This article presents a systematic overview of the work carried out in the area of pulsed laser deposition (PLD), characterization and device application aspects of high temperature superconducting YBCO thin films. The theoretical and experimental aspects of the pulsed laser deposition process for the synthesis of YBCO thin films have been described in detail. The deposition technique has emerged as a very powerful method to make composition and microstructure controlled superconducting YBCO films. Some unique features of this process are due to the rapid heating and evaporation of the target and the interaction of the laser beam with the evaporated materials leading to the formation of a high temperature plasma. Major advantages of PLD process are congruent evaporation and crystallinity due to the presence of high energy evaporants and fast response time. It has the potential to encompass a wide scope of physical vapor-deposition techniques from thermal evaporation to sputtering and MBE. The electrical and structural studies performed on laser deposited YBCO films have shown that films produced by PLD are superior than films produced by other thin film growth techniques. The major technical obstacle of particulates emission, encountered initially with PLD, has now greatly been solved. Some new developments with PLD technology such as large-area and nonplanar substrates coatings and growth of novel superconductors as oriented films have been demonstrated.

Pulsed laser deposition and characterization of high-Tc YBa2Cu3O7 − x superconducting thin films

This paper discusses the physics of ac losses in type-II superconductors and in multifilamentary superconducting-normal composite conductors. As an introduction, the ac eddy-current losses in a homogeneous normal conducting cylindrical specimen subjected to an alternating applied magnetic field are briefly reviewed. The ac losses in a superconducting cylindrical specimen are discussed in detail next, with emphasis on the losses in a specimen subjected to an alternating applied longitudinal magnetic field. In general, dissipation occurs via both flux-flow losses, which are analogous to eddy-current losses in the normal state, and hysteretic losses, which arise from bulk and surface pinning as well as annihilation of vortices of the opposite sense. Tests are suggested by which the dominant loss mechanisms can be identified. Special considerations are noted for the case of the high-temperature superconductors. Finally, the losses in multifilamentary superconducting-normal composite conductors are discussed.

Ac Losses in Type-II Superconductors

The real χ′ and imaginary χ″ components of the AC susceptibility for a ceramic sample of superconducting HgBa2CuO4+δ have been measured at HAC=0.1 Oe over the temperature range 80–300 K under purely hydrostatic pressure conditions. Upon cooling, a slight but sharp decrease in slope dχ′/dT occurs at the “upper” transition temperature, T u c ⋍98 K , which we attribute to the onset of superconductivity in a small volume (≁3%) of the sample which is nearly optimally doped. Below the “lower” transition temperature, T 1 c ⋍95 K , χ′(T) drops steeply, approaching for T d T c / d P⋍+1.75±0.1 K/GPa which is independent of whether the pressure is changed at room temperature or 80 K. These experiments allow us to speculate that the superconducting transition for optimally doped HgBa2CuO4+δ at ambient pressure probably lies near 98 K.

Dependence of the superconducting transition temperature of HgBa2CuO4+δ on hydrostatic pressure

The influence of the external field on the current–voltage characteristics and the critical current density J c and on the angular dependence of voltage V and resistivity ρ have been discussed in terms of the Lorentz force driven flux flow motion of the vortices. We have obtained analytical expressions for the field dependencies and angular dependencies of the critical current density, the current–voltage characteristics, voltage V and resistivity ρ which are in reasonable agreement with the experimental data published previously.

Influence of an external magnetic field on the current–voltage characteristics and transport critical current density

The dependence of the superconducting transition temperature [Tc](P) on purely hydrostatic pressure to 0.9 GPa has been determined in ac susceptibility studies in a He-gas pressure system for optimally doped ceramic samples of HgBa[sub 2]CaCu[sub 2]O[sub 6+[delta]] and HgBa[sub 2]Ca[sub 2]Cu[sub 3]O[sub 8+[delta]] with superconducting transitions at [Tc](0) [approx equal] 126.6 K and 133.9 K, respectively. [Tc] increases reversibly under hydrostatic pressure at the rates, d[Tc]/dP [approx equal] +1.80 [+-] 0.06 K/GPa and +1.71 [+-] 0.05 K/GPa, respectively. Within experimental error, these values are the same as found previously for optimally doped single-layered HgBa[sub 2]CuO[sub 4+[delta]], where d[Tc]/dP [approx equal] +1.72 [+-] 0.05 K/GPa. Remarkably, the logarithmic volume derivative of [Tc] is nearly identical for all three compounds, dln[Tc]/dlnV [approx equal] [minus]1.20 [+-] 0.05, even though the bulk modulus differs by more than 30%. This provides strong evidence that a common mechanism is responsible for the pressure dependence of the superconducting state in all three compounds.

Hydrostatic presure dependence of the superconducting transition temperature of HgBa2CaCu2O6+δ and HgBa2Ca2Cu3O8+δ

The onset of magnetic flux penetration through the wall of a tube of sintered YBa2Cu3O7 has been measured in the temperature range of 4 to 90 K. These measurements have been used to calculate the temperature dependence of the intergranular critical transport current employing three critical-state models. Good agreement was obtained between this calculated critical current and direct critical current measurements on rod-shaped samples of the same material. The critical-state magnetic field profiles were used to explain the observed voltage waveforms induced in a pick-up coil inside the bore of tubular samples when excited by an AC field. Best agreement was obtained using a model in which the critical current (jc) has a magnetic field dependence given by j c = j 0 H 2 0 (H 2 +H 2 0 ) where j0 and H0 depend only on temperature.

Flux pinning in tubes of a ceramic superconductor

A critical state model has been employed with an intergranular pinning force density which is independent of the local magnetic field, to calculate the critical transport current and the ac susceptibility of sintered Y Ba Cu O as a function of temperature. The assumed temperature dependence of the pinning force density is consistent with measurements on long tubular samples. The flux creep term of the effective intergranular pinning force causes a weak frequency dependence of the ac susceptibility.

Intergranular flux pinning in high temperature superconductors

The intra- and intergranular components of resistance in high- T c superconductors are modelled, after configurational averaging, as due to a single resistively shunted Josephson junction in each case. They are combined in series. A voltage develops across each junction due to thermally activated phase slippage. In the presence of a magnetic field slippage occurs at a faster rate and is accompanied by motion of the entire flux bundle in the junction. Data on thin films is fitted well by the model over a wide temperature range. The model predicts that in a flux density B the critical currents and activation energies at higher fields fall off as (o 0 /B) 1 2 /[D+(o 0 /B) 1 2 ] with D the junction width. Experimental data on thin films of YBa 2 Cu 3 O 7 agrees with this prediction for both types of junction. It also shows that neither the intra- nor the intergranular components of resistance are sensitive to the relative direction of macroscopic current and field down to low temperatures. Differences between the roles of the Lorentz force in flux creep and phase slippage models are discussed.

Phase slippage in a magnetic field at intra- and intergranular Josephson junctions in high-Tc superconductors

The critical state model with a pinning force independent of flux density is employed to interpret the measured ac magnetic flux response and the weak critical current density of ceramic Y-Ba-Cu-O material. The intergranular (Josephson) vortex plinning is found to be about 10 8 times weaker than that in conventional type II superconductors

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B.W. Ricketts

Papers

Flux pinning in tubes of a ceramic superconductor

Intergranular flux pinning in high temperature superconductors

Phase slippage in a magnetic field at intra- and intergranular Josephson junctions in high-Tc superconductors

Role of Flux Pinning in High Temperature Superconductors