Magnetotransport Properties of a Nodal Line Semimetal TiSi

We report the magnetoresistance(MR), de Haas-van Alphen(dHvA) effect and Hall effect measurements on a single crystal of TiSi, which is predicted to be a nodal line semimetal. With application of a magnetic field, a metal-to-insulator-like transition in ρ(T) and a nonsaturating MR are observed at low temperatures. The dHvA oscillations reveal a small Fermi-surface pocket with a nontrivial Berry phase. The analysis of the nonlinear Hall resistivity shows that TiSi is a multiband system with low carrier densities and high mobilities. All these results unambiguously prove the existence of Dirac fermions in TiSi.     

Re_3W的点接触安德烈夫反射谱研究

本文通过对不同晶体结构Re_3W样品的点接触测量和对比研究,证实具有中心对称结构和非中心对称结构的Re_3W都是弱耦合Bardeen-Cooper-Schrieffer超导体,同时发现在两个相表面都可以形成很理想的点接触结,即电子通过界面时受到的非弹性散射很弱.将Re_3W样品置于大气环境近六个月后重新进行测量,仍然能够得到类似的结果,表明Re_3W具有很好的稳定性. Re_3W的这种优良特性,不仅可通过点接触实验得到的参数推算出Re_3W两个相的费米速度,而且提供了一种简单的方法,可以在点接触实验中利用Re_3W来印证针尖材料的费米速度和测量其自旋极化率等.作为尝试,本文用Re_3W/Ni点接触结测量了铁磁性金属Ni的自旋极化率,得到了与前人报道一致的结果.     

Direct Microwave Synthesis of 11-Type Fe(Te,Se) Polycrystalline Superconductors with Enhanced Critical Current Density

We report a direct microwave synthesis method for the preparation of 11-type high quality Fe(Te,Se) polycrystalline superconductors. The bulk samples are rapidly synthesized under the microwave irradiation during several minutes, with a subsequent annealing process at 400℃. The samples exhibit a nearly single phase of the tetragonal PbO-type crystal structure with minor impurities. Morphology characterization shows high density, tight grain connectivity and large grain sizes around 100 μm with small cavities inside the sample. Resistivity and magnetization measurements both show similar superconducting transitions above 14 K. The magnetic hysteresis measurements display broad and symmetric loops without magnetic background, and a high critical current density J_c about 1.2 × 10~4 A/cm~2 at 2 K and 7 T is estimated by the Bean model. Compared with the solidstate reaction synthesized samples, these superconducting bulks from microwave-assisted synthesis are possibly free of the interstitial Fe due to smaller c-axis, higher T_c in magnetic transitions, better M–H loops without magnetic background and greatly enhanced J_c, and are promising as raw materials for the non-toxic Fe-based superconducting wires for large currents and high field applications.     

Pressure effect in the Kondo semimetal CeRu_4Sn_6 with nontrivial topology

Kondo semimetal CeRu_4Sn_6 is attracting renewed attention due to the theoretically predicted nontrivial topology in its electronic band structure. We report hydrostatic and chemical pressure effects on the transport properties of single-and poly-crystalline samples. The electrical resistivity ρ(T) is gradually enhanced by applying pressure over a wide temperature range from room temperature down to 25 mK. Two thermal activation gaps estimated from high-and low-temperature windows are found to increase with pressure. A flat ρ(T) observed at the lowest temperatures below 300 mK appears to be robust against both pressure and field. This feature as well as the increase of the energy gaps calls for more intensive investigations with respect to electron correlations and band topology.     

Superconductivity with a Violation of Pauli Limit and Evidences for Multigap in η-Carbide Type Ti4Ir2O

We report the synthesis, crystal structure, and superconductivity of Ti₄Ir₂O. The title compound crystallizes in an η-carbide type structure of the space group Fd3m（No. 227）, with lattice parameters a=b=c=11.6194（1） ?. The superconducting temperature T_c is found to be 5.1–5.7 K. Most surprisingly, Ti₄Ir₂O hosts an upper critical field of 16.45 T, which is far beyond the Pauli paramagnetic limit. Strong coupled superconductivity with evidence...     

过渡金属化合物中的竞争序与多体合作现象

文章简要介绍了过渡金属化合物所体现的层展现象和近几年作者所在研究组在几个不同过渡金属化合物系统开展的部分研究工作.     

Emergence of Superconductivity on the Border of Antiferromagnetic Order in RbMn6Bi5 under High Pressure:A New Family of Mn-Based Superconductors

We report the discovery of superconductivity on the border of antiferromagnetic order in a quasi-one-dimensional material RbMn₆Bi₅ via measurements of resistivity and magnetic susceptibility under high pressures.Its phase diagram of temperature versus pressure resembles those of many magnetism-mediated superconducting systems.With increasing pressure,its antiferromagnetic ordering transition with T_N=83 K at ambient pressure is first enhanced moderately and then s...     

Superconducting properties of the C15-type Laves phase ZrIr2 with an Ir-based kagome lattice

We report systematic studies on superconducting properties of the Laves phase superconductor ZrIr$_2$. It crystallizes in a C15-type (cubic MgCu$_2$-type, space group $Fd\overline{3}m$) structure in which the Ir atoms form a kagome lattice, with cell parameters $a=b=c=7.3596(1)$ Å. Resistivity and magnetic susceptibility measurements indicate that ZrIr$_2$ is a type-II superconductor with a transition temperature of 4.0 K. The estimated lower and upper critical fields are 12.8 mT and 4.78 T, respectively. Heat capacity measurements confirm the bulk superconductivity in ZrIr$_2$. ZrIr$_2$ is found to possibly host strong-coupled s-wave superconductivity with the normalized specific heat change $\Delta C_{\rm e}/\gamma T_{\rm c} \sim 1.86$ and the coupling strength $\Delta_0/k_{\rm B}T_{\rm c} \sim 1.92$. First-principles calculations suggest that ZrIr$_2$ has three-dimensional Fermi surfaces with simple topologies, and the states at Fermi level mainly originate from the Ir-5d and Zr-4d orbitals. Similar to SrIr$_2$ and ThIr$_2$, spin--orbit coupling has dramatic influences on the band structure in ZrIr$_2$.     

Optical properties of FeAs-based parent compound: A comparative study for polycrystalline EuFe2As2 and LaFeAsO

We report on a comparative optical investigation on EuFe₂As₂ and LaFeAsO polycrystalline samples. The spin-density wave (SDW) partial gap is observed for both compounds. The gap-like absorption peaks in the real part of conductivity for EuFe₂As₂ and LaFeAsO scale with their respective T _SDW. Most remarkably, the study reveals a substantial difference in optical reflectance spectra for the two prototype compounds of 122 and 1111 systems: EuFe₂As₂ shows a much higher reflectivity with fewer phonon modes than that of LaFeAsO. This yields optical evidence for much stronger anisotropy between ab-plane and c-axis in 1111-type compounds.     

Superconductivity in Hole-Doped （S1-xKx）Fe2As2

A series of layered （Sr1-xKx）Fe2As2 compounds with nominal x = 0-0.40 are synthesized by solid state reaction method. Similar to other parent compounds of iron-based pnictide superconductors, pure SrFe2As2 shows a strong resistivity anomaly near 210 K, which was ascribed to the spin-density-wave instability. The anomaly temperature is much higher than those observed in LaOFeAs and BaFe2As2, the two prototype parent compounds with ZrCuSiAs- and ThCr2Si2-type structures. K-doping strongly suppresses this anomaly and induces superconductivity. Like in the case of K-doped BaFe2As2, sharp superconducting transitions at Tc ～ 38 K is observed. We perform the Hall coefficient measurement, and confirm that the dominant carriers are hole-type. The carrier density is enhanced by a factor of 3 in comparison to F-doped LaOFeAs superconductor.