Shape- and orientation-dependence of surface barriers in single crystalline d-wave Bi_2Sr_2CaCu_2O_8+delta

7 pages, submitted to Phys. Rev. BMagneto-optical imaging and Hall-probe array magnetometry are used to measure the field of first flux entry, H_p, into the same Bi_2Sr_2CaCu_2O_8+delta single crystal cut to different crystal thickness-to-width ratios (d/w), and for two angles alpha between the edges and the principal in-plane crystalline (a,b) axes. At all temperatures, the variation with aspect ratio of H_p is qualitatively well described by calculations for the so-called geometric barrier [E.H. Brandt, Phys. Rev. B 60, 11939 (1999)]. However, the magnitude of H_p is strongly enhanced due to the square shape of the crystal. In the intermediate temperature regime (T < ~ 50 K) in which the Bean-Livingston barrier limits vortex entry, there is some evidence for a tiny crystal-orientation dependent enhancement when the sample edges are at an angle of 45° with respect to the crystalline axes, rather than parallel to them

Imaging flux distributions around superconductors: Geometrical susceptibility in the Meissner state

International audienceExperiment and analytical calculations show that the demagnetizing field of a superconductor is a sensitive probe of quantities otherwise difficult to measure, such as the sample-probe distance in flux-density imaging experiments, and the field of first flux penetration Hp. In particular, the ratio of the maximum field measured above the superconductor edge and the applied field can be determined unambiguously so as to define a linear "geometric" susceptibility. The evolution of this susceptibility with field depends on the regime of flux penetration, and can be used as a means to determine Hp and the effect of a parallel field component in magneto-optical imaging experiments

Microwave absorption in YBa[2]Cu[3]O[7-delta]-manganite superlattices

3 pages Erreur sur l'affiliation : CEA/DSM/IRAMIS et non CEA/DSM/DRECAMInternational audienceWe report on results of direct microwave absorption measurements of Re1−x Bx MnO3 /YBa2 Cu3 O7−δ half metal/d-wave superconductor superlattices (where Re - La and Pr) and (B - Sr and Ca) for microwave frequencies in the range from 9 to 20 GHz. The measurements of the entire heterostructure were performed mostly at temperatures below the superconducting transition. The obtained results strongly depend on the microwave frequency and can be qualitatively described within the theoretical model of high-frequency properties of Josephson junctions with a ferromagnetic barrier proposed by S. Takahashi, S. Hikino, M. Mori, J. Martinek, and S. Maekawa, Phys. Rev. Lett. 99, 057003 (2007)

Multifractal scaling of flux penetration in the Iron-based Superconductor Ba(Fe0.925_{0.925}Co0.075_{0.075})2_{2}As2_2

International audienceThe penetration of magnetic flux fronts in the optimally doped iron based superconductor Ba(Fe$_{0.925}$Co$_{0.075}$)$_{2}$As$_2$ is studied by means of high resolution magneto-optic imaging. The analysis of roughening and growth of the magnetic flux front reveals anomalous scaling properties. While higher-order spatial correlation functions reveal multifractal behavior for the roughening, the usual Kardar-Parisi-Zhang growth exponent is found. Both exponents are found to independent of temperature. We propose a scenario for vortex penetration based on 2D percolation and cluster aggregation in an inhomogeneously disordered superconductor

Microwave heating-induced DC magnetic flux penetration in YBa2_{2}Cu3_{3}O7−δ_{7-\delta} superconducting thin films

The magneto-optical imaging technique is used to visualize the penetration of the magnetic induction in YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ thin films during surface resistance measurements. The in-situ surface resistance measurements were performed at 7 GHz using the dielectric resonator method. When only the microwave magnetic field $H_{rf}$ is applied to the superconductor, no $H_{rf}$-induced vortex penetration is observed, even at high rf power. In contrast, in the presence of a constant magnetic field superimposed on $H_{rf}$ we observe a progression of the flux front as $H_{rf}$ is increased. A local thermometry method based on the measurement of the resonant frequency of the dielectric resonator placed on the YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ thin film shows that the $H_{rf}$--induced flux penetration is due to the increase of the film temperature.Comment: 6 pages, Journal of Applied Physic

Vortex creep and critical current densities in superconducting (Ba,K)Fe2_{2}As2_{2} single crystals

The surprisingly rapid relaxation of the sustainable current density in the critical state of single crystalline Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ is investigated for magnetic fields oriented parallel to the c-axis and to the $ab$--plane respectively. Due to the inadequacy of standard analysis procedures developed for flux creep in the high temperature superconducting cuprates, we develop a simple, straightforward data treatment technique that reveals the creep mechanism and the creep exponent $\mu$. At low magnetic fields, below the second magnetization peak, $\mu$ varies only slightly as function of temperature and magnetic flux density $B$. From the data, we determine the temperature- and field dependence of the effective activation barrier for creep. At low temperatures, the measured current density approaches the zero--temperature critical current density (in the absence of creep) to within a factor 2, thus lending credence to earlier conclusions drawn with respect to the pinning mechanism. The comparable values of the experimental screening current density and the zero-temperature critical current density reveals the limited usefulness of the widely used "interpolation formula".Comment: Physical Review B (2012) Accepte

Fully anisotropic superconducting transition in ion irradiated YBa2Cu3O7−delta with a tilted magnetic field

We consider the superconducting vortex solid-to-liquid transition in heavy ion-irradiated untwinned YBa2Cu3O7−delta single crystals in the case where the magnetic field direction does not coincide with that of the irradiation-induced linear columnar defects. For a certain range of angles, the resistivities measured in three orthogonal spatial directions vanish at the transition as three clearly different powers of reduced temperature. This is in contrast to the usual isotropic scaling of physical quantities close to a continuous phase transition, and to the Bose glass transition occurring for field parallel to the columns, where the scaling is anisotropic in one direction. Thus, our findings yield evidence for a new type of critical behavior with fully anisotropic critical exponents.

Disorder, critical currents, and vortex pinning energies in isovalently substituted BaFe2_{2}(As1−x_{1-x}Px_{x})2_{2}

We present a comprehensive overview of vortex pinning in single crystals of the isovalently substituted iron-based superconductor BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$, a material that qualifies as an archetypical clean superconductor, containing only sparse strong point-like pins [in the sense of C.J. van der Beek {\em et al.}, Phys. Rev. B {\bf 66}, 024523 (2002)]. Widely varying critical current values for nominally similar compositions show that flux pinning is of extrinsic origin. Vortex configurations, imaged using the Bitter decoration method, show less density fluctuations than those previously observed in charge-doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals. Analysis reveals that the pinning force and -energy distributions depend on the P-content $x$. However, they are always much narrower than in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$, a result that is attributed to the weaker temperature dependence of the superfluid density on approaching $T_{c}$ in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$. Critical current density measurements and pinning force distributions independently yield a mean distance between effective pinning centers $\bar{\mathcal L} \sim 90$ nm, increasing with increasing P-content $x$. This evolution can be understood as being the consequence of the P-dependence of the London penetration depth. Further salient features are a wide vortex free "Meissner belt", observed at the edge of overdoped crystals, and characteristic chain-like vortex arrangements, observed at all levels of P-substitution.Comment: 11 page

A vortex solid-to-liquid transition with fully anisotropic scaling

International audienceThe vortex solid-to-liquid transition has been studied in heavy ion irradiated untwinned single crystals of YBa2Cu3O7− with an inclined applied magnetic field. For magnetic fields tilted at angles about 45 away from the columnar defects, we find that the electric resistivity in the vortex liquid regime approaches zero with power laws in the reduced temperature T − Tc that have different exponents in all three spatial directions. Since the symmetry in the problem has been broken in two non-collinear directions by i) the direction of the columnar defects and ii) the direction of the applied magnetic field, our findings give evidence for a new type of critical behavior with fully anisotropic critical exponents. A possible view of the vortex topology for the transition is also suggested

Exploring the flux pinning performance of bulk FeSe by electron irradiation

OralInternational audienceBulk, polycrystalline samples of FeSe are interesting candidates for trapped field applications as the material can be prepared by simple solid-state sintering, is free of toxic elements and exhibits magnetic properties like the copper-based high-Tc superconductors, i.e., high upper critical fields Hc2, and relatively small anisotropy. Polycrystalline, sintered FeSe material shows further promising features such as strong grain coupling. The FeSe material consists of well-coupled, but randomly oriented, platelet-like round grains with a diameter between 1 and 6 µm.However, the critical currents of the FeSe system are still low, but comparable to sintered MgB2. The goal of the present work is to explore the possibilities of the FeSe system in application-type samples (i.e., polycrystalline material, not single crystals) using electron irradiation. Pieces of bulk, superconducting FeSe samples prepared by solid-state sintering were irradiated with 2.5 MeV electrons (T = 23.5 K) at SIRIUS facility using two different fluences, 2×10^19 electrons/cm2 and 4×10^19 electrons/cm2. The electron irradiation introduced point defects to the FeSe grains. The changes of Tc due to irradiation and the critical currents were measured using SQUID magnetometry. As result, the superconducting transition temperature, Tc, is slightly reduced, depending on the fluence, but the critical currents are increased by about 20-30%, which demonstrates that one can introduce additional disorder to FeSe to improve the flux pinning properties also in the polycrystalline material