High resolution magneto-optical studies of the intermediate state in thin film superconductors

The structure of the intermediate state in superconducting lead films has been investigated as a function of magnetic field and film thickness. The detection system utilized the high specific Faraday rotation in thin films of a mixture of EuS and EuF₂ in combination with a polarizing microscope, yielding a resolution of about 1 μm. The thickness of the Pb films ranged between 0.7 and 9 μm, thus including the critical film thickness at which the transition from the intermediate state to the vortex state occurs. At low fields a liquid-like mixed state of multi-quanta flux tubes was observed which appeared to be stable up to increasing magnetic fields with decreasing film thickness. The diameter of these flux tubes varied approximately with the square root of the film thickness. At intermediate fields the intermediate state pattern was found to persist down to a film thickness of 0.7 μm, the smallest thickness investigated. The periodicity length of the intermediate state structure was in reasonable agreement with the non-branching model of Landau. Just below H_c, small superconducting domains were observed in increasing field, whereas long threads of superconducting material were formed abruptly in decreasing field. These superconducting threads were absent in the specimentsthinner than 1–2μ, being replaced by a liquid-like mixed state of superconducting tubes. After the passage of a sufficiently high electrical current through the specimen, the flux structure was found to be rearranged into long domains oriented predominantly perpendicular to the current, leading to current hysteresis effects. Finally, some dynamic observations were made during current induced flux flow.     

Vortex-shaking effect in thin flat superconductors

We consider the vortex-shaking effect (a fast relaxation of irreversible currents) for a thin superconducting platelet placed in a large perpendicular dc magnetic field and in a weak ac magnetic field in the plane of the platelet. The relaxation of the magnetic moment of the sample and the redistribution of the magnetic induction in it during the shaking process are calculated. All these results can be verified experimentally.     

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.     

Spin glass state in high-temperature superconductors

The glass state of high-temperature superconductors in the models of Josephson weak links is studied. Reversibility phenomena in weak magnetic fields are outlined.     

Spin-glass state in high-T c superconductors

The high-temperature superconductor glass state in the Josephson weak-link model is studied. Reversibility phenomena in weak magnetic fields are outlined.     

Transport of high-temperature superconductors in the normal state

The impurity mechanism proposed earlier is used to interpret the transport properties of high-temperature superconductors in the normal state. Temperature and impurity concentration dependences of the resistance and the Seebeck and Hall coefficients are discussed. An explanation for the sharp drop in the Seebeck and Hall coefficients with impurity concentration and reducing them to zero in La_2–x Sr _x CuO₄ at the maximumx at which superconductivity exists is put foward.     

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.     

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.     

Ion migration in high-T c superconductors and the nature of the conducting state ind-wave superconductors

The available experimental data on the ion migration in superconducting Y-123 and Bi-2223 films under the effect of high transport currents are summarized. Irrespective of structural perfection, electrostimulated diffusion of heavy ions and oxygen gives rise to the formation of new, stable phases and partial degradation of superconductivity. A model taking into account the proximity of the superconducting, structural, and magnetic transitions in high-T _c materials is used to explain ion migration as the response of the system of ions and coherentd electrons to the external electric field. Thed-wave symmetry superconducting state is shown to possess combined, rotational + translational invariance, rotational symmetry being due to the motion of electrons in the curl field of the ions, playing the role of topological defects.     

Constraints on the pairing state of the cuprate superconductors

The pairing state of the cuprate superconductors is a key issue in understanding experiments and in clarifying possible mechanisms. A wide range of experiments now show that the gap function is predominantly in character. Here we discuss the relevant experiments, especially focusing on those which provide constraints on possible mixed pairing states, such as s+d and s+id.     

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.     

Inhomogeneous state of superconductors by intense tunnel injection of quasiparticles

Measurements of the current-voltage characteristics of a tunnel junction as a function of quasiparticle injection from a second tunnel junction are presented. Aluminum films (300 » thick) are studied from the transition temperature down to about 0.35 K. For a certain range of injection currents, there exists an inhomogeneous energy gap and tunnel current; the large gap approximates the undisturbed film, while the value of the small gap depends on the specific resistance of the injector junction in a regular and calculable way. There is an instability which leads to this inhomogeneous state; although these experiments cannot unambiguously determine the mechanism of the instability, we show that severe problems are found in trying to explain it in terms of existing models. On the basis of our results and analysis, the instability apparently occurs because of small spatial inhomogeneities in the films and the extremely nonlinear current-voltage characteristics of tunnel junctions, rather than intrinsic properties of the superconductor. This result does not disprove the other models of instability, but shows the limitations of tunneling for investigating the models.Based on work performed under the auspices of the U.S. Department of Energy.     

On the theory of the mixed state of type-II superconductors

The Ginzburg-Landau theory is generalized to the case when the Cooper pairs can form several condensates, i.e., accumulate in several different states. A homogeneous mixed state is obtained for type-II superconductors with a constant density of superconducting electrons and constant magentic field in a bulk sample. This state turns out to be thermodynamically more favorable than the vortex structure of Abrikosov.     

Spin-glass state in high-Tc superconductors

The high-temperature superconductor glass state in the Josephson weak-link model is studied. Reversibility phenomena in weak magnetic fields are outlined.     

Spatial structure of the inhomogeneous gap state in nonequilibrium superconductors

Work is reported on the gap instability in a superconductor under tunneling injection of quasiparticles at the gap sum voltage. Low-temperature scanning electron microscopy was used to investigate the spatial development of the inhomogeneous gap state and the influence of inhomogeneities in the injector junction on the nucleation process of the regions of small and large energy gap. Measurement of the small and the large gap for injector junctions with various normal state conductivities and use of the theory of Elesin and Kopaev made it possible to determine the electron phonon scattering time for PbIn.     

Thermomagnetic effects in the mixed state of high-T c superconductors

In the high-T_c superconductors a variety of thermomagnetic effects occurs above the irreversibility- or melting line. We present a brief overview of the experimental results, their systematics and their interpretation.