Synthesis Pressure–Temperature Effect on Pinning in MgB2-Based Superconductors

The volume pinning force, F _p(max), increases with increasing synthesis or sintering pressure (0.1 MPa–2 GPa) in materials prepared at high temperature (1050 °C) while it stays practically unchanged in those prepared at low temperature (800 °C). The position of F _p(max) can be shifted to higher magnetic fields by: (1) increasing the manufacturing pressure or decreasing the temperature (2) additions (Ti, SiC, or C, for example), and (3) in-situ preparation. Grain boundary pinning (GBP) dominates in materials prepared at low temperatures (600–800 °C), while high-temperature preparation induces strong point pinning (PP) or mixed pinning (MP) leading to outstanding properties. In materials produced by spark plasma sintering (SPS), the position of F _p(max) is higher than expected for both grain boundary and point pinning. The distribution of boron and oxygen in MgB₂ based material, which can changed by additions or the preparation conditions, significantly affects the type and strength of pining. Materials prepared under a pressure of 2 GPa with a nominal composition of Mg:7B or Mg:12B consist of 88.5 wt % MgB₁₂, 2.5 wt % MgB₂, 9 wt % MgO or 53 wt % MgB₁₂, 31 wt % MgB₂₀ 16 wt % MgO, respectively. Their magnetic shielding fractions at low temperatures are 10 % and 1.5 %, with a transition temperature, T _c of 37.4–37.6 K. Although their magnetic critical current density at zero field and 20 K was 2–5×10² A/cm², they were found to be insulating on the macroscopic level.     

Influence of Connectivity on Vortex-Dynamic Properties of Polycrystalline NdFeAsO0.88F0.12 Superconductors

Polycrystalline NdFeAsO_0.88F_0.12 samples were prepared by both high pressure (HP) and ambient pressure (AP) methods. Magnetic hysteresis loops (MHLs) as well as magnetic relaxation were measured to investigate the vortex dynamic properties of the two samples. Magnetic relaxation rate S combined with effective pinning barrier energies U _eff were calculated as a function of temperature and magnetic field. The results suggest that: (1) The samples with different connectivity display different properties of MHLs, indicating the coexistence of bulk superconductivity and granularity; (2) The different S(T) behaviors between sample AP and sample HP result from the transition from globality to granularity; (3) Field dependent S shows that the pinning mechanism of NdFeAsO_0.88F_0.12 can not be explained by collective pinning theory; (4) The anomalous temperature and magnetic field dependence of effective barrier energies as well as magnetic relaxation rate may be evoked by the competition between Bean-Livingstone (BL) surface pinning and bulk pinning.     

How to optimize high-Tc superconductive cuprates

Different ways to optimizeT _c 's and the magnetic properties of high-T _c superconductive cuprates are described and discussed. Oxygen intercalation-desintercalation phenomena and cationic substitutions (M³⁺ for M²⁺ or vice versa) lead to a variation of the hole carrier density and can have a drastic influence onT _c 's, as shown for bismuth, thallium, and lead-based cuprates, as well as for La₂CuO₄ and “123”-type superconductors. In the former, the role of hole reservoirs for the rock salt type layers is outlined. The modification of the hybridization of orbitals can also explain the variations ofT _c 's observed in some systems like Y_1?x Ca _x Ba₂Cu_3?x :Fe _x O₇ in which the oxidation state of copper remains constant. A critical current density enhancement can be realized by irradiating high-T _c materials by high-energy heavy ions because of the peculiar columnar structure of the heavy ion-induced tracks. The effects of such columnar defects, 70 Å in diameter, on flux pinning, magnetic relaxation, and location of the irreversibility line of bismuth-based 2212 crystals irradiated by 6-GeV Pb ions are reported. We observe a strong shift of the irreversibility line toward high fields and temperatures, indicating that pinning effects must be taken into account in the vortex lattice motion. Such induced changes are accompanied by a strong enhancement of the critical current density and a significant enlargement of the irreversibility region in theH,T plane.     

Combination of Ag Substrate Decoration with Introduction of?BaZrO3 Nano-Inclusions for?Enhancing Critical Current Density of?YBa2Cu3O7 Films

The combination of two methods: Ag substrate decoration and introduction of BZO nano-inclusions has been used in a pulsed laser deposition (PLD) method to increase the critical density (J _c ) of YBCO films. The films were deposited on single crystal SrTiO₃ (STO) substrates decorated with various architecture of Ag nano-dots. We have studied the diameter and density of Ag nano-dots and their influence on J _c of BZO-added YBCO films. We found that 15 laser pulses on the Ag target gives an optimum result in increasing J _c in comparison with BZO-doped YBCO films of the same thickness in self-field and low applied magnetic fields. A higher number of laser pulses on the Ag target led to increasing critical current density in high applied magnetic fields only (above 2 T). We have studied films of the thickness from 0.4 ??m to 3.8 ??m and found that the highest J _c at all applied fields investigated is achieved for a 1.2 ??m thick film. The transmission electron microscopy clearly shows BZO nano-rods that provide strong c-axis pinning centres in the films.     

Improvement in moisture durability of ZnO transparent conductive films with Ga heavy doping process

Moisture durability of ZnO transparent conductive films was achieved with Ga heavy doping by off-axis type rf magnetron sputtering. The resistivity of 10.9 at.% Ga-doped ZnO was 1.3 × 10⁻³ Ωcm and changed less than 5% of resistivity over a 9400-h reliability test at a temperature of 85 °C and humidity of 85%. The crystal structural analysis of the heavily Ga-doped ZnO films indicated that the c-axis was oriented in various directions as well as the perpendicular direction to the substrate surface. The heavily doped Ga disorders the crystal growth of ZnO films and forms a different crystal structure from conventional ZnO.     

Effect of Ce Substitution on the Magnetoresistivity and Flux Pinning Energy of the Bi2Sr2Ca1−x Ce x Cu2O8+δ Superconductors

In this study, the effect of Ce doping on the properties of Bi₂Sr₂Ca_1?x Ce _x Cu₂O_8+δ ceramic superconductors, with x=0.0, 0.01, 0.05, 0.1, and 0.25, has been investigated. Samples’ precursors were prepared using the conventional solid state method and subsequently textured using the Laser Floating Zone technique. The magnetoresistance measurements were studied under various applied magnetic fields. The activation energies, irreversibility fields (H _irr ), upper critical fields (H _c2) and coherence lengths at 0 K (ξ(0)) were calculated from the resistivity versus temperature (ρ–T) curves, under DC magnetic fields up to 5 T. The thermally activated flux flow model has been applied in order to calculate the flux pinning energies. The results indicated that H _c2(0) varied from 416.19 to 115 T and the flux pinning energies varied from 1.46 to 0.042 eV at 0 T.     

Study of Flux-Creep in Bulk Melt-Textured YBa2Cu3Ox in the Regime of Individual Pinning

Magnetic relaxation, with magnetic fields applied parallel to the c axis, was measured in a high-quality melt-textured YBa₂Cu₃O_x bar. Zero-field-cooled magnetization-versus-time data were obtained within a regime of field and temperatures dominated by single vortex pinning. The activation pinning energy U = U _M was calculated from experimental data and compared to U = U ₀ ln(J _c /J), where U, is a constant, J _c is the critical current density, and J the current density. This expression for U obtains within the individual pinning regime. A logarithmic dependence of U _M on J was found, and the data indicated that a scaling function of U _M , g(T/T _c ) = (1?T/T _c )^1/2, was appropriate for describing the single-vortex pinning regime.     

Li and Ce doping of melt-textured YBCO: Improved jc at medium fields

The top seeded melt textured growth method was applied to a Ce doped YBCO material with a partial substitution of Cu with Li. The amount of Li in the samples varied from 0.25 to 0.05 at% relative to the copper atoms. The change of T_c and j_c compared to a reference sample was measured using SQUID magnetometer. Field induced pinning was indicated by elevated j_c in the j_c(B) curve. The peak effect at 77 K appeared in the moderate field region around 0.5 T. Optimal lithium content was determined to be at 0.05 at%. Final Li content in textured bulk material was verified with atomic absorption.     

Preparation of High-Performance SmBa2Cu3O7−x Superconducting Films by Chemical Solution Deposition Approach

Pure and Co³⁺-doped SmBa₂Cu₃O_7?z (SmBCO) superconducting films were prepared on (00l) LaAlO₃ single crystal substrate by self-developed fluorine-free chemical solution deposition approach. According to the X-ray diffraction and SEM observation, SmBCO films with biaxial texture possess dense, smooth, and microcrack-free surface microstructures. However, critical transition temperature (T _c) of a Co³⁺-doped SmBCO film is lower than that of pure SmBCO film, which may be attributed to the Co³⁺ doping decreasing the concentration of hole carriers for doped film. In addition, the Co³⁺-doped film has higher normalized critical current densities (J _c) in the whole magnetic fields, indicating better magnetic flux pinning properties. These results show that Co³⁺ doping by this chemical method is one of the promising ways to prepare high-performance SmBCO films.     

Multiple Superconducting Gaps and Anisotropic Spin Fluctuations in Hole-Doped and Electron-Doped Iron-Pnictides: NMR Studies

We present extensive ⁷⁵As-NMR data on the electron-doped pnictides PrFeAsO_0.89F_0.11 (T _c=45 K), LaFeAsO_0.92F_0.08 (T _c=23 K), and the hole-doped Ba_0.72K_0.28Fe₂As₂ (T _c=31.5 K) single crystal. We find that the Fe antiferromagnetic spin fluctuations are anisotropic and are weaker compared to underdoped copper-oxides or cobalt-oxide superconductors. The spin lattice relaxation rate 1/T ₁ decreases below T _c with no coherence peak and shows a step-wise variation at low temperatures. The Knight shift decreases below T _c and shows a step-wise T variation as well. These results indicate spin-singlet superconductivity with multiple gaps.     

Effect of annealing temperature on magnetoresistivity, activation energy, irreversibility and upper critical field of the Cu-diffused MgB2 bulk superconductors

This study manifests the change of the pinning mechanism, superconducting and physical properties of Cu-diffused MgB₂ superconductors prepared at different annealing temperature from 650 to 850 °C by means of the magnetoresistivity measurements conducted at several applied magnetic fields in a range of 0–7 T as a function of temperature from 15 to 50 K. The onset (T _c ^onset ) and offset (T _c ^offset ) critical temperatures, activation energies (U₀), irreversibility fields (μ₀H_irr), upper critical fields (μ₀H_c2), residual resistivity ratios (R_R), cross-sectional area fractions (A_F), penetration depths (λ), coherence lengths (ξ) and electronic mean free path ( $ \ell $ ) of the samples are evaluated from the magnetoresistivity curves. Thermally activated flux creep (TAFC) model is used for the determination of the U₀ values; likewise, the μ₀H_irr and μ₀H_c2 values are obtained by resistivity criteria of 10 and 90 % normal-state resistivity, respectively. At absolute zero temperature (T = 0 K), the extrapolation of the μ₀H_irr(T) and μ₀H_c2(T) curves is used to find the μ₀H_irr(0) and μ₀H_c2(0) values of the samples. Moreover, the ξ values are deduced from the μ₀H_c2(0) values when the λ values are calculated from the Ginzburg–Landau parameter ( $ \kappa $ ). It is found that the superconducting and physical properties of the samples improve with the increment of the diffusion-annealing temperature; however, the presence of the magnetic field leading to the decrease of the flux pinning in the samples causes the reduction of these properties. Namely, the T _c ^onset and T _c ^offset values are found to increase from 38.4 to 39.3 K and 36.9 and 38.3 K with the enhancement in the diffusion-annealing temperature at zero filed. Similarly, the U₀ values increase significantly with the increase of the annealing temperature. In fact, the U₀ of 9,162 K belonging to the sample annealed at 850 °C is determined to be the maximum activation energy value. On the other hand, the minimum T _c ^onset of 30.9 K, T _c ^offset of 27.6 K and U₀ of 1,365 K at 7 T applied magnetic field are obtained for the pure sample, indicating that the sample annealed at 850 °C obtains much stronger flux pinning, better crystallinity and connectivity between grains compared to the other samples prepared. Based on these results, the superconducting and physical properties of the MgB₂ superconductors produced in this work are observed to enhance with the increase of the diffusion-annealing temperature as a result of the improvement of pinning abilities, crystallinity and connectivity between grains. The magnetic field and temperature dependence of the activation energy are also discussed.     

Electron-Boson Coupling Constant of Oxygen-Deficient Y0.7Ca0.3Ba2Cu3O7?�� Measured by Ultrafast Spectroscopy

The ultrafast dynamics in an (001) oxygen-deficient Y_0.7Ca_0.3Ba₂Cu₃O_7??? thin film were measured by using the ultrafast spectroscopy. According to the model of time-dependent distributions of electrons in the case of electron-phonon relaxation, the electron-boson coupling constant ?? could be directly obtained from the relaxation time of the transient reflectivity change (??R/R). For the overdoped case with T _c=61.3 K and the underdoped case with T _c=65.5 K, the relaxation time of ??R/R is around 4.32 ps and 3.66 ps, respectively, which correspond to ????0.06. However, the ?? become larger (??0.15) in the optimal doped region with T _c=82.1 K while the relaxation time of ??R/R shrinks to ??1.51 ps. These results indicate that the ?? is strongly hole-doping dependent, which may govern the T _c of cuprate superconductors.     

Study of the impact of HgO addition and low-field magnetic relaxation behaviour in granular high-T c superconductors

A series of high-T _c superconductors have been prepared with the HgO addition/substitution. Significant improvement in theT _conset as well asT _c0 was observed in all the cases. Substitution of Hg at the Sr site and Ba site in the case of (Bi, Pb)-Sr-Ca-Cu-O and Y-Ba-Cu-O systems, respectively over a range 0·01–0·6 at% helps in constructing an entire spectrum: improvement ofT _c0 up to 0·4 at % in the case of Bi-system and up to 0·03 at % in the case of Y123 system and slight drop inT _c thereafter. Such improvement is the result of abundant supply of highly reactive nascent oxygen all through the bulk. HgO decomposes and provides oxygen which helps in maintaining proper oxygen stoichiometry throughout the bulk. No Hg or Hg-based impure phases were observed in the X-ray diffraction spectra. Low-field (10–100 Oe) magnetic relaxation studies reveal faster relaxation of the intergranular critical state in the case of silver added samples as the grain boundary coupling energyE _J becomes quite uniform across the entire bulk which leads to smaller flux pinning energy. The distribution of the pinning energy is evaluated from the observed relaxation pattern and is found to be narrower in the case of silver added samples. It was also observed that the transportJ _c ∼ exp[−ΔT _c/T _c0] and the flux pinning energyU ∼ ΔT _c, where ΔT _c is the transition width and is a measure of the inhomogeneity within the sample. Such relationships may help in devising a strategy for achieving highJ _c, highU yet low ΔT _c. Silver addition turns out to be an effective tool in tailoring the sample properties depending on requirement.     

Improved Transport J c in MgB2 Tapes by Graphene Doping

Graphene is a special form of carbon which can effectively enhance the critical current density J _c of MgB₂. In this work, a systematic investigation on the impact of sintering conditions and doping level was carried out for graphene-doped MgB₂ tapes. It is found that an appropriate addition level, i.e., 8 at% in this work, is very critical to obtain a high J _c in graphene-doped samples. The critical field and pinning force are improved obviously due to the graphene doping. The magnetic J _c of samples sintered at 800 °C with 8 at% graphene doping reached 1.78 × 10⁴ A/cm², at 5 T, 20 K. At the same time, the transport J _c was up to 2.38 × 10⁴ A/cm² at 10 T, 4.2 K. The lattice distortion caused by C substitution and residual C at the grain boundaries were thought to be the major factors affecting the J _c of graphene-doped MgB₂ samples.     

High-Field Optical Spectroscopy of the Spin-Crossover Complex [MnIII(taa)]

In this study, we have observed the optical absorption spectra of the manganese(III) complex [Mn^III(taa)] (H₃taa = tris(1-(2-azolyl)-2-azabuten-4-yl)amine), in high magnetic fields. From the measurements at zero field, we have found a variation of the spectrum between 100 K and 4.2 K due to the thermally-induced spin-crossover transition, which occurs around T _c ≈47 K. Furthermore, from the high-field experiment, a magnetochromism, accompanying with the field-induced spin-crossover, has been observed just below T _c .     

Evolution of the Fishtail-Effect in Pure and Ag-doped MG-YBCO

We report on magnetic measurements carried out in a textured YBa₂Cu₃O_7?δ and YBa₂(Cu_1?x Ag _x )₃O_7?δ (at x≈ 0.02) crystals. The so-called fishtail-effect (FE) or second magnetization peak has been observed in a wide temperature range 0.4 <T/T _c < 0.8 for H ∥ c. The origin of the FE arises from the competition between surface barriers and bulk pinning. This is confirmed in a non-monotonically behavior of the relaxation rate R. The value H _max for Ag-doped crystals is larger than for the pure one due to the presence of additional pinning centers, above all on silver atoms.     

Unconventional Magnetic Transition in Na x CoO2 (x>0.7) Single Crystals

Na _x CoO₂ (x>0.7) single crystals with a typical dimension of 0.4×0.4 mm² have been prepared by a “rapid heat-up” technique. By the Benchtop Small Molecule X-ray Crystallography System, the samples have been confirmed to be a single phase of the hexagonal γ-Na _x CoO₂. Magnetic data from the crystal indicate a magnetic phase transition around T=30 K, and the transition is sensitive to the applied magnetic fields. This is most consistent with the formation of an antiferromagnetic spin-density-wave (SDW) with the easy axis for magnetization nearly along the c-axis. Weak and soft ferromagnetism has also been observed from the crystal at low temperature. This unconventional magnetic property may be ascribed to the effect of the small moments of Co^4?x?δ sites at low temperature.     

Upper limit ofT c for the copper oxides

The value of the critical temperature of the cuprates correlates with the doping level and is affected by the interplay of two competing factors: (1) the increase in carrier concentration, and (2) the pair-breaking effect of magnetic impurities. An analysis of the temperature dependence of the critical field leads to the conclusion that magnetic impurities are present even in a sample with the maximum observed value ofT _c. A new parameter, “intrinsic”T _c (T _cintr), which is its value in the absence of magnetic impurities, is introduced. The maximum value ofT _cintr, which corresponds to the maximum doping level, appears to be similar for different cuprates and to be equal to 160–170 K. This is the upper limit ofT _c in the cuprates.     

CeRu2Al10: Anomalous Magnetic Ordering and Its Field Stability

The intermetallic compound CeRu₂Al₁₀ orders in a new crystal structure type that is cage-like on account of its peculiar atomic arrangement and large interatomic distances—especially concerning the environment of the rare-earth element Ce. In previous work we showed that anomalous thermal, electronic, and magnetic properties of this compound coalesce into a phase transition at T ^*=27 K, which is exceptionally high for a cerium intermetallic compound. T ^* has been characterized through various temperature-dependent properties which suggest a multi-parameter nature of the ordering at T ^*. Here we report on continued investigations into this compound and focus in particular on the response to applied magnetic fields. Whereas transport properties in the ordered region are receptive to magnetic fields, the transition itself turns out to be more robust and is insensitive to static fields up to 14 T.     

Effect of Disorder Induced by Heavy-Ion Irradiation on CeCoIn5 Superconductivity

We present a study of the effect of heavy-ion irradiation on a thinned single crystal of the unconventional heavy fermion superconductor CeCoIn₅. Magnetic susceptibility and low-temperature specific heat results show that the superconducting transition temperature (T _c) changed only 4% with irradiation of 1×10¹² ions/cm², Energy=1 GeV, while the specific heat jump at T _c, ΔC, divided by C _n, where C _n is the normal state specific heat just above T _c, was reduced to 3.6 from ΔC/C _n=4.5 for unirradiated CeCoIn₅. The increase of low-temperature magnetic susceptibility and the saturation in magnetization suggest that the defects induced by heavy-ion irradiation are magnetic in nature, as was seen in the case of neutron irradiation on the heavy fermion superconductors, UBe₁₃ and UPt₃. The non-Fermi liquid behavior of the irradiated sample, based on the temperature dependence of the low temperature magnetic susceptibility, is significantly altered.