Thermodynamic properties of superconducting iridium

The superconducting transition temperature of pure Ir is found to be 0.1125 K (±0.0005 K). The critical magnetic field as a function of temperatureH _c(T) has also been measured. From these data it is determined thatH _c(0) is 16.00 G (1 G corresponds to 10^–4 T), (dH _c/dT)_T=Tc is 235 G/K, the linear coefficient of normal state electronic specific heat is 3.19 mJ/mole-K², and the energy gap anisotropy parameter a ² is 0.048. This value for a ² is the largest of any superconducting element so far observed, and its significance in determining the superconducting properties of Ir is discussed. By using the large supercooling effects noticed nearT _c, the Ginzburg-Landau parameter x₀ is found to be 8.6×10^–3. The effects of impurities onT _c and on the magnetic behavior of Ir are also discussed.     

Magnetic and superconducting properties of niobium oxides

Low temperature magnetic susceptibilities of niobium oxides have been measured. A homogeneous sample of NbO prepared by arc melting and checked by using X-ray and metallographic techniques exhibits no ferromagnetism but becomes superconducting at 1.20°K.     

Improved superconducting properties of multifilamentary niobium carbonitride wire

Transport and magnetization measurements have been made on 6μ, 720 filament niobium carbonitride yarn. The magnetization data is discussed in terms of the hysteretic loss, the temperature dependence of Jc, the matrix-superconductor bond quality and the doping effects on the high field Jc's. Transport properties are compared to properties obtained from magnetization data. In addition, weak coupling has been investigated in low conversion fibers. The V-I curves show numerous voltage steps, some corresponding to an order of magnitude increase in resistivity.     

Investigation of the properties of superconducting niobium nitride films

Niobium nitride films with a value for T_c of up to 17.3 K have been prepared by reactive magnetron and diode sputtering in Ar and N₂ gas mixture. Alteration of T_c and N₂ ion implantation into NbN films was studied. It is shown that small doses of implanted ions cause an abrupt decrease of T_c to 12.8 K. Annealing at 900°C restores the high T_c By the tunnel effect, the magnitude of the energy gap is 3.05 Me V at 1.56 K for NbN with T_c = 17.1 K and $\text{22/kT}{}_{\mathrm{c}}\text{= 4.14}$. The existence of an undersurface layer has been discovered, whose width is of the order of the coherence length, with a lower T_c, of 12.0–12.8 K. The superconductivity of this layer above T_c is due to the proximity effect. NbN tunnel junctions are made with a density of the Josephson current i=(1.7-2.87) · 10³ A cm^?2.     

Thermodynamic and spectral properties of ultrasmall superconducting grains

We study the problem of superconductivity in the canonicalensemble which applies to recent experiments on metallicnanoparticles. Spectral properties which can be expressed asuniversal functions allow a quantitative characterization ofthe crossover to superconductivity in finite metallicsystems. Parity effects in the ground state and excitationspectrum are discussed. We discuss the thermodynamicquantities as well. The susceptibility for odd grains shows areentrant behavior with temperature. This behavior may be anobservable signature of superconducting correlations inultrasmall grains.     

Equilibrium magnetization properties of irreversible superconducting niobium with peak effect

Measurements of the equilibrium magnetization curve of type II superconductors have usually only been possible on reversible specimens. In this paper, a method to measure the equilibrium magnetization curve of irreversible type II superconductors is described. Results obtained by this method from a hysteretic Nb specimen exhibiting the peak effect are reported. They show an irregularity in the shape of the equilibrium magnetization curve in the neighborhood ofH _c2 which corresponds to a minimum of the compressional modulusK of the flux line lattice. The minimum ofK also corresponds with the maximum of the critical current densityj _c, which strongly suggests that in this case the peak effect is originated by a “soft point” of the flux line lattice. The observed anomaly ofK is discussed in terms of a possible first-order phase transition in the flux line lattice.     

Miniature sapphire windows for superconducting niobium cavities

Miniature sapphire windows which allow coupling without ? degredation, have been designed for superconducting niobium cavities.     

Synthesis and characterization of superconducting nanocrystalline niobium nitride

Nanocrystalline niobium nitride (NbN0.9) was successfully synthesized at 600 degrees C through a solid-state reaction. The synthesis was carried out in an autoclave by using NbCl5 and NaN3 as the reactants. The X-ray powder diffraction pattern indicates the formation of cubic NbN0.9. Transmission electron microscopy images show that typical NbN0.9 crystallites are composed of uniform particles with an average size of about 30 nm and nanorod crystallites with a typical size of about 40 x 2500 nm. Magnetic measurements exhibited that a superconducting transition occurred at 15.4 K for the NbN0.9 product.     

Electrical resistivity and peak effect in superconducting niobium

The flux flow resistivity _f has been measured in foils of high purity niobium as a function of magnetic field and temperature. The magnetic field dependence of _f just belowH _C2 is much stronger than predicted by the recent theory of Maki for the pure limit. NearH _C2 a peak in the critical current is observed that is independent of any voltage criterion.Based on work performed partly under the auspices of the U.S. Atomic Energy Commission.     

Thermodynamic stability of superconducting magnetic states

The different magnetic states of superconductors are characterized by extensive and intensive parameters. The elements of the entropy matrix are determined from the derived total entropy density, which contains all possible interactions. The element of the conductivity matrix characterizing the vortex motion is also evaluated. Stability criteria for the Meissner, flux creep, and flux flow states and the value of the needed external generation are given, as are the nonequilibrium deviations, thermodynamical forces, currents and entropy productions for the different cases. The nature of the unstable flux jump state is discussed.     

Development of superconducting niobium accelerating structures for heavy ions

Results of tests of two new designs for superconducting niobium resonant cavities are presented. Both types resonate at 145.5 MHz and accelerate most efficiently for particle velocities β= v/c = 0.16. One resonator is of the split-ring type, but of a simpler design than a previously reported β=.16 unit. Although the surface fields are higher, the performance is somewhat better than for the earlier design: an accelerating field Ea= 4.3 MV/m has been obtained at 4.2K with 4w of rf input, where Eais defined as the energy gain per unit charge for a synchronous particle averaged over the interior resonator length. The other resonator is an 8-inch OD tapered coaxial quarter-wave line terminated with a drift tube of 1.50 inch aperture. At 4.2K, this resonator exhibits a low-level Q of 2×10⁹, and has achieved Ea= 4.7 MV/m with 2.8w of rf input.     

Precise slit-width control of niobium apertures for superconducting LEDs

We introduce a novel three-step procedure for precise niobium (Nb)-etching on the nanometer-scale, including the design of high contrast resist patterning and sacrifice layer formation under high radio frequency (RF) power. We present the results of precise slit fabrication using this technique and discuss its application for the production of superconducting devices, such as superconductor-semiconductor-superconductor (S-Sm-S) Josephson junctions. For the reactive ion etching (RIE) of Nb, we selected CF(4) as etchant gas and a positive tone resist to form the etching mask. We found that the combination of resist usage and RIE process allows for etching of thicker Nb layers when utilizing the opposite dependence of the etching rate (ER) on the CF(4) pressure in the case of Nb as compared to the resist. Precise slit-width control of 80 and 200 nm thick Nb apertures was performed with three kinds of ER control, for the resist, the Nb, and the underlying layer. S-Sm-S Josephson junctions were fabricated with lengths as small as 80 nm, which can be considered clean and short and thus exhibit critical currents as high as 50 μA. Moreover, possible further applications, such as for apertures of superconducting light emitting diodes (SC LEDs), are addressed.     

The lattice thermal conductivity of normal and superconducting niobium

The lattice thermal conductivity of superconducting and normal Nb as limited by the interaction of phonons with electrons has been deduced from measurements in the superconducting state. The results indicate that the mean free paths of transverse and longitudinal phonons are similar, ～4×10^?5 T ^?1 (cm K), in the normal state. Comparison is made with measurements on other metals. A compilation is included of the ratio of lattice conduction in the normal state to that in the superconducting state, based on the BCS theory of superconductivity.     

Study of non-equilibrium signals in superconducting single crystal niobium

We are developing a low temperature detector for dark matter based on single crystal superconducting Nb. This work requires a detailed knowledge of the behaviour of phonons and quasiparticles in Nb which arise from particle interactions. We discuss the use of superconducting tunnel junctions (STJs) and series connected arrays of superconducting tunnel junctions (SASTJs) to detect the high energy phonon flux arising from laser and particle interactions. The signal shapes and their variation with temperature for Al-Ox-Al and Sn-Ox-Sn junctions are discussed.     

50 Hz loss measurements on superconducting niobium coaxial conductors

A method is described for the measurement of ac losses in a cylindrical conductor forming part of a coaxial superconducting pair. Results given for 25 μm niobium are substantially lower than previous values obtained for this material when tested in the form of narrow strips, and show that, if used as the conductor in a superconducting power cable, the material could carry a peripheral current density of 360 A cm^?1 with tolerable losses (< 10 μW cm^?2) at temperatures of up to 7 K.     

Field history effect of the flux gradient in superconducting niobium

The flux density gradient in a plastically deformed niobium single crystal was measured. The profiles in increasing and decreasing field are not symmetric, and depend on the history of the magnetic field. This effect was observed up to 0.9H _c2, although it was more pronounced at lower fields. These results, as well as an observed asymmetry of minor hysteresis loops, cannot be understood by the critical state model. We suggest that they are related to the creation of dislocations in the flux line lattice.Work partially supported by FAPESP.     

Surface effect on the critical current density of superconducting niobium

Experiments are described which measure the critical current density of single and polycrystal niobium samples. The density measured is related to the ac losses of the sample caused by an applied ac field in the presence of a dc magnetic field. It was found that the critical current density depends upon the surface of the sample increasing with the surface smoothness.     

Deposition of superconducting niobium coatings on titanium from molten salts

Superconducting niobium coatings have been electrodeposited from molten salts onto titanium substrates with a copper, nickel, or molybdenum protective layer, and their structure and magnetic properties have been studied. The coatings are shown to be suitable as a starting material for the superconducting layer of the rotor of cryogenic gyroscopes.