Electronic structure,Dirac points and Fermi arc surface states in three-dimensional Dirac semimetal Na_3Bi from angle-resolved photoemission spectroscopy

The three-dimensional(3D) Dirac semimetals have linearly dispersive 3D Dirac nodes where the conduction band and valence band are connected. They have isolated 3D Dirac nodes in the whole Brillouin zone and can be viewed as a 3D counterpart of graphene. Recent theoretical calculations and experimental results indicate that the 3D Dirac semimetal state can be realized in a simple stoichiometric compound A_3Bi(A = Na, K, Rb). Here we report comprehensive high-resolution angle-resolved photoemission(ARPES) measurements on the two cleaved surfaces,(001) and(100), of Na_3Bi. On the(001) surface, by comparison with theoretical calculations, we provide a proper assignment of the observed bands, and in particular, pinpoint the band that is responsible for the formation of the three-dimensional Dirac cones. We observe clear evidence of 3D Dirac cones in the three-dimensional momentum space by directly measuring on the k_x–k_y plane and by varying the photon energy to get access to different out-of-plane k_zs. In addition, we reveal new features around the Brillouin zone corners that may be related with surface reconstruction. On the(100) surface, our ARPES measurements over a large momentum space raise an issue on the selection of the basic Brillouin zone in the(100) plane. We directly observe two isolated 3D Dirac nodes on the(100) surface. We observe the signature of the Fermi-arc surface states connecting the two 3D Dirac nodes that extend to a binding energy of ~150 me V before merging into the bulk band. Our observations constitute strong evidence on the existence of the Dirac semimetal state in Na_3Bi that are consistent with previous theoretical and experimental work. In addition, our results provide new information to clarify on the nature of the band that forms the3 D Dirac cones, on the possible formation of surface reconstruction of the(001) surface, and on the issue of basic Brillouin zone selection for the(100) surface.     

纸色谱法分离测定金属离子

纸色谱法是使用滤纸为载体的色谱法,固定相一般为纸纤维上吸附的水分,流动相为有机溶剂。如果使纸上吸留不与水混溶的有机物质作为固定相,以亲水性溶剂或溶液为流动相时。就称为纸上反相分配色谱法。     

为什么碱金属在过量空气中加热燃烧时,其主要产物为Li_2O、Na_2O_2、KO_2、RbO_2和CsO_2?

碱金属在过量空气中加热燃烧时,生成三种类型的氧化物,即正常氧化物Li_2O,过氧化物Na_2O_2和超氧化物KO_2、RbO_2和CsO_2。这是由于不同的碱金属所形成的各种类型氧化物的稳定性不同的缘故。一、从结构上来看 Li~+离子的半径很小,只是68pm,又是外     

Observation of a Flat Band in Silicene

Flat bands that exhibit weak band dispersion in crystals can host heavy fermions that are cru- cial to many strongly correlated phenomena, such as ferromagnetism and superconductivity. For topologically trivial flat bands, theories have proven the stability of the ferromagnetic ground state i.e., flat band ferromagnetism. In the presence of weak disorders, systems with flat bands are expected to exhibit unconventional Anderson localization due to the lifting of degeneracy. More interestingly, flat bands with nontrivial topology, i.e., topological flat bands, are expected to host a fractional quantum Hall effect at zero magnetic field. Hence, searching for topologically trivial/nontrivial flat bands （TFB） has been an interesting while challenging task so far.     

无机离子交换剂磷酸锆的制备及其在纸色谱法中的应用

磷酸错具有层状的结构,层与层之间借氢键结合,并在晶格中形成空腔,它的活性基因为-PO4H墓,其中的H+离子可在溶液中与金属离子进行交换反应.