Thermodynamic properties of superconducting niobium

The thermodynamic properties of superconducting Nb are calculated numerically from the solution of the Eliashberg equations on the imaginary axis for several possible electron-phonon spectral densities ²()F(). Comparison with experiment is made in order to see which spectrum gives the best agreement, and functional derivatives with respect to ²()F() are used to estimate how this agreement might be improved by small changes in ²()F(). Possible gap anisotropy effects are also considered with the help of a simple model anisotropy for the interaction.Research supported by Natural Science and Engineering Research Council of Canada.     

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.     

Magnetic properties of superconducting tin-europium-molybdenum sulfides

In an investigation of the possible coexistence of superconductivity and magnetism, we have measured the Mössbauer effect of¹⁵¹Eu and¹¹⁹Sn in Eu _z Sn _1–z Mo₆S _y (0z1,y7) between 0.05 and 4.2 K. The Eu spectra show a magnetic hyperfine splitting with a field of 27.3 T below 0.1 K; no fields in excess of 0.1 T have been observed at the Sn nuclei. For 0.1<T<0.5 K, the Eu Mössbauer spectra consist of a superposition of the static hyperfine spectrum observed forT0.1 K and a spectrum produced by electronic relaxation. In this intermediate temperature range the spectra can be understood as arising from ordered clusters in a paramagnetic surrounding, features we consider to be characteristic of a spin glass. The interaction between the Eu ions, and that between Eu ions and the superconducting Mod electrons, seem to be very weak.     

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.     

Magnetic properties of proximity-induced superconducting copper and silver

The magnetic properties of micrometer thick, proximity-induced superconductors have been investigated in the temperature range 5×10^–4 T _CNS T _CNS . HereT _CNS is the transition temperature of the NS proximity system. By means of ac susceptibility measurements and dc magnetization curves in copper and silver, we have determined the breakdown fieldH _b , the supercooled fieldH _sc and the superheated fieldH _sh , as functions of temperature and normal metal thicknessd _N . Precise magnetization curves of specimens withd _N <15 µm at the lowest temperatures gave the screening length as function of the field H. From these data, the spatial dependence of the local critical fieldH _c (x) and of the induced order parameter _N (x) in theN side were derived in the case _N (x)>k _B T and xK _N ^–1 . HereK _N ^–1 is the Cooper pair penetration length, andx the distance form the SN interface.     

Magnetic properties of pure superconducting vanadium foils

We report on the magnetic properties of very pure superconducting vanadium foils (a resistivity ratio of 1500 in the starting single-crystal rod and of 300 in the cold-rolled foils). The magnetization curves are analyzed to obtain, as a function of temperature, the critical magnetic fields, the Ginzburg-Landau parameters, and the characteristic superconducting lengths. The experimental data are compared with the theoretical calculations of Eilenberger and with other measurements reported in the literature.Research Fellow of the Interuniversity Institute for Nuclear Sciences, Belgium.     

Effect of metal oxides addition on the superconducting properties of YBaCuO

The superconducting properties of YBCO superconductor with ZrO2 addition prepared by different contents were prepared by a unique method so called thermal pyrolysis process to study the effect of the zirconium oxide on the electromagnetic properties and superconducting mechanism of the superconductor. The critical temperature of YBCO superconductor was not predominately dependent upon the zirconium oxide. The maximum magnetism was observed by adding 2% zirconium oxide. The addition of zirconium oxide forming a pinning center of magnetic flux due to ZrO2 phase which was related to the change of electromagnetic properties of the YBCO superconductor. The result indicates that magnetization is proportional to the number of magnetic flux lines passing through the sample. The added ZrO2 appear to contribute the increasing Vickers hardness.     

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.     

Magnetic properties of proximity-induced superconducting matrix in multifilamentary wires

Critical current densities J_cp of proximity-induced superconducting matrices in NbTi multifilamentary wires are estimated from measured twist-pitch dependence of magnetization. The values of J_cp are 2–4 orders of magnitude smaller than those of the superconducting filaments and decrease rapidly by raising temperature T and magnetic field B_e. The interfilamentary spacing d_N of submicrons results in zero screening length at measured temperatures ranging 4.5 to 8 K and the induced superconductivity is suggested to be type-II. The upper critical field B_c2p is obtained by applying the scaling law to J_cp data. B_c2p's at 4.5 K are 1.2–3.0 T for d_N = 0.20–0.59 m. The Cooper pair penetration length K _N ^–1 estimated from the temperature dependence of B_c2p shows the clean limit characteristic as T^–1 and 0.32 m for d_N = 0.59 m wire at 4.5 K. K _N ^–1 estimated from the temperature dependence of J_cp agrees consistently with those from B_c2p(T).     

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.     

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.     

Miniature sapphire windows for superconducting niobium cavities

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

Synthesis, structure, and properties of mixed niobium(IV,V) oxides

This review paper summarizes data on mixed oxides containing niobium in an intermediate valence state, between 4+ and 5+. The major preparation procedures are considered, and the effects of different process parameters—composition of the starting reagents, temperature, pressure, duration, and environment—are analyzed from the viewpoint of the preparation of bulk, single-phase materials with niobium in a controlled formal oxidation state. The optimal synthesis conditions are established for reduced niobates with different structures. The oxidation behavior of the niobates(IV,V) is examined as a function of the structure type. The structural and electrical properties of the reduced niobates are shown to correlate with the carrier concentration (formal oxidation state of niobium). The major factors determining the carrier concentration in the niobates are identified. The results are used to establish the conditions under which a semiconductor—metal transition is possible in reduced niobates     

Magnetic and superconducting nanowires

This article is focused on the use of electrodeposition and of various nanoporous templates for the fabrication of metallic nanowires made from single metals (Ni, Co, Pb, Sn), alloys (NiFe, CoFe, CoPt), and multilayers (Co/Cu, NiFe/Cu). An overview is given of our recent studies performed on both magnetic and superconducting nanowires. Using different approaches entailing measurements on both single wires and arrays, numerous interesting physical properties have been identified in relation to the nanoscopic dimensions of these materials. Finally, various novel applications of the nanowires are also discussed.     

Non-steady state solidification of superconducting oxides

The non-steady state solidification of RE123 (RE=Y, Nd) superconductive oxides was investigated by using an undercooling growth method. In both the Y- and Nd-systems, RE123 crystals grew steadily in the initial growth stage, but the growth rates decreased gradually in the later stage under the constant growth temperature. In the case of Nd-system, the substitution ratio of Hd/Ba in the grown Nd123 crystal was found to be gradually changed. The liquid compositions revealed that the liquid composition was close to the ternary equilibrium point after the growth rates decreased. This non-steady growth of Nd123 crystal is intrinsically caused since the composition of grown Nd123 solid solution is not on the 422-123 line and the residual liquid composition gradually shifts to the ternary equilibrium point to compensate for the mass balance during 123 growth. On the other hand, in the case of the Y-system, the evaluation of the volume fraction of Y211 particles showed that the volume fractions in the grown crystal were lower than those expected from the initial composition. On the contrary, those in liquid were higher and had similar values irrespective of the initial compositions and growth conditions. This accumulation of 211 phase particles near the 123/liquid interface would reduce the diffusion of the solute elements, which have the partition ratio of less than unity, away from the 123 interface. Therefore, such a large volume fraction would enhance the compositional shift close to the growth interface and cause locally a final transient to decrease the growth rate of the 123 crystal, if there is a compositional shift from the line connecting 123 and 211 compositions.     

Magnetic and transport properties of 214 oxides: Dependence on doping

We consider how doping can be described in terms of the charge-transfer insulator concept. We discuss and compare a few models for the band structure for the doped charges. This has led us to the conclusion that the band structure stability problem is one of the main issues in any correspondence between results for thet-J model and, say, the three-band model for the slightly doped layered oxides. The stability criterion is formulated and its implications discussed. Provided a phenomenological conduction band is chosen to satisfy the criterion of stability, a detailed picture of how dopants influence the spin wave spectrum atT=0 is presented. The basic physics for the destruction of the antiferromagnetic (AF) long-range order is rather model-independent: the long-range order (atT=0) disappears due to the Cerenkov effect when the Fermi velocity first exceeds the spin wave velocity. We then discuss the overall spectrum of spin excitations and see that the spin wave attenuation for x<x _c,T= 0 due to Landau damping appears in the range of magnon momentak(x)=2m ^* s±x. We also argue that in the presence of superconductivity, the Cerenkov effect is eliminated due to the gap in the spectrum. This may restore the role of the AF fluctuations as the main source of dissipation at the lowest temperatures. A brief discussion of how interaction with magnons may affect the hole spectrum concludes the paper.     

Magnetic properties of nickel and cobalt catalysts supported on nanoporous oxides

The aim of this work is to use magnetic measurements as a research tool in the study of possible metal-support interactions in nickel and cobalt nanoporous catalysts. Several physicochemical techniques, namely nitrogen adsorption, X-ray diffraction, temperature-programmed reduction and chemical analysis, were used to analyze the role of the preparation method and the nature of the support on the existence of such metal-support interactions and to relate them with the magnetic response of these nanoporous systems. The catalysts were prepared by incipient wetness impregnation and precipitation-deposition with two commercial oxides, gamma-Al2O3 and SiO2, as supports. The magnetic behavior of the catalysts is drastically affected by the existence of interactions between the metal and the support during the preparation procedure. The samples with weak metal-support interactions have characteristic magnetic behavior of antiferromagnetic metal oxide nanoparticles, while the ones having strong interactions display spin-glass like behavior.