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Here we show that a twisted graphene trilayer made by misoriented stacking of a graphene monolayer on top of a Bernal graphene bilayer can exhibit rich and tailored electronic properties. For the case that the graphene monolayer and bilayer are strongly coupled, both the massless Dirac fermions and massive chiral fermions coexist in the twisted trilayer, leading to unique in-plane chiral tunneling. For a weak coupling between the two graphene systems, the distinct chiralities and pseudospin textures of quasiparticles in monolayer and bilayer enable vertical valley-polarized quantum tunneling between them. Intriguingly, the polarity of the valley polarization can be inverted simply by either controlling the rotational angles between the two systems or tuning the Fermi levels of the two systems. Our result implies that layered van der Waals structures assembled from individual atomic planes can create materials that harbor unusual properties and alternative functionalities depending on the stacking configuration of the crystalline layers. [ABSTRACT FROM AUTHOR]

A pseudomagnetic field becomes an experimental reality after the observation of zero-field Landau level-like quantization in strained graphene, but it is not expected that the time-reversal symmetric pseudomagnetic fields will have any effect on the spin degree of freedom of the charge carriers. Here, we demonstrate that spin-orbit coupling (SOC) could act as a bridge between the pseudomagnetic field and spin. In quantum spin Hall (QSH) states, the direction of the spin of edge states is tied to their direction of motion because of the SOC. The pseudomagnetic field affects the clockwise and counterclockwise edge currents of the QSH states and consequently lifts the degenerate edge states of opposite spin orientation. Because of opposite signs of the pseudomagnetic field in two valleys of graphene, the one-dimensional charge carriers at the two opposite edges have different group velocities, and in some special cases, the edge states can only propagate at one edge of the nanoribbon, and the group velocity at the other edge becomes zero. [ABSTRACT FROM AUTHOR]

We consider the electronic structure of a slightly twisted graphene bilayer and show the coexistence of van Hove singularities (VHSs) and superlattice Dirac points in a continuum approximation. The graphene-on-graphene moiré pattern gives rise to a periodic electronic potential, which leads to the emergence of the superlattice Dirac points due to the chiral nature of the charge carriers. Because of the distinguishing real and reciprocal structures, the sublattice exchange even and sublattice exchange odd structures of the twisted graphene bilayer result in two different structures of the superlattice Dirac points. We calculate the effect of a strain on the low-energy electronic structure of the twisted graphene bilayer and demonstrate that the strain affects the position of the VHSs dramatically. [ABSTRACT FROM AUTHOR]

Recent studies show that the electronic structures of graphene can be modified by strain, and it was predicted that strain in graphene can induce peaks in the local density of states (LDOS), mimicking Landau levels (LLs) generated in the presence of a large magnetic field. Here we report the scanning tunneling spectroscopy (STS) observation of nine strain-induced peaks in LDOS at 77 K along a graphene ridge created when the graphene layer was cleaved from a sample of highly oriented pyrolytic graphite (HOPG). The energies of these peaks follow the progression of LLs of massless "Dirac fermions" (DFs) in a magnetic field of 230 T The results presented here suggest a possible route to realize zero-field quantum Hall-like effects at 77 K. [ABSTRACT FROM AUTHOR]

In strained graphene, lattice deformation can create pseudomagnetic fields affecting the behavior of massless Dirac fermions and result in zero-field Landau level-like quantization. In the presence of an external magnetic field, valley-polarized Landau levels are predicted to be observed because the strain-induced pseudomagnetic fields are of opposite directions in the K and K' valleys of graphene. However, an experimental verification of such a unique valley-polarized Landau quantization has not been reported so far. Here, we present experimental spectroscopic measurements in strained graphene on Rh foil by scanning tunneling microscopy. We directly observed the splitting of the Landau level in the quantum Hall regime and we interpreted the experimental result as the valley-polarized Landau level induced by the coexistence of the pseudomagnetic fields and external magnetic fields. The observed result paves the way to exploit novel electronic properties in graphene through the combination of the spatially varying strain (or the pseudomagnetic fields) and the external magnetic fields. [ABSTRACT FROM AUTHOR]

Currently there is a lively discussion concerning Fermi velocity renormalization in twisted bilayers and several contradicted experimental results are reported. Here we study electronic structures of the twisted bilayers by scanning tunneling microscopy (STM) and spectroscopy (STS). The interlayer coupling strengths between the adjacent bilayers are measured according to energy separations of two pronounced low-energy van Hove singularities (VHSs) in the STS spectra. We demonstrate that there is a large range of values for the interlayer interaction in different twisted bilayers. Below the VHSs, the observed Landau quantization in the twisted bilayers is identical to that of massless Dirac fermions in graphene monolayer, which allows us to measure the Fermi velocity directly. Our result indicates that the Fermi velocity of the twisted bilayers depends remarkably on both the twisted angles and the interlayer coupling strengths. This removes the discrepancy about the Fermi velocity renormalization in the twisted bilayers and provides a consistent interpretation of all current data. [ABSTRACT FROM AUTHOR]

The electron-hole symmetry in graphene monolayer, which is analogous to the inherent symmetric structure between electrons and positrons of the Universe, plays a crucial role in the chirality and chiral tunneling of massless Dirac fermions. Here we demonstrate that both strain and charged-defect scattering could break this symmetry dramatically in a graphene monolayer. In our experiment, the Fermi velocities of electrons veF and holes vhF are measured directly through Landau level spectroscopy. In strained graphene with lattice deformation and curvature, the veF and vhF are measured as (1.21 ± 0.03) × 106 m/s and (1.02 ± 0.03) × 106 m/s, respectively. This giant asymmetry originates from enhanced next-nearest-neighbor hopping in the strained region. Around positively charged defect, we observe opposite electron-hole asymmetry, and the veF and vhF are measured to be (0.86 ± 0.02) × 106 m/s and (1.14 ± 0.03) × 106 m/s, respectively. Such a large asymmetry is attributed to the fact that the massless Dirac fermions in a graphene monolayer are scattered more strongly when they are attracted to the charged defect than when they are repelled from it. [ABSTRACT FROM AUTHOR]

The interfacial atomic arrangement, which is different from that in the bulk form of the heterojunction, can induce a reconstruction of electrostatic field at the interface. For conventional semiconductor heterointerfaces, it is known that such reconstruction results in band bending, creating a quantum well in which the two-dimensional electron gas (2DEG) is formed. In this article, we show that this mechanism still works in a multivalent oxide heterojunction: for (110) LaA1O3/SrTiO3 (LAO/STO) heterojunctions, the coexistence of La and Ti in ABO3 perovskite unit cells at the interface reduces the valence of Ti, generating a local field leading to band bending in the interfacial STO layers. The extra free electrons are trapped in this bent conduction band forming a 2DEG. It unifies two independent mechanisms for 2DEG at LAO/STO interfaces, the "polar catastrophe" model and the "La1-xSrxTiO3" layers model, and is expected to end the decade-old controversy. This study opens insight into atomic-scale band engineering to control the behavior of complex oxide heterojunctions. [ABSTRACT FROM AUTHOR]

Non-Abelian gauge potentials are quite relevant in subatomic physics, but they are relatively rare in a condensed matter context. Here we report the experimental evidence for non-Abelian gauge potentials in twisted graphene bilayers by scanning tunneling microscopy and spectroscopy. At a magic twisted angle, θ ≈ (1.11 ± 0.05)°, a pronounced sharp peak, which arises from the nondispersive flat bands at the charge neutrality point, is observed in the tunneling density of states due to the action of the non-Abelian gauge fields. Moreover, we observe confined electronic states in the twisted bilayer, as manifested by regularly spaced tunneling peaks with energy spacing δE ≈ νF/D ≈ 70 meV (here νF is the Fermi velocity of graphene and D is the period of the moir'e patterns). This indicates that the non-Abelian gauge potentials in twisted graphene bilayers confine low-energy electrons into a triangular array of quantum dots following the modulation of the moiré patterns. Our results also directly demonstrate that the Fermi velocity in twisted bilayers can be tuned from about 106 m/s to zero by simply reducing the twisted angle of about 2°. [ABSTRACT FROM AUTHOR]

Electronic properties of surface areas decoupled from graphite are studied using scanning tunneling microscopy and spectroscopy. We show that it is possible to identify the decoupled graphene monolayer, the Bernal bilayer, and the Bernal trilayer on a graphite substrate according to their tunneling spectra in a high magnetic field. The decoupled monolayer and bilayer exhibit Landau quantization of massless and massive Dirac fermions, respectively. The substrate generates a sizable band gap ~35 meV in the Bernal bilayer, therefore, the eightfold degenerate Landau level at the charge neutrality point is split into two valley-polarized quartets polarized on each layer. In the decoupled Bernal trilayer, we find that both massless and massive Dirac fermions coexist and its low-energy band structure can be described quite well by taking into account only the nearest-neighbor intra- and interlayer hopping parameters. A strong correlation between the Fermi velocity of the massless Dirac fermions and the effective mass of the massive Dirac fermions is observed in the graphene trilayer. Our result demonstrates that the surface of graphite provides a natural ideal platform to probe the electronic spectra of graphene layers. [ABSTRACT FROM AUTHOR]

Recently, rhombohedral trilayer graphene (r-TLG) has attracted much attention because of its low-energy flat bands, which are predicted to result in many strongly correlated phenomena. However, there has been a need for more experimental evidence for these flat bands in the r-TLG, since the supporting substrates usually have strong destructive effects on the low-energy band structure of graphene systems. Here, we demonstrate that it is possible to directly probe the stacking order and electronic spectrum of the r-TLG on a graphite surface with scanning tunneling microscopy around a monoatomic step edge of the top graphene layer. The tunneling spectra of the r-TLG exhibit four adjacent peaks, which are generated by the low-energy flat bands, flanking the charge neutrality point. Based on these spectra, the true energy gap and the energy gap at the K point of the r-TLG are determined as about 9 and 23 meV, respectively. The observed features are well reproduced by a low-energy effective Hamiltonian. [ABSTRACT FROM AUTHOR]

Very recently, it was demonstrated that the carrier mobility of a molecular monolayer dioctylbenzothienobenzothiophene (C8-BTBT) on boron nitride can reach 10cm2/Vs, the highest among the previously reported monolayer molecular field-effect transistors. Here we show that the high-quality single crystal of the C8-BTBT monolayer may be the key origin of the record-high carrier mobility. We discover that the C8-BTBT molecules prefer layer-by-layer growth on both hexagonal boron nitride and graphene. The flatness of these substrates substantially decreases the C8-BTBT nucleation density and enables repeatable growth of large-area single crystal of the C8-BTBT monolayer. Our experimental result indicates that only out-of-plane roughness greater than 0.6 nm of the substrates could induce disturbance in the crystal growth and consequently affect the charge transport. This information would be important in guiding the growth of high-quality epitaxy molecular film. [ABSTRACT FROM AUTHOR]

Carrier density (ns) is a crucial parameter that governs the properties of correlated oxides, so the field-effect approach is an ideal tool to investigate the novel physics of the system. Here, the carrier-mediated transport of slightly doped anatase TiO2 epitaxial films were studied by electric double layer transistor (EDLT) gating. The ns has been increased hugely, and concomitantly, the channels of anatase TiO2 films undergo an insulator-metal transition with a decrease in resistivity by almost three orders of magnitude. More fascinating, the Kondo effect depends very strongly on ns, and the Hall mobility could be enhanced about one order of magnitude with increasing ns. This study shows that EDLT gating is a powerful method to stimulate and mediate novel phenomena of correlated oxides. [ABSTRACT FROM AUTHOR]

The creation of van der Waals heterostructures based on a graphene monolayer and other two-dimensional crystals has attracted great interest because the atomic registry of the two-dimensional crystals can modify the electronic spectra and properties of graphene. A twisted graphene bilayer can be viewed as a special van der Waals structure composed of two mutually misoriented graphene layers, where the sublayer graphene not only plays the role of a substrate, but also acts in an equivalent role as the top graphene layer in the structure. Here we report the electronic spectra of slightly twisted graphene bilayers studied by scanning tunneling microscopy and spectroscopy. Our experiment demonstrates that twist-induced van Hove singularities are ubiquitously present for rotation angles θ of less than about 3.5∘ , corresponding to moiré -pattern periods D longer than 4 nm. However, they totally vanish for θ>5.5 ∘ (D < 2.5 nm). Such a behavior indicates that the continuum models, which capture moiré-pattern periodicity more accurately at small rotation angles, are no longer applicable at large rotation angles. [ABSTRACT FROM AUTHOR]

Despite the fact that structures and properties of tilt grain boundaries of graphite surface and graphene have been extensively studied, their effect on the structures and electronic spectra of graphene layers has not been fully addressed. Here we study effects of one-dimensional tilt grain boundaries on structures and electronic spectra of graphene multilayers by scanning tunneling microscopy and spectroscopy. A tilt grain boundary of a top graphene sheet in graphene multilayers leads to a twist between consecutive layers and generates superstructures (Moiré patterns) on one side of the boundary. Our results demonstrate that the twisting changes the electronic spectra of Bernal graphene bilayer and graphene trilayers dramatically. We also study quantum-confined twisted graphene bilayer generated between two adjacent tilt grain boundaries and find that the band structure of such a system is still valid even when the number of superstructures is reduced to two in one direction. It implies that the electronic structure of this system is driven by the physics of a single Moiré spot. [ABSTRACT FROM AUTHOR]

It is well established that some kinds of lattice deformations in graphene monolayer, which change electron hopping in the sublattice and affect in-plane motion of electrons, may induce out-of-plane pseudomagnetic fields as large as 100 T. Here, we demonstrate that stacking misorientation in graphene bilayers mimics the effect of huge in-plane pseudomagnetic fields greater than 1000 T on the interlayer hopping of electrons. As well as addressing the similarity between the effect of in-plane pseudomagnetic fields and the twisting on the electronic band structure of the Bernal graphene bilayer, we point out that the in-plane magnetic fields (or twisting) could modify the low-energy pseudospin texture of the graphene bilayer (the pseudospin winding number is reduced from 2 to 1), thereby changing the chiralities of quasiparticles from those of spin 1 to those of spin 1/2. Our results illustrate the possibility of controllably manipulating electronic properties of Bernal graphene bilayer by introducing the in-plane magnetic field or twisting. [ABSTRACT FROM AUTHOR]

Theoretical research has predicted that a ripple of graphene generates an effective gauge field on its low-energy electronic structure and could lead to Landau quantization. Here, we demonstrate using a combination of an experimental method (scanning tunneling microscopy) and a theoretical approach (tight-binding approximation) that Landau levels will form when the effective pseudomagnetic flux per ripple Φ~(h²/la)Φ0 is larger than the flux quantum Φ0 (here, h is the height, l is the width of the ripple, and a is the nearest C-C bond length). The strain-induced gauge field in the ripple only results in one-dimensional (1D) Landau-level quantization along the ripple. Such 1D Landau quantization does not exist in two-dimensional systems in an external magnetic field. Its existence offers a unique opportunity to realize interesting electronic properties in strained graphene. [ABSTRACT FROM AUTHOR]

Recent studies show that periodic potentials can generate superlattice Dirac points at energies ±ħvF G 2 in graphene (where ħ is the reduced Planck constant, vp is the Fermi velocity of graphene, and G is the reciprocal superlattice vector). Here, we perform scanning tunneling microscopy and spectroscopy studies of a corrugated graphene monolayer on Rh foil. We show that the quasiperiodic ripples of nanometer wavelength in the corrugated graphene give rise to weak one-dimensional electronic potentials and thereby lead to the emergence of the superlattice Dirac points. The position of the superlattice Dirac point is space dependent and shows a wide distribution of values. We demonstrate that the space-dependent superlattice Dirac points are closely related to the space-dependent Fermi velocity, which may arise from the effect of the local strain and the strong electron-electron interaction in the corrugated graphene. [ABSTRACT FROM AUTHOR]

The electronic properties of twisted bilayer graphene on SiC substrate were studied via a combination of transport measurements and scanning tunneling microscopy. We report the observation of enhanced intervalley scattering from one Dirac cone to the other, which contributes to weak localization of the twisted bilayer graphene by increasing the interlayer coupling strength. Our experiment and analysis demonstrate that the enhanced intervalley scattering is closely related to the periodic AB stacked atoms (the A atom of layer 1 and the B atom of layer 2 that have the same horizontal positions) that break the sublattice degeneracy of graphene locally. We further show that these periodic AB stacked atoms affect intervalley but not intravalley scattering. The result reported here provides an effective way to atomically manipulate the intervalley scattering of graphene. [ABSTRACT FROM AUTHOR]

Using cryogenic scanning tunneling microscopy and scanning tunneling spectroscopy+ single-electron tunneling of Au nanoparticles (NPs) with 1.4 nm in radius was studied. We observe a gap ΔV ~ 2e/C (C is the capacitance of the Au particle and e the charge of an electron) around zero bias in the tunneling-conductancc spectrum, followed by a series of discrete single-electron tunneling peaks with peak spacing of EC ~ e/C at both negative and positive bias. Experimental data are well explained by taking into account both the shift o1 conductance peaks and the effect of exchange interaction of electrons on the single-electron tunneling of Au NPs. A conductance peak near zero bias was suppressed by the exchange-type zero-bias anomaly, which results in the gap Δ V ~ 2EC. According to our experiment and analysis, the shift of conductance peaks of Au NPs is attributed to self-biased gate and/or tip-induced gate. [ABSTRACT FROM AUTHOR]

The wave function of Dirac fermions is a two-component spinor. In graphene, a one-atom-thick film showing two-dimensional Dirac-like electronic excitations, the two-component representation, reflects the amplitude of the electron wave function on the A and B sublattices. This unique property provides unprecedented opportunities to image the two components of Dirac fermions spatially. Here, we report atomic resolution imaging of two-component Dirac-Landau levels in gapped graphene monolayers by scanning tunneling microscopy and spectroscopy. A gap of about 20 meV, driven by inversion symmetry breaking by the substrate potential, is observed in the graphene sheets on both SiC and graphite substrates. Such a gap splits the n=0 Landau level (LL) into two levels, 0+ and 0-. We demonstrate that the amplitude of the wave function of the 0+ LL is mainly on the A sites and that of the 0- LL is mainly on the B sites of graphene, characterizing the internal structure of the spinor of the n=0 LL. This provides direct evidence of the two-component nature of Dirac fermions. [ABSTRACT FROM AUTHOR]