arXiv subjects

Jijun Zhao

Publications and source records attributed to Jijun Zhao.

At least 37 records · Page 2Linked to original sources

arXiv2023

Room-temperature ferromagnetism in epitaxial bilayer FeSb/SrTiO3(001) terminated with a Kagome lattice

Two-dimensional (2D) magnets exhibit unique physical properties for potential applications in spintronics. To date, most 2D ferromagnets are obtained by mechanical exfoliation of bulk materials with van der Waals interlayer interactions, and the synthesis of single or few-layer 2D ferromagnets with strong interlayer coupling remains experimentally challenging. Here, we report the epitaxial growth of 2D non-van der Waals ferromagnetic bilayer FeSb on SrTiO3(001) substrates stabilized by strong coupling to the substrate, which exhibits in-plane magnetic anisotropy and a Curie temperature above 300 K. In-situ low-temperature scanning tunneling microscopy/spectroscopy and density-functional theory calculations further reveal that a Fe Kagome layer terminates the bilayer FeSb. Our results open a new avenue for further exploring emergent quantum phenomena from the interplay of ferromagnetism and topology for application in spintronics.

cond-mat.mtrl-sci↗

arXiv2023

Stationary Measures of Space-Inhomogeneous Three-State Quantum Walks on Line: Revisited

Of a quantum walk, its stationary measures play an important role in understanding its evolution behavior. In this paper we investigate stationary measures of two models of space-inhomogeneous three-state quantum walk on the line. By using the method of reduced matrix, we find out stationary measures of the two models under some mild conditions. Our results generalize the corresponding ones existing in the literature.

quant-ph↗

arXiv2023

Excellent HER and OER Catalyzing Performance of Se-vacancies in Defects-engineering PtSe2: From Simulation to Experiment

Facing with grave climate change and enormous energy demand, catalyzer gets more and more important due to its significant effect on reducing fossil fuels consumption. Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by water splitting are feasible ways to produce clean sustainable energy. Here we systematically explored atomic structures and related STM images of Se defects in PtSe2. The equilibrium fractions of vacancies under variable conditions were detailly predicted. Besides, we found the vacancies are highly kinetic stable, without recovering or aggregation. The Se vacancies in PtSe2 can dramatically enhance the HER performance, comparing with, even better than Pt(111). Beyond, we firstly revealed that PtSe2 monolayer with Se vacancies is also a good OER catalyst. The excellent bipolar catalysis of Se vacancies were further confirmed by experimental measurements. We produced defective PtSe2 by direct selenization of Pt foil at 773 K using a CVD process. Then we observed the HER and OER performance of defective PtSe2 is much highly efficient than Pt foils by a series of measurements. Our work with compelling theoretical and experimental studies indicates PtSe2 with Se defects is an ideal bipolar candidate for HER and OER.

cond-mat.mtrl-sci↗

arXiv2023

Robust Sandwiched B/TM/B Structures by Metal Intercalating into Bilayer Borophene Leading to Excellent Hydrogen Evolution Reaction

Bilayer borophene, very recently synthesized on Ag and Cu, possesses extremely flat large surface and excellent conductivity. Besides, the van der Waals gap of bilayer borophene can be intercalated by metal atoms, thereby tailoring the properties of bilayer borophene. Herein, we propose that sandwiched B/TM/B (TM=Co, Ni, Cu, Pd) could be a new 2D formation by transiton metal atoms intercalated into bilayer borophene network, it is quiet robust with both energetic, structural and thermal stability, and exhibits heat resistance of at least 1300 K. Besides, it is novel platform for electrocatalytic hydrogen evolution reaction (HER). The interecalation metal atom serves as single-atomic catalyst, which acting the nonmetal boron layers. Beyond that, the transtion metal is protected by outside boron layers from being corroded by acidic/alkaline solution. B/Cux/B, B/Pdx/B and B/Alx/B with different metal coverage exhibit defect-independent extremely low HER free energy in the range of -0.162 ~ 0.179 eV, -0.134 ~ 0.183 eV and -0.082 ~ 0.086 eV which are comparable to noble metal Pt. Combining excellent conduction, high structural and thermal stability, low resistance to intercalated behaviour, effortless water splitting process, excellent defect-independent catalytic performance, cheapness and abundance of raw materials, free of corrodation, 2D sandwiched B/TM/B (TM=Co, Ni, Cu, Pd) is believed to promising for electrocatalytic HER applications.

cond-mat.mtrl-sci↗

arXiv2023

Rich structural polymorphism of monolayer C60 from cluster rotation

The recent experimental fabrication of monolayer and few-layer C60 polymers paves the way for synthesizing two-dimensional cluster-assembled materials. Compared to atoms with the SO(3) symmetry, clusters as superatoms (e.g., C60) have an additional rotational degree of freedom, greatly enriching the phase spaces of superatom-assembled materials. Using first-principles calculations, we find the energy barriers of cluster rotation in quasi-tetragonal monolayer C60 structures are rather low (about 10 meV/atom). The small rotational energy barriers lead to a series of tetragonal C60 polymorphs with energies that are close to the experimental quasi-tetragonal (expt-qT) phase. Similarly, several dynamically stable quasi-hexagonal monolayer C60 structures are found to have energies within 7 meV/atom above the experimental quasi-hexagonal phase. Our calculations demonstrate photo-excited electron-hole pairs and electrostatic doping of electrons can effectively modulate the relative energies of quasi-tetragonal C60 polymorphs. Particularly, the unstable monolayer expt-qT phase becomes dynamically stable when it is electrostatically doped with electrons. In contrast, the relative energies between different quasi-hexagonal polymorphs are insensitive to electrostatic doping of electrons.

cond-mat.mtrl-sci↗

arXiv2023

Universal enhancement of vacancy diffusion by Mn inducing anomalous Friedel oscillation in concentrated solid-solution alloys

We present a proof-of-principle demonstration of a universal law for the element Mn, which greatly enhances vacancy diffusion through an anomalous Friedel Oscillation effect in a series of Ni-based concentrated solid-solution alloys, regardless of the type of atom involved. The antiferromagnetic element Mn possesses a unique half-filled 3d electron structure, creating split virtual bound states near the Fermi energy level and producing a large local magnetic moment after vacancy formation. The resultant electron spin oscillations reduce the number of electrons involved in charge density oscillations, destroying charge screening and lowering potential interaction at the saddle point between the vacancy and diffusing atom. This ultimately facilitates vacancy diffusion by reducing energy level variations of conduction band electrons during the diffusion process. These findings offer valuable insights into atom diffusion mechanisms and open up new avenues for manipulating defect properties through unique element design, thereby enabling the creation of high-performance alloys in a broad range of fields.

cond-mat.mtrl-sci↗

arXiv2022

Giant excitonic effects in bulk vacancy-ordered double perovskites

Using first-principles GW plus Bethe-Salpeter equation calculations, we identify anomalously strong excitonic effects in several vacancy-ordered double perovskites Cs2MX6 (M = Ti, Zr; X = I, Br). Giant exciton binding energies about 1 eV are found in these moderate-gap, inorganic bulk semiconductors, pushing the limit of our understanding of electron-hole (e-h) interaction and exciton formation in solids. Not only are the exciton binding energies extremely large compared with any other moderate-gap bulk semiconductors, but they are also larger than typical 2D semiconductors with comparable quasiparticle gaps. Our calculated lowest bright exciton energy agrees well with the experimental optical band gap. The low-energy excitons closely resemble the Frenkel excitons in molecular crystals, as they are highly localized in a single [MX6]2- octahedron and extended in the reciprocal space. The weak dielectric screening effects and the nearly flat frontier electronic bands, which are derived from the weakly bonded [MX6]2- units, together explain the significant excitonic effects. Spin-orbit coupling effects play a crucial role in red-shifting the lowest bright exciton by mixing up spin-singlet and spin-triplet excitons, while exciton-phonon coupling effects have minor impacts on the strong exciton binding energies.

physics.comp-ph↗

arXiv2022

Tetragonal Mexican-Hat Dispersion and Switchable Half-Metal State with Multiple Anisotropic Weyl Fermions in Penta-Graphene

In past decades, the ever-expanding library of 2D carbon allotropes has yielded a broad range of exotic properties for the future carbon-based electronics. However, the known allotropes are all intrinsic nonmagnetic due to the paired valence electrons configuration. Based on the reported 2D carbon structure database and first-principles calculations, herein we demonstrate that inherent ferromagnetism can be obtained in the prominent allotrope, penta-graphene, which has an unique Mexican-hat valence band edge, giving rise to van Hove singularities and electronic instability. Induced by modest hole-doping, being achievable in electrolyte gate, the semiconducting pentagraphene can transform into different ferromagnetic half-metals with room temperature stability and switchable spin directions. In particular, multiple anisotropic Weyl states, including type-I and type-II Weyl cones and hybrid quasi Weyl nodal loop, can be found in a sizable energy window of spin-down half-metal under proper strains. These findings not only identify a promising carbon allotrope to obtain the inherent magnetism for carbon-based spintronic devices, but highlight the possibility to realize different Weyl states by combining the electronic and mechanical means as well.

cond-mat.mtrl-sci↗

arXiv2022

Out-of-plane polarization and topological magnetic vortices in multiferroic CrPSe$_3$

Two-dimensional (2D) multiferroic materials are ideal systems for exploring new coupling mechanisms between different ferroic orders and producing novel quantum phenomena with potential applications. We employed first-principles density functional theory calculations to discover intrinsic ferroelectric and anti-ferroelectric phases of CrPSe$_3$, which show ferromagnetic order and compete with the centrosymmetric phase with an antiferromagnetic order. Our analysis show that the electrical dipoles of such type-I multiferroic phases come from the out-of-plane displacements of phosphorus ions due to the stereochemically active lone pairs. The coupling between polar and magnetic orders creates the opportunity for tunning the magnetic ground state by switching from the centrosymmetric to the ferroelectric phase using an out-of-plane electric field. In ferroelectric and antiferroelectric phases, the combination of easy-plane anisotropy and Dzyaloshinskii-Moriya interactions (DMI) indicate they can host topological magnetic vortices like meron pairs.

cond-mat.mtrl-sci↗

arXiv2022

Strong Dzyaloshinskii-Moriya Interaction in Monolayer CrI$_3$ on Metal Substrates

Dzyaloshinskii-Moriya interaction (DMI) is the primary mechanism for realizing real-space chiral spin textures, which are regarded as key components for the next-generation spintronics. However, DMI arises from a perturbation term of the spin-orbit interaction and is usually weak in pristine magnetic semiconductors. To date, large DMI and the resulting skyrmions are only realized in a few materials under stringent conditions. Using first-principles calculations, we demonstrate that significant DMI occurs between nearest-neighbor Cr atoms in two-dimensional (2D) magnetic semiconductor CrI$_3$ on Au or Cu substrates. This exceptionally strong DMI is generated by the interfacial charge transfer and weak chemical interactions between chromium halides and metal substrates, which break the spatial inversion symmetry. These findings highlight the significance of substrate effects in 2D magnets and expand the inventory of feasible materials with strong DMI.

cond-mat.mtrl-sci↗

arXiv2022

Mechanical Anisotropy and Multiple Direction-Dependent Dirac States in the Synthesized Ag3C20 Monolayer

Recently, a 2D orthorhombic silver-organic framework, Ag3C20 monolayer, was synthesized by assembling organic molecules linked with multiple aryl-metal bonds. Herein, via first-principles study, we demonstrate that owing to the unique bonding feature, Ag3C20 monolayer not only exhibits strong mechanical anisotropy, but also possesses various tunable direction-dependent Dirac states. Around the Fermi level blow, the intrinsic Dirac points form two antiparallel quasi type-III nodal lines protected by mirror symmetry, which can further evolve into hybrid nodal loops under tiny strains. Intriguingly, near the Fermi level above, a special semi-Dirac state can emerge under a critical strain by merging two type-I Dirac cone, which harbors direction-dependent strongly localized fermions, normal massive carries, and ultrafast Dirac fermions at the same time. These findings suggest that the mechanically sensitive Ag3C20 monolayer is a promising 2D material to realize the interesting Dirac physics and highly anisotropic multiple carries transport.

cond-mat.mtrl-sci↗

arXiv2022

Eliminating edge electronic and phonon states of phosphorene nanoribbon by unique edge reconstruction

Edge termination plays a vital role in determining the properties of 2D materials. By performing compelling ab initio simulations, a lowest-energy U-edge [ZZ(U)] reconstruction is revealed in the bilayer phosphorene. Such reconstruction reduces 60% edge energy compared with the pristine one and occurs almost without energy barrier, implying it should be the dominating edge in reality. The electronic band structure of phosphorene nanoribbon with such reconstruction resembles that of intrinsic 2D layer, exhibiting nearly edgeless band characteristics. Although ZZ(U) changes the topology of phosphorene nanoribbon (PNR), simulated TEM, STEM and STM images indicates it is very hard to be identified. One possible identify method is IR/Raman analyses because ZZ(U) edge alters vibrational modes dramatically. Beyond, it also increases the thermal conductivity of PNR 1.4 and 2.3 times than the pristine and Klein edges.

cond-mat.mtrl-sci↗

arXiv2021

Tuned Vacancy Diffusion by Mn via Anomalous Friedel Oscillations in NiCoFeCrMn High Entropy Alloys

Vacancy is known to play a critical role in solid state diffusion and determine many properties of the materials. We report a proof-of-principle result on Mn in the role of tuning vacancy diffusion by an anomalous Friedel Oscillations effect in an equivalent NiCoFeCrMn high entropy alloy. The vacancy induced Friedel Oscillations is destroyed by the presence of the first nearest neighboring Mn, opening a channel for long-range influence on the surrounding atoms from the vacancy, resulting in a lower vacancy diffusion barrier energy, despite of the atom type. The effects lead to a remarkable diffusion overlaps of the two type defects, vacancy and interstitial, enhancing their recombination, which can explain well for the experimental results with suppression of vacancy clusters under irradiation. This finding presents new understanding on the atomic diffusion mechanism and thus suggests novel concepts for tuning defect properties in the design of advanced materials with outstanding properties, in a broad field.

cond-mat.mtrl-sci↗

arXiv2020

Temperature-dependent hardness of diamond-structured covalent materials

Understanding temperature-dependent hardness of covalent materials is not only of fundamental scientific interest, but also of crucial importance for technical applications. In this work, a temperature-dependent hardness formula for diamond-structured covalent materials is constructed on the basis of the dislocation theory. Our results show that, at low temperature, the Vickers hardness is mainly controlled by Poisson's ratio and shear modulus with the latter playing a dominant role. With increasing temperature, the plastic deformation mechanism undergoes a transition from shuffle-set dislocation control to glide-set dislocation control, leading to a steeper drop of hardness at high temperature. In addition, an intrinsic parameter, a3G, is revealed for diamond-structured covalent materials, which measures the resistance to soften at high temperature. Our hardness model shows remarkable agreement with experimental data. Current work not only sheds lights on the physical origin of hardness, but also provides a direct principle for superhard materials design.

cond-mat.mtrl-sci↗

arXiv2020

Enhanced Ferromagnetism of CrI3 Bilayer by Self-Intercalation

Two-dimensional (2D) ferromagnets with high Curie temperature have long been the pursuit for electronic and spintronic applications. CrI3 is a rising star of intrinsic 2D ferromagnets, however, it suffers from weak exchange coupling. Here we propose a general strategy of self-intercalation to achieve enhanced ferromagnetism in bilayer CrI3. We showed that filling either Cr or I atoms into the van der Waals gap of stacked and twisted CrI3 bilayers can induce the double exchange effect and significantly strengthen the interlayer ferromagnetic coupling. According to our first-principles calculations, the intercalated native atoms act as covalent bridge between two CrI3 layers and lead to discrepant oxidation states for the Cr atoms. These theoretical results offer a facile route to achieve high-Curie-temperature 2D magnets for device implementation.

cond-mat.mtrl-sci↗

arXiv2020

High-Curie-temperature ferromagnetism in bilayer CrI3 on bulk semiconducting substrates

Two-dimensional (2D) ferromagnetic (FM) semiconductors with high Curie temperature have long been pursued for electronic and spintronic applications. Here we provide a general strategy to achieve robust FM state in bilayer CrI3 of the monoclinic stacking, which intrinsically has interlayer antiferromagnetic (AFM) order and weak in-plane FM coupling. We showed that the proximity effect from bulk semiconducting substrates induces electronic doping and significantly increases the FM nearest-neighbor exchange for bilayer CrI3, leading to the AFM-to-FM transition for the interlayer spin configuration as well as enhanced intralayer FM coupling. By first-principles calculations and Monte Carlo simulations, bulk and 2D semiconductors providing different interaction strengths from strong covalent bonding to weak van der Waals (vdW) interaction with CrI3 are compared to thoroughly address the substrate effect on magnetic behavior and Curie temperature of bilayer CrI3. These theoretical results offer a facile route for direct synthesis of 2D ferromagnets on proper semiconducting substrates to achieve high Curie temperature for device implementation.

cond-mat.mtrl-sci↗

arXiv2020

Ultra-high Hydrogen Storage Capacity of Holey Graphyne

Holey graphyne (HGY), a novel 2D single-crystalline carbon allotrope, was synthesized most recently by Castro-Stephens coupling reaction. The natural existing uniform periodic holes in the 2D carbon-carbon network demonstrate its tremendous potential application in the area of energy storage. Herein, we conducted density functional theory calculation to predict the hydrogen storage capacity of HGY sheet. It's found the Li-decorated single-layer HGY can serve as a promising candidate for hydrogen storage. Our numerical calculations demonstrate that Li atoms can bind strongly to the HGY sheet without the formation of Li clusters, and each Li atom can anchor four H2 molecules with the average adsorption energy about -0.22 eV/H2. The largest hydrogen storage capacity of the doped HGY sheet can arrive as high as 12.8 wt%, this value largely surpasses the target of the U. S. Department of Energy (9 wt%), showing the Li/HGY complex is an ideal hydrogen storage material at ambient conditions. In addition, we investigate the polarization mechanism of the storage media and and find that the polarization stemed from both the electric field induced by the ionic Li decorated on the HGY and the weak polarized hydrogen molecules dominated the H2 adsorption process.

physics.app-ph↗

arXiv2020

Selective C-C Coupling by Spatially Confined Dimeric Metal Centers

Direct conversion of carbon dioxide (CO2) to high-energy fuels and high-value chemicals is a fascinating sustainable strategy. For most of the current electrocatalysts for CO2 reduction, however, multi-carbon products are inhibited by large overpotentials and low selectivity. For practical applications, there remains a big gap of knowledge in proper manipulation of the C-C coupling process. Herein, we exploit dispersed 3d transition metal dimers as spatially confined dual reaction centers for selective reduction of CO2 to liquid fuels. Various nitrogenated holey carbon monolayers are shown to be promising templates to stabilize these metal dimers and dictate their electronic structures, allowing precise control of the catalytic activity and product selectivity. By comprehensive first-principles calculations, we screen the suitable transition metal dimers that universally have high activity for ethanol (C2H5OH). Furthermore, remarkable selectivity for C2H5OH against other C1 and C2 products is found for Fe2 dimer anchored on C2N monolayer. The correlation between the activity and d band center of the supported metal dimer as well as the role of electronic coupling between the metal dimer and the carbon substrates are thoroughly elucidated.

physics.chem-ph↗