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Single crystals of superconducting K_xFe_2−ySe₂ have been grown with the optical floating-zone technique under application of 8 bar of argon pressure. We found that large and high quality single crystals with dimensions of  ∼ 6 mm ×10 mm could be obtained at the termination of the grown ingot through quenching, while the remaining part of the ingot decomposed. As-grown single crystals commonly represent an intergrowth of two sets of c-axes characterized by slightly different lattice constants. Single-crystal K_0.80Fe_1.81Se₂ shows a superconducting transition at T_c = 31.6 K, leading to a near 100% expulsion of the external magnetic field in magnetization measurements. On the other hand, neutron diffraction data indicate that superconductivity in the sample coexists with a $\sqrt{5}\times \sqrt{5}$ iron-vacancy superstructure and static antiferromagnetic order. The anisotropic ratio of the upper critical field H_c2 for both H ∥ c and H ∥ ab configurations is  ∼ 3.46.

In this study we report the vortex-glass behavior of superconducting ternary germanide SrPd₂Ge₂ single crystals with a ThCr₂Si₂-type structure. We observed flux trapping and its nonexponential decay with time after the magnetic field was turned off at T = 2 K. In addition, we found that the diamagnetism in the zero field cooling (ZFC) mode below T_c was irreversible, depending on the temperature and field history, whereas the diamagnetism in the field-cooled warming (FCW) mode was reversible if the applied magnetic field was parallel to the c-axis. An irreversibility line T_r(H) was determined by the ZFC and FCW measurements at various magnetic fields, and the temperature dependence of T_r(H) was found to agree with the de Almeida–Thouless relation, H = H₀[1−T_r(H)/T_c(0)]^γ, where γ = 3/2. Including these vortex-glass behaviors, we discuss the critical current density, J_c(T), determined from isothermal magnetization measurements at various temperatures, and the pinning potential, determined from the slope of an Arrhenius plot, lnR(T,B) versus 1/T.

The pseudogap phase in high T_c superconductors has attracted intense research efforts in recent years. This phase is expected to be intimately linked with the state of an excess O hole doped in the CuO₂ plane. In the present paper, we investigate the nature of this single-hole state theoretically. Special attention is paid to the influences from the O sublattice. We argue that in the case of strong covalent bonding, the motions of O atoms can be very anisotropic: these atoms may find themselves in a single-well potential when moving perpendicular to the Cu–O–Cu bond directions, but in a double-well potential when moving along that direction. As a consequence, a two-plaquette localized polaron state can be realized for a doped hole in the CuO₂ plane. The energy of this state is evaluated via a variational method. It breaks several symmetries locally and these symmetries have been experimentally observed to be broken also in the pseudogap phase.

We investigated the impact of irreversible strain changes and filament cracking on the AC losses of several Nb₃Sn strands and a full-size ITER cable-in-conduit-conductor (CICC). The aim is to evaluate whether the presence of filament cracks in full-size ITER Nb₃Sn CICC (after cyclic loading) can be detected without extracting strands from the cable for microscopic observation. The strand AC loss was measured in a magnetometer in virgin condition and after an applied periodic and cyclic bending strain. The filament fracture pattern was determined afterwards by SEM analysis.

We found a significant decrease of the hysteresis loss in ITER bronze and internal-tin type strands with increasing filament fracture density. However, in the experimental comparison between a highly degraded section of a full-size ITER TF CICC sample subjected to high electromagnetic load and a section taken from the low magnetic field zone, no clear difference is observed in hysteresis loss but only in coupling loss.

The first measurement on a full-size ITER CICC sample indicates that the amount of cracks is at least restricted to an average crack density of 0.05 cracks/filament/mm but a higher accuracy of the CICC AC loss measurement is required for better precision. Further work is required to evaluate whether the observed degradation of the current sharing temperature and n-value is essentially attributed to strand deformation and associated periodic strain variations or filament cracks.

LReBaCuO (LRe=Nd, Sm, Gd) single domains possess a superior superconducting performance and have exhibited potential applications in the fields of superconducting bearings and permanent magnets. Therefore, the requirement of a large size for LReBaCuO single domains with high values of T_C and J_C is desirable. However, it has been found that some unexpected spontaneous nucleation actions always take place, which interrupt the growing process of the single domains. In this paper, the behavior of spontaneous nucleation in SmBaCuO–Ag solution has been investigated systematically. Experimental results reveal that the spontaneous nucleation that frequently takes place during the growing process could be eliminated greatly by reducing the cooling rate to a proper level. Based on the framework of the constitutional super-cooling theory, combined with the results obtained in experiments, we discuss the possible explanations for the spontaneous nucleation behavior in SmBaCuO–Ag solution. An assumption that the distribution of solute concentration C_L, as well as the solid–liquid phase transition temperature T_L, in SmBaCuO–Ag solution might be of a time-dependent nature is put forward. A graph of temperature versus time has been worked out based on the assumption, which matches the growth process behavior satisfactorily with the experimental results.

The transport AC losses (Q_tr) of YBCO pancake coils wound from n = 1, 2, 4 non-transposed parallel connected (n-tpc) tapes have been investigated experimentally and theoretically. It was found that the Q_tr AC losses of the coils with several n-tpc tapes consist of hysteresis AC losses (Q_h) and coupling AC losses (Q_cc). AC losses Q_h of these coils are nearly the same as the AC losses of single-tape coils (n = 1). In contrast, Q_cc are unique for coils with n > 1 n-tpc tapes. We found that the distribution of the coupling currents in the coil turns is defined by the magnetic flux penetrating between n-tpc tapes. This flux is related to the self-field component of the coil parallel to the tape surface, which is antisymmetrically distributed (an odd function) with respect to the winding center. This antisymmetrical distribution leads to a compensation effect of the coupling currents if n-tpc tapes are insulated along the turn length, in spite of low resistance between n-tpc tapes in the soldered coil ends. Therefore, AC losses Q_cc of such coils are negligibly small compared to AC losses Q_h. On the other hand, AC losses Q_cc per current cycle are frequency dependent and have a maximum defined by the time constant τ_eff if tapes are non-insulated along the turn length. In this paper we estimated τ_eff for several special cases. In particular, we found that Q_tr of YBCO pancake coils wound from several n-tpc tapes are comparable to Q_tr of single-tape coils if the operating frequency f is far from the characteristic frequency 1/τ_eff.

We analyzed the ITER TFEU5 cable-in-conduit conductor (CICC) after the full SULTAN conductor qualification test in order to explore whether Lorentz force induced strand movement inside the CICC produces any fracture of the brittle Nb₃Sn filaments. Metallographic image analysis was used to quantify the change in void fraction of each sub-cable (petal); strands move in the direction of the Lorentz force, increasing the void space on the low force side of the CICC and producing a densification on the high force side. Adjacent strand counting shows that local increases in void space result in lower local strand–strand support. Extensive metallographic sampling unambiguously confirms that Nb₃Sn filament fracture occurred in the TFEU5 CICC, but the filament fracture was highly localized to strand sections with high local curvature (likely produced during cabling, where strands are pivoted around each other). More than 95% of the straighter strand sections were free of filament cracks, while less than 60% of the bent strand sections were crack free. The high concentration of filament fractures on the tensile side of the strand–strand pivot points indicates that these pivot points are responsible for the vast majority of filament fracture. Much lower crack densities were observed in CICC sections extracted from a lower, gradient-field region of the SULTAN-tested cable. We conclude that localized filament fracture is induced by high Lorentz forces during SULTAN testing of this prototype toroidal field CICC and that the strand sections with the most damage are located at the petal corners of the high field zone.

Here we study the effects of varying split-melt processing (SMP) heat treatment temperatures on multifilamentary Bi₂Sr₂CaCu₂O_x (Bi2212) wires to understand the factors that influence the electrical transport properties. Sharp threshold behaviors result in an order-of-magnitude difference in critical current density and distinct difference in magnetization behavior. Quantitative image analysis of SEM micrographs shows a strong correlation between the content of 2–4 μm interfilamentary bridges and electrical performance. Avrami analysis of re-solidification indicates that Bi2212 grain nucleation is heterogeneous and site-saturation limited, with a transition in the reaction mechanism corresponding to the depletion of liquid. During the first SMP heat treatment, oxygen is fully recovered, accelerating Bi2201 and/or Bi2212 phase nucleation. Large, flat Bi2201 grains form from the Bi2201 liquid at the start of the second heat treatment and subsequently act as Bi2212 grain nucleation sites. During cooling from high temperature, Bi2212 grains form via a solid/liquid reaction that is enhanced by the liquid, that facilitates second phase dissolution and cation diffusion. At lower processing temperatures, and when the liquid is depleted, Bi2212 grain growth is from a slow solid-state reaction.

We identify a sharp crossover in the vortex pinning of a high-temperature superconductor with nanocolumnar stacks of precipitates as strong vortex pinning centers. Above a particular, temperature-dependent field B_X(T) the vortex response is no longer determined by the nanocolumns, and is instead determined by point-like pinning. This crossover is evident as a change in the dependence of the critical current density on the angle between the applied magnetic field and the nanocolumns. It also leads to the field-orientation-independent power law index n of the E–J curves. Below the transition, there is a strong maximum in J_C when the field is aligned parallel to the columns and n depends on field direction. Above the transition, n is independent of the field direction and there is a J_C minimum for H parallel to the columns. We discuss a possible mechanism for such behavior change, as well as testing and confirming a prediction that the crossover must become very broad at high temperatures and low fields.

C-doped MgB₂ tapes were fabricated using crystalline boron as one of the ball milled precursor powders. It was found that the ball milling process can strongly enhance the J_c values of MgB₂ tapes and reduce the anisotropy of the J_c. At 4.2 K, the transport J_c of the 80 h milled C-doped MgB₂ tapes sintered at 800 °C reaches 4.3 × 10⁴ A cm⁻² at 10 T and 2.2 × 10⁴ A cm⁻² at 12 T, respectively. These values are even higher than for tapes made from amorphous B powders. Ball milling can improve not only the upper critical field, by increasing the C substitution level in the MgB₂ lattice, but also the flux pinning, by increasing grain boundaries, both of which result in the enhancement of J_c performance. Moreover, the high MgB₂ phase fraction and large density of milled MgB₂ tapes are beneficial to the active area. These results indicate that the use of crystalline B in combination with a ball milling process may be an economic method to harness the excellent properties of MgB₂ for practical applications.

We demonstrate a new type of terahertz detector with superconducting tunnel junctions. The detector has two long junctions integrated on both wings of a log-periodic antenna. In this type of detector, the long junction is a lossy transmission line working based on the Cooper-pair breaking, as well as a standing-wave resonance line working based on the photon-assisted tunneling. A prototype detector using Nb/Al/AlO_x/Al/Nb junctions was fabricated, and the principle of the detector was verified. The prototype shows a gradual increase in sensitivity starting from 0.35 THz. We have also identified a resonance signal at 0.25 THz below the effective gap frequency.

Neutron scattering experiments are fundamental to the study of magnetic order and related phenomena in a range of superconducting and magnetic materials. Traditional methods of crystal growth, however, do not yield single crystals of sufficient size for practical neutron scattering measurements. In this paper, we demonstrate the growth of relatively pure, large Y Ba₂Cu₃O₇ single crystals up to 30 mm in diameter using a top seeded melt growth process. The characterization of the microstructural and magnetic properties of these crystals indicates that they contain <2% of impurity phases and, hence, exhibit only weak flux pinning behaviour.

In order to improve the signal-to-noise ratio (SNR) of low-field (LF) magnetic resonance imaging (MRI) measurements with a tuned high-T_c rf superconducting quantum interference device (SQUID) as a signal detector, we utilize a permanent magnet (PM) pair for sample pre-polarization. MRI images are acquired by using filtered back projection reconstruction. The projections are obtained by recording free induction decay or spin echo signals with the gradient field applied at different angles. For every projection, the sample is first pre-polarized in the gap of the PM pair and then mechanically transported to the measuring position underneath the tuned SQUID. Because of the strong magnetic polarization field of about 1 T and the highly sensitive detector with a noise floor of about 7 fT Hz^−1/2, two-dimensional LF-MRI images of water phantoms are obtained with sufficient SNR even without averaging. These images demonstrate the feasibility of LF-MRI based on a tuned SQUID detector, permanent magnet pre-polarization and a moving sample.

Though YBCO coils are stable against transient disturbances such as conductor motion, they suffer from thermal runaway at a current below the coil critical current due to continuous local disturbances attributed to partial degradation of the conductor in the coil winding. Continuous heat generation in the degraded layer induces thermal runaway in adjacent layers; thermal runaway does not occur in the degraded layer spontaneously due to the small n index of the degraded YBCO-coated conductor. The thermal runaway current depends on the cooling conditions of the winding. For a paraffin-impregnated YBCO coil under quasi-adiabatic conditions, the thermal runaway current is far below the coil critical current, while it is close to the coil critical current in the case of a dry-wound coil. The permissible temperature rise following a thermal runaway for YBCO conductors in the degraded layer is demonstrated to be 340 K. If the YBCO coils are operated at a temperature below 20 K, the current density, typically 600–800 A mm⁻², is much higher than that at 77 K. Therefore, the time interval between thermal runaway initiation and the melting temperature becomes less than 0.5 s, posing a difficult problem for protection; i.e., thermal runaway due to continuous local disturbances is hazardous to the safe operation of high current density YBCO coils.

Terahertz (THz) emission has been recently detected from intrinsic Josephson junction (IJJ) stacks made of the high critical temperature superconductor Bi₂Sr₂CaCu₂O_8+δ (BSCCO). The most employed structure is a mesa standing on a big pedestal of a single crystal with a thin gold layer as its top electrode. In this work, a large (300 × 50 × 1.2 μm³) IJJ stack with superconducting electrodes was fabricated and studied. The stack consisted of N ≈ 800 IJJs. It was prepared with a double-sided fabrication process, and significant THz emission was detected. The output power is comparable to the emission power detected from mesa structures, obviously not weakened by the superconducting upper electrode. The observation of THz emission from the double-sided structure suggests that off-chip THz emission from IJJs can be obtained not only from mesa structures and, most importantly, that the emission power can be potentially enhanced in integrated multi-stack radiation sources.

A thin layer of a vertically aligned nanocomposite (VAN) with separated phases of ferromagnetic Fe₂O₃ and non-magnetic CeO₂, arranged as alternating nanopillars, is introduced in YBa₂Cu₃O_7−δ (YBCO) thin films as either a cap or a buffer layer using a pulsed laser deposition method. Detailed microstructural characterization including XRD, high resolution XTEM and STEM is conducted and correlated with the superconducting properties to investigate the flux pinning properties introduced by the magnetic VAN layers. The T_c values of both doped samples are above 89 K and the ${J}_{\mathrm{c}}^{\mathrm{sf}}$ measured at 65 K increased to 150% of that of the reference YBCO prepared under the same conditions. As the measurement temperature decreases, the magnetic pinning effect increases and the field dependent J_c(H ∥ c) is further improved to more than 200% of the J_c value of the reference YBCO sample. This suggests that the Fe₂O₃:CeO₂ VAN can provide both ordered magnetic pillars and controlled defect density. Furthermore, the magnetic pillars are very effective pinning centers especially in the high field and low temperature regime.

Fast neutron irradiation is a powerful technique for introducing additional pinning centers into high temperature superconductors. The spherical defects with sizes of a few nanometers are considered to be effective pinning centers, enhancing J_c. Their morphology is well-known and has already been investigated by several authors in great detail. However, only very little is known about the nature and density of smaller and point defects, which are invisible in transmission electron microscopy. Positron annihilation lifetime spectroscopy was applied to investigate the nature and the concentration of small point-like defects. In this work, the influence of small point defects, such as vacancies and vacancy clusters, on the superconducting properties of YBa₂Cu₃O_7−x bulks was studied; these were introduced by irradiation in the TRIGA Mark II reactor in Vienna. J_c and T_c measurements were performed prior to and after each irradiation step. The samples were irradiated up to a fast neutron ( > 0.1 MeV) fluence of 6 × 10²¹ m⁻². The two kinds of defects—the large collision cascades and the small point-like defects—contribute to the decrease of T_c as well as to the J_c enhancement in astonishingly similar ways.