Bilayer Ground State Research Articles

Nonvolatile ferromagnetism in bilayer CrI3 induced by a heterointerface

The realization of a ferromagnetism in bilayer $\mathrm{Cr}{\mathrm{I}}_{3}$, an antiferromagnetic semiconductor in its ground state, recently has gained extensive attention for both fundamental and applied research. Despite much effort devoted to this topic, so far its nonvolatile ferromagnetism has not been achieved. Here, we report the realization of nonvolatile ferromagnetism in bilayer $\mathrm{Cr}{\mathrm{I}}_{3}$. Using first-principles calculations, we reveal that when interfacing with monolayer MoSeTe, the magnetic ground state of bilayer $\mathrm{Cr}{\mathrm{I}}_{3}$ transforms from antiferromagnetism to ferromagnetism. Also, depending on the interfaced surface of monolayer MoSeTe, bilayer $\mathrm{Cr}{\mathrm{I}}_{3}$ can be ferromagnetic semiconducting or half-metallic. We unveil that the obtained ferromagnetic transformation in bilayer $\mathrm{Cr}{\mathrm{I}}_{3}$ can be attributed to the competition between orbital-dependent interlayer antiferromagnetic super-super-exchange and ferromagnetic super-super-exchange. Our work not only provides a promising way to engineer nonvolatile ferromagnetism in bilayer $\mathrm{Cr}{\mathrm{I}}_{3}$, but also renders bilayer $\mathrm{Cr}{\mathrm{I}}_{3}$ an appealing platform for developing two-dimensional spintronics.

Ground state of bilayer hα-silica: mechanical and electronic properties

The family of two-dimensional (2D) crystals was recently joined by silica, one of the most abundant resources on earth. So far two different polymorphs of this material, namely a tetrahedra-shaped monolayer and a fully saturated bilayer structure, have been synthesized on various metal substrates and their fascinating properties enable 2D silica to hold promise in nanoelectronic device applications. In this paper a new ground state of bilayer—AAr-stacking hα-silica—has been discovered by first principles calculations. The new structure is featured with a formation of Si–Si bonds between all sp3 hybridized SiO3 triangular pyramids, lying respectively in different silica layers, with an intrinsic rotational angle of about 12.5° along the out-of-plane Si–Si bond. Due to the doubled number of Si–Si bonds in the new structure, the system energy is lowered by nearly three times more than that reported recently in literature (0.8 eV) (Özçelik et al 2014 Phys. Rev. Lett. 112 246803), when compared with the single layer hα-silica. A mechanical property investigation shows that the AAr-stacking bilayer hα-silica possesses high in-plane stiffness and a negative Poisson’s ratio, which stems from the intrinsic rotational angle of the SiO3 triangular pyramids. Strikingly, the negative Poisson’s ratio evolves into positive at a critical tensile strain ϵ ≈ 1.2%. Such negative-to-positive evolvement is associated with the adaptation of the rotational angle to the applied strain and the structure transition into the nearby valley of the energy landscape. The detailed transition process has been thoroughly analyzed. The electronic properties of the new ground state are also calculated, along with their response to the external strain. Our new ground state structure introduces a new member to the family of 2D bilayer silica materials and is expected to facilitate experimental studies identifying the related structures and exploring further physical and chemical properties of nanoscale systems.

Bilayer mapping of the paired quantum Hall state: Instability toward anisotropic pairing

One of the most dominant candidates for the paired quantum Hall (QH) state at filling factor $\ensuremath{\nu}=5/2$ is the Moore-Read (MR) Pfaffian state. A salient problem, however, is that it does not occur exactly at the Coulomb interaction, but rather at a modified interaction, which favors particle-hole-symmetry breaking. In an effort to find a better state, in this work, we investigate the possible connection between the paired QH state and the antisymmetrized bilayer ground state, which is inspired by the intriguing identity that the MR Pfaffian state is entirely equivalent to the antisymmetrized projection of the bilayer QH state called the Halperin (331) state, which is valid at interlayer distance, $d$, roughly equal to the magnetic length, ${l}_{\mathrm{B}}$. Specifically, by using exact diagonalization in the torus geometry, we show that the exact $5/2$ state at a given Haldane pseudopotential variation is intimately connected with the antisymmetrized bilayer ground state at a corresponding $d/{l}_{\mathrm{B}}$ via one-to-one mapping, which we call the bilayer mapping. One of the most important discoveries in this work is that the paired QH state occurring at the Coulomb interaction is mapped onto the antisymmetrized bilayer ground state at $d\ensuremath{\gg}{l}_{\mathrm{B}}$, which is equivalent to the antisymmetrized product state of two composite fermion seas at quarter filling, not the MR Pfaffian state. While maintaining high overlap with the paired QH state, the antisymmetrized bilayer ground state at $d\ensuremath{\gg}{l}_{\mathrm{B}}$ exhibits an abrupt change under the influence of small anisotropy. This suggests that the paired QH state occurring at the Coulomb interaction might be susceptible to anisotropic instability, opening up the possibility of anisotropic ${p}_{x}$- or ${p}_{y}$-wave pairing instead of ${p}_{x}\ifmmode\pm\else\textpm\fi{}i{p}_{y}$-wave pairing in the MR Pfaffian/anti-Pfaffian state.

Competing ordered states in bilayer graphene

We use a perturbative renormalization group approach with short-range continuum model interactions to analyze the competition between isotropic gapped and anisotropic gapless ordered states in bilayer graphene, commenting specifically on the role of exchange and on the importance of spin and valley flavor degeneracy. By comparing the divergences of the corresponding susceptibilities, we conclude that this approach predicts gapped states for flavor numbers $N=1,2,4$. We comment briefly on other approaches which can be used to shed light on the bilayer ground state and on the related ground state of chiral (ABC) trilayers.

Distinguishing Spontaneous Quantum Hall States in Bilayer Graphene

Chirally stacked N-layer graphene with N≥2 is susceptible to a variety of distinct broken symmetry states in which each spin-valley flavor spontaneously transfers charge between layers. In mean-field theory, one of the likely candidate ground states for a neutral bilayer is the layer antiferromagnet that has opposite spin polarizations in opposite layers. In this Letter, we analyze how the layer antiferromagnet and other competing states are influenced by Zeeman fields that couple to spin and by interlayer electric fields that couple to layer pseudospin, and comment on the possibility of using Zeeman responses and edge state signatures to identify the character of the bilayer ground state experimentally.

Charge density excitations in bilayer graphene in high magnetic field

The charge-density excitations in bilayer graphene at the filling-factor ν Open image in new window 1 at small momenta are considered in the frame of the Hartree-Fock approximation. The presence of small asymmetry of graphene layers is included. The dependence of the magnetoplasmon energy on the bilayer ground state is shown. The energy splitting proportional to \( \sqrt H \) for the symmetric case with half-filled zero-energy levels is found both for bilayer and monolayer graphene.

COLLECTIVE EXCITATIONS IN SEMICONDUCTOR AND GRAPHENE BILAYERS IN HIGH MAGNETIC FIELD

The effect of space symmetry on magnetoplasmon charge density and spin-density excitations in a weakly tunnel coupled bilayer electron system at the filling factor ν = 4 is considered. The optic magnetoplasmon energy as well as the energy of intralayer magnetoexciton is not affected by space symmetry. Three other collective magnetoplasmon modes as well as three magnetoexciton modes are determined by relations between the asymmetry parameter, tunnel parameter, and the difference in energies of intralayer and interlayer magnetoexcitons. The charge-density excitations in bilayer graphene at the filling-factor ν ≪ 1 are considered. The presence of small asymmetry of graphene layers is taken into account. It is demonstarted that the magnetoplasmon energy depends on the bilayer ground state.

Optical and magneto-optical far-infrared properties of bilayer graphene

We analyze the spectroscopic features of bilayer graphene determined by the formation of pairs of low-energy and split bands in this material. We show that the inter-Landau-level absorption spectrum in bilayer graphene at high magnetic field is much denser in the far-infrared range than that in monolayer material and that the polarization dependence of its lowest-energy peak can be used to test the form of the bilayer ground state in the quantum Hall-effect regime.

Spin-flip excitations in bilayer quantum Hall ferromagnets

Bilayer quantum Hall systems at ν=1 are believed to have a ground state with spontaneous spin-polarization and spontaneous interlayer phase coherence. In this paper, we address the spectrum of spin-flip collective excitations using a generalized lattice spin model that appeals to the quantum Hall gap to justify neglect of charge fluctuations. We find that the long wavelength spin-flip excitation spectrum consists of the global Larmor spin-wave mode and a higher energy mode in which spin-waves in the two-layers are out of phase. We address the dependence of the energy splitting of the two modes on quantum fluctuations in the bilayer ground state.