Epitaxial NdBa2Cu3O7-x films with a hybrid nanoscale defect structure comprised of BaZrO3 nanodot arrays aligned along the c-axis in one half of the film thickness and aligned perpendicular to the c-axis in the other half thickness of the film were fabricated. Transmission electron microscopy images confirm the orientation of the nanoscale defect structures. The angular dependence of critical current density, Jc, at 77 K, 1 T, shows significantly reduced angular variation of Jc. This study nicely demonstrates how pinning characteristics can be tuned by tuning the nanoscale defect structures within the films.
We report herein a high and isotropic critical-current density Jc for BaFe2(As,P)2 epitaxial films. The isotropy of Jc with respect to the magnetic-field direction was improved significantly by decreasing the film growth rate to 2.2 Å/s. The low growth rate served to preferentially align dislocations along the c-axis, which work well as c-axis vortex-pinning centers. Because of the intentional introduction of effective pinning, the absolute Jc at 9 T was larger than that obtained for other iron-based superconductors and conventional alloy superconducting wires.
We report on the isotropic pinning obtained in epitaxial Fe(Se,Te) thin films grown on CaF2 (001) substrate. High critical current density values larger than 1 MA/cm2 in self field in liquid helium are reached together with a very weak dependence on the magnetic field and a complete isotropy. Analysis through Transmission Electron Microscopy evidences the presence of defects looking like lattice disorder at a very small scale, between 5 and 20 nm, which are thought to be responsible for such isotropic behavior in contrast to what observed on SrTiO3, where defects parallel to the c-axis enhance…
The flux flow properties of epitaxial niobium films with different pinning strengths are investigated by dc electrical resistance measurements and mapped to results derived within the framework of a theoretical model. Investigated are the cases of weak random pinning in as-grown films, strong random pinning in Ga ion-irradiated films, and strong periodic pinning induced by a nanogroove array milled by focused ion beam. The generic feature of the current-voltage curves of the films consists in instability jumps to the normal state at some instability current density $j^\ast$ as the vortex latti…
We report on the transport properties of clean, epitaxial Fe(Se,Te) thin films prepared on Fe-buffered MgO (001) single crystalline substrates by pulsed laser deposition. Near Tc a steep slope of the upper critical field for H||ab was observed (74.1 T/K), leading to a very short out-of-plane coherence length, ξc, of 0.2 nm, yielding 2ξc(0) approximately 0.4 nm. This value is shorter than the interlayer distance (0.605 nm) between Fe-Se(Te) planes, indicative of modulation of the superconducting order parameter along the c-axis. An inverse correlation between the power law exponent N of the ele…
Superconductor/Ferromagnet (S/F) hybrid systems show interesting magneto-transport behaviors that result from the transfer of properties between both constituents. For instance, magnetic memory can be transferred from the F into the S through the pinning of superconducting vortices by the ferromagnetic textures. The ability to tailor this type of induced behavior is important to broaden its range of applications. Here we show that engineering the F magnetization reversal allows tuning the strength of the vortex pinning (and memory) effects, as well as the field range in which they appear. This…
We report a unique experimental approach to create topological superconductors by inducing superconductivity into epitaxial metallic thin film with strong spin-orbit coupling. Utilizing molecular beam epitaxy technique under ultra-high vacuum condition, we are able to achieve (111) oriented single phase of gold (Au) thin film grown on a well-oriented vanadium (V) s-wave superconductor film with clean interface. We obtained atomically smooth Au thin films with thicknesses even down to below a nanometer showing near-ideal surface quality. The as-grown V/Au bilayer heterostructure exhibits superc…
Symmetry-induced vortex-antivortex configurations in superconducting squares and triangles were predicted earlier; yet, they have not been resolved in experiment up to date. Namely, with vortex-antivortex states being highly unstable with respect to defects and temperature fluctuations, it is very unlikely that samples can be fabricated with the needed quality. Here we show how these drawbacks can be overcome by strategically placed nanoholes in the sample. As a result, (i) the actual shape of the sample becomes far less important, (ii) the stability of the vortex-antivortex configurations in …
The mixed-state Hall effect was examined in a Ba(Fe1-xCox)2As2 epitaxial film with a high critical current density. The transverse resistivity ρxy and the longitudinal resistivity ρxx follow power law scaling ρxy = Aρxxβ. In the temperature-sweep with a fixed field (T sweep), all of the β values are independent of magnetic field up to 9 T, and are lower than 2.0 (around 1.8). In contrast, the β values in the magnetic-field sweep with a fixed temperature (H sweep) change from 1.8 to 2.0 as the temperature increases from 13 to 16 K even in the T/H region that overlaps with the T sweep measuremen…
We report robust vortex matching effects in antidot arrays fabricated on thin films of NbN. The near absence of hysteresis between field sweep directions indicates a negligible residual pinning in the host thin film. Owing to the very small coherence length of NbN thin films ($ξ< 5 nm$), the observations suggests the possibility of probing physics of vortices at true nanometer length scales in suitably fabricated structures.
The angular-dependent critical current density, Jc(theta), and the upper critical field, Hc2(theta), of epitaxial Ba(Fe1-xCox)2As2 thin films have been investigated. No Jc(theta) peaks for H || c were observed regardless of temperatures and magnetic fields. In contrast, Jc(theta) showed a broad maximum at theta=90 degree, which arises from intrinsic pinning. All data except at theta=90 degree can be scaled by the Blatter plot. Hc2(theta) near Tc follows the anisotropic Ginzburg-Landau expression. The mass anisotropy increased from 1.5 to 2 with increasing temperature, which is an evidence for …
Biaxially textured FeSe1-xTex films have been realized on Fe-buffered MgO substrates by pulsed laser deposition. Similar to the Fe/BaFe2As2 bilayers, the crystalline quality of FeSe1-xTex films exhibit a sharp out-of-plane and in-plane texture less than 0.9°. The Fe/FeSe1-xTex bilayers showed high superconducting transition temperatures of over 17 K. The angular-dependent critical current densities exhibit peaks positioned at H // ab similar to other pnictide thin films. The volume pinning force of FeSe1-xTex in this direction is very strong compared with that of Co-doped BaFe2As2, due to a go…
Transport measurements along the node and anti-node directions in the a-b plane of optimally doped and epitaxial thin films of YBCO are reported. Low bias magnetoresistance measurements near and below T_c show that the flux flow resistivity along the node and anti-node directions versus magnetic field are indistinguishable. This result suggests that within the experimental error of our measurements, no correspondence is found between the flux pinning properties in YBCO and the d-wave nature of the order parameter.
We report measurements of the field and angular dependences of Jc of truly epitaxial Co-doped BaFe2As2 thin films grown on SrTiO3/(La,Sr)(Al,Ta)O3 with different SrTiO3 template thicknesses. The films show Jc comparable to Jc of single crystals and a maximum pinning force Fp(0.6Tc) > 5 GN/m3 at H/Hirr ~ 0.5 indicative of strong vortex pinning effective up to high fields. Due to the strong correlated c-axis pinning, Jc for field along the c-axis exceeds Jc for H//ab plane, inverting the expectation of the Hc2 anisotropy. HRTEM reveals that the strong vortex pinning is due to a high density of n…
We apply the vortex path model of critical currents to a comprehensive analysis of contemporary data on defect-engineered superconductors, showing that it provides a consistent and detailed interpretation of the experimental data for a diverse range of materials. We address the question of whether electron mass anisotropy plays a role of any consequence in determining the form of this data and conclude that it does not. By abandoning this false interpretation of the data, we are able to make significant progress in understanding the real origin of the observed behavior. In particular, we are a…
Epitaxial MgB2 thick films were grown on Al2O3 substrates at 600 oC by using the hybrid physical chemical vapor deposition (HPCVD) technique. In order to obtain a high magnesium vapor pressure around the substrates, we used a special susceptor having a susceptor cap and achieved a very high growth rate of 0.17 um/min. Hexgonal-shaped columnar structures were observed by cross-sectional and planar view transmission electron microscope (TEM) images. For the 1.7-um-thick film, the Tc was observed to be 40.5 K with a Jc of 1.5 x 10^6 A/cm^2 at 30 K. The vortex pinning mechanism by intercolumnar bo…
Dissipation-free current transport in high-temperature superconductors is one of the most crucial properties of this class of materials which is directly related to the effective inhibition of flux line movement by defect structures. In this respect epitaxially grown thin films of YBa2Cu3O7-d (YBCO) are proving to be the strongest candidates for many widescale applications that are close to realization. We show that the relation between different defect structures and flux line pinning in these films exhibits universal features which are clearly displayed in a detailed analysis of the temperat…
Understanding new superconductors requires high-quality epitaxial thin films to explore intrinsic electromagnetic properties, control grain boundaries and strain effects, and evaluate device applications. So far superconducting properties of ferropnictide thin films appear compromised by imperfect epitaxial growth and poor connectivity of the superconducting phase. Here we report novel template engineering using single-crystal intermediate layers of (001) SrTiO3 and BaTiO3 grown on various perovskite substrates that enables genuine epitaxial films of Co-doped BaFe2As2 with high transition temp…
Epitaxial niobium-nitride thin films with a critical temperature of Tc=16K and a thickness of 100nm were fabricated on MgO(100) substrates by pulsed laser deposition. Low-temperature magnetic force microscopy (MFM) images of the supercurrent vortices were measured after field cooling in a magnetic field of 3mT at various temperatures. Temperature dependence of the penetration depth has been evaluated by a two-dimensional fitting of the vortex profiles in the monopole-monopole model. Its subsequent fit to a single s-wave gap function results in the superconducting gap amplitude Delta(0) = 2.9 m…
Measurements of the nonlinear flux-flow resistivity $ρ$ and the critical vortex velocity $\rm v^*_ϕ$ at high voltage bias close to the instability regime predicted by Larkin and Ovchinnikov \cite{LO} are reported along the node and antinode directions of the d-wave order parameter in the \textit{a-b} plane of epitaxial $YBa_2Cu_3O_{7-δ}$ films. In this pinning-free regime, $ρ$ and $\rm v^*_ϕ$ are found to be anisotropic with values in the node direction larger on average by 10% than in the antinode direction. The anisotropy of $ρ$ is almost independent of temperature and field. We attribute th…
Square arrays of sub-micrometer columnar defects in thin YBa$_{2}$Cu$_{3}$O$_{7-δ}$ (YBCO) films with spacings down to 300 nm have been fabricated by a He ion beam projection technique. Pronounced peaks in the critical current and corresponding minima in the resistance demonstrate the commensurate arrangement of flux quanta with the artificial pinning landscape, despite the strong intrinsic pinning in epitaxial YBCO films. Whereas these vortex matching signatures are exactly at predicted values in field-cooled experiments, they are displaced in zero-field cooled, magnetic-field ramped experime…
Structural stability, electronic structure, and magnetism of the $d^9$ double infinite-layer La$_3$Ni$_2$O$_5$F under chemical pressure and epitaxial strain
Although nanoscale deformation, such as nanostrain in iron chalcogenide (FeSexTe1-x, FST) thin films, has attracted attention owing to the enhancement of general superconducting properties, including critical current density (Jc) and critical transition temperature, its formation has proven to be an extremely challenging and complex process thus far. Herein, we successfully fabricated an epitaxial FST thin film with uniformly distributed nanostrain by injection of a trace amount of CeO2 inside FST matrix using sequential pulsed laser deposition. Using transmission electron microscopy and geome…
An advanced mask-less nanofabrication technique, focused electron beam-induced deposition (FEBID), has been employed on epitaxial Nb thin films for their ferromagnetic decoration by an array of Co stripes. These substantially modify the non-patterned films' superconducting properties, providing a washboard-like pinning potential landscape for the vortex motion. At small magnetic fields B<0.1T, vortex lattice matching effects have been investigated by magneto-transport measurements. Peculiarities in the field dependencies of the films resistivity R(B) have been observed in particular for the vo…
We report on electrical transport properties of epitaxial Ba(Fe1-xNix)2As2 thin films grown by pulsed laser deposition in static magnetic fields up to 35 T. The thin film shows a critical temperature of 17.2 K and a critical current density of 5.7x10^5 A/cm^2 in self field at 4.2 K while the pinning is dominated by elastic pinning at two-dimensional nonmagnetic defects. Compared to single-crystal data, we find a higher slope of the upper critical field for the thin film at a similar doping level and a small anisotropy. Also an unusual small vortex liquid phase was observed at low temperatures,…
Studies involving vortex dynamics and their interaction with pinning centers are an important ingredient to reach higher critical currents in superconducting materials. The vortex distribution around arrays of engineered defects, such as blind and through holes, may help to improve the superconducting properties. Thus, in this work, we used the time-dependent Ginzburg-Landau theory to investigate the vortex dynamics in superconductors of mesoscopic dimensions with a large central square defect with three different configurations: (i) a hole which passes through the sample (interface with the v…
Field angle dependent critical current, magneto-optical microscopy, and high resolution electron microscopy studies have been performed on YBa2Cu3O7-d thin films grown on mis-cut substrates. High resolution electron microscopy images show that the films studied exhibited clean epitaxial growth with a low density of anti-phase boundaries and stacking faults. Any anti-phase boundaries (APBs) formed near the film substrate interface rapidly healed rather than extending through the thickness of the film. Unlike vicinal films grown on annealed substrates, which contain a high density of anti-phase …
The defect-rich morphology of YBa$_{2}$Cu$_{3}$O$_{7-δ}$ (YBCO) thin films leads to a glass-like arrangement of Abrikosov vortices which causes the resistance to disappear in vanishing current densities. This vortex glass consists of entangled vortex lines and is identified by a characteristic scaling of the voltage-current isotherms. Randomly distributed columnar defects stratify the vortex lines and lead to a Bose glass. Here, we report on the observation of an ordered Bose glass in a YBCO thin film with a hexagonal array of columnar defects with 30 nm spacings. The periodic pinning landscap…
This paper reports the nanoscale texture and microstructure of a high-performance NdFeAs(O,F) superconducting thin film grown by molecular beam epitaxy on a textured MgO/Y$_2$O$_3$/Hastelloy substrate. The NdFeAs(O,F) film forms a highly textured columnar grain structure by epitaxial growth on the MgO template. Although the film contains stacking faults along the $ab$-plane as well as grain boundaries perpendicular to the $ab$-plane, good superconducting properties are measured: a critical temperature, $T _{\rm c}$, of 46 K and a self-field critical current density, $J_{\rm c}$, of $2 \times 1…
