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We reported that bulk superconductivity in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ can be induced by annealing in Se and S vapor. The well-annealed samples show a large critical current density, emph{J}$_c$ $\sim$ 2 - 4 $\times$ 10$^5$ A/cm$^2$. Combined with our previous reports about O$_2$ and Te annealing effects, we proved that bulk superconductivity can be successfully induced in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ by annealing in atmosphere of all chalcogens, such as O, S, Se and Te., Comment: 2 pages, 3 figures

Flux penetrations into three-dimensional Nb superconducting strip arrays, where two layers of strip arrays are stacked by shifting a half period, are studied using a magneto-optical imaging method. Flux avalanches are observed when the overlap between the top and bottom layers is large even if the width of each strip is well below the threshold value. In addition, anomalous linear avalanches perpendicular to the strip are observed in the shifted strip array when the overlap is very large and the thickness of the superconductor is greater than the penetration depth. We discuss possible origins for the flux avalanches, including linear ones, by considering flux penetration calculated by the Campbell method assuming the Bean model., 11 pages, 6 figures

We report 175Lu and 29Si NMR results of the ternary-iron silicide superconductor, Lu2Fe3Si5, with \(T_{\text{c}}\sim 6.1\) K. We confirm the decrease of the Knight shift and the existence of coherence peak of nuclear spin–lattice relaxation rate, \(1/T_{1}\) of 29Si below \(T_{\text{c}}\), indicating apparently that the superconducting (SC) pairing symmetry is of the spin-singlet. The observed 175Lu-NMR signals are attributed to the Zeeman split Φ\(|{\pm} 1/2\rangle\) transition under a large nuclear quadrupole interaction. The temperature dependence of \(1/T_{1}\) of 175Lu obeys a Curie–Weiss behavior in the normal state. In the SC state, \(1/T_{1}\) decreases rapidly, and follows the \(T_{1}T=\text{const.}\) relation below \(T=1\) K, which is suggestive of the existence of the residual density of states of the conduction electrons at the Fermi level. This indicates that the small SC gap is completely suppressed even at small field of \(H\)

The critical current density, Jc, in high-quality single crystals of iron-based superconductors (IBS) has reached larger than 105 A/cm2 at 4.2 K. This large value of Jc urged the development of potential superconducting wires or tapes based on IBS. We have fabricated superconducting wires and tapes of IBS using several techniques and made an extensive characterization by structural, transport, and magnetic measurements. The powder-in-tube method is used for (Ba, K)Fe2As2 and Ba(Fe, Co)2As2 wires and the diffusion method is used for FeSe tape. Intra- and inter-granular Jc's have been evaluated by direct transport and indirect magnetization measurements as well as using magneto-optical (MO) imaging. The intergranular Jc's at 4.2 K under self-field for (Ba, K)Fe2As2 and Ba(Fe, Co)2As2 wires and FeSe tapes are 1.3 × 104 A/cm2, 3.8 × 103 A/cm2, and 600 A/cm2, respectively. On the other hand, MO images have demonstrated that the intragranular Jc is at least one order of magnitude larger than the intergranular Jc, which is consistent with the magnetic evaluation.

SmFeAsO1−xFx possesses the highest superconducting transition temperature (Tc) among the iron-based superconductors, with a very high upper critical field and critical current density (Jc). All these are desirable for practical applications. Although tremendous progress has been made in improving the Jc values for Sr(Ba)-122 tapes or wires [1–3], the transport Jc of Sm1111 tapes or wires is still very low. Here we report on large transport critical current densities observed in Sn-added SmFeAsO1−xFx/Fe tapes produced by an ex situ powder-in-tube (PIT) method. It was found that the loss of fluorine was reduced and the intergranular coupling was enhanced in the Sn-added samples. As a result, the onset and offset Tc values for 10 wt%, 20 wt% and 30 wt% Sn-added samples were 52 K and 35 K, 53 K and 39 K, and 54 K and 41.5 K, respectively. In particular, a transport Jc as large as 22 000 A cm−2 (Ic = 160 A) at 4.2 K under self-field was achieved in 30 wt% Sn-added specimens, which is the highest value reported so far for SmFeAsO1−xFx wires and tapes. Magneto-optical imaging further showed that there were large global currents over the whole sample, which is consistent with the results obtained by the four-probe method.

The temperature dependence of the microwave complex conductivity σ(T)=σ1(T)-iσ1(T) was determined at 9.6 and 14 GHz in a single crystal of CeNiSn from the surface impedance measurements. Below about 10 K the real part σ1 is suppressed in comparison with the dc conductivity and the imaginary part σ2 increases rapidly. These results indicate a rapid decrease of scattering of quasiparticles, whose excitation spectrum is drastically changed from the heavy-fermion to a gapped state at low temperatures.

We measured the temperature dependence of the in-plane (λ ab (T)) and out-of-plane (λC(T)) penetration depths in high-quality single crystals of Bi2Sr2CaCu2O8+y in a wide lange of oxygen content using a superconducting cavity resonator at 9.6 GHz. At relatively low oxygen content, the linear temperature dependence of the change in the penetration depth, Δλ(T) = λ(T) — λ(0), was observed at low temperatures both in the CuO2 planes and in the c direction, consistent with a clean d-wave superconductor. In the overdoped regime, however, both Aλab(T) and Δλc(T) are proportional to T 2 at low temperatures, which is considered as a sign of a gapless superconductor. The present results suggest that the pair breaking effect is induced by oxygen doping in this system.

Time evolution of the induction profile across a Bi2Sr2CaCu2O8+δ crystal was magneto-optically recorded in two experiments: during ramping the external magnetic field or after a sudden application of the field. The results of these experiments reveal a front-like propagation or retreat of a transient disordered vortex state, respectively. Analysis of the front velocity shows that while the retreat is continuously decelerated, the propagation is accelerated, approaching a constant velocity when the front induction reaches the vortex order–disorder phase transition field Bod. The latter observation suggests a method for determining the value of Bod, eliminating transient state effects.

Arrangements of pancake vortices in Bi 2 Sr 2 CaCu 2 O 8+ y are studied by means of the Bitter decoration in the presence of dense Josephson vortices. Vortex chains are visualized in in-plane fields up to 1000 Oe. Increase of out-of-plane fields up to 33 G smears the vortex chains. This smearing is quantitatively in reasonable agreement with a theoretically predicted transition from chain + lattice to deformed lattice state.

We have studied detailed anisotropic superconducting properties of a new layered intermetallic compound CaAlSi with AlB 2 structure grown by the floating-zone method. The anisotropy parameter γ (= H c2 ab / H c2 c ) determined from both magnetic and transport measurements is about 2.0. Despite the small value of γ , angular dependence of H c2 shows cusp-like feature for fields parallel to the superconducting plane, suggesting the presence of two-dimensional superconducting units. These apparently contradictory results can be partly explained by the presence of superstructure along c -axis. The resistivity anisotropy ( ρ c / ρ ab ) is 3.1 at room temperature, consistent with that estimated from H c2 .

Time relaxation of the local persistent current in a Bi2Sr2CaCu2O8+δ crystal was magneto-optically recorded after suddenly exposing the crystal to a constant magnetic field between 140 and 840 G. A remarkable behavior of the relaxation is revealed for fields below the order–disorder vortex phase transition: The relaxation starts at a slow rate, it then accelerates for a short period of time, after which a slow relaxation rate is resumed. We show that this acceleration marks a local transition of a transient disordered vortex state to a thermodynamically favored quasi-ordered vortex phase. The slow relaxation rates, before and after the transition, reflect ordinary thermally activated flux creep process in the disordered and quasi-ordered vortex phases, respectively.

Local magnetization has been studied in a phase-separated manganite (La 1− y Pr y ) 0.7 Ca 0.3 MnO 3 ( y =0.7) by a magneto-optical imaging technique simultaneously with resistivity measurements. The observed image demonstrates a region in which magnetization changes corresponding to a telegraph noise in resistive fluctuation. This observation is the first direct evidence for a switching domain in phase-separated manganites.

A micro-Hall probe is used to search for spontaneous magnetic field at the edge of chiral domains in the time-reversal-symmetry-breaking state in Sr2RuO4. We did not detect any additional magnetic signal larger than 20 mG at low fields, which is much smaller than the theoretical estimate for an isolated chiral domain. This fact suggests that either the spontaneous magnetization is absent or the chiral domains are smaller than the size of Hall probe, 5 μm. In the latter case, magnetic history dependent measurements of the vortex pinning force strongly suggest that the chiral domains can be easily flipped by magnetic fields.

We present a study of structural change of the magnetic domains in La1.36Sr1.64Mn2O7 using magneto-optical effect of a garnet film mounted on the ab-plane of a crystal. The maze domain formed below 65 K changes into the stripe domain elongated along field direction by applying in-plane field. Under an offset field perpendicular to the plane, however, the domain pattern changes into bubbles. These domain patterns are stable even after removing the field. The in-plane field needed for such structural change becomes small at higher temperatures. These changes are similar to those observed in bubble garnet films.

We have studied Y 2 PdGe 3− x Si x superconductor using magnetization and specific heat measurements. The compounds are very sensitive to changes in the honeycomb layers by Si substitution. The critical temperature shows a maximum of T c =3.55 K near x =0.3 and decreases with further increase in Si concentration. The changes in the Debye frequency and electron–phonon coupling have combined effect to control the critical temperature.

AC local permeability is measured for a strongly anisotropic superconductor Bi 2 Sr 2 CaCu 2 O 8+ y under tilted fields. In addition to the peak in local permeability at the vortex-lattice melting transition, step-like change accompanied by hysteresis is observed under large and nearly constant in-plane field. This anomaly naturally extrapolates to the novel peak effect reported by Ooi et al. [Phys. Rev. B 63 (2001) 020501] at low temperatures under tilted fields. This observation indicates that the vortex solid phase in the crossing-lattices state changes into a new state with less shielding in the presence of dense Josephson vortices before melting into vortex liquid.

We study the Josephson plasma resonance (JPR) in Bi 2 Sr 2 CaCu 2 O 8+ y (BSCCO) with inhomogeneous phase coherence caused by partial introduction of columnar defects. In these BSCCO, we observed several resonances, which cannot be regarded as a superposition of those from constituent parts. By calculating linearized sine-Gordon equation, we can reproduce the above results as well as the existence of a crossover frequency ( ω cr ) at which the character of JPR changes from irradiated- to pristine-type in partially irradiated BSCCO.

We performed magneto-optical imaging on Pr 1−x Ca x Mn 1−y Cr y O 3 (x=0.35, y=0.03) concomitantly with transport measurements. Non-uniform distribution of magnetization due to phase-separation is visualized. Temporal evolution of the images revealed much faster relaxation in highly magnetized region than the average over the sample. In the inhomogeneous phase-separated state, we observed telegraph noise in resistivity which suggests the existence of switcher domains.

We have prepared high-quality FeSe tape through diffusion method. The transport critical current density evaluated by the current-voltage characteristics is 600 A/cm2 at 4.2 K under zero field. The upper critical field of the FeSe tape at T = 0 K estimated by Werthamer-Helfand-Hohenberg theory is 243 kOe, which is similar to that of single crystals.

We have systematically investigated the effects of 300 MeV Xe irradiation along c-axis in optimally Co-doped BaFe2As2. Reflecting the cascade defects morphology induced by the irradiation, Tc in this system is reduced while the value of ΔT/Tc is small. Here we report the details of suppression rates of Tc experimentally without an ambiguity of piece dependencies. We found that the reduction is systematic if we assume sample thickness dependence in creating defects. We also discuss the observed saturation behavior of Jc in terms of defect density.

Flux penetration into a superconducting film with holes is simulated by considering nonlinear current-voltage dependence for flux creep. The film has a pattern of a square superconducting network (SCN) which has square lattice of square holes. The simulated flux penetration into SCN with the ratio of w/a = 0.5 shows a pattern extending along the parallel and diagonal direction where a is lattice constant and w is line width. The simulated pattern is similar to some of the experimentally observed the flux penetration patterns.

We have systematically investigated and compared different methods to induce superconductivity in the iron chalcogenide Fe1+yTe0.6Se0.4, including annealing in a vacuum, N2, O2 and I2 atmospheres and immersing samples into acid and alcoholic beverages. Vacuum and N2 annealing are proved to be ineffective in inducing superconductivity in a Fe1+yTe0.6Se0.4 single crystal. Annealing in O2 and I2 and immersion in acid and alcoholic beverages can induce superconductivity by oxidizing the excess Fe in the sample. Superconductivity in O2 annealed samples is of a bulk nature, while I2, acid and alcoholic beverages can only induce superconductivity near the surface. By comparing the different effects of O2, I2, acid and alcoholic beverages we propose a scenario to explain how the superconductivity is induced in the non-superconducting as-grown Fe1+yTe0.6Se0.4.

We report a study on the anisotropy of superconducting properties in single crystalline Ca10(Pt4-{\delta}As8)(Fe1.79Pt0.21As2)5 with Tc ~13.6 K. Under a field of 5 Oe, the sample reaches fully-diamagnetic state at ~10 K for H//c, and ~8 K for H//ab, indicating the presence of slight inhomogeneities. The magnetization measurements reveal fish-tail effect in the hysteresis loop for both H//c and H//ab. Averaged critical current densities at low magnetic fields along the c-axis and ab-plane Jc(H//c) and Jc(H//ab) at 5 K are estimated to be 0.9*10^5 and 0.7*10^5 A/cm2, respectively. Resistive transitions under magnetic field show broadening, which is consistent with a relatively large anisotropy of upper critical field. Magneto-optical images reveal homogenous current flow within the ab-plane., Comment: 16 pages, 5 figures. arXiv admin note: substantial text overlap with arXiv:1203.0099

The effect of proton irradiation in Ba(Fe0.93Co0.07)2As2 single crystals is reported. We analyze temperature dependence of the current density and normalized flux relaxation rate in the framework of the collective-creep model. The glassy exponent and barrier height for flux creep are directly determined by Maley's method. Our model functions for barrier height and critical current density in the absence of flux creep are explained by the superposition of δTc and δl pinnings. We also approach true critical current density by means of the generalized inversion scheme, and the obtained result is in reasonable agreement with our model function. The proton-irradiation effect on temperature dependence of the current density and normalized relaxation rate can be summarized as doubling of the barrier height at the beginning of flux creep.

Using high sensitivity magneto-optical imaging we find evidence for a jump in local vortex density associated with a vortex liquid to solid phase transition just above the lower critical field in a single crystal of Bi2Sr2CaCu2O8. We find the regions of the sample where the jump in vortex density occurs are associated with low screening currents. In the field - temperature vortex phase diagram we identify phase boundaries demarcating a dilute vortex liquid phase and the vortex solid phase. The phase diagram also identifies a coexistence regime of the dilute vortex liquid and solid phases and shows the effect of pinning on the vortex liquid to solid phase transition line. We find the phase boundary lines can be fitted to the theoretically predicted expression for the low-field portion of the phase boundary delineating a dilute vortex solid from a vortex liquid phase. We show that the same theoretical fit can be used to describe the pinning dependence of the low-field phase boundary lines provided a dependence of the Lindemann number on pinning strength is considered., 16 pages and 6 figures (Published)

Various kinds of energetic particles are irradiated into iron-based superconductors, and their effects on the critical current density (Jc) and vortex dynamics have been systematically studied. It is found that Jc is enhanced and vortex dynamics is strongly suppressed by energetic particles having a sufficient energy deposition rate, similar to the case of high temperature cuprate superconductors. The enhancement of Jc, in general, persists up to much higher irradiation doses than in cuprates. However, details of the effect of irradiation depend on the kind of ion species and their energies. Even with the same ions and energies, the effect is not universal for different kinds of iron-based superconductors. The correlated nature of defects created by heavy-ion irradiation is confirmed by the angular dependence of irreversible magnetization.

Various kinds of energetic particles are irradiated into iron-based superconductors, and their effects on the critical current density (Jc) and vortex dynamics have been systematically studied. It is found that Jc is enhanced and vortex dynamics is strongly suppressed by energetic particles having a sufficient energy deposition rate, similar to the case of high temperature cuprate superconductors. The enhancement of Jc, in general, persists up to much higher irradiation doses than in cuprates. However, details of the effect of irradiation depend on the kind of ion species and their energies. Even with the same ions and energies, the effect is not universal for different kinds of iron-based superconductors. The correlated nature of defects created by heavy-ion irradiation is confirmed by the angular dependence of irreversible magnetization.

The shrinkage of magnetic domains in a superconductor/ferromagnet bilayer is studied experimentally using Pb as a superconductor and a garnet film with maze domains as a ferromagnet. The effects of trapped flux in a superconductor and the pinning of magnetic domain boundaries are carefully minimized by using a high-quality Pb single crystal and an AC demagnetization technique. A temperature independent shrinkage of magnetic domains by a factor ~ 0.89 is obtained below the superconducting transition of Pb. The slightly suppressed shrinkage ratio as compared with the theoretical prediction can be explained by a finite gap between the Pb crystal and the garnet film.

We present scanning tunneling microscopy/spectroscopy (STM/STS) on iron-based superconductor Ba(Fe 0.93 Co 0.07 ) 2 As 2 single crystals. The STM topography shows that the low-temperature cleaved surface consists of two types of ordered atomic structures: (1) the 2×1 surface, formed by half of the Ba atoms remaining on the As layer, and (2) the \(\sqrt{2}\times\sqrt{2}\) surface of the As layer from which Ba atoms are completely removed. The superconducting spectra with the energy gap (2Δ≃6.9 k B T c ) are observed only on the 2×1 surface. This value is approximately twice that for weak coupling s -wave BCS superconductors, indicating a strong pairing interaction similar to that observed in cuprate superconductors. We also find that vortices form a disordered structure and that the vortex core spectroscopy does not show a bound state, suggesting a strong disorder effect on the vortex pinning and quasiparticle scattering.

The c-axis resistive transitions were examined as functions of magnetic field H and angle θ between H and the basal plane for a single-crystal (La1-xSrx)2CuO4 (x=0.12). The data followed the extended 3D fluctuation scaling form in a wide transition region at the reduced field h>2T. Relevant parameters are obtained by this scaling treatment. The anisotropy ratio(γ2) is estimated to be about 50 at T=25.8K by scaling the in-plane resistivity in terms of the h. Small γ -value and the 3D transition character are explained as resulting from strengthening of the coupling between the layers by the Sr “overdoping” in this system.

We report resistivity, Hall coefficient, current?voltage characteristics and magneto-optical imaging measurements of epitaxial Co-doped BaFe2As2 thin films deposited on an MgO(001) substrate. The Hall resistivity of the films has a substantial contribution arising from the anomalous Hall effect of ferromagnetic components. The critical current density (Jc) of the films is ~ 2?MA?cm ? 2 at low temperatures. Differential magneto-optical images of the remnant state give similar Jc values and also exhibit the presence of extended defects in the film.

Low temperature transport properties have been studied on the two types of samples of La1.875Ba0.125CuO4 in order to elucidate the relation between low temperature phase transition and superconductivity. These two samples, being different in normal transport properties, have revealed similar pressure dependence of the characteristic temperatures related to the low temperature phase transition. Possible scenarios of electronic states are discussed.

Transport in the mixed state of YBa2Cu3O7 (YBCO) and Bi2Sr2CaCu2O8 (BSCCO) thin films in parallel magnetic field configuration is studied with precision angular alignment of magnetic field and layer plane. For YBCO, an anomalous resistive component independent of the macroscopic Lorentz force is separated from an ordinary component. For BSCCO, a new feature is found at elevated current densities near the parallel configuration.

We report the temperature dependence of resistivity and Hall coefficient RH for YBa2Cu3O6.9Hx and Bi2Sr2CaCu2O8+yHx. The Hall density nH=V/RHe in the latter decreases linearly with x by nearly one per one hydrogen. The rate of the decrease of nH in the former is doubled, but the onset Tc remains constant at 93 K up to x=0.4. The analysis of these results suggests that H+ locating in the Cu(1)O plane of YBCO induces a localization of the neighboring carriers. On the other hand, H+ near the CuO2 or BiO plane in BSCCO donates one electron to the hole band, which results in an enhancement of Tc from 80 to 90 K.

Transient vortex states, created after a sudden exposure of a Bi 2 Sr 2 CaCu 2 O 8+δ crystal to a steady magnetic field, are observed using a high-temporal resolution magneto-optical imaging system. The images reveal dynamic coexistence of a disordered phase near the sample edges and a quasi-ordered phase in the sample interior, prior to the establishment of a quasi-ordered state throughout the entire sample as dictated by the thermodynamic conditions. Different points on the borderline between these two phases move toward the sample edges at different rates, depending on the local induction.

Vortex matter phase transitions in Bi 2 Sr 2 CaCu 2 O 8+ y (BSCCO) under fields tilted from the c -axis is studied by using Hall probes. Vortex lattice melting ( H m (θ)) and peak effect ( H p (θ)) are found to follow a novel angular scaling; transition fields linearly depends on the in-plane component of the field. The novel scaling is valid in the vortex solid phase up to doping-level-dependent angle, θ*. New anomalies in the magnetization hysteresis curve, H * p and H * cp , appear only at large tilting angles. Above θ*, the angular dependence of H p , H * p , and H * cp changes drastically, suggesting the presence of vortex phase transition at this angle.

Local magnetization measurements of Bi2Sr2CaCu2O8+δ single crystals with various shape and size were performed using a local GaAs/AlGaAs Hall probe. The vortex penetration field is proportional to the − 1 4 power of the sample area, independent of sample geometry. The second magnetization peak (SMP) is observed only below a size dependent temperature, which may not support the 2D–3D vortex solid transition as the intrinsic origin of the SMP.

We report measurements of the magnetic penetration depth \lambda_m(T) in the presence of a DC magnetic field in optimally doped BSCCO-2212 single crystals. Warming, after magnetic field is applied to a zero-field cooled sample, results in a non-monotonic \lambda_m(T), which does not coincide with a curve obtained upon field cooling, thus exhibiting a hysteretic behaviour. We discuss the possible relation of our results to the vortex decoupling, unbinding, and dimensional crossover.

Vortex states in heavily Pb-substituted Bi 2 Sr 2 CaCu 2 O 8+ y (BSCCO) with planar defects (PDs) from overdoped to underdoped region is studied using micro Hall probes. Angular dependence of magnetization reveals that in a restricted angle and field region, magnetization first increases when the field is tipped away from the c axis and then decreases again. This is understood as a manifestation of vortex channeling along the PDs. In addition, we observe an enhancement of magnetization (peak effect) at nearly constant c -axis component of the field and the peak field is almost independent of temperature and the doping level. The peak field has systematic correlation with the average spacing of PDs suggesting that a kind of matching effect is responsible for this phenomenon.

Simultaneous transport and magnetization measurements in Bi 2 Sr 2 CaCu 2 O 8 crystals are reported. The transport current is injected at the top surface, while Hall sensor arrays measure the local magnetic "eld across the crystals at the bottom. Surprisingly, at high-currents we "nd xnite resistivity well below the magnetic irreversibility line, where the vortices are pinned. This resistivity is non-monotonic with temperature and highly non-linear. We interpret the new observation as a shear-induced decoupling, in which the vortices #ow only in the top few layers and are decoupled from the pinned vortices in the rest of the crystal. ( 2000 Elsevier Science B.V. All rights reserved.

Nature of the pairing and vortex states are studied in Sr 2 RuO 4 using a Hall probe array. No clear evidence for the spontaneous magnetic field, accompanying the time-reversal symmetry breaking states, has been obtained. Magnetization with H a ∥ c shows an anomaly close to H c2 and a peak near H a =0. We interpret the former to be due to the synchronization peak effect. H c1 or change in the vortex lattice structure could be relevant to the latter.

Distribution of a local field B(x) on the surface of Bi2Sr2CaCu2O8+d single crystal and its time dependence are measured using a micro Hall-probe array with 7 elements. From these data, the activation energy dependence on B(x) and on the current density J are estimated. The three-dimensional mapping of U(B, J) in B, J and U space gives us information on spatial behavior of U(B, J) in the sample.

Local magnetization and local permeability in the mixed state of Bi 2 Sr 2 CaCu 2 O 8+ y single crystals have been measured by using a micro-Hall-probe. When temperature is decreased in the vortex lattice phase, local magnetic induction shows step-like decrease, indicating that vortices are expelled from the sample discontinuously. This behavior suddenly disappears at the so-called depinning temperature. In addition to this, broad band low-frequency noise in the local permeability is observed in the region where the discontinuous vortex expulsion occurs. The noise spectrum shows a power-law behavior. Magnetic field dependence of the local permeability suggests that these phenomena are related to the bulk pinning. The discontinuous vortex expulsion and the power-law behavior of the local permeability noise are consistently explained by the vortex avalanches in the vortex lattice phase.

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the undoped and electron-doped iron pnictides BaFe2\_{-x}CoxAs2 (Ba122) (x=0, 0.14) and studied the Fermi surfaces (FSs) and band dispersions near the Fermi level. The FS sheets we observed are consistent with the shrinkage of the hole-like pockets around the Brillouin Zone (BZ) center and the expansion of the electron pockets around the BZ corner in the electron-doped compound as compared to the undoped parent compound. Band dispersions and FSs around the BZ center strongly depend on the photon energy, indicating the three-dimensional (3D) electronic structure. This observation suggests that the antiferromagnetism and superconductivity in the pnictides may have to be considered including the orbital-dependent 3D electronic structure, where FS nesting is not necessarily strong.

Thermal transport measurements have been performed on single-crystalline Co-doped BaFe2As2 down to 0.1 K and under magnetic fields up to 7 T. Significant peak anomalies are observed in both thermal conductivity and thermal Hall conductivity below Tc as an indication of the enhancement of the quasiparticle mean-free path. Moreover, we find a sizable residual T-linear term in thermal conductivity, possibly due to a finite quasiparticle density of states in the superconducting gap induced by impurity pair breaking. Our findings support a pairing symmetry compatible with the theoretically predicted sign-reversing s-wave state., 4 pages, 4 figures, to be published in Journal of the Physical Society of Japan

We report on generation of spatiotemporal oscillations of magnetic flux in a Bi2Sr2CaCu2O8+δ crystal irradiated in part with 2.2 GeV Au ions. Flux oscillations are spontaneously excited after exposing the sample to a steady magnetic field near the order-disorder vortex phase transition line. The oscillations originate at the border between the irradiated and nonirradiated parts of the sample and propagate into the nonirradiated region toward the sample edge. Previously reported flux oscillations were observed in the vicinity of undefined defects in as grown Bi2Sr2CaCu2O8+δ crystals. Observation of spontaneous oscillations in partially irradiated samples present the first attempt to generate such oscillations in a controlled manner.

Surface field of a thin superconductor YBa2Cu3O7-δ in mixed state is measured by a Hall probe array. To reproduce the measured field profiles, shielding current distributions are determined by numerical iterative calculations without supposing any models for field dependence of critical current density Jc (B). Utilizing the estimated local current density and local magnetic field for x – y coordinates, a field variation of current density is plotted. Though any model for Jc (B) is not used for numerical calculations, the field variation roughly shows a dependence like Kim model.

Magneto-optical imaging was employed to study magnetic flux propagation through an interface separating between heavy-ion irradiated and non-irradiated parts of the same Bi2Sr2CaCu2O8+δ crystal. As the flux front hits the interface the local induction increases abruptly at a rate considerably larger than that of the external field. This induction jump is accompanied by the appearance of a finger-like flux pattern at the interface. At low temperatures or high field ramp-rates, the induction at the interface increases gradually, finger patterns do not form and the flux front crosses the interface smoothly. We explain the flux behavior at the interface on the basis of the magnetic diffusion equation, and propose that the induction jump triggers the formation of finger patterns.

Local nonlinear AC magnetic measurements were employed in the study of the vortex phase diagram in Bi2Sr2CaCu2O8+x. The melting and the Bragg-Glass to Vortex-Glass (solid-solid) transitions are manifested by a sharp peak and a sharp drop in the third harmonic response, respectively. The peak at the solid-liquid transition signifies the hysteretic nature of the first-order melting transition. The disappearance of the nonlinear response in the vicinity of the solid-solid phase transition line suggests the existence of an intermediate soft lattice vortex state in-between the ordered and disordered vortex phases.

The magnetic properties of a superconducting disk change dramatically when its dimensions become mesoscopic. Unlike large disks, where the screening currents induced by an applied magnetic field are strong enough to force vortices to accumulate in a 'puddle' at the centre, in a mesoscopic disk the interaction between one of these vortices and the edge currents can be comparable to the intervortex repulsion, resulting in a destruction of the ordered triangular vortex lattice structure at the centre. Vortices instead form clusters which adopt polygonal and shell-like structures which exhibit magic number states similar to those of charged particles in a confining potential, and electrons in artificial atoms. We have fabricated mesoscopic high temperature superconducting Bi2Sr2CaCu2O8+δ disks and investigated their magnetic properties using magneto-optical imaging (MOI) and high resolution scanning Hall probe microscopy (SHPM). The temperature dependence of the vortex penetration field measured using MOI is in excellent agreement with models of the thermal excitation of pancake vortices over edge barriers. The growth of the central vortex puddle has been directly imaged using SHPM and magic vortex numbers showing higher stability have been correlated with abrupt jumps in the measured local magnetisation curves.