arXiv subjects

Xianqing Lin

Publications and source records attributed to Xianqing Lin.

14 recordsLinked to original sources

arXiv2024

Ferroelectricity in oxygen-terminated 2D carbides of lanthanide elements

We investigate the properties of oxygen-functionalized carbides of lanthanide elements with the composition M2CO2 (M=Gd, Tb,Dy) that form two-dimensional (2D) structures. Our ab initio calculations reveal that oxygen termination turns M2C monolayers into semiconductors with two dynamically stable phases. Of these, the energetically favored alpha-phase becomes ferroelectric, whereas the beta-phase turns anti-ferroelectric. Applying in-plane biaxial strain may transform one phase into the other, changes the ferroelectric polarization of the alpha-phase in a linear fashion, and modifies the size and nature of the fundamental band gap from direct to indirect. The structure with a direct band gap exhibits in-plane isotropic electronic and optical properties. This previously unexplored class of systems also exhibits excellent photon absorption in the ultraviolet range.

cond-mat.mtrl-sci↗

arXiv2023

Collective excitations of the Chern-insulator states in commensurate double moir\'{e} superlattices of twisted bilayer graphene on hexagonal boron nitride

We study the collective excitation modes of the Chern insulator states in magic-angle twisted bilayer graphene aligned with hexagonal boron nitride (TBG/BN) at odd integer fillings ($\nu$) of the flat bands. For the $1 \times 1$ commensurate double moir\'{e} superlattices in TBG/BN at three twist angles ($\theta'$) between BN and graphene, self-consistent Hartree-Fock calculations show that the electron-electron interaction and the broken $C_{2z}$ symmetry lead to the Chern-insulator ground states with valley-spin flavor polarized HF bands at odd $\nu$. In the active-band approximation, the HF bands in the same flavor of TBG/BN are much more separated than those of the pristine TBG with TBG/BN having a larger intra-flavor band gap so that the energies of the lowest intra-flavor exciton modes of TBG/BN computed within the time-dependent HF method are much higher than those of TBG and reach about 20 meV, and the exciton wavefunctions of TBG/BN become less localized than those of TBG. The inter-flavor valley-wave modes in TBG/BN have excitation energies higher than 2.5 meV which is also much larger than that of TBG, while the spin-wave modes all have zero excitation gap. In contrast to TBG with particle-hole symmetric excitation modes for positive and negative $\nu$, the excitation spectrums and gaps of TBG/BN at positive $\nu$ are rather different from those at negative $\nu$. The quantitative behavior of the excitation spectrum of TBG/BN also varies with $\theta'$. Full HF calculations demonstrate that more HF bands besides the two central bands can have rather large contributions from the single-particle flat-band states, then the lowest exciton modes that determine the optical properties of the Chern insulator states in TBG/BN are generally the ones between the remote and flat-like bands, while the valley-wave modes have similar energies as those in the active-band approximation.

cond-mat.mes-hall↗

arXiv2022

Energetic stability and spatial inhomogeneity in the local electronic structure of relaxed twisted trilayer graphene

We study the energetic stability and the local electronic structure of the general twisted trilayer graphene (TTG) with the top and bottom layers rotated with respect to the middle layer respectively by $θ$ and $θ'$. Approximate supercells of the moiré-of-moiré superlattices with $θ$ and $θ^{\prime}$ within $1^{\circ}\sim 2^{\circ}$ are established to describe the structural and electronic properties of relaxed TTG with the periodic boundary condition. Full relaxation demonstrates that the commensurate TTG with $θ=θ^{\prime}$ has the local minimum total energy ($E_{tol}$) at a fixed $θ$, while $E_{tol}$ first reaches a local maximum and begins to drop with decreasing $θ^{\prime}$ for $θ^{\prime} < θ$. Some regions exhibit enhanced in-plane relaxation in the top and bottom layers but suppressed relaxation in the middle layer and form a hexagonal network with the moiré-of-moiré length scale. The stacking configurations with the atoms in the three layers vertically aligned at the origin of the relaxed TTG supercells at $θ$ around $1.6^{\circ}$ and $θ^{\prime}$ around $1.4^{\circ}$ have a high density of states (DOS) near the Fermi level ($E_F$), which can reach that of the mirror symmetric TTG with equal twist angles of about $1.7^{\circ}$. In contrast, some other stackings can have rather low DOS around $E_F$. The significant stacking dependence of DOS for some TTG supercells demonstrates that the local electronic structure of TTG can exhibit strong spatial inhomogeneity when the twist angles are slightly away from those of the small supercells with large variations of DOS among different stackings. Moreover, the structural relaxation of TTG plays a crucial role in the high DOS and its strong stacking dependence.

cond-mat.mes-hall↗

arXiv2021

Emergence of intrinsically isolated flat bands and their topology in fully relaxed twisted multi-layer graphene

We study the electronic structure and band topology of fully relaxed twisted multi-layer graphene (TMLG). Isolated flat bands emerge in TMLG with the number of layers [$M+N$ with $M$ the layer number of the bottom few-layer graphene (FLG)] up to 10 and with various stacking orders, and most of them are on the hole side. The touched bands of FLGs around the Fermi level are split by the moiré coupling through the electron-hole asymmetry in low-energy bands of FLGs and by the vertical hopping between next-nearest layers. The full structural relaxation leads to global gaps that completely isolate a flat band. For TMLG with given $M$ and $N$, the highest magnitude of Chern numbers ($|C|$) of the separable flat bands reaches $M+N-1$ and can be hosted by certain isolated bands. The $|C|=9$ occurs in the isolated flat valence band of several configurations with 10 layers. Such high $|C|$ originates from the lifting of the band-state degeneracy in the weak regime of moiré coupling or from the topological phase transitions induced by the strong moiré coupling. Moreover, large orbital magnetic moments arise in isolated flat bands with high $|C|$ and depend on the structural configurations of TMLG.

cond-mat.mes-hall↗

arXiv2020

Moiré effects in graphene--hBN heterostructures

Encapsulating graphene in hexagonal Boron Nitride has several advantages: the highest mobilities reported to date are achieved in this way, and precise nanostructuring of graphene becomes feasible through the protective hBN layers. Nevertheless, subtle effects may arise due to the differing lattice constants of graphene and hBN, and due to the twist angle between the graphene and hBN lattices. Here, we use a recently developed model which allows us to perform band structure and magnetotransport calculations of such structures, and show that with a proper account of the moiré physics an excellent agreement with experiments can be achieved, even for complicated structures such as disordered graphene, or antidot lattices on a monolayer hBN with a relative twist angle. Calculations of this kind are essential to a quantitative modeling of twistronic devices.

cond-mat.mes-hall↗

arXiv2020

Misalignment instability in magic-angle twisted bilayer graphene on hexagonal boron nitride

We study the stability and electronic structure of magic-angle twisted bilayer graphene on the hexagonal boron nitride (TBG/BN). Full relaxation has been performed for commensurate supercells of the heterostructures with different twist angles ($θ'$) and stackings between TBG and BN. We find that the slightly misaligned configuration with $θ' = 0.54^\circ$ and the AA/AA stacking has the globally lowest total energy due to the constructive interference of the moiré interlayer potentials and thus the greatly enhanced relaxation in its $1 \times 1$ commensurate supercell. Gaps are opened at the Fermi level ($E_F$) for small supercells with the stackings that enable strong breaking of the $C_2$ symmetry in the atomic structure of TBG. For large supercells with $θ'$ close to those of the $1 \times 1$ supercells, the broadened flat bands can still be resolved from the spectral functions. The $θ' = 0.54^\circ$ is also identified as a critical angle for the evolution of the electronic structure with $θ'$, at which the energy range of the mini-bands around $E_F$ begins to become narrower with increasing $θ'$ and their gaps from the dispersive bands become wider. The discovered stablest TBG/BN with a finite $θ'$ of about $0.54^\circ$ and its gapped flat bands agree with recent experimental observations.

cond-mat.mes-hall↗

arXiv2020

Symmetry breaking in the double moiré superlattices of relaxed twisted bilayer graphene on hexagonal boron nitride

We study the atomic and electronic structures of the commensurate double moiré superlattices in fully relaxed twisted bilayer graphene (TBG) nearly aligned with the hexagonal boron nitride (BN). The single-particle effective Hamiltonian ($\hat{H}^0$) taking into account the relaxation effect and the full moiré Hamiltonian introduced by BN has been built for TBG/BN. The mean-field (MF) band structures of the self-consistent Hartree-Fock (SCHF) ground states at different number ($ν$) of filled flat bands relative to the charge neutrality point (CNP) are obtained based on $\hat{H}^0$ in the plane-wave-like basis. The single-particle flat bands in TBG/BN become separated by the opened gap at CNP due to the symmetry breaking in $\hat{H}^0$. We find that the broken $C_2$ symmetry in $\hat{H}^0$ mainly originates from the intralayer inversion-asymmetric structural deformation in the graphene layer adjacent to BN, which introduces spatially non-uniform modifications of the intralayer Hamiltonian. The gapped flat bands have finite Chern numbers. For TBG/BN with the magic twist angle, the SCHF ground states with $|ν|$ = 1-3 are all insulating with narrow MF gaps. When the flat conduction bands are filled, the gap at $ν$ = 1 is smaller than that at $ν$ = 3, suggesting that the nontrivial topological properties associated with the flat Chern bands are more likely to be observed at $ν= 3$. This is similar for negative $ν$ with empty valence bands. The dependence of the electronic structure of TBG/BN on positive $ν$ is roughly consistent with recent experimental observations.

cond-mat.mes-hall↗

arXiv2020

Pressure induced gap modulation and topological transitions in twisted bilayer and double bilayer graphene

We study the electronic and topological properties of fully relaxed twisted bilayer (TBG) and double bilayer (TDBG) graphene under perpendicular pressure. An approach has been proposed to obtain the equilibrium in-plane structural deformation and out-of-plane corrugation in moiré superlattices under pressure. We find that the in-plane relaxation becomes much stronger under higher pressure, while the corrugation height in each layer is maintained. The comparison between band structures of relaxed and rigid structures demonstrates that not only the gaps on the electron and hole sides ($Δ_e$ and $Δ_h$) are significantly underestimated without relaxation but also the detailed dispersions of the middle bands of rigid structures are rather different from those of relaxed systems. $Δ_e$ and $Δ_h$ in TBG reach maximum values around critical pressures with narrowest middle bands. Topological transitions occur in TDBG under pressure with the middle valence and conduction bands in one valley touching and their Chern numbers transferred to each other. The pressure can also tune the gap at the neutrality point of TDBG, which becomes closed for a pressure range and reopened under higher pressure. The behavior of electronic structure of supertlattices under pressure is sensitive to the twist angle $θ$ with the critical pressures generally increase with $θ$.

cond-mat.mes-hall↗

arXiv2020

Periodically Gated Bilayer Graphene as an Electronic Metamaterial

We study ballistic transport in periodically gated bilayer graphene as a candidate for a 2D electronic metamaterial. Our calculations use the equilibrium Green function formalism and take into account quantum corrections to charge density changes induced by a periodically modulated top gate voltage. Our results reveal an intriguing interference-like pattern, similar to that of a Fabry-Perot interferometer, in the resistance map as a function of the voltage $V_{BG}$ applied to the extended bottom gate and $V_{TG}$ applied to the periodic top gate.

cond-mat.mes-hall↗

arXiv2019

Electron-hole Hybridization in Bilayer Graphene

Band structure determines the motion of electrons in a solid, giving rise to exotic phenomena when properly engineered. Drawing an analogy between electrons and photons, artificially designed optical lattices indicate the possibility of a similar band modulation effect in graphene systems. Yet due to the fermionic nature of electrons, modulated electronic systems promise far richer categories of behaviors than those found in optical lattices. Here, we uncovered a strong modulation of electronic states in bilayer graphene subject to periodic potentials. We observed for the first time the hybridization of electron and hole sub-bands, resulting in local band gaps at both primary and secondary charge neutrality points. Such hybridization leads to the formation of flat bands, enabling the study of correlated effects in graphene systems. This work may also offer a viable platform to form and continuously tune Majorana zero modes, which is important to the realization of topological quantum computation.

cond-mat.mes-hall↗

arXiv2018

Is Twisted Bilayer Graphene Stable under Shear?

In twisted bilayer graphene (TBLG), extremely small deviations from the magic twist angle $θ_m{\approx}1.08^\circ$ change its electronic structure near the Fermi level drastically, causing a meV-wide flat band to appear or disappear. In view of such sensitivity to minute structural deformations, we investigate the combined effect of shear and atomic relaxation on the electronic structure. Using precise experimental data for monolayer and bilayer graphene as input in a simplified formalism for the electronic structure and elastic energy, we find TBLG near $θ_m$ to be unstable with respect to global shear by the angle $α{\approx}0.08^\circ$. In TBLG, the effect of shear on the electronic structure is as important as that of atomic relaxation. Under optimum global shear, calculated $θ_m$ is reduced by $0.04^\circ$ and agrees with the observed value.

cond-mat.mes-hall↗

arXiv2018

Minimum model for the electronic structure of twisted bilayer graphene and related structures

We introduce a minimum tight-binding model with only three parameters extracted from graphene and untwisted bilayer graphene. This model reproduces quantitatively the electronic structure of not only these two systems and bulk graphite near the Fermi level, but also that of twisted bilayer graphene including the value of the magic angle, at which bands at $E_F$ flatten without overlap and two gaps open, one above and one below $E_F$. The Hamiltonian is sufficiently transparent and flexible to be adopted to other twisted layered systems.

cond-mat.mes-hall↗

arXiv2018

Microscopic Mechanism of the Helix-to-Layer Transformation in Elemental Group VI Solids

We study the conversion of bulk Se and Te, consisting of intertwined a helices, to structurally very dissimilar, atomically thin two-dimensional (2D) layers of these elements. Our ab initio calculations reveal that previously unknown and unusually stable δ- and η-2D allotropes may form in an intriguing multi-step process that involves a concerted motion of many atoms at dislocation defects. We identify such a complex reaction path involving zipper-like motion of such dislocations that initiate structural changes. With low activation barriers <0.3 eV along the optimum path, the conversion process may occur at moderate temperatures. We find all one-dimensional (1D) and 2D chalcogen structures to be semiconducting.

cond-mat.mes-hall↗

arXiv2012

Flat Bands near Fermi Level of Topological Line Defects on Graphite

Flat bands play an important role in the study of strongly correlated phenomena, such as ferromagnetism, superconductivity, and fractional quantum Hall effect. Here we report direct experimental evidence for the presence of flat bands, close to the Fermi level, in one-dimensional topological defects on graphite seen as a pronounced peak in the tunnelling density of states. Our ab initio calculations indicate that the flat bands with vanishing Fermi velocity originate from sp2 dangling bonds (with antibonding nature) of undercoordinated carbon atoms at the edges of the defects. We further demonstrate that the presence of flat bands could be a universal behavior of 1D defects of graphene/graphite with undercoordinated carbon atoms at the edges of the defects.

cond-mat.mes-hall↗