arXiv subjects

Lin He

Publications and source records attributed to Lin He.

At least 73 records · Page 4Linked to original sources

arXiv2021

Spatial and Magnetic Confinement of Massless Dirac Fermions

The massless Dirac fermions and the ease to introduce spatial and magnetic confinement in graphene provide us unprecedented opportunity to explore confined relativistic matter in this condensed-matter system. Here we report the interplay between the confinement induced by external electric fields and magnetic fields of the massless Dirac fermions in graphene. When the magnetic length lB is larger than the characteristic length of the confined electric potential lV, the spatial confinement dominates and a relatively small critical magnetic field splits the spatial-confinement-induced atomic-like shell states by switching on a pi Berry phase of the quasiparticles. When the lB becomes smaller than the lV, the transition from spatial confinement to magnetic confinement occurs and the atomic-like shell states condense into Landau levels (LLs) of the Fock-Darwin states in graphene. Our experiment demonstrates that the spatial confinement dramatically changes the energy spacing between the LLs and generates large electron-hole asymmetry of the energy spacing between the LLs. These results shed light on puzzling observations in previous experiments, which hitherto remained unaddressed.

cond-mat.mes-hall↗

arXiv2021

Direct observation of magneto-electric Aharonov-Bohm effect in moiré-scale quantum paths of minimally twisted bilayer graphene

Aharonov-Bohm (AB) effect, the well-known archetype of electron-wave interference phenomena, has been explored extensively through transport measurements. However, these techniques lack spatial resolution that would be indispensable for studying the magnetic and electrostatic AB oscillations at the nanometer scale. Here, we demonstrated that scanning tunneling microscopy (STM) can be used as an AB interferometer operating on nanometer length scales and the magneto-electric Aharonov-Bohm effect in minimally twisted bilayer graphene (TBG) was directly measured by using STM. In the minimally TBG, there is a triangular network of chiral one-dimensional states hosted by domain boundaries due to structural reconstruction. Taking advantage of the high spatial resolution of the STM, both the magnetic and electrostatic AB oscillations arising from electron interference along moiré-scale triangular quantum paths in the minimally TBG were measured. Our work enables measure and control of the AB effect and other electron-wave interference at the nanoscale.

cond-mat.mes-hall↗

arXiv2021

Local Measurements of Shubnikov-de Haas Oscillations in Graphene Systems

Shubnikov-de Haas (SdH) oscillations, the most well-known magneto-oscillations caused by the quantization of electron energy levels in the presence of magnetic fields in two-dimensional (2D) electron systems, can be used to determine Fermi-surface properties and directly measure the Berry phase of the 2D systems. It is usually thought that transport measurements are required to measure the SdH oscillations. Contradicting this belief, we demonstrate that the SdH oscillations can be measured in graphene systems by carrying out scanning tunneling spectroscopy (STS) measurements. The energy-momentum dispersions and Berry phases of monolayer, Bernal-stacked bilayer, and ABC-stacked trilayer graphene are obtained according to the measured SdH oscillations in the STS spectra. It is possible to obtain the SdH oscillations when the size of the 2D systems is larger than the magnetic length and, importantly, no gate electrode is required in the STS measurement, therefore, the reported method in this work is applicable to a wide range of materials.

cond-mat.mes-hall↗

arXiv2020

Pseudospin-mediated Atomic-scale Vortices and Their Quantum Interferences in Monolayer Graphene

Vortex is a universal and significant phenomenon that has been known for centuries. However, creating vortices to the atomic limit has remained elusive because that the characteristic length to support a vortex is usually much larger than the atomic scale. Very recently, it was demonstrated that intervalley scattering induced by the single carbon defect of graphene leads to phase winding over a closed path surrounding the defect. Motivated by this, we demonstrate, in this Letter, that the single carbon defects at A and B sublattices of graphene can be regarded as pseudospin-mediated atomic-scale vortices with angular momenta l = +2 and -2, respectively. The quantum interferences measurements of the interacting vortices indicate that the vortices cancel each other, resulting in zero total angular momentum, in the |A| = |B| case, and they show aggregate chirality and angular momenta similar to a single vortex of the majority in the |A| not equal to |B| case, where |A| (|B|) is the number of vortices with angular momenta l = +2 (l = -2).

cond-mat.mtrl-sci↗

arXiv2020

Stellar X-ray activity across the Hertzsprung-Russell diagram. I. Catalogs

Stellar magnetic activity provides substantial information on the magnetic dynamo and the coronal heating process. We present a catalog of X-ray activity for about 6000 stars, based on the $Chandra$ and $Gaia$ DR2 data. We also classified more than 3000 stars as young stellar objects, dwarf stars, or giant stars. By using the stars with valid stellar parameters and classifications, we studied the distribution of X-ray luminosity ($L_X$) and the ratio of X-ray-to-bolometric luminosities ($R_X$), the positive relation between $L_X$, $R_X$, and hardness ratio, and the long-term X-ray variation. This catalog can be used to investigate some important scientific topics, including the activity-rotation relation, the comparison between different activity indicators, and the activities of interesting objects (e.g., A-type stars and giants). As an example, we use the catalog to study the activity-rotation relation, and find that the young stellar objects, dwarfs, and giants fall on a single sequence in the relation $R_X$ versus Rossby number, while the giants do not follow the relation $R_X$ versus $P_{\rm rot}^{-2}R^{-4}$ valid for dwarfs.

astro-ph.SR↗

arXiv2020

Twistronics in graphene-based van der Waals structures

The electronic properties of van der Waals (vdW) structures can be substantially modified by the moire superlattice potential, which strongly depends on the twist angle among the compounds. In twisted bilayer graphene (TBG), two low-energy Van Hove singularities (VHSs) move closer with decreasing twist angles and finally become highly non-dispersive flat bands at the magic angle (~ 1.1 degree). When the Fermi level lies within the flat bands of the TBG near the magic angle, Coulomb interaction is supposed to exceed the kinetic energy of the electrons, which can drive the system into various strongly correlated phases. Moreover, the strongly correlated states of flat bands are also realized in other graphene-based vdW structures with an interlayer twist. In this article, we mainly review the recent experimental advances on the strongly correlated physics of the magic-angle TBG (MATBG) and the small-angle twisted multilayer graphene. Lastly we will give out a perspective of this field.

cond-mat.mtrl-sci↗

arXiv2020

Tunable lattice reconstruction and bandwidth of flat bands in magic-angle twisted bilayer graphene

The interplay between interlayer van der Waals interaction and intralayer lattice distortion can lead to structural reconstruction in slightly twisted bilayer graphene (TBG) with the twist angle being smaller than a characteristic angle θc. Experimentally, the θc is demonstrated to be very close to the magic angle (θ ~ 1.05°). In this work, we address the transition between reconstructed and unreconstructed structures of the TBG across the magic angle by using scanning tunnelling microscopy (STM). Our experiment demonstrates that both the two structures are stable in the TBG around the magic angle. By applying a STM tip pulse, we show that the two structures can be switched to each other and the bandwidth of the flat bands, which plays a vital role in the emergent strongly correlated states in the magic-angle TBG, can be tuned. The observed tunable lattice reconstruction and bandwidth of the flat bands provide an extra control knob to manipulate the exotic electronic states of the TBG near the magic angle.

cond-mat.mes-hall↗

arXiv2020

Robust two-dimensional ice on graphene built from finite-length water molecular chains

Interfacial ice on graphene has attracted much attention because it is a model system to study two-dimensional (2D) ice structures on chemically inert substrates. While water-graphene interaction was usually assumed to be negligible, the structures of the 2D ice are believed to be not appreciably perturbed by the graphene substrate. Here we report atomic-resolved characterizations of an exotic 2D ice structure on graphene built from water molecular chains with finite lengths. Our experiments demonstrated that the water molecular chains are exactly orientated along zigzag directions of the graphene substrate, which evidences an anomalously strong interlayer interaction between the 2D ice and the graphene substrate. Moreover, the length of the water molecular chains closely links to the number of graphene layers, indicating layer-number-dependent water-graphene interfacial interactions. Our work highlights the important role of the 2D ice structures on the water-graphene interfacial interactions.

cond-mat.mtrl-sci↗

arXiv2020

Spectroscopic evidence for a spin and valley polarized metallic state in a non-magic-angle twisted bilayer graphene

In the magic-angle twisted bilayer graphene (MA-TBG), strong electron-electron (e-e) correlations caused by the band-flattening lead to many exotic quantum phases such as superconductivity, correlated insulator, ferromagnetism, and quantum anomalous Hall effects, when its low-energy van Hove singularities (VHSs) are partially filled. Here our high-resolution scanning tunneling microscope and spectroscopy measurements demonstrate that the e-e correlation in a non-magic-angle TBG with a twist angle θ = 1.49 still plays an important role in determining its electronic properties. Our most interesting observation on that sample is that when one of its VHS is partially filled, the one associated peak in the spectrum splits into four peaks. Our analysis based on the continuum model suggests that such a one-to-four split of the VHS originates from the formation of an interaction-driven spin-valley-polarized metallic state near the VHS, lifting both the spin and valley degeneracies. Our results for this non-magic-angle TBG reveal a new symmetry-breaking phase, which has not been identified in the MA-TBG or in other systems.

cond-mat.mtrl-sci↗

arXiv2020

Intervalley quantum interference and measurement of Berry phase in bilayer graphene

Chiral quasiparticles in Bernal-stacked bilayer graphene have valley-contrasting Berry phases of 2π. This nontrival topological structure, associated with the pseudospin winding along a closed Fermi surface, is responsible for various novel electronic properties, such as anti-Klein tunneling, unconventional quantum Hall effect, and valley Hall effect1-6. Here we show that the quantum interference due to intervalley scattering induced by atomic defects/impurities provides further insights into the topological nature of the bilayer graphene. The scattered chiral quasiparticles between distinct valleys with opposite chirality undergoes a rotation of pseudospin that results in the Friedel oscillation with wavefront dislocations. The number of dislocations reflects the information about pseudospin texture and hence can be used to measure the Berry phase7. As demonstrated both experimentally and theoretically, the Friedel oscillation, depending on the atomic defect/impurity at different sublattices, can exhibit N = 4, 2, or 0 additional wavefronts, characterizing the 2π Berry phase of the bilayer graphene. Our results not only provide a comprehensive study of the intervalley quantum interference in bilayer graphene, but also shed lights on the pseudospin physics.

cond-mat.mtrl-sci↗

arXiv2020

Nonlinear stability of rarefaction waves for a viscous radiative and reactive gas with large initial perturbation

We investigate the time-asymptotically nonlinear stability of rarefaction waves to the Cauchy problem of an one-dimensional compressible Navier-Stokes type system for a viscous, compressible, radiative and reactive gas, where the constitutive relations for the pressure $p$, the specific internal energy $e$, the specific volume $v$, the absolute temperature $θ$, and the specific entropy $s$ are given by $p=Rθ/v +aθ^4/3$, $e=C_vθ+avθ^4$, and $s=C_v\ln θ+ 4avθ^3/3+R\ln v$ with $R>0$, $C_{v}>0$, and $a>0$ being the perfect gas constant, the specific heat and the radiation constant, respectively. For such a specific gas motion, a somewhat surprising fact is that, general speaking, the pressure $\widetilde{p}(v,s)$ is not a convex function of the specific volume $v$ and the specific entropy $s$. Even so, we show in this paper that the rarefaction waves are time-asymptotically stable for large initial perturbation provided that the radiation constant $a$ and the strength of the rarefaction waves are sufficiently small. The key point in our analysis is to deduce the positive lower and upper bounds on the specific volume and the absolute temperature, which are uniform with respect to the space and the time variables, but are independent of the radiation constant $a$.

math.AP↗

arXiv2020

Naive Gabor Networks for Hyperspectral Image Classification

Recently, many convolutional neural network (CNN) methods have been designed for hyperspectral image (HSI) classification since CNNs are able to produce good representations of data, which greatly benefits from a huge number of parameters. However, solving such a high-dimensional optimization problem often requires a large amount of training samples in order to avoid overfitting. Additionally, it is a typical non-convex problem affected by many local minima and flat regions. To address these problems, in this paper, we introduce naive Gabor Networks or Gabor-Nets which, for the first time in the literature, design and learn CNN kernels strictly in the form of Gabor filters, aiming to reduce the number of involved parameters and constrain the solution space, and hence improve the performances of CNNs. Specifically, we develop an innovative phase-induced Gabor kernel, which is trickily designed to perform the Gabor feature learning via a linear combination of local low-frequency and high-frequency components of data controlled by the kernel phase. With the phase-induced Gabor kernel, the proposed Gabor-Nets gains the ability to automatically adapt to the local harmonic characteristics of the HSI data and thus yields more representative harmonic features. Also, this kernel can fulfill the traditional complex-valued Gabor filtering in a real-valued manner, hence making Gabor-Nets easily perform in a usual CNN thread. We evaluated our newly developed Gabor-Nets on three well-known HSIs, suggesting that our proposed Gabor-Nets can significantly improve the performance of CNNs, particularly with a small training set.

eess.IV↗

arXiv2020

Electron interactions in strain-induced zero-energy flat band in twisted bilayer graphene near the magic angle

In the vicinity of the magic angle in twisted bilayer graphene (TBG), the two low-energy van Hove singularities (VHSs) become exceedingly narrow1-10 and many exotic correlated states, such as superconductivity, ferromagnetism, and topological phases, are observed11-16. Heterostrain, which is almost unavoidable in the TBG, can modify its single-particle band structure and lead to novel properties of the TBG that have never been considered so far. Here, we show that heterostrain in a TBG near the magic angle generates a new zero-energy flat band between the two VHSs. Doping the TBG to partially fill the zero-energy flat band, we observe a correlation-induced gap of about 10 meV that splits the flat band. By applying perpendicular magnetic fields, a large and linear response of the gap to magnetic fields is observed, attributing to the emergence of large orbital magnetic moments in the TBG when valley degeneracy of the flat band is lifted by electron-electron interactions. The orbital magnetic moment per moire supercell is measured as about 15 uB in the TBG.

cond-mat.str-el↗

arXiv2020

Valley Polarization and Inversion in Strained Graphene via Pseudo-Landau Levels, Valley Splitting of Real Landau Levels and Confined States

It is quite easy to control spin polarization and spin direction of a system via magnetic fields. However, there is no such a direct and efficient way to manipulate valley pseudospin degree of freedom. Here, we demonstrate experimentally that it is possible to realize valley polarization and valley inversion in graphene by using both strain-induced pseudomagnetic fields and real magnetic fields. The pseudomagnetic fields, which are quite different from real magnetic fields, pointing in opposite directions at the two distinct valleys of graphene. Therefore, coexistence of the pseudomagnetic fields and the real magnetic fields leads to imbalanced effective magnetic fields at two distinct valleys of graphene. This allows us to control the valley in graphene as convenient as the electron spin. In this work, we report consistent observation of valley polarization and inversion in strained graphene via pseudo-Landau levels, splitting of real Landau levels and valley splitting of confined states using scanning tunneling spectroscopy. Our results highlight a pathway to valleytronics in strained graphene-based platforms.

cond-mat.mes-hall↗

arXiv2020

Nanoscale Probing of Broken-Symmetry States in Graphene Induced by Individual Atomic Impurities

Inherent symmetries of a system lead to multiple degeneracies of its energy spectra. Introducing individual atomic impurities can locally break these symmetries, which is expected to lift the degenerate degrees of freedom around the impurities. Although central to our understanding of the fundamental properties of solids, the broken-symmetry states induced by individual atomic impurities have so far eluded observation. Here, we report nanoscale probing of the broken-symmetry states in graphene induced by two types of individual atomic impurities, i.e., isolated nitrogen dopants and isolated hydrogen atoms chemisorbed on graphene. Our experiments demonstrate that both types of atomic impurities can locally break sublattice symmetry of graphene and generate valley-polarized states, which extends several nanometers around the impurities. For the isolated hydrogen atom chemisorbed on graphene, the enhanced spin-orbit coupling, which arises from the sp3 distortion of graphene due to the hydrogen chemisorption, further lifts the spin degeneracy, resulting in a fully spin and valley polarized states within about 1 nm around the hydrogen atom. Our result paves the way to control various broken-symmetry states at the nanoscale by various atomic impurities.

cond-mat.mes-hall↗

arXiv2020

Breaking whispering-gallery modes of massless Dirac fermions in graphene quantum dots by Coulomb interaction

Coulomb interaction is of central importance in localized energy levels (bound states) or electronic flat bands and could result in many exotic quantum phases, such as magnetic, superconducting, and topological phases in graphene systems1-14. In graphene monolayer, the relativistic massless Dirac fermion nature of the charge carriers enables us to realize unprecedented quasibound states, which are trapped temporarily via whispering-gallery modes (WGMs) with the lifetime (trapping time) of ~ 10 fs, in circular graphene quantum dots (GQDs)15-20. Here we show that Coulomb interaction still plays a dominating role in determining the electronic properties of the temporarily-confined quasibound states. Our scanning tunneling microscopy (STM) and spectroscopy (STS) measurements demonstrate that the discrete quasibound state in a GQD will split into two peaks when it is partially filled. The energy separation of the two split peaks increases linear with inverse effective radius of the GQDs, indicating that the splitting arises from the Coulomb interaction. Moreover, we show that the real space distribution of the two split states separates in different regions of the GQD to reduce the Coulomb interaction, leading to the breaking of the WGM of the quasibound states.

cond-mat.mes-hall↗

arXiv2019

Experimental evidence for orbital magnetic moments generated by moiré-scale current loops in twisted bilayer graphene

A remarkable property of twisted bilayer graphene (TBG) with small twist angle is the presence of a well-defined and conserved low-energy valley degrees of freedom1, which can potentially bring about new types of valley-associated spontaneous-symmetry breaking phases. Electron-electron (e-e) interactions in the TBG near the magic angle 1.1 degree can lift the valley degeneracy, allowing for the realization of orbital magnetism and topological phases2-11. However, direct measurement of the orbital-based magnetism in the TBG is still lacking up to now. Here we report evidence for orbital magnetic moment generated by the moire-scale current loops in a TBG with a twist angle θ ~ 1.68 degree. The valley degeneracy of the 1.68 degree TBG is removed by e-e interactions when its low-energy van Hove singularity (VHS) is nearly half filled. A large and linear response of the valley splitting to magnetic fields is observed, attributing to coupling to the large orbital magnetic moment induced by chiral current loops circulating in the moire pattern. According to our experiment, the orbital magnetic moment is about 10.7 uB per moire supercell. Our result paves the way to explore magnetism that is purely orbital in slightly twisted graphene system.

cond-mat.mtrl-sci↗

arXiv2019

Spectroscopic Characterization of Landau Level Splitting and the Intermediate v = 0 Phase in Bilayer Graphene

Despite various novel broken symmetry states have been revealed in bilayer graphene (BLG) experimentally, the atomic-scale spectroscopic investigation has been greatly limited. Here, we study high-resolution spectroscopic characteristics of high-quality BLG and observe rich broken-symmetry-induced Landau level (LL) splittings, including valley, spin and orbit, by using ultralow-temperature and high-magnetic-field scanning tunneling microscopy and spectroscopy (STM and STS). Our experiment demonstrates that both the spin and orbital splittings of the lowest n = (0,1) LL depend sensitively on its filling and exhibit an obvious enhancement at partial-filling states. More unexpectedly, the splitting of a fully-filled and valley-polarized LL is also enhanced by partial filling of the LL with the opposite valley. These results reveal significant many-body effects in this system. At half filling of the n = (0,1) LL (filling factor v = 0), a single-particle intermediate v = 0 phase, which is the transition state between canted antiferromagnetic and layer-polarized states in the BLG, is measured and directly visualized at the atomic scale. Our atomic-scale STS measurement gives direct evidence that this intermediate v = 0 state is the predicted orbital-polarized phase.

cond-mat.mes-hall↗