Search

In order to enhance J c in (Ba,Na)Fe2As2 wires and tapes further by improving texturing, we examined the following approaches; (1) Ag-sheathed HIP tapes, (2) AgSn-sheathed cold-pressed tapes, and (3) multifilamentary HIP wires with each filament having tape-like cross section. In addition, we attempted to increase I c by fabricating (Ba,Na)Fe2As2 wires with larger core areas, and constructed a small demo coil using one of these wires with length up to 9 m. J c characteristics of these new wires and the performance of the demo coil with and without an electrical insulation are reported.

Effects of columnar defects created by 320 MeV Au irradiation on 2H-NbSe2 single crystals with a dose equivalent matching field up to 16 T were studied. Critical temperature is found to be suppressed almost linearly at a rate of 0.07 K/T. At the same time, the lattice parameters a and c are found to be expanded at rates of 0.016%/T and 0.030%/T, respectively. Such a lattice expansion should work as negative pressure to affect Tc. By separating the effect of heavy-ion irradiation on Tc suppression through lattice expansion and disorder, it is found that Tc is suppressed more by lattice expansion rather than by disorder.

When exposed to high magnetic fields, certain materials manifest an exotic superconducting (SC) phase that has attracted considerable attention. A proposed explanation for the origin of the high-field SC phase is the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. This state is characterized by inhomogeneous superconductivity, where the Cooper pairs have finite center-of-mass momenta. Recently, the high-field SC phase was observed in FeSe, and it was deemed to originate from the FFLO state. Here, we synthesize FeSe single crystals with different levels of disorder. The level of disorder is expressed by the ratio of the mean free path to the coherence length and ranges between 35 and 1.2. The upper critical field \textit{B}$_{\rm{c}2}$ was obtained by both resistivity and magnetic torque measurements over a wide range of temperatures, which went as low as $\sim$0.5 K, and magnetic fields, which went up to $\sim$38 T along the \textit{c} axis and in the \textit{ab} plane. In the high-field region parallel to the \textit{ab} plane, an unusual SC phase was confirmed in all the crystals, and the phase was found to be robust against disorder. This result suggests that the high-field SC phase in FeSe is not a conventional FFLO state., 11 pages, 11 figures

Na-substituted 122-type iron-based superconductors were synthesized and fabricated into superconducting tapes and wires. Good J c performance of Ag-sheathed Sr0.45Na0.55Fe2As2 pressed tapes was demonstrated, and it was further improved by using AgSn as a sheath material. The improved J c characteristics are due to the reduction of the optimum sintering temperature by ∼100 °C, which suppressed the reaction between the sheath and the core. However, the fabrication of good Sr0.45Na0.55Fe2As2 round wire using the hot isostatic press method failed, due possibly to the segregation of Na in the core. To avoid the segregation of Na, we fabricated round wires employing (Ba,Na)Fe2As2 rather than underdoped (Sr,Na)Fe2As2. J c in Ba0.6Na0.4Fe2As2 round wire processed 700 °C under the pressure of 175 MPa reached 1.5 × 105 and 2.0 × 104 A/cm2 at self-field and 100 kOe, respectively. These values are more than fifty percent of the largest J c in (Ba,K)Fe2As2 round wires, demonstrating the excellent performance of (Ba,Na)Fe2As2 wires.

A spontaneous vortex structure in magnetic domains (vortex domains) has been visualized by the decoration technique near the ferromagnetic transition temperature (TC = 18 K) in a ferromagnetic superconductor EuFe2(As0.79P0.21)2 single crystal with the superconducting transition temperature (TSC = 22 K). Meissner domains previously observed by magnetic force microscopy have also been observed in a narrow temperature range (ΔT ∼ 1 K) near TC. A domain structure with the alternating direction of the magnetic flux has been observed upon cooling below TC. Experimental results are in reasonable agreement with a theoretical estimate of the periods of domain structures and the temperature interval below TC in which they are observed.

We report the fabrication of small (Ba,A)Fe2As2 (A: Na, K) coils using 10 m-class long round wires, fabricated by powder-in-tube (PIT) method. Coils are sintered using hot-isostatic-press (HIP) technique after glass-fiber insulations are installed. Critical current (Ic) of the whole coil using (Ba,Na)Fe2As2 and (Ba,K)Fe2As2 are 60 A and 66 A under the self-field, and the generated magnetic fields at the center of the coil reach 2.6 kOe and 2.5 kOe, respectively. Furthermore, the largest transport critical current density (Jc) and Ic in (Ba,Na)Fe2As2 wires picked up from the coil reach 54 kAcm-2 and 51.8 A at T = 4.2 K under a magnetic field of 100 kOe, respectively. This value exceeds transport Jc of all previous iron-based superconducting round wires. Texturing of grains in the core of the wire due to the improvement of the wire drawing process plays a key role for the enhancement of Jc., 25 pages, 1 table, 9 figures, accepted in Superconductor Science and Technology

Static (DC) and dynamic (AC, at 14 MHz and 8 GHz) magnetic susceptibilities of single crystals of a ferromagnetic superconductor, EuFe2(As1−xPx)2 (x = 0.23), were measured in pristine state and after different doses of 2.5 MeV electron or 3.5 MeV proton irradiation. The superconducting transition temperature, Tc(H), shows an extraordinarily large decrease. It starts at Tc(H=0)≈24K in the pristine sample for both AC and DC measurements, but moves to almost half of that value after moderate irradiation dose. Remarkably, after the irradiation not only Tc moves significantly below the FM transition, its values differ drastically for measurements at different frequencies, ≈16 K in AC measurements and ≈12 K in a DC regime. We attribute such a large difference in Tc to the appearance of the spontaneous internal magnetic field below the FM transition, so that the superconductivity develops directly into the mixed spontaneous vortex-antivortex state where the onset of diamagnetism is known to be frequency-dependent. We also examined the response to the applied DC magnetic fields and studied the annealing of irradiated samples, which almost completely restores the superconducting transition. Overall, our results suggest that in EuFe2(As1−xPx)2 superconductivity is affected by local-moment ferromagnetism mostly via the spontaneous internal magnetic fields induced by the FM subsystem. Another mechanism is revealed upon irradiation where magnetic defects created in ordered Eu2+ lattice act as efficient pairbreakers leading to a significant Tc reduction upon irradiation compared to other 122 compounds. On the other hand, the exchange interactions seem to be weakly screened by the superconducting phase leading to a modest increase of Tm (less than 1 K) after the irradiation drives Tc to below Tm. Our results suggest that FM and SC phases coexist microscopically in the same volume.

We report a systematic study of the superconducting (SC) and normal-state anisotropy of ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{0.6}{\mathrm{Se}}_{0.4}$ single crystals with controlled amounts of excess Fe ($y$ = 0, 0.07, and 0.14). The SC state anisotropy ${\ensuremath{\gamma}}_{H}$ was obtained by measuring the upper critical fields under high magnetic fields over 50 T for both $H\ensuremath{\parallel}ab$ and $H\ensuremath{\parallel}c$. On the other hand, the normal state anisotropy ${\ensuremath{\gamma}}_{\ensuremath{\rho}}$ was obtained by measuring the resistivity with current flowing in the $ab$ plane (${\ensuremath{\rho}}_{ab}$) and along the $c$ axis (${\ensuremath{\rho}}_{c}$). To precisely measure ${\ensuremath{\rho}}_{ab}$ and ${\ensuremath{\rho}}_{c}$ in the same part of a specimen avoiding the variation dependent on pieces or parts, we adopt a new method using a microfabricated bridge with an additional neck part along the $c$ axis. The ${\ensuremath{\gamma}}_{H}$ decreases from a value dependent on the amount of excess Fe at ${T}_{\mathrm{c}}$ to a common value $\ensuremath{\sim}1$ at 2 K. The different ${\ensuremath{\gamma}}_{H}$ at ${T}_{\mathrm{c}}$ ($\ensuremath{\sim}1.5$ for $y$ = 0, and 2.5 for $y$ = 0.14) suggests that the anisotropy of effective mass ${m}_{c}^{*}/{m}_{ab}^{*}$ increases from $\ensuremath{\sim}2.25$ ($y$ = 0) to 6.25 ($y$ = 0.14) with the excess Fe. The almost isotropic ${\ensuremath{\gamma}}_{H}$ at low temperatures is due to the strong spin paramagnetic effect at $H\ensuremath{\parallel}ab$. By contrast, the ${\ensuremath{\gamma}}_{\ensuremath{\rho}}$ shows a much larger value of $\ensuremath{\sim}17$ ($y$ = 0) to $\ensuremath{\sim}50$ ($y$ = 0.14) at the temperature just above ${T}_{\mathrm{c}}$. Combined the results of ${\ensuremath{\gamma}}_{H}$ and ${\ensuremath{\gamma}}_{\ensuremath{\rho}}$ near ${T}_{\mathrm{c}}$, we found out that the discrepant anisotropies between the SC and normal states originates from a large anisotropy of scattering time ${\ensuremath{\tau}}_{ab}/{\ensuremath{\tau}}_{c}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}$ 7.8. The ${\ensuremath{\tau}}_{ab}/{\ensuremath{\tau}}_{c}$ is found to be independent of the excess Fe.

We fabricated a compact (13$\times$12$\times$12.5 mm${^3}$) trapped-field magnet by stacking 200 pieces of EuBa${_2}$Cu${_3}$O${_7}$ coated conductors with BaHfO${_3}$ nanorods as artificial pinning centers. It was magnetized by field-cooling method using a 18 T superconducting magnet, and the maximum field of 14.8 T was trapped at 10 K at a field-ramp rate of 0.1 T/min. At higher temperatures, although the trapped field was decreased, we could trap the field with much faster ramp rate. In order to understand these results, we also performed calculations of the trapped field based on $J_\mathrm{c}-H$ characteristics of the coated conductor. The calculated trapped field value was larger than the experimental value at 10 K. This discrepancy can be understood by considering the reduction of the effective size of the stacked coated conductors due to the existence of extended regions of irregular flux penetration called "flux jets"., 19 pages, 9 figures, to be published in Superconductor Science and Technology

FeSe$_{1-x}$Te$_{x}$ superconductors manifest some intriguing electronic properties depending on the value of $x$. In FeSe single crystal, the nematic phase and Dirac band structure have been observed, while topological surface superconductivity with the Majorana bound state was found in the crystal of $x \sim 0.55$. Therefore, the electronic properties of single crystals with $0 < x \leq 0.5$ are crucial for probing the evolution of those intriguing properties as well as their relations. However, this study is still left blank due to the lack of single crystals because of phase separation. Here, we report the synthesis, magnetization, electronic transport properties, and hydrostatic pressure effect of FeSe$_{0.67}$Te$_{0.33}$ single crystals free of phase separation. A structural (nematic) transition is visible at $T_{s} = 39$ K, below which the resistivity exhibits a Fermi-liquid behavior. Analysis of upper critical fields suggests that spin-paramagnetic effect should be taken into account for both $H \parallel c$ axis and $H \parallel ab$ plane. A crossover from the low-$H$ quadratic to the high-$H$ quasi-linear behavior is observed in the magnetoresistance, signifying the possible existence of Dirac-cone state. Besides, the strong temperature dependence of Hall coefficient, violation of (modified) Kohler's rule, and two-band model analysis indicate the multiband effects in FeSe$_{0.67}$Te$_{0.33}$ single crystals. Hydrostatic pressure measurements reveal that $T_{s}$ is quickly suppressed with pressure while $T_{c}$ is monotonically increased up to 2.31 GPa, indicating the competition between nematicity and superconductivity. No signature of magnetic order that has been detected in FeSe$_{1-x}$S$_{x}$ is observed. Our findings fill up the blank of the knowledge on the basic properties of FeSe$_{1-x}$Te$_{x}$ system with low-Te concentrations., 10 pages, 5 figures

We report on ion irradiation experiments performed on compounds belonging to the $$\hbox {BaFe}_2\hbox {As}_2$$ BaFe 2 As 2 family, each one involving the partial substitution of an atom of the parent compound (K for Ba, Co for Fe, and P for As), with an optimal composition to maximize the superconducting critical temperature $$T_c$$ T c . Employed ion beams were 3.5-MeV protons, 250-MeV Au ions, and 1.2-GeV Pb ions, but additional data from literature are also considered, thus covering a wide range of ions and energies. Microwave characterization based on the use of a coplanar waveguide resonator allowed us to investigate the irradiation-induced $$T_c$$ T c degradation, as well as the increase of normal state resistivity and London penetration depth. The damage was quantified in terms of displacements per atom (dpa). From this broad and comprehensive set of experimental data, clear scaling laws emerge, valid in the range of moderate irradiation-induced disorder (dpa up to 5 $$\times$$ × 10 $$^{-3}$$ - 3 were investigated). In these conditions, linear trends with dpa were found for all the modification rates, while a power law dependence on the ion energy was found for heavy-ion irradiation. All these scaling laws are reported and discussed throughout the paper.

We present a systematic study of electrical resistivity, Hall coefficient, magneto-optical imaging, magnetization, and scanning transmission electron microscopy (STEM) analyses of $\mathrm{K}{\mathrm{Ca}}_{2}{\mathrm{Fe}}_{4}{\mathrm{As}}_{4}{\mathrm{F}}_{2}$ single crystals. Sharp diamagnetic transition and magneto-optical imaging reveal homogeneity of single crystal and prominent Bean-like penetrations of vortices. Large anisotropy of electrical resistivity, with ${\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{ab}g100$, and semiconductorlike ${\ensuremath{\rho}}_{c}$ suggest that the electronic state is quasi-two-dimensional. Hall effect measurements indicate that $\mathrm{K}{\mathrm{Ca}}_{2}{\mathrm{Fe}}_{4}{\mathrm{As}}_{4}{\mathrm{F}}_{2}$ is a multiband system with holes as main carriers. Magnetization measurements reveal significantly larger ${J}_{\mathrm{c}}$ compared with that in other iron-based superconductors with different values of ${J}_{\mathrm{c}}$ depending on the direction of magnetic field. The origin of these ${J}_{\mathrm{c}}$ characteristics is discussed based on microstructural observations using STEM. In addition, further enhancement of ${J}_{\mathrm{c}}$ in $\mathrm{K}{\mathrm{Ca}}_{2}{\mathrm{Fe}}_{4}{\mathrm{As}}_{4}{\mathrm{F}}_{2}$ for future high-field application is demonstrated in terms of heavy-ion irradiation.

In order to enhance the critical current density ($J_c$) of superconductors, introduction of columnar defects (CDs) through swift-particle irradiation is effective. By dispersing the direction of CDs (splayed CDs), not only further enhancement of $J_c$ has been confirmed but also an anomalous peak effect (APE) in $J_c$ at a certain magnetic field determined by the irradiation dose was observed. It has been proposed that the APE arises from the suppression of kink motion and/or the effects of vortex entanglement in systems with splayed CDs. In this study, we measure $J_c$ properties of optimally K-doped Ba-122-type iron-based superconductor Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ single crystals with splayed CDs that are introduced asymmetrically with respect to the $c$-axis by irradiating 2.6 GeV U ions. We discuss the significance of the average direction of splayed CDs to the APE. We also discuss the relationship between the APE and the entanglement of vortices., 19 pages, 7 figures, to be published in Journal of the Physical Society of Japan

We investigate the superconducting gap function of topological superconductor PbTaSe$_2$. Temperature, magnetic field, and three-dimensional (3D) field-angle dependences of the specific heat prove that the superconductivity of PbTaSe$_2$ is fully-gapped, with two isotropic $s$-wave gaps. The pair-breaking effect is probed by systematically increasing non-magnetic disorders through H$^+$-irradiations. The superconducting transition temperature, $T_{\rm{c}}$, is found to be robust against disorders, which suggests that the pairing should be sign-preserved rather than sign-reversed., 9 pages, 7 figures

The change in vortex ensemble in BaFe$_{2}$(As$_{0.67}$P$_{0.33}$)$_2$, an isovalently doped iron-based superconductor (IBS), is studied through global magnetization measurements and single vortex imaging before and after 6 MeV proton irradiation. The field dependence of the critical current density ($J_\mathrm{c}$) is analyzed through the strong pinning model, with which the pristine sample is consistent. After the irradiation, the $J_\mathrm{c}$ aberrates from the strong pinning field dependence of $B^{-5/8}$, and evolves to a weaker $B^{-1/3}$ dependence with an anomalous two-step behavior creating a cusp like feature. The cusp coincides with the field of the local minima in the normalized relaxation rate ($S$), manifested by increased pinning due to increased intervortex interactions followed by fast vortex dynamics caused by flux activation at higher fields. Furthermore, single vortex imaging reveals that while long-range triangular correlation of the Abrikosov vortex lattice is observed in pristine samples, irradiated samples exhibit a highly disordered glassy vortex state which is more densely packed with increased pinning force. These artificial defects incorporated via proton irradiation have the same pinning effect as Co doping which not only shifts the pinning force to a higher degree but also broadens its distribution, in contrast to P doping which only broadens the pinning distribution without inducing a shift. All in all, through this investigation, we provide a systematic understanding of the pinning behavior in BaFe$_{2}$(As$_{0.67}$P$_{0.33}$)$_2$ through carefully controlling the defects in the system.

Self-field imaging of current distribution in Ba0.6K0.4Fe2As2 superconductor is used to study the effect of drive on the low-field vortex solid to liquid melting phase transformation. At low fields, the current induced drive on the vortices aids in thermally destabilizing the vortex state thereby shifting the low field melting phase boundary. We show that the current induced drive shifts solid-liquid boundaries and prepones the vortex melting phenomenon compared to the equilibrium situation. The analysis shows that for currents above 50 mA, Joule heating effects shift the melting line while below 50 mA, an effective temperature concept for driven system viz., a drive dependent shaking temperature, explains the shift. The observation of a transformation from inhomogeneous to homogeneous current flow in the sample at low fields as a function of the driving force is reconciled via an inverse dependence of the shaking temperature on vortex velocity, which is incorporated in our analysis.

The strong modulation of the AC magnetic response in the magnetic field-temperature region of the DC magnetization peak effect (PE) occurring just below the DC irreversibility line (IL) of weakly pinned superconducting single crystals is well known. However, it is not yet clear if the AC magnetic signal registered at usual frequencies and amplitudes is significantly affected by the presence of the second magnetization peak (SMP) in the DC magnetic hysteresis curves, since the SMP appears in specimens with stronger pinning and is located well below the IL. We investigated the AC magnetic response of several La2−xSrxCuO4 and 122-type pnictide superconducting single crystals exhibiting a pronounced DC SMP. It was found that in the case of small demagnetization effects, the AC magnetic signal across the SMP remains in the linear regime, with no detectable distortions in the SMP range (within the high sensitivity of a Physical Property Measurement System). This indicates reduced values of the Campbell penetration depth compared with the London magnetic penetration depth. A clearly nonmonotonic, SMP-related variation has only been obtained for the DC field dependence of the out of phase component of the AC magnetic moment in the nonlinear regime. The nonlinear AC Bean regime far below the IL has been attained in the conditions of enhanced demagnetization effects (plate-like specimens in perpendicular magnetic fields).

Topological insulators and semimetals as well as unconventional iron-based superconductors have attracted major recent attention in condensed matter physics. Previously, however, little overlap has been identified between these two vibrant fields, even though the principal combination of topological bands and superconductivity promises exotic unprecedented avenues of superconducting states and Majorana bound states (MBSs), the central building block for topological quantum computation. Along with progressing laser-based spin-resolved and angle-resolved photoemission spectroscopy (ARPES) towards high energy and momentum resolution, we have resolved topological insulator (TI) and topological Dirac semimetal (TDS) bands near the Fermi level ($E_{\text{F}}$) in the iron-based superconductors Li(Fe,Co)As and Fe(Te,Se), respectively. The TI and TDS bands can be individually tuned to locate close to $E_{\text{F}}$ by carrier doping, allowing to potentially access a plethora of different superconducting topological states in the same material. Our results reveal the generic coexistence of superconductivity and multiple topological states in iron-based superconductors, rendering these materials a promising platform for high-$T_{\text{c}}$ topological superconductivity., Comment: 9 pages, 6 figures. arXiv admin note: substantial text overlap with arXiv:1803.00846, arXiv:1803.00845

Inhomogeneous superconductivity in rare-earth (RE)-doped CaFe2As2 (Ca122) compounds leads to a novel state of matter in which the superconducting and magnetic states can be simultaneously controlled by using an electric current (I). Both La- and Ce-doped Ca122 single crystals show a very broad superconducting transition width (ΔTc) due to their non-bulk nature. Surprisingly, ΔTc becomes sharper or broader after an electric current larger than a threshold value (It) is applied, with a concomitant change in the normal-state magnetism. The sharpened (broadened) ΔTc is accompanied by a decrease (an increase) in the amplitude of the ferromagnetic signals. The sensitive changes in the superconductivity and magnetism that occur when an external current is applied are related to the inhomogeneous electronic states that originate from the Fe magnetic state and/or self-organized superconducting/magnetic composites in Ca122 compounds. These discoveries shed new light on the role of Fe in Fe-based superconductors and will provide new ideas for the design of novel superconducting devices. Clues to the superconducting behavior of a complex, iron-based substance can be revealed with a technique based on electric current. When rare-earth atoms are incorporated into calcium–iron–arsenic (CaFe2As2) crystals, simultaneous magnetic and superconductive properties emerge after cooling below 50 K. Soon-Gil Jung and Tuson Park from South Korea’s Sungkyunkwan University, Suwon, and colleagues report that passing milliamp-level electric currents through rare-earth-doped CaFe2As2 causes unexpected shifts in superconducting transition temperatures and magnetic field responses. In some samples, a sharpening of the superconductivity onset point was accompanied by a loss of magnetic signals, while in others magnetism improved at the expense of well-defined superconducting temperatures. The researchers conclude that these changes arise from inhomogeneities within doped CaFe2As2, possibly from iron magnetic states or tiny self-assembled nanocomposites that become unstable past a current threshold. Large inhomogeneous electronic states in rare-earth-doped CaFe2As2 produce striking results of manipulating the superconducting phases via current-driven magnetic state. Magnetization hysteresis loops at superconducting state (2 K) and normal state (50 K) for La-doped CaFe2As2 are largely changed by the electric current because their high-Tc regions are localized. Current path between high-Tc regions is considered as a long wire, thus current-induced large magnetic field around the path can modulate the magnetic state in normal/weak superconducting regions. These observations provide new insights into the role of Fe in the Fe-based superconductors and ideas for the design of new superconducting devices.

A microwave technique suitable for investigating the AC magnetic susceptibility of small samples in the GHz frequency range is presented. The method—which is based on the use of a coplanar waveguide resonator, within the resonator perturbation approach—allows one to obtain the absolute value of the complex susceptibility, from which the penetration depth and the superfluid density can be determined. We report on the characterization of several iron-based superconducting systems, belonging to the 11, 122, 1144, and 12442 families. In particular, we show the effect of different kinds of doping for the 122 family, and the effect of proton irradiation in a 122 compound. Finally, the paradigmatic case of the magnetic superconductor EuP-122 is discussed, since it shows the emergence of both superconducting and ferromagnetic transitions, marked by clear features in both the real and imaginary parts of the AC susceptibility.

We have successfully trapped a field of 17.89 T at 6.5 K at the center of a compact coated-conductor (CC) stacks (13 × 12 × 11.7 mm3) within 75 min by suppressing flux jumps. The CC stack consists of 200 sheets of EuBa2Cu3O7 CCs with BaHfO3 nanorods to increase the critical current density at high fields and low temperatures. To enhance thermomagnetic stability, the central 50 CCs are coated with 1 µm thick Pb with large specific heat at low temperatures. Numerical calculations based on the actual J c–H characteristics reproduces the trapped field quantitatively. New directions for achieving even higher trapped field at higher temperatures and making use of the trapped field are discussed.

We report the critical current density (Jc) and vortex pinning properties in single crystals of a novel iron-based superconductor (IBS) KCa2Fe4As4F2 with large Jc in the pristine state, before and after introduction of artificial defects by swift-particle irradiation. The effects of 2.6 GeV U and 3 MeV proton irradiations in KCa2Fe4As4F2 single crystals on transition temperature Tc and Jc, including its dose dependence, are systematically studied. Jc~8 MA/cm2 under a self-field at 2 K in the pristine crystal is strongly enhanced up to 19.4 and 17.5 MA/cm2 by irradiation of 2.6 GeV U-ions and 3 MeV protons, respectively. Suppression of Tc and dose dependence of Jc in KCa2Fe4As4F2 is different from that in a representative IBS of (Ba,K)Fe2As2, which can be explained by considering the presence of embedded defects in pristine KCa2Fe4As4F2. The vortex dynamics in the pristine and proton irradiated KCa2Fe4As4F2 single crystals are also investigated from the analyses of the field dependence of Jc and the normalized magnetic relaxation rate. In addition to the contribution of embedded defects, weak collective pinning is considered for comprehensive analyses. Vortex dynamics in KCa2Fe4As4F2 is similar to those in (Ba,K)Fe2As2 to some extent, and different from that in anisotropic Li0.8Fe0.2OHFeSe. Large anisotropy, due to the presence of insulating blocking layers in KCa2Fe4As4F2, which leads to much lower irreversibility field (Hirr) compared with 122-type IBSs, strongly affect the vortex dynamics.

High-performance Fe–Se–Te (FST) films are favorable for revealing the relationships between the crystal structure and the superconducting properties of the material because of their simple structures. Therefore, it is particularly important to prepare high-quality “11” thin films. Herein, we focused on the thickness and deposition temperature dependence of $T^{\rm {zero}}_{\rm{c}}$ and found that the thickness of about 200 nm and the deposition temperature of 300 °C are the optimal parameters for growing high-quality FST films on CaF2 and SrTiO3 substrates, which exhibit $T^{\rm {zero}}_{\rm{c}}$ of 18.8 K and 17 K, respectively. Magnetic Jc values of FST films grown on CaF2 and SrTiO 3 are both larger than 2 × 106 A·cm–2 at 4.2 K and self-field, significantly higher than that of FST single crystals.

Critical state field profiles and thermomagnetic instabilities are studied in two kinds of three-dimensional nanostructured superconductors. We find that the critical state field profiles in some simple bi-layer systems are not simple superpositions of critical states in the two layers. Competition between the divergence of the local field at the edges of the film and the shielding by the neighboring layer makes novel critical state field profiles. We also studied flux avalanches in shifted strip arrays (SSAs) with layer numbers up to six. Various forms of avalanches either perpendicular or parallel to the strip are observed when the overlap between strips in neighboring layers is large. We also find that introduction of asymmetry in various forms to SSA affects the shape of flux avalanches sensitively.

Theoretically, the vortex melting phenomenon occurs at both low and high magnetic fields at a fixed temperature. While the high-field melting has been extensively investigated in high-${T}_{c}$ cuprates, the low-field melting phenomena in the presence of disorder has not been well explored. Using bulk magnetization measurements and a high-sensitivity differential magneto-optical imaging technique, we detect a low-field vortex melting phenomenon in a single crystal of ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$. The low-field melting is accompanied with a significant change in local magnetization, \ensuremath{\sim}3 G, which decreases with increasing applied field. The observed vortex melting phenomenon is traced on a field-temperature phase diagram, which lies very close to theoretically predicted Lindemann criteria based low-field melting line. Our analysis shows a Lindemann number ${c}_{L}=0.14$ associated with the low-field melting. Imaging of low-field vortex melting features shows that the process nucleates via formation of extended fingerlike projections which spreads across the sample with increasing field or temperature, before entering into an interaction-dominated vortex solid phase regime. Magnetization scaling analysis and ngular dependence of bulk magnetization hysteresis loop shows extended pins naturally present in the sample. These defects create a low-field glassy vortex phase sustaining a finite critical current present in the phase diagram below the low-field liquid phase. We construct a vortex matter phase diagram, which identifies boundaries demarcating a low-field glassy vortex state from a dilute vortex liquid phase and a weakly interacting solid phase above it. All these phases are shown to be present well below the interaction dominated vortex state shown in the phase diagram.

We fabricated (Ba,Na)Fe2As2 superconducting wires using the powder-in-tube method and hot isostatic pressing. By improving powder synthesis processes compared with previous studies, highly pure raw materials for the wire fabrication were obtained. The largest transport critical current density (Jc) reached 40 kAcm-2 at T = 4.2 K under a magnetic field of 100 kOe. This value exceeds not only the value of transport Jc of the previous (Ba,Na)Fe2As2 wire but also those of all iron-based superconducting round wires. Improvements of polycrystalline powder synthesis play a key role for the enhancement of Jc. Furthermore, it was clarified that higher densification by high-pressure sintering and the texturing of grains in the core of the wire due to drawing also increased Jc effectively. Advantages of (Ba,Na)Fe2As2 wires compared with (Ba,K)Fe2As2 wires are also discussed., Comment: 22 pages, 8 figures

This paper presents a microwave analysis of the interplay between magnetism and superconductivity in an iron-based ferromagnetic superconductor. By comparing the complex rf susceptibility with magnetic force images, the authors discuss the nature of the observed phase transitions and the possible presence of a quantum critical point.

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 Sr_{x}Bi_{2}Se_{3}. 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.

We report on the combined experimental and theoretical analysis of the microwave-frequency electromagnetic response of ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ single crystals with different substitutions: K in the Ba site (hole doping), Co in the Fe site (electron doping), and P in the As site (isovalent substitution). Measurements using a coplanar resonator technique lead to the experimental determination of the penetration depth and microwave conductivity as a function of temperature. The whole set of data is analyzed within a self-consistent three-band ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave Eliashberg approach, able to account for all the main observed features in the different properties. Besides the validation of the model itself, the comparison between experiment and theory allows discussing the possible role of the Fe-As planes in defining the superconducting properties of these compounds, the relevance of coherence effects, and the presence of nodes in the superconducting order parameter.

Three independent components of critical current density, one for the $H//c$ axis and the other two for the $H//ab$ plane, have been studied in ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$ single crystals. When the magnetic field is applied along the $c$ axis, we observed fish-tail-like peaks in the $M\ensuremath{-}H$ hysteresis loop, and the magnetization at higher temperatures exceeds that at lower temperatures at high fields. When the field is applied parallel to the $ab$ plane, a dip structure is observed in the $M\ensuremath{-}H$ hysteresis loop near the self-field. In addition, for the $H//ab$ plane, we succeeded in separately evaluating the large and significantly anisotropic in-plane and out-of-plane ${J}_{\mathrm{c}}$. Transmission electron microscopy revealed the presence of planar defects parallel to the $ab$ plane in ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$, which have not been observed in any other iron-based superconductors. We discuss the possible relationship between the anomalous ${J}_{\mathrm{c}}$ behavior and the planar defects.

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 Sr_{x}Bi_{2}Se_{3}. 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.

Enhancement and non-monotonic field dependence of critical current density (Jc ), called anomalous peak effect, were observed in Ba1−x K x Fe2As2 with splayed columnar defects. In principle, anisotropy of in-plane Jc should be present in this superconductor since the introduced splayed columnar defects introduce anisotropy in the plane. This study reports in-plane anisotropy of Jc observed in 2.6 GeV U irradiated Ba0.6K0.4Fe2As2 with splay angles of ±15° and ±20°. Results show that Jc component parallel to the splay plane is larger than the perpendicular component, and the parallel component contribute to the anomalous peak effect at around B Φ/3 more.

We report the fabrication and the transport measurements of FeTe1-xSex (x=0.4, 1) nanobridges along the c axis, toward the appearance of Josephson effects in single-crystal devices. Cross sectional areas of both FeTe0.6Se0.4 and FeSe nanobridges were systematically reduced to 0.06 μm2 by using a new method based on the focused ion beam (FIB) techniques. The critical current Ic measured by the current-voltage characteristics is roughly two orders of magnitude smaller than that for the conventional microbridges with larger cross sections, while the IcRn product, where R n is the normal-state resistance along the c axis, is still 3-4 times larger than the theoretical value for the appearance of Josephson effects. We argue the importance of the development of single-crystal Josephson devices and the comparison between FeTe0.6Se0.4 and FeSe nanobridges.

We report the fabrication of 10 m-class (Ba, Na)Fe2As2 long round wires and coils using these round wires. Round wires are fabricated by powder-in-tube (PIT) method and hot-isostatic-press (HIP) technique. To increase Ic keeping the diameter of the wire constant, the cross section area of the superconducting core is enlarged by changing the dimensions of metal sheaths. At 4.2 K under a high field of 100 kOe, Jc of a short segment of HIP wire reaches 42 kAcm-2, which is comparable to the highest Jc of IBS round wires of 44 kAcm-2. Furthermore, Ic of the HIP wire at 4.2 K under 10 kOe and 100 kOe reaches 95 A and 54 A, respectively, which is about twice the value of the previous report. We also fabricated a small demonstration superconducting coil using 12.5 m long PIT wires by HIP method. Ic of the whole coil is ∼60 A under the self-field, and magnetic field at the center of the coil reaches 2.6 kOe. The coil generated the magnetic field of 43 Oe/A, which is almost consistent with the calculated field from the coil dimensions.

Effects of three kinds of columnar defects in NbSe2 single crystals introduced by 320 MeV Au irradiation with a dose equivalent matching field B Φ up to 4 T were studied. Through the magnetization measurement using a SQUID magnetometer, two pronounced peak effects in the external field dependence of critical current density (J c) were observed in crystals introduced with splayed and tilted columnar defects. In crystals with splayed columnar defects, J c at the peak field reaches the maximum for splay angle θ CD = ±10°, which is a little different from that found in YBa2Cu3O7 and iron-based superconductors, where optimal θ CD is equal to ±5°.

CaKFe4As4 is a new member of the 1144-type iron-based superconductors (IBSs) which is expected to show remarkable superconducting properties because its crystal structure is similar to that of 122-type IBSs. Recently, substantial anisotropy between the in-plane and out-of-plane critical current densities (J C), with peculiar temperature dependence of in-plane J C at high fields, has been reported in CaKFe4As4 single crystals. The anisotropy of J C was attributed to the presence of planar defects, which were shown to be confined to the ab plane. However, the origin of these defects remains unclear. To elucidate the origin of these planar defects, we herein present our results on atomic-scale microstructure analysis of high-quality CaKFe4As4 single crystals. Using high-resolution scanning transmission electron microscopy (STEM), along with electron energy loss spectroscopy, we have demonstrated that these planar defects consist of one- or two-layer KFe2As2 step network, which is spread over the periodically ordered KFe2As2 and CaFe2As2 monolayers. Further, we report that the dark contrast regions observed via conventional STEM are local strain distributions, which originate from the substitution of Ca by K in a unit cell.

Magnetic flux structure on the surface of EuFe$_2$(As$\rm_{1-x}$P$\rm_x$)$_2$ single crystals with nearly optimal phosphorus doping levels $x=0.20$, and $x=0.21$ is studied by low-temperature magnetic force microscopy and decoration with ferromagnetic nanoparticles. The studies are performed in a broad temperature range. It is shown that the single crystal with $x=0.21$ in the temperature range between the critical temperatures $T_{\rm SC}=22$ K and $T_{\rm C}=17.7$ K of the superconducting and ferromagnetic phase transitions, respectively, has the vortex structure of a frozen magnetic flux, typical for type-II superconductors. The magnetic domain structure is observed in the superconducting state below $T_{\rm C}$. The nature of this structure is discussed., 6 pages, 3 figures