scholarly journals Spectroscopy of a tunable moiré system with a correlated and topological flat band

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Xiaomeng Liu ◽  
Cheng-Li Chiu ◽  
Jong Yeon Lee ◽  
Gelareh Farahi ◽  
Kenji Watanabe ◽  
...  
Keyword(s):  
Band Structure ◽  
Twist Angle ◽  
Bilayer Graphene ◽  
Scanning Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Flat Band ◽  
Correlated Electron ◽  
Partial Filling ◽  
Valley Polarization ◽  
Spectroscopic Signatures

AbstractMoiré superlattices created by the twisted stacking of two-dimensional crystals can host electronic bands with flat energy dispersion in which enhanced interactions promote correlated electron states. The twisted double bilayer graphene (TDBG), where two Bernal bilayer graphene are stacked with a twist angle, is such a moiré system with tunable flat bands. Here, we use gate-tuned scanning tunneling spectroscopy to directly demonstrate the tunability of the band structure of TDBG with an electric field and to show spectroscopic signatures of electronic correlations and topology for its flat band. Our spectroscopic experiments are in agreement with a continuum model of TDBG band structure and reveal signatures of a correlated insulator gap at partial filling of its isolated flat band. The topological properties of this flat band are probed with the application of a magnetic field, which leads to valley polarization and the splitting of Chern bands with a large effective g-factor.

scholarly journals Flat band carrier confinement in magic-angle twisted bilayer graphene

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Nikhil Tilak ◽  
Xinyuan Lai ◽  
Shuang Wu ◽  
Zhenyuan Zhang ◽  
Mingyu Xu ◽  
...  
Keyword(s):  
Twist Angle ◽  
Bilayer Graphene ◽  
Scanning Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Flat Band ◽  
Magic Angle ◽  
Energy Bands ◽  
Carrier Confinement ◽  
Correlated Electron ◽  
Twisted Bilayer Graphene

AbstractMagic-angle twisted bilayer graphene has emerged as a powerful platform for studying strongly correlated electron physics, owing to its almost dispersionless low-energy bands and the ability to tune the band filling by electrostatic gating. Techniques to control the twist angle between graphene layers have led to rapid experimental progress but improving sample quality is essential for separating the delicate correlated electron physics from disorder effects. Owing to the 2D nature of the system and the relatively low carrier density, the samples are highly susceptible to small doping inhomogeneity which can drastically modify the local potential landscape. This potential disorder is distinct from the twist angle variation which has been studied elsewhere. Here, by using low temperature scanning tunneling spectroscopy and planar tunneling junction measurements, we demonstrate that flat bands in twisted bilayer graphene can amplify small doping inhomogeneity that surprisingly leads to carrier confinement, which in graphene could previously only be realized in the presence of a strong magnetic field.

scholarly journals Flat band carrier confinement in magic-angle twisted bilayer graphene

2020 ◽  
Author(s):  
Nikhil Tilak ◽  
xinyuan lai ◽  
Shuang Wu ◽  
Zhenyuan Zhang ◽  
Mingyu Xu ◽  
...  
Keyword(s):  
Twist Angle ◽  
Bilayer Graphene ◽  
Scanning Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Flat Band ◽  
Magic Angle ◽  
Energy Bands ◽  
Angle Variation ◽  
Carrier Confinement ◽  
Twisted Bilayer Graphene

Abstract Magic angle twisted bilayer graphene has emerged as a powerful platform for studying strongly correlated electron physics, owing to its almost dispersionless low-energy bands and the ability to tune the band filling by electrostatic gating. Techniques to control the twist angle between graphene layers have led to rapid experimental progress, but improving sample quality is essential for separating the delicate correlation physics from disorder effects. Owing to the 2D nature of the system and the relatively low carrier density, the samples are highly susceptible to small doping inhomogeneity which can drastically modify the local potential landscape. This potential disorder is distinct from the twist-angle variation which has been studied elsewhere. Understanding and mitigating the effects of such disorder is important. Here, we demonstrate using low temperature scanning tunneling spectroscopy and planar tunneling junction measurements, how flat bands in twisted bilayer graphene can amplify small doping inhomogeneity leading to carrier confinement, thus obscuring magic-angle physics.

scholarly journals Dynamic band structure and capacitance effects in scanning tunneling spectroscopy of bilayer graphene

2019 ◽  
Vol 115 (18) ◽  
pp. 181601 ◽  
Author(s):  
Gregory R. Holdman ◽  
Zachary J. Krebs ◽  
Wyatt A. Behn ◽  
Keenan J. Smith ◽  
K. Watanabe ◽  
...  
Keyword(s):  
Band Structure ◽  
Bilayer Graphene ◽  
Scanning Tunneling Spectroscopy ◽  
Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Capacitance Effects

Evolution of polypyrrole band structure: a scanning tunneling spectroscopy study

1992 ◽  
Vol 96 (3) ◽  
pp. 1428-1430 ◽  
Author(s):  
R. Yang ◽  
W. H. Smyrl ◽  
D. F. Evans ◽  
W. A. Hendrickson
Keyword(s):  
Band Structure ◽  
Scanning Tunneling Spectroscopy ◽  
Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Spectroscopy Study

scholarly journals Moiréless correlations in ABCA graphene

2021 ◽  
Vol 118 (4) ◽  
pp. e2017366118 ◽  
Author(s):  
Alexander Kerelsky ◽  
Carmen Rubio-Verdú ◽  
Lede Xian ◽  
Dante M. Kennes ◽  
Dorri Halbertal ◽  
...  
Keyword(s):  
Theoretical Calculations ◽  
Van Der Waals ◽  
Bilayer Graphene ◽  
Scanning Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Van Hove Singularity ◽  
Natural Interfaces ◽  
Simple Material ◽  
Electronic Band ◽  
Topological Quantum

Atomically thin van der Waals materials stacked with an interlayer twist have proven to be an excellent platform toward achieving gate-tunable correlated phenomena linked to the formation of flat electronic bands. In this work we demonstrate the formation of emergent correlated phases in multilayer rhombohedral graphene––a simple material that also exhibits a flat electronic band edge but without the need of having a moiré superlattice induced by twisted van der Waals layers. We show that two layers of bilayer graphene that are twisted by an arbitrary tiny angle host large (micrometer-scale) regions of uniform rhombohedral four-layer (ABCA) graphene that can be independently studied. Scanning tunneling spectroscopy reveals that ABCA graphene hosts an unprecedentedly sharp van Hove singularity of 3–5-meV half-width. We demonstrate that when this van Hove singularity straddles the Fermi level, a correlated many-body gap emerges with peak-to-peak value of 9.5 meV at charge neutrality. Mean-field theoretical calculations for model with short-ranged interactions indicate that two primary candidates for the appearance of this broken symmetry state are a charge-transfer excitonic insulator and a ferrimagnet. Finally, we show that ABCA graphene hosts surface topological helical edge states at natural interfaces with ABAB graphene which can be turned on and off with gate voltage, implying that small-angle twisted double-bilayer graphene is an ideal programmable topological quantum material.

scholarly journals Correlated insulating and superconducting states in twisted bilayer graphene below the magic angle

2019 ◽  
Vol 5 (9) ◽  
pp. eaaw9770 ◽  
Author(s):  
Emilio Codecido ◽  
Qiyue Wang ◽  
Ryan Koester ◽  
Shi Che ◽  
Haidong Tian ◽  
...  
Keyword(s):  
Energy Gap ◽  
Twist Angle ◽  
High Energy ◽  
Bilayer Graphene ◽  
Unit Cell ◽  
Flat Band ◽  
Magic Angle ◽  
Strongly Correlated ◽  
New Information ◽  
Twisted Bilayer Graphene

The emergence of flat bands and correlated behaviors in “magic angle” twisted bilayer graphene (tBLG) has sparked tremendous interest, though its many aspects are under intense debate. Here we report observation of both superconductivity and the Mott-like insulating state in a tBLG device with a twist angle of ~0.93°, which is smaller than the magic angle by 15%. At an electron concentration of ±5 electrons/moiré unit cell, we observe a narrow resistance peak with an activation energy gap ~0.1 meV. This indicates additional correlated insulating state, and is consistent with theory predicting a high-energy flat band. At doping of ±12 electrons/moiré unit cell we observe resistance peaks arising from the Dirac points in the spectrum. Our results reveal that the “magic” range of tBLG is in fact larger than what is previously expected, and provide a wealth of new information to help decipher the strongly correlated phenomena observed in tBLG.

scholarly journals Competing correlated states and abundant orbital magnetism in twisted monolayer-bilayer graphene

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Minhao He ◽  
Ya-Hui Zhang ◽  
Yuhao Li ◽  
Zaiyao Fei ◽  
Kenji Watanabe ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Anomalous Hall Effect ◽  
Bilayer Graphene ◽  
Flat Band ◽  
Zero Field ◽  
First Order ◽  
Orbital Magnetism ◽  
Valley Polarization ◽  
The Rich ◽  
Topological Phase Transitions

AbstractFlat band moiré superlattices have recently emerged as unique platforms for investigating the interplay between strong electronic correlations, nontrivial band topology, and multiple isospin ‘flavor’ symmetries. Twisted monolayer-bilayer graphene (tMBG) is an especially rich system owing to its low crystal symmetry and the tunability of its bandwidth and topology with an external electric field. Here, we find that orbital magnetism is abundant within the correlated phase diagram of tMBG, giving rise to the anomalous Hall effect in correlated metallic states nearby most odd integer fillings of the flat conduction band, as well as correlated Chern insulator states stabilized in an external magnetic field. The behavior of the states at zero field appears to be inconsistent with simple spin and valley polarization for the specific range of twist angles we investigate, and instead may plausibly result from an intervalley coherent (IVC) state with an order parameter that breaks time reversal symmetry. The application of a magnetic field further tunes the competition between correlated states, in some cases driving first-order topological phase transitions. Our results underscore the rich interplay between closely competing correlated ground states in tMBG, with possible implications for probing exotic IVC ordering.

scholarly journals Abnormal conductivity in low-angle twisted bilayer graphene

2020 ◽  
Vol 6 (47) ◽  
pp. eabc5555
Author(s):  
Shuai Zhang ◽  
Aisheng Song ◽  
Lingxiu Chen ◽  
Chengxin Jiang ◽  
Chen Chen ◽  
...  
Keyword(s):  
Density Functional ◽  
Twist Angle ◽  
Van Der Waals ◽  
Density Functional Theory Calculations ◽  
Bilayer Graphene ◽  
Abnormal Behavior ◽  
Scanning Tunneling ◽  
Tunneling Microscopy ◽  
Van Der Waals Materials ◽  
The Impact

Controlling the interlayer twist angle offers a powerful means for tuning the electronic properties of two-dimensional (2D) van der Waals materials. Typically, the electrical conductivity would increase monotonically with decreasing twist angle owing to the enhanced coupling between adjacent layers. Here, we report a nonmonotonic angle-dependent vertical conductivity across the interface of bilayer graphene with low twist angles. More specifically, the vertical conductivity enhances gradually with decreasing twist angle up to a crossover angle at θc ≈ 5°, and then it drops notably upon further decrease in the twist angle. Revealed by density functional theory calculations and scanning tunneling microscopy, the abnormal behavior is attributed to the unusual reduction in average carrier density originating from local atomic reconstruction. The impact of atomic reconstruction on vertical conductivity is unique for low-angle twisted 2D van der Waals materials and provides a strategy for designing and optimizing their electronic performance.

scholarly journals Electronic Structure of Oxygen-Deficient SrTiO3 and Sr2TiO4

Crystals ◽  
2019 ◽  
Vol 9 (11) ◽  
pp. 580 ◽  
Author(s):  
Ali Al-Zubi ◽  
Gustav Bihlmayer ◽  
Stefan Blügel
Keyword(s):  
Density Functional ◽  
Scanning Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Conductive Atomic Force Microscopy ◽  
Force Microscopy ◽  
The Core ◽  
Atomic Force ◽  
Level Shifts ◽  
Spectroscopic Signatures ◽  
Gap States

The conductive behavior of the perovskite SrTiO 3 is strongly influenced by the presence of oxygen vacancies in this material, therefore the identification of such defects with spectroscopic methods is of high importance. We use density functional theory to characterize the defect-induced states in SrTiO 3 and Sr 2 TiO 4 . Their signatures at the surface, the visibility for scanning tunneling spectroscopy and locally conductive atomic force microscopy, and the core-level shifts observed on Ti atoms in the vicinity of the defect are studied. In particular, we find that the exact location of the defect state (e.g., in SrO or TiO 2 planes relative to the surface) are decisive for their visibility for scanning-probe methods. Moreover, the usual distinction between Ti 3 + and Ti 2 + species, which can occur near defects or their aggregates, cannot be directly translated in characteristic shifts of the core levels. The width of the defect-induced in-gap states is found to depend critically on the arrangement of the defects. This also has consequences for the spectroscopic signatures observed in so-called resistive switching phenomena.

scholarly journals Visualizing delocalized correlated electronic states in twisted double bilayer graphene

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Canxun Zhang ◽  
Tiancong Zhu ◽  
Salman Kahn ◽  
Shaowei Li ◽  
Birui Yang ◽  
...  
Keyword(s):  
Local Density ◽  
Coulomb Repulsion ◽  
Bilayer Graphene ◽  
Differential Conductance ◽  
Scanning Tunneling ◽  
Flat Band ◽  
Tunneling Microscopy ◽  
Hartree Fock ◽  
Half Filling ◽  
Graphene Devices

AbstractThe 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.

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