S.P. Pai

Large critical currents and improved epitaxy of laser ablated Ag‐doped YBa2Cu3O7−δ thin films

Microstructure and critical current densities of laser ablated YBa2Cu3O7−δ thin films doped with 2–20u2009wt.u2009% Ag have been studied. A critical current density as high as 1.4×107 Au2009cm−2 at 77 K has been realized on 〈100〉 SrTiO3 substrates with YBaCuO films doped with 5 wt.u2009% Ag which has been found to be the optimum. Evidence indicates that the improved microstructure and epitaxy which is a consequence of grain enlargement and alignment caused by Ag is responsible for the high values of critical currents observed.

Optimization of KrF laser ablation parameters for in-situ growth of Y1Ba2Cu3O7−δ thin films

Abstract Using a KrF pulsed excimer laser, various interrelated deposition parameters governing the quality of laser-ablated Y 1 Ba 2 Cu 3 O 1−δ (123) thin films have been systematically studied. Modification of the 123 target with increasing laser exposure has been found to affect the plume stability, and the axis of the plume has been found to shift slowly towards the direction of the laser beam. Small laser spots exposing a relatively large diameter annular track of the rotating target have been found to give better plume stability than larger spots exposing the same diameter track. Because of better plume stability and larger plume expansion, smaller laser spots have been found to give significantly better quality 123 films as compared with large spots under optimised growth conditions.The effects of varying O 2 pressure and target-substrate distance have been found to be similar and the location of the substrates at or close to the tip of the plume has been found to be important for the realization of film stoichiometry and high quality. Results have shown that under optimised conditions of 3 J cm −2 fluence, 200 m TorrO 2 pressure and 4.5 cm target-substrate distance, films with T c = 90 K,Δ T ∽1 K and critical current density, J c ≥2×10 6 A cm at 77 K can be reproducibly realized on 〈100〉 MgO substrates with small (3 mm × 0.8 mm) laser spots.

Role of Ag in laser-ablated YBa2Cu3O7−δ thin films: Evidence for grain enlargement and alignment

Abstract Microstructural and superconducting properties of YBa 2 Cu 3 O 7−δ thin films grown in situ by pulsed laser ablation using YBa 2 Cu 3 O 7−δ targets doped with 5 wt.% Ag have been studied. Ag-doped films grown on 〈100〉 LaAlO 3 and 〈100〉 SrTiO 3 substrates at temperatures in the range 650–700°C have shown very significant improvements over the undoped YBa 2 Cu 3 O 7−δ films grown under identical conditions. A critical current density of 1.4×10 7 A cm -2 at 77 K has been observed on SrTiO 3 substrates. Microwave measurements carried out using a microstrip resonator at the X-band have shown a low microwave surface resistance of 190 μΩ at 77 K and an absence of dependence on microwave power up to 13 dBm. Evidence found through scanning electron microscopic observation of Ag-doped films has indicated a substantial grain enlargement and alignment. Hence, the improved transport and microwave transmission properties of Ag-doped films have been attributed to grain enlargement and low-angle brain boundaries due to Ag. Availability of active O to the growing YBa 2 Cu 3 O 7−δ lattice, due to the dissociation of Ag 2 O on the substrate, and the increased mobility of Ag atoms segragating out of the growing grains are presumed to be the major mechanism responsible for the enlargement and alignment of grains in Ag-doped YBa 2 Cu 3 O 7−δ films.

Oxygen order, structure and superconductivity in YBa2Cu3O7- delta films during in situ formation

We have studied the structure and oxygen incorporation in YBa2Cu3O7- delta thin films during in situ formation by pulsed laser deposition. A quenching technique has been used for the first time to freeze the nature of in situ films immediately on termination of growth. In situ resistance and transition-temperature measurements, and X-ray diffraction studies on undoped and Ag-doped films, have shown that an oxygen-deficient orthorhombic phase is indeed possible during growth due to the presence of atomic and ionized oxygen species in the laser-induced plasma.

Effect of thermal cycling on normal state thermal properties of YBa2Cu3O7−x films

Abstract The photopyroelectric (PPE) technique has been used to study the effect of thermal cycling on normal state thermal properties of YBa 2 Cu 3 O 7− x thin films. For an optically opaque and thermally thin sample, the PPE voltage amplitude ( V ) is inversely proportional to the specific heat of the sample. Hysteresis in V as a function of temperature is observed and annealing of films at T = 325 K enhances the magnitude of hysteresis. For annealed films, the magnitude of hysteresis reduces slowly as a function of time and in some films it disappears after a period of more than 24 h at 295 K. Two peaks in V −1 at T = 110 to 140 K and at T = 200 to 240 K have been observed. The peak at T = 200 to 240 K has been observed only during the heating path for samples cooled to a temperature less than 150 K. No hysteresis in the resistance as a function of temperature is observed, but annealing at 325 K reduces the magnitude of the resistance.

Growth of CaF2 buffer on Si using low energy cluster beam deposition technique and study of its properties

Calcium Fluoride buffer lavers were grown on Si substrates using the low energy cluster beam deposition technique. The films were annealed at various temperatures ranging between 500oC and 700oC. The SEM studies showed that as-deposited films were well oriented along the c-axis and had very smooth surface morphology. The annealed films on the contarary, showed lowering of peak intensities and roughening of the surface. The dielectric constant derived from the C-V measurements at 1 MHz were 2.01 and 18 for as-deposited and annealed films respectively.

Self-injection length in La0.7Ca0.3MnO3–YBa2Cu3O7−δ ferromagnet–superconductor multilayer thin films

We have carried out extensive studies on the self-injection problem in barrierless heterojunctions between La0.7Ca0.3MnO3 (LCMO) and YBa2Cu3O7-δ (YBCO) thin films. The heterojunctions were formed in situ by sequentially growing LCMO and YBCO films on 〈100〉 LaAlO3 (LAO) substrate using a pulsed laser deposition (PLD) system. YBCO micro-bridges with 64 µm width were patterned both on the LAO (control) and LCMO side of the substrate. Critical current, Ic, was measured at 77 K on both the control side as well as the LCMO side for different YBCO film thickness. It was observed that while the control side showed a Jc of ∼ 2 × 106 A/cm2, the LCMO side showed about half the value for the same thickness (1800 Å). The difference in Jc indicates that a certain thickness of YBCO has become ‘effectively’ normal due to self-injection. From the measurement of Jc at two different thicknesses (1800 Å and 1500 Å) of YBCO films both on the LAO as well as the LCMO side, the value of self-injection length (at 77 K) was estimated to be ∼ 900 Å. To the authors’ best knowledge, this is the first time that self-injection length has been quantified. A control experiment carried out with LaNiO3 deposited by PLD on YBCO did not show any evidence of self-injection.

Oxygen incorporation duringin situ growth of YBCO films on both sides of substrates

Oxygen out-diffusion during cooling and heating ofin situ grown {ie685-1} (YBCO) films in low oxygen pressure used during growth by pulsed laser deposition was studied in the temperature range 700−450°C usingin situ resistance measurements. Results indicate that irrespective of the number of cooling and heating cycles seen by the films, full oxygenation of the films can be realized by the final cooling from the growth temperature in 500 torr oxygen pressure. This result has been successfully used to sequentially grow high quality YBCO films on both sides of LaAlO3 substrates. These films have been used for the fabrication of X-band microstrip resonators with superconducting ground plane.

Substrate bias effects during in situ growth of Y1Ba2Cu3O7−δthin films by RF magnetron sputtering

Abstract Superconducting Y1Ba2Cu3O7−δ thin films have been grown in situ by RF magnetron sputtering using a planar YBaCuO target with and without RF bias on the substrate. RF bias on the substrate has been found to have a significant effect on the composition of the sputtered film. While the Y and Ba contents in the film have been found to slightly decrease with increasing RF bias on the substrate, the Cu content has been found to increase significantly and approach that of the target at — 30 V substrate bias. The increase in Cu content of the film with increasing substrate bias has been attributed to a decrease in secondary electron bombardment of the film which increases the sticking coefficient of Cu atoms and does not affect that of Ba atoms at the applied substrate bias and temperature range. Nevertheless, low substrate power levels of 20 W have given films of improved thickness and compositional uniformity. Films grown in situ at 650°C on 〈100〉 MgO and at 680° on 〈100〉 SrTiO3 substrates have shown Tc0 values of 80–82 K and a critical current density of 105 A cmue5f82 at 70 K.

Superconductor ferromagnet bilayers; experimental investigation of spin polarized transport across the interface

Deposition of high temperature superconducting thin films by pulsed laser deposition (PLD) started a flurry of activities in the area of rare earth class of oxide manganites. Studies have been carried out by injection of the polarized spins from a ferromagnetic metal, into the superconductor via a thin insulating barrier. However because of the presence of an insulating layer possible role of joule heating in limiting the spin polarized transport across the interface cannot be ruled out. We would like to present some of the results of our work carried out on LaAlO3/La0.7Ca0.3MnO3/YBa2Cu3O7-δ (LAO/LCMO/YBCO) LCMO on top and LaAlO3//YBa2Cu3O7-δ La0.7 Ca0.3 MnO3 (LAO/YBCO/LCMO) YBCO on top heterostructures deposited on LAO substrates without any insulating layer separating the two. PLD technique was used to deposit a 500 micron wide line of LCMO followed by deposition of YBCO layer which was subsequently patterned into a 500 micron line directly on top of the LCMO line. Here we report the suppression in the critical current Ic of the superconductor due to pair-breaking phenomenon when polarized spins are injected into YBCO in case of LAO/LCMO/YBCO (LCMO on top) sample. Surface and interface characterization of these bilayers was performed by SQUID, AFM, SIMS and four probe techniques.