arXiv subjects

Lin He

Publications and source records attributed to Lin He.

At least 109 records · Page 6Linked to original sources

arXiv2018

Two-dimensional spinodal interface in one-step grown graphene-molybdenum carbide heterostructures

Heterostructures made by stacking different materials on top of each other are expected to exhibit unusual properties and new phenomena. Interface of the heterostructures plays a vital role in determining their properties. Here, we report the observation of a two-dimensional (2D) spinodal interface in graphene-molybdenum carbide (α-Mo2C) heterostructures, which arises from spinodal decomposition occurring at the heterointerface, by using scanning tunneling microscopy. Our experiment demonstrates that the 2D spinodal interface modulates graphene into whispering gallery resonant networks filled with quasi-bound states of massless Dirac fermions. Moreover, below the superconducting transition temperature of the underlying α-Mo2C, the 2D spinodal interface behaves as disorders, resulting in the breakdown of the proximity-induced superconductivity in graphene. Our result sheds new light on tuning properties of heterostructures based on interface engineering.

cond-mat.mtrl-sci↗

arXiv2018

High-resolution Tunneling Spectroscopy of ABA-stacked Trilayer Graphene

ABA-stacked trilayer graphene (TLG), the simplest system consisting of both massless and massive Dirac fermions, is expected to exhibit many interesting broken-symmetry quantum Hall states and interaction-induced phenomena. However, difficulties in microscopically identifying the stacking order of the TLG and limited spectroscopic resolution have stymied experimental probes of these interesting states and phenomena in scanning tunneling microscopy (STM) studies. Here we studied the detailed features of the electronic structure in the ABA TLG by using high-resolution STM measurements. Landau-level (LL) crossings of the massless and massive Dirac fermions, and effective mass renormalization of the massive Dirac fermions were observed, indicating strong electron-electron interactions in the ABA TLG. Most unexpectedly, we observed unconventional splittings of the lowest LLs for both the massless and massive Dirac fermions in high perpendicular magnetic fields. These splittings of the LLs, which are beyond the description of tight-binding calculations, reveal unexplored broken-symmetry quantum Hall states in the ABA TLG induced by many-body effects.

cond-mat.mtrl-sci↗

arXiv2018

Segregation growth and self-organization of ordered S atomic superlattices confined at interface between graphene and substrates

Ordered atomic-scale superlattices on surface hold great interest both for basic science and for potential applications in advanced technology. However, controlled fabrication of superlattices down to atomic scale has proven exceptionally challenging. Here we demonstrate the segregation-growth and self-organization of ordered S atomic superlattices confined at the interface between graphene and S-rich Cu substrates. Scanning tunneling microscope (STM) studies show that, by finely controlling the growth temperature, we obtain well-ordered S (sub)nanometer-cluster superlattice and monoatomic superlattices with various periods at the interface. These atomic superlattices are stable in atmospheric environment and robust even after high-temperature annealing (~ 350 oC). Our experiments demonstrate that the S monoatomic superlattice can drive graphene into the electronic Kekulé distortion phase when the period of the ordered S adatoms is commensurate with graphene lattice. Our results not only open a road to realize atomic-scale superlattices at interfaces, but also provide a new route to realize exotic electronic states in graphene.

cond-mat.mtrl-sci↗

arXiv2018

The UV Emission of Stars in LAMOST Survey I. Catalogs

We present the ultraviolet magnitudes for over three million stars in the LAMOST survey, in which 2,202,116 stars are detected by $GALEX$. For 889,235 undetected stars, we develop a method to estimate their upper limit magnitudes. The distribution of (FUV $-$ NUV) shows that the color declines with increasing effective temperature for stars hotter than 7000 K in our sample, while the trend disappears for the cooler stars due to upper atmosphere emission from the regions higher than their photospheres. For stars with valid stellar parameters, we calculate the UV excesses with synthetic model spectra, and find that the (FUV $-$ NUV) vs. $R'_{\mathrm{FUV}}$ can be fitted with a linear relation and late-type dwarfs tend to have high UV excesses. There are 87,178 and 1,498,103 stars detected more than once in the visit exposures of $GALEX$ in the FUV and NUV, respectively. We make use of the quantified photometric errors to determine statistical properties of the UV variation, including intrinsic variability and the structure function on the timescale of days. The overall occurrence of possible false positives is below 1.3\% in our sample. UV absolute magnitudes are calculated for stars with valid parallaxes, which could serve as a possible reference frame in the NUV. We conclude that the colors related to UV provide good criteria to distinguish between M giants and M dwarfs, and the variability of RR Lyrae stars in our sample is stronger than that of other A and F stars.

astro-ph.SR↗

arXiv2017

Magnetic-field-controlled negative differential conductance in graphene npn junction resonators

Negative differential conductance (NDC), characterized by the decreasing current with increasing voltage, has attracted continuous attention for its various novel applications. The NDC typically exists in a certain range of bias voltages for a selected system and controlling the regions of NDC in curves of current versus voltage (I-V) is experimentally challenging. Here, we demonstrate an unusual magnetic-field-controlled NDC in graphene npn junction resonators. The magnetic field not only can switch on and off the NDC, but also can continuously tune the regions of the NDC in the I-V curves. In the graphene npn junction resonators, magnetic fields generate sharp and pronounced Landau-level peaks with the help of the Klein tunneling of massless Dirac fermions. A tip of scanning tunneling microscope induces a relatively shift of the Landau levels in graphene beneath the tip. Tunneling between the misaligned Landau levels results in the magnetic-field-controlled NDC that may have potential applications for future graphene-based technology.

cond-mat.mes-hall↗

arXiv2017

Realization of room-temperature ferromagnetic semiconducting state in graphene monolayer

Room-temperature ferromagnetic semiconductor is vital in nonvolatile digital circuits and it can provide an idea system where we can make use of both charge and spin of electrons. However, seeking room-temperature ferromagnetic semiconductors is still just an appealing idea that has never been realized in practice up to now. Here we demonstrate that graphene monolayer, hybridized with underlying Ni(111) substrate, is the room-temperature ferromagnetic semiconductor that has been continuously searched for decades. Our spin-polarized scanning tunnelling microscopy (STM) experiments, complemented by first-principles calculations, demonstrate explicitly that the interaction between graphene and the Ni substrate generates a large gap in graphene and simultaneously leads to a relatively shift between majority- and minority-spin bands. Consequently, the graphene sheet on the Ni substrate exhibits a spin-polarized gap with energy of several tens meV even at room-temperature. This result makes the science and applications of room-temperature ferromagnetic semiconductors achievable and raises hopes of graphene-based novel information technologies.

cond-mat.mtrl-sci↗

arXiv2017

Scanning tunneling microscopy and spectroscopy of twisted trilayer graphene

Twist, as a simple and unique degree of freedom, could lead to enormous novel quantum phenomena in bilayer graphene. A small rotation angle introduces low-energy van Hove singularities (VHSs) approaching the Fermi level, which result in unusual correlated states in the bilayer graphene. It is reasonable to expect that the twist could also affect the electronic properties of few-layer graphene dramatically. However, such an issue has remained experimentally elusive. Here, by using scanning tunneling microscopy/spectroscopy (STM/STS), we systematically studied a twisted trilayer graphene (TTG) with two different small twist angles between adjacent layers. Two sets of VHSs originating from the two twist angles were observed in the TTG, indicating that the TTG could be simply regarded as a combination of two different twisted bilayer graphene. By using high-resolution STS, we observed split of the VHSs and directly imaged spatial symmetry breaking of electronic states around the VHSs. These results suggest that electron-electron interactions play an important role in affecting the electronic properties of graphene systems with low-energy VHSs.

cond-mat.mtrl-sci↗

arXiv2017

The tunnelling spectra of quasi-free-standing graphene monolayer

With considering the great success of scanning tunnelling microscopy (STM) studies of graphene in the past few years, it is quite surprising to notice that there is still a fundamental contradiction about the reported tunnelling spectra of quasi-free-standing graphene monolayer. Many groups observed V-shape spectra with linearly vanishing density-of-state (DOS) at the Dirac point, whereas, the others reported spectra with a gap of 60 meV pinned to the Fermi level in the quasi-free-standing graphene monolayer. Here we systematically studied the two contradicted tunnelling spectra of the quasi-free-standing graphene monolayer on several different substrates and provided a consistent interpretation about the result. The gap in the spectra arises from the out-of-plane phonons in graphene, which mix the Dirac electrons at the Brillouin zone corners with the nearly free-electron states at the zone center. Our experiment indicated that interactions with substrates could effectively suppress effects of the out-of-plane phonons in graphene and enable us to detect only the DOS of the Dirac electrons in the spectra. We also show that it is possible to switch on and off the out-of-plane phonons of graphene at the nanoscale, i.e., the tunnelling spectra show switching between the two distinct features, through voltage pulses applied to the STM tip.

cond-mat.mtrl-sci↗

arXiv2017

Scanning tunneling microscopy and spectroscopy of nanoscale twisted bilayer graphene

Nanoscale twisted bilayer graphene (TBG) is quite instable and will change its structure to Bernal (or AB-stacking) bilayer with a much lower energy. Therefore, the lack of nanoscale TBG makes its electronic properties not accessible in experiment up to now. In this work, a special confined TBG is obtained in the overlaid area of two continuous misoriented graphene sheets. The width of the confined region of the TBG changes gradually from about 22 nm to 0 nm. By using scanning tunnelling microscopy, we studied carefully the structure and the electronic properties of the nanoscale TBG. Our results indicate that the low-energy electronic properties, including twist-induced van Hove singularities (VHSs) and spatial modulation of local density-of-state, are strongly affected by the translational symmetry breaking of the nanoscale TBG. Whereas, the electronic properties above the energy of the VHSs are almost not influenced by the quantum confinement even when the width of the TBG is reduced to only a single moire spot.

cond-mat.mes-hall↗

arXiv2017

Generating nanoscale and atomically-sharp p-n junctions in graphene via monolayer-vacancy-island engineering of Cu surface

Creation of high quality p-n junctions in graphene monolayer is vital in studying many exotic phenomena of massless Dirac fermions. However, even with the fast progress of graphene technology for more than ten years, it remains conspicuously difficult to generate nanoscale and atomically-sharp p-n junctions in graphene. Here, we employ monolayer-vacancy-island engineering of Cu surface to realize nanoscale p-n junctions with atomically-sharp boundaries in graphene monolayer. The variation of graphene-Cu separations around the edges of the Cu monolayer-vacancy-island affects the positions of the Dirac point in graphene, which consequently lead to atomically-sharp p-n junctions with the height as high as 660 meV in graphene. The generated sharp p-n junctions isolate the graphene above the Cu monolayer-vacancy-island as nanoscale graphene quantum dots (GQDs) in a continuous graphene sheet. Massless Dirac fermions are confined by the p-n junctions for a finite time to form quasi-bound states in the GQDs. By using scanning tunneling microscopy, we observe resonances of quasi-bound states in the GQDs with various sizes and directly visualize effects of geometries of the GQDs on the quantum interference patterns of the quasi-bound states, which allow us to test the quantum electron optics based on graphene in atomic scale.

cond-mat.mes-hall↗

arXiv2017

Evidence of electron-electron interactions around Van Hove singularities of a graphene Moiré superlattice

A variety of new and interesting correlated states have been predicted in graphene monolayer doped to Van Hove singularities (VHSs) of its density-of-state (DOS). However, tuning the Fermi energy to reach a VHS of graphene by either gating or chemical doping is prohibitively difficult, owning to their large energy distance (3 eV). Therefore, these correlated states, which arise from effects of strong electron-electron interactions at the VHSs, have remained experimentally elusive. Here, we report experimental evidences of electron-electron interactions around the VHSs of a twisted bilayer graphene (TBG) through scanning tunneling microscopy measurements. By introducing a small twisted angle between two adjacent graphene sheets, we are able to generate low-energy VHSs arbitrarily approaching the Fermi energy. The split of the VHSs are observed and the symmetry breaking of electronic states around the VHSs are directly visualized. These results experimentally demonstrate the important effects of electron-electron interactions on electronic properties around the VHSs of the TBG, therefore providing motivation for further theoretical and experimental studies in graphene systems with considering many-body interactions.

cond-mat.mes-hall↗

arXiv2017

Landau quantization of Dirac fermions in graphene and its multilayer

When electrons are confined in a two dimensional (2D) system, typical quantum mechanical phenomena such as Landau quantization can be detected. Graphene systems, including the single atomic layer and few-layer stacked crystals, are ideal 2D materials for studying a variety of quantum mechanical problems. In this article, we review the experimental progress in the unusual Landau quantized behaviors of Dirac fermions in monolayer and multilayer graphene by using scanning tunneling microscopy(STM) and scanning tunneling spectroscopy(STS). Through STS measurement of the strong magnetic fields, distinct Landau-level spectra and rich level splitting phenomena are observed in different graphene layers. These unique properties provide an effective method for identifying the number of layers, as well as the stacking orders, and investigating the fundamentally physical phenomena of graphene. Moreover, in the presence of a strain and charged defects, the Landau quantization of graphene can be significantly modified, leading to unusual spectroscopic and electronic properties.

cond-mat.mes-hall↗

arXiv2017

One-step synthesis of van der Waals heterostructures of graphene and 2D superconducting a-Mo2C

Assembling different two-dimensional (2D) crystals, covering a very broad range of properties, into van der Waals (vdW) heterostructures enables the unprecedented possibilities for combining the best of different ingredients in one objective material. So far, metallic, semiconducting, and insulating 2D crystals have been used successfully in making functional vdW heterostructures with properties by design. Here, we expand 2D superconducting crystals as a building block of the vdW hererostructures. A one-step growth of large-scale high-quality vdW heterostructures of graphene and 2D superconducting a-Mo2C by using chemical vapor deposition (CVD) method is reported. The superconductivity and its 2D nature of the heterostructures are characterized by our scanning tunneling microscopy (STM) measurements. This adds the 2D superconductivity, the most attractive property of condensed matter physics, to the vdW heterostructures.

cond-mat.mtrl-sci↗

arXiv2017

Imaging the dynamics of individual hydrogen atom intercalated between two graphene sheets

The interlayer gallery between two adjacent sheets of van der Waals materials is expected to modify properties of atoms and molecules confined at the atomic interfaces. Here, we directly image individual hydrogen atom intercalated between two graphene sheets and investigate its dynamics by scanning tunnelling microscope (STM). The intercalated hydrogen atom is found to be remarkably different from atomic hydrogen chemisorbed on external surface of graphene. Our STM measurements, complemented by first-principles calculations, show that the hydrogen atom intercalated between two graphene sheets has dramatically reduced potential barriers for elementary migration steps. Especially, the confined atomic hydrogen dissociation energy from graphene is reduced to 0.34 eV, which is only about a third of a hydrogen atom chemisorbed on graphene. This offers a unique platform for direct imaging of the atomic dynamics of confined atoms. Our results suggest that the atomic interfaces of van der Waals materials may provide a confined environment to tune the interfacial chemical reactions.

cond-mat.mes-hall↗

arXiv2017

Bound States in Nanoscale Graphene Quantum Dots in a Continuous Graphene Sheet

Considerable efforts have been made to trap massless Dirac fermions in graphene monolayer, but only quasi-bound states are realized in continuous graphene sheets up to now. Here, we demonstrate the realization of bound states in nanoscale graphene quantum dots (GQDs) in a continuous graphene sheet. The GQDs are electronically isolated from the surrounding continuous graphene sheet by circular boundaries, which are generated by strong coupling between graphene and substrate. By using scanning tunneling microscopy (STM), we observe single-electron charging states of the GQDs, seen as Coulomb oscillations in the tunneling conductance. Evolution of single-electron tunneling of the GQDs between the Coulomb blockade regime and the Coulomb staircase regime is observed by tuning the STM tip-sample distances. Spatial maps of the local electronic densities reveal concentric rings inside the GQDs with each ring corresponding to a single Coulomb oscillation of the tunneling spectra. These results indicate explicitly that the electrons are completely trapped inside the nanoscale GQDs.

cond-mat.mes-hall↗

arXiv2017

Massless Dirac Fermions Trapping in a Quasi-one-dimensional npn Junction of a Continuous Graphene Monolayer

Massless Dirac fermions in graphene provide unprecedented opportunities to realize the Klein paradox, which is one of the most exotic and striking properties of relativistic particles. In the seminal theoretical work [Katsnelson et al., Nat. Phys. 2 620 (2006)], it was predicted that the massless Dirac fermions can pass through one-dimensional (1D) potential barriers unimpededly at normal incidence. Such a result seems to preclude confinement of the massless Dirac fermions in graphene by using 1D potential barriers. Here, we demonstrate, both experimentally and theoretically, that massless Dirac fermions can be trapped in quasi-1D npn junction of a continuous graphene monolayer. Because of highly anisotropic transmission of the massless Dirac fermions at n-p junction boundaries (the so-called Klein tunneling in graphene), charge carries incident at large oblique angles will be reflected from one edge of the junction with high probability and continue to bounce from the opposite edge. Consequently, these electrons are trapped for a finite time to form quasi-bound states in the quasi-1D npn junction. The quasi-bound states seen as pronounced resonances are probed and the quantum interference patterns arising from these states are directly visualized in our scanning tunneling microscope measurements.

cond-mat.mes-hall↗

arXiv2016

Stacking transition in bilayer graphene caused by thermally activated rotation

Crystallographic alignment between two-dimensional crystals in van der Waals heterostructures brought a number of profound physical phenomena, including observation of Hofstadter butterfly and topological currents, and promising novel applications, such as resonant tunnelling transistors. Here, by probing the electronic density of states in graphene using graphene-hexagonal boron nitride tunnelling transistors, we demonstrate a structural transition of bilayer graphene from incommensurate twisted stacking state into a commensurate AB stacking due to a macroscopic graphene self-rotation. This structural transition is accompanied by a topological transition in the reciprocal space and by pseudospin texturing. The stacking transition is driven by van der Waals interaction energy of the two graphene layers and is thermally activated by unpinning the microscopic chemical adsorbents which are then removed by the self-cleaning of graphene.

cond-mat.mes-hall↗

arXiv2016

Corrugation induced stacking solitons with topologically confined states in gapped bilayer graphene

Graphene, as an atomic-thick ultrasoft membrane, almost has no resistance against out-of-plane deformations and, therefore, it is always wrinkled to a certain degree. Recently, corrugated structures and their effects on the electronic properties of monolayer graphene have been studied extensively. However, similar experimental studies in bilayer graphene have yet to be reported. Here, we show that corrugations in bilayer graphene can generate incommensurate stacking solitons (domain walls) between commensurate Bernal-stacked domains. By using scanning tunneling microscopy, we microscopically study electronic structures of a corrugation-induced stacking soliton that separates two adjacent AB and BA stacked bilayer regions with a uniform interlayer potential difference. Both topological gapless edge states and quasi-localized gapped quantum-well-like states are observed in the nanoscale corrugation. Atomic resolution mapping of the topological edge states along the stacking soliton reveals the existence of intervalley scattering between them, which could explain recent experiments where the conductance along domain walls of gapped graphene bilayer is smaller than a quantized value 4e2 h.

cond-mat.mes-hall↗