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We report the fabrication of (Sr,Na)Fe2As2 superconducting tapes by the powder-in-tube technique and their characteristics, including the transport critical current density Jc at 4.2 K up to 140 kOe, the magnetic Jc estimated from magnetic hysteresis curves, magneto-optical (MO) images, and scanning electron microscopy images. In a tape sintered at 875 °C for 1 h, the transport Jc reaches 26 kA/cm2 at 4.2 K and 100 kOe for a field perpendicular to the tape surface. When the field is parallel to the tape surface, the magnetic Jc exceeds the practical level of 100 kA/cm2 at 4.2 K below 25 kOe. Analysis of the MO images reveals clear current discontinuity lines in the core, indicating that the current flows homogeneously and the connections between grains are strong in the core. [ABSTRACT FROM AUTHOR]

We fabricated (Ba,K)Fe2As2 superconducting wires and tapes using the powder-in-tube method and hot isostatic pressing (HIP). HIP wires and tapes showed a high value of transport critical current density (Jc) exceeding 100 kAcm−2 at T = 4.2 K and the self-field. Transport Jc in the HIP wire reached 38 kAcm−2 in a high magnetic field of 100 kOe. This value is almost twice larger than the previous highest value of Jc among round wires using iron-based superconductors. Enhancement of Jc in the wires and tapes was caused by improvement of the drawing process, which caused degradation of the core, formation of microcracks, weak links between grains, and random orientation of grains. Details of the effect of the improved fabrication processes on the Jc are discussed. [ABSTRACT FROM AUTHOR]

(Ba,K)Fe2As2 superconducting wires and tapes are fabricated by using hot isostatic pressing (HIP) technique, and their superconducting properties are studied. In the HIP round wire, transport critical current density (Jc) at 4.2 K has achieved record-high value of 175 kA cm−2 at zero field, and exceeds 20 kA cm−2 even at 100 kOe. Improvement of polycrystalline powder synthesis may play a key role for the enhancement of Jc. In the HIP tape, even larger transport Jc of 380 kA cm−2 is realized at zero field. Based on magnetization and magneto-optical measurements, possible further enhancement of Jc is discussed. [ABSTRACT FROM AUTHOR]

We examine the critical current density (Jc) of CaLaxFe(AsSby)2, a 112-type iron-based superconductor (IBS) with 47 K, via magneto-optical imaging and magnetization measurements. We assert that the large self-field Jc of A cm−2 at 2 K is a strong indication that it is a bulk superconductor with spatially homogeneous superconductivity. A 2.8-fold enhancement in Jc to A cm−2 was achieved through artificially engineering pinning centers by irradiating 3 MeV protons with a total dosage of . The results not only demonstrate the potential of 112-type IBSs for application but also enrich the current understanding of the role of artificial defects in IBSs. [ABSTRACT FROM AUTHOR]

Magneto-optical (MO) characterizations and the angular-dependent critical current density (Jc(Θ)) of epitaxial FeSe0.5Te0.5 (FST) thin films grown on CaF2 single-crystalline substrates were performed. The MO images show typical rooftop patterns in the remanent state from which a large, homogeneous, and almost isotropic self-field Jc over 2 × 106 A cm−2 at 8 K was obtained. The vortex pinning mechanism is investigated measuring the magnetic field and angular-dependent critical current density Jc. The FST films exhibit small anisotropy of Jc in the whole applied magnetic field range below 15 K. The Dew-Hughes model and angular scaling analyses prove that pointlike normal cores, which are distributed randomly in the FST film, dominate the pinning in the FST films on CaF2 substrates. [ABSTRACT FROM AUTHOR]

The evolution of the superconducting properties of round wires of (Ba,K)Fe2As2 fabricated by the powder-in-tube (PIT) method is systematically studied. After establishing the method to obtain the largest transport critical current density (Jc) in round wires using the hot isostatic press technique, we investigated how the transition temperature (Tc), Jc, and microstructures change at each step of the wire fabrication. Unexpectedly, we find that superconducting properties of the wire core are significantly damaged by the drawing process. Systematic measurements of Jc and Tc of the core superconductor after each drawing and sintering process clarified the evolution of degradation by the drawing process and recovery by heat treatment. [ABSTRACT FROM AUTHOR]

We report a comprehensive study of the effect of H+ irradiation on the critical current density Jc and vortex pinning in an FeSe single crystal. The value of Jc for FeSe is enhanced by more than a factor of 2 after 3-MeV H+ irradiation, which is explained by the introduction of point pinning centers. Vortex creep rates are found to be strongly suppressed after irradiation. Detailed analyses of the pinning energy based on collective-creep-theory and an extended Maley’s method show that the H+ irradiation enhances the value of Jc before the flux creep and also reduces the size of the flux bundle, which suppresses the field dependence of Jc owing to vortex motion. [ABSTRACT FROM AUTHOR]

The screening current density (J) and vortex pinning properties for are studied in optimally hole-doped Ba0.6K0.4Fe2As2 single crystals with a superconducting transition of Tc = 38.6 K. We utilize the irreversible magnetization and its relaxation to evaluate the screening current density and vortex dynamics before and after the introduction of point defects created by 3-MeV proton (H+) irradiation with a dose of /cm2. In the as-grown crystal, both J and vortex dynamics are strongly influenced by temperature and magnetic field, indicating that the predominant pinning is not the weak-collective pinning but the strong pinning. With an introduction of point defects, J in the -irradiated crystal at 2 K reaches A cm−2, which is one of the largest values in iron-based superconductors. In contrast to the as-grown sample, the sensitivity to the temperature and magnetic field becomes quite mild, and the vortex system is described by a simple weak-collective pinning scenario. [ABSTRACT FROM AUTHOR]

We investigated the superconducting and transport properties in FeTeSex (0.1 x 0.4) single crystals prepared by O2-annealing. The sharp superconducting transition width observed in the magnetization measurement and the small residual resistivity prove the high quality of the crystals. All the crystals manifest large, homogeneous, and isotropic critical current density Jc with a self-field value over 105 A cm−2 at 2 K. The large and field-robust critical current densities prove that the superconductivity in FeTeSex (0.1 x 0.4) is in bulk nature. The values of anisotropy parameter close to Tc for crystals with different Se doping levels all reside in the range of 2–3. Hall coefficients RH remain positive and at an almost constant value at high temperatures, followed by a sudden decrease before reaching Tc, which indicates that the electron-type charge carriers become dominant at low temperatures. Furthermore, the characteristic temperature for the sudden decrease in RH gradually increases with Se doping. [ABSTRACT FROM AUTHOR]

We performed magneto-optical (MO) measurements on FeTe0.5Se0.5 thin films grown on LaAlO3 (LAO) and Yttria-stabilized zirconia (YSZ) single-crystalline substrates. These thin films show superconducting transition temperature Tc ∼19 K, 4 K higher than the bulk sample. Typical roof-top patterns can be observed in the MO images of thin films grown on LAO and YSZ, from which a large and homogeneous critical current density Jc over at 5 K was obtained. Magnetic flux penetration measurement reveals that the current is almost isotropically distributed in the two thin films. Compared with bulk crystals, FeTe0.5Se0.5 thin film demonstrates not only higher Tc, but also much larger Jc, which is attractive for applications. [ABSTRACT FROM AUTHOR]

The article offers information on a study regarding Domain Meissner state and spontaneous vortex-antivortex generation in the ferromagnetic superconductor EuFe2. It mentions that internal exchange energy in ferromagnets is usually significantly larger than the condensation energy of conventional superconductivity. It presents information on the superconducting quantum interference device (SQUID).

We report detailed study of the transport properties of FeSe1-xSx (x = 0 and 0.14) single crystals grown by vapor transport method. 14% S doping is found to significantly suppress the structural transition from Ts∼ 86 K in FeSe to ∼49 K, although the superconducting transition temperature Tc is only slightly affected. A pronounced linear magnetoresistance (MR) is observed in both FeSe and FeSe0.86S0.14 single crystals, which is found to be triggered by the structural transition. The linear MR and related discussion indicate the possible existence of Dirac-cone-like state, which may come from the band shift induced by ferro-orbital order. The mobility of the Dirac-cone-like band is found to decrease after S doping. Besides, the invalid Kohler's scaling of MR is found for temperature below Ts in both crystals, however, the reestablishment of the Kohler's scaling at temperatures below 30 K is observed in FeSe, but not in FeSe0.86S0.14. All these observations above support that the orbital ordering causes the band reconstruction in FeSe, and also that the orbital ordering in FeSe is suppressed by the chemical pressure from S doping. [ABSTRACT FROM AUTHOR]

We present a comprehensive study of the vortex pinning and dynamics in a high-quality FeSe single crystal which is free from doping-introduced inhomogeneities and charged quasiparticle scattering because of its innate superconductivity. The critical current density Jc is found to be almost isotropic and reaches a value of ~3 ×104 A/cm² at 2 K (self-field) for both H∥c and ab. The normalized magnetic relaxation rate S (=∣d ln M/d lnt ∣) shows a temperature-insensitive plateau behavior in the intermediate temperature range with a relatively high creep rate (S~0.02 under zero field), which is interpreted in the framework of the collective creep theory. A crossover from the elastic to plastic creep is observed, while the fishtail effect is absent for both H∥c and ab. Based on this observation, the origin of the fishtail effect is also discussed. Combining the results of Jc and S, the vortex motion in the FeSe single crystal is found to be dominated by sparse, strong pointlike pinning from nanometer-sized defects or imperfections. The weak collective pinning is also observed and proved in the form of large bundles. Besides, the vortex phase diagram of FeSe is also constructed and discussed. [ABSTRACT FROM AUTHOR]

We investigated the transport properties of Fe1+yTe0.6Se0.4 single crystals with different amounts of excess Fe prepared by O2 annealing. The O2 annealing remarkably improved transport properties. In particular, a strongly nonlinear Hall resistivity was observed only in the fully annealed crystal, and the magnetoresistance (MR) was drastically enhanced after annealing, reaching a value larger than 17% at 16 K and 14 T. The obvious change of transport properties after the annealing indicates that the band structure of Fe1+yTe0.6Se0.4 is affected by the excess Fe. The nonlinear Hall resistivity and violation of (modified) Kohler's scaling of the large MR prove the multiband effects in the Fe1+yTe0.6Se0.4 single crystal. The MR for the fully annealed crystal develops linearly against the magnetic field from the intermediate field (e.g., 2 T at 16 K) to the measurement limit of 14 T. This phenomenon is interpreted by the existence of the Dirac cone state, in which all the Dirac fermions occupy only the lowest Landau level in the quantum limit. [ABSTRACT FROM AUTHOR]

The angular dependence of magnetization was measured in an optimally Co-doped BaFe2As2 single crystal, which is irradiated by 2.6 GeV uranium to introduce columnar defects (CDs). At low fields (H), we observe the irreversible magnetization Mirr insensitive to the angle θ around H‖CDs. The field dependence of the angular range 2θL is proportional to 1/H, quite consistent with the expectation of the lock-in transition. This manifests the first observation of the fingerprint of the Bose glass phase in iron-based superconductors. [ABSTRACT FROM AUTHOR]

Pair-breaking effects induced by 3-MeV proton irradiations are examined in underdoped, optimally doped, and overdoped Ba1-xKxFe2As2 single crystals in terms of suppression of the superconducting critical temperature Tc. The small residual resistivity (RR) in as-grown crystals shows the presence of negligible intrinsic scatterings, which makes this material a model system for studying the effect of artificially introduced scatterings. The RR and Tv change linearly with the proton dose. As in the case of proton irradiation in Co-doped BaFe2As2, we do not detect any low-temperature upturns in resistivity attributable to magnetic scattering or localization. Regardless of K doping levels, the critical value of the normalized scattering rate is much higher than that expected in s±-wave superconductors. [ABSTRACT FROM AUTHOR]

In the electronic nematic state, an electronic system has a lower symmetry than the crystal structure of the same system. Electronic nematic states have been observed in various unconventional superconductors such as cuprate, iron-based, heavy-fermion, and topological superconductors. The relation between nematicity and superconductivity is a major unsolved problem in condensed matter physics. By angle-resolved specific heat measurements, we report bulk quasiparticle evidence of nematicity in the topological superconductor SrxBi2Se3. The specific heat exhibited a clear twofold symmetry despite the threefold symmetric lattice. Most importantly, the twofold symmetry appeared in the normal state above the superconducting transition temperature. This is explained by the angle-dependent Zeeman effect due to the anisotropic density of states in the nematic phase. Such results highlight the interrelation between nematicity and unconventional superconductivity. [ABSTRACT FROM AUTHOR]

Quasiparticle excitations in FeSe were studied by means of specific heat (C) measurements on a high-quality single crystal under rotating magnetic fields. The field dependence of C shows three-stage behavior with different slopes, indicating the existence of three gaps (Δ1,Δ2, and Δ3). In the low-temperature and low-field region, the azimuthal angle (ϕ) dependence of C shows a fourfold symmetric oscillation with a sign change. On the other hand, the polar angle (θ) dependence manifests as an anisotropy-inverted twofold symmetry with unusual shoulder behavior. Combining the angle-resolved results and the theoretical calculation, the smaller gap Δ1 is proved to have two vertical-line nodes or gap minima along the kz direction, and is determined to reside on the electron-type ɛ band. Δ2 is found to be related to the electron-type δ band, and is isotropic in the ab plane but largely anisotropic out of the plane. Δ3 residing on the hole-type α band shows a small out-of-plane anisotropy with a strong Pauli paramagnetic effect. [ABSTRACT FROM AUTHOR]

We report a study on the superconducting pairing mechanism of FeSe via the pair-breaking effects induced by H+ irradiation combined with low-temperature specific heat measurements. A multigap structure with nodes or gap minima is suggested in a clean FeSe by the specific heat results. The suppression of critical temperature Tc with increasing the defect density manifests a two-step behavior. When the increase in the residual resistivity is small, Δρ0≲4.5μΩcm, Tc is gradually suppressed with increasing the density of the scattering centers, suggesting the presence of symmetry-unprotected nodes or gap minima. However, for Δρ0≳4.5μΩcm, Tc is almost independent of the scattering, which indicates that the nodes or gap minima are lifted and the order parameter becomes almost isotropic without a sign change. Thus, the superconductivity in FeSe is found to be realized in a symmetry-unprotected nodal or highly anisotropic s++ state. [ABSTRACT FROM AUTHOR]

We report the effects of heavy-ion irradiation on FeSe single crystals by irradiating uranium up to a dose-equivalent matching field of Bϕ = 16 T. Almost continuous columnar defects along the c axis with a diameter of ~10 nm are confirmed by high-resolution transmission electron microscopy. Tc is found to be suppressed by introducing columnar defects at a rate of dTc/dBϕ ~ -0.29 K/T, which is much larger than those observed in iron pnictides. This unexpected large suppression of Tc in FeSe is discussed in relation to the large diameter of the columnar defects as well as its unique band structure with a remarkably small Fermi energy. The critical current density is first dramatically enhanced with irradiation reaching a value over ~2×105A/cm² (~5 times larger than that of the pristine sample) at 2 K (self-field) with Bϕ=2 T, then gradually suppressed with increasing Bϕ. The δl pinning associated with charge-carrier mean-free-path fluctuations and the δTc pinning associated with spatial fluctuations of the transition temperature are found to coexist in the pristine FeSe, while the irradiation increases the contribution from δl pinning and makes it dominant over Bϕ = 4 T. [ABSTRACT FROM AUTHOR]

We report detailed study of angular-dependent magnetoresistance (AMR) with tilting angle θ from c axis ranging from 0∘ to 360∘ on a high-quality FeSe single crystal. A pronounced AMR with twofold symmetry is observed, which is caused by the quasi-two-dimensional (quasi-2D) Fermi surface. The pronounced AMR is observed only in the orthorhombic phase, indicating that the quasi-2D Fermi surface is induced by the structural transition. Details about the influence of the multiband effect to the AMR are also discussed. Besides, the angular response of a possible Dirac-cone-like band structure is investigated by analyzing the detailed magnetoresistance at different θ. The obtained characteristic field (B∗) can be also roughly scaled in the 2D approximation, which indicates that the Dirac-cone-like state is also 2D in nature. [ABSTRACT FROM AUTHOR]