Single crystals of Fe 1+x Te 1−y Se y have been grown with a controlled Fe excess and Se doping, and the crystal structure has been refined for various compositions. The systematic investigation of magnetic and superconducting properties as a function of the structural parameters shows how the material can be driven into various ground states, depending on doping and the structural modifications. Our results prove that the occupation of the additional Fe site, Fe2, enhances the spin localization. By reducing the excess Fe, the antiferromagnetic ordering is weakened, and the superconducting ground state is favored. We have found that both Fe excess and Se doping in synergy determine the properties of the material and an improved 3-dimensional phase diagram is proposed.

An extensive study of the magnetic properties of FeTe 0.7 Se 0.3 crystals in the superconducting state is presented. We show that weak collective pinning, originating from spatial variations of the charge carrier mean free path (δl pinning), rules in this superconductor. Our results are compatible with the nanoscale phase separation observed on this compound and indicate that in spite of the chemical inhomogeneity, spatial fluctuations of the critical temperature are not important for pinning. A power-law dependence of the magnetization vs time, generally interpreted as the signature of a single-vortex creep regime, is observed in magnetic fields up to 8 T. For magnetic fields applied along the c axis of the crystal, the magnetization curves exhibit a clear peak effect whose position shifts when varying the temperature, following the same dependence as observed in YBa 2 Cu 3 O 7−δ . The time and temperature dependence of the peak position has been investigated. We observe that the occurrence of the peak at a given magnetic field determines a specific vortex configuration that is independent on the temperature. This result indicates that the influence of the temperature on the vortex-vortex and vortex-defect interactions leading to the peak effect in FeTe 0.7 Se 0.3 is negligible in the explored range of temperatures.

We present measurements of resistivity and thermopower S of Fe 1+x Te 1−y Se y single crystalline samples with y = 0, 0.1, 0.2, 0.3, and 0.45 in zero field and in a magnetic field B = 8 T. We find that the shape of thermopower curves appears quite peculiar in respect to that measured in other Fe-based superconducting families. We propose a qualitative analysis of the temperature behavior of S, where the samples are described as almost compensated semimetals: different electron and hole bands with similar carrier concentrations compete and their relative contribution to the thermoelectric transport depends on the respective filling, mobility, and coupling with phonons. For y Ն 0.2, superconductivity occurs and the optimum Se-doping level for a maximum T c of 13 K turns out to be y = 0.3. At low temperatures, evidence of a contribution to S by an excitation-drag mechanism is found while at high temperatures a strikingly flat behavior of S is explained within a narrowband Hubbard model.

Abstract:We report first principles calculations of the structural, electronic, elastic and vibrational properties of the semiconducting orthorhombic ZnSb compound. We study also the intrinsic point defects in order to eventually improve the thermoelectric properties of this already very promising thermoelectric material.Concerning the electronic properties, in addition to the band structure, we show that the Zn (Sb) crystallographically equivalent atoms are not exactly equivalent from the electronic point of view. Lattice dynamics, elastic and thermodynamic properties are found to be in good agreement with the experiments and they confirm the non equivalency of the zinc and antimony atoms from the vibrational point of view. The calculated elastic properties show a relatively weak anisotropy and the hardest direction is the y direction. We observe the presence of low energy modes involving both Zn and Sb atoms at about 5-6 meV, similarly to what has been found in Zn 4 Sb 3 and we suggest that the interactions of these modes with acoustic phonons could explain the relatively low thermal conductivity of ZnSb. Zinc vacancies are the most stable defects and this explains the intrinsic p-type conductivity of ZnSb.