Flux pinning in columnar-structured and single crystalline MgB2 films

We have investigated the flux pinning effect of columnar grain boundary in columnar-structured and single crystalline MgB₂ films. The MgB₂ films with columnar structure showed much higher J_c than that of single crystalline thin film, and sample having smaller grain size had a higher J_c in high magnetic fields. At 5 K, the MgB₂ film with grain size of 460 nm showed an abnormal double-peak behavior in pining force density, F_p(B), caused by competition of different types of pinning sites, such as planar defects and point defects. Field dependences of F_p in columnar-structured films suggest that the columnar grain boundary is a strong pinning source in the MgB₂ film and it plays a crucial role in enhancing J_c over a wide range of magnetic fields and temperatures.     

Local impurity phase pinning and pinning force in charge density waves

Starting from the static Fukuyama-Lee-Rice equation for a three-dimensional incommensurate charge density wave (CDW) in quasi one-dimensional conductors a solvable model for local phase pinning by impurities is defined and studied. We find that average CDW energy and average pinning force show critical behaviour with respect to the pinning parameter h. Specifically the pinning force exhibits a threshold at h=1 with exponent . Our model exemplifies a general concept of local impurity pinning in which the force exerted by the impurity on the periodic CDW structure becomes multivalued and metastable states appear beyond a threshold. It is found that local impurity pinning becomes less effective at low temperatures and may eventually cease completely. These results are independent of spatial dimensionality as expected for local impurity pinning. Comparison with Larkin's model is also made. Received 8 July 1998     

Effect of W-addition on pinning property of MgB2

The 5d transition metal W was added into the MgB₂ superconductor. The Mg, B and W were sintered at 1173 K for 30 min under H₂/Ar atmosphere in the electric furnace. The W_x(MgB₂)_1?x samples were prepared in the W concentration range of 0 ? x ? 0.05. Temperature and field dependences of magnetization were measured by the SQUID magneto-meter. The field and x dependences of J_c at 20 K were analyzed by the extended critical state model. The enhancement of J_c became maximum for the x = 0.02 sample.     

Anisotropic pinning in Nb thin films with triangular pinning arrays

A superconducting thin film with regular triangular arrays has been fabricated to explore the anisotropic vortex pinning behavior. We found that the critical currents for these films depend strongly on the current directions. Some interesting temperature-dependent phenomena are observed and discussed. We also made molecular dynamic simulations to study the dynamics of the vortex motion. The simulation results confirm the anisotropic features we found in the experiments.     

Statics and dynamics of 2D colloidal crystals in a random pinning potential

We report the first experimental study of a model system of a two-dimensional colloidal crystal in a random pinning potential. The colloidal crystal consists of monodispersed charged polystyrene microspheres suspended in deionized aqueous media and confined near a rough charged surface. It is found that the static orientational correlation function g6(r) decays exponentially for intermediate and strong pinning, in agreement with theories. The driven depinning is dominated by thermally activated creep motion along 1D-like channels between regions with short-range order. A coexistence model is proposed for describing the observations.     

Soliton pinning and the central peak

The soliton contribution to the dynamical structure factor for the one-dimensional λψ⁴ model is studied. While a soliton is pinned with some defect, the central peak width is of order of $\text{α}\text{Γ}$, where α is the impurity potential amplitude, Γ the soft mode damping rate.     

Adsorption-induced Fermi level pinning

The adsorption-induced Fermi level pinning is shown to occur at the coverages Θ* corresponding to a shallow extremum or saturation of the work function of the system. Equations for Θ* are derived within a modified Anderson-Newns adsorption model. Experimental data on the adsorption of atoms of alkali, alkaline-earth (Ba), and rare-earth metals and hydrogen on semiconductors (silicon, gallium arsenide, and titanium dioxide) are analyzed. The position of the Fermi level on the surface of a semiconductor is estimated.     

Correlated vortex pinning in Si-nanoparticle doped MgB2

The magnetoresistivity and critical current density of well characterized Si-nanoparticle doped and undoped Cu-sheathed MgB₂ tapes have been measured at temperatures T≥28 K in magnetic fields B≤0.9 T. The irreversibility line B_irr(T) for doped tape shows a stepwise variation with a kink around 0.3 T. Such B_irr(T) variation is typical for high-temperature superconductors with columnar defects (a kink occurs near the matching field B_?) and is very different from a smooth B_irr(T) variation in undoped MgB₂ samples. The microstructure studies of nanoparticle doped MgB₂ samples show uniformly dispersed nanoprecipitates, which probably act as a correlated disorder. The observed difference between the field variations of the critical current density and pinning force density of the doped and undoped tape supports the above findings.     

Effects of crossed columnar defects on vortex pinning in Bi2Sr2CaCu2O8

To investigate the existence of a splay effect in Bi₂Sr₂CaCu₂O₈ (Bi-2212), vortex pinning has been studied in different configurations of strongly inclined columnar defects (75 from the c axis), installed by heavy-ion irradiation. It is shown that the symmetry of the track setting with respect to the field direction is a more influent parameter than the presence of a dispersion in the track directions. We claim that the enhanced pinning efficiency which is observed in some splayed configurations of columnar defects in Bi-2212 can be interpreted without invoking a splay effect. Received 27 December 1999     

Flux pinning and superconducting properties of melt-textured NEG-123 superconductor with TiO2 addition

We studied the effect of TiO₂ doping on flux pinning and superconducting properties of a melt-grown (Nd_0.33Eu_0.33Gd_0.33) Ba₂Cu₃O_y + 35 mol% Gd₂BaCuO₅ (70 nm in size) composite (NEG-123) processed in Ar–1% O₂ atmosphere. As indicated by similar, sharp superconducting transitions, the small quantities of TiO₂ used in our experiments did not deteriorate superconducting properties of the NEG material. Transmission electron microscopy (TEM) analysis found 20–50 nm Ti-based particles in the NEG-123 matrix. However, we have not observed the clouds of <10 nm sized particles in the NEG-123 matrix, as in the case of recently reported NEG-123 composites doped by Mo and Nb nanoparticles. Nevertheless, quite a good J_c–B performance in the 0.1 mol% Ti-doped sample, namely 550 kA/cm² at the self-field and at the secondary peak field (4.5 T) was achieved at 65 K, while 320 kA/cm² was obtained at zero-field at 77 K, and 50 kA/cm² at 90.2 K. The pinning effectiveness decreased with increasing Ti content above 0.2 mol%. The analysis of the pinning force showed that higher concentration of Ti (>0.2 mol%) increased the amount of normal pins (δl pinning), indicated by the Fp(h) peak shift from h = 0.42–0.36. The maximum pinning effect in a broad field range could be achieved by optimizing Ti content and adding sub-micron Gd-211 particles.