M. Murugesan

Microcrack-free epitaxy of thick YBa2Cu3O7−δ films on vicinal r-cut sapphire buffered with CeO2

YBa2Cu3O7−δ (YBCO) films were fabricated by pulsed laser deposition on deliberately miscut Al2O3(11¯02) buffered with CeO2. Scanning electron microscopy observations demonstrated that 1-μm-thick YBCO films were microcrack free. Characterization of the films revealed porous morphology and high crystalline quality of YBCO with CuO planes tilted vertically and terminated frequently. These features are considered to be a contributing factor to the strain-relieving mechanism responsible for the increase in film thickness without microcracking. Microcrack-free thick YBCO films revealed Tc=90.5±0.5K, Jc(77.3K,0T)=2.0–3.0×106A∕cm2, and a substantial enhancement of Jc×t(77.3K,0T) up to 246 A/cm.

Influence of MgO substrate annealing on the microwave properties of laser ablated YBa2Cu3Oz thin films

We have systematically investigated the dependence of the crystal structure and the microwave response of laser ablated YBa2Cu3Oz (YBCO) thin films on MgO substrate microstructure formed upon thermal annealing. A dramatic improvement in both crystallinity as well as microwave surface resistance Rs (of almost an order of magnitude) is observed for the films grown on substrates annealed for the initial 2 h at 1000 ◦ Ci n fl owing O 2 ambient compared to films deposited on non-annealed substrates. However, for further annealing of the MgO substrate we found no further pronounced improvement in either Rs or the crystalline quality of YBCO. The observed improvement in Rs for the film grown on an annealed substrate can be explained by the graphoepitaxial growth of YBCO on MgO that results in very good crystallinity of YBCO film. The ratio of 0 ◦ and 45 ◦ grains, D(45 ◦ ) and the degree of in-plane alignment, �φ , have a non-monotonic relation with the substrate annealing time. At the same time, a strong correlation is observed between Rs and �φ ,a ndD(45 ◦ ), which themselves are dependent

High corrosion stability of DyBa2Cu3Oz thin films relative to YBa2Cu3Oz films: A possible relation between corrosion and c-axis correlated extended defects

An enhanced stability towards corrosion was noted for DyBa2Cu3Oz relative to YBa2Cu3Oz. Between pure and Ca doped films, Ca doped films were found to be more corrosion resistant towards aqueous environment. The observed high corrosion stability in DyBa2Cu3Oz can be explained by the improved microstructure and the absence of c-axis correlated extended defects (EDs), while the remarkable increase in corrosion stability for the Ca doped films is presumed to be due to the notable improvement in the grain boundary region and the absence of ED. It is confirmed that the films with c-axis correlated ED are highly prone to corrosion by moisture.

Microcrack-free and high Jc thick YBCO films on vicinal r-cut sapphire buffered with CeO2

By growing CeO2 buffer layers on deliberately miscut Al2O3 (102) surfaces, we have successfully fabricated stoichiometric YBa2Cu3O7-δ (YBCO) thick films by pulsed laser deposition. Atomic force microscopy and scanning electron microscopy observations demonstrated that YBCO films with thickness up to t ~ 1.0 µm were microcrack-free. Characterization of the films revealed a porous morphology, consisting of interconnected islands and deep holes (pores). This feature is considered as one of the sources contributing to the strain-relieving mechanism responsible for the increase in film thickness without microcracking. Microcrack-free thick YBCO films revealed Tc = 90.5 ± 0.5 K, Jc(77.3 K, 0 T ) = 2.0-3.0 × 106 A/cm2, and a substantial enhancement of Jc × t (77.3 K, 0 T ) up to 246 A/cm.

Microwave surface resistance of under doped Co substituted YBCO films

Abstract Laser ablated under doped YBa 2 Cu 3 O z (YBCO) films grown on LaAlO 3 substrates have been studied for its crystal structure and microwave surface resistance ( R s ). The carrier content of YBCO film was varied by site selective substitution for Cu at chain site by Co atom. It is inferred that the low laser repetition rate facilitates the alignment of grains along c -axis, while deteriorates the in-plane alignment of grains in YBa 2 Cu 2.97 Co 0.03 O z (YBCCO) films. The deterioration in in-plane alignment with laser repetition rate is attributed to the changes in Co content or the uneven distribution of Co in YBCCO films. The temperature dependent R s of YBCO film is increased with Co substitution. The observed high R s value combined with broad transition in R s ( T ) curves for the Co substituted samples is thought to be primarily due to the increase in penetration depth caused by the reduced carrier (hole) concentration in the Co substituted samples.

The influence of external factors on the corrosion resistance of high temperature superconductor thin films against moisture

High temperature superconductor (HTS) thin films have been systematically investigated for their corrosion resistance against moisture by studying the role of external factors such as temperature (T), relative humidity (RH), and the type of substrates in the corrosion. In general, (i) the corrosion is progressed monotonously with increasing T as well as RH, (ii) a threshold level of water vapor is needed to cause degradation, and (iii) between T and RH, the influence of T is more dominant. HTS films on SrTiO3 and CeO2 buffered sapphire (cbs) substrates showed better corrosion stability and a low rate of degradation in the critical current density as compared to that of the film grown on MgO substrate. Between DyBa2Cu3Oz (DBCO) and YBa2Cu3Oz, the former is reproducibly found to have many fold higher corrosion resistance against moisture. This observed enhancement in the corrosion resistance in DBCO could be explained by the improved microstructure in the films and the better lattice matching with the subst...

The combined effect of thermal annealing of MgO substrate and Ca substitution on the surface resistance of YBa2Cu3Oz thin films

Single-layer Y1−xCaxBa2Cu3Oz (YCBCO) thin films (x=0.00, 0.02, 0.05, and 0.10) grown on annealed as well as unannealed MgO substrates have been systematically investigated for their carrier concentration, critical current density Jc, and microwave surface resistance Rs. For x⩽0.05, the grain growth follows a three-dimensional-spiral growth mechanism, while for x=0.10 we observed a mainly two-dimensional-like growth of grains. The results of Hall data reveal that the x=0.05 film is overdoped while films with x=0.02 and 0.10 are underdoped with respect to the x=0.00 film. However, the Hall mobility μH is highly enhanced for the x=0.02 film. Thermal annealing of MgO substrates prior to film deposition results to an improvement in the overall superconducting properties of the film such as suppression of normal-state resistivity, enhancement of Jc, and minimization of Rs both for pure as well as Ca-substituted films. Annealing of MgO substrates enhances the Jc value to a magnitude (i) nearly doubled for x=0.00...

Effect of underdoping on the microwave surface resistance of YBa2Cu3Oz thin films

The effect of underdoping on the microwave surface resistance (Rs) and the Hall number (nH) in the Co-substituted YBa2Cu3Oz (YBCO) films grown on LaAlO3 substrates has been investigated, and the results are compared with oxygen-deficient YBCO films. From the analysis of Hall data, it is inferred that the nH is suppressed upon the Co substitution. As the Co concentration increases, the in-plane resistivity (ρ) increases and ρ(T) develops a downward curvature below room temperature, whereas the nH decays exponentially. Microwave studies on both the Co-substituted YBCO as well as the oxygen-deficient YBCO reveal that the reduction in carrier concentration of YBCO, by any means, leads to an undesirable effect on Rs. The Rs(T) value of Co-substituted films increases as the Co concentration is increased. Near the superconducting transition temperature, Tc, the observed high Rs value and a broad transition in the Rs(T) curve in the Co-substituted films are presumed to be due to an increase in the penetration dep...

Correlative study of Rs and Jc for 123 superconducting thin films

We have studied the relationship between the microwave surface resistance, Rs (measured by a dielectric resonator technique at 22?GHz), and the critical current density, Jc (obtained via a transport method), for various superconducting thin films belonging to the 123 family. It is found that Rs = 4.29 ? 107Jc?0.98 for YBa2Cu3Oz (YBCO) films on MgO, which is in good agreement with the reported value (Rs,22?GHz = 1.8 ? 107Jc?1) for YBCO on BaSnO3 buffered MgO. On the other hand, we observed Rs = 6.8 ? 1023Jc?2.55 and Rs = 2.2 ? 105Jc?0.76, respectively, for Y0.90Ca0.10Ba2Cu3Oz on MgO and Dy0.40Ho0.60Ba2Cu3Oz ((D, H)BCO) on LaAlO3 films, where Rs is in ? and Jc is in A?m?2. The observed deviation in the Rs?Jc relations (from the simple relation, which holds good for pure YBCO films) for (D, H)BCO films may be attributed to the existing different temperature dependence of Rs for these films. On the other hand, in the case of Y1?xCaxBa2Cu3Oz (YCBCO) films, the very low Jc and the microstructural imperfections are considered a plausible cause for the observed deviation from the simple relation. Though the pre-exponential factor, m, in Rs = mJc?n is believed to be highly sample dependent, it is inferred from the correlative results (obtained from eight?films, i.e., YBCO (two?films); (D, H)BCO (three?films); YCBCO (three?films with x = 0.02, 0.05, 0.10)) that m and n in the Rs = mJc?n correlation are related to each other in such a way that log(m) varies linearly with n. An m value of the order of 107 is observed typically when the n value is 1.

Transport properties of pulsed laser deposition grown Dy0.4Ho0.6Ba2Cu3Oz thin films

Pulsed laser deposition (PLD) grown Dy 0 . 4 Ho 0 . 6 Ba 2 Cu 3 O z [(D, H)BCO] superconducting films on LaAlO 3 substrates have been studied for their critical current density (J c ) and microwave surface resistance (R s ) in comparison with YBCO films. It is inferred that the best films (grown at 800 °C) with transition temperature (T c ) 92 K showed a higher J c value above 80 K compared to that of YBCO films. Also, at 77 K, the field dependent J c value of (D, H)BCO films (5 kA cm - 2 at 8 T) is 2.5 times higher than that of YBCO films (2 kA cm - 2 at 8 T). The best (D, H)BCO films showed lower R s in the liquid nitrogen (N 2 ) temperature region (4.2 mΩ at 77 K) than YBCO films (7.65 mΩ at 77 K) while in the low temperature region (below 60 K) YBCO films exhibit better microwave performance irrespective of their deposition temperature. These significantly improved values (both the J c as well as the R s values above 77 K) reveal that a rare earth (RE) mixed 123 thin film is a rather better candidate than YBCO thin film for use in fabricating microwave passive devices operating at high fields and at liquid N 2 temperature.