Real space vortex imaging methods are powerful tools to understand pinning mechanisms and identify features that increase the critical current. Mostly used probes, as for instance Bitter decoration and magnetic scanning probe techniques, are sensitive to the spatial variations of the magnetic properties around vortices. But these are restricted to relatively low magnetic fields, where the magnetic contrast is large enough.
Scanning tunneling spectroscopy measures the spatial dependence of the superconducting density of states—giving vivid images of vortex cores. Spatial resolution is superior to magnetic contrast techniques and the vortex lattice is imaged from well separated vortices up to the upper critical field. Experiments can be made from very low temperatures up to the critical temperature, and currents can be also applied during imaging.
However, STM is a surface sensitive technique and its use has been traditionally limited to materials with excellent surface properties. In particular, imaging studies in nanostructured superconductors are notoriously lacking, in spite of the need to understand nanoscale vortex interactions for controlling dissipative behavior. Here I will present a recent work where we image up to several thousands of vortices one by one in a nanostructured superconducting thin film at 100 mK [1]. The nanostructuring consists of a one-dimensional modulation created by focused ion beam. We visualize the order-disorder transition of the lattice, obtaining the critical exponents of positional and angular correlations. This fully characterizes the transition, driven by non-thermal (zero temperature) modifications of intervortex interactions induced by the magnetic field. Our results show that the one-dimensional nanostructure favors the ordered hexagonal lattice in a large range of magnetic fields. I will also discuss ongoing work, including direct imaging of local vortex dissipation maps in presence of currents and vortex arrangements in tilted magnetic fields.
[1] I. Guillamón et al., Nature Physics 10, 851–856 (2014)

Work supported by Spanish MINECO, CIG Marie Curie grant, Axa Research Fund and FBBVA.

A new vortex pinning landscape has arisen from the finely tuning of the nanostructure of superconducting CSD-YBCO films with the insertion of randomly oriented nanoparticles [1]. Angular dependent transport measurements and advanced STEM investigations have been performed to deeply understand the superconducting performance of nanocomposites. In particular, we have measured IV curves over an extensive range of temperatures, magnetic fields and orientations that have provided very conclusive information about the dominating pinning and dynamics of vortices in the different H-T regions of the phase diagram.
We have developed a new methodology to quantify the effectiveness of the intergrowths-originated strains in terms of vortex dynamics and we have also deeply studied the activation energies of different pinning contributions. A characteristic vortex pinning energy associated to diverse pinning centers present in the sample (point defects, nanostrain, twin boundaries, stacking faults and intrinsic pinning) has been determined, regardless the particular nanocomposite considered. The higher vortex pinning energies have been found for anisotropic pinning centers, minimum energies for the isotropic weak pinning contributions and intermediate energies for isotropic-strong defects. The total contribution of each defect to the vortex pinning has been found to be strongly dependent on the sample nanostructure and on the H-T region evaluated.
We have established a correlation between processing, microstructure and physical properties by analysing samples with different stacking fault length and density. Our results show that besides the enhancement of pinning performance at intermediate temperatures due to isotropic strong pinning contribution, effective isotropic-weak pinning defects are needed to obtain the best performances at low temperatures. Anisotropic defects are dominating the pinning phase diagram at high temperatures.
[1] Llordés et al, Nature Materials 11, 329-336 (2012)

We acknowledge the financial support from the EU-FP7 NMP-LA-2012-280432 EUROTAPES and COST action MP1201, and MAT2011-28874-C02, MAT2014-51778-C2-1R

Jc of REBa2Cu3O7 (REBCO: RE=Y, Gd, Sm, etc.) coated conductors is increased by BZO, BSO, and BHO nanorods. However, it decreases significantly when the magnetic field is inclined from the c-axis of the films, leading to the appearance of Jc peak at B || c. On the contrary, Jc of random point pins such as Y2O3, BZO, and Y211 nanoparticles, whose diameters are several times of coherence length, is hardly changed; the Jc angular dependence is isotropic without any peak at B || c. Thinking about applications of REBCO coated conductors, desired features of Jc are isotropic and high enough under magnetic fields. In this framework, nanoparticle pinning is interesting because the in-field Jc at temperatures below 65 K is high and its Tc is not so suppressed by nanoparticle insertion. However, the detailed distribution control of nanoparticles is very difficult, so that the Bose glass state is destroyed, resulting in the decrease of Birr(T) and Jc -B characteristics. These results suggest that the intermediate anisotropy range of flux pinning between both limits should be examined. Hybrid APCs, combined with differently shaped pinning centers such as nanorods and nanoparticles, holds a key for obtaining higher Birr(T) and Jc -B characteristics in REBCO coated conductors. We reports the present efforts to control Jc anisotropy and Jc -B characteristics at 65-77 K range as well as Birr(T) by using several kinds of nanorods and nanoparticles. It was very difficult so far to theoretically predict broad Jc peak at B || c in Jc(θ) angular dependence. We propose a new theoretical model based on the energy gain by flux pinning to derive angular dependent Jc(θ) of REBCO films with hybrid APCs. Obtained results are in good agreement with experimental ones.

This work was supported by KAKENHI, Grant-in-Aid for Science Research (S), Grant Number 23226014.

The critical current densities Jcs at magnetic field increased by introducing BaMO3 (BMO) pinning centers (M=Zr, Sn, Hf) into SmBCO films using the various PLD techniquie. For example, BaHfO3 (BHO)-doped SmBa2Cu3Ox films by the conventinal PLD process in improving its straight of BHO nanorod on the LaAlO3 substrate has high Fp=27 GN/m3 and Birr =15.0T (77K). The low-angle annular dark-field STEM observes slightly bright contrast regions around BHO nanorods on a BHO-doped SmBa2Cu3Ox. The dislocation positions estimated by the Geometric Phase Analysis (GPA) of STEM images are corresponding to the origin on the radial bright contrast. We considered that the slightly bright contrast region spread in radial is the strain field.
Using the low temperature growth technique with the seed layer(LTG), we can design and control the higher density and fine inclined　BHO nanorods of SmBCO films, as changing lower substrate temperature. The optimum level of BHO doping was found to be 5.6 vol%, which achieved Maximum Fp = 14.2 GN/m2 at B=6T, 77K, that is less than the high Fp PLD sample. On the other hand, at low measurement temperatures, the LTG film shows high flux pinning force ;FpMAX is 407 GN/m3 at 40 K, 10 T and 779 GN/m3 @ 20 K, 9 T, respectively. Furthermore BHO-doped GdBa2Cu3Ox layers on IBAD-MgO tapes shows the high irreversibility filed (Birr) over 15T at 77.3K.

This research was partially supported by “Japan Society for the Promotion of Science (JSPS): Grant-in-Aid for Scientific Research (23226014)”, “Japan Science and Technology Agency (JST): Advanced Low Carbon Technology Research and Development Program (ALCA)”

Nanorods are introduced into REBCO films to improve J_c at high temperature and under high magnetic field. In the last decade, the growth technique of the nanorods has been enthusiastically studied from the chemical point of view. Finally, highly dense BaHfO₃ (BHO) nanorods are successfully introduced into SmBCO films with a slight suppression of T_c by using the low temperature growth (LTG) technique. In this work, we discuss vortex pinning as an approach on the physical point of view by considering the Bose glass theory established in 1990’s.
We fabricated SmBCO films with BHO nanorods using the alternating-targets technique and the low temperature growth technique with pulsed laser deposition, and measured J_c and the resistivity using a transport method. Critical exponents calculated from the temperature dependences of the resistivity are ~ 4 at magnetic fields less than the matching fields, indicating the Bose glass phase is possibly realized. According to J_c properties, the flux pinning strength drastically decreases in a high temperature region, where a delocalization of vortices occurs. As a result, the coexisting behavior of the correlated pinning and random pinning can be seen as the double peak in the flux pinning force density F_p curves above the delocalization temperature T_dl. Since the T_dl value strongly depends on the distance between nanorods, we can optimize the nanorod introduction for the application temperature.

Nanoinclusions (nanoparticles, NPs, [1] or nanorods [2]), can induce strong vortex pinning in YBa₂Cu₃O_7-x (YBCO). The effects are observed in dc measurements of J_c [3] and also in vortex dynamic measurements in a wide range of frequencies, from intermediate 1-10⁴ Hz [4] to -surprisingly- microwaves [5]. The comparison of pinning at different frequency regimes shed light on the state of the vortex matter when nanorods are present [6]. Recently, it has been shown that strong pinning in solution-derived YBCO films with secondary-phase NPs (BYTO, BZO…) is associated to the creation of a network of nanostrained regions emerging from the NP surfaces, acting as isotropic-strong pinning centers [7]. We present here an extensive investigation of the pinning properties in YBCO and YBCO/BYTO nanostructured superconducting thin films, as measured in the high-frequency regime (microwave surface impedance, 48 GHz). The analysis of the field-dependent microwave data (µ₀H ≤ 0.8 T, T ≥ 63 K) using the generalized model of relaxational vortex dynamics [8] yielded the elastic-to-dissipative impedance ratio r—parameter and the Labusch parameter k_p, the elastic vortex restoring force in the pinning well. We find that, even at our very high measuring frequency, the flux lines are very strongly pinned in YBCO/BYTO. YBCO and YBCO/BYTO exhibit different dependences r(H) and k_p(H), demonstrating different vortex regimes. Results are compared and discussed with dc transport measurements of J_c(H,T) on twin samples.
[1] J. Gutiérrez et al., Nature Mater. 6, 367 (2007)
[2] J. L. Macmanus-Driscoll et al., Nature Mater. 3, 439 (2004)
[3] A. Palau et al. Supercond. Sci. Technol. 24, 125010 (2011)
[4] E. Bartolomé et al., Phys. Rev. B. 81, 184530 (2010)
[5] N. Pompeo et al., Appl. Phys. Lett. 91, 182507 (2007)
[6] N. Pompeo et al., Appl. Phys. Lett. 103, 022603 (2013)
[7] A. Llordés et al., Nature Mater. 11, 329 (2012)
[8] N. Pompeo and E. Silva, Phys. Rev. B 78, 094503 (2008)

This work has been partially supported by Eurofusion and by EUROTAPES FP7 NMPLA2012280432

The controlled growth of self-assembled secondary-phase nanostructures, such as vertically aligned nanorods, has provided an essential means for enhancing pinning properties of High Tc superconducting films. The nanorod diameter is a critical parameter and controlling it is important to achieving an optimal pinning through selecting the compatible nanorod materials for a given superconducting matrix. Such a control requires an understanding of the underlying physics of the formation of the nanorods. We have thus studied the dependence of the lattice strain energy due to the formation of the nanorods on the elastic properties of the secondary phase oxide and the lattice mismatch between the film and nanorods. The analytical solution of the strain energy is obtained by solving equilibrium equations of the lattice strain under the condition of coherent interfaces between the film and nanorods. An energy balance model for the formation of the nanorods is used to evaluate the importance of the strain energy and the film/nanorod interface energy to the nanorod diameter. A correlation between the nanorod diameter and the elastic properties of the secondary phase oxides in YBCO/nanorod composite films has been discovered, which can be used to predict the energetically-preferred nanorod diameter for a given secondary phase oxide. Besides a direct comparison with experimentally demonstrated nanorods in YBCO films, such as BaZrO3, BaSnO3, and BaHfO3, a few examples of new oxides have been identified that could yield the nanorods of different diameters. This elastic energy model can be used as a guidance in the quest of the self-assembled nanorods with different diameters in YBCO films.