G. Hammerl

Superconducting interfaces between insulating oxides.

At interfaces between complex oxides, electronic systems with unusual electronic properties can be generated. We report on superconductivity in the electron gas formed at the interface between two insulating dielectric perovskite oxides, LaAlO3 and SrTiO3. The behavior of the electron gas is that of a two-dimensional superconductor, confined to a thin sheet at the interface. The superconducting transition temperature of ≅ 200 millikelvin provides a strict upper limit to the thickness of the superconducting layer of ≅ 10 nanometers.

Enhanced supercurrent density in polycrystalline YBa2Cu3O7-δ at 77 K from calcium doping of grain boundaries

With the discovery of high-temperature superconductivity, it seemed that the vision of superconducting power cables operating at the boiling point of liquid nitrogen (77 K) was close to realization. But it was soon found that the critical current density Jc of the supercurrents that can pass through these polycrystalline materials without destroying superconductivity is remarkably small. In many materials, Jc is suppressed at grain boundaries, by phenomena such as interface charging and bending of the electronic band structure. Partial replacement (‘doping’) of the yttrium in YBa2Cu3O7-δ with calcium has been used to increase grain-boundary Jc values substantially, but only at temperatures much lower than 77 K (ref. 9). Here we show that preferentially overdoping the grain boundaries, relative to the grains themselves, yields values of J c at 77 K that far exceed previously published values. Our results indicate that grain-boundary doping is a viable approach for producing a practical, cost-effective superconducting power cable operating at liquid-nitrogen temperatures.

Evidence of Doping-Dependent Pairing Symmetry in Cuprate Superconductors

Scanning tunneling spectroscopy studies reveal long-range spatial homogeneity and predominantly d(x(2)-y(2))-pairing spectral characteristics in under- and optimally doped YBa2Cu 3O (7-delta) superconductors, whereas STS on YBa2(Cu 0.9934Zn 0.0026Mg (0.004))3O (6.9) exhibits microscopic spatial modulations and strong scattering near the Zn or Mg impurity sites, together with global suppression of the pairing potential. In contrast, in overdoped (Y 0.7Ca (0.3))Ba 2Cu 3O (7-delta), (d(x(2)-y(2))+s)-pairing symmetry is found, suggesting significant changes in the superconducting ground state at a critical doping value.

Microlithography of electron gases formed at interfaces in oxide heterostructures

Submicron wide structures of conducting quasi-two-dimensional electron gases generated at SrTiO3∕LaAlO3 interfaces have successfully been patterned by modulating the thickness of the LaAlO3 layers with unit cell resolution. This technique allows the authors to structure the electron gases without exposing them to the environment and without incorporating other materials at the edges. The structured electron gases have resistances of ≈200Ω∕◻ (4.2K) and mobilities of ≈700cm2∕Vs (4.2K), while the resistances of the areas patterned to be insulating exceed 1010Ω∕◻.

Possible solution of the grain-boundary problem for applications of high-Tc superconductors

It is shown that the critical current density of high-Tc wires can be greatly enhanced by using a three-fold approach, which consists of grain alignment, doping, and optimization of the grain architecture. According to model calculations, current densities of 4×106 A/cm2 can be achieved for an average grain alignment of 10° at 77 K. Based on this approach, a road to competitive high-Tc cables is proposed.

Ca-doping-induced enhancement of the critical currents of coated conductors grown by ion-beam-assisted deposition

One of the most promising technologies for the fabrication of high-Tc cables is the ion-beam-assisted deposition (IBAD) technique. The performance of the superconductors fabricated by IBAD, and the fabrication costs, are to a great extent determined by the critical current densities of the superconductors’ grain boundaries. Since, in bicrystalline samples, overdoping has been found to improve the transport properties of grain boundaries in high-Tc superconductors, we have explored whether overdoping also enhances the critical currents of IBAD samples. The measurements show that, depending on the critical current density of the superconducting film, Jc (77 K) is increased by factors up to 2.2, also in applied magnetic fields of several tesla.

Conduction and magnetoresistance in doped manganite grain boundaries

Grain boundary diffusion has been used to increase selectively the doping in grain boundaries of doped lanthanum manganite thin films. We show that this doping strongly modifies the temperature dependence of the grain boundary resistance and the form of the conductance versus voltage characteristic. The low field magnetoresistance associated with the grain boundary is altered following doping, but is not necessarily enhanced. This behavior is interpreted in terms of a doping-induced suppression of the local Curie temperature in the region of the grain boundary.

Investigating the pairing state of cuprate superconductors via quasiparticle tunneling and spin injection

Abstract Scanning tunneling spectroscopic studies of cuprate superconductors revealed that the most eminent phenomena of hole-doped (p-type) cuprates, including the predominant d x 2 − y 2 pairing symmetry, spin fluctuations and pseudogap, are absent in the infinite-layer electron-doped (n-type) cuprates Sr 1− x La x CuO 2 . The optimally doped Sr 0.9 La 0.1 CuO 2 exhibits s-wave pairing symmetry, with a superconducting transition temperature T c =43.0 K, a large energy gap Δ =13.0±1.0 meV, and no pseudogap in the normal state. The response of Sr 0.9 La 0.1 CuO 2 to quantum impurities is also conventional, being less sensitive to non-magnetic impurities than to magnetic impurities, in sharp contrast to the strong effects induced by static non-magnetic impurities in the p-type cuprates. The dynamic injection of spin-polarized quasiparticles into the p-type cuprates is found to incur significant suppression of superconductivity, probably due to the strong influence on the spin fluctuations.

Large grain boundary area superconductors

Abstract:For many applications of polycrystalline high-Tc superconductors the small critical currents of the grain boundaries pose a severe problem. To solve this problem, we derive novel designs for the microstructure of coated conductors.

Doping-induced enhancement of grain boundary critical currents

The critical-current density of grain boundaries in high-T/sub c/ superconductors was enhanced to values exceeding the previously known limits both at 4.2 K and at 77 K. Noting the importance of space-charge layers and of the d(x/sup 2/-y/sup 2/)-wave pairing symmetry on grain-boundary transport, we have established a model that provides a comprehensive description of the grain boundaries and proposes ways for their improvement, such as overdoping of the grains and of their boundaries. Exploring as example the effects of overdoping of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// with Ca, we enhanced significantly the critical current densities and decreased the normal-state resistivities of grain boundaries to unprecedented values. By introducing doping heterostructures to overdope grain boundaries selectively over a few nanometers by benefiting from grain boundary diffusion, the enhancement of the critical-current density is achieved at all temperatures up to T/sub c/. At 77 K, critical current densities are obtained which before had been found only at 4.2 K. This concept is proposed as a practical and cost-effective route to enhance the performance of high-T/sub c/ coated conductors fabricated by ion beam assisted deposition (IBAD) or by the rolling assisted biaxially aligned substrate process (RABITS).