Corresponding authors: W.J. (email: wji@ruc.edu.cn) and Z.Z. (email: zhong@nimte.ac.cn) † These authors contributed equally to this work.Diverse interlayer tunability of physical properties of two-dimensional layers mostly lies in the covalent-like quasi-bonding that is significant in electronic structures but rather weak for energetics. Such characteristics result in various stacking orders that are energetically comparable but may significantly differ in terms of electronic structures, e.g. magnetism. Inspired by several recent experiments showing interlayer antiferromagnetically coupled CrI3 bilayers, we carried out first-principles calculations for CrI3 bilayers. We found that the anti-ferromagnetic coupling results from a new stacking order with the C2/m space group symmetry, rather than the graphene-like one with 3 as previously believed. Moreover, we demonstrated that the intra-and interlayer couplings in CrI3 bilayer are governed by two different mechanisms, namely ferromagnetic super-exchange and direct-exchange interactions, which are largely decoupled because of their significant difference in strength at the strong-and weakinteraction limits. This allows the much weaker interlayer magnetic coupling to be more feasibly tuned by stacking orders solely. Given the fact that interlayer magnetic properties can be altered by changing crystal structure with different stacking orders, our work opens a new paradigm for tuning interlayer magnetic properties with the S2 freedom of stacking order in two dimensional layered materials.Introduction.-Magnetism in two dimensions has received growing attention since the two ferromagnetic monolayers, namely CrI3 [1] and Cr2Ge2Te6 [2], were successfully fabricated in 2017. The ferromagnetism in these two layers was believed to be stabilized by magnetic anisotropy as enhanced by spin-orbit coupling or external magnetic fields.Their Curie temperatures were up to ~50 K. Very recently, a room-temperature Tc were achieved in monolayer VSe2 [3] and MnSex [4], two members of the transition-metal dichalcogenides family. This shed considerable light on the search for high Tc ferromagnetic (FM) magnets. However, the tunability of magnetism has been emerging as a new challenge. The coupling strengths of two-dimensional (2D) materials are significantly different between intra-and inter-layer interactions. Such difference may offer diverse magnetic coupling mechanisms at strong and weak interacting limits. The interlayer magnetic coupling is of peculiar interest, as the effective coupling is relatively weak and confined within few atomic layers, which is much easier to model and more feasible to tune than strong and periodic couplings in three-dimension.Recent experiments demonstrated that the anti-ferromagnetic (AFM) interlayer order in bilayer CrI3 can be manipulated to a FM order by electric gating or reasonably large magnetic fields [5][6][7][8][9][10][11][12]. As a consequence, a magnetic tunnel junction with giant tunneling magnetoresistance values was achieved in bilayer CrI3 d...

Manipulating physical properties using the spin degree of freedom constitutes a major part of modern condensed matter physics and is a key aspect for spintronics devices. Using the newly discovered two-dimensional van der Waals ferromagnetic CrI as a prototype material, we theoretically demonstrated a giant magneto band-structure (GMB) effect whereby a change of magnetization direction significantly modifies the electronic band structure. Our density functional theory calculations and model analysis reveal that rotating the magnetic moment of CrI from out-of-plane to in-plane causes a direct-to-indirect bandgap transition, inducing a magnetic field controlled photoluminescence. Moreover, our results show a significant change of Fermi surface with different magnetization directions, giving rise to giant anisotropic magnetoresistance. Additionally, the spin reorientation is found to modify the topological states. Given that a variety of properties are determined by band structures, our predicted GMB effect in CrI opens a new paradigm for spintronics applications.

The discovery of infinite layer nickelate superconductor marks the new era in the field of superconductivity. In the rare-earth (Re) nickelates ReNiO2, although the Ni is also of d 9 electronic configuration, analogous to Cu d 9 in cuprates, whether electronic structures in infinite-layer nickelate are the same as cuprate and possess the single band feature as well are still open questions. To illustrate the electronic structure of rare-earth infinite-layer nickelate, we perform first principle calculations of LaNiO2 and NdNiO2 compounds and compare them with that of CaCuO2 using hybrid functional method together with Wannier projection and group symmetry analysis. Our results indicate that the Ni-dx 2 -y 2 in the LaNiO2 has weak hybridization with other orbitals and exhibits characteristic single band feature, whereas in NdNiO2,the Nd-f orbital hybridizes with Ni-dx 2 -y 2 and is a non-negligible ingredient for transport and even high-temperature superconductivity. Given that the Cu-dx 2 -y 2 in cuprate strongly hybridizes with O-2p, the calculated band structures of nickelate imply some new band characters which is worth to gain more attentions.

Molybdenum disulfide (MoS 2 ) has been proved to be a potential electromagnetic wave (EMW) absorber. However, the limited EMW attenuation mechanisms and conductivity have always been recognized as the major challenges impeding their further developments. In this study, a new dielectric tuning strategy giving rise to high EMW attenuation performance by manipulating phase content (with 0, 24, 50, and 100 wt% 1T phase) toward MoS 2 is demonstrated. The greatly introduced 2H/1T interfaces facilitate the dipole distribution dynamics, and the metal-semiconductor mixed phase enhances the electron transfer ability. Benefiting from the structural merits, the MoS 2 with 50 wt% 1T absorber delivers the maximum reflection loss of −45.5 dB and effective absorbing bandwidth of ≈3.89 GHz, corresponding to nearly ten times higher than that of pure 2H counterpart. Moreover, the Computer Simulation Technology (CST) simulation and Lorentz transmission electron microscope are performed to visualize the structural advantages of MoS 2 absorbers with mixed 2H/1T phases. By manipulating the phase compositions, this study provides a deep understanding and opens an avenue in developing efficient and high performance transition metal dichalcogenides (e.g., WS 2 , MoSe 2 , and WSe 2 ) absorbers.

The thermoelectric properties of MoS2 armchair nanoribbons with different width are studied by using first-principles calculations and Boltzmann transport theory, where the relaxation time is predicted from deformation potential theory. Due to the dangling bonds at the armchair edge, there is obvious structure reconstruction of the nanoribbons which plays an important role in governing the electronic and transport properties. The investigated armchair nanoribbons are found to be semiconducting with indirect gaps, which exhibit interesting width-dependent oscillation behavior. The smaller gap of nanoribbon with width N = 4 leads to a much larger electrical conductivity at 300 K, which outweighs the relatively larger electronic thermal conductivity when compared with those of N = 5, 6. As a results, the room temperature ZT values can be optimized to 2.7 (p-type) and 2.0 (n-type), which significantly exceed the performance of most laboratory results reported in the literature

As a new carbon allotrope, the recently fabricated graphdiyne has attracted much attention due to its interesting two-dimensional character. Here we demonstrate by multiscale computations that, unlike graphene, graphdiyne has a natural band gap, and simultaneously possess high electrical conductivity, large Seebeck coefficient, and low thermal conductivity. At a carrier concentration of 2.74×10 11 cm -2 for holes and 1.62×10 11 cm -2 for electrons, the room temperature ZT value of graphdiyne can be optimized to 3.0 and 4.8, respectively, which makes it an ideal system to realize the concept of "phonon-glass and electron-crystal" in the thermoelectric community.

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Few-layer black phosphorus has recently emerged as a promising candidate for novel electronic and optoelectronic device. Here we demonstrate by first-principles calculations and Boltzmann theory that, black phosphorus could also have potential thermoelectric applications and a fair ZT value of 1.1 can be achieved at elevated temperature. Moreover, such value can be further increased to 5.4 by substituting P atom with Sb atom, giving nominal formula of P 0.75 Sb 0.25 . Our theoretical work suggests that high thermoelectric performance can be achieved without using complicated crystal structure or seeking for low-dimensional systems.