Table of contents

Superlattices based on standard high temperature superconductors (HTS) such as YBa₂Cu₃O_7−x and Bi₂Sr₂Cu_nCa_n−1O_x have been extensively used to investigate many fundamental issues in HTS physics such as dimensionality, vortex dynamics, proximity effects, epitaxial strain effects, interplay between superconductivity and magnetism. Recently, a different approach based on thin layers of the (Ca, Sr, Ba)CuO₂ compounds (infinite layers compounds) has been applied to the synthesis of superconducting cuprate heterostructures. Ideal infinite layer (IL) compounds alone are not superconducting; however, in IL-based heterostructures, the occurrence of charge transfer among layers containing different alkaline earth ions can give rise to superconductivity. Structural features of these heterostructures can be engineered in a wide range and, consequently, their superconducting properties are studied. The highlights of these investigations are reviewed here.

Nonlinear optimization problems in the field of electromagnetics have been successfully solved by means of sequential quadratic programming (SQP) and the finite element method (FEM). For example, the combination of SQP and FEM has been proven to be an efficient tool in the optimization of low temperature superconductors (LTS) superconducting magnetic energy storage (SMES) magnets. The procedure can also be applied for the optimization of HTS magnets. However, due to a strongly anisotropic material and a slanted electric field, current density characteristic high temperature superconductors HTS optimization is quite different from that of the LTS. In this paper the volumes of solenoidal conduction-cooled Bi-2223/Ag SMES magnets have been optimized at the operation temperature of 20 K. In addition to the electromagnetic constraints the stress caused by the tape bending has also been taken into account. Several optimization runs with different initial geometries were performed in order to find the best possible solution for a certain energy requirement. The optimization constraints describe the steady-state operation, thus the presented coil geometries are designed for slow ramping rates. Different energy requirements were investigated in order to find the energy dependence of the design parameters of optimized solenoidal HTS coils. According to the results, these dependences can be described with polynomial expressions.

Using a fluorine-free sol–gel method, both c- and a-axis textured YBCO thin films were synthesized in low oxygen partial pressure. The experimental results of both x-ray diffraction 2θ scan and pole figure showed well epitaxial grown films on LaAlO₃ (LAO) single crystal substrates. When the processing condition was altered, the scanning electron microscopy indicated a large fraction of a-axis oriented grains in the film. High-resolution transmission electron microscopy was performed on both types of films and revealed interface lattice structural characteristics. A classic nucleation and growth model was used to explain the texturing mechanism. Other possible mechanisms for the a-axis oriented grains are also discussed.

We have studied a newly designed BPSCCO-2223 bulk composed of mono-cored BPSCCO-2223/Ag sheath filaments. The composite structure design of the combination of BPSCCO-2223 and mono-cored filaments was newly developed with the aim of protection when bulk material is used as a current feeder for large-scale applications. The composites were made by alternately stacking BPSCCO-2223 oxide-cored Ag sheath filaments and the oxide powder by the powder-in-tube (PIT) method, and then the prepared samples were sintered at 840 °C for 50 h. Then, a cold isosatic pressing (CIP) process was applied and they were re-sintered at 840 °C for 50 h. As a result, the maximum transport current (I_c) value of the composite bulk, which is composed of 24 mono-cored sheath filaments of 0.4 mm in diameter, was estimated to be about 240 A at 4.2 K and 0 T. This I_c value was about three times higher than that of a conventional bulk, and the value of the PIT filaments composite bulk was also higher than that of the Ag wires composite bulk. This is why good c-axis oriented and densely structured BPSCCO-2223 plate-like grains were formed on both the outer and inner interface between the oxide and Ag in the PIT filaments. Furthermore, we confirmed that transport current was flowed into the PIT filaments composite bulk after forcing a fracture by the bending test. We guessed that the PIT filaments could act as a bypass for the fracture of the bulk. We thought that a new design of the composite bulk in this study was interesting in terms of safety precautions for large-scale applications.

Fe-sheathed MgB₂ tapes were prepared by the in situ powder-in-tube technique by 5 at% ZrSi₂, ZrB₂ and WSi₂ doping, respectively. The doping effect of these compounds on the microstructure and superconducting properties of MgB₂ tapes has been investigated by using x-ray diffraction, scanning electron microscope, transport measurements and dc susceptibility measurements. Compared to the undoped samples, J_c for all the doped samples were much improved; the best result in terms of J_c was achieved for ZrSi₂ doping, by up to a factor of 3.4 at 4.2 K in magnetic fields up to 12 T. Moreover, these dopants did not significantly decrease the transition temperature. The J_c–B curves of WSi₂-doped tapes show better performance in higher magnetic fields in comparison to undoped tapes, suggesting that pinning centres effective in a high-field region were possibly introduced.

We study the hysteresis and relaxation of both intragranular and intergranular properties of TlBa₂Ca₂Cu₃O_y (Tl-1223) superconducting polycrystals between 80 and 120 K. The samples have been prepared using a technique involving the mixing of grains of different sizes before the final sintering. The grains show a sizeable reversible magnetization, while vortices inside the grain behave as three-dimensional objects. The transport critical current is strongly hysteretic, with features that distinguish our Tl-1223 samples from 'standard' YBa₂Cu₃O_7−δ (YBCO), (Hg, Re)Ba₂Ca₂Cu₃O_8+δ (HBCCO) and (Bi, Pb)₂Sr₂Ca₂Cu₃O_10−δ (BSCCO) polycrystals. The preparation method improves the transport properties of the samples. The relaxation of the transport critical current density, in the presence of trapped fields, is reported here for the first time in this system, as far as we know. A phenomenological model can qualitatively describe the transport properties, where the intragrain magnetization affects the intergranular junctions, but a precise quantitative description is not achieved. The differences in the shape of the transport measurements, for different polycrystalline systems, are also well described by the model.

The high temperature reaction between the superconducting Nd123(x) (Nd_1+xBa_2−xCu₃O_7−δ+x/2) phase and Nd422(z) (Nd_4−2zBa_2+2zCu_2−zO_10−2z), leading to substitutions of Nd in the Ba sites of Nd123, was analysed. For this purpose, differential thermal analysis (DTA) was performed on powders with different Nd422 addition. Measurement results were compared with the values, reported in the literature, of the peritectic temperatures in function of Nd123 substitution. The superconducting transition critical temperature analysis of melt-textured samples shows that the use of Barium rich Nd422 allows significant improvements of the superconducting parameters of produced samples, avoiding the occurrence of the Nd–Ba substitutions.

Analytical investigations of the critical state are carried out for a superconducting stripline consisting of two individual coplanar strips with an arbitrary distance between them. Two different cases are considered: a stripline with transport current and strips exposed to a perpendicular magnetic field. In the second case, the obtained solutions correspond to 'field-like' (for unclosed strips) and 'current-like' (for a long rectangular superconducting loop) states in an isolated strip to which both a transport current and a magnetic field are applied with constant ratio.

The elemental composition and depth profiles of MgB₂ films prepared by successive e-beam evaporation as well as by thermal co-deposition of Mg and B components were investigated by Rutherford backscattering spectrometry (RBS). In the case of films deposited by e-beam evaporation we studied both Mg-B precursors and appropriate MgB₂ films grown on glassy carbon, Si(100) and J-sapphire substrates annealed in situ. For the films co-deposited by thermal evaporation on R-sapphire substrates and annealed ex situ we investigated superconducting MgB₂ films only. The T_co values of all MgB₂ films ranged from 21 to 30 K. Because of a very fine granular structure of the annealed films, confirmed also by SEM observations, we could not identify any MgB₂ phase from x-ray diffraction (XRD) patterns. On the other hand, Mg₂Si phase has been detected by XRD on the film–substrate interface for the superconducting film deposited on Si(100) substrate. The RBS measurements were performed with a 3.1 MeV ⁴He⁺ beam. Under these conditions, the ¹⁶O(α,α)¹⁶ elastic resonance allowed us to detect oxygen in all studied samples especially in B layers. The depth profiles of precursors prepared by successive e-beam evaporation showed clearly the multilayer film structure consisting of B and Mg layers. A strong interdiffusion between B and Mg layers may be observed after an in situ annealing, but still some degree of non-homogeneous component distribution may be observed. On the other hand, the MgB₂ films co-deposited by thermal evaporation and annealed ex situ are much more homogeneous, but a higher content of oxygen is present.

In this work, effects of varying mechanical deformations on the relationship between mesotexture and current percolation in (Bi, Pb)₂Sr₂Ca₂Cu₃O_10+x (Bi2223) tapes are investigated. Electron backscattered diffraction analysis demonstrates that the mesotexture distribution characteristics influence critical current density (J_c) as results of the processing variations. The disorientation angle distribution dependence of J_c is also discussed using current percolation theory. The results show that improving the mesotexture distribution in central region of Bi2223 tapes through optimization of the mechanical deformation processing can significantly increase J_c.

High-T_c superconducting YBa₂Cu₃O_7−δ (YBCO) films were prepared by a unique pulsed-laser deposition using crossed-laser-beam technique onto LaAlO₃ substrates in order to develop a basic technology for microwave-device application. The pulsed-laser deposition set-up uses two Nd–YAG lasers with 1064 nm wavelength and 0.3 J/pulse power. The unique feature of this method is a possibility to get a plasma plume with a low level of 'droplet', which essentially improves the film-surface quality. Transmission-electron microscopy and x-ray data show that YBCO films deposited at substrate temperature (T_s) of 780 °C exhibit a minimal density of dislocations in the crystal. YBCO films deposited at T_s = 780 °C with a cooling rate of V = 3 K min⁻¹ have the smallest surface resistance below the transition temperature.

To look at a quasi-particle injection into high T_c superconductors, YBa₂Cu₃O_7−δ/insulator/LaNiO₃ (YBCO/I/LNO) trilayers have been grown on (100) SrTiO₃ substrates in situ by pulsed laser deposition (PLD), where, an insulator (I), SrTiO₃ (STO), CeO₂ (CO) or Eu₂CuO₄ (ECO) are used. All the deposited films are highly oriented. The surface morphology of LNO and the insulating films is investigated during fabrications after the films are made, revealing small roughness and low droplet density. The top YBCO layer exhibits good superconducting properties with a critical temperature above 82 K and a transition width as sharp as 1.5 K.

SrTiO₃ buffer layers have been deposited on rolled Ni substrates using metal organic deposition (MOD) for HTS-coated conductors. The MOD process is based on the formation of the SrTiO₃ layer coated on the surface of a Ni substrate using dip-coating as well as spin-coating techniques from a solution-based precursor, which is prepared by dissolving strontium acetate and titanium (IV) butoxide in acetic acid and methanol. The films were annealed at 950 °C for 2 h under 5% H₂–Ar gas flow. X-ray diffraction (XRD) shows that the buffer layers on the Ni tape are highly oriented. The pole figure indicates a single cube-on-cube texture in addition to SEM and AFM observations revealing a continuous, dense, smooth and crack-free microstructure for the coated buffer. These results offer the potential of further manufacturing coated conductors with high current density.

Rb₃C₆₀ crystals with a size of several tenths of a millimetre were prepared by doping C₆₀ crystals in rubidium vapour. Experiments with different annealing times and various annealing temperatures and temperature gradients were performed to assess the optimal conditions for homogeneous doping. The crystals were characterized by ac magnetization measurements. Doping at 463 K with 10% excess of rubidium and annealing at 685 K for 27 days were found to produce the best crystals with almost 100% of the superconducting Rb₃C₆₀ phase.

We report our studies on ultrafast voltage transients of optically thick superconducting YBa₂Cu₃O_7−x microbridges biased with nanosecond supercritical current pulses (I > I_c) and, simultaneously, illuminated with 100 fs optical pulses (810 nm wavelength) from a Ti:sapphire laser. The pulsed current bias created a resistive flux-flow state in the superconductor, while the laser pulse transferred it into a state of more intensive flow of vortices within a time of less than 100 ps. The light-enhanced flux state remained constant until the end of the biasing pulse. The amplitude of the photoresponse signal increased rapidly with the increase of laser fluence in the range from 8 × 10⁶ to 2 × 10⁸ photons per pulse, as well as with the current-bias increase up to 2I_c, exhibiting the maximum, when the light illuminated the entire area of the microbridge. Maximal repetition rate of optically excited YBa₂Cu₃O_7−x photoresponse signals was found to be in the GHz range, appropriate for applications of YBa₂Cu₃O_7−x thin-film microbridges as high-power ultrafast switches.

Temperature dependence of far-infrared transmission of NbN thin films deposited on MgO and Si substrates was measured at several frequencies from 0.4 to 4.3 THz. Activated exponential increase of relative penetration depth at low temperatures and a peak in transmission near T_c were observed for frequencies below the optical gap. On the other hand, the transmission measured at frequencies above the gap exhibits only flat, almost linear temperature dependence. This behaviour is consistent with the BCS theory of superconductivity. Similar measurements were performed also on YBa₂Cu₃O_7−δ thin films deposited on MgO and sapphire substrates. Low-temperature variation of transmission indicates the d-wave symmetry. The peak below T_c predicted by the BCS theory is not observed. Flat temperature dependence of transmission at higher frequencies shows that the photon energy was sufficient for excitation over the optical gap. The s-wave BCS theory is adequate for NbN films but not for the YBa₂Cu₃O_7−δ materials. Using the theoretical BCS model we show that the transmission peak is not correlated to the coherence peak in real conductivity at given frequency and other parameters relevant to experiment.

We investigate the effect of Pr substitution on the superconductivity and interlayer coupling of the Bi₂Sr₂Ca₁Cu₂O_y system. Magnetic and transport measurements were performed for the purposes of characterization. The superconducting transition temperature T_c first increases and then decreases until it becomes zero at x = 0.6. The effective superconducting volume also decreases due to Pr substitution. From the fluctuation conductivity analysis, it is found that the interlayer coupling constant J decreases monotonically with the increase of the Pr content. This result shows that the Pr doping weakens the CuO₂ interlayer coupling of the Bi2212 system due to the loss of local superconductivity in the CuO₂ layers.

Effect of SiO₂ and SiC nano-powder doping was investigated for the powder-in-tube processed MgB₂/Fe tapes. Mg or MgH₂ powder was used as the Mg source of starting materials, and heat treatment was carried out at 600 °C for 1 h. These heat treatment conditions of lower temperature and shorter heating time are advantageous from the aspect of practical production processes. MgH₂ powder improved the connection of MgB₂ grains and prevented oxidation of MgB₂. SiC and SiO₂ doping greatly enhanced the critical current density (J_C) values of the tapes prepared with Mg + B powder. However, only the SiC doping was effective in enhancing J_C values for MgH₂ + B powder. SiC doping decreased magnetic field sensitivity of J_C, while SiO₂ doping did not change the field dependence of J_C. The SiC doped tape showed transport J_C value of about 6 500 A cm⁻² at 4.2 K and in the magnetic field of 12 T. The irreversibility field increased from 17 T to 23 T by the SiC doping.

Te-doping of (Tl_0.54Pb_0.5−xTe_x)(Sr_0.85Ba_0.15)₂Ca₂Cu₃O_y superconductors with x ranging from 0 to 0.1 has been investigated. Small amounts of Te (x < 0.05) strongly favoured the formation of the Tl-1223 phase in bulk samples, leading to phase-pure material. The concentration of the Tl-1212 phase was below the detection limit of the x-ray diffraction. The critical temperatures of bulk phase Tl-1223 for x = 0.02 were 120 K. The transition widths narrowed and the critical current densities for bulk phase material increased upon addition of small amounts of Te. At higher doping levels, Ba₂CuTeO₆ appeared as a secondary phase but the remaining material was still high-purity Tl-1223 phase. (Tl_0.54Pb_0.48Te_0.02)(Sr_0.85Ba_0.15)₂Ca₂Cu₃O_y films on lanthanum aluminate substrates prepared by screen printing of the precursor and followed by ex situ thallination, lead to highly c-axis oriented and ab-aligned samples with critical temperatures T_c(0) of 117 K and transition widths ΔT_(90–10%) of 1 K. Critical current densities of 1.15 × 10¹⁰ A m⁻² were achieved for 1.5 µm thick films in self-field at 77 K. The critical current densities at 0.8 T and 77 K were 7.4 × 10⁹ A m⁻² for H||ab and 0.7 × 10⁻⁹ A m⁻² for H||c.

We present a systematic approach to the time-dependent generalization of the Ginzburg–Landau Lagrangian for a lossless medium. We consider a Lagrangian, which contains four arbitrary scalar functions and admits two alternative terms determining the time dependence. Standard variational technique then yields Ginzburg–Landau equations, which coincide with the equations obtained from the corresponding Hamiltonian and determine the energy conservation law. The alternative time-dependent terms lead either to the first, or to the second order time derivatives in the equations. By introducing the gauge-invariant potentials and choosing a gauge, which differs slightly from the classical Lorentz one, we are able to simplify significantly the theory. The results are discussed and compared to some earlier propositions. When the problem involving first order time derivative is reduced to the static case, the results are found to be in perfect agreement with those reported recently by Kolácek et al (Kolácek, Lipavský and Brandt 2001 Phys. Rev. Lett.86 312), which may be considered as an indirect indication that this particular alternative form of the theory is better adapted to treat the physical problems.

The photo-induced linear electro-optics effect (EOE) has been revealed in Hg_0.89Pb_0.17Ba_0.11Ba₂CuO_4+δ (Hg-1201) single crystalline films coated on an MgO surface. Investigations have been performed using time-resolved measurements of the photovoltage, photocurrent, photoresistivity and linear EOE. A time delay shift between the non-linear optical response and photocurrent has been found. Applying external static magnetic fields (up to 1.7 T) we achieve a substantial decrease of the linear electro-optics coefficients. We discuss the possible origin of the observed phenomenon.

The rising industrial demand for a high-quality power supply has resulted in a growing interest in superconducting magnetic energy storage (SMES). These devices have a storage capacity ranging from fractions of MWs up to about 1 MWh. The commercialized units are based on NbTi technology at liquid helium temperature. The development of mechanical cooling systems has been very active during the recent years. The cooling capacities of cryocoolers are well around 10 W at 10 K. Based on this fact, a conduction-cooled Nb₃Sn SMES concept has been built and tested (uninterrupted power supply) application. The cryogenic interface of the coil also provides the protection scheme of the magnet which helps speed up the current decay during normality and thus diminishes the hot spot temperature in the winding. The test results are presented and discussed together with problems related to the cryointegration of the magnet. The magnet system has been demonstrated to compensate a short term loss of power.

We report magnetization studies on MgB₂ superconducting thin films in a temperature range 4.2–40 K and magnetic field range 0–6 T. Thin films prepared by both pulsed laser deposition (PLD) and electron beam evaporation (EBE) methods were investigated. In addition, both films were studied before and after heavy ion irradiation by 200 MeV Ag ions with a dose of 10¹¹ ions cm⁻². Variation of sweep rates during the measurement of the magnetization loop reveals the presence of flux creep in both films. The PLD film, after irradiation, shows a severe degradation of T_c, critical current densities (J_c) in low fields and irreversibility line (B*(T)). In contrast, the EBE film shows a slight enhancement in T_c, and nearly no change in J_c(B) and the position of irreversibility line after irradiation. For both pristine films, the obtained volume pinning forces F_p versus reduced field b = B/B* shows a good scaling for T ≤ 10 K, which matches well with the theoretical curve based on the flux line shear (FLS) pinning model. These and other results can be interpreted in terms of grain boundaries in MgB₂ films acting as FLS channels.

(Tl_0.5Pb_0.5)(Sr_(0.75–0.9)Ba_(0.1–0.25))₂Ca₂Cu₃O_xF_y bulk superconductors have been prepared by partial replacement of CaO by CaF₂ and of BaO by BaF₂ in the starting materials via the mixed oxide method. The addition of either CaF₂ or BaF₂ as a fluoride ion source yielded predominantly the SrF₂ secondary phase. The critical temperatures decreased by 5 to 20 K compared to the critical temperature of the samples prepared without the addition of CaF₂ or BaF₂. Annealing at 820 °C yielded comparable T_c(0) values between 114 and 119 K for both the fluoride containing and fluoride-free samples. The critical current densities at 77 K decreased. Partial or complete substitution of Tl₂O₃ by TlF in the starting material led to critical temperatures (T_c(0)) between 115 and 117 K and the formation of large grains (up to 200 µm). The concentration of fluoride ions in the superconducting crystallites was below the detection limit of the energy dispersive x-ray fluorescence analysis. Excellent results were obtained when TlF was used in thallium sources for in situ preparation of screen-printed thick films on single-crystal LaAlO₃. The x-ray diffraction pattern showed that very well textured Tl-1223 crystallites were formed. The critical temperature T_c(0) for these films was about 117 K with transition widths (ΔT_c(90–10%)) smaller than 1 K. The films prepared at ambient pressure yielded critical current densities up to 370 000 A cm⁻² at 77 K in the self-field.

By applying high temperature superconductors (HTS) in generators, transformers and synchronous motors it is possible to improve their efficiency. Higher efficiency saves electrical energy and thus reduces greenhouse gas (GHG) emissions as well. The reduction of GHG emissions is becoming a topical issue due to the Kyoto Protocol which requires the European Union (EU) to reduce its emissions by 8% from the 1990 levels between 2008 and 2012. This environmental viewpoint can accelerate the commercialization of HTS applications if certain efficiency and sufficiently large power range are reached. In this paper, a detailed study about the replacement of existing devices by HTS ones is made in order to find the efficiency level and the power range where HTS becomes competitive. Finland is taken, as an example of an EU country, to present accurate figures of saved electricity. The structure of energy production and consumption was investigated and the emission data from different types of power plants were screened. The potential savings were allocated to the reduced usage of coal. Finally, an expanded view towards the possible emissions reduction gained by superconducting technology in the whole EU is presented. A market penetration model was introduced to investigate the time-scale in which conventional devices can be replaced with HTS devices.