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Thallium-based cuprates are known to exhibit high critical temperatures and currents. Usually, partial substitution of copper by transition elements leads to the suppression of superconducting properties. This contribution reports on the studies of Tl2Ba2Ca2Cu3-xCoxOy. All specimens were found to be superconductors with critical temperatures of 102.4 K, 112.9 K, 112.2 K, 113.5 K, 112.8 K, and 94.8 K for x = 0.005, 0.010, 0.020, 0.030, 0.040, and 0.050, respectively. The samples were characterised by scanning electron microscopy, X-ray diffraction, AC susceptibility, resistance, and vibrating sample magnetometry.

The paper reports on the temperature dependencies of the electrical resistivity and the a.c. magnetic susceptibility of the Tl0.8Bi0.3Sr1.8Ba0.2Ca2Cu3Ox bulk superconductor measured in the area of the transition from the metallic to the superconducting state. From the resistivity data, the onset temperature $T_{\mathrm {c,onset}} =$ 121.6 K, the critical temperature $T_{\mathrm {c50\% }} =$ 118.8 K, the zero resistance critical temperature obtained using $E =$ 10− 6 V/cm criterion $T_{\mathrm {c0}} =$ 117.9 K, and the width of superconducting transition ${\Delta } T =$ 1.6 K were derived. The thermal fluctuations were examined using the formulae for the conductance: ${\Delta } \sigma =K\varepsilon ^{-\lambda }$ , and for the magnetic susceptibility: χ = Ce−λ, where ${\Delta } \sigma = 1/R(T)-1/R_{R}(T)$ , $\varepsilon =~|$ (T − Tc)/Tc|, λ is the critical exponent, and $K $ and C are constants. The critical exponents in the transition region from the normal to the superconducting state were determined and analyzed. The three-dimensional fluctuations were found to occur in the vicinity of the zero-resistance critical temperature.

This article details the solid-state synthesis of high-temperature superconducting YBa2Cu3O7−x. Tests were carried out on samples formed at different pressures (200, 400, 600 and 800 MPa) before being annealed under pure oxygen. The X-ray diffraction method showed that, regardless of the forming pressure, the samples contain about 97 wt.% of Y-123 phase. SEM images showed a polycrystalline structure of samples of similar grain size and number of pores (intergranular spaces). The values of critical temperatures (Tc0), determined from magnetoresistance measurements, are about 91.5 K for all samples, and the Tc0 temperatures do not depend on sample-forming pressure. Magnetoresistance measurements have shown that samples formed with higher pressures exhibit smaller changes in Tc0 and superconducting transition width ΔT due to the influence of the HDC magnetic field, than the samples formed with lower pressures. Values of specific resistance determined by the use of van der Pauw method at 300 K is about 2 mΩcm for all samples. The critical temperatures (T cintra) of grains and critical current densities at 77 K were determined from AC magnetic susceptibility measurements, and they are about 91.6 K and 400 A cm−2, respectively.

The (Bi1−x Pb x )2Sr 2Ca 2Cu 3 O y (x = 0.2, 0.4) films were obtained directly on silver substrates by the sedimentation process. The temperature dependencies of the AC susceptibility of the films and bulk materials were measured and analyzed. The critical temperatures of the grains T cg as well as the Josephson junctions T cJ of these films were obtained from the dispersion part of AC susceptibility and they are T cg = 106.5 K, T cJ = 100.2, T cg = 106.1 K, and T cJ = 97 K, respectively. The thickness of these films is of the order of a hundred micrometers. The critical currents were derived from the absorption part of AC susceptibility using the Bean’s critical state model. The temperature dependencies of the critical currents were analyzed using the Ginzburg–Landau strong coupling limit approach.

The temperature dependencies of the magnetoresistance of Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O x bulk superconductor were measured within the transition from normal to the superconducting state. The critical temperature of the sample was found to be T c50% = 107.1 K. The thermal fluctuations were studied using the following formula of conductance: Δ σ = K e − λ where e = ( T − T c )/ T c , λ is the critical exponent and K is a constant. The critical exponents in the transition region were determined and their magnetic field dependencies were analysed. The crossovers from 1D to 3D and from 3D to 1D dimensional fluctuations were found within the regions of transition from the normal to the superconducting state.

The magnetoresistance and irreversibility fields of commercial bismuth-based 1G tape were studied on the basis of the temperature dependencies of the magnetoresistance at the two relative orientations of magnetic field and superconductor plane. The critical temperatures of this tape are the following: T c50 % = 110.3 K and T c0 = 109.9 K, and the width of superconducting transition is ΔT = 0.5 K. The widths of the transition to the superconducting state versus applied magnetic fields were derived for both orientations. The experimental data were fitted using the formula ΔT = C H m + ΔT 0. The irreversibility field values were obtained and successfully fitted as a function of temperature. At 77 K, they were found to amount to H i r r = 72.8 kOe and H i r r = 5.5 kOe for the parallel and perpendicular directions, respectively.

The critical currents of commercial bismuth based superconducting tape were determined in the two ways. In the rst one the transport critical current density was measured by the four points method using the dc current power supply at the liquid nitrogen temperature. In the second one the critical current densities were obtained from the absorption part of ac susceptibility measurements using the Bean model near the critical temperature. The temperature dependence of the critical current densities was tted to take advantage of the Ginzburg Landau strong-coupling limit approach. Using the t parameters the critical current density at 77 K was calculated. The critical temperature of this tape (Tc = 110.5 K) was determined from the ac susceptibility measurements.

The two extended formulae in the power form of the Kim critical state model were used to fit the critical currents versus the dc applied magnetic field. Two samples of thallium based superconductors: the (Tl0.6Pb0.24Bi0.16)(Sr0.9Ba0.1)2Ca2Cu3Oy film on single-crystalline lanthanum aluminate and the bulk (Tl0.5Pb0.5)Sr2(Ca1.8Gd0.2)Cu2Oy were chosen to test the models. The formulae were compared to the percolation model described by the exponential expression. The first model fits the experimental data better for the thallium based film whereas the second approach is better for the thallium based bulk sample.

Superconducting (Tl 0.5 Pb 0.5 )(Sr 0.85 Ba 0.15 ) 2 Ca 2 Cu 3 O 2 films were prepared on highly polished, untextured silver substrates. A superconducting transition temperature (T c ) of 114.9 ± 0.1 K was obtained from the resistivity versus temperature measurements for different applied ac currents. The temperature dependence of the critical current was fitted with a power law: J c (T) = J c (0)[1-T/T c ] n with n = 2. The critical current in self-field at 77 K was found to be 8 x 10 3 A/cm 2 . The temperature dependence of the real and imaginary parts of ac susceptibility was measured for different amplitudes of ac magnetic field. Only a small differences of the ac susceptibility for parallel and perpendicular fields with respect to the film surface were observed.

The temperature and magnetic field dependences of the ac resistance and the critical currents of a c -axis oriented, ab aligned (Tl 0.6 Pb 0.24 Bi 0.16 )(Ba 0.1 Sr 0.9 ) 2 Ca 2 Cu 3 O y film on (1 0 0) single-crystalline lanthanum aluminate substrate were measured and analysed. The magnetic field and the temperature dependences of the width of the resistive transition to the superconducting state and of the irreversibility fields were analysed both within the flux-creep model and the superconducting liquid vortex state model. The temperature width of the resistive transition was explained on the basis of the superconducting Josephson weak links model of resistance and the barrier of vortex motion. The irreversibility field was described by an exponential formula. The temperature dependence of the critical current was described by a power law; the magnetic field dependence of the critical current was fitted to a Kim-type power law as well as to an exponential relation from the percolation model.

Polycrystalline samples of YBa2Cu3O7 were studied by the microwave absorption technique at the low magnetic fields in the temperature range from 5 to 80 K. The direct microwave absorption and the field modulated microwave absorption were measured at 77 K. Analysis of the microwave absorption curves indicates that the field penetration into the sample starts from the lowest values of applied magnetic field within the whole temperature range.

The YBa2Cu3Oδ lms were obtained directly on silver substrates by the sedimentation processes. The thicknesses of these lms are of the order of several tens μ. The temperature dependences of the AC susceptibility of the lms were measured and analyzed. The critical temperatures of these specimens obtained from the dispersion part of AC susceptibility vary from 89.2 K to 92.5 K. The critical currents were calculated from the absorption part of AC susceptibility using the Bean model. The temperature dependences of the critical currents were tted using the Ginzburg Landau strong coupling limit approach.

Magnetic field dependences of the direct and the magnetically modulated microwave absorption of a number of Y-, Bi- and Tl-based high-temperature superconductors were measured at 4.2 and 77 K by X-band microwave spectrometer. Correlation between value of the low field maximum of the absorption and the inter-grain critical currents of Josephson junctions was found. Results were analyzed and compared to theoretical models of microwave absorption available in the literature yielding good agreement.

A thin film of a (Tl0.5Pb0.5)(Sr0.85Ba0.15)2Ca2Cu3Oz superconductor was prepared on highly polished silver substrates. Its superconducting transition temperature (Tc,50%) is 114.7 ± 0.3 K and the critical current density is 11.4 × 103 A cm−2 at 77 K in the self-field. The transport critical currents densities were measured as a function of temperature for several applied magnetic fields. The magnetization versus the dc applied magnetic field and temperature were also measured and the critical current densities were calculated. The temperature dependence of the critical currents obeys the Ginzburg–Landau power-law. Reasons for this behaviour were discussed taking into account the pinning force and anisotropy of the critical currents associated with the c-axis grain orientation of the film. The magnetization as a function of the angle between the applied magnetic field and the a–b plane showed a periodic dependence due to changes of the demagnetizing field.

Bulk (Tl0.5Pb0.5)Sr2(Ca1−xGdx)Cu2Oz superconductors with x = 0.1,0.2 and 0.3 were prepared by a wet chemical gel technique followed by a special heat-treatment procedure. X-ray diffraction analysis revealed practically phase-pure samples with tetragonal structure (space group P/4mmm). The temperature dependence of the real and imaginary parts of the ac susceptibility and the dependence on the ac applied magnetic field were investigated to learn about the superconducting and magnetic properties of bulk (Tl0.5Pb0.5)Sr2(Ca1−xGdx)Cu2Oz superconductors. The critical current densities of the samples were derived from Bean's model. The temperature dependence of the critical current was successfully fitted based on a power law from the thermally activated magnetic flux creep model. From magneto-resistance measurements the temperature dependences of the transport critical currents and the irreversibility fields were found. The dc magnetic field dependence of the width of superconducting transitions and the critical currents was also obtained. These properties of the superconductors were analyzed within available theoretical models.

The microwave absorption and ac susceptibility vs. magnetic field and temperature were measured for YBaCuO type high- T c superconductors which were prepared by the sol-gel method. Some characteristic fields were found and their temperature dependences were measured. The grain size and porosity effects on the penetration of a magnetic field into the sample were also studied.

The a.c. susceptibility and microwave absorption as a function of applied magnetic field and of temperature were carried out for the solgel YBaCuO type high- temperature superconductors. The mixed states associated with the intergrain weak superconducting links and the intragrain regions were examined yielding the characteristic lines separating reversible and irreversible regions as well as the temperature dependencies of the lower critical fields inside the field- temperature HT phase diagram. The data were analyzed in the frame of the giant flux creep and the vortex glassy state models.

The magnetoresistance, irreversibility fields, and critical current density were studied for a commercial 2G tape at the two relative orientations of magnetic field and superconductor plane. The critical temperatures of this tape of T c50 % = 91.5 K and T c0 = 90 K and the width of superconducting transition of ΔT = 1.5 K were obtained. The widths of the transition from the normal to the superconducting state do not increase at the applied magnetic field up to 90 kOe and do not depend on the orientation of the magnetic field with respect to the tape plane. The irreversibility field values were obtained and successfully fitted as a function of temperature using the formula: $H_{\text {irr}} =H_{\text {irr0}} \left ({1-\frac {T}{T_{\text {c0}} }} \right )^{n}$ . The irreversibility fields show an anisotropy, and at the liquid nitrogen temperature, they reach H irr = 430 kOe and H irr = 106 kOe for the parallel and perpendicular directions, respectively. The anisotropy ratio amounts to γ = 4 at 77 K and is small in comparison with other high-temperature superconducting materials. The critical current density of this tape was found to be of the order of 106 A cm−2 at 77 K in the self-magnetic field.