X. X. Xi

Giant magnetoresistance in epitaxial Nd0.7Sr0.3MnO3−δ thin films

The magnetoresistance of epitaxial Nd0.7Sr0.3MnOδ thin films has been studied. A giant magnetoresistance, with more than 4 orders of magnitude change in resistance (−ΔR/RH≳106%), was obtained at ∼60 K and a magnetic field of 8 T. This giant magnetoresistance (GMR) ratio is about one order of magnitude larger than the highest value reported previously which was observed in La–Ca–Mn–O film. We have also obtained a large GMR ratio with −ΔR/RH≳3000% for H=5 T in an in situ Nd0.7Sr0.3MnOδ thin film, a much larger effect than the previous results in doped manganese oxide films in which a large GMR ratio was obtained only in postannealed samples. Our results also show that the GMR effect in these films can be strongly influenced by the thin‐film preparation conditions.

Electric field effect in high Tc superconducting ultrathin YBa2Cu3O7−x films

A multilayer high Tc superconducting field‐effect transistor‐like structure was made from ultrathin YBa2Cu3O7−x films. An epitaxially grown dielectric SrTiO3 insulation layer, which had a forward bias breakdown voltage of about 20 V, allowed an electric field induced change in the channel layer of 1.25×1013 carrier/cm2 per volt of the gate voltage. A significant modulation of the normal state and superconducting properties was observed in samples with YBa2Cu3O7−x channel layers of a few unit cells thick. By applying gate voltage of different polarities, Tc was both suppressed and enhanced by ∼1 K. The resistance was modulated by as much as 20% in the normal state and by over 1500% near the zero resistance temperature.

Dielectric properties of SrTiO3 thin films used in high Tc superconducting field‐effect devices

The dielectric constant ■ of thin epitaxial SrTiO3 films used as dielectric layers in high Tc superconducting field‐effect devices has been studied. ■ in these thin films was found to have much weaker dependence on temperature and applied electric field than the bulk material. The breakdown characteristics of gate structures containing SrTiO3 dielectric layers are strongly influenced by the metal used as the gate electrode.

Voltage‐current characteristics of a high Tc superconducting field effect device

The source‐drain voltage‐current characteristics of a high Tc field‐effect device consisting of a YBa2Cu3O7−x/SrTiO3/Au multilayer structure are measured. Similarities of the V‐I curves to those of a metal‐oxide‐semiconductor field‐effect transistor (MOSFET) are observed. However, we emphasize that the superconducting channel exhibits significantly lower dissipation in comparison with the inversion layer in a MOSFET. The device has a 50 A thick YBa2Cu3O7−x channel layer (Tc=43 K) with a high critical current density Jc (5 K) over 1×105 A/cm2. A transconductance of over 0.2 mS/mm and a voltage gain of 0.9 are obtained in the device.

High critical current densities in ultrathin YBa2Cu3O7−δ films sandwiched between (PrxY1−x)Ba2Cu3O7−δ layers

YBa2Cu3O7−δ (YBCO) films with nominal thicknesses of 1–4 unit cells were grown by pulsed laser deposition using (PrxY1−x)Ba2Cu3O7−δ [(PrxY1−x)BCO] (1≥x≥0) as buffer layers and cap layers. The films of 1 unit cell thick were superconducting for all the x values while Tc increased when x was reduced. For adjacent layers of (Pr0.6Y0.4)BCO which is semiconducting, a Tc of 43 K and Jc of 2×106 A/cm2 for B∥ab and 4×105 A/cm2 for B⊥ab at B=7 T and 4.2 K were obtained in a one‐unit cell thick YBCO layer. The Jc values of a few unit cell thick YBCO layers nearly approached that of thick YBCO films. The results suggest the absence of significant weak‐link effects in these films.

Sr2AlTaO6 films for multilayer high‐temperature superconducting device applications

Thin films of Sr2AlTaO6 (SAT) and multilayers of YBa2Cu3O7 (YBCO)/SAT have been grown by pulsed laser deposition on [001] LaAlO3 substrates. X‐ray diffraction shows that SAT grows on [001] LaAlO3 with the c‐axis oriented normal to the substrate plane. X‐ray rocking curve and Rutherford backscattering channeling measurements on SAT films yield full width at half maximum of <0.3° and minimum backscattering yield χmin of 5%–7%, respectively, indicating good crystallinity. The real part of the dielectric constant er is found to be ∼23–30, with ∼6×107 V/m static breakdown electric field. Both the dielectric constant and the static breakdown field show negligible temperature dependence between 10 and 300 K. A 100 nm×10 μm×50 μm YBCO film on SAT shows zero‐field critical current density of ∼1.3×106 A/cm2 at 77 K, with a superconducting transition temperature Tc0 of ∼89.2 K.

Effect of dc electric field on the effective microwave surface impedance of YBa2Cu3O7/SrTiO3/YBa2Cu3O7 trilayers

We have studied the effect of a dc electric field on the effective microwave surface impedance of a thin film YBa2Cu3O7/SrTiO3/YBa2Cu3O7 (YBCO/STO/YBCO) trilayer by a dielectric resonator technique. At large dc electric fields (≳105 V/cm), both the effective surface resistance and reactance of the sample decrease monotonically with increasing dc voltage applied to the to YBCO film, yielding at 25 K and 24.7 GHz ‖δRs/δVdc‖∼0.25 μΩ/V and ‖δXs/δVdc‖∼1.8 μΩ/V, respectively. A two‐fluid analysis indicates that the changes in the surface impedance can be explained in terms of field induced changes in the superconducting carrier density of the top YBCO film.

Superconducting Y1Ba2Cu3O7-x/Nd1.85Ce0.15CuO4-y bilayer thin films

Bilayers of Y1Ba2Cu3O7−x(YBCO)/Nd1.85Ce0.15CuO4−y(NCCO), in which both layers were superconducting, were fabricated in situ by pulsed laser deposition. Using N2O as the reactive gas, we were able to meet the conflicting processing requirements, oxygenation for YBCO, and reduction for NCCO, in the same sample with a specific cooling procedure. The bilayer thin films were c‐axis oriented with a transition temperature, Tco, of 88.5 K for YBCO and 18.5 K for NCCO. Supercurrent was observed across the interface of the YBCO and NCCO superconducting layers, indicating the phase coherence between these superconductors.

Deposition and reduction of Nd1.85Ce0.15CuO4−y superconducting thin films

Superconducting thin films of Nd1.85Ce0.15CuO4−y (NCCO) were grown at 720–820u2009°C by pulsed laser deposition in N2O atmosphere. The reduction subsequent to the deposition was found to be critical, and was studied systematically for various temperature, atmosphere, and duration. An ac susceptibility technique was used to stringently characterize the film quality. Very high quality films were made with optimized reduction conditions which showed a Tc(R=0) of 22.4 K, with a transition width of 0.2 K and a Jc (4.2 K) of 8×105/cm2 at zero field. Microwave surface resistance Rs was measured at 9.6 GHz and a value of ∼3 mΩ was obtained at 4.2 K in a 5000 A thick NCCO film.

Characteristics of oxygen over‐reduced Nd1.85Ce0.15CuO4−y films

We have studied the oxygen doping effect in thin Nd1.85Ce0.15CuO4−y (NCCO) films. Contrary to the common believe that the disappearance of superconductivity in the over‐reduced NCCO samples is a consequence of phase decomposition, we found that there exists an over‐reduced region in which the crystallographic structure of the film remains essentially the same as in the optimum‐oxygen doped region, but the film becomes insulating. Rutherford backscattering spectroscopy channeling analysis reveals that the oxygen over‐reduction increases atomic distortion in the lattice. This work provides strong support to previous transport studies by Jiang et al. [Phys. Rev. Lett. 73, 1291 (1994)], which suggested the existence of both electron and hole carriers in superconducting NCCO.