We report our study of the evolution of superconductivity and the phase diagram of the ternary Fe(Se 1-x Te x ) 0.82 (0 ≤ x ≤ 1.0) system. We discovered a new superconducting phase with T c , max = 14 K in the 0.3 < x < 1.0 range. This superconducting phase is suppressed when the sample composition approaches the end member FeTe 0.82 , which exhibits an incommensurate antiferromagnetic order. We discuss the relationship between the superconductivity and magnetism of this material system in terms of recent results from neutron scattering measurements. Our results and analyses suggest that superconductivity in this new class of Fe-based compounds is associated with magnetic fluctuations, and therefore may be unconventional in nature.

We have investigated the effect of Fe nonstoichiometry on properties of the Fe 1+y (Te, Se) superconductor system by means of resistivity, Hall coefficient, magnetic susceptibility, and specific heat measurements. We find that the excess Fe at interstitial sites of the (Te, Se) layers not only suppresses superconductivity, but also results in a weakly localized electronic state. We argue that these effects originate from the magnetic coupling between the excess Fe and the adjacent Fe square planar sheets, which favors a short-range magnetic order.

We have determined the resistive upper critical field H c2 for single crystals of the superconductor Fe 1.11 Te 0.6 Se 0.4 using pulsed magnetic fields of up to 60 T. A rather high zero-temperature upper critical field of 0 H c2 ͑0͒Ϸ47 T is obtained in spite of the relatively low superconducting transition temperature ͑T c Ϸ 14 K͒. Moreover, H c2 follows an unusual temperature dependence, becoming almost independent of the magnetic field orientation as the temperature T → 0. We suggest that the isotropic superconductivity in Fe 1.11 Te 0.6 Se 0.4 is a consequence of its three-dimensional Fermi-surface topology. An analogous result was obtained for ͑Ba, K͒Fe 2 As 2 , indicating that all layered iron-based superconductors exhibit generic behavior that is significantly different from that of the "high-T c " cuprates.

After growing successfully TaP single crystal, we measured its longitudinal resistivity (ρxx) and Hall resistivity (ρyx) at magnetic fields up to 9T in the temperature range of 2-300K. It was found that at 2K its magnetoresistivity (MR) reaches to 3.28×105 %, at 300K to 176% at 8T, and both do not appear saturation. We confirmed that TaP is indeed a low carrier concentration, hole-electron compensated semimetal, with a high mobility of hole µ h =3.71×105 cm 2 /V s, and found that a magnetic-field-induced metal-insulator transition occurs at room temperature. Remarkably, as a magnetic field (H ) is applied in parallel to the electric field (E ), the negative MR due to chiral anomaly is observed, and reaches to -3000% at 9T without any signature of saturation, too, which distinguishes with other Weyl semimetals (WSMs). The analysis on the Shubnikov-de Haas (SdH) oscillations superimposing on the MR reveals that a nontrivial Berry's phase with strong offset of 0.3958 realizes in TaP, which is the characteristic feature of the charge carriers enclosing a Weyl nodes. These results indicate that TaP is a promising candidate not only for revealing fundamental physics of the WSM state but also for some novel applications. [8] compound, in which fine-tuning the chemical composition is necessary for breaking inversion symmetry, a WSM has not realized experimentally in any of these compounds due to either no enough large magnetic domain or difficulty to tune the chemical composition within 5%. Very recently, the theoretical proposal [9,10] for a WSM in a class of stoichiometric materials, including TaAs, TaP, NbAs and NbP, which break crystalline inversion symmetry, has been soon confirmed by the experiments [11][12][13][14], except for TaP due to difficulty to grow large crystal. The exotic transport properties exhibiting in these materials ignite an extensive interesting in both the condensed matter physics and material science community, especial for their extremely large magnetoresistance (MR) and ultrahigh mobility of charge carriers.Materials with large MR have been used as magnetic sensors [16], in magnetic memory [17], and in hard drives [18] at room temperature. Large MR is an uncommon property, mostly of magnetic compounds, such as a giant magnetoresistance (GMR) [19] emerging in Fe/Cr thin-film, and colossal magnetoresistance (CMR) in the manganese based perovskites [20,21]. In contrast, ordinary MR, a relatively weak effect, is commonly found in non-magnetic compounds and elements [22]. Magnetic materials typically have negative MR. Positive MR is seen in metals, usually at the level of a few percent, and in some semiconductors, such as 200% at room temperature in Ag 2+δ (Te,Se) [30], comparable with those of materials showing CMR [24], and semimetals, such as high-purity bismuth, graphite [25], and 4.5×10 4 % in WTe 2 [26]. In the semimetals, very high MR is attributed to a balanced hole-electron "resonance" condition, as described in Ref. [26]. WSM provides another possibility to realize extremely large MR, ...