We studied ab-plane transport properties in single crystals of the superconductor β-FeSe up to 16 T. In the normal state, below 90 K, the crystals present a strongly anisotropic positive magnetoresistance that becomes negligible above that temperature. In the superconducting state (Tc=8.87(5) K) the upper critical field anisotropy Hc2 ab/ Hc2 c changes with temperature and the angular dependence of the dissipation for fixed temperatures and fields reflects a strongly anisotropic behavior. Our results make evident that multiband effects are needed to describe the measured transport properties. We model the magnetoresistance and upper critical field behavior with a two-band model showing that the diffusivities ratio parameter remains unchanged going from the normal to the superconducting state.

BaFe2S3 is a quasi one-dimensional Mott insulator that orders antiferromagnetically below 117(5) K. The application of pressure induces a transition to a metallic state, and superconductivity emerges. The evolution of the magnetic behavior on increasing pressure has up to now been either studied indirectly by means of transport measurements, or by using local magnetic probes only in the low pressure region. Here, we investigate the magnetic properties of BaFe2S3 up to 9.9 GPa by means of synchrotron 57 Fe Mössbauer spectroscopy experiments, providing the first local magnetic phase diagram. The magnetic ordering temperature increases up to 185(5) K at 7.5 GPa, and is fully suppressed at 9.9 GPa. The low-temperature magnetic hyperfine field is continuously reduced from 12.9 to 10.3 T between 1.4 and 9.1 GPa, followed by a sudden drop to zero at 9.9 GPa indicating a first-order phase transition. The pressure dependence of the magnetic order in BaFe2S3 can be qualitatively explained by a combination of a bandwidth-controlled insulator-metal transition as well as a pressure enhanced exchange interaction between Fe-atoms and Fe 3d -S 3p hybridization.

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Abstract. We study the angular dependence of the dissipation in the superconducting state of FeSe and Fe(Se 1-x Tex) through electrical transport measurements, using crystalline intergrown materials. We reveal the key role of the inclusions of the non superconducting magnetic phase Fe 1-y (Se 1-x Tex), growing into the Fe(Se 1-x Tex) pure β-phase, in the development of a correlated defect structure. The matching of both atomic structures defines the growth habit of the crystalline material as well as the correlated planar defects orientation.

We present a comparative study of electrical transport properties in the normal state and in the dissipative superconducting state between pure β-FeSe phase and Fe deficient Fe 1−y Se crystals. We discuss the influence of the intergrowth of the magnetic hexagonal phase (Fe 7 Se 8 ) in Fe deficient samples when compared to pure β-FeSe samples. In the superconducting state, we measured the ab-plane electrical resistivity with magnetic field up to 16 T and the electrical resistivity as a function of the angle between the c axis and the applied field. The angular dependence at fixed temperature below the superconducting critical temperature, T c (H = 0), is very different for both sets of crystals. The Fe deficient samples display a vortex pinningrelated feature at ∼57 • off the plane while the pure β-FeSe phase samples show the persistence of a strong angular-dependent magnetoresistance characteristic of the normal state electronic structure.

Abstract.We report on the critical current density Jc and the vortex dynamics of pristine and 3 MeV proton irradiated (cumulative dose equal to 2×10 16 cm −2 ) β-FeSe single crystals.We also analyze a remarkable dependence of the superconducting critical temperature Tc, Jc and the flux creep rate S on the sample mounting method. Free-standing crystals present Tc=8.4(1)K, which increases to 10.5(1)K when they are fixed to the sample holder by embedding them with GE-7031 varnish. On the other hand, the irradiation has a marginal effect on Tc. The pinning scenario can be ascribed to twin boundaries and random point defects. We find that the main effect of irradiation is to increase the density of random point defects, while the embedding mainly reduces the density of twin boundaries. Pristine and irradiated crystals present two outstanding features in the temperature dependence of the flux creep rate: S(T ) presents large values at low temperatures, which can be attributed to small pinning energies, and a plateau at intermediate temperatures, which can be associated with glassy relaxation. From Maley analysis, we observe that the characteristic glassy exponent µ changes from ∼ 1.7 to 1.35-1.4 after proton irradiation.

We have investigated the correlation between structural and transport properties in sputtered β -FeSe films grown onto SrTiO 3 (100). The growth parameters, such as substrate temperature and thickness, have been varied in order to explore different regimes. In the limit of textured thick films, we found promising features like an enhanced T c ∼ 12 K, a relatively high H c2 and a low anisotropy. By performing magnetoresistance and Hall coefficient measurements, we investigate the influence of the disorder associated with the textured morphology on some features attributed to subtle details of the multi-band electronic structure of β -FeSe. Regarding the superconductor-insulator transition (SIT) induced by reducing the thickness, we found a non-trivial evolution of the structural properties and morphology associated with a strained initial growth and the coalescence of grains. Finally, we discuss the origin of the insulating behavior in high-quality stressed epitaxial thin films. We found that a lattice distortion, described by the Poisson's coefficient associated with the lattice parameters a and c, may play a key role.

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