Superconductivity in Topological Semimetal θ-TaN at High Pressure

Recently,θ-TaN was proposed to be a topological semimetal with a new type of triply degenerate nodal points.Here,we report studies of pressure dependence of transport,Raman spectroscopy and synchrotron x-ray diffraction on θ-TaN up to 61 GPa.We find that θ-TaN becomes superconductive above 24.6 GPa with T_c at 3.1 K.The θ-TaN is of n-type carrier nature with carrier density about 1.1 × 10~(20)/cm~3 at 1.2 GPa and 20 K, while the carrier density increases with the pressure and saturates at about 40 GPa in the measured range.However,there is no crystal structure transition with pressure up to 39 GPa,suggesting the topological nature of the pressure induced superconductivity.     

Single crystal growth,structural and transport properties of bad metal RhSb_2

We have successfully grown an arsenopyrite marcasite type RhSb2 single crystal, and systematically investigated its crystal structure, electrical transport, magnetic susceptibility, heat capacity, and thermodynamic properties. We found that the temperature-dependent resistivity exhibits a bad metal behavior with a board peak around 200 K. The magnetic susceptibility of RhSb2 shows diamagnetism from 300 K to 2 K. The low-temperature specific heat shows a metallic behavior with a quite small electronic specific-heat coefficient. No phase transition is observed in both specific heat and magnetic susceptibility data. The Hall resistivity measurements show that the conduction carriers are dominated by electrons with ne = 8.62 × 10¹⁸ cm^-3 at 2 K, and the electron carrier density increases rapidly above 200 K without change sign. Combining with ab-initio band structure calculations, we showed that the unusual peak around 200 K in resistivity is related to the distinct electronic structure of RhSb_2. In addition, a large thermopower S(T) about -140 μV/K is observed around 200 K, which might be useful for future thermoelectric applications.     

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.     

压力诱导酌拓扑绝缘体化合物Bi2Te3常压相的超导性

通过高压电阻测量,发现了拓扑绝缘体化合物BizTe3压力诱导的超导性,在3-6GPa的压力范围内,超导临界温度L约为3K．高压下原位同步辐射的结果证明这个超导相来源于常压相结构．通过霍尔效应的测量,发现超导的Bi2Te3样品的载流子为P型．对高压同步辐射结果Reitveld精修得到的晶格参数和原子位置,并以此进行第一性...     

从反常霍尔效应到量子反常霍尔效应

 美国物理学家霍尔 (Edwin H.Hall) 在1879 年发现,当电流垂直于外磁场通过导体时,在导体的垂直于磁场和电流方向的两个边界之间会出现电势差(图1)。这个现象被称作霍尔效应。在当时要理解这一重要的现象还非常困难,因为电子的概念在18年后才被提出来。     

磁性拓扑绝缘体中的量子化反常霍尔效应——无需外磁场的量子化霍尔效应

文章从平常霍尔效应出发,介绍了反常霍尔效应及其内秉物理机制,并在此基础上介绍了其量子化版本——量子化反常霍尔效应.然后从拓扑有序态的角度,重点讨论了量子化反常霍尔效应与量子霍尔效应、量子自旋霍尔效应、拓扑绝缘体等之间的区别与内在联系.最后介绍了通过在拓扑绝缘体(Bi2Se3,Bi2Te3和Sb2Te3)薄膜中掺杂过渡金属元素(Cr或Fe)实现量子化反常霍尔效应的方法.     

Band Structure of the Non—Oxide Perovskite MgCNi3

Using the self-consistent full-potential linearized augmented plane wave method,we carry out the electronic structure calculations of MgCNi3 in the local spin density approximation(LSDA) of the density-functional theory.The LSDA solution is metal.The magnetic moment on the Ni ion is only 0.014uB.There is a peak in the density of state just below the Fermi energy(Ef),which can be strongly correlated with various instabilities.After including the strong electron-electron correlation effects on the Ni 3d state by the on-site Coulomb interaction correction,the density of state is greatly redistributed.and the peak just below Ef disappears.The magnetic moment on the Ni ion becomes 0.66uB.Both LSDA and LSDA U have shown that the electroic states near the Fermi surface are dominated by the 3d orbital of Ni.     

Topological nodal line semimetals

We review the recent,mainly theoretical,progress in the study of topological nodal line semimetals in three dimensions.In these semimetals,the conduction and the valence bands cross each other along a one-dimensional curve in the three-dimensional Brillouin zone,and any perturbation that preserves a certain symmetry group(generated by either spatial symmetries or time-reversal symmetry) cannot remove this crossing line and open a full direct gap between the two bands.The nodal line(s) is hence topologically protected by the symmetry group,and can be associated with a topological invariant.In this review,(ⅰ) we enumerate the symmetry groups that may protect a topological nodal line;(ⅱ) we write down the explicit form of the topological invariant for each of these symmetry groups in terms of the wave functions on the Fermi surface,establishing a topological classification;(ⅲ) for certain classes,we review the proposals for the realization of these semimetals in real materials;(ⅳ) we discuss different scenarios that when the protecting symmetry is broken,how a topological nodal line semimetal becomes Weyl semimetals,Dirac semimetals,and other topological phases;and(ⅴ) we discuss the possible physical effects accessible to experimental probes in these materials.     

Electronic Structure of the Weak Topological Insulator Candidate Zintl Ba3Cd2Sb4

One of the greatest triumph of condensed matter physics in the past ten years is the classification of materials by the principle of topology.The existence of topological protected dissipationless surface state makes topological insulators great potential for applications and hotly studied.However,compared with the prosperity of strong topological insulators,theoretical predicted candidate materials and experimental confirmation of weak topological insulators(WTIs) are both extremely rare.By com...