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Frontiers of Physics

  • Cover Illustration

    2018, Vol.13  No.3

    Since the discovery of graphene in 2004, many other 2D materials with unique properties and promising applications have been developed, which directly resulted in this special topic on Two-Dimensional Nanomaterials (Eds. Changzheng Wu & Xiaojun Wu).. Besides experimental investigations, computational/theoretical studies also played an important role in 2D materials research. In the cover are the newly predicted aluminum monoxide (AlO) nanosheets and severa [Detail] ...

    Table of Contents

, Volume 13 Issue 3

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First-principles investigation of quantum transport in GeP3 nanoribbon-based tunneling junctions
Qiang Wang, Jian-Wei Li, Bin Wang, Yi-Hang Nie
Front. Phys. . 2018, 13 (3): 138501 .
Abstract   PDF (2630KB)

Two-dimensional (2D) GeP3 has recently been theoretically proposed as a new low-dimensional material [Nano Lett. 17(3), 1833 (2017)]. In this manuscript, we propose a first-principles calculation to investigate the quantum transport properties of several GeP3 nanoribbon-based atomic tunneling junctions. Numerical results indicate that monolayer GeP3 nanoribbons show semiconducting behavior, whereas trilayer GeP3 nanoribbons express metallic behavior owing to the strong interaction between each of the layers. This behavior is in accordance with that proposed in two-dimensional GeP3 layers. The transmission coefficient T(E) of tunneling junctions is sensitive to the connecting formation between the central monolayer GeP3 nanoribbon and the trilayer GeP3 nanoribbon at both ends. The T(E) value of the bottom-connecting tunneling junction is considerably larger than those of the middle-connecting and top-connecting ones. With increases in gate voltage, the conductances increase for the bottom-connecting and middle-connecting tunneling junctions, but decrease for the top-connecting tunneling junctions. In addition, the conductance decreases exponentially with respect to the length of the central monolayer GeP3 nanoribbon for all the tunneling junctions. IV curves show approximately linear behavior for the bottom-connecting and middle-connecting structures, but exhibit negative differential resistance for the top-connecting structures. The physics of each phenomenon is analyzed in detail.

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Strategies for discovery and optimization of thermoelectric materials: Role of real objects and local fields Collection
Hao Zhu, Chong Xiao
Front. Phys. . 2018, 13 (3): 138112 .
Abstract   PDF (1232KB)

Thermoelectric materials provide a renewable and eco-friendly solution to mitigate energy shortages and to reduce environmental pollution via direct heat-to-electricity conversion. Discovery of the novel thermoelectric materials and optimization of the state-of-the-art material systems lie at the core of the thermoelectric society, the basic concept behind these being comprehension and manipulation of the physical principles and transport properties regarding thermoelectric materials. In this mini-review, certain examples for designing high-performance bulk thermoelectric materials are presented from the perspectives of both real objects and local fields. The highlights of this topic involve the Rashba effect, Peierls distortion, local magnetic field, and local stress field, which cover several aspects in the field of thermoelectric research. We conclude with an overview of future developments in thermoelectricity.

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Self-folding mechanics of graphene tearing and peeling from a substrate Collection
Ze-Zhou He, Yin-Bo Zhu, Heng-An Wu
Front. Phys. . 2018, 13 (3): 138111 .
Abstract   PDF (12256KB)

Understanding the underlying mechanism in the tearing and peeling processes of graphene is crucial for the further hierarchical design of origami-like folding and kirigami-like cutting of graphene. However, the complex effects among bending moduli, adhesion, interlayer interaction, and local crystal structure during origami-like folding and kirigami-like cutting remain unclear, resulting in challenges to the practical applications of existing theoretical and experimental findings as well as to potential manipulations of graphene in metamaterials and nanodevices. Toward this end, classical molecular dynamics (MD) simulations are performed with synergetic theoretical analysis to explore the tearing and peeling of self-folded graphene from a substrate driven by external force and by thermal activation. It is found that the elastic energy localized at the small folding ridge plays a significant role in the crack trajectory. Due to the extremely small bending modulus of monolayer graphene, its taper angle when pulled by an external force follows a scaling law distinct from that in case of bilayer graphene. With the increase in the initial width of the folding ridge, the self-folded graphene, motivated by thermal fluctuations, can be self-assembled by spontaneous self-tearing and peeling from a substrate. Simultaneously, the scaling law between the taper angle and adhesive energy is independent of the motivations for thermal activation-induced self-assembly and external force tearing, providing effective insights into the underlying physics for graphene-based origami-like folding and kirigami-like cutting.

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Ultrathin nanosheets of Mn3O4: A new two-dimensional ferromagnetic material with strong magnetocrystalline anisotropy Collection
Jun-Chi Wu, Xu Peng, Yu-Qiao Guo, Hao-Dong Zhou, Ji-Yin Zhao, Ke-Qin Ruan, Wang-Sheng Chu, Changzheng Wu
Front. Phys. . 2018, 13 (3): 138110 .
Abstract   PDF (22798KB)

Two-dimensional (2D) materials with robust ferromagnetism have played a key role in realizing nextgeneration spin-electronic devices, but many challenges remain, especially the lack of intrinsic ferromagnetic behavior in almost all 2D materials. Here, we highlight ultrathin Mn3O4 nanosheets as a new 2D ferromagnetic material with strong magnetocrystalline anisotropy. Magnetic measurements along the in-plane and out-of-plane directions confirm that the out-of-plane direction is the easy axis. The 2D-confined environment and Rashba-type spin-orbit coupling are thought to be responsible for the magnetocrystalline anisotropy. The robust ferromagnetism in 2D Mn3O4 nanosheets with magnetocrystalline anisotropy not only paves a new way for realizing the intrinsic ferromagnetic behavior in 2D materials but also provides a novel candidate for building next-generation spin-electronic devices.

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Two-dimensional aluminum monoxide nanosheets: A computational study Collection
Shiru Lin, Yanchao Wang, Zhongfang Chen
Front. Phys. . 2018, 13 (3): 138109 .
Abstract   PDF (6035KB)

By means of density functional theory (DFT) computations and particle-swarm optimization (PSO) structure searches, we herein predict five low-lying energy structures of two-dimensional (2D) aluminum monoxide (AlO) nanosheets. Their high cohesive energy, absence of imaginary phonon dispersion, and good thermal stability make them feasible targets for experimental realization. These monolayers exhibit diverse structural topologies, for instance, PmA- and Pmm-AlO possess buckled four- and sixmembered AlO rings, whereas P62-, PmB-, and P6m-AlO have pores of varied sizes. Interestingly, the most energetically preferred monolayers, PmA- and Pmm-AlO, feature wide band gaps (2.45 and 5.13 eV, respectively), which are promising for green and blue light-emitting devices (LEDs) and photodetectors.

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Computational study on the half-metallicity in transition metal–oxide-incorporated 2D g-C3N4 nanosheets Collection
Qian Gao (高乾), Hui-Li Wang (王会丽), Li-Fu Zhang (张丽芙), Shuang-Lin Hu (胡双林), Zhen-Peng Hu (胡振芃)
Front. Phys. . 2018, 13 (3): 138108 .
Abstract   PDF (4189KB)

In this study, based on the first-principles calculations, we systematically investigated the electronic and magnetic properties of the transition metal–oxide-incorporated 2D g-C3N4 nanosheet (labeled C3N4– TM–O, TM= Sc–Mn). The results suggest that the TM–O binds to g-C3N4 nanosheets strongly for all systems. We found that the 2D C3N4–TM–O framework is ferromagnetic for TM= Sc, Ti, V, Cr, while it is antiferromagnetic for TM= Mn. All the ferromagnetic systems exhibit the half-metallic property. Furthermore, Monte Carlo simulations based on the Heisenberg model suggest that the Curie temperatures (Tc) of the C3N4–TM–O (TM= Sc, Ti, V, Cr) framework are 169 K, 68 K, 203 K, and 190 K, respectively. Based on Bader charge analysis, we found that the origin of the half-metallicity at Fermi energy can be partially attributed to the transfer of electrons from TM atoms to the g-C3N4 nanosheet. In addition, we found that not only electrons but also holes can induce half-metallicity for 2D g-C3N4 nanosheets, which may help to understand the origin of half-metallicity for graphitic carbon nitride.

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Dissociation of liquid water on defective rutile TiO2 (110) surfaces using ab initio molecular dynamics simulations Collection
Hui-Li Wang (王会丽), Zhen-Peng Hu (胡振芃), Hui Li (李晖)
Front. Phys. . 2018, 13 (3): 138107 .
Abstract   PDF (26486KB)

In order to obtain a comprehensive understanding of both thermodynamics and kinetics of water dissociation on TiO2, the reactions between liquid water and perfect and defective rutile TiO2 (110) surfaces were investigated using ab initio molecular dynamics simulations. The results showed that the free-energy barrier (~4.4 kcal/mol) is too high for a spontaneous dissociation of water on the perfect rutile (110) surface at a low temperature. The most stable oxygen vacancy (Vo1) on the rutile (110) surface cannot promote the dissociation of water, while other unstable oxygen vacancies can significantly enhance the water dissociation rate. This is opposite to the general understanding that Vo1 defects are active sites for water dissociation. Furthermore, we reveal that water dissociation is an exothermic reaction, which demonstrates that the dissociated state of the adsorbed water is thermodynamically favorable for both perfect and defective rutile (110) surfaces. The dissociation adsorption of water can also increase the hydrophilicity of TiO2.

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Epitaxial growth of highly strained antimonene on Ag(111) Collection
Ya-Hui Mao, Li-Fu Zhang, Hui-Li Wang, Huan Shan, Xiao-Fang Zhai, Zhen-Peng Hu, Ai-Di Zhao, Bing Wang
Front. Phys. . 2018, 13 (3): 138106 .
Abstract   PDF (38981KB)

The synthesis of antimonene, which is a promising group-V 2D material for both fundamental studies and technological applications, remains highly challenging. Thus far, it has been synthesized only by exfoliation or growth on a few substrates. In this study, we show that thin layers of antimonene can be grown on Ag(111) by molecular beam epitaxy. High-resolution scanning tunneling microscopy combined with theoretical calculations revealed that the submonolayer Sb deposited on a Ag(111) surface forms a layer of AgSb2 surface alloy upon annealing. Further deposition of Sb on the AgSb2 surface alloy causes an epitaxial layer of Sb to form, which is identified as antimonene with a buckled honeycomb structure. More interestingly, the lattice constant of the epitaxial antimonene (5 Å) is much larger than that of freestanding antimonene, indicating a high tensile strain of more than 20%. This kind of large strain is expected to make the antimonene a highly promising candidate for roomtemperature quantum spin Hall material.

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Recent progress on borophene: Growth and structures Collection
Longjuan Kong, Kehui Wu, Lan Chen
Front. Phys. . 2018, 13 (3): 138105 .
Abstract   PDF (72890KB)

Boron is the neighbor of carbon on the periodic table and exhibits unusual physical characteristics derived from electron-deficient, highly delocalized covalent bonds. As the nearest neighbor of carbon, boron is in many ways similar to carbon, such as having a short covalent radius and the flexibility to adopt sp2 hybridization. Hence, boron could be capable of forming monolayer structural analogues of graphene. Although many theoretical papers have reported finding two-dimensional allotropes of boron, there had been no experimental evidence for such atom-thin boron nanostructures until 2016. Recently, the successful synthesis of single-layer boron (referred to as borophene) on the Ag(111) substrate opens the era of boron nanostructures. In this brief review, we will discuss the progress that has been made on borophene in terms of synthetic techniques, characterizations and the atomic models. However, borophene is just in infancy; more efforts are expected to be made in future on the controlled synthesis of quality samples and tailoring its physical properties.

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SiP monolayers: New 2D structures of group IV-V compounds for visible-light photohydrolytic catalysts Collection
Zhinan Ma (马志楠), Jibin Zhuang (庄吉彬), Xu Zhang (张旭), Zhen Zhou (周震)
Front. Phys. . 2018, 13 (3): 138104 .
Abstract   PDF (3718KB)

Because of graphene and phosphorene, two-dimensional (2D) layered materials of group IV and group V elements arouse great interest. However, group IV–V monolayers have not received due attention. In this work, three types of SiP monolayers were computationally designed to explore their electronic structure and optical properties. Computations confirm the stability of these monolayers, which are all indirect-bandgap semiconductors with bandgaps in the range 1.38–2.21 eV. The bandgaps straddle the redox potentials of water at pH= 0, indicating the potential of the monolayers for use as watersplitting photocatalysts. The computed optical properties demonstrate that certain monolayers of SiP 2D materials are absorbers of visible light and would serve as good candidates for optoelectronic devices.

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Interfacial properties of black phosphorus/transition metal carbide van der Waals heterostructures Collection
Hao Yuan, Zhenyu Li
Front. Phys. . 2018, 13 (3): 138103 .
Abstract   PDF (11277KB)

Owing to its outstanding electronic properties, black phosphorus (BP) is considered as a promising material for next-generation optoelectronic devices. In this work, devices based on BP/MXene (Zrn+1CnT2, T= O, F, OH, n = 1, 2) van der Waals (vdW) heterostructures are designed via first-principles calculations. Zrn+1CnT2 compositions with appropriate work functions lead to the formation of Ohmic contact with BP in the vertical direction. Low Schottky barriers are found along the lateral direction in BP/Zr2CF2, BP/Zr2CO2H2, BP/Zr3C2F2, and BP/Zr3C2O2H2 bilayers, and BP/Zr3C2O2 even exhibits Ohmic contact behavior. BP/Zr2CO2 is a semiconducting heterostructure with type-II band alignment, which facilitates the separation of electron-hole pairs. The band structure of BP/Zr2CO2 can be effectively tuned via a perpendicular electric field, and BP is predicted to undergo a transition from donor to acceptor at a 0.4 V/Å electric field. The versatile electronic properties of the BP/MXene heterostructures examined in this work highlight their promising potential for applications in electronics.

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Penta-P2X (X=C, Si) monolayers as wide-bandgap semiconductors: A first principles prediction Collection
Mosayeb Naseri, Shiru Lin, Jaafar Jalilian, Jinxing Gu, Zhongfang Chen
Front. Phys. . 2018, 13 (3): 138102 .
Abstract   PDF (20239KB)

By means of density functional theory computations, we predicted two novel two-dimensional (2D) nanomaterials, namely P2X (X=C, Si) monolayers with pentagonal configurations. Their structures, stabilities, intrinsic electronic, and optical properties as well as the effect of external strain to the electronic properties have been systematically examined. Our computations showed that these P2C and P2Si monolayers have rather high thermodynamic, kinetic, and thermal stabilities, and are indirect semiconductors with wide bandgaps (2.76 eV and 2.69 eV, respectively) which can be tuned by an external strain. These monolayers exhibit high absorptions in the UV region, but behave as almost transparent layers for visible light in the electromagnetic spectrum. Their high stabilities and exceptional electronic and optical properties suggest them as promising candidates for future applications in UV-light shielding and antireflection layers in solar cells.

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First-principles calculations of nitrogen-doped antimony triselenide: A prospective material for solar cells and infrared optoelectronic devices
Sajid-ur- Rehman, Faheem K. Butt, Chuanbo Li, Bakhtiar Ul Haq, Zeeshan Tariq, F. Aleem
Front. Phys. . 2018, 13 (3): 137805 .
Abstract   PDF (3534KB)

This study is focused on calculation of the electronic structure and optical properties of non-metal doped Sb2Se3 using the first-principles method. One and two N atoms are introduced to Sb and Se sites in a Sb2Se3 crystal. When one and two N atoms are introduced into the Sb2Se3 lattice at Sb sites, the electronic structure shows that the doping significantly modifies the bandgap of Sb2Se3 from 1.11 eV to 0.787 and 0.685 eV, respectively. When N atoms are introduced to Se sites, the material shows a metallic behavior. The static dielectric constants ε1(0) for Sb16Se24, Sb15N1Se24, Sb14N2Se24, Sb16Se23N1, and Sb16Se22N2 are 14.84, 15.54, 15.02, 18.9, and 39.29, respectively. The calculated values of the refractive index n(0) for Sb16Se24, Sb15N1Se24, Sb14N2Se24, Sb16Se23N1, and Sb16Se22N2 are 3.83, 3.92, 3.86, 4.33, and 6.21, respectively. The optical absorbance and optical conductivity curves of the crystal for N-doping at Sb sites show a significant redshift towards the short-wave infrared spectral region as compared to N-doping at Se sites. The modulation of the static refractive index and static dielectric constant is mainly dependent on the doping level. The optical properties and bandgap narrowing effect suggest that the N-doped Sb2Se3is a promising new semiconductor and can be a replacement for GaSb due to its very similar bandgap and low cost.

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Fano resonances in heterogeneous dimers of silicon and gold nanospheres
Qian Zhao, Zhong-Jian Yang, Jun He
Front. Phys. . 2018, 13 (3): 137801 .
Abstract   PDF (4856KB)

We theoretically investigate the optical properties of dimers consisting of a gold nanosphere and a silicon nanosphere. The absorption spectrum of the gold sphere in the dimer can be significantly altered and exhibits a pronounced Fano profile. Analytical Mie theory and numerical simulations show that the Fano profile is induced by constructive and destructive interference between the incident electric field and the electric field of the magnetic dipole mode of the silicon sphere in a narrow wavelength range. The effects of the silicon sphere size, distance between the two spheres, and excitation configuration on the optical responses of the dimers are studied. Our study reveals the coherent feature of the electric fields of magnetic dipole modes in dielectric nanostructures and the strong interactions of the coherent fields with other nanophotonic structures.

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Theoretical studies of superconductivity in doped BaCoSO
Shengshan Qin, Yinxiang Li, Qiang Zhang, Congcong Le, Jiangping Hu
Front. Phys. . 2018, 13 (3): 137502 .
Abstract   PDF (2931KB)

We investigate superconductivity that may exist in the doped BaCoSO, a multi-orbital Mott insulator with a strong antiferromagnetic ground state. The superconductivity is studied in both t-J type and Hubbard type multi-orbital models by mean field approach and random phase approximation (RPA) analysis. Even if there is no C4 rotational symmetry, it is found that the system still carries a d-wave like pairing symmetry state with gapless nodes and sign changed superconducting order parameters on Fermi surfaces. The results are largely doping insensitive. In this superconducting state, the three t2g orbitals have very different superconducting form factors in momentum space. In particular, th intra-orbital pairing of the dx2y2 orbital has an s-wave like pairing form factor. The two methods also predict very different pairing strength on different parts of Fermi surfaces. These results suggest that BaCoSO and related materials can be a new ground to test and establish fundamental principles for unconventional high temperature superconductivity.

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Discrete ellipsoidal statistical BGK model and Burnett equations
Yu-Dong Zhang, Ai-Guo Xu, Guang-Cai Zhang, Zhi-Hua Chen, Pei Wang
Front. Phys. . 2018, 13 (3): 135101 .
Abstract   PDF (14176KB)

A new discrete Boltzmann model, the discrete ellipsoidal statistical Bhatnagar–Gross–Krook (ESBGK) model, is proposed to simulate nonequilibrium compressible flows. Compared with the original discrete BGK model, the discrete ES-BGK has a flexible Prandtl number. For the discrete ES-BGK model in the Burnett level, two kinds of discrete velocity model are introduced and the relations between nonequilibrium quantities and the viscous stress and heat flux in the Burnett level are established. The model is verified via four benchmark tests. In addition, a new idea is introduced to recover the actual distribution function through the macroscopic quantities and their space derivatives. The recovery scheme works not only for discrete Boltzmann simulation but also for hydrodynamic ones, for example, those based on the Navier–Stokes or the Burnett equations.

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Periodic synchronization in a system of coupled phase oscillators with attractive and repulsive interactions
Di Yuan, Jun-Long Tian, Fang Lin, Dong-Wei Ma, Jing Zhang, Hai-Tao Cui, Yi Xiao
Front. Phys. . 2018, 13 (3): 130504 .
Abstract   PDF (1184KB)

In this study we investigate the collective behavior of the generalized Kuramoto model with an external pinning force in which oscillators with positive and negative coupling strengths are conformists and contrarians, respectively. We focus on a situation in which the natural frequencies of the oscillators follow a uniform probability density. By numerically simulating the model, it is shown that the model supports multistable synchronized states such as a traveling wave state, π state and periodic synchronous state: an oscillating π state. The oscillating π state may be characterized by the phase distribution oscillating in a confined region and the phase difference between conformists and contrarians oscillating around π periodically. In addition, we present the parameter space of the oscillating π state and traveling wave state of the model.

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Practical pulse engineering: Gradient ascent without matrix exponentiation
Gaurav Bhole, Jonathan A. Jones
Front. Phys. . 2018, 13 (3): 130312 .
Abstract   PDF (140KB)

Since 2005, there has been a huge growth in the use of engineered control pulses to perform desired quantum operations in systems such as nuclear magnetic resonance quantum information processors. These approaches, which build on the original gradient ascent pulse engineering algorithm, remain computationally intensive because of the need to calculate matrix exponentials for each time step in the control pulse. In this study, we discuss how the propagators for each time step can be approximated using the Trotter–Suzuki formula, and a further speedup achieved by avoiding unnecessary operations. The resulting procedure can provide substantial speed gain with negligible costs in the propagator error, providing a more practical approach to pulse engineering.

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