Z. A. Ren

An anomalous tail effect in the resistivity transition and weak-link behavior of superconducting NdFeAsO0.88F0.12

Temperature dependent resistivity of the iron-based superconductor NdFeAsO0.88F0.12 was measured under different applied fields and excitation currents. Arrhenius plot shows an anomalous tail effect, which contains obvious two resistivity dropping stages. The first is caused by the normal superconducting transition, and the second is supposed to be related to the weak-link between the grains. A model for the resistivity dropping related to the weak-link behavior is proposed, which is based on the Josephson junctions formed by the impurities in grain boundaries like FeAs, Sm2O3 and cracks together with the adjacent grains. These Josephson junctions can be easily broken by the applied fields and the excitations currents, leading to the anomalous resistivity tail in many polycrystalline iron-based superconductors. The calculated resistivity dropping agrees well with the experimental data, which manifests the correctness of the explanation of the obtained anomalous tail effect.The temperature dependent resistivity of the iron-based superconductor NdFeAsO0.88F0.12 was measured under different applied fields and excitation currents. The Arrhenius plot shows an anomalous tail effect, which contains two obvious stages of decreasing resistivity. The first is caused by the normal superconducting transition, and the second is supposed to be related to the weak link between the grains. A model for the resistivity decrease related to the weak-link behavior is proposed, which is based on the Josephson junctions formed by the impurities in grain boundaries like FeAs, Sm2O3 and cracks together with the adjacent grains. These Josephson junctions can be easily broken by the applied fields and the excitation currents, leading to an anomalous resistivity tail in many polycrystalline iron-based superconductors. The calculated drop in resistivity agrees well with the experimental data, which indicates the correctness of the explanation for the obtained anomalous tail effect.

Magnetotransport properties of the triply degenerate node topological semimetal tungsten carbide

We report the magnetoresistance (MR), Hall effect, and de Haas-van Alphen (dHvA) effect studies of the single crystals of tungsten carbide, WC, which is predicted to be a new type of topological semimetal with triply degenerate nodes. With the magnetic field rotated in the plane perpendicular to the current, WC shows field induced metal to insulator like transition and large nonsaturating quadratic MR at low temperature. As the magnetic field parallel to the current, a pronounced negative longitudinal MR only can be observed when the current flows along the certain direction. Hall effect indicates WC is a perfect compensated semimetal, which may be related to the large nonsaturating quadratic MR. The analysis of dHvA oscillations reveals that WC is a multiband system with small cross-sectional areas of Fermi surface and light cyclotron effective masses. Our results indicate that WC is an ideal platform to study the recently proposed New Fermions with triply degenerate crossing points.

Pressure dependence of the thermoelectric power of the iron-based high-Tc superconductor SmFeAsO0.85

We have measured the thermoelectric power S of the iron-based superconductor SmFeAsO0.85 as a function of temperature at various pressures up to 1.1 GPa. The magnitude of thermoelectric power increases with decreasing temperature and exhibits a maximum at a characteristic temperature T* (~110 K at ambient pressure), whereas the temperature dependence of the resistance shows metallic behavior. The superconducting transition temperature Tc and T* decrease monotonically with pressure. We discuss our results in terms of the effects of Fermi-surface nesting and orbit degeneracy in the new iron-oxypnictides.

Electronic behavior of superconducting SmFeAsO0.75

High-quality polycrystalline SmFeAsO0.75 was synthesized with a superconducting transition width less than 1 K, and the electronic behavior was systematically studied by transport and specific heat measurements. An obvious superconducting jump was witnessed, together with a very small normalized superconducting jump, ΔC/γnTc ∼ 0.2, which is much smaller than expected from the BCS theory. A strong temperature-dependent Hall coefficient was found and attributed to the partial gapping of the Fermi surface up to the temperature of 160 K, which was predicted and supported by the emergence of the pseudogap. The charge-carrier density as well as the effective mass were also obtained and discussed in detail.

Superconductivity in Pd-intercalated charge-density-wave rare earth poly-tellurides RETe n

The interplay between magnetism and superconductivity is one of the dominant themes in the study of unconventional superconductors, such as high-Tc cuprates, iron pnictides and heavy fermions. In such systems, the same d- or f-electrons tend to form magnetically ordered states and participate in building up a high density of states at the Fermi level, which is responsible for the superconductivity. Charge-density-wave (CDW) is another fascinating collective quantum phenomenon in some low dimensional materials, like the prototypical transition-metal poly-chalcogenides, in which CDW instability is frequently found to accompany with superconducting transition at low temperatures. Remarkably, similar to the antiferromagnetic superconductors, superconductivity can also be achieved upon suppression of CDW order via chemical doping or applied pressure in 1T-TiSe2. However, in these CDW superconductors, the two ground states are believed to occur in different parts of Fermi surface (FS) sheets, derived mainly from chalcogen p-states and transition metal d-states, respectively. The origin of superconductivity and its interplay with CDW instability has not yet been unambiguously determined. Here we report on the discovery of bulk superconductivity in Pd-intercalated CDW RETen (RE=rare earth; n=2.5, 3) compounds, which belong to a large family of rare-earth poly-chalcogenides with CDW instability usually developing in the planar square nets of tellurium at remarkably high transition temperature and the electronic properties are also dominated by chalcogen p-orbitals. Our study demonstrates that the intercalation of palladium leads to the suppression of the CDW order and the emergence of the superconductivity. Our finding could provide an ideal model system for comprehensive studies of the interplay between CDW and superconductivity.

Josephson effects in MgB2 break junctions and MgB2/Nb point contacts

The existence of the irreversibility line is one of the most intrigue features of high temperature superconductors. The irreversibility line is resulted from the short coherence length, large anisotropy and high working temperature of high-T. oxides. For low temperature superconductors, however, there has been experimental evidence of the existence of the irreversibility line even though it is previously thought that the irreversibility line is too close to the upper critical field line to be observed. In this work, systematic measurements of dc magnetization, ac susceptibility and magnetoresistivity as a function of applied field and temperature have been performed on Nb alloy samples in order to investigate the irreversibility line in low temperature superconductors. The results show that there exits an observable region below the mean-field critical field, H-c2, where the magnetization is reversible during a cycle of increasing and decreasing field. Furthermore, results obtained on other low temperature superconductors such as the Chevrel phase superconductor PbMo6S8, Ba0.25Pb0.75BiO3, the newly discovered intermetallic compound MgB2 and overdoped cuprate Tl2Ba2CuO6+delta (T-c similar to 20 K) are also discussed