Hironori Wakana

EuBa 2Cu 3O 7-δ Thin Films Grown on Sapphires with Epitaxial CeO 2 Buffer Layers

Epitaxial growth of CeO2 films on Al2O3 (102) substrates and the effect of CeO2 buffer layers on the growth orientation and superconducting properties of EuBa2Cu3O7-δ (EBCO) were investigated. CeO2 and EBCO films were prepared by rf and dc magnetron sputtering, respectively, and were characterized by X-ray diffraction (θ-2θ scan and scan), atomic force microscopy (AFM) and high resolution scanning electron microscopy. Epitaxial (001) CeO2 films were obtained at an off-center distance (D on-off) of 3.5 cm and a substrate temperature of 660°C. The structural and superconducting properties of EBCO films deposited at 650°C depended on the thickness of the CeO2 buffer layer. The EBCO films deposited on CeO2 50-400-A-thick buffer layers had T ces of 90 K or above. The high-T c EBCO films had in-plane epitaxial orientation relationships of Al2O3 [110]varparallelCeO2 [100]varparallelEBCO[110]. The EBCO films on the thin CeO2 buffer layers had rectangular grains similar to those on MgO(001) substrates. The critical current density of the EBCO films with T ce=90 K was about 6 ×105 A/cm2 in zero field at 77.3 K. The T ce varied largely and decreased with increasing CeO2 buffer layer thickness above 500 A. AFM observation of a 1000-A-thick CeO2 film showed growth of bamboo-like crystal grains 1700 A long and 300 A wide along the direction of CeO2 [110]. The ravine depths were about 100 A. The EBCO films on the thick CeO2 buffer layer (>500 A) exhibited poor superconducting behavior and gave (103) or (110) diffraction peaks.

HIGH ENERGY HEAVY ION IRRADIATION DAMAGE IN OXIDE SUPERCONDUCTOR EUBA2CU3OY

Defect production and recovery are studied in oxide superconductor, EuBa2Cu3Oy, irradiated with high energy (80 MeV–3.80 GeV) heavy ions. X-ray diffraction measurements show that the c-axis lattice parameter increases linearly with increasing ion-fluence. The rate of the increase in lattice parameter can be scaled with the primary ionization rate, dJ/dx, and not with the electronic stopping power, Se. This result implies that a Coulomb explosion process triggers the atomic displacements in EuBa2Cu3Oy. From the electrical resistivity change at 100 K as a function of ion-fluence, the diameter and the resistivity of a cylindrically-shaped damage region can be determined. Irradiation induced resistivity change is markedly recovered during the annealing of the specimen up to 300 K. The dJ/dx dependence of the diameter and the resistivity of damaged region is discussed.

Defect Production Induced by Primary Ionization in Ion-Irradiated Oxide Superconductors.

Resistivity as a function of ion-fluence has been measured in situ in thin films of oxide superconductor EuBa 2 Cu 3 O y irradiated with various heavy ions in a wide energy range of 90 MeV–3.84 GeV. From the resistivity measurements, the quantities characterizing the irradiation-induced columnar defects, such as the diameter of the columnar defect and the inside-resistivity (the resistivity inside the columnar defect), have been estimated. It is found that they are not scaled with the electronic stopping power, while they are strongly correlated with the primary ionization rate which is the number of atoms primarily ionized by an incident ion per unit path length. This result suggests that the defect production is triggered by Coulomb repulsion of ionized atoms. Based on the results of thermal annealing of irradiated samples, structure of the columnar defects has also been discussed.

Se-scaling of lattice parameter change in high ion-velocity region (v⩾2.6×109 cm/s) in ion-irradiated EuBa2Cu3Oy

Abstract Swift heavy ions ( 35 Cl– 238 U) with wide energy range of 80 MeV–3.84 GeV have been irradiated to EuBa 2 Cu 3 O y oxide superconductor films, and the lattice parameter change due to electronic excitation has been measured. In the high ion-velocity region ( v ⩾2.6×10 9 cm/s), the change in crystallographic c -axis lattice parameter per unit ion-fluence varies as the 4th power of S e . However, in the low ion-velocity region ( v ⩽1.7×10 9 cm/s), the deviation from the 4th power dependence is observed. The S e scaling in the high ion-velocity region cannot be explained by the thermal spike model that is based on a radial distribution of energy deposition by secondary electrons. The change in c -axis lattice parameter per unit ion-fluence varies as the 4th power of the primary-ionization rate, d J /d x , in the whole ion-velocity region. The result supports that the Coulomb explosion triggers the atomic displacements.

Non-c-axis-Oriented EuBa2Cu3O7-δ Thin Films Grown on Al2O3(11̄02) Substrates with CeO2 Buffer Layers

EuBa2Cu3O7-δ (EBCO) thin films with different growth orientations were prepared on Al2O3(102) substrates with CeO2 buffer layers by dc magnetron sputtering. The EBCO thin films were deposited immediately after off-axis rf magnetron sputtering of CeO2(001) films. The effects of substrate temperature and oxygen concent on epitaxial orientation of EBCO thin films were examined. With the increase in oxygen concentration, the surface roughness of an EBCO thin film increased. An appropriate oxygen concentration existed. It was clarified that the orientation of an EBCO thin film depended on CeO2 film thickness. The (100)- and (110)-oriented EBCO thin films were obtained on CeO2 buffer layers 30–90 A thick and more than 700 A thick, respectively. The (100)- and (110)-oriented EBCO films had in-plane epitaxial orientation relationships of Al2O3[120] ∥CeO2[100] ∥EBCO[013] and Al2O3[110] ∥CeO2[100] ∥EBCO[10], respectively. The (100)-oriented EBCO films deposited on 50-A-thick CeO2 (001) buffer layers had Tces of 72.0 K, and (110)-oriented EBCO films deposited on 750-A-thick CeO2(001) buffer layers had Tces of 70.0 K. The Tce decreased with increasing CeO2 buffer layer thickness above 800 A. The a-axis-oriented EBCO thin films exhibited Tces of about 85.4 K on the 50-A-thick CeO2 buffer layers prepared by a self-template method.

High-quality a-axis oriented EuBa2Cu3O7−δ films on sapphire substrates

Abstract The superconducting properties of a-axis oriented EuBa2Cu3O7−δ (EBCO) films on Al2O3 (1 1 0 2) substrates with CeO2 buffer layers have been improved by introducing a template layer of PrBa2Cu3Ox (PBCO). The a-axis oriented PBCO films were grown epitaxially on 50 Axa0thick CeO2 (0 0 1) buffer layers using DC magnetron sputtering. The high-quality a-axis oriented PBCO films grew under sputtering conditions of a substrate temperature (Ts) of 620°C and a deposition rate (Rd) of 35 A/min. The a-axis oriented EBCO films were grown on the a-axis oriented PBCO template layers above 700 A in thickness. The high-quality a-axis oriented EBCO films were deposited on sapphire⧹CeO2 (0 0 1) with a-axis oriented 1000 A thick PBCO template layers at Ts=650°C and exhibited Tc endpoints (Tce) of about 86.7 K.

Recovery Treatment of EuBa2Cu3O7-δ Thin Films with Activated Oxygen Plasma

Using high-quality c-axis-oriented EuBa2Cu3O7-δ (EBCO) thin films with transition temperatures (Tce) of about 90 K, a deterioration treatment by removal of oxygen atoms and a recovery treatment were performed in a sputtering chamber. The removal of oxygen atoms by heating in a mixture gas of Ar+7.5%O2 at 7 Pa resulted in an expansion of lattice constants (c0) of about 11.86 A and semiconductive features. Two recovery methods using pure oxygen and activated oxygen plasma were examined. For the treatment with pure oxygen, signs of recovery in structural and transport properties appeared at annealing temperatures (Tsa) above 300°C at an oxygen pressure (PO2(a)) of 30 Pa, but the as-grown state was not obtained even at PO2(a)=2000 Pa. For the activated oxygen plasma treatment, where the deteriorated films were exposed to oxygen plasma and oxygen gas was subsequently introduced into the chamber, an almost complete recovery of superconducting and structural properties was achieved. The appropriate conditions for the recovery were Tsas of 500–700°C, plasma exposure time (tp) above 30 min, and oxygen partial pressures (PO2(pc)) above 200 Pa. Atomic force microscope (AFM) images of EBCO films treated at Tsas below 550°C showed the same spiral structure as those of as-grown films. The image-deformation and growth of outgrowths became obvious with increasing annealing temperature. The change in the surface morphology revealed the substantial movement of constituent atoms from Tsas of about 600°C. Because the films exhibit optimum superconducting properties in spite of the rapid cooling, it is considered that the movement of constituent atoms induces trilayered perovskite structures with an excess oxygen atom deficiency at high temperatures.

Preparation of CeO2\SrTiO3 bilayers as a barrier material for SIS Josephson junctions

Abstract CeO 2 , SrTiO 3 (STO) and CeO 2 ⧹STO as barrier materials were deposited on c -axis-oriented EuBa 2 Cu 3 O 7− δ (EBCO) films with T c endpoints ( T ce ) above 90 K, and the effects on the superconducting properties of the lower-layer EBCO were examined. For several 100-A-thick CeO 2 sputter depositions, the EBCO did not deteriorate, but the CeO 2 film contained many pinholes. On the other hand, a 50-A-thick STO deteriorated an EBCO film to T ce =52 K. When a CeO 2 ⧹STO multi-barrier was deposited, the deterioration of EBCO was reduced, and the generation of pinholes was suppressed. Activated oxygen plasma treatment was effective for the recovery of EBCO covered by a CeO 2 ⧹STO multi-barrier. In the differential characteristics of an I – V curve from a junction with a CeO 2 (50 A)⧹STO (200 A) multi-barrier, a gap structure was observed.

Irradiation temperature dependence of swift heavy ion induced defects in oxide superconductor EuBa2Cu3Oy

Abstract Thin films of oxide superconductor EuBa2Cu3Oy have been irradiated with 80–90 MeV I ions at various temperatures (281, 100, 50 and 12 K). To decide the diameter and the inside-resistivity (resistivity inside the columnar defect), the ion-fluence dependence of electrical resistivity was measured in-situ at 100 and 281 K. The irradiation at 281 K results in the production of columnar defects with much higher inside-resistivity and smaller diameter than those produced by irradiation at 100 K. The difference in the defect structure between these two irradiations can be explained as due to the difference in stability of defects primarily produced by electronic excitation. The result of annealing the sample up to room temperature after irradiation at 100 K supports the above explanation. The irradiation at 50 and 12 K, which are below the superconducting transition temperature, Tc, does not show any difference in irradiation effect from the irradiation above Tc. Effects of irradiation at 100 K and subsequent annealing up to room temperature on the Hall coefficient have also been studied.

Lattice parameter change due to electronic excitation in oxygen-deficient EuBa2Cu3Oy

Abstract The films of EuBa2Cu3Oy (EBCO) having different oxygen contents are irradiated with 125 MeV Br, 200 MeV I and 200 MeV Au, and their irradiation-induced change in c-axis lattice parameter is measured. Although the electrical resistivity of EBCO is drastically changed by varying oxygen content from y=7 to 6.1, almost the same slope of c-axis lattice parameter as a function of fluence is observed. This result shows that the electrical resistivity is not necessarily a dominant parameter that determines the electronic excitation effect.