Yanpeng Qi

Superconductivity in Weyl semimetal candidate MoTe2

Transition metal dichalcogenides have attracted research interest over the last few decades due to their interesting structural chemistry, unusual electronic properties, rich intercalation chemistry and wide spectrum of potential applications. Despite the fact that the majority of related research focuses on semiconducting transition-metal dichalcogenides (for example, MoS2), recently discovered unexpected properties of WTe2 are provoking strong interest in semimetallic transition metal dichalcogenides featuring large magnetoresistance, pressure-driven superconductivity and Weyl semimetal states. We investigate the sister compound of WTe2, MoTe2, predicted to be a Weyl semimetal and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that bulk MoTe2 exhibits superconductivity with a transition temperature of 0.10 K. Application of external pressure dramatically enhances the transition temperature up to maximum value of 8.2 K at 11.7 GPa. The observed dome-shaped superconductivity phase diagram provides insights into the interplay between superconductivity and topological physics.

Signature of type-II Weyl semimetal phase in MoTe2

Topological Weyl semimetal (TWS), a new state of quantum matter, has sparked enormous research interest recently. Possessing unique Weyl fermions in the bulk and Fermi arcs on the surface, TWSs offer a rare platform for realizing many exotic physical phenomena. TWSs can be classified into type-I that respect Lorentz symmetry and type-II that do not. Here, we directly visualize the electronic structure of MoTe2, a recently proposed type-II TWS. Using angle-resolved photoemission spectroscopy (ARPES), we unravel the unique surface Fermi arcs, in good agreement with our ab initio calculations that have nontrivial topological nature. Our work not only leads to new understandings of the unusual properties discovered in this family of compounds, but also allows for the further exploration of exotic properties and practical applications of type-II TWSs, as well as the interplay between superconductivity (MoTe2 was discovered to be superconducting recently) and their topological order.

High transport critical current densities in textured Fe-sheathed Sr1−xKxFe2As2+Sn superconducting tapes

We report the realization of grain alignment in Sn-added Sr1−xKxFe2As2 superconducting tapes with Fe sheath prepared by ex-situ powder-in-tube method. At 4.2 K, high transport critical current densities Jc of 2.5 × 104 A/cm2 (Ic = 180 A) in self-field and 3.5 × 103 A/cm2 (Ic = 25.5 A) in 10 T have been measured. These values are the highest ever reported so far for Fe-based superconducting wires and tapes. We believe the superior Jc in our tape samples are due to well textured grains and strengthened intergrain coupling achieved by Sn addition. Our results demonstrated an encouraging prospect for application of iron based superconductors.

The role of silver addition on the structural and superconducting properties of polycrystalline Sr0.6K0.4Fe2As2

The effect of Ag addition (0–20 wt%) on polycrystalline Sr0.6K0.4Fe2As2 superconductor has been investigated. It is found that the critical transition temperature Tc was not depressed, and the irreversibility field Hirr and hysteresis magnetization were significantly enhanced upon Ag addition. A characterization study reveals that larger grains are observed in the Ag-added samples. Moreover, the formation of a glassy phase as well as an amorphous layer, which are present in almost all the grain edges and boundaries in pure samples, are suppressed by Ag addition. The improvement of superconducting properties in Ag-added samples may originate from the enlargement of grains as well as better connections between grains

Superconductivity at 34.7 K in the iron arsenide Eu0.7Na0.3Fe2As2

EuFe2As2 is a member of the ternary iron arsenide family. Similar to BaFe2As2 and SrFe2As2, EuFe2As2 exhibits a clear anomaly in resistivity near 200 K. Here, we report the discovery of superconductivity in Eu0.7Na0.3Fe2As2 by partial substitution of the europium site with sodium. ThCr2Si2 tetragonal structure, as expected for EuFe2As2, is formed as the main phase for the composition Eu0.7Na0.3Fe2As2. Resistivity measurements reveal that the transition temperature Tc as high as 34.7 K is observed in this compound. The rate of Tc suppression with the applied magnetic field is 3.87 T K−1, giving an extrapolated zero-temperature upper critical field of 100 T. It demonstrates a very encouraging application of the new superconductors.

Topological Quantum Phase Transition and Superconductivity Induced by Pressure in the Bismuth Tellurohalide BiTeI

A pressure-induced topological quantum phase transition has been theoretically predicted for the semiconductor bismuth tellurohalide BiTeI with giant Rashba spin splitting. In this work, evolution of the electrical transport properties in BiTeI and BiTeBr is investigated under high pressure. The pressure-dependent resistivity in a wide temperature range passes through a minimum at around 3 GPa, indicating the predicted topological quantum phase transition in BiTeI. Superconductivity is observed in both BiTeI and BiTeBr, while resistivity at higher temperatures still exhibits semiconducting behavior. Theoretical calculations suggest that superconductivity may develop from the multivalley semiconductor phase. The superconducting transition temperature, Tc , increases with applied pressure and reaches a maximum value of 5.2 K at 23.5 GPa for BiTeI (4.8 K at 31.7 GPa for BiTeBr), followed by a slow decrease. The results demonstrate that BiTeX (X = I, Br) compounds with nontrivial topology of electronic states display new ground states upon compression.

Development of Powder-in-Tube Processed Iron Pnictide Wires and Tapes

The development of PIT fabrication process of iron pnictide superconducting wires and tapes has been reviewed. Silver was found to be the best sheath material, since no reaction layer was observed between the silver sheath and the superconducting core. The grain connectivity of iron pnictide wires and tapes has been markedly improved by employing Ag or Pb as dopants. At present, critical current densities in excess of 3750 A/cm2 (Ic=37.5A) at 4.2 K have been achieved in Ag-sheathed SrKFeAs wires prepared with the above techniques, which is the highest value obtained in iron-based wires and tapes so far. Moreover, Ag-sheathed Sm-1111 superconducting tapes were successfully prepared by PIT method at temperatures as low as ~900°C, instead of commonly used temperatures of ~1200°C. These results demonstrate the feasibility of producing superconducting pnictide composite wires, even grain boundary properties require much more attention.

Synthesis and properties of La-doped CaFe2As2 single crystals with Tc=42.7 K

Large single crystals of La-doped CaFe2As2 were successfully synthesized by the FeAs self-flux method. The X-ray diffraction patterns suggest high crystalline quality and c-axis orientation. By substitution of trivalent La3+ ions for divalent Ca2+, the resistivity anomaly in the parent compound CaFe2As2 is completely suppressed and the superconducting transition temperature reaches the value of 42.7 K, which is higher than that of about 30 K reported in Saha S. R. et al., arXiv:1105.4798v1. The upper critical field has been determined with the magnetic field along the ab-plane and c-axis, yielding an anisotropy of about 3.3.

Enhanced critical current properties in Ba0.6K0.4+xFe2As2 superconductor by overdoping of potassium

Phase-pure polycrystalline Ba0.6K0.4+xFe2As2 with 0≤x≤0.1 were prepared using a one-step solid-state reaction method. We found that overdoping of potassium can improve the critical current density (Jc). High-field Jc for samples with x=0.1 is three times higher than that for samples with x=0. Overdoping of K has minimal effect on the critical transition temperature (Tc). Less than 0.5 K degradations in Tc was measured for samples with x=0.1. Transmission electron microscopy (TEM) revealed high concentration of dislocations in samples with x=0.1, resulting in enhanced flux pining. Further analyses on magnetization loops for powder samples confirm that K overdoping can promote intragrain Jc. Our results indicate that slight excess of K in Ba0.6K0.4Fe2As2 superconductor is beneficial to high-field applications.

Direct observation of nanometer-scale amorphous layers and oxide crystallites at grain boundaries in polycrystalline Sr1−xKxFe2As2 superconductors

We report here an atomic resolution study of the structure and composition of the grain boundaries in polycrystalline Sr0.6K0.4Fe2As2 superconductor. A large fraction of grain boundaries contain amorphous layers larger than the coherence length, while some others contain nanometer-scale crystallites sandwiched in between amorphous layers. We also find that there is significant oxygen enrichment at the grain boundaries. Such results explain the relatively low transport critical current density (Jc) of polycrystalline samples with respect to that of bicrystal films.