Ion irradiation effects in EuBa2Cu3O y thin film

The effect of He ion irradiation on the pinning potential in EuBa₂Cu₃O _y , thin film was investigated by measuring the temperature dependence of resistivity in magnetic fields. The pinning potential decreased as the ion fluence increased. A slower decrease of pinning potential was observed in higher magnetic field in the fluence region <3.5×10¹⁵ cm^?2.     

Magnetic relaxation and critical current in Bi2Sr2CaCu2O8+x single crystals

The anisotropy of critical current densityJ _c in Bi₂Ba₂CaCu₂O_8+x single crystals has been investigated as a function both of the temperature and of the applied magnetic field. An anisotropic behavior ofJ _c has been found. The decay of the remanent magnetization has been studied for fields applied both parallel and perpendicular to thec axis. A logarithmic behavior was found. A pinning energyU ₀ of about 0.01 eV, independent of the direction of the applied field, was obtained.     

Physical properties of a new cuprate superconductor Pr2Ba4Cu7O15−δ

We present studies of the thermal, magnetic, and electrical transport properties of reduced polycrystalline Pr₂Ba₄Cu₇O_15−δ (Pr247) showing a superconducting transition at T_c=10–16 K, and compare them with those of as-sintered non-superconducting Pr247. The electrical resistivity in the normal state exhibited T² dependence up to approximately 150 K. A clear specific heat anomaly was observed at T_c for Pr247 reduced in a vacuum for 24 h, proving the bulk nature of the superconducting state. By the reduction treatment, the magnetic ordering temperature T_N of Pr moments decreased from 16 to 11 K, and the entropy associated with the ordering increased, while the effective paramagnetic moments obtained from the DC magnetic susceptibility varied from 2.72 to 3.13μ_B. The sign of Hall coefficient changed from positive to negative with decreasing temperature in the normal state of a superconducting Pr247, while that of the as-sintered one was positive down to 5 K. The electrical resistivity under high magnetic fields was found to exhibit T^α dependence (α=0.08–0.4) at low temperatures. A possibility of superconductivity in the so-called CuO double chains is discussed.     

High-capacity superconducting current leads of Y1Ba2Cu3O7?x

We have fabricated and measured a high-capacity superconducting current lead composed of a Y₁Ba₂Cu₃O_7–x cylinder, 20 cm long and 0.9 cm² cross section. A steady-state, d.c., critical current of 225 A at a temperature of 77 K was measured in this sample, using a voltage criterion of 2×10^–7 V/cm (p = 8×10^–10 ohm-cm). This current was limited by the currentinduced, self magnetic field. To our knowledge this is the largest d.c. critical current so far reported in a Y₁Ba₂Cu₃O_7–x sample and demonstrates the possibility of using hightemperature superconducting HTS materials for current leads to low-temperature superconducting LTS magnets or in power distribution systems.     

High Critical Current Density YBa2Cu3Ox Tapes Using the RABiTs Approach

Progress in the fabrication of epitaxial, high-J _c, biaxially aligned YBCO thick films on Rolling-assisted biaxially textured substrates (RABiTs) is reported. RABiT substrates comprise a biaxially textured metal substrate with epitaxial oxide buffer layers suitable for growth of superconductors. Oxide buffer layers have been deposited using three techniques: laser ablation, electron-beam evaporation, and sputtering. Epitaxial YBCO films grown using laser ablation on such substrates have critical current densities approaching 3 × 10⁶ A/cm² at 77 K in zero field and have field dependences similar to epitaxial films on single crystal ceramic substrates. Critical current densities in excess of 0.2 MA/cm² have been obtained on stronger, nonmagnetic substrates. In addition, samples with J _e of 12.5 kA/cm² at 77 K have been fabricated. The highest strain tolerence obtained so far is 0.7% in compression and 0.25% in tension. Deposited conductors made using this technique offer a potential route for the fabrication of long lengths of high-J _c wire capable of carrying high currents in high magnetic fields and at elevated temperatures.     

Current progress in ternary LREBa2Cu3Oy materials and their application

The J_c and H_irr values at 77 K of ternary light-rare-earth compounds, LREBa₂Cu₃O_y “LRE-123”, are usually high enough to serve in various applications. Several sources of vortex pinning can be in these composites tailored to fit the needs of the particular application. The list comprises LRE/Ba solid solution, oxygen vacancies, large particles of secondary phases, twin planes, nanoscale lamellas, etc. By means of the latter defects one can achieve a very high irreversibility field. Refinement of secondary phase particles and the optimal choice of their amount enhance the electromagnetic performance in a broad temperature range, up vicinity of T_c, allowing levitation at liquid oxygen, 90.2 K. An optimum content of MoO₃ doubles the self-field super-current at 77 K, H||c-axis. Altogether, the pinning tailoring in ternary LRE-123 materials provides a flexible and reliable way to fit the electromagnetic performance with the needs of sophisticated high-temperature and high-magnetic-field applications.     

Anisotropy in the resistive superconducting transition under magnetic fields in single crystal Pb2Sr2Ho0.5Ca0.5Cu3O8

Anisotropie properties of the single crystal Pb₂Sr₂Ho_0.5Ca_0.5Cu₃O₈ have been investigated by measuring the electrical resistivity in theab-plane _ab (H, ,T), which depends on the angle between theab-plane and the magnetic-field direction, in various constant fieldsH perpendicular to the current direction. All the angle-dependent values of _ab (H, ,T) at a constant temperature are scaled to be on one curve as a function of reduced field. The anisotropic parameter (m _c ^* /m _ab ^* )^1/2 is estimated as 12–13, which is larger than that of YBa₂Cu₃O₇ and much smaller than that of Bi₂Sr₂CaCu₂O₈. It has been concluded that the anisotropy does not always depend on the thickness of the blocking layer but seems to depend on the overlap of the electronic wave functions along thec-axis. Anisotropy in the pinning potential has also been discussed from the resistive tail in the temperature dependence of _ab (H,,T).     

Investigations of Bi substitution in NdBa2Cu3Oy

Attempts to substitute Bi for Nd in orthorhombic NdBa₂Cu₃O _y , prepared in air or oxygen at about 950°C led instead to formation of Ba₂NdBiO₆, a new cubic compound witha=0.8703 nm. The possibility was then explored of preparing superconducting (Nd_1–x Bi _x )Ba₂Cu₃O _y , by first forming the tetragonal phase at 880–950°C in nitrogen or argon followed by reheating in oxygen or air at 250–500°C in order to insert the additional oxygen required to yield the orthorhombic form while avoiding oxidation of Bi³⁺ to Bi⁵⁺. X-ray diffraction studies, electrical conductivity measurements, and thermogravimetric analysis of products indicate that Bi does not enter the NdBa₂Cu₃O _y , lattice in either the tetragonal or the orthorhombic phase. Ba₂NdBiO₆ clearly forms on reheating in oxygen or air even at low temperatures, and evidence is presented that a poorly crystallized oxygen-deficient form of this compound is already present prior to the reheating.     

Study on structural phase transitions and relaxation processes of Cs2Co(SO4)2·6H2O and Cs2Zn(SO4)2·6H2O crystals

The structural phase transitions and relaxation processes of Cs₂Co(SO₄)₂·6H₂O and Cs₂Zn(SO₄)₂·6H₂O single crystals were investigated, with the phase transitions of both crystals being determined from NMR data. The spin–lattice relaxation time, T₁, of the ¹³³Cs nucleus in two crystals undergoes a significant change near the phase transition temperature, T_C, and these changes coincide with the changes in the splitting of the ¹³³Cs resonance lines. The variations in the temperature dependence for the splitting of the ¹³³Cs resonance lines and T₁ near T_C are related to changes in the symmetry of surrounding Cs⁺. In addition, the ¹³³Cs T₁ of Cs₂Co(SO₄)₂·6H₂O, which contains paramagnetic ions, was found to be shorter than that of Cs₂Zn(SO₄)₂·6H₂O. This relaxation time is inversely proportional to the square of the magnetic moment of the paramagnetic ions. The differences between the ¹³³Cs T₁ of these compounds are probably due to the differences between the electronic structures of their metal ions.     

A New Method for the Estimation of Hc2 Anisotropy in Polycrystalline MgB2 Samples

By using isothermal magnetization measurements in polycrystalline MgB₂ samples, we estimate the H ^c _c2 in the interval [0, T _c]. By combining these measurements to the estimated H ^ab _c2 from the onset of the diamagnetic transition in isofield and isothermal magnetic measurements, an estimation of the anisotropy parameter can be achieved. The H ^c _c2 values coming from high quality polycrystalline samples agrees nicely to those obtained on single crystals. Our results show a temperature variation of the (T ) = H ^ab _c2/H^c2 with (T _c) 3.     

Interaction between the Ferromagnetic Dots and Vortices: Numerical Calculation and Experimental Results

Enhancing the pinning force in high-T _c superconductors can be achieved by externally introduced periodic magnetic dots. We numerically calculate the interaction between ferromagnetic dots and vortices in high-T _c superconductors. The London equation is used to generate two-dimensional vortex lattice. In the matching condition, we calculate the attraction force between magnetic dots and vortices. It is found that in an ideal condition, the pinning force of the magnetic dot reaches 2.5×10⁻¹¹ N that is more than one order magnitude stronger than the intrinsic pinning force in YBa₂Cu₃O₇ thin films. In the experimental side, we use a novel nano-technique to deposit periodic submicron Ni dots on YBa₂Cu₃O₇ thin films. The current versus voltage characteristics of an YBa₂Cu₃O₇ thin film strip with uniform Ni dots are measured at various temperatures and magnetic fields. They are compared with the current versus voltage characteristics of a bare YBa₂Cu₃O₇ thin film strip without magnetic dots. It is found the critical current value of the strip with Ni dots reduces with a much slower pace as the magnetic field strength increases in comparison with the value of the bare sample.     

Vortex motion in coupled DyBa2Cu3O7/(Y0.6Pr0.4)Ba2Cu3O7 heterostructures

Multilayer structures containing 24 Å thick DyBa₂Cu₃O₇ layers, separated by 96 Å of an (Y_0.6Pr_0.4)Ba₂Cu₃O₇ alloy, are studied to investigate the effect of coupling on vortex dynamics. With the magnetic field perpendicular to theab plane, and as a function of the number of superconducting layers in the structure, we find that the activation energy for flux motion increases, first linearly, and then saturates. This linear increase is taken as evidence that pancake vortices belonging to different DyBa₂Cu₃O₇ layers are stacked and have a coupled motion. Above a characteristic number of superconducting layers,N _c , shear of the vortex structures becomes important and the thermally activated process only displaces a stack ofN _c pancake vortices, meaning that the vortex lattice is turning three dimensional. In these structures we findN _c to be 2 to 3.     

Superconductivity and Ferromagnetism of the Ru-1232 Compounds in the (Ru1?xNbx)Sr2(GdCe1.8Sr0.2)Cu2Oz System

The Ru-1232 compounds have been synthesized in the (Ru_1–xNb _x )Sr₂(GdCe_1.8Sr_0.2)Cu₂O _z system, and effects of Nb substitution for Ru on superconductivity and ferromagnetism of the Ru-1232 compounds have been investigated. First, X-ray powder diffraction study shows that nearly the single 1232 phase samples can be obtained in the x composition range from 0.0 to 0.3. Then, from the electrical resistivity study, it is found that each of the samples shows resistivity dropping phenomenon at two temperatures of T _c ^l and T _c ^h, which originates from superconductivity of the Ru-1232 phase and the Ru-1222 one, respectively. Both of the starting temperatures are lowering with increasing Nb content x. Lastly, from the magnetic susceptibility study, it is found that superconducting transition temperature T _c is 20 K for the Ru-1232 sample with x = 0.0 and the ferromagnetic transition temperature T _m is about 90 K. This study also shows that both of the values of T _c and T _m become low with increasing x from 0.0 to 0.3.     

Dependency of Flux Jump on Temperature and Field-Sweep Rate for Textured (Nd1/3Eu1/3Gd1/3)Ba2Cu3O7- δ Bulk Superconductor with Gd-211 Particles

Anisotropy flux jumps in the mixed state of a textured (Nd_0.33Eu_0.33Gd_0.33)Ba₂Cu₃O_7-δ bulk superconductor with Gd-211 doping particles have been studied by means of magnetization measurements in the model of the magnetic field paralleling and perpendicular to the c axis. A typical anisotropy flux jump was observed at a temperature ranging from 2.0 to 3.0 K. Under the magnetic field perpendicular to c axis, no flux jump was found at whole temperature range until the sweep rate of 200 Oe/sec. For the magnetic field paralleling to the c axis, the number of flux jumps decreased with the increase of temperature, and the third quadrant of the M–H curve is the most flux-instability quadrant. The magnetic field sweep rate dependences of flux jumps were also studied and the influence of flux creep on flux jumps was also discussed.     

Luminescence properties of Sn2P2Se6 crystals

A large family of Sn_2yPb_2(1−y)P₂S_6xSe_6(1−x) semiconductor-ferroelectric crystals were obtained by the Bridgman technique. The photoluminescence properties of the Sn_2yPb_2(1−y)P₂S_6xSe_6(1−x) family crystals strongly depend on their chemical composition, excitation energy and temperature. The influence of the Pb → Sn and S → Se isovalent substitutions on the luminescence properties of a crystal with the Sn₂P₂Se₆ basic composition was investigated. A broad emission band observed in the Sn₂P₂Se₆ crystal with a maximum roughly at 600 nm (at T = 8.6 K) was assigned to a band-to-band electron-hole recombination, whereas broad emission bands, peaked near 785 nm (at T = 8.6 K) and 1025 nm (at T = 44 K) were assigned to an electron-hole recombination from defect levels localised within the bandgap. Possible types of recombination defect centres and specific mechanisms of luminescence in the Sn₂P₂Se₆ semiconductor-ferroelectric crystals were considered and discussed on the basis of the obtained results and the referenced data.     

Effect of Micro-Phase Separation on Magnetic Response in Pr-Doping YBa2Cu3O7−δ Single Crystals

Pr-doping Y_0.928Pr_0.072Ba₂Cu₃O_7−δ superconducting single crystal was studied systemically by the magnetic response measurement below T _c . DC magnetization shows a possible existence of micro-phase separation caused by the non-uniform distribution of Pr ions in microstructure during the growth procedure. The magnetic field dependence of critical current density J _c and pinning force density F _p had been obtained from the magnetization hysteresis loop. Flux pinning mechanisms in different magnetic fields had been discussed using the scaling function as well as the ratio of the pinning force field at H _on and H _peak. We find that the peak effect is influenced by the micro-phase separation which is caused by the non-uniform distribution of Pr ions in microstructure during the growth procedure. The interface between the different T _c superconductivity areas can be the effective pinning center which is useful to the appearance of peak effect. The value of the ratio of the maximum pinning force field to the irreversibility field is discussed. The conclusion is consistent with the model constructed by Wen et al.; we finally draw out the thermomagnetic phase diagram for our samples.      

The Jc Limit of Multifilamentary Ag/Bi-2223 Tapes

To improve on present critical current (J _c) performance, multifilamentary Ag/Bi-2223 tapes with a large range of reduction rates were manufactured. The relative core mass density D was calculated, dependent on the measured geometric dimensions of the tapes. Experimental results, D vs. J _c, D vs. maximum pinning force density F _max , and D vs. irreversible magnetic field B _irr, are quantitatively formatted. In particular, the magnetic field dependence of J _c is critically dependent on its core density. If the core density increases by 10%, J _c of the tapes in this experiment is enhanced by as much as 100%. Therefore, in the present state of the technological process for manufacturing Ag/Bi-2223 tape, increasing the core density is clearly a significant strategy in improving the electronic and magnetic properties of the tapes and enhancing the capacity for carrying current at high magnetic fields. The limit of the bulk self-field-J _c can be calculated by the relationships of J _c vs. D. The limit is estimated to be on the order of 200 kA/cm² for multifilamentary Bi-2223 tapes, which was supported by magneto-optical (MO) magnetization measurements results. It is a hard task to approach this limit with the present state of the art in manufacturing Ag/Bi-2223 tape, and it is the time to suggest some new ideals for Bi-2223 tapes to promote large-scale applications.     

Enhancement of Pinning Role by Nonstoichiometry in YBa2Cu3O7?y Superconductors

Considerably improved flux pinning and critical current density J c values have been achieved in Y-deficient Y-123 superconductors by directional solidification in air. In comparison with the regular Y-123 composition, Y-deficient one also has an orthorhombic structure and Y-123 main crystal phase remains in it. Whereas with the shortage of Y, Y_1–x Ba₂Cu₃O_7–y can be regarded as (YBa₂Cu₃)_1–x O_7–t(Ba₂Cu₃) _x O _t–y or (YBa₂Cu₃O_7–z )( _x ^YO _z–y ), so there may develop several kinds of microstructure defects as pinning sites in the system, such as highly dense, fine-scale, and faultlike defects, as well as localized superstructure, which are able to induce the increasing in flux pinning and J c values in higher external magnetic fields. This kind of simple nonstoichiometric route could lead to a commercial technique for flux-pinning enhancement in Y-123 bulk materials.     

Optical Conductivity of La1.875Ba0.04Sr0.085CuO4

The in-plane optical conductivity and dc resistivity of La_1.875Ba_0.125–YSr _Y CuO₄ with y = 0.085 was measured from 30 to 20,000 cm^–1 and from 295 to 8 K. A strong extra-Drude absorption at finite frequency is detected both in the normal and superconducting state, and remains unchanged when the free carriers condensate. The present results confirm and extend recent infrared data on La_2–xSr_xCuO₄.     

The Influence of Bi Doping on the In Situ Growth of TlBa2Ca2Cu3O9 High-Tc Films

The in situ process—laser ablation in combination with thermal evaporation of Tl₂O—has turned out to be a preparation method for single-phase and epitaxial TlBa₂Ca₂Cu₃O₉ (1223) thin films with T _c values up to 109 K. It was found by several groups that a partial substitution of Tl by Bi simplifies the phase development of the 1223 compound in the usual two-step process. We have investigated the influence of the Bi doping on the in situ growth. X-ray measurements show that the films consisted mainly of the 1223 compound. In 300-nm thin films there was no evidence of a Bi amount in the crystal structure, but thinner films (80 nm) show a small amount of Bi. We concluded that Bi doping supports the phase development of the 1223 compound only in an early stage of the film growth. The Bi-doped films have higher T _c values up to 114 K, higher j _c values up to 6 × 10⁵ A/cm² (77 K, 0 T), and lower surface resistances of 56 m (77 K, 87 GHz) than the undoped films.