No abstract

Flux pinning properties of pinning centres having correlation along the c-axis in epitaxial YBCO films were investigated by measuring the magnetic-field angle ψ-dependence of the critical current density JC and the E–J-characteristics. YBCO films were prepared by using the pulsed-laser-deposition method on four different substrates at three different target-to-substrate distances D. The ψ-dependence of JC showed large peaks when magnetic field B was applied parallel to the c-axis (), and we observed two types of JC-peak: that is, a broad peak for the films deposited at small D (50–60 mm), and a narrow peak for the films deposited at large D (112 and 142 mm). The E–J-characteristics followed the power law, , and the ψ-dependence of the n-value also showed broad peaks around for the films deposited at D = 50–60 mm, and narrow peaks for the film deposited at D = 112 mm. Based on these results and our previous microstructural observations by AFM and TEM, we confirm that the broad-angle flux pinning effect around may be attributed to a high density of elongated precipitates, and the narrow-angle pinning effect around may be attributed to dense planar defects parallel to the c-axis.

Niobium nitride-niobium nitride Josephson tunnel junctions have been fabricated using amorphous magnesium oxide (a-MgO) films as barriers. These junctions have excellent tunneling characteristics. For example, a large gap voltage (Vg=5.1 mV), a large product of the maximum critical current and the normal tunneling resistance (IcRn=3.25 mV), and a small subgap leakage current (Vm=45 mV, measured at 3 mV) have been obtained for a NbN/a-MgO/NbN junction. The critical current of this junction remains finite up to 14.5 K.

No abstract

Magnetic-field and temperature dependence of the critical current density J c is investigated in epitaxial Bi2Sr2Ca2Cu30 x thin films. For the magnetic field H applied parallel to the c axis, the flux pinning force density F p (=J C B) exhibits clear scaling behavior when H is normalized by the irreversibility field //*. The maximum pinning force density scales linearly with //*. This is the first observed scaling of F p in high-quality thin films of Bi oxides, which we can reasonably explain with flux-creep theory by assuming that the activation energy is proportional to the flux line spacing. PACS numbers: 74.60.Ge, 74.60.Jg, 74.72.Hs, 74.76.BzBi oxide cuprate superconductors, Bi2Sr2CaCu20 x (Bi-2212) and Bi 2 Sr 2 Ca2Cu30 x (Bi-2223), are technologically important because relatively high transport critical current densities, J c -~ 10 8 -10 9 A/m 2 , can be obtained in polycrystalline samples such as Ag-sheathed tapes [1,2]. J c , however, declines precipitously with magnetic field at moderate temperatures (> 30 K) because of weak flux pinning [1]. The mechanism controlling J c in the Bi oxides is one of the key issues for the application of this material. The irreversibility temperatures and the irreversibility fields H* above which magnetization becomes reversible and hence J c becomes zero are much lower than those observed in YBa 2 Cu 3 0 7 (YBCO) [3].Recently, we prepared high-quality, epitaxial Bi-2223 thin films by metalorganic chemical vapor deposition, showing high critical temperatures T c of 92-97 K [4]. These films have the highest reported J c for the Bi-oxide system: J c > 10 9 A/m 2 at 77.3 K in high magnetic fields (1-8 T, H\\a-b plane) [4,5]; zero field / c = 1.3xl0 10 A/m 2 at 70 K and 10 11 A/m 2 at 30 K [6]. In this Letter we report clear scaling behavior over a wide temperature range, 20-60 K, for the flux pinning force density F p , calculated from J c values measured in fields parallel to the c axis. Such F p scaling has been observed in the transport J c of YBCO polycrystalline thin films [7] and epitaxial films [8,9] in limited temperature ranges, but until now was not reported for high-quality Bi-oxide thin films. The F p scaling, which is technologically important in predicting the J c behavior in magnetic fields, is reasonably explained using flux-creep theory by assuming that the activation energy is proportional to the flux line spacing tfo.