1PO4-20 Deposition and Characterization of Superconducting SmBa₂Cu₃O₇ films

Q.X. Jia, S.R. Foltyn, Y. Fan, and J.F. Smith, Superconducting Technology Center, MS K763, Los Alamos National Laboratory, Los Alamos, NM 87545

Presenting Author: S.R. Foltyn

It is essential to deposit smooth high temperature superconducting films for many electronic device applications. A few recent studies have suggested that SmBa₂Cu₃O₇ (Sm123) films deposited by pulsed laser deposition show much smoother surface compared to YBa₂Cu₃O₇ films. We have systematically studied the processing conditions to grow epitaxial superconducting Sm123 films by pulsed-laser deposition. The deposition temperature plays a big role in determining the superconducting properties. At a deposition temperature in the range of 750 °C - 835 °C, the transition temperature increases with the deposition temperature whereas the transition width decreases. Further increase of deposition temperature up to 875 °C does not obviously improve or degrade the transition temperature and transition width. We have routinely deposited Sm123 films with a transition temperature above 91 K and a transition width less than 0.6 K. Scanning electron microscopy reveals that these films are very smooth. In this presentation, we will also discuss our effort by using these smoother Sm123 films for electronic device applications.

1PO4-21 Some Results on the SmBaCuO Thin Films Formation by Off-Axis Pulsed Laser Deposition

A. Di Trolio and A. Morone, CNR-Area di Ricerca di Potenza Istituto per i Materiali Speciali, I85050 Tito Scalo, Italy

Presenting Author: M.A. Morone

The SmBaCuO is a relevant material for different superconductivity applications, such as high-field magnets and devices. The growth of SmBaCuO superconducting films via off-axis Pulsed Laser Depostion has not been investigated. We report preliminary results on thin film's structure and chemical composition change as a function of the O₂ pressure. We discuss the growth of SmBaCuO film on different substrates such as MgO and SrTiO₃. Finally we show the parameters to growth of SmBaCuO thin film in the correct phase.

1PO4-22 Strain and Doping Dependence of Properties of La_2-xSr_xCuO_4+d (0<x<0.25) Thin Films

Weidong Si and Xiaoxing Xi, Department of Physics, The Pennsylvania State University, University Park, PA 16802

Presenting Author: W. Si

We have measured normal state and superconducting properties of La_2-xSr_xCuO_4+d (0<x<0.25) thin films, deposited by Pulsed Laser Deposition, as a function of film thickness, doping level x, lattice strain and oxygen content. Strain was controlled by depositing SrLaAlO₄ buffer layers of different thickness on SrTiO₃ substrates, and characterized using a-axis lattice mismatch, which was obtained through X-ray diffraction analysis. An ozone/molecular oxygen mixture was used during cooling to achieve better oxygen uptake. We found that the compressive strain can improve the oxygen uptake in the La_2-xSr_xCuO_4+d thin films and accordingly, T_c can increase significantly. Especially in the undoped or lightly-doped level, we have observed superconductor-insulator transition by only tuning the strain. On the other hand, tensile strain would decrease the T_c, and even destroy the superconductivity of the film. As to the doping dependence, we have observed systematic change from underdoped sample to overdoped sample. We also noticed the so-called 1/8 anomaly, which is, to some extent, the suppression of superconductivity around x=0.125.

1PO4-23 Observation of Step-flow Growth in Laser-Ablated Thin Filmsof the T'-Phase Compound Pr₂CuO₄

J.T. Markert 1, T.C. Messina 1, B. Dam 2, J. Huijbregste 2, J.H. Rector 2, and R. Griessen 2. 1 Department of Physics, University of Texas at Austin, Austin, TX 78712 USA. 2 Department of Physics and Astronomy, Vrije Universiteit, 1081 HV Amsterdam, Netherlands.

Presenting Author: J.T. Markert

We report laser-ablation studies of highly-oriented thin films of two parent compounds of the electron-doped superconductors: Ln₂CuO₄ (Ln = Pr, Sm) on SrTiO₃(001). The primary synthesis variables were substrate temperature, laser fluence, and oxygen pressure. The films were characterized by x-ray diffraction and atomic force microscopy (AFM). For substrate temperatures in the range 450–850°C, the smoothest films were obtained at high temperatures, T = 800–850°C. Laser fluence had little apparent effect on film quality above a threshhold of 0.8 J/cm² to the maximum fluence of 2.0 J/cm². Oxygen pressure during ablation had significant effects on microstructure as measured by AFM: below 0.05 mbar O₂, small grains (< 50 nm) and deep trenches were typical; above 0.15 mbar, growth of high islands was common. For intermediate oxygen pressure (0.05–0.15 mbar), very flat films (rocking curves with FWHM < 0.10°) with broad terraces (~250 nm) were typical. A true step-fow growth mode was observed with steps descending 0.22° along the (110) direction; this was accompanied by a coherent tilt boundary oriented at 0.11°, half the step angle. It is argued that such growth is driven by relief of epitaxial stress.

1PO4-24 Superconductive Bi₂Sr₂CuO_6+x Thin Films Prepared by Laser Ablation

Tetsuji Uchiyama and Zhen Wang, Kansai Advanced Research Center, Communications Research Laboratory, MPT, 588-2 Iwaoka, Iwaoka-cho, Nishi-Ku, Kobe 651-2492, Japan

Presenting Author: T. Uchiyama

Superconductive Bi₂Sr₂CuO_6+x (Bi-2201) thin films were prepared by laser ablation using the 4th harmonics of a Nd:YAG pulse laser beam (wave length l = 266 nm) and a KrF excimer one (l = 249 nm). The Nd:YAG laser is solid-state laser and is operated safer than the KrF excimer laser. However, the laser power of commercial Nd:YAG laser is less than 100 mJ/shot, that is low to activate molecular oxygen in the chamber. In this work, we fixed the laser power at 80 - 90 mJ/shot and 300 mJ/shot for the Nd:YAG laser ablation and KrF excimer one and investigated the laser power dependence of crystallinity and superconductivity for the films. The crystallinity and superconductivity of the films were estimated by X-ray diffraction and resistant measurements as a function of temperature, respectively. As results, in the case of Nd:YAG, as-deposited Bi-2201 thin films showed high crystallinity, semiconductive normal-conductivity and the zero resistance temperature (T_c0) ~ 5 K, while as-deposited Bi-2201 thin films fabricated by the KrF excimer laser showed high crystallinity, metallic normal-conductivity till a onset of a critical temperature and T_c0 ~ 10 K. This difference may be deduced from a deviation from the stoichiometry.

1PO4-26 Fabrication of YBa₂Cu₃O_x (123) Thick Films on MgO Single Crystal Substrates

Mingji Jin 1, Sang-Chul Han 2, Young-Hee Han 2, Sang-Joon Kim 2, Tae-Hyun Sung 2, and Kwangsoo No 1. 1 Electronic and Optical Materials Laboratory, Dep. of Mat. Sci. & Eng.Korea Advanced Institute of Science and Technology 373-1 Kusung-dong, Yusung-gu, Taejon, Korea 305-701. 2 Power System Laboratory, Korea Electric Power Research Institute, 103-16 Munji-dong, Yusung-gu, Taejon, Korea 305-380.

Presenting Author: E. Kim

YBa₂Cu₃O_x thick films were fabricated on MgO single crystal substrates to induce the preferred orientation by screen printing of powder mixture of Y₂BaCuO₅ (211), BaCuO₂ (011) and CuO, partial melting and controlled cooling process. The precursor powders were prepared with the solid state sintering and attrition milling, and the powder mixture was prepared with attrition milling. The paste of the powder mixture was fabricated using a standard paste making process, and the thick films of few ten microns were fabricated multiple screen printing. The films were heated at different temperatures from 990 to 1050C and cooled down at different cooling schedules. To reduce a possible low temperature reaction, the sample was put into the furnace maintained at the objective temperature. The microstructure and phases of the thick films fabricated with different temepratures, different thickness and cooling schedules were analyzed using XRD, pole figure, phi scan, rocking curve, SEM, EDXS and polarizing microscope. The superconducting properties were analyzed using four point probe method

1PO4-27 Thermal stability, synthesis and Mossbauer characterization of the prospective Pr_1.9Ba_1.1Cu₃O_7+y substrate material

A.V. Kravchenko 1, I.S. Bezverkhy 2, E.A. Goodilin 2, and Yury D.Tretyakov 2. 1 Kurnakov’s Institute of General and Inorganic Chemistry, Moscow. 2 Department of Chemistry, Moscow State University, Moscow 119899, Russia.

Presenting Author: Y.D. Tretyakov

In the present work Pr_1+xBa_2-xCu₃O_7+y substrate materials (Pr123ss) were studied to control a mismatch with deposited superconducting films by the x parameter adjustment. The homogeneity field of Pr 123ss was investigated by XRD, DTA, in situ resistivity measurements and high-temperature optical microscopy, and a new solution with x=0.9 (a=3.848, b=3.900, c=11.581Å) was prepared by freeze-drying followed by sintering at 950° C in air for 60h. The solution stayed permanently orthorhombic and possessed a small oxygen nonstoichiometry. To probe local oxygen arrangement, the sample was doped with 57Fe (Pr_1.9Ba_1.1Cu_2.85Fe_0.15O_7+y) at a temperature (1000° C) above the thermal stability of the pure phase (955-958° C). Its Moessbauer spectrum can be fitted by a superposition of two doublets (IS1=0.04; IS2=0.23 mm/c; QS1=0.50, QS2 =1.49 mm/s). The spectral contribution of the second doublet rises up from 53 to 65% after full oxygenation at 350° C in oxygen. It can be assumed that iron atoms are localized in pyramids formed by the CuO₂ plane and apical oxygens; another part of 57Fe becomes five-coordinated after oxygenation. Therefore extra oxygen atoms exist in the basal plane at fixed positions prohibited in the x=0 case. The Pr_1.9Ba_1.1Cu₃O_7+y phase is isostructural with Nd_1.9Ba_1.1Cu₃O_7+y, and can be applied as a new substrate material.

This work is supported by RFBR (grant 98-03-32575).

*1PO4-28 Time-Resolved Optical Studies of BSCCO films

Gino P. Segre and J. Orenstein, Physics Dept., University of California, Berkeley, MS 2-300 Lawrence Berkeley Lab, Berkeley,CA 94720

Presenting Author: G.P. Segre

We have examined the temperature dependence of the transient optical response of BSCCO films to ultrashort Ti:Sapph laser pulses. The behavior of the fractional change in reflectivity (D R/R) is clearly associated with the onset of superconductivity. Both the amplitude of the response and its characteristic decay time increase with decreasing temperature, consistent with an increase in superfluid density. We discuss the temperature dependence well above T_c and its relation to the formation of the pseudogap. Finally we observe a nonlinear pump power dependence suggestive of a bimolecular pathway for quasiparticle recombination.

1PO4-29 Electrical Properties of Y-Doped Bi2212 Thin Film

Kazuo Sakai, Kazuhiko Hayashi, Yang Qi, Hironaru Murakami, and Toshimichi Ito, Dept. of Electrical Eng., Osaka University, 2-1 Yamada-oka, Suita-shi, Osaka 565-0871, Japan

Presenting Author: K. Sakai

Recently much attention has directed to an underdoped region in high-T_c superconducting oxides for the purpose of solving the superconducting mechanism. However, the replacement of Ca²⁺ to Y³⁺ ions partly must be carried out to obtain the underdoped Bi2212 single crystal despite the crystal quality obtaining in the latter method is disintegrated.

As the Y-doped Bi2212 thin film has not been reported yet, to our knowledge, we tried to fabricate its thin film and to investigate the electrical properties. The important requirement to lead this trail to the success is dependent on whether the fabrication technique of high quality thin film of the Bi2212 single phase is completed. Nowadays we have been engaged in the high quality Bi2212 thin film fabrication using the ion beam sputtering or MBE method, and the basic fabrication conditions for theBi2212 thin film fabrication has been reported. The Y-doped Bi2212 thin film has been grown up under such background.

Temperature dependence of Hall coefficient and resistance in these thin films obtained were measured. Although the reduction of carrier concentrations in the part of them were certificated ranging 1020 to 1019 /cm³, the reproducibility is checked at present since the systematic relationship between T_c and the carrier concentration is not obtained yet.