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In two-dimensional transitional metal dichalcogenides, tuning the spin-valley-layer coupling via changing layer numbers and stacking orders remains desirable for their application in valleytronics. Herein, six-point star-like MoSe2 nanoflakes simultaneously containing different atom registration regions from monolayer to bilayer with 2H and 3R stacking order were fabricated, and the valley polarizations were comparably investigated by circular polarized photoluminescent spectroscopy. The degree of valley polarization was detected to be about 12.5% in the monolayer and 10% in the 2H bilayer, but greatly upgraded to about 40% in the 3R bilayer MoSe2. This enhancement was attributed to the multiband spin splitting and generation of spin-dependent layer polarization for the 3R MoSe2 bilayer, which is well evidenced by our ab initio calculations of the energy band structures. Our results demonstrate that preparing TMD crystals with controllable stacking orders and interlayer coupling is a promising route to tune the valley index in TMDs for developing valleytronics technology.

The origin of the quantum superconductor to metal transition at zero temperature in two-dimensional superconductors is still an open problem, which has caused intensely discussion. Here, we report the observation of a quantum superconductor-to-metal transition in LaAlO3/KTaO3 (111) interface, driven by magnetic field. When a small magnetic field perpendicular to the film plane is applied, the residual saturated resistance is observed, indicating the emergence of an anomalous metallic state associated with a failed superconductor. The dependence of saturated resistance on magnetic field at low temperature indicates that the observed metal state is a Bose metal state. From our findings, magnetic field regulating LaAlO3/KTaO3 (111) interface emerges as a platform to scrutinize the details of the anomalous metallic state in a controllable way.

Flexoelectricity has been proven to be an effective way in tailoring a material’s properties. Here, we report the flexoelectricity-induced transport modulation to the two-dimensional electron gas a...

The incorporation of two-dimensional MoS2 nanostructures with other semiconductors used in photoelectrocatalytic (PEC) water-splitting for hydrogen (H2) production has been a very promising and eco...

The planar Hall effect (PHE) has remarkable advantages for applications to spintronic devices and the study of intrinsic magnetic properties because of its high signal-to-noise ratio and low thermal drift. Here, the magnetic field controllable PHE in ${\mathrm{Sr}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}/\mathrm{SrTi}{\mathrm{O}}_{3}$ (001) films is presented. The PHE transforms from a ``sinusoidal-shaped'' twofold symmetry to fourfold oscillations with decrease in the magnetic field at 35 K. Furthermore, the PHE is approximately two to three orders of magnitude larger than the giant PHE associated with ferromagnetic semiconductors as the temperature decreases. Based on subsequent measurements of the magnetic transport properties and calculations of the angle between the magnetization and current, the unexpected fourfold-symmetric PHE can be attributed to magnetocrystalline anisotropy and spin-orbital coupling. These observations of PHE facilitate a deeper understanding of the magnetic interactions in ${\mathrm{Sr}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ films and indicate potential applications in spintronic or magnetic-sensor devices.

The domain morphology in the growth of transition-metal dichalcogenides (TMDCs) is mostly triangular but rarely dendritic. Here, we report a robust chemical vapor deposition method to fabricate atomic-thin 2H-phase MoS2 dendrites on several single-crystalline substrates with different lattice structures, such as rutile-TiO2(001), SrTiO3(001), and sapphire(0001). It is found that by tuning the concentration of Mo adatoms, the morphology of MoS2 domains on these substrates evolves from tridentate dendrites at a low Mo concentration to semicompact fractal domains at an intermediate Mo concentration, and to a compact triangular shape at a high Mo concentration. First-principles calculations reveal that the edge diffusion barrier of Mo is comparable to the attachment barrier, inhibiting fast Mo atom diffusion along the edge. Kinetics Monte Carlo simulations with varying Mo concentrations well reproduce the experimental results. Our combined experimental and theoretical analyses evidently show that the growth of MoS2 dendritic domains at a low Mo concentration is a nonequilibrium process, which is dominated by the kinetics of Mo adatoms. Our study presents an effective route to control the morphology of TMDCs by simply tuning the transition-metal adatom concentration.

High mobility quasi two-dimensional electron gas (2DEG) found at the CaZrO3/SrTiO3 nonpolar heterointerface is attractive and provides a platform for the development of functional devices and nanoelectronics. Here we report that the carrier density and mobility at low temperature can be tuned by gate voltage at the CaZrO3/SrTiO3 interface. Furthermore, the magnitude of Rashba spin–orbit interaction can be modulated and increases with the gate voltage. Remarkably, the diffusion constant and the spin–orbit relaxation time can be strongly tuned by gate voltage. The diffusion constant increases by a factor of ∼ 19.98 and the relaxation time is reduced by a factor of over three orders of magnitude while the gate voltage is swept from –50 V to 100 V. These findings not only lay a foundation for further understanding the underlying mechanism of Rashba spin–orbit coupling, but also have great significance in developing various oxide functional devices.

Wafer-scale growth of the unidirectional graphene monolayer on Ge surfaces has rejuvenated the intense study of the surfaces and interfaces of semiconductors underneath graphene. Recently, it was reported that the Ge atoms in the Ge(110) surface beneath a graphene monolayer underwent a rearrangement and formed an ordered (6 × 2) reconstruction. However, a plausible atomic model related to this (6 × 2) reconstruction is still lacking. Here, by using scanning tunnelling microscopy/spectroscopy (STM/S) and density functional theory (DFT) calculations, we deeply investigated the structural and electronic properties of the Ge(110) (6 × 2) surface encapsulated by a graphene monolayer. The (6 × 2) surface reconstruction was confirmed for the post-annealing-graphene-covered Ge(110) surface via STM, and was found to be quite air-stable, owing to the protection of the graphene monolayer against surface oxidation. Our study disclosed that the topographic features of the topmost graphene monolayer and the Ge(110) surface could be selectively imaged by utilizing suitable scanning biases. According to the STM results and DFT calculations, a rational ball-and-stick model of the (6 × 2) reconstruction was successfully provided, in which an elemental building block comprising two Ge triangles and two isolated Ge atoms adsorbed on the unreconstructed ideal Ge(110) surface. Local density of states of the graphene/Ge surface was explored via scanning tunneling spectroscopy (STS), presenting four well-defined differential conductance (dI/dV) peaks, protruding at energies of 0.2, 0.4, 0.6 and 0.8 eV, respectively. The four peaks predominantly originated from the surface states of the reconstructing adatoms and were well reproduced by our theoretical simulation. This result means that the Ge surface is very robust after being encapsulated by the epitaxial graphene, which could be advantageous for directly fabricating graphene/Ge-hybrid high-speed electronics and optoelectronics based on conventional microelectronics technology.

The grain boundaries (GBs) of a graphene surface were extensively studied because GBs with specific defect configurations result in the formation of new curved structures, which can be treated as new carbon allotropes. We studied the structures and electronic spectra of two periodic GBs in graphene on highly oriented pyrolytic graphite (HOPG) surfaces using scanning tunneling microscopy and spectroscopy (STM/S). Our results demonstrated that a GB consisting of dual parallel periodic dislocation cores of pentagonal-heptagonal (5-7) carbon rings gives rise to an enhanced localized state at 0.45 eV above the Dirac point in graphene surfaces, which is attributed to van Hove singularities (VHSs). Moreover, the energy positions of the localized states are varied between 0.40 and 0.47 eV depending on the site and eventually decayed to 0.36 eV. The variation of the energy positions is induced by two parallel GBs because of the higher electron density at the GB as a defect center and the lower electron away from the GB. Another periodic GB with a symmetric zigzag structural feature induced VHSs at -0.038 and 0.12 eV near the Fermi level. Moreover, intervalley scattering was observed on both these GBs. This means that carrier concentration and thus, the conductance around the periodic GBs can be significantly enhanced by localized density of states. This finding suggests that graphene with a properly embedded ordered GB is promising for improving the performance of graphene-based electronic devices., 21 pages, 4 figures

Experimentally, we found the percentage of low valence cations, the ionization energy of cations in film, and the band gap of substrates to be decisive for the formation of two-dimensional electron gas at the interface of amorphous/crystalline oxide (a-2DEG). Considering these findings, we inferred that the charge transfer from the film to the interface should be the main mechanism of a-2DEG formation. This charge transfer is induced by oxygen defects in film and can be eliminated by the electron-absorbing process of cations in the film. Based on this, we propose a simple dipole model that successfully explains the origin of a-2DEG, our experimental findings, and some important properties of a-2DEG.

Electron-doped $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ has been the subject of much recent attention because of its similarities with hole-doped $\mathrm{L}{\mathrm{a}}_{2}\mathrm{Cu}{\mathrm{O}}_{4}$. However, direct evidence of its superconductivity has not yet been observed, which has led to various unresolved issues related to this material. In this study, a 14-nm-thick La-doped $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ film was grown on a (001) $\mathrm{SrTi}{\mathrm{O}}_{3}$ substrate. We found that the fourfold component of the in-plane anisotropic magnetoresistance disappears and is replaced by a twofold symmetry as the temperature increases. Variable-temperature x-ray diffraction and spatially resolved optical second-harmonic generation experiments eliminate the possibility of crystal structural distortion. The temperature dependence of direct current magnetization measurements indicates that a change in the spin structure may account for the symmetry breaking, similar to the nematic ordering of iron-based and other superconductors. Remarkably, the transition temperature of the symmetry breaking is well consistent with the previously reported onset (50 K) of the low-energy gap in K-doped $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ observed by scanning tunneling spectroscopy. Our results provide a perspective on these materials that helps elucidate the nature of magnetic interactions and explores the expected superconductivity of iridates.

The two-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 (LAO/STO) interface exhibits many emergent properties, such as high mobility, giant photoconductivity, the light-enhanced gating effect, etc., which are invaluable for basic research and potential applications. In this work, we report an unusual enhancement of persistent photoconductivity (PPC) by electrostatic gating (EG) at the LAO/STO interface. Under the influence of pre-EG processing, the 2DEG at the LAO/STO interface is triggered into a resistance state showing a more pronounced optical response and greatly enhanced PPC under light illumination (LI) at room temperature. These observations are found to be attributed to EG-induced interfacial polarization and the subsequent LI-accelerated depolarization at the interface. Based on these findings, a nonvolatile memory device made by this interface is proposed. Our work offers a new perspective for tuning the photoelectrical properties at the oxide interface, which is helpful for designing advanced photoelectric devices with high performance and multifunctionality.

In this paper, the conductivity of La-doped Sr2IrO4 thin films is enhanced by covering amorphous LaAlO3. Electrons of the order of 1020 cm−3 can be transferred into La-doped Sr2IrO4 thin film from the top amorphous LaAlO3 layer. This value is close to that of the largest polarization induced by the field effect. The conductivity of La-doped Sr2IrO4 thin film with a fixed thickness will be enhanced as the thickness of the amorphous LaAlO3 film increases until which reaches a critical value. For a certain thickness of amorphous LaAlO3, the thinner the La-doped Sr2IrO4 film, the more significant the resistance reduction. Furthermore, the conductivity and carrier density can be controlled by varying the growth oxygen pressure of amorphous LaAlO3 layer. This work provides a permanent and controllable method to enhance the conductivity of La-doped Sr2IrO4 film without changing the conduction mechanism and magnetic properties and facilitates subsequent research on the intrinsic properties of La-doped Sr2IrO4.

Weyl semimetal WTe2 has attracted considerable attention owing to its extremely large, unsaturated and quadratic magnetoresistance. Here, we study the magnetotransport properties of WTe2-δ thin film, which shows an unsaturated and linear magnetoresistance of up to ∼1650%. A more complex and accurate method, known as the maximum entropy mobility spectrum, is used to analyze the mobility and density of carriers. The results show that linear magnetoresistance can be explained by the classical disorder model because the slope of linear magnetoresistance and the crossover field are proportional to the mobility and inverse mobility, respectively. Furthermore, the validity of the maximum entropy mobility spectrum is validated by the Shubnikov-de Haas oscillations. Moreover, at low temperature, we determined that the unsaturated and near-quadratic magnetoresistance in the WTe1.93 thin film can be explained by charge compensation. Note that the electron-hole compensation is broken in the WTe1.42 thin film, which indicates that the carrier scattering induced by the disorder may suppress the charge compensation in the WTe2 sample with defects/dopants. To summarize, the discovery of disorder-induced linear magnetoresistance allows us to explain different magnetoresistance behaviors of WTe2.

Electrical transport property is closely related to the dimensionality of carriers' distribution. In this work, we succeed in tuning the carriers' distribution and the Rashba spin-orbit coupling at LaAlO3/SrTiO3 interface by varying the oxygen pressure (c-P O2) adopted in crystalline LaAlO3 growth. Measurements of the in-plane anisotropic magnetoresistance and the conducting-layer thickness indicate that the carriers' distribution changes from three to two dimensions with c-P O2 increasing, i.e. the electron confinement gets stronger. Importantly, by measuring the low-temperature out-of-plane magnetoresistance and analyzing the weak localization/weak anti-localization, we find that the strength of Rashba spin-orbit coupling can be enhanced by electron confinement. The electron confinement is a manifestation of breaking of spatial inversion symmetry. Therefore, our work reveals the intimate relationship between spatial inversion symmetry breaking and Rashba spin-orbit coupling at the LaAlO3/SrTiO3 interface, and provides a new method to tune the Rashba spin-orbit coupling, which is valuable in the application of oxide-spintronics.

The 2D electron systems of SrTiO3/NdGaO3 (STO/NGO) and amorphous-LaAlO3/SrTiO3/NdGaO3 (a-LAO/STO/NGO) heterojunctions were explored. An obvious interaction between in-gap states (IGSs) and carriers was found. The IGSs can trap a large number of carriers and enhance carrier scattering. As a result of the high density of IGSs in STO, the conductivity of STO/NGO was severely weakened. However, for a-LAO/STO/NGO heterojunctions, the high carrier density can reduce the effect of IGSs through the electrostatic screening effect. The competition between IGSs and the screening effect of carriers results in an insulator-metal transition and a strange temperature dependence of carrier density. We also explored the interaction between IGSs and carriers theoretically. A mathematical description was proposed and the calculated results showed good agreement with experimental findings.

The out-of-plane magnetoresistance of a $\mathrm{LaAl}{\mathrm{O}}_{3}$/$\mathrm{SrTi}{\mathrm{O}}_{3}$ interface is commonly positive due to significant orbital effect. In this paper, a large negative magnetoresistance is observed at a $\mathrm{LaAl}{\mathrm{O}}_{3}$ and Fe-doped $\mathrm{SrTi}{\mathrm{O}}_{3}$ interface upon the application of an out-of-plane magnetic field. Such large negative magnetoresistance is attributed to enhanced two-dimensional weak localization and Kondo effect. We think that these two effects are mainly the consequences of intentional Fe dopants in the substrate, which introduce holes, disorders, and spin scattering centers into the electronically conducting interface, directly modifying the spin-orbit coupling, orbital effect, scale of electrons motion, and local magnetic moments at the $\mathrm{LaAl}{\mathrm{O}}_{3}$/$\mathrm{SrTi}{\mathrm{O}}_{3}$ interface. The ${\mathrm{LaAlO}}_{3}\text{/Fe-doped}\phantom{\rule{0.28em}{0ex}}{\mathrm{SrTiO}}_{3}$ interfacial system provides a prospective platform for studying quantum interference between strongly correlated electrons. The substrate doping method may facilitate the application of $\mathrm{LaAl}{\mathrm{O}}_{3}$/$\mathrm{SrTi}{\mathrm{O}}_{3}$ interfaces in versatile electronic devices.

We explored in-gap states (IGSs) in perovskite oxide heterojunction films. We report that IGSs in these films play a crucial role in determining the formation and properties of interfacial two-dimensional electron gas (2DEG). We report that electron trapping by IGSs opposes charge transfer from the film to the interface. The IGS in films yielded insulating interfaces with polar discontinuity and explained low interface carrier density of conducting interfaces. An ion trapping model was proposed to explain the physics of the IGSs and some experimental findings, such as the unexpected formation of 2DEG at the initially insulating LaCrO3/SrTiO3 interface and the influence of substitution layers on 2DEG.

By comparing the in-plane and out-of-plane orbitally dependent magnetoresistance under a magnetic field of 9 T, the conductive layer thickness of the ${\mathrm{LaAlO}}_{3}$/${\mathrm{SrTiO}}_{3}$ interface is determined for temperatures between 45 and 300 K. As the influence of spin-orbit coupling cannot be ignored when considering the conductive layer thickness dependence at temperatures below 45 K, a lower magnetic field is employed. The results show that the thickness increases gradually and then remains constant as the temperature decreases. Such a tendency is well reproduced theoretically by considering the equilibrium between drift and diffusion of electrons near the interface. We discover that the thickness mainly depends on the relative permittivity of ${\mathrm{SrTiO}}_{3}$ and electron mobility, and it is independent of the sheet carrier density. Our investigations show that the dimensionality of the confined zone for electrons can be controlled and can be used as a reference for other interfacial systems in condensed matter physics.

We measure planar Hall effect (PHE) and longitudinal anisotropic magnetoresistance (AMR) with a magnetic field rotating in the a-b plane in the type-II Dirac semimetal PdTe2. The measured PHE and AMR curves can be fitted by the theoretical equations; however, a detailed analysis of the extracted data demonstrates that the parameter related to PHE and AMR has no relationship with the chiral anomaly due to the absence of negative longitudinal magnetoresistance (MR) when the electric and magnetic fields are parallel to each other. Meanwhile, we prove that the origin of PHE in PdTe2 is the anisotropic orbital MR. Our work suggests that negative longitudinal MR is necessary to identify chiral anomaly, and we cannot in general use PHE as a signal for the presence of the chiral anomaly in Dirac/Weyl semimetals.

In this work the interaction between alkali metal ions and a graphene surface with the absence and the presence of an external electric field applied perpendicular to the surface was investigated. M05-2X/6-31G(d) DFT calculations were performed to describe the adsorption properties. Results show that the electric field pushes the positively charged ion closer to the graphene, where the charge transfer between the alkali metal cations and the electron rich graphene surface increases. At a species-dependent certain strength of the electric field, the excess electrons cause negative charge on the alkali metal ion. This effect will promote the removal of the ion from the surface.

The fundamental parameters of the superconducting state such as coherence length and pairing strength are essential for understanding the nature of superconductivity. These parameters can be estimated by measuring critical parameters such as upper critical field, Hc2. In this work, Hc2 of a superconducting (110) LaAlO3/SrTiO3 interface is determined through magnetoresistive measurements as a function of the gate voltage, VG. When VG increases, the critical temperature has a dome-like shape, while Hc2 monotonically decreases. This relationship of independence between the variation of Tc and of Hc2 suggests that the Cooper pairing potential is stronger in the underdoped region and the coherence length increases with the increase of VG. The result is as for high temperature superconducting cuprates and it is different than for conventional low temperature superconductors.

Multiwalled carbon nanotubes (MWCNTs) were etched by EtOH treating. Such treated MWCNTs were deposited from solution phase onto the nanostructured CeO2 films. High resolution images by both transmission and scanning electron microscopy showed that the nanographite fractions are existed in the solution and they were also reorganized on the surface. On the nanostructured surface of specific morphology, the nanographite fractions are deposited into the valleys between the CeO2 islands and therefore a selective pattern of nanographite was formed. Molecular dynamics calculations highlighted that adsorption and reorganization properties of the nanographite layers have considerable dependence on the morphology of CeO2 nanostructures.

Here we reported a simple method to synthesize transition metal oxide nanowires. Copper oxide (CuO), zinc oxide (ZnO), and cobalt oxide (Co3O4) nanowires were synthesized by heating the copper, zinc, and cobalt substrates under atmosphere condition. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to analyze the morphology and microstructure of the nanowires. According to our experimental results, self-catalysis growth mechanism was proposed to explain the growth of the nanowires. The temperature window for the growth of nanowires was estimated by taking into account the Gibbs free energy of reaction. The synthesis approach observed in our experiment could be applied to synthesize other one-dimensional structures, such as FeSe and Bi2Te3 nanowires.

Multiwalled carbon nanotubes (MWCNTs) were deposited from solution phase onto the nanostructured CeO2 films grown on a sapphire substrate. High-resolution images by scanning electron microscopy showed that MWCNTs laid on the top of CeO2 islands. Raman shifts of these combinatorial materials showed considerable dependence on the morphology of the underlying CeO2 nanostructures. Quantum chemical calculations validated the resonant nanotube vibrations, especially for the stationary longitudinal wave at typical surface morphology, as the origin of this unexpected property of the Raman signal. Considering that the CeO2 surface morphology is fully controlled by the experimental setup of growth, properties of the CNT layers are tunable toward the requirements of practical applications.

In this work, we investigate the electrical transport property and electronic structure of oxide heterostructure LaCrO3/SrTiO3 (111). The interface grown under relatively low oxygen partial pressure is found to be metallic with a conducting critical thickness of 11 unit cells of LaCrO3. This criticality is also observed by x-ray photoelectron spectroscopy, in which the Ti3+ signal intensity at the spectrum edge of the Ti-2p3/2 core level increases rapidly when the critical thickness is reached. The variations of the valence band offset and full width at half maximum of the core-level spectrum with LaCrO3 thickness suggest that the built-in fields exist both in LaCrO3 and in SrTiO3. Two possible origins are proposed: the charge transfer from LaCrO3 and the formation of a quantum well in SrTiO3. Our results shed light on the understanding of the doping mechanism at the polar/non-polar oxide interface. Moreover, due to the interesting lattice and spin structure of LCO in the (111) direction, our work provides a basis for further exploring the novel topological quantum phenomena in this system.

According to the aim to compose combinatorial material by adsorption of carbon nanotubes onto the structured CeO 2 surface the interaction of the armchair (5,5) and zigzag (8,0) nanotubes with the (0 0 1) and (1 1 1) surfaces of CeO 2 islands have been investigated by theoretical methods. The thermodynamics of the adsorption were studied at the low surface coverage region. The interaction energy between the nanotube and the different CeO 2 surfaces shows significant increase when the size of the interface reaches 7–8 unit cells of CeO 2 and it remains unchanged in the larger interface region. However, the entropy term of the adsorption is significantly high when the distances of CeO 2 islands are equal to 27 nm (adsorption of armchair (5,5) nanotube) or 32 nm (adsorption of zigzag (8,0) nanotube). This property supports adsorption of nanotubes onto CeO 2 surfaces which possesses a very specific surface morphology. A long-wave vibration of nanotubes was identified as background of this unexpected phenomenon. This observation could be applicable in the development of such procedures where the nanotube adsorption parallel to the surface is aimed to perform.

Microcrack-free thick YBa2Cu3O7-delta (YBCO) films were successfully fabricated by pulsed laser deposition on deliberately miscut AI2O3 (1102) (5.22deg off towards [1102]) buffered with CeO2. Characterization of the films revealed a porous morphology, consisting of interconnected islands and deep holes (pores). The microstructure of the YBCO films was further investigated by cross-section (cut along both AI2O3 [1101] and [1120] transmission electron microscopy (TEM). No apparent interface reaction is seen for all the TEM observations. A high density of linear defects aligned near c-axis are frequently observed. All the defects are initiated from the YBCO / CeO2 interface, indicating the importance of the microstructure of the CeO2 buffer layer and the sapphire substrate for the growth of YBCO. In addition, numerous small stacking faults lying in the alpha - b-plane of YBCO were produced by the linear defects. The high density of the growth-induced defects may act as strong pinning centers in YBCO films and therefore increase Jc of the film.

We use the tip of a scanning tunneling microscope (STM) to manipulate single weakly bound nanometer-sized sheets on a highly oriented pyrolytic graphite (HOPG) surface through artificially increasing the tip and sample interaction by pretreatment of the surface using a liquid thiol molecule. By this means it is possible to tear apart a graphite sheet against a step and fold this part onto the HOPG surface and thus generate graphene superlattices with hexagonal symmetry. The tip and sample surface interactions, including the van der Waals force, electrostatic force and capillary attraction force originating from the Laplace pressure due to the formation of a highly curved fluid meniscus connecting the tip and sample, are discussed quantitatively to understand the formation mechanism of a graphene superlattice induced by the STM tip. The capillary force plays a key role in manipulating the graphite surface sheet under humid conditions. Our approach provides a simple and feasible route to prepare controllable superlattices and graphene nanoribbons and also to better understand the process of generation of a graphene superlattice on the surface of HOPG with the tip.

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 tunnelling of massless Dirac fermions. Here we demonstrate that both strain and charged-defect scattering could break this symmetry dramatically in graphene monolayer. In our experiment, the Fermi velocities of electrons and holes are measured directly through Landau level spectroscopy. In strained graphene with lattice deformation and curvature, the and are measured as 1.2 x 106 m/s and 1.02 x106 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 and are measured to be 0.86x 106 m/s and 1.14 x106 m/s, respectively. Such a large asymmetry is attributed to the fact that the massless Dirac fermions in graphene monolayer are scattered more strongly when they are attracted to the charged-defect than when they are repelled from it., 4 Figures in main text

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., 4 Figures in main text

Recently, the 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. Here, we demonstrate that it is possible to probe the stacking order and electronic spectrum of the r-TLG directly with a 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 meV and 23 meV, respectively. The observed features are well reproduced by a low-energy effective Hamiltonian., 3 Figures

Electronic properties of surface areas decoupled from graphite are studied using scanning tunnelling microscopy and spectroscopy. We show that it is possible to identify decoupled graphene monolayer, Bernal bilayer, and Bernal trilayer on graphite surface according to their tunnelling spectra in 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 trilayer. Our result demonstrates that the surface of graphite provides a natural ideal platform to probe the electronic spectra of graphene layers., Comment: 5 figures

The wavefunction of massless 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 wavefunction on the A and B sublattices. This unique property provides unprecedented opportunities to image the two components of massless Dirac fermions spatially. Here we report atomic resolution imaging of the two-component Dirac-Landau levels in a gapped graphene monolayer by scanning tunnelling microscopy and spectroscopy. A gap of about 20 meV, driven by inversion symmetry breaking by the substrate potential, is observed in the graphene 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 wavefunction of the 0- LL is mainly at the A sites and that of the 0+ LL is mainly at 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 massless Dirac fermions., Comment: 4 Figures in main text and 4 Figures in SI

By growing CeO2 buffer layers on deliberately miscut Al2O3 (102) surfaces, we have successfully fabricated stoichiometric YBa2Cu3O7-δ (YBCO) thick films by pulsed laser deposition. Atomic force microscopy and scanning electron microscopy observations demonstrated that YBCO films with thickness up to t ~ 1.0 µm were microcrack-free. Characterization of the films revealed a porous morphology, consisting of interconnected islands and deep holes (pores). This feature is considered as one of the sources contributing to the strain-relieving mechanism responsible for the increase in film thickness without microcracking. Microcrack-free thick YBCO films revealed Tc = 90.5 ± 0.5 K, Jc(77.3 K, 0 T ) = 2.0-3.0 × 106 A/cm2, and a substantial enhancement of Jc × t (77.3 K, 0 T ) up to 246 A/cm.

CeO2 buffer layers were grown on deliberately miscut Al2O3 ð1 ¯ 102 Þ surfaces by pulsed laser deposition. Atomic force microscopy observations revealed that their morphology was very smooth. High lattice perfection of the CeO2 layer was shown by X-ray diffraction. Thick YBa2Cu3O7� d (YBCO) films have been grown on such vicinal surfaces by laser ablation. The resultant YBCO films were stoichiometric and microcrack-free with a porous morphology (confirmed by scanning electron microscopy), consisting of interconnected islands and deep holes (pores). The porous feature is considered as one of the sources contributing to the strain-relieving mechanism responsible for the increase in film thickness without microcracking. Microcrack-free thick YBCO films revealed T c ¼ 90:5 � 0: 5K , Jcð77: 3K ; 0T Þ¼ 1:5 � 3:0 � 10 6 A=cm 2

Relatively thick YBCO thin films (thickness d>0.5 /spl mu/m) ideal for fault current limiter as well as microwave applications have been successfully prepared by large-area pulsed laser deposition (PLD) on CeO/sub 2/-buffered sapphire substrates. The attainment of an unusually high film thickness (up to 2.0 /spl mu/m) without microcracking is attributed in part to the presence of pores correlated with yttrium-rich composition in the films. The effect of using YBCO targets with varied Y:Ba:Cu ratios on film porosity and surface morphology was investigated in detail. Using the optimum target composition, uniform critical current densities (J/sub c/) ranging from /spl sim/4.5 MA/cm/sup 2/ at d/spl sim/0.23 /spl mu/m to /spl sim/1.75 MA/cm/sup 2/ at d/spl sim/0.6 /spl mu/m have been achieved. Characterization of a film with d/spl sim/0.5 /spl mu/m showed low microwave surface resistance values [R/sub s/(77.3 K)/spl ap/2.09 m/spl Omega/ and R/sub s/(20 K)/spl ap/227 /spl mu//spl Omega/ at 22 GHz] comparable to the best YBCO films reported by other studies.

Stoichiometric YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films were fabricated by pulsed laser deposition on deliberately miscut Al/sub 2/O/sub 3/ (11_02) buffered with CeO/sub 2/. Atomic force microscopy observations demonstrated that 600 nm-thick YBCO films were microcrack-free. Characterization of the films revealed a porous morphology, consisting of interconnected islands and deep holes (pores). This feature is considered as one of the sources contributing to the strain-relieving mechanism responsible for the increase in film thickness without microcracking. We observed T/sub c/=90/spl plusmn/0.5 K, J/sub c/ (77.3 K, 0 T)=1.5-2.0/spl times/10/sup 6/ A/cm/sup 2/, and J/sub c/t (77.3 K, 0 T)/spl sim/120 A/cm. In addition, this new approach, using the deliberately miscut sapphire as a substrate for YBCO, is also applicable to other kinds of deposition techniques.

We evaluated for the first time the thickness dependence of the critical current density (Jc) of micrometre thick YBCO films on CeO2-buffered sapphire. YBCO films were successfully grown in microcrack-free form up to a thickness of by large-area pulsed laser deposition. Jc was found to decrease exponentially with YBCO thickness. Results suggest that the reduction in Jc with film thickness can be attributed to an evolving film microstructure as a function of thickness, as well as a corresponding change in the defect structures responsible for flux pinning. It was observed that film porosity and roughness increased with film thickness due to the growth and encroachment of the BaY2O4 phase. To clarify the flux pinning mechanism, we measured the angular dependence of Jc for films of different thicknesses and correlated this with the defect structure as revealed from the etch pit method and atomic force microscopy (AFM) observations. An unusually prominent Jc peak was observed when , which is due to correlated extended defects parallel to the c-axis of YBCO. Examination of the film microstructure revealed two defect types that give rise to the Jc peak: linear defects in the form of screw and edge dislocations, and planar defects possibly in the form of stacking faults. The density of linear defects decreased with film thickness whereas that of the planar defects increased considerably. From the behaviour of Jc with film thickness, these results suggest that linear defects may be more effective pinning centres than planar defects, or that an overabundance of planar defects may offset the increase of Jc for thick films.

Chinese-Hungarian Intergovernmental ST 973 Program of China [2012CB619301, 2013CB921701]; National Natural Science Foundation of China [51172029, 91121012, 11204254, 61227009, 90921002]; Fundamental Research Funds for the Central Universities of China; [TAMOP-4.2.2.A-11/1/KONV-2012-0065]

Flux pinning properties of pinning centres having correlation along the c-axis in epitaxial YBCO films were investigated by measuring the magnetic-field angle ψ-dependence of the critical current density JC and the E–J-characteristics. YBCO films were prepared by using the pulsed-laser-deposition method on four different substrates at three different target-to-substrate distances D. The ψ-dependence of JC showed large peaks when magnetic field B was applied parallel to the c-axis (), and we observed two types of JC-peak: that is, a broad peak for the films deposited at small D (50–60 mm), and a narrow peak for the films deposited at large D (112 and 142 mm). The E–J-characteristics followed the power law, , and the ψ-dependence of the n-value also showed broad peaks around for the films deposited at D = 50–60 mm, and narrow peaks for the film deposited at D = 112 mm. Based on these results and our previous microstructural observations by AFM and TEM, we confirm that the broad-angle flux pinning effect around may be attributed to a high density of elongated precipitates, and the narrow-angle pinning effect around may be attributed to dense planar defects parallel to the c-axis.

Flux pinning centres having correlation along the c-axis in epitaxial YBCO films were investigated by measuring the magnetic-field angle ? dependence of the critical current density JC and by observing the microstructure. First, c-axis oriented YBCO films were prepared on four different substrates at three different target-to-substrate distances D by using a pulsed-laser-deposition method. Films deposited on LaAlO3 (100) and CeO2/Al2O3 with large D (112 or 142 mm) showed narrow peaks in the JC versus ? curves when the magnetic field was applied parallel to the c-axis (B//c). In these films, a high density (up to 5.5 ?m?1) of planar defects, most probably stacking faults parallel to the c-axis, were evident. The ratio of JC (B//c) to JC (B//a?b) significantly increased as the density of the planar defects increased. In contrast, films deposited on SrTiO3 (100), LaAlO3 (100) and NdGaO3 (110) with small D (50?60 mm) showed high, broad JC peaks around B//c, and had a high density (up to 430 ?m?2) of nanometre-sized precipitates that were elongated along the c-axis. The ratio of JC (B//c) to JC (B//a?b) increased as the density of these small elongated precipitates increased. In conclusion, planar defects such as stacking faults parallel to the c-axis, and the small elongated precipitates are effective as c-axis correlated flux pinning centres.

By very short time rf sputtering in certain deposition conditions we have grown three-dimensional Ag nanodots on the substrate prior to the growth of Tl-based superconducting films. These nanodots create pinning centers, leading to an increase in the critical current density about 10 times. From the frequency dependence of the critical current density we estimated also the activation energy for the flux jumps, which resulted to be several times higher. The rate of the thermally-activated flux jumps decreases several orders of magnitude. We suggest that our method can be used for the reduction of thermal noise in high-T/sub c/ dc SQUID's.

Carrier density $({n}_{\mathrm{s}})$ 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 ${\mathrm{TiO}}_{2}$ epitaxial films were studied by electric double layer transistor (EDLT) gating. The ${n}_{\mathrm{s}}$ has been increased hugely, and concomitantly, the channels of anatase ${\mathrm{TiO}}_{2}$ 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 ${n}_{\mathrm{s}}$, and the Hall mobility could be enhanced about one order of magnitude with increasing ${n}_{\mathrm{s}}$. This study shows that EDLT gating is a powerful method to stimulate and mediate novel phenomena of correlated oxides.