arXiv subjects

Chengyong Zhong

Publications and source records attributed to Chengyong Zhong.

At least 19 recordsLinked to original sources

arXiv2024

Strain-tunable Dirac semimetal phase transition and emergent superconductivity in a borophane

A two-dimensional (2D) Dirac semimetal with concomitant superconductivity has been long sought but rarely reported. It is believed that light-element materials have the potential to realize this goal owing to their intrinsic lightweight and metallicity. Here, based on the recently synthesized $β_{12}$ hydrogenated borophene [Science 371, 1143 (2021)], we investigate its counterpart named $β_{12}$-$ \rm {B_5H_3}$. Our first-principles calculations suggest it has good stability. $β_{12}$-$ \rm {B_5H_3}$ is a scarce Dirac semimetal demonstrating a strain-tunable phase transition from three Dirac cones to a single Dirac cone. Additionally, $β_{12}$-$ \rm {B_5H_3}$ is also a superior phonon-mediated superconductor with a superconducting critical temperature of 32.4 K and can be further boosted to 42 K under external strain. The concurrence of Dirac fermions and superconductivity, supplemented with dual tunabilities, reveals $β_{12}$-$ \rm {B_5H_3}$ is an attractive platform to study either quantum phase transition in 2D Dirac semimetal or the superconductivity or the exotic physics brought about by their interplay.

cond-mat.mtrl-sci↗

arXiv2021

Theoretical design of all-carbon networks with intrinsic magnetism

To induce intrinsic magnetism in the nominally nonmagnetic carbon materials containing only $s$ and $p$ electrons is an intriguing yet challenging task. Here, based on first-principles electronic structure calculations, we propose a universal approach inspired by Ovchinnikov's rule to guide us the design of a series of imaginative magnetic all-carbon structures. The idea is to combine the differently stacked graphene layers via the acetylenic linkages (-C$\equiv$C-) to obtain a class of two-dimensional (2D) and three-dimensional (3D) carbon networks. With first-principles electronic structure calculations, we confirm the effectiveness of this approach via concrete examples of double-layer ALBG-C14, triple-layer ALTG-C22, and bulk IALG-C30. We show that these materials are antiferromagnetic (AFM) semiconductors with intralayer Néel and interlayer AFM couplings. According to the above idea, our work not only provides a promising design scheme for magnetic all-carbon materials, but also can apply to other $π$-bonding network systems.

cond-mat.mtrl-sci↗

arXiv2021

Predication of topological states in the allotropes of group-IV elements

Three-dimensional topological insulators have been studied for approximately fifteen years, but those made from group-IV elements, especially Ge and Sn, seem particularly attractive owing to their nontoxicity, sizable intrinsic spin-orbit coupling (SOC) strength and natural compatibility with the current semiconductor industry. However, group-IV elemental TIs have rarely been reported, except for the low temperature phase of alpha-Sn under strain. Here, based on first-principles calculations, we propose new allotropes of Ge and Sn, named T5-Ge(Sn), as desirable TIs. These new allotropes are also highly anisotropic Dirac semimetals if the SOC is turned off. To the best of our knowledge, T5-Ge(Sn) are the first 3D allotropes of Ge(Sn) that possess topological states in their equilibrium state at room temperature. Additionally, their isostructures of C and Si are metastable indirect and direct semiconductors. Our work not only reveals two promising TIs, but more profoundly, we justify the advantages of group-IV elements as topological quantum materials (TQMs) for fundamental research and potential practical applications, and thus reveal a new direction in the search for desirable TQMs.

cond-mat.mtrl-sci↗

arXiv2019

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

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

cond-mat.mtrl-sci↗

arXiv2019

Grapheayne: a class of low-energy carbon allotropes with diverse optoelectronic and topological properties

A series of carbon allotropes with novel optoelectronic and rich topological properties is predicted by systematic first-principles calculations. These fascinating carbon allotropes can be derived by inserting acetylenic linkages (-C$\equiv$C-) into graphite, hence they are termed as grapheaynes. Grapheaynes possess two different space groups, $P$2/$m$ or $C$2/$m$, and contain simultaneously the $sp$, $sp^2$, and $sp^3$ chemical bonds. They have formation energies lower than the already experimentally synthesized graphdiyne and other theoretically predicted carbon allotropes with acetylenic linkages. Particularly, when the width $n$ of grapheayne-$n$ exceeds 15, its cohesive energy is lower than that of diamond, and approaches that of graphite with increasing $n$. Remarkably, we find that some grapheaynes behave as semiconductors with direct narrow band gaps and own the highest absorption coefficients among all known semiconducting carbon allotropes, while some others are topological semimetals with nodal lines. Especially, some grapheaynes can be engineered with tunable direct band gaps in the range of 1.07-1.87 eV and have ideal properties for photovoltaic applications. Our work not only uncovers the unique atomic arrangement and prominent properties of the grapheayne family, but also offers a treasury that provides promising materials for catalyst, energy storage, molecular sieves, solar cell, and electronic devices.

cond-mat.mtrl-sci↗

arXiv2019

Two-dimensional honeycomb borophene oxide: A promising anode material offering super high capacity for Li/Na-ion batteries

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 (less than 1 V), and small lattice change (less than 6.2%) during intercalation. Most remarkably, its theoretical storage capacity is extremely high, reaching up to 2137 mAh/g for Li and 1425 mAh/g for Na. Its Li storage capacity is more than six times higher than graphite (372 mAh/g), and is actually 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.

cond-mat.mtrl-sci↗

arXiv2019

Hexagonal supertetrahedral boron: A topological metal with multiple spin-orbit-free emergent fermions

We predict a new three-dimensional (3D) boron allotrope based on systematic first-principles electronic structure calculations. This allotrope can be derived by substituting each carbon atom in a hexagonal diamond lattice with a B$_{4}$ tetrahedron and possesses the same space group $P6_{3}/mmc$ as hexagonal diamond, hence it is termed as H-boron. We show that H-boron has good stability and excellent mechanical property. Remarkably, we find that H-boron is a topological metal with rich types of spin-orbit-free emergent fermions, including semi-Dirac fermion, quadratic and linear triple-point fermion, nodal-line fermion, and nodal-surface fermion. We clarify their symmetry protections and characterize them by constructing the corresponding low-energy effective models. Our work not only discovers a new boron allotrope with excellent properties, it also offers a platform to explore interesting physics of new kinds of emergent fermions.

cond-mat.mtrl-sci↗

arXiv2019

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

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

cond-mat.mtrl-sci↗

arXiv2018

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, the 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.

cond-mat.mtrl-sci↗

arXiv2018

Three-dimensional honeycomb carbon: Junction line distortion and novel emergent fermions

Carbon enjoys a vast number of allotropic forms, each possessing unique properties determined by the lattice structures and bonding characters. Here, based on first-principles calculations, we propose a new three-dimensional carbon allotrope--hC28. We show that hC28 possesses exceptional energetic, dynamical, thermal, and mechanical stability. It is energetically more stable than most other synthesized or proposed carbon allotropes. The material has a relatively small bulk modulus, but is thermally stable at temperatures as high as 2000 K. The structural, mechanical, x-ray diffraction, and electronic properties are systematically investigated. Particularly, we show that its low-energy band structure hosts multiple unconventional emergent fermions, including the quadratic-contact-point fermions, the birefringent Dirac fermions, and the triple-point fermions. We construct effective models to characterize each kind of fermions. Our work not only discovers a new carbon allotropic form, it also reveals remarkable mechanical and electronic properties for this new material, which may pave the way towards both fundamental studies as well as practical applications.

physics.comp-ph↗

arXiv2018

Ferromagnetism and Wigner crystallization in Kagome graphene and related structures

Interaction in a flat band is magnified due to the divergence in the density of states, which gives rise to a variety of many-body phenomena such as ferromagnetism and Wigner crystallization. Until now, however, most studies of the flat band physics are based on model systems, making their experimental realization a distant future. Here, we propose a class of systems made of real atoms, namely, carbon atoms with realistic physical interactions (dubbed here as Kagome graphene/graphyne). Density functional theory calculations reveal that these Kagome lattices offer a controllable way to realize robust flat bands sufficiently close to the Fermi level. Upon hole doping, they split into spin-polarized bands at different energies to result in a flat-band ferromagnetism. At a half filling, this splitting reaches its highest level of 768 meV. At smaller fillings, e.g., when ν=1/6, on the other hand, a Wigner crystal spontaneously forms, where the electrons form closed loops localized on the grid points of a regular triangular lattice. It breaks the translational symmetry of the original Kagome lattice. We further show that the Kagome lattices exhibit good mechanical stabilities, based on which a possible route for experimental realization of the Kagome graphene is also proposed.

cond-mat.mtrl-sci↗

arXiv2018

A review of thermal transport and electronic properties of borophene

In recent years, two-dimensional boron sheets (borophene) have been experimentally synthesized and theoretically observed as promising conductor or transistor with novel thermal and electronic properties. We first give a general survey of some notable electronic properties of borophene, including the superconductivity and topological characters. We then mainly review the basic approaches, thermal transport, as well as the mechanical properties of borophene with different configurations. This review gives a general understanding of some of the crucial thermal transport and electronic properties of borophene, and also calls for further experimental investigations and applications on certain scientific community.

cond-mat.mtrl-sci↗

arXiv2018

Nodal surface semimetals: Theory and material realization

We theoretically study the three-dimensional topological semimetals with nodal surfaces protected by crystalline symmetries. Different from the well-known nodal-point and nodal-line semimetals, in these materials, the conduction and valence bands cross on closed nodal surfaces in the Brillouin zone. We propose different classes of nodal surfaces, both in the absence and in the presence of spin-orbit coupling (SOC). In the absence of SOC, a class of nodal surfaces can be protected by spacetime inversion symmetry and sublattice symmetry and characterized by a $\mathbb{Z}_2$ index, while another class of nodal surfaces are guaranteed by a combination of nonsymmorphic two-fold screw-rotational symmetry and time-reversal symmetry. We show that the inclusion of SOC will destroy the former class of nodal surfaces but may preserve the latter provided that the inversion symmetry is broken. We further generalize the result to magnetically ordered systems and show that protected nodal surfaces can also exist in magnetic materials without and with SOC, given that certain magnetic group symmetry requirements are satisfied. Several concrete nodal-surface material examples are predicted via the first-principles calculations. The possibility of multi-nodal-surface materials is discussed.

cond-mat.mes-hall↗

arXiv2017

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 concatenated 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.

cond-mat.mtrl-sci↗

arXiv2016

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

The enchanting Dirac fermions in graphene stimulated us to seek for other two-dimensional (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 them possesses Dirac electrons. Herein, by means of density functional theory (DFT) 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, 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. The unique electronic properties induced by special bond characteristics also imply that this boron monolayer may be a good superconductor.

cond-mat.mes-hall↗

arXiv2016

Type-I and type-II Dirac fermions in graphene with nitrogen-molecule line defects

Recently, type-II Dirac fermions characterized by strongly titled Dirac cones have been proposed. The new fermions exhibit unique physical properties different from the type-I Dirac fermions in graphene, and thus attract tremendous attentions. Up to date, all type-II fermions are only found in the heavy compounds with strong spin obit coupling. Here, we propose that both type-I and type-II Dirac fermions can exist in the graphene embedding nitrogen-molecule line defects (NMLDs). While the types of Dirac fermions are determined by the size W of graphene nanoribbons between the line defects. By comparing the two types of Dirac fermions, their different physical properties and originations are revealed directly. Remarkably, the type-I Dirac points induce one Fermi arc corresponding to edge states along the armchair direction, while the type-II Dirac points induce two Fermi arcs corresponding to two sets of edge states along the zigzag direction. These results not only expand our views on the Dirac fermions in two-dimensional structures, but also extend their applications in electronics.

cond-mat.mtrl-sci↗

arXiv2016

Semi-Dirac Semimetal in Silicene Oxide

Semi-Dirac semimetal is a material exhibiting 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 matters. 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 chemical formula of Si2O to be a candidate of semi-Dirac semimetal. Our approach is backed by the analysis and understanding of the effect of p-orbital frustration on the band structure of the graphene-like materials.

cond-mat.mes-hall↗

arXiv2016

Electron and phonon properties and gas storage in carbon honeycomb

A new kind of three-dimensional carbon allotropes, termed carbon honeycomb (CHC), has recently been synthesized [PRL 116, 055501 (2016)]. Based on the experimental results, a family of graphene networks are constructed, and their electronic and phonon properties are calculated by using first principles methods. All networks are porous metal with two types of electron transport channels along the honeycomb axis and they are isolated from each other: one type of channels is originated from the orbital interactions of the carbon zigzag chains and is topologically protected, while the other type of channels 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. 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 experiment.

cond-mat.mtrl-sci↗