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.

Scanning tunneling spectroscopy of a magnetic adatom on graphene

2014 ◽  
Vol 28 (31) ◽  
pp. 1450225 ◽  
Author(s):  
Yang Gao ◽  
Kai-He Ding
Keyword(s):  
Gate Voltage ◽  
Local Density ◽  
Scanning Tunneling Spectroscopy ◽  
Tunneling Spectroscopy ◽  
Differential Conductance ◽  
Scanning Tunneling ◽  
Density Of State ◽  
Tunneling Microscopy ◽  
Substitutional Site ◽  
Kondo Resonance

We present a theoretic study on scanning tunneling spectroscopy (STS) of a magnetic adatom on graphene. Three typical configurations of adatoms on graphene are considered explicitly: the adatom is on the top of a carbon atom (TC), in a substitutional site (SC), or above the center of the honeycomb hexagon (HC). Based on the nonequilibrium Green's function method, we derive the local density of state (LDOS) for the adatom and the differential conductance through the scanning tunneling microscopy (STM) device. Our results show that in comparison with the cases of the TC and SC, there exists an anomalous broadening of the local adatom energy level in the HC, which pushes the adatom energy to first cross the Fermi level, leading to the appearance of an antiresonance in the LDOS due to the interference between the Kondo resonance and the broadened adatom level. Correspondingly, the bias dependence of the differential conductance in the HC exhibits a more asymmetric sharp Kondo peak pinned to the gate voltage, and its height still remains significantly large compared to that for the other two cases. Additionally, with decreasing the gate voltage, the Kondo peak in the differential conductance gradually decays, and eventually vanishes in the absence of the gate voltage.

scholarly journals Evolution of quasi-bound states in the circular n–p junction of bilayer graphene under magnetic field

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
Haijiao Ji ◽  
Yueting Pan ◽  
Haiwen Liu
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Bound States ◽  
Local Density ◽  
Energy Levels ◽  
Bilayer Graphene ◽  
Scanning Tunneling ◽  
Tunneling Microscopy ◽  
Weak Magnetic Fields ◽  
The Magnetic Field

Abstract Electron in gapless bilayer graphene can form quasi-bound states when a circular symmetric potential is created in bilayer graphene. These quasi-bound states can be adjusted by tuning the radius and strength of the potential barrier. We investigate the evolution of quasi-bound states spectra in the circular n–p junction of bilayer graphene under the magnetic field numerically. The energy levels of opposite angular momentum split and the splitting increases with the magnetic field. Moreover, weak magnetic fields can slightly shift the energy levels of quasi-bound states. While strong magnetic fields induce additional resonances in the local density states, which originates from Landau levels. We demonstrate that these numerical results are consistent with the semiclassical analysis based on Wentzel–Kramers–Brillouin approximation. Our results can be verified experimentally via scanning tunneling microscopy measurements.

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.

In-Plane Asymmetries on the Ge(111)-c(2×8) Surface Mapped with the Scanning Tunneling Microscope

1992 ◽  
Vol 295 ◽  
Author(s):  
P. Molinàs-Mata ◽  
J. Zegenhagen ◽  
M. Böhringer ◽  
N. Takeuchi ◽  
A. Selloni
Keyword(s):  
Local Density ◽  
Experimental Studies ◽  
Surface States ◽  
Small Sample ◽  
Constant Current ◽  
Scanning Tunneling ◽  
Tunneling Microscopy ◽  
Low Energy Electron ◽  
Tunneling Currents ◽  
Insight Into

AbstractWe report on new experimental studies of the Ge(111)-c(2×8) reconstruction performed with low-energy electron diffraction. (LEED) and scanning tunneling microscopy (STM). Weak quarter-order reflections are present in the c(2 × 8) LEED pattern in agreement with previous observations and results of ab initio calculations. In order to gain insight into the predicted splitting of dangling bond states, we compare constant current topographs (CCT's) performed at high-tunneling currents (40.nA) with first-principles calculations of the local density of states (LDOS) 1Å above the surface adatoms and obtain good qualitative agreement. We finally discuss to what extent the STM CCT's at high tunneling currents (small sample-tip distances (STD)) are sensitive to surface states outside the Г point.

Density fluctuations as door-opener for diffusion on crowded surfaces

Science ◽  
2019 ◽  
Vol 363 (6428) ◽  
pp. 715-718 ◽  
Author(s):  
Ann-Kathrin Henß ◽  
Sung Sakong ◽  
Philipp K. Messer ◽  
Joachim Wiechers ◽  
Rolf Schuster ◽  
...  
Keyword(s):  
High Speed ◽  
Density Functional ◽  
Local Density ◽  
Variable Temperature ◽  
Industrial Process ◽  
Density Fluctuations ◽  
Scanning Tunneling ◽  
Fast Diffusion ◽  
Tunneling Microscopy ◽  
Adsorption Sites

How particles can move on a catalyst surface that, under the conditions of an industrial process, is highly covered by adsorbates and where most adsorption sites are occupied has remained an open question. We have studied the diffusion of O atoms on a fully CO-covered Ru(0001) surface by means of high-speed/variable-temperature scanning tunneling microscopy combined with density functional theory calculations. Atomically resolved trajectories show a surprisingly fast diffusion of the O atoms, almost as fast as on the clean surface. This finding can be explained by a “door-opening” mechanism in which local density fluctuations in the CO layer intermittently create diffusion pathways on which the O atoms can move with low activation energy.

Scanning tunneling microscopy study of the quasicrystalline 30° twisted bilayer graphene

2D Materials ◽  
2019 ◽  
Vol 6 (4) ◽  
pp. 045041 ◽  
Author(s):  
Chao Yan ◽  
Dong-Lin Ma ◽  
Jia-Bin Qiao ◽  
Hao-Yuan Zhong ◽  
Lin Yang ◽  
...  
Keyword(s):  
Scanning Tunneling Microscopy ◽  
Microscopy Study ◽  
Bilayer Graphene ◽  
Scanning Tunneling ◽  
Tunneling Microscopy ◽  
Twisted Bilayer Graphene ◽  
Scanning Tunneling Microscopy Study

Local Electronic Structure in Ordered Aggregates of Carbon Nanotubes: Scanning Tunneling Microscopy/scanning Tunneling Spectroscopy Study

1998 ◽  
Vol 13 (9) ◽  
pp. 2389-2395 ◽  
Author(s):  
D. L. Carroll ◽  
P. M. Ajayan ◽  
S. Curran
Keyword(s):  
Carbon Nanotubes ◽  
Electronic Structure ◽  
Scanning Tunneling Microscopy ◽  
Local Density ◽  
Scanning Tunneling Spectroscopy ◽  
Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Tunneling Microscopy ◽  
Spatially Resolved ◽  
Local Electronic Structure

The recent application of tunneling probes in electronic structure studies of carbon nanotubes has proven both powerful and challenging. Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), local electronic properties in ordered aggregates of carbon nanotubes (multiwalled nanotubes and ropes of single walled nanotubes) have been probed. In this report, we present evidence for interlayer (concentric tube) interactions in multiwalled tubes and tube-tube interactions in singlewalled nanotube ropes. The spatially resolved, local electronic structure, as determined by the local density of electronic states, is shown to clearly reflect tube-tube interactions in both of these aggregate forms.

scholarly journals Theory of correlated insulating behaviour and spin-triplet superconductivity in twisted double bilayer graphene

2019 ◽  
Vol 10 (1) ◽  
Author(s):  
Jong Yeon Lee ◽  
Eslam Khalaf ◽  
Shang Liu ◽  
Xiaomeng Liu ◽  
Zeyu Hao ◽  
...  
Keyword(s):  
Displacement Field ◽  
Twist Angle ◽  
Vertical Displacement ◽  
Bilayer Graphene ◽  
Magic Angle ◽  
Hartree Fock ◽  
Flat Bands ◽  
Half Filling ◽  
Spin Triplet ◽  
Experimental Findings

AbstractTwo graphene monolayers twisted by a small magic angle exhibit nearly flat bands, leading to correlated electronic states. Here we study a related but different system with reduced symmetry - twisted double bilayer graphene (TDBG), consisting of two Bernal stacked bilayer graphenes, twisted with respect to one another. Unlike the monolayer case, we show that isolated flat bands only appear on application of a vertical displacement field. We construct a phase diagram as a function of twist angle and displacement field, incorporating interactions via a Hartree-Fock approximation. At half-filling, ferromagnetic insulators are stabilized with valley Chern number $${C}_{{\rm{v}}}=\pm 2$$Cv=±2. Upon doping, ferromagnetic fluctuations are argued to lead to spin-triplet superconductivity from pairing between opposite valleys. We highlight a novel orbital effect arising from in-plane fields plays an important role in interpreting experiments. Combined with recent experimental findings, our results establish TDBG as a tunable platform to realize rare phases in conventional solids.

PHOTON EMISSION FROM TRANSITION METAL SURFACES IN SCANNING TUNNELING MICROSCOPY

1993 ◽  
Vol 07 (01n03) ◽  
pp. 516-519 ◽  
Author(s):  
RICHARD BERNDT ◽  
JAMES K. GIMZEWSKI
Keyword(s):  
Scanning Tunneling Microscope ◽  
Metal Surfaces ◽  
Light Emission ◽  
Local Density ◽  
Photon Emission ◽  
Scanning Tunneling ◽  
Local Density Of States ◽  
Tunneling Microscopy ◽  
Photon Intensity ◽  
Plasmon Modes

Light emission from noble metal surfaces excited by a scanning tunneling microscope has been interpreted as arising from in elastic tunneling excitation of tip-induced plasmon modes. We have extended this work to study the adsorption of oxygen on Ti and have observed the formation of structures with subnanometer lateral dimensions which give rise to clear contrasts in STM topographs and photon intensity maps. The experimental results strongly indicate that these contrasts are due to oxygen-induced variations of the local density of states.

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