Surface doping manipulation of the insulating ground states in Ta_(2)Pd_(3)Te_(5) and Ta_(2)Ni_(3)Te_(5)

Manipulating emergent quantum phenomena is a key issue for understanding the underlying physics and contributing to possible applications.Here we study the evolution of insulating ground states of Ta_(2)Pu_(3)Te_(5) and Ta_(2)Ni_(3)Te_(5) under in-situ surface potassium deposition via angle-resolved photoemission spectroscopy.Our results confirm the excitonic insulator character of Ta_(2)d_(3)Te_(5).Upon surface doping,the size of its global gap decreases obviously.After a deposition time of more than 7 min,the potassium atoms induce a metal-insulator phase transition and make the system recover to a normal state.In contrast,our results show that the isostructural compound Ta_(2)Ni_(3)Te_(5) is a conventional insulator.The size of its global gap decreases upon surface doping,but persists positive throughout the doping process.Our results not only confirm the excitonic origin of the band gap in Ta_(2)Pd_(3)Te_(5),but also offer an effective method for designing functional quantum devices in the future.

Ferromagnetic MnSn Monolayer Epitaxially Grown on Silicon Substrate

Two-dimensional(2D)ferromagnetic materials have been exhibiting promising potential in applications,such as spintronics devices.To grow epitaxial magnetic films on silicon substrate,in the single-layer limit,is practically important but challenging.In this study,we realized the epitaxial growth of Mn Sn monolayer on Si(111)substrate,with an atomically thin Sn/Si(111)-231/2×231/2-buffer layer,and controlled the Mn Sn thickness with atomic-layer precision.We discovered the ferromagnetism in Mn Sn monolayer with the Curie temperature(Tc)of^54 K.As the Mn Sn film is grown to 4 monolayers,Tc increases accordingly to^235 K.The lattice of the epitaxial Mn Sn monolayer as well as the Sn/Si(111)-231/2×231/2 is perfectly compatible with silicon,and thus an sharp interface is formed between Mn Sn,Sn and Si.This system provides a new platform for exploring the 2D ferromagnetism,integrating magnetic monolayers into silicon-based technology,and engineering the spintronics heterostructures.

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.

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 I4_1 md→P6 m2→P2_1/c→Pm3 m, 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 P6 m2-NbAs, there are coexistingWeyl points and triple degenerate points, similar to those found in high-pressure P6 m2-TaAs.

Evidence for a Dirac nodal-line semimetal in SrAs_3

Dirac nodal-line semimetals with the linear bands crossing along a line or loop, represent a new topological state of matter. Here, by carrying out magnetotransport measurements and performing first-principle calculations, we demonstrate that such a state has been realized in high-quality single crystals of SrAs_3.We obtain the nontrivial ∏ Berry phase by analysing the Shubnikov-de Haas quantum oscillations. We also observe a robust negative longitudinal magnetoresistance induced by the chiral anomaly.Accompanying first-principles calculations identifies that a single hole pocket enclosing the loop nodes is responsible for these observations.

Determining dimensionalities and multiplicities of crystal nets

Low-dimensional materials have attracted significant attention over the past decade.To discover new low-dimensional materials,high-throughput screening methods for structures with target dimensionality have been applied in different materials databases.For this purpose,the reliability of dimensionality identification is therefore highly important.In this work,we find that the existence of self-penetrating nets may lead to incorrect results by previous methods.Instead of this,we use the quotient graph to analyse the topologies of structures and compute their dimensionalities.

非常规材料:电子分布不在原子上

在拓扑量子化学中,研究人员发展了空间群下完备的基本能带表示,既对应于实空间的局域轨道,又对应于倒空间的一组不可约表示.通过计算实际材料的高对称k点的不可约表示,再利用基本能带表示分解来判断材料的拓扑性质.本研究发现拓扑平庸的材料可以分为电子电荷中心在原子上的常规材料和电荷中心不在原子上的“非常规材料”.这种非常规的电子分布特征在特定晶体对称群下可以方便地由能带表示分解来判断:占据态是基础能带表示的组合,但不是原子轨道对应的能带表示的组合.同时作者引入空位上的(必要)能带表示和实空间不变量来标定这一非常规电子分布特性.利用这一基于对称性本征值的能带表示分解方法,本文通过高通量搜索找到了423个非常规材料,并讨论了这些材料热电、储氢、催化、超导等特殊的化学性质与非常规电荷分布的相关性.

Unprotected quadratic band crossing points and quantum anomalous Hall effect in FeB_(2) monolayer

Quadratic band crossing points(QBCPs)and quantum anomalous Hall effect(QAHE)have attracted the attention of both theoretical and experimental researchers in recent years.Based on first-principle calculations,we find that the FeB_(2) monolayer is a nonmagnetic semimetal with QBCPs at K.Through symmetry analysis and k·p invariant theory,we find that the QBCP is not protected by rotation symmetry and consists of two Dirac points with the same chirality(Berry phase of 2π).Once introducing Coulomb interactions,we find that there is a spontaneous-time-reversal-breaking instability of the spinful QBCPs,which gives rise to a C=2 QAH insulator with orbital moment ordering.