Hard Superconductors Research Articles

On Electron Tunneling from a Hard Superconductor in the Mixed State

Gorkov's derivation of the Ginzburg-Landau equation from BCS theory is reformulated in such a form that one can apply it to dynamical processes involving quasiparticle excitations. As the simplest example, the method is applied to electron tunneling from an intrinsic hard superconductor in Abrikosov's mixed state. It is shown that near the transition point one obtains the usual tunneling current simply averaged over Abrikosov's distribution of energy gap. Some numerical estimates are made of the dependence of I-V characteristics on magnetic field in the case of Nb.

Experimental evidence of degradation effects in short samples of hard superconductors : Fakan, John C., Lewis Research Center, and Edward R. Schrader, Radio Corp. of America, NASA Technical Note, NASA TN D-2345, June 1964, 11 pp.

It is practicable to obtain accurate measurements of magnetic field intensity in non-uniform fields by the nuclear resonance method with the aid of a set of current-carrying coils which neutralize the field gradient. For use on an electromagnet, a method of controlling the compensating currents by reference to the magnet current is described. This method simplifies the adjustment of the compensating currents. Field gradients up to 150 gauss/cm may be neutralized by the equipment described, which has been used so far in fields ranging from 2 to 10 kilogauss. In the instruments constructed, it has only been necessary to compensate for variations of the field along the two axes normal to the field itself. However, it should be possible to compensate for variation along the axis parallel to the field as well, without difficulty, and the coil arrangement required for this is described also.

Observations on the behaviour and structure of flux jumps in a hard superconductor

Observations on the behaviour and structure of flux jumps in a hard superconductor

Alternating Current Power Losses in Superconducting Nb-Zr Alloys

A calorimetric technique, involving measurement of the thermal conductivity, was used to measure non-Ohmic ac power losses in short, 0.25-mm-diam samples of superconducting Nb-25% Zr and Nb-33% Zr wire as a function of frequency, current amplitude and waveform, and temperature. The observed power losses at 4.2°K ranged from 6×10−11 to 10−7 W/cm. The losses depended linearly on frequency in the range 60–40 000 cps and decreased approximately as T2.5 for temperatures T in the range 2.0°–4.2°K. Peak currents from 1 to 8 A were used to determine that the power loss was a function of the peak current Im rather than the rms current and also was proportional to Imn, where n varied from 2.6 to 4.5. These results indicate a hysteresis loss mechanism with no detectable relaxation effects for the entire frequency range covered. Models which have been proposed to account for hysteresis losses in hard superconductors are discussed. The results of this experiment are compared with earlier measurements on Nb-Zr alloys. There is considerable disagreement between the present results and the published results of Zar in magnitude and in the frequency and current dependences of the ac power losses.

Flux instabilities in hard superconductors

Abstract This letter reports the observation of flux jumps in specimens where the fluxoid pinning is associated with small normal regions. Conditions under which macroscopic flux jumps will be observed are discussed and two mechanisms are suggested which will lead to instability.

Preparation and Properties of Thin-Film Hard Superconductors

Films of thickness the order of 1000 Å of Nb-Zr, Nb-Ti, Ti-V, V-Si, Nb-Sn, and Nb have been prepared by dc sputtering and by electron bombardment evaporation. Measurement of the critical current Jc as a function of external magnetic field H was carried out at 4.2°K by using pulsed magnetic fields and pulsed currents. The films have Jc vs H characteristics and transition temperatures very similar to those reported for bulk hard superconductors. An exception was the sputter-deposited Nb, Tc≈9°K, which was superconducting at 70 kOe and 103 A/cm2. More structure was observed in the Jc vs H characteristic for longitudinal configuration than previously reported.

The penetration depth in several hard superconductors

The magnetic penetration depth in the superconducting intermetallic compounds V 3Si, V 3Ga, V 3Ge and V 3Pt and in tantalum was studied by means of a coil whose inductance was made to depend on this parameter. Difficulty was experienced in finding samples of the intermetallic compounds with sharp enough superconducting transitions to allow quantitative measurements to be made. However, quantitative results were obtained for V 3Si and tantalum which showed that below the transition region, in the temperature range studied, the magnetic field penetration depth varied linearly with y = (1 − (T/T c) 4) − 1 2 . For V 3Si the slope of this line, λ 0, is found to be 2,300 Å to an accuracy +10 and −30 per cent. For tantalum, λ 0 is 1,500 Å ±50 per cent. From these figures, the BCS theory is used to find a zero temperature penetration depth λ(0), and a coherence distance, ξ 0. The relative values of λ(0) and ξ 0 confirm that these materials are hard superconductors in the negative surface energy sense.

Flux compression in hard superconductors

Flux compression in hard superconductors

Rare-Earth—Conduction Electron Interaction in 4d and 5d Laves Phases Compounds

A systematic investigation has been made of the sign and magnitude of the g shifts of Gd impurities in the AB2 intermetallic Laves phase compounds. Gd was partially substituted in the A site for the elements Sc, Y, La, Th, U, and Zr. The B sites are occupied by elements of the 4d and 5d group whose positions in the periodic table are such as to correspond to an almost filled d band (e.g., Ru, Os, and Rh). Negative Gd g shifts, Δg, up to 7% were observed; the highest was that found in LaRu2. It was found that the closer a B element was to having a filled d shell the larger was the g value. It appears, in keeping with our previous EPR measurements on Gd in Rh-Pd, Pd-Ag alloys and in Cu5Ca intermetallic compounds, that without exception the g shifts are negative when the d band is nearly filled in the host alloy and that the g value has a minimum as a function of the atomic position in the host d band metal. In very fine powders of the hard superconductors (Ce0.99Gd0.01) Ru2 and (Th0.99Gd0.01) Ru2 no change in the Gd g shift was observed in going from the normal to the superconductor state. For the superconductor (La0.15Ce0.85) Ru2 the La Knight shift was also observed not to change in going from well above to well below the superconducting transition temperature. In both experiments Ho≪the upper critical field.

The penetration depth in several hard superconductors

The present work reports the elaboration and physical investigation of new compounds of the following composition Ba1−xNax(Ti1−ySny)1−xNbxO3 (BTSnNxy). The studied ternary system presents some continuous solid solutions between the next 3 phases: the NaNbO3 antiferroelectric phase that becomes easily ferroelectric at low rate substitutions, the BaTiO3 ferroelectric phase and the paraelectric stannate phase BaSnO3. Two different dielectric behaviors can be observed once some substitutions are made either in A or B sites of an ABO3 perovskite. These substitutions modify the dielectric properties of the material. The introduction of Sn4+ and Ti4+ cations in the B site favors, respectively, a decrease of the transition temperature and an increase in the value of the real dielectric permittivity. The transition temperature should be modulated by varying the rate of cationic substitution. Some relaxor materials can be obtained at a temperature around room temperature.

Superconductors in instrumentation

During the last two decades the unique and unusual properties of superconductors have begun to be extensively exploited. Some of the applications, both suggested and actual, are reviewed in this paper. There are several which have lead to `instruments' in the broadest sense. The superconducting bolometer depending on the steep resistance-temperature characteristic in the transition is an example. Superconducting galvanometers of very high sensitivity have also been developed. The destruction of superconductivity by a magnetic field leads to a variety of switches which have zero resistance when `closed' and finite resistance when `open'. The most well known is probably the cryotron which has received a great deal of attention with a view to computer applications. Switches of this type provide the basis of laboratory modulators and amplifiers which are capable of detecting extremely small voltages. However the most significant development is the discovery that it is possible to generate powerful magnetic fields using solenoids built from hard worked superconductors, such as the niobium-zirconium alloys or the compound niobium-tin. Not only does this enable powerful fields to be available for laboratory use, but it makes possible the successful development of any solid state device depending on low temperatures and powerful magnetic fields. A new infra-red detector is quoted as an example. There is no doubt that the biggest obstruction to getting such devices widely used has been the lack of suitable small closed circuit helium refrigerators, although these are now beginning to appear.

Hard Superconductivity: Theory of the Motion of Abrikosov Flux Lines

The basic ideas of flux-creep theory (i.e., theory of motion of quantized flux lines) for hard superconductors are reviewed and evaluated. The qualitative predictions of this theory are compared with experimentally observed phenomena. (T.F.H.)

Resistive States of Hard Superconductors

Resistive States of Hard Superconductors

Hysteresis in Hard Superconductors

To incorporate the effects of pinning due to defects and predict size- dependence and hysteresis in the magnetization curves of hard superconductors, the AbrikosovCoodman theory is developed. The interaction forces between fluxoid filaments are determined semiempirically. The method developed is illustrated by application to a simple case. (C.E.S.)

Filamentary structure in hard superconductors

Several theories of the structure of hard superconductors are presented, and resolution is attempted. The training effect, the critical current density and inhomogeneity, directional effects of the field containing the superconductor on the critical current density, resistance in the mixed state, and the abnormal drop of resistance near the normalstate-transformation field value are discussed. (R.E.U.)

Flux Creep in Hard Superconductors

Resistive states of hard superconductors have been investigated by tube magnetization and resistance measurements. The flux-creep theory of Anderson is very effective in accounting for the experimental observations reported herein. Resistive phenomena were observed in the presence of transport current density $J$ and magnetic field $B$ perpendicular to $J$. It is found that the whole spectrum of resistive states can be represented in terms of a single parameter $\ensuremath{\alpha}=J(B+{B}_{0})$, where ${B}_{0}$ is a constant of the material. This parameter represents essentially the Lorentz force or the magnetic pressure gradient in the material. While a wide range of $\ensuremath{\alpha}$ values is possible, under given experimental conditions superconductivity usually can not be maintained above a critical value. In tube magnetization, the critical value ${\ensuremath{\alpha}}_{c}$ is determined primarily by the rate with which the persistent current $J$ decays. If $\ensuremath{\alpha}$ is raised beyond ${\ensuremath{\alpha}}_{c}$, $J$ decays rapidly and $\ensuremath{\alpha}$ quickly falls near to ${\ensuremath{\alpha}}_{c}$. $\ensuremath{\alpha}$ continues to decrease slowly, but proportional to the logarithm of time as predicted by the theory. The observed temperature dependence of ${\ensuremath{\alpha}}_{c}$ is accounted for by the theory. Discrete, stochastic changes in field anticipated from the motion of flux bundles have been detected through pickup coils placed in close proximity to the superconducting tube. In resistance measurements, voltages appearing across 3Nb-Zr wire samples were measured by supplying $J$ externally in the presence of a perpendicular field $H$. The voltage observed is interpreted as a manifestation of an uncompensated emf arising from flux creep. At a given temperature, voltage readings obtained over a wide range of $J$ and $H$ are found to be a function of $\ensuremath{\alpha}=J(H+{B}_{0})$ only. $V(\ensuremath{\alpha}, T)$ follows qualitatively a form expected from the theory. In resistance measurements, the critical value ${\ensuremath{\alpha}}_{p}$ is determined by the power dissipation in the material. If $\ensuremath{\alpha}$ is raised beyond ${\ensuremath{\alpha}}_{p}$, thermal conduction lags the power dissipation and the sample undergoes a catastrophic transition to the normal state.

Alternating current losses in superconductors of the second kind

The power (heating) losses occurring when an alternating current in a hard superconductor is used to produce a pulsed or alternating magnetic field are estimated. Heat losses for a solenoid are proportional to the conductor thickness, dictating the use of thin films for second-kind superconductors carrying alternating current. (D.C.W.)

Behaviour of the magnetisation and transport current in NbZr wire

The behavior of the magnetization and transport current in Nb- Zr wire in transverse fields was investigated. The hard superconductor model of Beam provides a qualitative understanding of the magnetization. Possible explanations for some phenomena associated with Nb-- Zr solenoids are suggested by the data. Relations between the transport current, critical current, magnetization, field and current changes, etc., suggest the invalidity of pulsed field-pulsed current measurements of the critical current. (D.C.W.)

Influence of Transport Current on the Magnetization of a Hard Superconductor

The influence of transport current on the magnetization of cold-worked Nb--Zr wire in a transverse field was studied at 4.2 deg K. The magnetic moment is reduced by the transport current and tends to zero as the critical current is approached. At any given field the curve of magnetic moment vs transport current is not single-valued, but is seen to depend on the sequence of application of the current and field and also on the history of the transport current at the final field. (C.E.S.)

Measurement Of Low Resistance and the ac Resistance of Superconductors

The resistance of highly conductive materials is measured by an induction method that does not require a direct electrical connection to the test specimen. Instead, the test sample is used as a short-circuited secondary winding of a transformer. The method is suited to specimens where lead correction errors would introduce difficulty, or where the temperature or field environment requires that the sample be located at some distance from the instruments. Resistances of the test specimen from 10−5 to 10−2 Ω are suitable for this method. It has been applied to determining the resistance of hard superconductors at 4.2° and at frequencies of 100 to 30 000 cps. Various niobium-zirconium alloys all followed a characteristic resistivity curve with ρ proportional to f0.5 at high frequencies and ρ proportional to f1.7 at f<500 cps.