Intermediate state of thin superconductors

The purpose of this study was to examine the state of a thin type-I superconductor in a magnetic field to determine its dependence on the specimen thickness and on the value of the Ginzburg-Landau parameter . Three materials (aluminum, indium, and In_0.99Pb_0.01) with values of the Ginzburg-Landau parameter ranging from 0.19 to 0.34 were studied. Specimen thicknesses ranged from 200 to 200,000 Å. All of the materials studied were type-I or nonlocal superconductors in the bulk. Yet, it has been predicted that they would behave in ways characteristic of type-II or local superconductors if the specimen were sufficiently thin. For intermediate thicknesses the specimens were expected to be in one of many possible states. We have inferred from critical field studies that the structure of the intermediate state in thin type-I superconductors is equivalent to the type-II vortex state for very thin films (d ₀), and to the type-I macroscopic domain state for very thick films d 2(t). For thicknesses between these limits the intermediate-state structure takes on many forms as the area of each normal domain and the amount of flux threading it increases with increasing thickness.Work supported in part by the Army Research Office (Durham) under Grant No. DAHC04-70-G-0009 and also a grant from the Research Foundation, State University of New York.     

Current hysteresis in the intermediate state of thin-film superconductors

We have studied the current hysteresis at 4.2 K in the intermediate state of superconducting lead films using electrical resistance measurements and high-resolution magnetooptical observations of the magnetic flux structure. At low magnetic fields the liquidlike arrangement of the flux structure in the form of multiquanta flux tubes was found to persist following the application of a high electrical transport current with an average current density of 10⁵–10⁶ A/cm². In this field range areduction of the resistive voltage was observed following the application of a high transport current. This inverse current hysteresis appears to be associated with a shift of the magnetic flux structure in the direction of the current-induced flux motion resulting in an extended fluxfree region along the sample edge where flux tubes enter the film during flux motion. At higher magnetic fields, where the laminar flux structure is the stable configuration, the usual resistive voltage enhancement has been observed following the application of a high transport current. In this regime the laminae rearrange themselves under the influence of a transport current resulting in a preferential orientation perpendicular to the current.Based on work performed under the auspices of the U.S. Atomic Energy Commission.     

Dynamic resistance of the current-induced intermediate state in superconductors

We have measured the in-phase component of the low-frequency ac impedance of the dc current-induced intermediate state in wires of pure and impure type I superconductors. We find that this dynamic resistance of the intermediate state is greater than the normal state resistance of the wire. The linear response theory for the dynamic resistance developed by Gauthier and Rochon provides a reasonable qualitative understanding of our experimental results.This work has been supported by the Chief of Research and Development, Department of National Defence, Canada through his contribution No. 3610-320.     

Preparation techniques of thin film ITC superconductors

This paper considers the several deposition techniques currently in use for deposition of thin film superconductors with reasonably good results.     

Landau intermediate state structure in type I superconductors

Landau laminar structures are discussed for cases where the normal-superconducting (N-S) boundary is curved and where it is straight throughout a sample. A numerical calculation shows that the free energy for a curved N-S boundary is less than that for a straight one, except for values ofd/ less than about 8 and a small region of magnetic fields close toh . Here is the surface energy parameter andh is the perpendicular critical field corresponding to a sample thicknessd. The expression forh for the straight N-S boundary is different from that obtained by Simonin and López. It is shown that the data ofh reported to date are insufficient to verify the formation of laminar structures with a straight N-S boundary.This work was supported by the Fonds National Suisse de la Recherche Scientifique.On leave from the Department of Physics, Faculty of Science, Kyushu University, Fukuoka, Japan.     

The resistivity of the intermediate state of type-I superconductors

The interaction between transport current and flux spots in the intermediate state of a superconductor is investigated. It is shown that the force on the flux spots depends on factors such as current lead configuration, and it is therefore not possible to define a unique flow resistivity for type-I superconductors. The conclusions are supported by experimental evidence.     

High resolution electron microscopy of superconductors

Until recently, in the field of superconductivity, electron microscopy has played only a minor role in studies of structure and properties of existing superconductors (except Nb and its alloys) and in the development of new and improved superconductors. This situation has changed in recent years. Electron microscopy is now being extensively used to study the growth, microstructure, and kinetics of many superconductors, especially of A15 type. Electron microscopy has also contributed to the understanding of the nature of radiation damage in these materials. These contributions will be reviewed with selected examples.     

On the london model of the intermediate state in current-carrying superconductors

The London picture for the destruction of superconductivity by a current is considered. Equipotential S regions are taken as being placed periodically along the wire, and the magnetic field at the phase boundary is taken to have the critical value. From a solution of the Laplace equation for the electric potential near the contact point of two S regions a relationship between the angular aperture of the conical S region and the period of the structure is found. A numerical solution is given for the phase boundary and for the resistance at large currents. The volume of the S phase is found to be much smaller than that in the original London model. As a consequence the resistance of the wire after destruction of the uniform S state differs from that for the uniform N state by only a few percent.     

Effect of doping on the ultrasonic attenuation in the intermediate state of superconductors

The attenuation of longitudinal ultrasonic waves has been measured in single crystals of indium (99.999%) and of indium doped with 0.003 at % of tin in the intermediate state over the frequency range 10–30 MHz. Two phases have been identified in the superconducting layers. The fundamental frequency for one phase is approximately constant in both the specimens for the same field, but increases appreciably with the increase in the magnetic field from 0.7 H _cto 0.9 H _c; the ₀ value for the other phase, on the other hand, increases with increase in the concentration of the dopant and is independent of the magnetic field. The origin of the two phases is discussed.     

Effect of strain content on the ultrasonic attenuation in the intermediate state of type I superconductors

The attenuation of longitudinal ultrasonic waves has been measured in single-crystal indium doped with 0.002 at % of bismuth in various physical states of the specimen in the intermediate state over the frequency range 23–30 MHz. Two phases have been identified in the superconducting layers. The fundamental frequency v₀ for one phase is approximately constant for various physical states of the specimen for the same field, but increases with increase in magnetic field from 0.7H _cto 0.9H _c; the fundamental frequency v₀ for the other phase decreases with decrease in dislocation density. The origin of the first phase is due to the laminar structure of the intermediate state. The second phase is due to the formation of microstructure in the superconducting layers due to the impurity and strain content in the specimen. The thickness of the microstructure has been calculated utilizing Granato and Lucke's vibrating string model of dislocation for various physical states of the specimen.     

Nanoindentation studies of thin film coated systems

In this paper we report the results of ultralow load indentation (“nanoindentation”) tests carried out on a range of thin film coated systems. In addition to characterizing the properties of each system at high resolutions, a further principal aim has been to explore the extent to which it is possible to generalize the nanoindentation response of the various systems in terms of the relative mechanical properties of the coating and the substrate. In this case we classify the substrate and coating in terms of the relative elastic and plastic responses (i.e. the ratio of yield stress to Young's modulus). Some of the different possible responses are shown together with the transitions in response (elasticity-dominated to plasticity-dominated and vice versa) which can occur. We also report on the various steps which can occur in loading curves and our attempts to relate them, via high resolution scanning electron microscopy, to the fracture patterns (through-thickness cracks and interfacial cracks) within and around the indentations.     

Electrochemical protection of thin film electrodes in solid state nanopores

Solid state nanopores are a core element of next-generation single molecule tools in the field of nano-biotechnology. Thin film electrodes integrated into a pore can interact with charges and fields within the pore. In order to keep the nanopore open and thus functional electrochemically induced surface alteration of electrode surfaces and bubble formation inside the pore have to be eliminated. This paper provides electrochemical analyses of nanopores drilled into TiN membranes which in turn were employed as thin film electrodes. We studied physical pore integrity and the occurrence of water decomposition yielding bubble formation inside pores by applying voltages between -4.5 and +4.5 V to membranes in various protection stages continuously for up to 24 h. During potential application pores were exposed to selected electrolyte-solvent systems. We have investigated and successfully eliminated electrochemical pore oxidation and reduction as well as water decomposition inside nanopores of various diameters ranging from 3.5 to 25 nm in 50 nm thick TiN membranes by passivating the nanopores with a plasma-oxidized layer and using a 90% solution of glycerol in water as KCl solvent. Nanopore ionic conductances were measured before and after voltage application in order to test for changes in pore diameter due to electrochemical oxidation or reduction. TEM imaging was used to confirm these observations. While non-passivated pores were electrochemically oxidized, neither electrochemical oxidation nor reduction was observed for passivated pores. Bubble formation through water decomposition could be detected in non-passivated pores in KCl/water solutions but was not observed in 90% glycerol solutions. The use of a protective self-assembled monolayer of hexadecylphosphonic acid (HDPA) was also investigated.     

Heat conduction in the intermediate state of superconductors: Observation of a new size effect in lead

Experimental evidence is presented for the existence of a size effect predicted by Andreev as a consequence of his quasiparticle reflection law at the superconducting-normal interphase boundaries. In high-purity lead we observe a reduction of the thermal conductivity in the intermediate state when the heat current is parallel to the laminar structure. This work was supported by the Fonds National Suisse de la Recherche Scientifique.