arXiv subjects

Junjie He

Publications and source records attributed to Junjie He.

49 records · Page 3Linked to original sources

arXiv2022

Ultrafast optically induced magnetic state transition in 2D antiferromagnets

Manipulating spin in antiferromagnetic (AFM) materials has great potential in AFM opto-spintronics. Laser pulses can induce a transient ferromagnetic (FM) state in AFM metallic systems, but have never been proven in two-dimensional (2D) AFM semiconductors and related van der Waals (vdW) heterostructures. Here, using 2D vdW heterostructures of FM MnS2 and AFM MXenes as prototypes, we investigated optically induced interlayer spin transfer dynamics based on the real-time time-dependent density functional theory (rt-TDDFT). We observed that laser pulses induce significant spin injection and the interfacial atom-mediated spin transfer from MnS2 to Cr2CCl2. In particular, we first demonstrated the transient FM state in semiconducting AFM/FM heterostructures during photoexcited processes. Because the proximity magnetism breaks the magnetic symmetry of Cr2CCl2 in heterostructures. Our results provide the microscopic understanding for optically controlled interlayer spin dynamics in 2D magnetic heterostructures and open a new way to manipulate magnetic orders in ultrafast opto-spintronics.

cond-mat.mtrl-sci↗

arXiv2021

Machine-learning-based identification for initial clustering structure in relativistic heavy-ion collisions

$α$-clustering structure is a significant topic in light nuclei. A Bayesian convolutional neural network (BCNN) is applied to classify initial non-clustered and clustered configurations, namely Woods-Saxon distribution and three-$α$ triangular (four-$α$ tetrahedral) structure for $^{12}$C ($^{16}$O), from heavy-ion collision events generated within a multi-phase transport (AMPT) model. Azimuthal angle and transverse momentum distributions of charged pions are taken as inputs to train the classifier. On multiple-event basis, the overall classification accuracy can reach $95\%$ for $^{12}$C/$^{16}$O + $^{197}$Au events at $\sqrt{S_{NN}} =$ 200 GeV. With proper constructions of samples, the predicted deviations on mixed samples with different proportions of both configurations could be within $5\%$. In addition, setting a simple confidence threshold can further improve the predictions on the mixed dataset. Our results indicate promising and extensive possibilities of application of machine-learning-based techniques to real data and some other problems in physics of heavy-ion collisions.

hep-ph↗

arXiv2021

Doping isolated one-dimensional antiferro-magnetic semiconductor Vanadium tetrasulfide ($VS_4$) nanowires with carriers induces half-metallicity

Quasi one-dimensional (1D) vanadium tetrasulfide ($VS_4$) nanowires (NWs) are synthetic semiconductors which combine with each other through Van der Waals interactions to form bulk phases. However, the properties of these individual nanowires remain unknown. Nevertheless, our calculations of their stability indicate that $VS_4$) NWs can be separated from their bulk structures. Accordingly, we theoretically investigated the geometrical, electronic, and magnetic properties of bulk phase and isolated $VS_4$ NWs. Our results indicate that both bulk phase and isolated $VS_4$ NWs are semiconductors with band gaps of 2.24 and 2.64 eV, respectively, and that they prefer the antiferromagnetic (AFM) ground state based on DFT calculations. These calculations also suggested that isolated $VS_4$ NWs show half-metallic antiferromagnetism upon electron and hole doping because carrier doping splits the spin degeneracy to induce local spin polarisation. As a result, spin polarisation currents in isolated $VS_4$ NWs can be manipulated with locally applied gate voltage. Therefore, these 1D AFM materials have a high potential for advancing both fundamental research and spintronic applications because they are more resistant to magnetic perturbation than their 1D ferromagnetic counterparts.

cond-mat.mtrl-sci↗

arXiv2020

Remarkably enhanced Curie temperature in monolayer CrI3 by hydrogen and oxygen adsorption: A first-principles calculations

wo-dimensional (2D) materials unique properties and their promising applications in post-silicon microelectronics have attracted significant attention in the past decade. Recently, ferromagnetic order with out-of-plane easy axis in a monolayer of CrI3 has been observed and reported, with a Curie temperature of 45 K. Here we study, using density functional theory (DFT) calculations, how hydrogen and oxygen adsorption affects the structural, electronic, and magnetic properties of a CrI3 monolayer. Our results show that while the structure remains almost unchanged by the adsorption of hydrogen, adsorption of oxygen completely distorts it. We have also found that both the adsorption of hydrogen and oxygen atoms significantly influences the electronic and magnetic properties of the monolayer. While hydrogen quenches the magnetic moments of Cr atoms, oxygen introduces an impurity band in the gap. Interestingly, we find a strong enhancement of the Curie temperature by full hydrogenation, while the results are not conclusive for O. This result suggests a simple and effective approach to manipulate the electronic and magnetic properties of 2D magnets for spintronics applications.

cond-mat.mtrl-sci↗

arXiv2020

Optically driven ultrafast magnetic order transitions in two-dimensional ferrimagnets

Laser-induced switching and manipulation of the spins in magnetic materials are of great interest to revolutionize future magnetic storage technology and spintronics with fastest speed and least power dissipative. Inspired by the recent discovery of intrinsic two-dimensional (2D) magnets, which provide unique platform to explore the new phenomenon for light-control magnetism in the 2D limit, we propose to realize light can efficiently tune magnetic properties of 2D ferrimagnets in early time. Here, using the 2D ferrimagnetic MXenes as prototype systems, our real-time density functional theory (TDDFT) simulation show that laser pulses can directly induce ultrafast spin-selective charge transfer between two magnetic sublattices on a few femtoseconds, and further generate dramatic changes in the magnetic structure of these MXenes, including a magnetic order transition from ferrimagnetic (FiM) to transient ferromagnetic (FM). The microscopic mechanism underpinning this ultrafast switching of magnetic order in MXenes is governed by optically induced inter-site spin transfer (OISTR) effect, which theoretically enables the ultrafast direct optical manipulation of the magnetic state in MXenes-based materials. Our results open new opportunities to optically manipulate the spin in 2D magnets.

cond-mat.mes-hall↗

arXiv2020

Clustering structure effect on Hanbury-Brown-Twiss correlation in $^{12}$C + $^{197}$Au collisions at 200 GeV}

Through $^{12}$C + $^{197}$Au collisions at $\sqrt{s_{NN}} =$ 200 GeV using a multiphase transport (AMPT) model, the azimuthal angle dependences of the Hanbury Brown-Twiss (HBT) radii relative to the second- and third-order participant plane from $π$-$π$ correlations are discussed. Three initial geometric configurations of $^{12}$C, namely three-$α$-cluster triangle, three-$α$-cluster chain and Woods-Saxon distribution of nucleons, are taken into account, and their effects on the correlations are investigated. The ratio of the third- to the second-order HBT radii $R_{o(s),3}^2/R_{o(s),2}^2$ is shown to be a clear probe for three configurations. In addition, this work presents the hadronic rescattering time evolution of the azimuthally dependent HBT radii. From the present study, one can learn that the HBT correlation from identical particles at freeze-out is able to provide the information of different initial configurations as collective flow proposed before.

nucl-th↗

arXiv2019

Control of spintronic and electronic properties of bimetallic and vacancy-ordered vanadium carbide MXenes via surface functionalization

MXenes are 2D transition metal carbides with high potential for overcoming limitations of conventional two-dimensional electronics. In this context, various MXenes have shown magnetic properties suitable for applications in spintronics, yet the number of MXenes reported so far is far smaller than their parental MAX phases. Therefore, we have studied the structural, electronic and magnetic properties of bimetallic and vacancy-ordered MXenes derived from a new $(V_{2/3}Zr_{1/3})_2AlC $ MAX phase to assess whether MXene exfoliation would return stable magnetic materials. In particular, we have investigated the properties of pristine and surface-functionalized $(V_{2/3}Zr_{1/3})_2CX_2$ bimetallic and $(V_{2/3} \square_{1/3})_2CX2$ vacancy-ordered MXenes with X = O, F and OH. Our density functional theory (DFT) calculations showed that modifying the MXene stoichiometry and/or MXene surface functionalization changes MXene properties. After testing all possible combinations of metallic motifs and functionalization, we identified $V_{2/3}Zr_{1/3})_2CX_2$, $(V_{2/3}\square_{1/3})_2CF_2$ and $(V_{2/3}\square_{1/3})_2C(OH)_2$ as stable structures. Among them, $(V_{2/3}Zr_{1/3})_2CO_2$ MXene is predicted to be an FM intrinsic half-semiconductor with a remarkably high Curie temperature ($T_C$)) of 270 K. The $(V_{2/3}Zr_{1/3})_2C(OH)_2$ MXene exhibits a rather low work function (WF) (1.37 eV) and is thus a promising candidate for ultra-low work function electron emitters. Conversely, the $(V_{2/3}\square_{1/3})_2CF_2$ MXene has a rather high WF and hence can be used as a hole injector for Schottky-barrier-free contact applications. Overall, our proof-of-concept study shows that theoretical predictions of MXene exfoliation and properties support further experimental research towards developing spintronics devices.

cond-mat.mtrl-sci↗

arXiv2019

Three-dimensional graphene networks modified with acetylenic linkages for high-performance optoelectronics and Li-ion battery anode material

Searching for three-dimensional(3D) semiconducting carbon allotropes with proper bandgaps and excellent optoelectronic properties is always the chasing goal for the new emerging all-carbon optoelectronics. On the other side, 3D carbon materials have also been recognized as promising anode materials superior to commercialized graphite in Li-ion batteries (LIBs). Here, using first-principles calculations, we propose two novel 3D carbon allotropes through acetylenic linkages modification of two structurally intimately correlated 3D carbon structures - carbon kagome lattice(CKL)and interpenetrated graphene network(IGN). The modified CKL is a truly direct-gap semiconductor and possibly possesses the strongest optical transition coefficient amongst of all semiconducting carbon allotropes. The suitable bandgap and small effective masses also imply it can be a good electron transport material(ETM) for perovskite solar cells. As for the modified IGN, it is a topological nodal-line semimetal and shows greatly enhanced specific capacity as anode materials in LIBs comparing to that of IGN. Our work not only finds two new 3D carbon phases with fabulous physical and chemical properties for high-performance optoelectronics and Li-ion anode material, we also offer a fresh view to create various carbon structures with versatile properties.

cond-mat.mtrl-sci↗

arXiv2019

Two-dimensional honeycomb borophene oxide: Strong anisotropy and nodal loop transformation

The search for topological semimetals is mainly focused on heavy-element compounds as following the footsteps of previous research on topological insulators, with less attention on light-element materials. However, the negligible spin orbit coupling with light elements may turn out to be beneficial for realizing topological band features.Here, using first-principles calculations, we propose a new two-dimensional light-element material-the honeycomb borophene oxide (h-B2O), which has nontrivial topological properties.The proposed structure is based on the recently synthesized honeycomb borophene on Al (111) substrate [Sci. Bull. 63, 282 (2018)]. The h-B2O monolayer is completely flat, unlike the oxides of graphene or silicene. We systematically investigate the structural properties of h-B2O, and find that it has very good stability and exhibits significant mechanical anisotropy. Interestingly, the electronic band structure of h-B2O hosts a nodal loop centered around the Y point in the Brillouin zone, protected by the mirror symmetry. Furthermore, under moderate lattice strain, the single nodal loop can be transformed into two loops, each penetrating through the Brillouin zone. The loops before and after the transition are characterized by different Z*Z topological indices. Our work not only predicts a new two-dimensional material with interesting physical properties, but also offers an alternative approach to search for new topological phases in 2D light-element systems.

cond-mat.mtrl-sci↗

arXiv2016

NiCl3 Monolayer: Dirac Spin-Gapless Semiconductor and Chern Insulator

The great obstacle for practical applications of the quantum anomalous Hall (QAH) effect is the lack of suitable QAH materials (Chern insulators) with large non-trivial band gap, room-temperature magnetic order and high carrier mobility. The Nickle chloride (NiCl3) monolayer characteristics are investigated herein using first-principles calculations. It is reported that NiCl3 monolayers constitute a new class of Dirac materials with Dirac spin-gapless semiconducting and high-temperature ferromagnetism (~400K). Taking into account the spin-orbit coupling, the NiCl3 monolayer becomes an intrinsic insulator with a large non-trivial band gap of ~24 meV, corresponding to an operating temperature as high as ~280K at which the quantum anomalous Hall effect could be observed. The calculated large non-trivial gap, high Curie temperature and single-spin Dirac states reported herein for the NiCl3 monolayer lead us to propose that this material give a great promise for potential realization of a near-room temperature QAH effect and potential applications in spintronics. Last but not least the calculated Fermi velocities of Dirac fermion of about 4x105 m/s indicate very high mobility in NiCl3 monolayers.

cond-mat.mtrl-sci↗

arXiv2013

Half-Semiconductor antiferromagnets and Spin-Gapless-Semiconductor antiferromagnets

We propose a concept of half-semiconductor antiferromagnets in which both spin-polarized valence and conduction bands belong to the same spin channel with completely compensated spontaneous magnetization. Using density functional theory plus Hubbard U (DFT+U) methods, we find a viable approach to achieve the half-semiconductor antiferromagnets through the transition metal (TM) Fe and Cr codoped boron nitride(BN) sheet. Moreover, spin gapless semiconductor antiferromagnets with zero magnetic moment are also achieved in such systems.

cond-mat.mtrl-sci↗

arXiv2013

Magnetic Exchange Coupling and Anisotropy of 3d Transition-Metal Nanowire on the Surface of Graphyne Sheet

Using density functional theory plus Hubbard-U (DFT+U) approach, we find that quasi one-dementation(1D) 3d transition metal(TM) zigzag nanowire can be constructed by TM adsorbed on the surface of graphyne sheet. The results show that the TM exchange coupling of the zigzag nanowire mediated by sp hybridized carbon atoms gives rise to long range ferromagnetic order except for Cr with anti-ferromagnetic order. The magnetic exchange interaction of TM chains follows like-Zener's p_z-d exchange mechanism: the coexistence of out-of plane p_z-d and in-plane p_x-y-d exchange. Finally, by including spin-orbit interactions within spin-DFT, we calculate the magnetic anisotropy energy of the TM chain on graphyne. We find that the Fe and Co chains show considerable magnetic anisotropy energy (MAE) and orbital magnetic moment. The easy axis of V, Cr, Mn and Fe chains is perpendicular to the surface, whereas the easy axis of Co lies in the surface. Moreover, only V chain shows relatively larger in-plane anisotropy. Our results open a new route to realize the applications of graphyne in spintronics.

cond-mat.mtrl-sci↗

arXiv2013

Magnetic Properties of Single Transition-Metal Atom Absorbed Graphdiyne and Graphyne Sheet

The electronic and magnetic properties of single 3d transition-metal(TM) atom (V, Cr, Mn, Fe, Co, and Ni) adsorbed graphdiyne (GDY) and graphyne (GY) are systematically studied using first-principles calculations within the density functional framework. We find that the adsorption of TM atom not only efficiently modulates the electronic structures of GDY/GY system, but also introduces excellent magnetic properties, such as half-metal and spin-select half-semiconductor. Such modulation originates from the charge transfer between TM adatom and the GDY/GY sheet as well as the electron redistribution of the TM intra-atomic s, p, and d orbitals. Our results indicate that the TM adsorbed GDY/GY are excellent candidates for spintronics.

cond-mat.mtrl-sci↗