Formation of the YBa 2 Cu 2 NbO y phase in thin films
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Robert A. Wheeler Materials & Manufacturing Directorate (UES), Wright-Patterson Air Force Base, Ohio 45433 (Received 11 April 2008; accepted 25 September 2008)
We report the systematic substitution of Nb at the Cu1 site of YBa2Cu3Oy in thin films to form a new phase of YBa2Cu2NbO8. These films were deposited on SrTiO3(100) crystals using pulsed laser deposition and deposited at an optimal temperature of 850 C. Films were characterized using x-ray diffraction (XRD), atomic force microscopy, x-ray photoelectron spectroscopy (XPS), micro-Raman spectroscopy, and transmission electron microscopy. XRD of these films indicate c-axis oriented YBa2Cu2NbOy formation. XPS and micro-Raman spectroscopy analysis suggests Cu exists in the þ2 state.
I. INTRODUCTION
Our group has previously investigated alternate functional oxide materials as possible buffer layers on metallic substrates for the high-temperature superconducting (HTS) YBa2Cu3O7–d (YBCO) coated conductor applications. In this context, the growth of YBa2CuxMyOz (YBCMO, M ¼ transition metals) phases is considered for thin films. The coated conductors can also potentially be used for both direct current (dc) and alternating current (ac) applications. However, the optimal architectural configurations for these two applications can differ. In the actolerant configuration, the buffer layer is ideally nonconducting to avoid eddy current and coupling losses, whereas in dc applications, a conducting buffer may be desirable for use of the substrate as a stabilization layer.1,2 Identifying a metal-to-insulating transition in the oxide buffer by means of chemical doping can serve as a dual-use buffer. Previous investigations have studied the effects of the partial substitution of copper in the YBCO compound as YBa2Cu3–xMxO7–z and have demonstrated that such substitutions degrade the superconducting properties of the material.3–7 Preferential occupancy cations in the two nonequivalent copper sites have shown to substantially affect superconductivity in the YBCO thin films, significantly depressing Tc for a 0.2 molar substitution.3–5 For the use considered here, complete suppression is necessary with additional doping while maintaining the appropriate epitaxial properties for subsequent YBCO growth. The particular substitution of Nb can potentially produce a metal-to-insulating transformation in the electrical and structural properties. It has been reported by several researchers that a new structure of dielectric a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0009
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J. Mater. Res., Vol. 24, No. 1, Jan 2009 Downloaded: 30 Mar 2015
buffer layers consisting of both insulator grains and superconducting grains can be produced by Nb doping at the copper sites in the YBCO structure.6,7 Srinivas et al.8 have reported Nb doping at copper site in thin films deposited on SrTiO3(100) substrates and the addition of V2O5 and Nb2O5 into YBCO thin films. Additionally, recent initial results have demon
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