K. B. Traito

Temperature dependence of the Meissner penetration length in submicron vortex-free and vortex-trapped YBaCuO particles: test for applicability of the modified London equation

To test the applicability of the modified London equation (MLE) the method based on comparison of the temperature dependencies of the penetration length λab in the Meissner and the vortex states is proposed. For experimental testing we investigate the magnetic properties of submicron superconducting YBaCuO particles prepared by a citrate gel modification of the sol-gel technique. It is found that the dependence λab2(0)λab2(T) obeys a power law with the exponent n = 2.0 ± 0.2 and n = 2.5 ± 0.05 in the Meissner and the vortex states, respectively. This difference indicates that MLE is not valid in the vortex state at least in the scale of the used particles i.e. of some hundred A. Possible reasons for this are discussed.

Temperature dependence of the magnetic field penetration depth versus oxygen ordering in submicron YBCO particles

Abstract The influence of oxygen ordering on the temperature dependence of the magnetic field penetration depth λ is investigated in submicron YBa 2 Cu 3 O 6+ x particles obtained by low-temperature ( T s = 800 − 850°C) synthesis from a sol-gel precursor. Additional OO bonds created at low temperature synthesis hinder the oxygen ordering in Cu(1)O plane despite of its high content ( x > 0.82). Oxygen exchange as well as additional annealing stimulate tetra-ortho transition and oxygen ordering. A wide linear range is observed in λ ab −2 ( T ) for samples with highly disordered oxygen. This is attributed to strong phase fluctuations caused by local strains. A gauge-glass model with random distribution of order parameter phases describes the behavior of λ ab −2 ( T ), revealed at initial stage of the hindered tetra-ortho transition. Transformation of the linear λ ab −2 ( T ) fuction to a quadratic one with oxygen ordering is related to decreasing of local deformations and to transformation from a glass state to the usual superconducting orthophase. Two essentially different ways of oxygen ordering, reoxidation and additional annealing, result in the same final state with a similar power law of λ ab −2 ( T ).

Magnetization of submicron superconducting YBCO particles: isolated single vortices versus critical state model

Abstract Strong suppression of the ratio of the thermoremanent (MTR) and the field-cooled (MFC) magnetizations MTR/MFC, exceeding two orders of magnitude, when decreasing the size of superconducting YBCO particles from micron to submicron scale has been observed. A high saturation field (Hs > 8 kOe) of MTR (H) in submicron particles excludes the applicability of critical state models for interpretation of the results. A model of trapping and relaxation of isolated single vortices in particles having size D ⪅ λ (λ is the magnetic field penetration depth) is introduced showing that interaction between a vortex and its mirror image results in a fast relaxation of MTR with size-dependent relaxation time τ ∝ D2. The reduction of the remanence and its temperature and field dependencies in submicron particles are explained by the fast relaxation resulting in vortex-free particles and threshold fields of vortex trapping.

Reentrant reversible remanent magnetization in superconducting submicron YBaCuO particles

Abstract Reversible properties of the remanent magnetic moment, M rem , in superconducting submicron- and micron-size Y–Ba–Cu–O particles and thin films are investigated with a SQUID magnetometer. In micron-size particles and thin films a monotonic increase of M rem was observed when decreasing the temperature. In submicron particles, the same monotonic behaviour is observed at first but at low temperatures an unusual reentrant behaviour was found: M rem decreased when cooling the sample below 6 K. This phenomenon is independent of the direction of the thermal cycle. Possible mechanisms of this effect, such as midgap states in the quasiparticle excitation spectrum and defect-induced localization of Cooper pairs, are discussed.

IMPACT OF THE ORDER PARAMETER SYMMETRIES ON THE VORTEX CORE STRUCTURE IN IRON-BASED SUPERCONDUCTORS

Effects of the order parameter symmetries on the cutoff parameter ξh (determining the magnetic field distribution) in the mixed state are investigated in the framework of quasiclassical Eilenberger theory for isotropic s±, s++ and anisotropic dx2-y2-wave superconducting pairings. These symmetries are proposed for the pairing state of the Fe-pnictides. In s± pairing symmetry, the gap function has opposite sign at the electron and hole pockets of the Fermi surface, it is connected with interband antiferromagnetic spin fluctuations. In s++ pairing symmetry, the gap function has the same sign at the Fermi surface, it is mediated by moderate electron–phonon interaction due to Fe-ion oscillation and the critical orbital fluctuation. The dx2-y2 pairing symmetry can rise from intraband antiferromagnetic spin fluctuation in strongly hole overdoped iron pnictide KFe2As2 and ternary chalcogenides. The s± pairing symmetry results in different effects of intraband (Γ0) and interband (Γπ) impurity scattering on ξh. It is found that ξh/ξc2 value decreases with Γ0 leading to the values much less than those predicted by the analytical Ginzburg–Landau (AGL) theory for high Γ0. At very high Γ0, the interband scattering suppresses ξh/ξc2 considerably below one in the whole field range making it flat for both s± and s++ pairing symmetries. Scaling of the cutoff parameter with the electromagnetic coherence length shows the importance of the nonlocal effects in mixed state. The small values of ξh/ξc2 were observed in μSR measurements of Co-doped BaFe2As2. If Γ0 and Γπ are small and equal than the ξh/ξc2(B/Bc2) dependence for s± symmetry behaves like that of the AGL model and shows a minimum with value much more than that obtained for s++ superconductors. With high Γπ, the ξh/ξc2(B/Bc2) dependence resides above the AGL curve for s± pairing symmetry, as observed in SANS measurements of stoichiometrical LiFeAs compound. In d-wave superconductors, ξh/ξc2 always increases with Γ similar to the s± symmetry case with Γ0 = Γπ.

Superfluid Density in Ultrathin YBCO Films

We studied the transition of 8 nm-thick YBCO films deposited with and without YBaCuNbO buffer from the superconducting to resistive state due to the temperature or transport current increase. For both film types, the current –voltage characteristics and resistive transition followed the Kosterlitz-Thouless transition model. The superfluid densities were found from the Kosterlitz-Thouless transition temperature T KT to be about 3% and 1.3% of the total carrier density in YBCO with and without buffer, respectively. We believe that the low superfluid densities indicate the presence of weak links in the film.

Line shape of the field dependence of the trapped moment in Y–Ba–Cu–O superconducting powders in low magnetic fields: manifestation of vortex polaron

Abstract The structure of trapped magnetic flux is investigated in small superconducting Y–Ba–Cu–O particles, attributing the trapped moment Mpol to an ensemble of vortex polarons which consists of vortices surrounded by diamagnetic matter. To distinguish between the contributions of the vortices and their diamagnetic surrounding to the magnetic moment, measurements of the field dependence of Mpol are made with a SQUID magnetometer around zero magnetic field H. It is found that the value of Mpol grows with increasing field and has a narrow dip at H=0. A similar line shape but with opposite sign is found in the field dependence of the zero field cooled susceptibility confirming the polaron nature of the trapped flux.

Model of single vortices and magnetization of submicron YBCO particles

A model of vortex trapping and relaxation in particles with sizeD<λ (λ is the magnetic field penetration depth) distinguished from critical state model by accounting size-dependent relaxation rate, threshold field and current is considered. Experimentally observed strong suppression of remanent magnetization in submicron YBCO particles is in accordance with this model.