Erratum:Superconducting Single-Layer T-Graphene and Novel Synthesis Routes[Chin.Phys.Lett.36(2019)097401]

In Fig.4(a)of the article,the range ofλ(w)exhibited by the horizontal coordinate of the middle panel should be from 0 to 2,instead of 0 to 1.Figure 4(a)should be as follows:We note that this mistake does not affect the conclusion of our report,and apologize to readers for any inconvenience caused by our oversight.

Superconducting Single-Layer T-Graphene and Novel Synthesis Routes

Single-layer superconductors are ideal materials for fabricating superconducting nano devices.However,up to date,very few single-layer elemental superconductors have been predicted and especially no one has been successfully synthesized yet.Here,using crystal structure search techniques and ab initio calculations,we predict that a single-layer planar carbon sheet with 4-and 8-membered rings called T-graphene is a new intrinsic elemental superconductor with superconducting critical temperature(Tc)up to around 20.8 K.More importantly,we propose a synthesis route to obtain such a single-layer T-graphene,that is,a T-graphene potassium intercalation compound(C4 K with P4/mmm symmetry)is firstly synthesized at high pressure(>11.5 GPa)and then quenched to ambient condition;and finally,the single-layer T-graphene can be either exfoliated using the electrochemical method from the bulk C4 K,or peeled off from bulk T-graphite C4,where C4 can be obtained from C4 K by evaporating the K atoms.Interestingly,we find that the calculated Tc of C4 K is about 30.4 K at 0 GPa,which sets a new record for layered carbon-based superconductors.The present findings add a new class of carbon-based superconductors.In particular,once the single-layer T-graphene is synthesized,it can pave the way for fabricating superconducting devices together with other 2 D materials using the layer-by-layer growth techniques.

Partially Diffusive Helium-Silica Compound under High Pressure

Helium is the second most abundant element in the universe, and together with silica, they are important components of giant planets. Exploring the reactivity and state of helium and silica under high pressure is crucial for understanding of the evolution and internal structure of giant planets. Here, using first-principles calculations and crystal structure predictions, we identify four stable phases of a helium-silica compound with seven/eight-coordinated silicon atoms at pressure of 600–4000 GPa, corresponding to the interior condition of the outer planets in the solar system. The density of He Si O2 agrees with current structure models of the planets.This helium-silica compound exhibits a superionic-like helium diffusive state under the high-pressure and hightemperature conditions along the isentropes of Saturn, a metallic fluid state in Jupiter, and a solid state in the deep interiors of Uranus and Neptune. These results show that helium may affect the erosion of the rocky core in giant planets and may help to form a diluted core region, which not only highlight the reactivity of helium under high pressure but also provide evidence helpful for building more sophisticated interior models of giant planets.

Spatio-temporal control of the phase separation of chemically active immotile colloids

Understanding and controlling phase separation in nonequilibrium colloidal systems are of both fundamental and applied importance.In this article,we investigate the spatiotemporal control of phase separation in chemically active immotile colloids.We show that a population of silver colloids can spontaneously phase separate into dense clusters in hydrogen peroxide(H_(2)O_(2))due to phoretic attraction.The characteristic length of the formed pattern was quantified and monitored over time,revealing a growth and coarsening phase with different growth kinetics.By tuning the trigger frequency of light,the lengths and growth kinetics of the clusters formed by silver colloids in H_(2)O_(2)can be controlled.In addition,structured light was used to precisely control the shape,size,and contour of the phase-separated patterns.This study provides insight into the microscopic details of the phase separation of chemically active colloids induced by phoretic attraction,and presents a generic strategy for controlling the spatiotemporal evolution of the resulting mesoscopic patterns.

钾氨电子化合物中的准二维自旋-派尔斯相变

Electron-phonon interactions and electron-electron correlations represent two crucial facets of condensed matter physics.For instance,in a half-filled spin-1/2 anti-ferromagnetic chain,the lattice dimerization induced by electron-nucleus interaction can be intensified by onsite Coulomb repulsion,resulting in a spin-Peierls state.Through first-principles calculations and crystal structure prediction methods,we have identified that under mild pressures,potassium and ammonia can form stable compounds:R3m K(NH_(3))_(2),Pm3 m K(NH_(3))_(2),and Cm K_(2)(NH_(3))_(3).Our predictions suggest that the R3 m K(NH_(3))_(2)exhibits electride characteristics,marked by the formation of interstitial anionic electrons(IAEs)in the interlayer space.These IAEs are arranged in quasi-two-dimensional triangular arrays.With increasing pressure,the electronic van-Hove singularity shifts toward the Fermi level,resulting in an augmented density of states and the onset of both Peierls and magnetic instabilities.Analyzing these instabilities,we determine that the ground state of the R3 m K(NH_(3))_(2)is the dimerized P2_(1)/m phase with zigzag-type anti-ferromagnetic IAEs.This state can be described by the triangular-lattice antiferromagnetic Heisenberg model with modulated magnetic interactions.Furthermore,we unveil the coexistence and positive interplay between magnetic and Peierls instability,constituting a scenario of spin-Peierls instability unprecedented in realistic 2D materials,particularly involving IAEs.This work provides valuable insights into the coupling of IAEs with the adjacent lattice and their spin correlations in quantum materials.

Three-Dirac-fermion approach to unexpected universal gapless surface states in van der Waals magnetic topological insulators

Layered van der Waals(vdW) topological materials, especially the recently discovered MnBi_(2)Te_(4)-family magnetic topological insulators(TIs), have aroused great attention. However, there has been a serious debate about whether the surface states are gapped or gapless for antiferromagnetic(AFM) TI MnBi_(2)Te_(4), which is crucial to the prospect of various magnetic topological phenomena. Here, a minimal three-Dirac-fermion approach is developed to generally describe topological surface states of nonmagnetic/magnetic vdW TIs under the modulation of the interlayer vdW gap. In particular, this approach is applied to address the controversial issues concerning the surface states of vdW AFM TIs. Remarkably, topologically protected gapless Dirac-cone surface states are found to arise due to a small expansion of the interlayer vdW gap on the surface, when the Chern number equals zero for the surface ferromagnetic layer;while the surface states remain gapped in all other cases. These results are further confirmed by our first-principles calculations on AFM TI MnBi_(2)Te_(4). The theorectically discovered gapless Dirac-cone states provide a unique mechanism for understanding the puzzle of the experimentally observed gapless surface states in MnBi_(2)Te_(4).This work also provides a promising way for experiments to realize the intrinsic magnetic quantum anomalous Hall efect in MnBi_(2)Te_(4) films with a large energy gap.

Metallic Aluminum Suboxides with Ultrahigh Electrical Conductivity at High Pressure

Aluminum,as the most abundant metallic elemental content in the Earth's crust,usually exists in the form of alumina(Al_(2)O_(3)).However,the oxidation state of aluminum and the crystal structures of aluminum oxides in the pressure range of planetary interiors are not well established.

A novel superhard tungsten nitride predicted by machine-learning accelerated crystal structure search

Transition metal nitrides have been suggested to have both high hardness and good thermal stability with large potential application value, but so far stable superhard transition metal nitrides have not been synthesized. Here, with our newly developed machine-learning accelerated crystal structure searching method, we designed a superhard tungsten nitride, h-WN6, which can be synthesized at pressure around 65 GPa and quenchable to ambient pressure. This h-WN6 is constructed with single-bonded armchair-like N6 rings and presents ionic-like features, which can be formulated as W^2.4＋N^2.4-. It has a band gap of 1.6 eV at 0GPa and exhibits an abnormal gap broadening behavior under pressure. Excitingly, this h-WN6 is found to be the hardest among transition metal nitrides known so far （Vickers hardness around 57 GPa） and also has a very high melting temperature （around 1,900 K）. Additionally, the good gravimet- ric （3.1 kJ/g/and volumetric （28.0 kJ/cm3） energy densities make this nitrogen-rich compound a potential high-energy-density material, These predictions support the designing rules and may stimulate future experiments to synthesize superhard and high-energy-density material.

High-pressure phases of Weyl semimetals NbP, NbAs, TaP,and TaAs

In this study, we used the crystal structure search method and first-principles calculations to systematically explore the highpressure phase diagrams of the TaAs family （NbP, NbAs, TaP, and TaAs）. Our calculation results show that NbAs and TaAs have similar phase diagrams, the same structural phase transition sequence I41md→Pδm2→}P21/c→Pm3m, and slightly different transition pressures. The phase transition sequence of NbP and TaP differs somewhat from that of NbAs and TaAs, in which new structures emerge, such as the Cmcm structure in NbP and the Pmmn structure in TaP. Interestingly, we found that in the electronic structure of the high-pressure phase Pδm2-NbAs, there are coexisting Weyl points and triple degenerate points, similar to those found in high-pressure Pδm2-TaAs.

Anomalous Andreev reflection on a torus-shaped Fermi surface

Andreev reflection(AR)refers to the electron-hole conversion at the normal metal-superconductor interface.In a threedimensional metal with a spherical Fermi surface,retro(specular)AR can occur with the sign reversal of all three(a single)components of particle velocity.Here,we predict a novel type of AR with the inversion of two velocity components,dubbed"anomalous Andreev reflection"(AAR),which can be realized in a class of materials with a torus-shaped Fermi surface,such as doped nodal line semimetals.For its toroidal circle perpendicular to the interface,the Fermi torus doubles the AR channels and generates multiple AR processes.In particular,the AAR and retro AR are found to dominate electron transport in the light and heavy doping regimes,respectively.We show that the AAR visibly manifests itself as a ridge structure in the spatially resolved nonlocal conductance,in contrast to the peak structure for the retro AR.Our work opens a new avenue for the AR spectroscopy and offers a clear transport signature of the torus-shaped Fermi surface.

Tunable dynamical magnetoelectric effect in antiferromagnetic topological insulator MnBi_(2)Te_(4) films

Axion was postulated as an elementary particle to solve the strong charge conjugation and parity puzzle,and later axion was also considered to be a possible component of dark matter in the universe.However,the existence of axions in nature has not been confirmed.Interestingly,axions arise out of pseudoscalar fields derived from the Chern–Simons theory in condensed matter physics.In antiferromagnetic insulators,the axion field can become dynamical due to spin-wave excitations and exhibits rich exotic phenomena,such as axion polariton.However,antiferromagnetic dynamical axion insulator has yet been experimentally identified in realistic materials.Very recently,MnBi_(2)Te_(4)was discovered to be an antiferromagnetic topological insulator with a quantized static axion field protected by inversion symmetry P and magnetic-crystalline symmetry S.Here,we studied MnBi_(2)Te_(4)films in which both the P and S symmetries are spontaneously broken and found that substantially enhanced dynamical magnetoelectric effects could be realized through tuning the thickness of MnBi_(2)Te_(4)films,temperature,or element substitutions.Our results show that thin films of MnBi_(2)Te_(4)and related compounds could provide a promising material platform to experimentally study axion electrodynamics.

Composite topological nodal lines penetrating the Brillouin zone in orthorhombic AgF_(2)

It has recently been found that nonsymmorphic symmetries can bring many exotic band crossings.Here,based on symmetry analysis,we predict that materials with time-reversal symmetry in the space group of Pbca(No.61)possess rich symmetry-enforced band crossings,including nodal surfaces,fourfold degenerate nodal lines and hourglass Dirac loops,which appear in triplets as ensured by the cyclic permutation symmetry.We take Pbca AgF_(2) as an example in real systems and studied its band structures with ab initio calculations.Specifically,in the absence of spin-orbit coupling(SOC),besides the above-mentioned band degeneracies,this system features a nodal chain and a nodal armillary sphere penetrating the Brillouin zone(BZ).While with SOC,we find a new configuration of the hourglass Dirac loop/chain with the feature traversing the BZ,which originates from the splitting of a Dirac loop confined in the BZ.Furthermore,guided by the bulk-surface correspondence,we calculated the surface states to explore these bulk nodal phenomena.The evolution of these interesting nodal phenomena traversing the BZ under two specific uniaxial strains is also discussed.