Flux pinning in high-Tc superconductive films

Magnetization measurements have been performed onc-axis oriented Y- and Gd-based superconductive films in a wide range of the temperaturesT (4.2–85 K) and magnetic fieldsH (0–8 T) withH c-axis. The influence of flux creep on both the temperature dependence of critical current densityJ _cm and the scaling behavior of flux pinning forceF _p has been discussed in detail. The experimental results show that Y and Gd films have different pinning mechanism. Flux pinning-force peaks in high fields are observed in Gd film at high temperatures and can be considered as evidence for collective pinning.     

High Vortex Depinning Temperatures in YBCO Films with BZO Nanorods

The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts the depinning of vortices due to the renormalization of the vortex pinning barriers by thermal fluctuations. For YB₂Cu₃O₇ (YBCO) in external magnetic fields H oriented along the columnar pins generated by various techniques theoretical estimates give a depinning temperature T _dp very close to the critical temperature T _c (T _dp～0.95T _c), whereas the results of standard magnetization relaxation experiments are repeatedly interpreted in terms of a much lower T _dp (～0.5T _c). We investigated the temperature T variation of the normalized magnetization relaxation rate S for YBCO thin films containing BaZrO₃ (BZO) nanorods preferentially oriented along the c axis, with H along the nanorods. The nonmonotonous S(T) variation below the matching field observed up to close to T _c does not support a low T _dp. The often considered S(T) maximum occurring at relatively low T (which was connected to a disappointing T _dp) is related to the occurrence of thermomagnetic instabilities. We show that the accommodation of vortices to the columnar pins in the presence of the T dependent macroscopic currents induced in the sample is signaled by a pronounced S(T) deep located at high T, in agreement with a T _dp close to T _c. By increasing the film thickness and using the substrate decoration the BZO nanorods splay out, leading to the inhibition of (detrimental) vortex excitations involving double vortex kink or superkink formation, characteristic for high-quality thin films and single crystals with columnar pins along the c axis.     

Experimental analysis and finite element modelling of nano-scratch test applied on 40-120 nm SiCN thin films deposited on Cu/Si substrate

In this work, the nano-scratch test is used to characterize the interfacial adhesion of amorphous SiCN thin films deposited by plasma enhanced chemical vapour deposition on Cu/Si substrates. The experimental results show that the critical load F_c is directly related to the rupture of the SiCN/Cu interface. A strong linear dependence of F_c to the SiCN thickness independently to the adhesion is also put in evidence. A three-dimensional finite element model of the test is then built. The results show a clear relation between the stresses into the coating and the cracking and buckling of the film observed experimentally. We then discuss how the interfacial tensile stresses can explain the increase of F_c with the film thickness.     

Fluctuations of the Order Parameter's Phase in Layered Superconductors

The influence of the quantum fluctuations of the order parameter's phase on the critical temperature T _c is studied for a Josephson coupled layered superconductor. Two characteristic critical temperatures exist for a system, namely the superconducting critical temperature T ⁽²⁾ _c for a single layer estimated by the mean-field theory and the transition temperature for the outset or the superconducting phase coherence T* _c . The true critical temperature T _c is shown to vary inside the intervals T* _c ≤ T _c ≤ T ⁽²⁾ _c . For a strong quantum phase fluctuation limit, the superconducting layers become decoupled.     

Depinning of Two-Dimensional Vortex System with Square Pinning Array at the Second Matching Field

Depinning of a two-dimensional vortex system at the second matching field B=2B _? in the presence of square pinning array was studied by molecular dynamics simulations. The annealed ground structures and depinning properties are found to be dependent on pinning strength. For strong pinning, we find a two-step depinning transition with a lower depinning force F _c1 and a higher one F _c2. At F _c1 only the interstitial vortices start to move, while at F _c2 the vortices at pinning sites are depinned and all vortices move.     

Ginzburg-Landau Analysis of the Critical Temperature and the Upper Critical Field for Three-Band Superconductors

Using the microscopic formalism of Eilenberger equations, a three-band Ginzburg-Landau theory for the intraband dirty limit and clean interband scattering case is derived. Within the framework of this three-band Ginzburg-Landau theory, expressions for the critical temperature T _c and the temperature dependence of the upper critical field H _c2 are obtained. Based on some special cases of the matrix of interaction constants, we demonstrate the influence of the sign of the interband interaction on the critical temperature and the upper critical field as compared with a two-band superconductor where it plays no role. We study also analytically and numerically the effect of its magnitude.     

Phase Diagram and Upper Critical Field of Homogeneously Disordered Epitaxial 3-Dimensional NbN Films

We report the evolution of superconducting properties with disorder, in 3-dimensional homogeneously disordered epitaxial NbN thin films. The effective disorder in NbN is controlled from moderately clean limit down to Anderson metal?Cinsulator transition by changing the deposition conditions. We propose a phase diagram for NbN in temperature-disorder plane. With increasing disorder, we observe that as k _F l??1 the superconducting transition temperature (T _c ) and normal state conductivity in the limit T??0 (?? ₀) go to zero. The phase diagram shows that in homogeneously disordered 3-D NbN films, the metal?Cinsulator transition and the superconductor?Cinsulator transition occur at a single quantum critical point, k _F l??1.     

Computer simulation of vortex pinning in type II superconductors. II. Random point pins

Pinning of vortices in a type II superconductor by randomly positioned identical point pins is simulated using the two-dimensional method described in a previous paper (Part I). The system is characterized by the vortex and pin numbers (N _v ,N _p ), the vortex and pin interaction ranges (R _v ,R _p ), and the amplitude of the pin potentialA _p . The computation is performed for many cases: dilute or dense, sharp or soft, attractive or repulsive, weak or strong pins, and ideal or amorphous vortex lattice. The total pinning forceF as a function of the mean vortex displacementX increases first linearly (over a distance usually much smaller than the vortex spacing and thanR _p ) and then saturates, fluctuating about its average \(\bar F\) . We interpret \(\bar F\) as the maximum pinning forcej _c B of a large specimen. For weak pins the prediction of Larkin and Ovchinnikov for two-dimensional collective pinning is confirmed: \(\bar F\) =const· \(\bar W\) /R _p c ₆₆, where \(\bar W\) is the mean square pinning force andc ₆₆ is the shear modulus of the vortex lattice. If the initial vortex lattice is chosen highly defective (“amorphous”) the constant is 1.3–3 times larger than for the ideal triangular lattice. This finding may explain the often observed “history effect”. The function \(\bar F\) (A _p ) exhibits a jump, which for dilute, sharp, attractive pins occurs close to the “threshold value” predicted for isolated pins by Labusch. This jump reflects the onset of plastic deformation of the vortex lattice, and in some cases of vortex trapping, but isnot a genuine threshold. For strong pins \(\bar F\) ～(N _p \(\bar W\) )^1/2 approaches the direct summation limit. For both weak and strong pinningj _c B is related to the mean squareactual (not maximum) force of each pin. This mean square in general is not proportional toA _p ² but, due to relaxation of the vortex lattice, may be smaller or larger than its rigid-lattice limit. Therefore, simple power lawsj _c ～n _p A _p ² orj _c ～n _p A _p in general donot hold except for very weak or unphysically strong pinning.     

The J c Limit of Multifilamentary Ag/Bi-2223 Tapes

To improve on present critical current (J _c) performance, multifilamentary Ag/Bi-2223 tapes with a large range of reduction rates were manufactured. The relative core mass density D was calculated, dependent on the measured geometric dimensions of the tapes. Experimental results, D vs. J _c, D vs. maximum pinning force density F _max , and D vs. irreversible magnetic field B _irr, are quantitatively formatted. In particular, the magnetic field dependence of J _c is critically dependent on its core density. If the core density increases by 10%, J _c of the tapes in this experiment is enhanced by as much as 100%. Therefore, in the present state of the technological process for manufacturing Ag/Bi-2223 tape, increasing the core density is clearly a significant strategy in improving the electronic and magnetic properties of the tapes and enhancing the capacity for carrying current at high magnetic fields. The limit of the bulk self-field-J _c can be calculated by the relationships of J _c vs. D. The limit is estimated to be on the order of 200 kA/cm² for multifilamentary Bi-2223 tapes, which was supported by magneto-optical (MO) magnetization measurements results. It is a hard task to approach this limit with the present state of the art in manufacturing Ag/Bi-2223 tape, and it is the time to suggest some new ideals for Bi-2223 tapes to promote large-scale applications.     

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.     

Electrical and Magnetic Behaviour of PrFeAsO_{\mathbf{0.8}}F_{\mathbf{0.2}} Superconductor

The superconducting and ground state samples of PrFeAsO_0.8F_0.2 and PrFeAsO have been synthesised via the easy and versatile single step solid state reaction route. X-ray and Reitveld refine parameters of the synthesised samples are in good agreement to the earlier reported value of the structure. The ground state of the pristine compound (PrFeAsO) exhibited a metallic-like step in resistivity below 150 K followed by another step at 12 K. The former is associated with the spin density wave (SDW)-like ordering of Fe spins and later to the anomalous magnetic ordering for Pr moments. Both the resistivity anomalies are absent in case of the superconducting PrFeAsO_0.8F_0.2 sample. Detailed high field (up to 12 Tesla) electrical and magnetization measurements are carried out for the superconducting PrFeAsO_0.8F_0.2 sample. The PrFeAsO_0.8F_0.2 exhibited superconducting onset ( $T_{c}^{\mathrm{onset}}$ ) at around 47 K with T _c (ρ=0) at 38 K. Though the $T_{c}^{\mathrm{onset}}$ remains nearly invariant, the T _c (ρ=0) is decreased with applied field, and the same is around 23 K under an applied field of 12 Tesla. The upper critical field (H _c2) is estimated from the Ginzburg–Landau equation (GL) fitting, which is found to be ～182 Tesla. Critical current density (J _c ), being calculated from high field isothermal magnetization (MH) loops with the help of Beans critical state model, is found to be of the order of 10³ A/cm². Summarily, the superconductivity characterization of the single step synthesised PrFeAsO_0.8F_0.2 superconductor is presented.     

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.     

Superconducting wires of pbmo6s8

Using a gas diffusion technique, we have produced PbMo₆S₈ in the form of wires. The critical temperature of these wires is strongly dependent on the reaction conditions. Optimal conditions give a T_c of 14.4 K (ΔT_c ≈ 0.25 K). The lower limit for the critical current density at 4.2 K is 4.10⁴ A cm^?2 in a field of 4.2 T.     

Relationship Between Annealing Time and Magnetic Properties in Bi-2212 Textured Composites

In this study, we report the physical and magnetic properties of Bi₂Sr₂CaCu₂O _x textured materials prepared by a LFZ melting technique and annealed for different times (60, 72, 96 and 120 h). SEM images of the annealed samples for 96 and 120 hours indicate very good alignment with the longitudinal rod axis. In all cases, X-ray diffraction patterns show that the Bi-2212 phase is the major one. The magnetization measurements have been carried out as a function of the magnetic field up to 9 kOe. J _c values of the samples were calculated by using the Bean model. The results indicate that the different annealing time has no significant effects on the T _c values but, significant change on the critical current values of samples, J _c, has been obtained for sample annealed at 96 hours. We also found that the maximum critical density of J _c is 5.5×10⁵ A/cm² at 10 K for the 96 hours annealed sample.     

Shedding of Vortex Rings from an Oscillating Sphere in Superfluid 4He Below 0.5 K

The onset of turbulent flow around an oscillating sphere is known to occur at a critical velocity $v_{c} \sim\sqrt{\kappa\,\omega}$ where κ is the circulation quantum and ω is the oscillation frequency. However, in a small interval of driving force amplitudes F (or corresponding velocity amplitudes of few percent above v _c ) the turbulent flow is found to be unstable. The flow pattern switches intermittently between potential flow and turbulence. The lifetimes of the turbulent phases have an exponential distribution and the mean lifetimes τ grow very rapidly with increasing driving force, namely as τ(F)～exp[(F/F ₁)²]. In this work this experimental result is analyzed in more detail than before, in particular the force F ₁ is identified. As a result, the turbulent drag force can be ascribed quantitatively to the shedding of vortex rings having the size of the sphere.     

Excess Conductivity Analysis and the Critical Region in Be-Doped Cu0.5Tl0.5Ba2Ca1?y Be y Cu0.5Zn1.5O8?δ Superconductors

The excess conductivity analysis of as-prepared and oxygen-annealed Be-doped Cu_0.5Tl_0.5Ba₂Ca_1?y Be _y (Cu_0.5Zn_1.5)O_8??? (y=0, 0.15, 0.3, 0.45) superconducting samples is carried out in the critical regimes and beyond. Various superconducting parameters, like the zero resistance critical temperature T _c , mean field critical temperature $T_{c}^{mf}$ , onset temperature of superconductivity T ^?, the crossover temperature T _G of the critical and 3D fluctuations and dimensionality of fluctuations are determined. From the 3D to 2D crossover, and at the termination of 2D fluctuations, the coherence length ?? _c (0) along the c-axis, the coupling constant?J, the phase relaxation time ?? _?? , the Fermi velocity V _F of the carriers and Fermi energy E _F are calculated. Using the Ginzburg?CLandau (G?CL) equations in the critical regime, the thermodynamic critical magnetic field B _c(0), the lower critical field B _c1(0), the upper critical field B _c2(0) and the critical current density J _c(0) are also calculated from these analyses. Four fluctuation regions above T _c are observed for all of the samples, namely the critical (cr), three-dimensional (3D), two-dimensional (2D) and zero-dimensional (0D) fluctuations regions. It is observed that with the increased Be content at Ca sites, ?? _c (0), V _F and the coupling constant J are increased, which shows that the inter-planar coupling is improved by the incorporation of Be atoms at the Ca sites.     

An important parameter in high-temperature superconductivity

It is widely held that the single most important parameter describing the strength of the electron-phonon interaction in superconductivity is the mass enhancement factor λ, and that the maximum possible transition temperature for a given class of materials (e.g., the polyvalent metals) is attained for a very large value, λ≈3, of the parameter. We show that it is not likely that the highest attainable values ofT _c will reflect such large values of λ. Instead, we expect that they will be associated with values of λ that do not differ greatly from a value of about 1.5, and with very large values of the parameterA, whereA is the area under the electron-phonon interaction spectrum α² F(v). This is certainly the case forNb ₃ Sn, which has aT _c of 18 K.     

Kondo effect in superconducting diluteMoCo,MoFe, andMoMn alloys

The superconducting critical fieldsH _c and the critical temperaturesT _c have been measured down to 0.05 K in a series of dilute alloys ofMo Co,Mo Fe, andMo Mn. These systems haveT _K /T _c0 of 50, 0.3, and 15, respectively, whereT _K is the Kondo temperature of the alloy andT _c0 is the critical temperature of pure Mo. The critical fields atT=0 and the specific heat jump atT _c have been estimated from the observedH _c and compared with the theories of the magnetic impurity effect in superconductors. The behavior of these quantities and the density of states are found to depend strongly uponT _K /T _c0 in agreement with theory. From the results it is inferred that in the limit of largeT _K the superconducting properties of a Kondo alloy would tend to those of the Abrikosov-Gor'kov theory.     

Synthesis and Superconductivity of New BiS2 Based Superconductor PrO0.5F0.5BiS2

We report synthesis and superconductivity at 3.7 K in PrO_0.5F_0.5BiS₂. The newly discovered material belongs to the layered sulfide based REO_0.5F_0.5BiS₂ compounds having a ZrCuSiAs-type structure. The bulk polycrystalline compound is synthesized by the vacuum encapsulation technique at 780 ^°C in a single step. Detailed structural analysis has shown that the as synthesized PrO_0.5F_0.5BiS₂ is crystallized in a tetragonal P4/nmm space group with lattice parameters a=4.015(5) Å, c=13.362(4) Å. Bulk superconductivity is observed in PrO_0.5F_0.5BiS₂ below 4 K from magnetic and transport measurements. Electrical transport measurements showed superconducting transition temperature (T _c ) onset at 3.7 K and T _c (ρ=0) at 3.1 K. The hump at T _c related to the superconducting transition is not observed in the heat capacity measurement and rather a Schottky-type anomaly is observed at below ～6 K. The compound is slightly semiconducting in a normal state. Isothermal magnetization (MH) exhibited typical type II behavior with a lower critical field (H _c1) of around 8 Oe.     

Magneto-transport and Magnetic Susceptibility of SmFeAsO1−x F x (x=0.0 and 0.20)

We report superconductivity in the SmFeAsO_1?x F _x for the x=0.2 system being synthesized using the single step solid state reaction route. Rietveld analysis of room temperature X-ray diffraction (XRD) data shows the studied samples, SmFeAsO_1?x F _x with x=0.0 and x=0.2, are crystallized in a single phase tetragonal structure with space group P4/nmm. The resistivity measurement shows superconductivity for the x=0.20 sample with T _c (onset) ～51.7 K. The upper critical field, [H _c2(0)] is estimated ～3770 kOe by Ginzburg–Landau (GL) theory. Broadening of superconducting transition in magnetotransport is studied through thermally activated flux flow in an applied field up to 130 kOe. The flux flow activation energy (U/k _B ) is estimated ～1215 K for 1 kOe field. Magnetic measurements exhibited bulk superconductivity with lower critical field (H _c1) of ～1.2 kOe at 2 K. In the normal state, the paramagnetic nature of compound confirms no trace of magnetic impurity, which orders ferromagnetically. AC susceptibility measurements have been carried out for SmFeAsO_0.80F_0.20 sample at various amplitude and frequencies of applied AC drive field. The intergranular critical current density (J _c ) is estimated. Specific heat [C _p (T)] measurement showed an anomaly at around 140 K due to the SDW ordering of Fe, followed by another peak at 5 K corresponding to the antiferromagnetic (AFM) ordering of Sm⁺³ ions in the SmFeAsO compound. Interestingly, the change in entropy (marked by the C _p transition height) at 5 K for Sm⁺³ AFM ordering is heavily reduced in the case of the superconducting SmFeAsO_0.80F_0.20 sample.