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Chen C, Song Z, Zhao JZ, Chen Z, Yu ZM, Sheng XL, Yang SA. Universal Approach to Magnetic Second-Order Topological Insulator. PHYSICAL REVIEW LETTERS 2020; 125:056402. [PMID: 32794859 DOI: 10.1103/physrevlett.125.056402] [Citation(s) in RCA: 19] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/06/2020] [Accepted: 07/06/2020] [Indexed: 06/11/2023]
Abstract
We propose a universal practical approach to realize magnetic second-order topological insulator (SOTI) materials, based on properly breaking the time reversal symmetry in conventional (first-order) topological insulators. The approach works for both three dimensions (3D) and two dimensions (2D), and is particularly suitable for 2D, where it can be achieved by coupling a quantum spin Hall insulator with a magnetic substrate. Using first-principles calculations, we predict bismuthene on EuO(111) surface as the first realistic system for a two-dimensional magnetic SOTI. We explicitly demonstrate the existence of the protected corner states. Benefitting from the large spin-orbit coupling and sizable magnetic proximity effect, these corner states are located in a boundary gap ∼83 meV, and hence can be readily probed in experiment. By controlling the magnetic phase transition, a topological phase transition between a first-order TI and a SOTI can be simultaneously achieved in the system. The effect of symmetry breaking, the connection with filling anomaly, and the experimental detection are discussed.
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Kowalczyk PJ, Brown SA, Maerkl T, Lu Q, Chiu CK, Liu Y, Yang SA, Wang X, Zasada I, Genuzio F, Menteş TO, Locatelli A, Chiang TC, Bian G. Realization of Symmetry-Enforced Two-Dimensional Dirac Fermions in Nonsymmorphic α-Bismuthene. ACS NANO 2020; 14:1888-1894. [PMID: 31971774 DOI: 10.1021/acsnano.9b08136] [Citation(s) in RCA: 13] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Two-dimensional (2D) Dirac-like electron gases have attracted tremendous research interest ever since the discovery of free-standing graphene. The linear energy dispersion and nontrivial Berry phase play a pivotal role in the electronic, optical, mechanical, and chemical properties of 2D Dirac materials. The known 2D Dirac materials are gapless only within certain approximations, for example, in the absence of spin-orbit coupling (SOC). Here, we report a route to establishing robust Dirac cones in 2D materials with nonsymmorphic crystal lattice. The nonsymmorphic symmetry enforces Dirac-like band dispersions around certain high-symmetry momenta in the presence of SOC. Through μ-ARPES measurements, we observe Dirac-like band dispersions in α-bismuthene. The nonsymmorphic lattice symmetry is confirmed by μ-low-energy electron diffraction and scanning tunneling microscopy. Our first-principles simulations and theoretical topological analysis demonstrate the correspondence between nonsymmorphic symmetry and Dirac states. This mechanism can be straightforwardly generalized to other nonsymmorphic materials. The results enlighten the search of symmetry-enforced Dirac fermions in the vast uncharted world of nonsymmorphic 2D materials.
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Hu J, Zhong C, Wu W, Liu N, Liu Y, Yang SA, Ouyang C. 2D honeycomb borophene oxide: a promising anode material offering super high capacity for Li/Na-ion batteries. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2020; 32:065001. [PMID: 31631885 DOI: 10.1088/1361-648x/ab4f4d] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Rational design of novel two-dimensional (2D) electrode materials with high capacity is crucial for the further development of Li-ion and Na-ion batteries. Herein, based on first-principles calculations, we systemically investigate Li and Na storage behaviors in the recently discovered 2D topological nodal-loop metal-the honeycomb borophene oxide (h-B2O). We show that h-B2O is an almost ideal anode material. It has good conductivity before and after Li/Na adsorption, fast ion diffusion with diffusion barrier less than 0.5 eV, low open-circuit voltage (<1 V), and small lattice change (<6.2%) during intercalation. Most remarkably, its theoretical storage capacity is extremely high, reaching up to 2137 mAh · g-1 for Li and 1425 mAh · g-1 for Na. Its Li storage capacity is more than six times higher than graphite (~372 mAh · g-1), and is almost the highest among all 2D materials discovered to date. Our results strongly suggest that 2D h-B2O is an exceedingly promising anode material for both Li- and Na-ion batteries with super high capacity.
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Hua C, Li S, Xu Z, Zheng Y, Yang SA, Lu Y. Tunable Topological Energy Bands in 2D Dialkali-Metal Monoxides. ADVANCED SCIENCE (WEINHEIM, BADEN-WURTTEMBERG, GERMANY) 2020; 7:1901939. [PMID: 32099757 PMCID: PMC7029633 DOI: 10.1002/advs.201901939] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/27/2019] [Revised: 11/13/2019] [Indexed: 05/25/2023]
Abstract
2D materials with nontrivial energy bands are highly desirable for exploring various topological phases of matter, as low dimensionality opens unprecedented opportunities for manipulating the quantum states. Here, it is reported that monolayer (ML) dialkali-metal monoxides, in the well-known 2H-MoS2 type lattice, host multiple symmetry-protected topological phases with emergent fermions, which can be effectively tuned by strain engineering. Based on first-principles calculations, it is found that in the equilibrium state, ML Na2O is a 2D double Weyl semimetal, while ML K2O is a 2D pseudospin-1 metal. These exotic topological states exhibit a range of fascinating effects, including universal optical absorbance, super Klein tunneling, and super collimation effect. By introducing biaxial or uniaxial strain, a series of quantum phase transitions between 2D double Weyl semimetal, 2D Dirac semimetal, 2D pseudospin-1 metal, and semiconductor phases can be realized. The results suggest monolayer dialkali-metal monoxides as a promising platform to explore fascinating physical phenomena associated with novel 2D emergent fermions.
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Yu ZM, Guan S, Sheng XL, Gao W, Yang SA. Valley-Layer Coupling: A New Design Principle for Valleytronics. PHYSICAL REVIEW LETTERS 2020; 124:037701. [PMID: 32031831 DOI: 10.1103/physrevlett.124.037701] [Citation(s) in RCA: 17] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/15/2019] [Indexed: 06/10/2023]
Abstract
The current valleytronics research is based on the paradigm of time-reversal-connected valleys in two-dimensional (2D) hexagonal materials, which forbids the fully electric generation of valley polarization by a gate field. Here, we go beyond the existing paradigm to explore 2D systems with a novel valley-layer coupling (VLC) mechanism, where the electronic states in the emergent valleys have a valley-contrasted layer polarization. The VLC enables a direct coupling between a valley and a gate electric field. We analyze the symmetry requirements for a system to host VLC, demonstrate our idea via first-principles calculations and model analysis of a concrete 2D material example, and show that an electric, continuous, wide-range, and switchable control of valley polarization can be achieved by VLC. Furthermore, we find that systems with VLC can exhibit other interesting physics, such as valley-contrasting linear dichroism and optical selection of the valley and the electric polarization of interlayer excitons. Our finding opens a new direction for valleytronics and 2D materials research.
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Liu X, Zhang S, Guo S, Cai B, Yang SA, Shan F, Pumera M, Zeng H. Advances of 2D bismuth in energy sciences. Chem Soc Rev 2020; 49:263-285. [PMID: 31825417 DOI: 10.1039/c9cs00551j] [Citation(s) in RCA: 62] [Impact Index Per Article: 15.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
Since graphene has been successfully exfoliated, two-dimensional (2D) materials constitute a vibrant research field and open vast perspectives in high-performance applications. Among them, bismuthene and 2D bismuth (Bi) are unique with superior properties to fabricate state-of-the-art energy saving, storage and conversion devices. The largest experimentally determined bulk gap, even larger than those of stanene and antimonene, allows 2D Bi to be the most promising candidate to construct room-temperature topological insulators. Moreover, 2D Bi exhibits cyclability for high-performance sodium-ion batteries, and the enlarged surface together with the good electrochemical activity renders it an efficient electrocatalyst for energy conversion. Also, the air-stability of 2D Bi is better than that of silicene, germanene, phosphorene and arsenene, which could enable more practical applications. This review aims to thoroughly explore the fundamentals of 2D Bi and its improved fabrication methods, in order to further bridge gaps between theoretical predictions and experimental achievements in its energy-related applications. We begin with an introduction of the status of 2D Bi in the 2D-material family, which is followed by descriptions of its intrinsic properties along with various fabrication methods. The vast implications of 2D Bi for high-performance devices can be envisioned to add a new pillar in energy sciences. In addition, in the context of recent pioneering studies on moiré superlattices of other 2D materials, we hope that the improved manipulation techniques of bismuthene, along with its unique properties, might even enable 2D Bi to play an important role in future energy-related twistronics.
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Yang TY, Wan Q, Yan DY, Zhu Z, Wang ZW, Peng C, Huang YB, Yu R, Hu J, Mao ZQ, Li S, Yang SA, Zheng H, Jia JF, Shi YG, Xu N. Directional massless Dirac fermions in a layered van der Waals material with one-dimensional long-range order. NATURE MATERIALS 2020; 19:27-33. [PMID: 31591532 DOI: 10.1038/s41563-019-0494-1] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/25/2019] [Accepted: 08/28/2019] [Indexed: 06/10/2023]
Abstract
One or a few layers of van der Waals (vdW) materials are promising for applications in nanoscale electronics. Established properties include high mobility in graphene, a large direct gap in monolayer MoS2, the quantum spin Hall effect in monolayer WTe2 and so on. These exciting properties arise from electron quantum confinement in the two-dimensional limit. Here, we use angle-resolved photoemission spectroscopy to reveal directional massless Dirac fermions due to one-dimensional confinement of carriers in the layered vdW material NbSi0.45Te2. The one-dimensional directional massless Dirac fermions are protected by non-symmorphic symmetry, and emerge from a stripe-like structural modulation with long-range translational symmetry only along the stripe direction as we show using scanning tunnelling microscopy. Our work not only provides a playground for investigating further the properties of directional massless Dirac fermions, but also introduces a unique component with one-dimensional long-range order for engineering nano-electronic devices based on heterostructures of vdW materials.
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Sheng XL, Chen C, Liu H, Chen Z, Yu ZM, Zhao YX, Yang SA. Two-Dimensional Second-Order Topological Insulator in Graphdiyne. PHYSICAL REVIEW LETTERS 2019; 123:256402. [PMID: 31922761 DOI: 10.1103/physrevlett.123.256402] [Citation(s) in RCA: 54] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/29/2019] [Indexed: 06/10/2023]
Abstract
A second-order topological insulator (SOTI) in d spatial dimensions features topologically protected gapless states at its (d-2)-dimensional boundary at the intersection of two crystal faces, but is gapped otherwise. As a novel topological state, it has been attracting great interest, but it remains a challenge to identify a realistic SOTI material in two dimensions (2D). Here, based on combined first-principles calculations and theoretical analysis, we reveal the already experimentally synthesized 2D material graphdiyne as the first realistic example of a 2D SOTI, with topologically protected 0D corner states. The role of crystalline symmetry, the robustness against symmetry breaking, and the possible experimental characterization are discussed. Our results uncover a hidden topological character of graphdiyne and promote it as a concrete material platform for exploring the intriguing physics of higher-order topological phases.
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Yang Y, Xia JP, Sun HX, Ge Y, Jia D, Yuan SQ, Yang SA, Chong Y, Zhang B. Observation of a topological nodal surface and its surface-state arcs in an artificial acoustic crystal. Nat Commun 2019; 10:5185. [PMID: 31729392 PMCID: PMC6858449 DOI: 10.1038/s41467-019-13258-3] [Citation(s) in RCA: 28] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2019] [Accepted: 10/30/2019] [Indexed: 11/27/2022] Open
Abstract
Three-dimensional (3D) gapless topological phases can be classified by the dimensionality of the band degeneracies, including zero-dimensional (0D) nodal points, one-dimensional (1D) nodal lines, and two-dimensional (2D) nodal surfaces. Both nodal points and nodal lines have been realized recently in photonics and acoustics. However, a nodal surface has never been observed in any classical-wave system. Here, we report on the experimental observation of a twofold symmetry-enforced nodal surface in a 3D chiral acoustic crystal. In particular, the demonstrated nodal surface carries a topological charge of 2, constituting the first realization of a higher-dimensional topologically-charged band degeneracy. Using direct acoustic field measurements, we observe the projected nodal surface and its Fermi-arc-like surface states and demonstrate topologically-induced robustness of the surface states against disorders. This discovery of a higher-dimensional topologically-charged band degeneracy paves the way toward further explorations of the physics and applications of new topological semimetal phases.
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Xu X, Chen B, Hu J, Sun B, Liang X, Li N, Yang SA, Zhang H, Huang W, Yu T. Heterostructured TiO 2 Spheres with Tunable Interiors and Shells toward Improved Packing Density and Pseudocapacitive Sodium Storage. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2019; 31:e1904589. [PMID: 31566277 DOI: 10.1002/adma.201904589] [Citation(s) in RCA: 21] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/17/2019] [Revised: 09/16/2019] [Indexed: 05/17/2023]
Abstract
Insertion-type anode materials with beneficial micro- and nanostructures are proved to be promising for high-performance electrochemical metal ion storage. In this work, heterostructured TiO2 shperes with tunable interiors and shells are controllably fabricated through newly proposed programs, resulting in enhanced pseudocapacitive response as well as favorable Na+ storage kinetics and performances. In addition, reasonably designed nanosheets in the extrinsic shells are also able to reduce the excess space generated by hierarchical structure, thus improving the packing density of TiO2 shperes. Lastly, detailed density functional theory calculations with regard to sodium intercalation and diffusion in TiO2 crystal units are also employed, further proving the significance of the surface-controlled pseudocapacitive Na+ storage mechanism. The structure design strategies and experimental results demonstrated in this work are meaningful for electrode material preparation with high rate performance and volume energy density.
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Jiao Y, Wu W, Ma F, Yu ZM, Lu Y, Sheng XL, Zhang Y, Yang SA. Room temperature ferromagnetism and antiferromagnetism in two-dimensional iron arsenides. NANOSCALE 2019; 11:16508-16514. [PMID: 31453618 DOI: 10.1039/c9nr04338a] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
The discovery of two-dimensional (2D) magnetic materials with high critical temperature and intrinsic magnetic properties has attracted significant research interest. By using swarm-intelligence structure search and first-principles calculations, we predict three 2D iron arsenide monolayers (denoted as FeAs-I, II and III) with good energetic and dynamical stabilities. We find that FeAs-I and II are ferromagnets, while FeAs-III is an antiferromagnet. FeAs-I and III have sizable magnetic anisotropy comparable to the magnetic recording materials such as the FeCo alloy. Importantly, we show that FeAs-I and III have critical temperatures of 645 K and 350 K, respectively, which are above room temperature. In addition, FeAs-I and II are metallic, while FeAs-III is semiconducting with a gap comparable to Si. For FeAs-III, there exist two pairs of 2D antiferromagnetic Dirac points below the Fermi level, and it displays a giant magneto band-structure effect. The superior magnetic and electronic properties of the FeAs monolayers make them promising candidates for spintronics applications.
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Zhang J, Du L, Feng S, Zhang RW, Cao B, Zou C, Chen Y, Liao M, Zhang B, Yang SA, Zhang G, Yu T. Enhancing and controlling valley magnetic response in MoS 2/WS 2 heterostructures by all-optical route. Nat Commun 2019; 10:4226. [PMID: 31530805 PMCID: PMC6748949 DOI: 10.1038/s41467-019-12128-2] [Citation(s) in RCA: 26] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/12/2019] [Accepted: 08/20/2019] [Indexed: 11/24/2022] Open
Abstract
Van der Waals heterostructures of transition metal dichalcogenides with interlayer coupling offer an exotic platform to realize fascinating phenomena. Due to the type II band alignment of these heterostructures, electrons and holes are separated into different layers. The localized electrons induced doping in one layer, in principle, would lift the Fermi level to cross the spin-polarized upper conduction band and lead to strong manipulation of valley magnetic response. Here, we report the significantly enhanced valley Zeeman splitting and magnetic tuning of polarization for the direct optical transition of MoS2 in MoS2/WS2 heterostructures. Such strong enhancement of valley magnetic response in MoS2 stems from the change of the spin-valley degeneracy from 2 to 4 and strong many-body Coulomb interactions induced by ultrafast charge transfer. Moreover, the magnetic splitting can be tuned monotonically by laser power, providing an effective all-optical route towards engineering and manipulating of valleytronic devices and quantum-computation. Van der Waals heterostructures may offer a suitable platform for all-optical manipulation of valleytronic devices. Here, the authors observe a strong enhancement of the valley magnetic response in MoS2, and magnetic tuning of the polarization of MoS2 direct optical transition
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Tang F, Ren Y, Wang P, Zhong R, Schneeloch J, Yang SA, Yang K, Lee PA, Gu G, Qiao Z, Zhang L. Three-dimensional quantum Hall effect and metal–insulator transition in ZrTe5. Nature 2019; 569:537-541. [DOI: 10.1038/s41586-019-1180-9] [Citation(s) in RCA: 150] [Impact Index Per Article: 30.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2018] [Accepted: 02/12/2019] [Indexed: 11/09/2022]
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Yesilyurt C, Siu ZB, Tan SG, Liang G, Yang SA, Jalil MBA. Electrically tunable valley polarization in Weyl semimetals with tilted energy dispersion. Sci Rep 2019; 9:4480. [PMID: 30872691 PMCID: PMC6418200 DOI: 10.1038/s41598-019-40947-2] [Citation(s) in RCA: 20] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2018] [Accepted: 02/18/2019] [Indexed: 11/16/2022] Open
Abstract
Tunneling transport across electrical potential barriers in Weyl semimetals with tilted energy dispersion is investigated. We report that the electrons around different valleys experience opposite direction refractions at the barrier interface when the energy dispersion is tilted along one of the transverse directions. Chirality dependent refractions at the barrier interface polarize the Weyl fermions in angle-space according to their valley index. A real magnetic barrier configuration is used to select allowed transmission angles, which results in electrically controllable and switchable valley polarization. Our findings may pave the way for experimental investigation of valley polarization, as well as valleytronic and electron optic applications in Weyl semimetals.
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Hu J, Ouyang C, Yang SA, Yang HY. Germagraphene as a promising anode material for lithium-ion batteries predicted from first-principles calculations. NANOSCALE HORIZONS 2019; 4:457-463. [PMID: 32254098 DOI: 10.1039/c8nh00333e] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Finding electrode materials with high capacity is a key challenge for developing lithium-ion batteries (LIBs). Graphene was once expected to be a promising candidate, but it turns out to be too inert to interact with Li. Here, by using first-principles calculations, we predict that germanium doped graphene, termed as Germagraphene, which has been achieved in a recent experiment, is a promising LIB anode material. We find that at the optimal Ge concentration, which corresponds to the chemical formula C17Ge, the specific capacity for Germagraphene can be as high as 1734 mA h g-1, over four times larger than that of graphite. We show that the material has good electrical conduction before and after Li adsorption. We also investigate the diffusion process of Li on Germagraphene, and find that the diffusion barrier is low (∼0.151 eV), implying fast Li diffusion. The calculated average intercalation potential is very low (∼0.03 V), which is beneficial for increasing the working voltage for full-cells. In addition, during the process of Li intercalation, the lattice change for the material is quite small (∼0.48%), implying a good cycling performance. These results suggest that Germagraphene could be a promising high-capacity anode material for LIBs.
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Zhong C, Wu W, He J, Ding G, Liu Y, Li D, Yang SA, Zhang G. Two-dimensional honeycomb borophene oxide: strong anisotropy and nodal loop transformation. NANOSCALE 2019; 11:2468-2475. [PMID: 30671570 DOI: 10.1039/c8nr08729f] [Citation(s) in RCA: 20] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
The search for topological semimetals is mainly focused on heavy-element compounds by 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 an Al (111) substrate [W. Li, L. Kong, C. Chen, J. Gou, S. Sheng, W. Zhang, H. Li, L. Chen, P. Cheng and K. Wu, Sci. Bull., 2018, 63, 282-286]. 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 [Doublestruck Z] × [Doublestruck 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.
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Yu ZM, Liu Y, Yao Y, Yang SA. Unconventional Pairing Induced Anomalous Transverse Shift in Andreev Reflection. PHYSICAL REVIEW LETTERS 2018; 121:176602. [PMID: 30411955 DOI: 10.1103/physrevlett.121.176602] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/19/2017] [Revised: 03/12/2018] [Indexed: 06/08/2023]
Abstract
Superconductors with unconventional pairings have been a fascinating subject of research, for which a central issue is to explore effects that can be used to characterize the pairing. The process of Andreev reflection-the reflection of an electron as a hole at a normal-metal-superconductor interface-offers a basic mechanism to probe the pairing. Here we predict that in Andreev reflection from unconventional superconductors, the reflected hole acquires an anomalous spatial shift normal to the plane of incidence, arising from the unconventional pairing. The transverse shift is sensitive to the superconducting gap structure, exhibiting characteristic features for each pairing type, and can be detected as voltage signals. Our work not only unveils a fundamentally new effect with a novel underlying mechanism, but also suggests a possible new technique capable of probing the structure of unconventional pairings.
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Wang H, Wu M, Tian Z, Liu G, Xu B, Yang SA, Ouyang C. Electron-donor doping enhanced Li storage in electride Ca 2N monolayer: a first-principles study. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2018; 30:345501. [PMID: 30020082 DOI: 10.1088/1361-648x/aad446] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
It was reported by Hu et al (2015 ACS Appl. Mater. Interfaces 7 24016) that Li adsorption on the Ca2N monolayer is energetically unfavorable, although it was found to be a good candidate for Na storage. In this paper, from first-principles calculations, it is shown that compressive stain can greatly enhance the interactions between Li adsorbate and the Ca2N host, which is beneficial to prevent Li from clustering on the surface and thus Li storage becomes possible. Charge distribution analysis further shows that the enhanced Li-adsorption is a result of the increased surface charge density and the more confined charge distribution under compressive strain. Inspired by this observation, several electron-donor-doped Ca2N materials are considered as anode materials for Li-ion batteries. Results show that the O and F doped Ca2N have the best performance, with predicted Li storage capacity reaching about 567.9 and 565.9 mAh g-1 for O and F doped cases, respectively. It is also demonstrated that electron-donor doping does not change the metallic electronic structures and the low Li-ion migration energy barriers on the surface of the Ca2N monolayer, and thus the rate performance of the doped Ca2N can be as good as the undoped case. Our study offers a deep understanding of the Li interactions with 2D materials and provides an approach of material modification to 2D electrides for battery applications.
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Wang SS, Liu Y, Yu ZM, Sheng XL, Yang SA. Hourglass Dirac chain metal in rhenium dioxide. Nat Commun 2017; 8:1844. [PMID: 29184139 PMCID: PMC5705673 DOI: 10.1038/s41467-017-01986-3] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/22/2017] [Accepted: 10/30/2017] [Indexed: 11/22/2022] Open
Abstract
Nonsymmorphic symmetries, which involve fractional lattice translations, can generate exotic types of fermionic excitations in crystalline materials. Here we propose a topological phase arising from nonsymmorphic symmetries-the hourglass Dirac chain metal, and predict its realization in the rhenium dioxide. We show that ReO2 features hourglass-type dispersion in the bulk electronic structure dictated by its nonsymmorphic space group. Due to time reversal and inversion symmetries, each band has an additional two-fold degeneracy, making the neck crossing-point of the hourglass four-fold degenerate. Remarkably, close to the Fermi level, the neck crossing-point traces out a Dirac chain-a chain of connected four-fold-degenerate Dirac loops-in the momentum space. The symmetry protection, the transformation under symmetry-breaking, and the associated topological surface states of the Dirac chain are revealed. Our results open the door to an unknown class of topological matters, and provide a platform to explore their intriguing physics.
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Zhang Y, Wu W, Wang Y, Yang SA, Ma Y. Pressure-Stabilized Semiconducting Electrides in Alkaline-Earth-Metal Subnitrides. J Am Chem Soc 2017; 139:13798-13803. [DOI: 10.1021/jacs.7b07016] [Citation(s) in RCA: 32] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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Sheng XL, Yu ZM, Yu R, Weng H, Yang SA. d Orbital Topological Insulator and Semimetal in the Antifluorite Cu 2S Family: Contrasting Spin Helicities, Nodal Box, and Hybrid Surface States. J Phys Chem Lett 2017; 8:3506-3511. [PMID: 28693321 DOI: 10.1021/acs.jpclett.7b01390] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
We reveal a class of three-dimensional d orbital topological materials in the antifluorite Cu2S family. Derived from the unique properties of low-energy t2g states, their phases are solely determined by the sign of the spin-orbit coupling (SOC): topological insulator (TI) for negative SOC and topological semimetal for positive SOC, both having Dirac cone surface states but with contrasting helicities. With broken inversion symmetry, the semimetal becomes one with a nodal box consisting of butterfly-shaped nodal lines that are robust against SOC. Further breaking the tetrahedral symmetry by strain leads to an ideal Weyl semimetal with four pairs of Weyl points. Interestingly, the Fermi arcs coexist with a surface Dirac cone on the (010) surface, as required by a [Formula: see text] invariant.
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Chang TR, Xu SY, Sanchez DS, Tsai WF, Huang SM, Chang G, Hsu CH, Bian G, Belopolski I, Yu ZM, Yang SA, Neupert T, Jeng HT, Lin H, Hasan MZ. Type-II Symmetry-Protected Topological Dirac Semimetals. PHYSICAL REVIEW LETTERS 2017; 119:026404. [PMID: 28753359 DOI: 10.1103/physrevlett.119.026404] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/20/2016] [Indexed: 06/07/2023]
Abstract
The recent proposal of the type-II Weyl semimetal state has attracted significant interest. In this Letter, we propose the concept of the three-dimensional type-II Dirac fermion and theoretically identify this new symmetry-protected topological state in the large family of transition-metal icosagenides, MA_{3} (M=V, Nb, Ta; A=Al, Ga, In). We show that the VAl_{3} family features a pair of strongly Lorentz-violating type-II Dirac nodes and that each Dirac node can be split into four type-II Weyl nodes with chiral charge ±1 via symmetry breaking. Furthermore, we predict that the Landau level spectrum arising from the type-II Dirac fermions in VAl_{3} is distinct from that of known Dirac or Weyl semimetals. We also demonstrate a topological phase transition from a type-II Dirac semimetal to a quadratic Weyl semimetal or a topological crystalline insulator via crystalline distortions.
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Zhang S, Zhou W, Ma Y, Ji J, Cai B, Yang SA, Zhu Z, Chen Z, Zeng H. Antimonene Oxides: Emerging Tunable Direct Bandgap Semiconductor and Novel Topological Insulator. NANO LETTERS 2017; 17:3434-3440. [PMID: 28460176 DOI: 10.1021/acs.nanolett.7b00297] [Citation(s) in RCA: 104] [Impact Index Per Article: 14.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/26/2023]
Abstract
Highly stable antimonene, as the cousin of phosphorene from group-VA, has opened up exciting realms in the two-dimensional (2D) materials family. However, pristine antimonene is an indirect band gap semiconductor, which greatly restricts its applications for optoelectronics devices. Identifying suitable materials, both responsive to incident photons and efficient for carrier transfer, is urgently needed for ultrathin devices. Herein, by means of first-principles computations we found that it is rather feasible to realize a new class of 2D materials with a direct bandgap and high carrier mobility, namely antimonene oxides with different content of oxygen. Moreover, these tunable direct bandgaps cover a wide range from 0 to 2.28 eV, which are crucial for solar cell and photodetector applications. Especially, the antimonene oxide (18Sb-18O) is a 2D topological insulator with a sizable global bandgap of 177 meV, which has a nontrivial Z2 topological invariant in the bulk and the topological states on the edge. Our findings not only introduce new vitality into 2D group-VA materials family and enrich available candidate materials in this field but also highlight the potential of these 2D semiconductors as appealing ultrathin materials for future flexible electronics and optoelectronics devices.
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Zhong C, Chen Y, Yu ZM, Xie Y, Wang H, Yang SA, Zhang S. Three-dimensional Pentagon Carbon with a genesis of emergent fermions. Nat Commun 2017; 8:15641. [PMID: 28580929 PMCID: PMC5465352 DOI: 10.1038/ncomms15641] [Citation(s) in RCA: 89] [Impact Index Per Article: 12.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2016] [Accepted: 04/14/2017] [Indexed: 12/19/2022] Open
Abstract
Carbon, the basic building block of our universe, enjoys a vast number of allotropic structures. Owing to its bonding characteristic, most carbon allotropes possess the motif of hexagonal rings. Here, with first-principles calculations, we discover a new metastable three-dimensional carbon allotrope entirely composed of pentagon rings. The unique structure of this Pentagon Carbon leads to extraordinary electronic properties, making it a cornucopia of emergent topological fermions. Under lattice strain, Pentagon Carbon exhibits topological phase transitions, generating a series of novel quasiparticles, from isospin-1 triplet fermions to triply degenerate fermions and further to Hopf-link Weyl-loop fermions. Its Landau level spectrum also exhibits distinct features, including a huge number of almost degenerate chiral Landau bands, implying pronounced magneto-transport signals. Our work not only discovers a remarkable carbon allotrope with highly rare structural motifs, it also reveals a fascinating hierarchical particle genesis with novel topological fermions beyond the Dirac and Weyl paradigm.
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75
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Xiao C, Wang F, Wang Y, Yang SA, Jiang J, Yang M, Lu Y, Wang S, Feng Y. Layer-dependent semiconductor-metal transition of SnO/Si(001) heterostructure and device application. Sci Rep 2017; 7:2570. [PMID: 28566756 PMCID: PMC5451440 DOI: 10.1038/s41598-017-02832-8] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2017] [Accepted: 04/19/2017] [Indexed: 11/09/2022] Open
Abstract
As the downscaling of electronic devices continues, the problems of leakage currents and heat dissipation become more and more serious. To address these issues, new materials and new structures are explored. Here, we propose an interesting heterostructure made of ultrathin SnO layers on Si(001) surface. Our first-principle calculations show that a single layer of SnO on Si(001) surface is a semiconductor, but a bilayer SnO on the same surface is metallic. This metal-semiconductor dichotomy allows construction of single-2D-material-based electronic devices with low contact resistance and low leakage currents. In particular, due to the interaction between Sn and the Si substrate, the semiconducting monolayer-SnO/Si(001) has a highly anisotropic band structure with a much lighter hole effective mass along one direction than that of Si and most other 2D materials, indicating a high carrier mobility. Furthermore, by combining density functional theory and nonequilibrium Green's function method, we directly investigate the transport characteristics of a field effect transistor based on the proposed heterostructures, which shows very low contact resistance, negligible leakage current, and easy gate control at a compact channel length.
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