Session L10: Focus Session: Electronic and Vortex Mechanisms for Higher Performing Superconductors

Exploring the limits of critical currents in superconductors

Mechanisms, which determine the ultimate limit of the critical current density $J_c(T,B)$ in superconductors are discussed. The talk is mostly focused on the extreme strong pinning limit of highly deformed vortex segments, the role of anisotropy, current-blocking effects of pinning centers and grain boundaries, thermal fluctuations of vortices in high-$T_c$ superconductors. In particular, the design of optimum pinning nanostructures, which produce the maximum $J_c$ is addressed. The results are applied to YBCO thick-film coated conductors with insulating nanoprecipitates, for which several groups have reported very high $J_c$ values, up to 12-20 $\%$ of the depairing current density. Requirements for a putative room-temperature superconductor to be useful in high-field applications are discussed.   

Current density in YBCO-based Tapes Studied over 8 Decades of Dissipation

Many applications of superconductors require conduction of high density electric currents in a magnetic field, with minimal dissipation. We investigated the dependence of current density $J$ on electric field $E$ due to motion of depinned vortices, over a range of $\sim $10$^{8}$ in $E$. The materials are pre-commercial YBa$_{2}$Cu$_{3}$O$_{\sim 7}$ coated conductors (3.5$\mu $m) on buffered Hastelloy substrates prepared by SuperPower, Inc. Experimental methods include conventional 4-probe electrical transport at the highest $E$ fields; inductive measurements of magnetic moment $m\sim J$ using a swept magnetic field d$H$/d$t\sim E$ at lower $E$ fields; and time dependent ``flux creep'' measurement where d$m$/d$t\sim E$. At $T$ = 77 K, a power law variation $E\sim J^{n}$ is found. The resulting $E(J)$ dependencies become steeper, i.e., the characteristic $n$-value increases, as $J$ is reduced, reflecting a diverging activation energy for vortex movement. The inductive studies are easily extended to lower temperatures and a wide range of magnetic fields. Implications for applications will be discussed.   

Isotropic critical currents in anisotropic superconductors: a simple physical model

Critical current densities, $J_{c}$, that are nearly independent of magnetic field orientation can be observed in intrinsically anisotropic high-temperature superconductors that have specific, very strong flux pinning nanostructure. The phenomenon is observed to occur at specific temperature dependent fields, $H$*($T)$. The possibility of such isotropic behavior can be described by a simple physical model based on the orientation dependence of the irreversibility field $H_{irr}(\theta)$ and the power-law decay exponent $\alpha (\theta )$, where $J_{c}\propto H^{-\alpha }$ in the intermediate field regime. An analysis will be discussed that elucidates necessary conditions for occurrence of the effect, and provides possible predictive tools for tailoring of $H$*($T)$ to practical fields and temperatures by means of defect engineering.   

Exploring the limits to vortex pinning in superconductors

Vortices in type II superconductors sit on a potential energy landscape created by material inhomogeneities. In the presence of an electrical current these inhomogeneities produce a restoring force that precludes vortex motion, thus allowing dissipation-free transport, as long as the current density does not exceed the critical current density $J_{c}$. Based on present theoretical understanding, by introducing the appropriate type of pinning centers it should be possible to attain $J_{c}$ values (for low vortex densities) as large as the physical limit determined by the depairing current density $J_{0}$. However, after decades of large efforts and resources dedicated to pinning enhancement (which has obvious technological relevance) we are far below that limit. Presently, the largest $J_{c}$/$J_{0}$ ratios have been obtained for very thin epitaxial YBa$_{2}$Cu$_{3}$O$_{7}$ films and are $\sim $0.3, slightly higher than in the conventional superconductor Nb-Ti ($J_{c}$/$J_{0}\sim $0.25). I will analyze the possible reasons for this limitation and discuss possible ways to circumvent it. I will particularly focus on the influence of thermal fluctuations, which promote some level of vortex motion even below $J_{c}$, resulting in a temporal decay of the supercurrents and consequently lower $J_{c}$ values as determined by standard experimental techniques. Based on general principles, I will discuss what pinning performance we may expect in yet-to-be-discovered superconductors with high $T_{c}$.   

Magnetic field and temperature specific isotropic critical currents in strong-pinning high-temperature superconductors

We report the observation of a unique temperature-dependent magnetic field, $H$*($T)$, at which the critical current of (R)BaCuO (R=rare earth) films with strong $c$-axis pinning can be nearly isotropic. That is, $J_{c}(\theta $, $H$*) $\cong $ constant over nearly the entire interval of sample orientation from \textit{H$\vert \vert $c} to \textit{H$\vert \vert $ab }(in the full Lorentz force configuration). The phenomenon is observed in classes of HTS coatings that contain self-assembled, strongly pinning columnar stacks of second-phase precipitates, BaZrO$_{3}$, oriented near the $c$ axis, and appears to originate from the combination of and offsetting effects of material anisotropies. Systematics of this behavior will be explored and several important control parameters will be identified.   

AC losses in multifilamentary YBCO thin films

Striation of superconducting tape allows the reduction of hysteresis losses. We studied the effect of an ac current as a function of the frequency and of a static magnetic field on the flux behavior in ultifilamentary YBa$_{2}$Cu$_{3}$O$_{7-x}$ (YBCO) thin films. The current density, the magnetic and electric field profiles are determined quantitatively during the cycle. The shielding and transport current distribution in the filaments are affected by hysteresis and inductive effects that depend on the number and the distribution of the filaments. Time resolved magneto-optical imaging measurements reveal a cross-talk between adjacent conducting filaments that affects the overall hysteresis losses. This new, quantitative and fast method allows us to determine a of dynamic parameters, such as mapping the transport current density and electric field distribution during the ac cycle, that are important for practical superconducting applications and complementary to conventional transport measurement techniques.   

Evaluation of YBCO to 45T over wide temperature range

The ability to tune the vortex pinning of YBCO in coated conductor form is both an enormous benefit to future superconducting materials applications and a challenge to understanding the properties over a broad range on particular samples. We have been doing such characterizations in support of the design of 30 T magnets. In recent work we have mapped the $J_{c}$ to fields of 33T at temperatures down to 4K and measured the angular dependent $J_{c}$ in similar fields. We see that it is possible to enhance $H_{irr}$ by about 15{\%} as compared to standard YBCO at 55K, when a high density of RE$_{2}$O3 nanoprecipitates is formed in the microstructure These precipitate arrays produce $J_{c}$ 50 MA/cm$^{2}$ almost 20{\%} of the depairing current density.   

Vortex phases and dynamics in YBa$_{2}$Cu$_{3}$O$_{7}$+BaZrO$_{3}$ films as a function of angle and field up to 50 Tesla

Studying the vortex solid-liquid transition (resistivity=0) in high T$_{c}$ superconductors is scientifically and technologically relevant. We have used low current transport measurements to study the melting line of YBa$_{2}$Cu$_{3}$O$_{7}$ films with and without BaZrO$_{3}$ additions in fields up to 50 T. Samples with mostly extended particle defects, mostly columnar defects, or a mixture of both will be compared. Plain YBa$_{2}$Cu$_{3}$O$_{7}$ shows correlated pinning along the crystalline axes and the emergence of a smectic phase when field is aligned with the a-b plane. Inclusion of BaZrO$_{3}$ not only alters the angular dependence of the irreversibility line indicating the stronger influence of c-axis correlated pinning, but also affects dissipation in the vortex-liquid state over the entire angular range. We will discuss the results in terms of vortex pinning, the corresponding types of phase transitions, micro-structural analysis, and information obtained from critical current measurements.   

Vortex pinning landscape in MOD-TFA YBCO nanostroctured films

A methodology of general validity to study vortex pinning in YBCO based on J$_{c}$ transport measurements is described. It permits to identify, separate and quantify three basic vortex pinning contributions associated to anisotropic-strong, isotropic-strong and isotropic-weak pinning centers. Thereof, the corresponding vortex pinning phase diagrams are built up. This methodology is applied to the new solution-derived YBCO nanostructured films, including controlled interfacial pinning by the growth of nanostructured templates by means of self-assembled processes [1] and YBCO-BaZrO$_{3}$ nanocomposites prepared by modified solution precursors. The application of the methodology and comparison with a standard solution-derived YBCO film [2], enables us to identify the nature and the effect of the additional pinning centers induced. The nanostructured templates films show c-axis pinning strongly increased, controlling most of the pinning phase diagram. On the other hand, the nanocomposites have achieved so far, the highest pinning properties in HTc-superconductors [3], being the isotropic-strong defects contribution the origin of their unique properties. [1] M. Gibert et al, Adv. Mat. vol 19, p. 3937 (2007) [2] Puig.T et al, SuST EUCAS 2007 (to be published) [3] J. Gutierrez et al, Nat. Mat. vol. 6, p. 367 (2007) * Work supported by HIPERCHEM, NANOARTIS and MAT2005-02047   

Flux dynamics in a two-band superconductor with delocalized electric fields

In conventional flux flow, vortex dissipation is localized to the vicinity of the vortex core leading to a viscous coefficient $\eta$ that is independent of flux density $B$ and a flux-flow resistance $R_f \propto B$. This causes a progressive broadening with $B$ of $I$-$V$ and $R$-$T$ curves, which in turn degrades a superconductor's performance in switching applications. An anomalous behavior arises when a substantial quasiparticle population exists away from the cores and when the electric field and dissipation extend into those regions---a scenario that is realized in a disordered two-band superconductor with slow branch-imbalance relaxation. In this case $\eta$ rises linearly with $B$ and $R_f$ becomes independent of $B$, as observed in disordered magnesium diboride. Such an intrinsically field indifferent mixed-state response makes this system especially suited for magnetic-field induced switching.   

In-Field Critical Current by Correlated Anti-Pins in Type-II Superconductors

The critical current shown by films of YBa$_2$Cu$_3$O$_y$ that contain naturally occuring linear pinning centers aligned parallel to the c-axis decays with increasing magnetic field that is also aligned in parallel as an inverse-square-root power law. Recent theoretical work based on 2D collective pinning of the vortex lattice by such material line defects recovers this dependence on magnetic field in the weak-pinning limit[1]. It further predicts an in-field critical current for correlated {\it antipins} that decays with magnetic field more slowly, as an inverse power law characterized by an exponent below 1/2. We test these predictions by performing Langevin dynamics simulations of the corresponding 2D vortex lattice driven by the Lorentz force. Long-range logarithmic interactions between vortices are assumed, while (anti)pinning centers are arranged in a ``liquid'' fashion. We find indeed that collective pinning by antipins result in a significant critical current. More detailed comparisons with theory in relation to the power-law decay with magnetic field will be made. \newline [1] J.P. Rodriguez and M.P. Maley, Phys. Rev. B {\bf 73}, 094502 (2006).