
structure determines the behaviour of electron-doped twisted monolayer-bilayer
graphene. Through gate- and magnetic field-dependent measurements, we observe
local spectroscopic signatures indicating a quantum anomalous Hall insulating
state with a total Chern number of at a doping level of three electrons
per moir\'e unit cell. We show that the sign of the Chern number and associated
magnetism can be electrostatically switched only over a limited range of twist
angle and sample hetero-strain values. This results from a competition between
the orbital magnetization of filled bulk bands and chiral edge states, which is
sensitive to strain-induced distortions in the moir\'e superlattice.Comment: Article 14 pages, 4 figures & Supplementary Information 13 pages, 9
figure
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Visualizing delocalized correlated electronic states in twisted double bilayer graphene.
The discovery of interaction-driven insulating and superconducting phases in moiré van der Waals heterostructures has sparked considerable interest in understanding the novel correlated physics of these systems. While a significant number of studies have focused on twisted bilayer graphene, correlated insulating states and a superconductivity-like transition up to 12 K have been reported in recent transport measurements of twisted double bilayer graphene. Here we present a scanning tunneling microscopy and spectroscopy study of gate-tunable twisted double bilayer graphene devices. We observe splitting of the van Hove singularity peak by ~20 meV at half-filling of the conduction flat band, with a corresponding reduction of the local density of states at the Fermi level. By mapping the tunneling differential conductance we show that this correlated system exhibits energetically split states that are spatially delocalized throughout the different regions in the moiré unit cell, inconsistent with order originating solely from onsite Coulomb repulsion within strongly-localized orbitals. We have performed self-consistent Hartree-Fock calculations that suggest exchange-driven spontaneous symmetry breaking in the degenerate conduction flat band is the origin of the observed correlated state. Our results provide new insight into the nature of electron-electron interactions in twisted double bilayer graphene and related moiré systems