P. M. Tedrow

MEASUREMENTS OF THE KINETIC INDUCTANCE OF SUPERCONDUCTING LINEAR STRUCTURES.

The kinetic inductance Lk is associated with the inertial mass of the current carriers. For a long superconductor of length l and very small cross‐sectional area σ, Lk = (m* / nse*2) (l/σ) and is the main contribution to the temperature‐dependent inductance LT. We have measured LT in superconducting tin wires and thin film meander lines by a technique which uses a counter to determine frequency changes of a 15 MHz tunnel‐diode oscillator whose tank circuit contains LT. The measured frequency changes are proportional to z = [λ(t)/λ(0)]BCS for wires and thick films; for thin films the frequency changes are proportional to z2. This result agrees with calculations of LT from the London theory and the values of λ(0) agree well with the values expected for these samples. This technique allows measurement of the carrier concentration or penetration depth in thin films over the whole H, T plane of the superconducting state and can be independent of the penetration law assumed. Frequency shifts observed with thin film alumimum meander lines have been used to detect changes of temperature of 5∼10−7 °K, changes of magnetic field of 10−5 G, and changes of current associated with one quantum in a flux quantum magnetometer.

Tunneling characteristics of amorphous Si barriers

Tunnel junctions using a barrier of amorphous silicon (a‐Si) between normal and superconducting metals were studied at temperatures from 300 to 0.45 K. These junctions were reliably made by depositing on a 77 K glass substrate Ni or Au, a‐Si 60 to 100 A thick, and then Al. It was demonstrated that the dominant conduction process was elastic tunneling by the presence of structure caused by the superconducting energy gap of Al, and by comparing measurements of the voltage, temperature, and barrier thickness dependence of the conductance with theory. The effective barrier heights were grouped close to 2×10−2 eV. A semiquantitative argument suggests that the barriers controlling the elastic tunneling are much the same as those controlling the phonon‐activated variable range tunneling at higher temperatures. Although a‐Si barriers can be formed reliably and have low leakage, the low barrier height leads to large nonlinearity even at low voltages.

Longitudinal magnetoresistance of CrO2 thin films

The longitudinal magnetoresistance (LMR) of thin films of the putative half-metallic ferromagnet CrO2 deposited by chemical vapor deposition onto TiO2/Si(111) substrates has been measured in the temperature range 0.55 K<T<4.2 K in magnetic fields up to 7 T. The dependence of the LMR on small changes in the angle between the applied field and the substrate plane was also measured at T=1.3 K. X-ray diffraction measurements showed that these films have a textured rutile structure with a axis and 〈110〉 orientations normal to the substrates. The LMR of these films at about 3 T was about twice that of similar films that showed only a 〈100〉 orientation normal to their ZrO2 substrates. A change in the substrate orientation relative to the applied field direction of 10° caused a 25% decrease in the magnitude of the LMR at 3 T. The results suggest that the existence of the 〈110〉 crystallite structure strongly affects the magnetoresistance.

Properties of Very Thin Aluminum Films

Thin films of aluminum have been produced in the thickness range of 1000–30 A in small area samples with photoetched edges. The superconducting transition temperature Tc, the critical magnetic field Hc, and the room‐temperature conductivity have been measured as a function of thickness d. The results indicate that films as thin as 30 A act essentially as uniform layers in which the crystal size is approximately equal to the film thickness. The transition temperature was found to vary linearly with d−1. Hc (T) was measured from Tc to 0.4°K. For thickness from 1000 to 200 A, Hc∼d−3/2, as expected from the Ginzburg‐Landau theory. For d <200 A, Hc is paramagnetically limited to about 49 kOe = 19.6Tc, slightly above the Clogston limit.

Superconductor‐insulator‐superconductor tunneling in Ba1−xKxBiO3 grain boundaries

We have prepared single grain boundaries of the superconductor Ba1−xKxBiO3 by growing epitaxial thin films of this compound on SrTiO3 bicrystal substrates. The four‐terminal current‐voltage characteristics of the grain boundaries showed clear superconductor‐ insulator‐superconductor (SIS) tunneling behavior. The leakage at zero bias was smaller than 0.3% at 4.2 K. The differential conductance displayed sharp symmetric peaks at 2Δ close to 6.5 mV. At higher bias, an increase in conductance proportional to V2 was observed. The temperature dependence of the conductance was found to be in qualitative accord with conventional SIS theory.

Two-dimensional character of the magnetoresistance in Nd1.85Ce0.15CuO4−δ thin films

Abstract The magnetoresistance of in situ thin films of Nd 1.85 Ce 0.15 CuO 4−δ in a metallic but non-superconducting state due to excess oxygen was measured at low temperatures in transverse fields. At 4.2 K and for fields perpendicular to the CuO 2 planes, we find a reduction of the resistance by 16% in a field of 13.2T, with no sign of saturation. The effect in parallel fields is much smaller, and the field dependence is different. The perpendicular field data are fit by the theory of weak localization in two dimensions. The fits indicate a low spin-orbit scattering rate. In situ superconducting samples were measured in a perpendicular field up to 30 T, and also show negative magnetoresistance and an upturn of the normal state resistance at low temperature.

Improved laser-ablated thin films of NdCeCuO by use of N2O

Thin films of the electron‐doped superconductor Nd1.85Ce0.15CuO4−δ were prepared by laser ablation in O2 and N2O ambients. Although films prepared in an O2 ambient need to be reduced to become superconducting, the use of a lower oxygen pressure during deposition produces inferior films. A more metallic character can be obtained by increasing the O2 pressure during deposition. The substitution of N2O for O2 results in films that are superconducting without a separate reduction step, with a sharp transition at about 16 K. A short vacuum anneal at the deposition temperature yields reproducible films with Tc=20 K, without change in the transition width.

In situ deposition of superconducting Bi‐Pb‐Sr‐Ca‐Cu‐O thin films on MgO(100) substrates by rf magnetron sputtering and laser ablation

Superconducting thin films of Bi‐Pb‐Sr‐Ca‐Cu‐O (BPSCCO) have been prepared in situ by magnetron sputtering and laser ablation onto heated MgO (100) substrates. Post‐deposition annealing at higher temperatures was not required to obtain superconducting BPSCCO films, and a narrow temperature window in a very similar temperature region was observed for the growth of oriented films for both the laser ablation and magnetron sputtering technique. The best films were smooth and shiny with their c axes oriented perpendicular to the substrate surface but scanning electron microscope analysis showed particulates on the film surface. Room‐temperature resistivity values of the order of 2–3 mΩ cm were obtained for these films. At 70 K and 8 T, the resistivity ratio for the H⊥ and ∥ to the plane of the film was ∼25 for an applied current density J of 0.8 A/cm2. Rutherford backscattering analysis on the laser‐ablated and sputtered films indicates that the BPSCCO films had 2223 composition while the x‐ray diffraction stu...

Proximity effect in ferromagnetic metals

Abstract Spin polarized tunneling measurements show that ferromagnetism is suppressed in films of Ni thinner than three atomic layers when in contact with Al. In contrast, ferromagnetism was found in Fe, Co, Gd, and Tm films only one atomic layer thick. Present results on Ni agree with anomalous Hall effect measurements, but do not agree with photoemission results.

Measurement of the proximity effect in ferromagnetic‐normal metal thin film sandwiches

A spin polarized tunneling measurement shows that ferromagnetism is suppressed in films of Ni thinner than three atomic layers when in contact with Al. In contrast, ferromagnetism was found in Fe, Co, Gd, and Tm films only one atomic layer thick when backed with Al. The present results agree with measurements of Ni, Fe, and Co using the anomalous Hall effect, but do not agree with photoemission results on Ni.