A topological nodal line semi-metal with a negative Poisson's ratio in a three-dimensional carbon network with sp2 hybridization.

In carbon allotropes, a series of topological semi-metals have been predicted, but both novel electronic properties and mechanical characteristics, e.g., a negative Poisson's ratio (NPR), are rarely discovered in the same sp2 type system. Here, a new three-dimensional carbon network, named WZGN, constructed from distorted one-dimensional zigzag graphene nanoribbons is proposed. The stability of the system is fully ensured by the phonon dispersion, AIMD simulation, and binding energy calculations. Besides, it is found that the system holds both topologically protected nodal line semi-metal properties together with an NPR property. Especially, the value of the NPR can exceed -0.36 when 21% uniaxial tensile strain along the c'-direction is applied. Our findings point out that nodal line semi-metals can be compatible with intrinsic NPR properties in a wide strain range in carbon systems with sp2 hybridization, suggesting possible applications in mechanical and electronics fields.

Spin Hall effect modulated by an electric field in asymmetric two-dimensional MoSiAs2Se.

Spin current generation from charge current in nonmagnetic materials promises an energy-efficient scheme for manipulating magnetization in spintronic devices. In some asymmetric two-dimensional (2D) materials, the Rashba and valley effects coexist owing to strong spin-orbit coupling (SOC), which induces the spin Hall effect due to spin-momentum locking of both effects. Herein, we propose a new Janus structure MoSiAs2Se with both valley physics and the Rashba effect and reveal an effective way to modulate the properties of this structure. The results demonstrated that applying an external electric field is an effective means to modulating the electronic properties of MoSiAs2Se, leading to both type I-II phase transitions and semiconductor-metal phase transitions. Furthermore, the coexistence of the Rashba and valley effects in monolayer MoSiAs2Se contributes to the spin Hall effect (SHE). The magnitude and direction of spin Hall conductivity can also be manipulated with an out-of-plane electric field. Our results enrich the physics and materials of the Rashba and valley systems, opening new opportunities for the applications of 2D Janus materials in spintronic devices.

Enhancement and modulation of valley polarization in Janus CrSSe with internal and external electric fields.

The valley polarization, induced by the magnetic proximity effect, in monolayer transition metal dichalcogenides (TMDCs), has attracted significant attention due to the intriguing fundamental physics. However, the enhancement and modulation of valley polarization for real device applications is still a challenge. Here, using first-principles calculations we investigate the valley polarization properties of monolayer TMDCs CrS2 and CrSe2 and how to enhance the valley polarization by constructing Janus CrSSe (with an internal electric field) and modulate the polarization in CrSSe by applying external electric fields. Janus CrSSe exhibits inversion symmetry breaking, internal electric field, spin-orbit coupling, and compelling spin-valley coupling. A magnetic substrate of the MnO2 monolayer can induce a modest magnetic moment in CrSe2, CrSe2, and CrSSe. Notably, the Janus structure with an internal electric field has a much larger valley p compared with its non-Janus counterparts. Moreover, the strength of valley polarization can be further modulated by applying external electric fields. These findings suggest that Janus materials hold promise for designing and developing advanced valleytronic devices.

Coupling double flat bands in a quadrangular-star lattice.

Most special two-dimensional (2D) lattices, such as Kagome and Lieb lattices, can only generate a single flat band. Here, we propose a 2D lattice named a quadrangular-star lattice (QSL). It can produce coupling double flat bands, which indicates that there exists stronger electronic correlation than in the systems with only one flat band. Moreover, we suggest some 2D carbon allotropes (e.g. CQSL-12 and CQSL-20), made of carbon rings and dimers, to realize QSL in real materials. By calculating the band structures of the carbon materials, we find that there are indeed two coupling flat bands around the Fermi level. Hole doping leads to strong magnetism of the carbon materials. When the two flat bands are half filled, i.e., in the cases of one- and three-hole doping, the magnetic momentums mainly distribute on the atoms of the carbon rings and dimers, respectively. Even in the case of two-hole doping, the carbon structure also shows ferromagnetic characteristics, and the total magnetic moments are larger than the former two cases.

Super high-performance 7-atomic-layer thermoelectric material ZrGe2N4.

7-Atomic-layer materials have attracted much attention recently because of their rich structures and more-abundant properties than 3- and 5-atomic-layer materials. However, the thermoelectric properties of the new monolayer materials have not been explored yet. Here, we investigate the thermoelectric conversion efficiency of a 7-atomic-layer structure ZrGe2N4, which is selected from a series of 7-atomic-layer structures according to their stabilities and thermoelectric properties. The results indicate that this material is an excellent candidate for high-performance thermoelectric materials. Its figure of merit ZT value is close to 4.0 at high temperature. The high efficiency originates from two factors: one is the lower thermal conductivity of ZrGe2N4 and the other is the decoupling of electron and phonon transport in the 7-atomic-layer structures.

Critical topological nodal points and nodal lines/rings in Kagome graphene.

Critical topological phases, possessing flat bands, provide a platform to study unique topological properties and transport phenomena under a many-body effect. Here, we propose that critical nodal points and nodal lines or rings can be found in Kagome lattices. After the C3 rotation symmetry of a single-layer Kagome lattice is eliminated, a quadratic nodal point splits into two critical nodal points. When the layered Kagome lattices are stacked into a three-dimensional (3D) structure, critical nodal lines or rings can be generated by tuning the interlayer coupling. Furthermore, we use Kagome graphene as an example to identify that these critical phases could be obtained in real materials. We also discuss flat-band-induced ferromagnetism. It is found that the flat band splits into two spin-polarized bands by hole-doping, and as a result the Dirac-type critical phases evolve into Weyl-type phases.

Squeezed metallic droplet with tunable Kubo gap and charge injection in transition metal dichalcogenides.

Shrinking the size of a bulk metal into nanoscale leads to the discreteness of electronic energy levels, the so-called Kubo gap δ. Renormalization of the electronic properties with a tunable and size-dependent δ renders fascinating photon emission and electron tunneling. In contrast with usual three-dimensional (3D) metal clusters, here we demonstrate that Kubo gap δ can be achieved with a two-dimensional (2D) metallic transition metal dichalcogenide (i.e., 1T'-phase MoTe2) nanocluster embedded in a semiconducting polymorph (i.e., 1H-phase MoTe2). Such a 1T'/1H MoTe2 nanodomain resembles a 3D metallic droplet squeezed in a 2D space which shows a strong polarization catastrophe while simultaneously maintaining its bond integrity, which is absent in traditional δ-gapped 3D clusters. The weak screening of the host 2D MoTe2 leads to photon emission of such pseudometallic systems and a ballistic injection of carriers in the 1T'/1H/1T' homojunctions which may find applications in sensors and 2D reconfigurable devices.

Thermal transports of one-dimensional ultrathin carbon structures.

Carbon atomic chain, linear benzene polymers, and carbon nanothreads are all one-dimensional (1D) ultrathin carbon structures. They possess excellent electronic and mechanical properties; however, their thermal transport properties have been rarely explored. Here, we systematically study their thermal conductance by combining the nonequilibrium Green's function and force field methods. The thermal conductance varies from 0.24 to 1.00 nW K-1 at 300 K, and phonon transport in the linear benzene polymers and carbon nanothreads is strongly dependent on the connectivity styles between the benzene rings. We propose a simple 1D model, namely force-constant model, that explains the complicated transport processes in these structures. Our study not only reveals intrinsic mechanisms of phonon transport in these carbon structures, but also provides an effective method to analyze thermal properties of other 1D ultrathin structures made of only several atomic chains.

Spindle nodal chain in three-dimensional α' boron.

Topological metals/semimetals (TMs) have emerged as a new frontier in the field of quantum materials. A few two-dimensional (2D) boron sheets have been suggested as Dirac materials, however, to date TMs made of three-dimensional (3D) boron structures have not been found. Herein, by means of systematic first principles computations, we discovered that a rather stable 3D boron allotrope, namely 3D-α' boron, is a nodal-chain semimetal. In momentum space, six nodal lines and rings contact each other and form a novel spindle nodal chain. This 3D-α' boron can be formed by stacking 2D wiggle α' boron sheets, which are also nodal-ring semimetals. In addition, our chemical bond analysis revealed that the topological properties of the 3D and 2D boron structures are related to the π bonds between boron atoms, however, the bonding characteristics are different from those in the 2D and 3D carbon structures.

Nodal-chain network, intersecting nodal rings and triple points coexisting in nonsymmorphic Ba3Si4.

Coexistence of topological elements in topological metals/semimetals (TMs) has gradually attracted attention. However, non-topological factors always interfere with the Fermi surface and cover interesting topological properties. Here, we find that Ba3Si4 is a "clean" TM which contains coexisting nodal-chain networks, intersecting nodal rings (INRs) and triple points, in the absence of spin-orbit coupling (SOC). Moreover, the nodal rings in the topological phase exhibit diverse types: from type-I and type-II to type-III rings according to band dispersions. All of the topological elements are generated by crossings of three energy bands, and thus they are correlated rather than mutually independent. When some structural symmetries are eliminated by an external strain, the topological phase evolves into another phase including a Hopf link, a one-dimensional nodal chain and new INRs.

Double Kagome Bands in a Two-Dimensional Phosphorus Carbide P2C3.

The interesting properties of Kagome bands, consisting of Dirac bands and a flat band, have attracted extensive attention. However, materials with only one Kagome band around the Fermi level cannot possess physical properties of Dirac Fermions and strong correlated Fermions simultaneously. Here, we propose a new type of band structure, double Kagome bands, which can realize coexistence of the two kinds of Fermions. Moreover, the new band structure is found to exist in a new two-dimensional material, phosphorus carbide P2C3. The carbide material shows good stability and unusual electronic properties. Strong magnetism appears in the structure by hole doping of the flat band, which results in spin splitting of the Dirac bands. The edge states induced by Dirac and flat bands coexist on the Fermi level, indicating outstanding transport characteristics. In addition, a possible route to experimentally grow P2C3 on some suitable substrates such as the Ag(111) surface is also discussed.

Symmorphic Intersecting Nodal Rings in Semiconducting Layers.

The unique properties of topological semimetals have strongly driven efforts to seek for new topological phases and related materials. Here, we identify a critical condition for the existence of intersecting nodal rings (INRs) in symmorphic crystals, and further classify all possible kinds of INRs which can be obtained in the layered semiconductors with Amm2 and Cmmm space group symmetries. Several honeycomb structures are suggested to be topological INR semimetals, including layered and "hidden" layered structures. Transitions between the three types of INRs, named as α, ß, and γ type, can be driven by external strains in these structures. The resulting surface states and Landau-level structures, more complicated than those resulting from a simple nodal loop, are also discussed.

Three-dimensional Pentagon Carbon with a genesis of emergent fermions.

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.

Dirac Nodal Lines and Tilted Semi-Dirac Cones Coexisting in a Striped Boron Sheet.

The enchanting Dirac fermions in graphene stimulated us to seek other 2D Dirac materials, and boron monolayers may be a good candidate. So far, a number of monolayer boron sheets have been theoretically predicted, and three have been experimentally prepared. However, none of intrinsic sheets possess Dirac electrons near the Fermi level. Herein, by means of density functional theory computations, we identified a new boron monolayer, namely, hr-sB, with two types of Dirac fermions coexisting in the sheet: One type is related to Dirac nodal lines traversing Brillouin zone (BZ) with velocities approaching 106 m/s, and the other is related to tilted semi-Dirac cones with strong anisotropy. This newly predicted boron monolayer consists of hexagon and rhombus stripes. With an exceptional stability comparable to the experimentally achieved boron sheets, it is rather optimistic to grow hr-sB on some suitable substrates such as the Ag (111) surface.

Semi-Dirac semimetal in silicene oxide.

A semi-Dirac semimetal is a material that exhibits linear band dispersion in one direction and quadratic band dispersion in the orthogonal direction and, therefore, hosts massless and massive fermions at the same point in the momentum space. While a number of interesting physical properties have been predicted in semi-Dirac semimetals, it has been rare to realize such materials in condensed matter. Based on the fact that some honeycomb materials are easily oxidized or chemically absorb other atoms, here, we theoretically propose an approach of modifying their band structures by covalent addition of group-VI elements and strain engineering. We predict a silicene oxide with the chemical formula of Si2O to be a candidate semi-Dirac semimetal. Our approach is backed by the analysis and understanding of the effect of p-orbital frustration on the band structure of graphene-like materials.

Phonon transport in single-layer boron nanoribbons.

Inspired by the successful synthesis of three two-dimensional (2D) allotropes, the boron sheet has recently been one of the hottest 2D materials around. However, to date, phonon transport properties of these new materials are still unknown. By using the non-equilibrium Green's function (NEGF) combined with the first principles method, we study ballistic phonon transport in three types of boron sheets; two of them correspond to the structures reported in the experiments, while the third one is a stable structure that has not been synthesized yet. At room temperature, the highest thermal conductance of the boron nanoribbons is comparable with that of graphene, while the lowest thermal conductance is less than half of graphene's. Compared with graphene, the three boron sheets exhibit diverse anisotropic transport characteristics. With an analysis of phonon dispersion, bonding charge density, and simplified models of atomic chains, the mechanisms of the diverse phonon properties are discussed. Moreover, we find that many hybrid patterns based on the boron allotropes can be constructed naturally without doping, adsorption, and defects. This provides abundant nanostructures for thermal management and thermoelectric applications.

Electron and phonon properties and gas storage in carbon honeycombs.

A new kind of three-dimensional carbon allotrope, termed carbon honeycomb (CHC), has recently been synthesized [PRL 116, 055501 (2016)]. Based on the experimental results, a family of graphene networks has been constructed, and their electronic and phonon properties are studied by various theoretical approaches. All networks are porous metals with two types of electron transport channels along the honeycomb axis and they are isolated from each other: one type of channel originates from the orbital interactions of the carbon zigzag chains and is topologically protected, while the other type of channel is from the straight lines of the carbon atoms that link the zigzag chains and is topologically trivial. The velocity of the electrons can reach ∼10(6) m s(-1). Phonon transport in these allotropes is strongly anisotropic, and the thermal conductivities can be very low when compared with graphite by at least a factor of 15. Our calculations further indicate that these porous carbon networks possess high storage capacity for gaseous atoms and molecules in agreement with the experiments.

Towards three-dimensional Weyl-surface semimetals in graphene networks.

Graphene as a two-dimensional topological semimetal has attracted much attention for its outstanding properties. In contrast, three-dimensional (3D) topological semimetals of carbon are still rare. Searching for such materials with salient physics has become a new direction in carbon research. Here, using first-principles calculations and tight-binding modeling, we propose a new class of Weyl semimetals based on three types of 3D graphene networks. In the band structures of these materials, two flat Weyl surfaces appear in the Brillouin zone, which straddle the Fermi level and are robust against external strain. Their unique atomic and electronic structures enable applications in correlated electronics, as well as in energy storage, molecular sieves, and catalysis. When the networks are cut, the resulting slabs and nanowires remain semimetallic with Weyl lines and points at the Fermi surfaces, respectively. Between the Weyl lines, flat surface bands emerge with possible strong magnetism. The robustness of these structures can be traced back to a bulk topological invariant, ensured by the sublattice symmetry, and to the one-dimensional Weyl semimetal behavior of the zigzag carbon chain.

A theoretical prediction of super high-performance thermoelectric materials based on MoS2/WS2 hybrid nanoribbons.

Modern society is hungry for electrical power. To improve the efficiency of energy harvesting from heat, extensive efforts seek high-performance thermoelectric materials that possess large differences between electronic and thermal conductance. Here we report a super high-performance material of consisting of MoS2/WS2 hybrid nanoribbons discovered from a theoretical investigation using nonequilibrium Green's function methods combined with first-principles calculations and molecular dynamics simulations. The hybrid nanoribbons show higher efficiency of energy conversion than the MoS2 and WS2 nanoribbons due to the fact that the MoS2/WS2 interface reduces lattice thermal conductivity more than the electron transport. By tuning the number of the MoS2/WS2 interfaces, a figure of merit ZT as high as 5.5 is achieved at a temperature of 600 K. Our results imply that the MoS2/WS2 hybrid nanoribbons have promising applications in thermal energy harvesting.

Nanostructured Carbon Allotropes with Weyl-like Loops and Points.

Carbon allotropes are subject of intense investigations for their superb structural, electronic, and chemical properties, but not for topological band properties because of the lack of strong spin-orbit coupling (SOC). Here, we show that conjugated p-orbital interactions, common to most carbon allotropes, can in principle produce a new type of topological band structure, forming the so-called Weyl-like semimetal in the absence of SOC. Taking a structurally stable interpenetrated graphene network (IGN) as example, we show, by first-principles calculations and tight-binding modeling, that its Fermi surface is made of two symmetry-protected Weyl-like loops with linear dispersion along perpendicular directions. These loops are reduced to Weyl-like points upon breaking of the inversion symmetry. Because of the topological properties of these band-structure anomalies, remarkably, at a surface terminated by vacuum there emerges a flat band in the loop case and two Fermi arcs in the point case. These topological carbon materials may also find applications in the fields of catalysts.