arXiv subjects

Xieyu Zhou

Publications and source records attributed to Xieyu Zhou.

7 recordsLinked to original sources

arXiv2022

Strain Effect on Air-Stability of Monolayer CrSe2

The discovery of two dimensional (2D) magnetic materials has brought great research value for spintronics and data storage devices. However, their air-stability as well as the oxidation mechanism has not been unveiled, which limits their further applications. Here, by first-principles calculations, we carried out a detailed study on the oxidation process of monolayer CrSe2 and biaxial tensile strain effect. We found dissociation process of O2 on pristine CrSe2 sheet is an endothermic reaction with a reaction energy barrier of 0.53 eV, indicating its thermodynamics stability. However, such a process becomes exothermic under a biaxial tensile strain reaching 1%, accompanying with a decreased reaction barrier, leading to reduced stability. These results manifest that in-plane strain plays a significant role in modifying air-stability in CrSe2 and shed considerable light on searching appropriate substrate to stabilize 2D magnetic materials.

cond-mat.mtrl-sci↗

arXiv2020

Bethe-Slater-curve-like behavior and interlayer spin-exchange coupling mechanisms in two-dimensional magnetic bilayers

Layered magnets have recently received tremendous attention, however, spin-exchange coupling mechanism across their interlayer regions is yet to be revealed. Here, we report a Bethe-Slater-curve (BSC) like behavior in nine transition metal dichalcogenide bilayers (MX2, M=V, Cr, Mn; X=S, Se, Te) and established interlayer spin-exchange coupling mechanisms at their van der Waals gaps using first-principle calculations. The BSC-like behavior offers a distance-dependent interlayer anti-ferromagnetic (AFM) to ferromagnetic (FM) transition. This phenomenon is explained with the spin-exchange coupling mechanisms established using bilayer CrSe2 as a prototype in this work. The Se pz wavefunctions from two adjacent interfacial Se sublayers overlap at the interlayer region. The spin alignment of the region determines interlayer magnetic coupling. At a shorter interlayer distance, Pauli repulsion at the overlapped region dominates and thus favors anti-parallel oriented spins leading to interlayer AFM. For a longer distance, kinetic energy gain of polarized electrons across the bilayer balances the Pauli repulsion and the bilayer thus prefers an interlayer FM state. In light of this, the AFM-FM transition is a result of competition between Pauli and Coulomb repulsion and kinetic energy gain. All these results open a new route to tune interlayer magnetism and the revealed spin-exchange coupling mechanisms are paramount additions to those previously established ones.

cond-mat.mtrl-sci↗

arXiv2020

Direct Measurement of the Electronic Structure and band gap nature of atomic-layer-thick 2H-MoTe2

The millimeter sized monolayer and bilayer 2H-MoTe2 single crystal samples are prepared by a new mechanical exfoliation method. Based on such high-quality samples, we report the first direct electronic structure study on them, using standard high resolution angle-resolved photoemission spectroscopy (ARPES). A direct band gap of 0.924eV is found at K in the rubidium-doped monolayer MoTe2. Similar valence band alignment is also observed in bilayer MoTe2,supporting an assumption of a analogous direct gap semiconductor on it. Our measurements indicate a rather large band splitting of 212meV at the valence band maximum (VBM) in monolayer MoTe2, and the splitting is systematically enlarged with layer stacking, from monolayer to bilayer and to bulk. Meanwhile, our PBE band calculation on these materials show excellent agreement with ARPES results. Some fundamental electronic parameters are derived from the experimental and calculated electronic structures. Our findings lay a foundation for further application-related study on monolayer and bilayer MoTe2.

cond-mat.mtrl-sci↗

arXiv2019

Phase-controllable growth of ultrathin 2D magnetic FeTe crystals

Two-dimensional (2D) magnets with intrinsic ferromagnetic/antiferromagnetic (FM/AFM) ordering are highly desirable for future spintronics devices. However, the synthesis of 2D magnetic crystals, especially the direct growth on SiO2/Si substrate, is just in its infancy. Here, we report a chemical vapor deposition (CVD)-based rational growth approach for the synthesis of ultrathin FeTe crystals with controlled structural and magnetic phases. By precisely optimizing the growth temperature (Tgrowth), FeTe nanoplates with either layered tetragonal or non-layered hexagonal phase can be controlled with high-quality. The two controllable phases lead to square and triangular morphologies with a thickness down to 3.6 and 2.8 nm, respectively. More importantly, transport measurements reveal that tetragonal FeTe is antiferromagnetic with a Neel temperature (TN) about 71.8 K, while hexagonal FeTe is ferromagnetic with a Curie temperature (TC) around 220 K. Theoretical calculations indicate that the ferromagnetic order in hexagonal FeTe is originated from a concomitant lattice distortion and the spin-lattice coupling. This study represents a major step forward in the CVD growth of 2D magnetic materials on SiO2/Si substrates and highlights on their potential applications in the future spintronic devices.

cond-mat.mtrl-sci↗

arXiv2019

A family of high-temperature ferromagnetic monolayers with locked spin-dichroism-mobility anisotropy: MnNX and CrCX (X=Cl, Br, I; C=S, Se, Te)

Two-dimensional magnets have received increasing attention since Cr2Ge2Te6 and CrI3 were experimentally exfoliated and measured in 2017. Although layered ferromagnetic metals were demonstrated at room temperature, a layered ferromagnetic semiconductor with high Curie temperature (Tc) is yet to be unveiled. Here, we theoretically predicted a family of high Tc ferromagnetic monolayers, namely MnNX and CrCX (X=Cl, Br and I; C=S, Se and Te). Their Tc values were predicted from over 100 K to near 500 K with Monte Carlo simulations using an anisotropic Heisenberg model. Eight members among them show semiconducting bandgaps varying from roughly 0.23 to 1.85 eV. These semiconducting monolayers also show extremely large anisotropy, i.e. ~101 for effective masses and ~102 for carrier mobilities, along the two in-plane lattice directions of these layers. Additional orbital anisotropy leads to a spin-locked linear dichroism, in different from previously known circular and linear dichroisms in layered materials. Together with the mobility anisotropy, it offers a spin-, dichroism- and mobility-anisotropy locking. These results manifest the potential of this 2D family for both fundamental research and high performance spin-dependent electronic and optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2018

Two ultra-stable novel allotropes of Tellurium few-layers

At least four two- or quasi-one- dimensional allotropes and a mixture of them were theoretically predicted or experimentally observed for low-dimensional Te, namely the α, \b{eta}, γ, δ and chiral-α+δ phases. Among them the γ and α phases were found the most stable phases for monolayer and thicker layers, respectively. Here, we found two novel low-dimensional phases, namely the ε and ζ phases. The ζ phase is over 29 meV/Te more stable than and the ε phase shows comparable stability with the most stable monolayer γ phase. The energetic difference between the ζ and α phases reduces with respect to the increased layer thickness and vanishes at the four-layer (12-sublayer) thickness, while this thickness increases under change doping. Both ε and ζ phases are metallic chains and layers, respectively. The ζ phase, with very strong interlayer coupling, shows quantum well states in its layer-dependent bandstructures. These results provide significantly insight into the understanding of polytypism in Te few-layers and may boost tremendous studies on properties of various few-layer phases.

cond-mat.mtrl-sci↗

arXiv2018

Layer- and doping-tunable long range ferromagnetic order-ing in two-dimensional CrS2

Interlayer coupling is of vital importance for manipulating physical properties, e.g. electronic bandgap, in two-dimensional materials. However, tuning magnetic proper-ties in these materials is yet to be addressed. Here, we found a striped antiferromag-netic (sAFM) to ferromagnetic (FM) transition undergoing from monolayer to bilayer and thicker CrS2. This transition is attributed to charge sharing of interlayer S atoms and its resulting charge transfer from Cr d eg to t2g orbitals. The transferred charge reduces a portion of Cr4+ to Cr3+, which enhances the double-exchange mechanism favoring FM rather than sAFM ordering. Doping of electrons shares the same effect with stacking induced charge sharing. Therefore, charge doping could either manip-ulate the magnetic ordering between sAFM and FM or tune CrS2 between p- and n-doped magnetic semiconductors. We also proposed several prototype devices ena-bling to use external electric field for manipulating magnetic orderings of CrS2 layers. These results manifest the role of interlayer coupling in modifying magnetic proper-ties of layered materials

cond-mat.mtrl-sci↗