HighT c SmBa2Cu3O7−δ superconductors

The superconducting transition with onset temperature of 95K has been confirmed in SmBa₂Cu₃O_7−δ system by resistivity and AC susceptibility measurements. Pressure dependence ofT _co andT _cf of the title superconductor is described. The superconducting transition onset temperature and complete transition temperature increases with increase of pressure.     

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.     

Crystal structure and resistivity studies on PrBa2 −x Pr x Cu3O7 −y oxide system

The compound PrBa₂Cu₃O_{7 −y} is not superconducting while most other RBa₂Cu₃O_{7 −y} (R=rare earth) compounds exhibit superconductivity in the 90K range. The system PrBa_{2 −x} Pr _x Cu₃O_{7 −x} has been prepared to study the effect of excess Pr at the Ba site on the structure, resistivity and magnetic behaviour of this system. It is observed that single-phase compounds in the above series form forx=0·8—that is up to the composition Pr_1·8Ba_1·2Cu₃O_{7 −y} . While stoichiometric PrBa₂Cu₃O_{7 −y} is orthorhombic, the compounds with excess Pr show tetragonal structure. Four-probe dc resistivity measurements show that all the single-phase compounds in the above series do not exhibit superconductivity and are semiconducting down to 12 K. Magnetic susceptibility measurements reveal deviation from Curie-Weiss behaviour starting at a characteristic temperature, which is taken to be the ordering temperature (T _N ) of the Pr moments. BothT _N and overall resistivity decrease with increasingx and may have a common origin.     

Fluctuation-induced excess conductivity in the compounds CaREBaCu3O7−y (RE=La and Sm)

The compounds CaREBaCu₃O_7−y (RE=La and Sm) are tetragonal at room temperature withT _c between 60 and 70 K. The single-phase compounds were prepared by solid-state reaction. The resistivity was measured by a four-probe technique in a continuous flow cryostat with the temperature being controlled to an accuracy of 10 mK. The resistivity vs temperature showed a break in slope around 180 K in CaLaBaCu₃O_7−y and around 220 K in CaSmBaCu₃O_7−y . The results were analysed for fluctuation conductivity from 180 K downwards. A plot of dρ/dT vsT showed a sharp peak atT _m =69·69 K for La compound and 66·00 K for the Sm compound. Detailed analysis of the resistivity in the regionT _on to 180 K was carried out using the procedure due to Veira and Vidal. The results are discussed in this paper.     

Improvement of the High-Magnetic-Field Critical Current Density of the <Emphasis Type="Italic">Ex-Situ</Emphasis> Annealed MgB<Subscript>2</Subscript> Thick Films by Oxygen Doping

This paper presents a very simple way to synthesis MgB₂ thick films with high critical current density in a magnetic field by ex-situ annealing precursor B films in air with excessive Mg in a sealed quartz tube. The films show a significant improvement of critical current density in a magnetic field compared to the high purity films annealed in vacuum, while its zero-resistance transition temperature T _c ^zero and normal state resistivity still maintain about 38 K and 17 μΩcm. The results demonstrate MgB₂ thick films have great potential applications in superconducting coated conductors.      

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.

PACS: 74.72.Jt; 74.25.Bt; 75.40.Cx; 74.25.Fy     

Abnormal Structural Behavior of Y0.8Ca0.2Ba1.8Nd0.2Cu3Oy at Low Temperature

The variation of structure and transport property for the nominal composition of Y_0.8Ca_0.2Ba_1.8Nd_0.2Cu₃O _y (YCBNC) at temperature range of 78 K < T < 284 K were studied. YCBNC shows a superconducting transition temperature (zero resistivity) of T _c0 83 K. There is no phase transition detected in the accuracy of our experiments in the whole temperature range. The dependences of lattice constants upon temperature can be discussed in two temperature regions. The other atomic parameters, or the deduced structural parameters, such as the orthorhombic strain, the lattice strain, some selected bond lengths and bond angles all depend on temperature. The dependences of the atomic structural parameters on temperature indicate that these atomic structural parameters may relate to the superconductivity to some extent.     

Processing of single-phase ceramic 123 YBaCu-oxide superconductor by hot pressing

High-quality, single-phase, polycrystalline samples of high-temperature superconducting 123 YBaCu-oxide have been processed by hot pressing of specially prepared charge material at elevated temperatures. The samples are orthorhombic with unit cell dimensionsa = 3.824(1) Å,b = 3.888(1) Å, andc = 11.679(2) Å. They exhibit: (i) densities up to 82% of the theoretical value; (2) a sharp phase transition between the normal and superconducting states at 91.6 K; (3) a reduced dependence on thermal cycling effects and on excitation currents in resistivity measurements; and (4) a reduction in surface deterioration due to water, air, and acid attack. The hot-pressed samples supported current densities at least two orders of magnitude greater than those found in cold-pressed samples measured under similar conditions.     

Superconducting transition temperatures in the Ru-1232 system, RuSr2(Gd1? x Ln x Ce1.8Sr0.2)Cu2O z (Ln = Sm, Dy, and Ho)

We have investigated effects of the lanthanide element Ln and the composition changes on the superconducting transition temperatureT _c in the Ru-1232 system, RuSr₂(Gd_1−x Ln _x Ce_1.8Sr_0.2)Cu₂O _z (Ln = Sm, Dy, and Ho). At first, in the case of the samples with Ln = Sm among almost the single 1232 phase samples, the values of the superconducting onset temperatureT _co are almost the same forx=0.00−0.15, and each of the lattice parametersa andc is almost constant. While, in each of the cases of the samples with Ln = Dy and Ho, the sample withx=0.05 shows the maximum values for both the superconducting onset temperatureT _co and the zero resistivity temperatureT _cz. Especially for the sample with Ln = Dy, the values ofT _co andT _cz are 18.5 and 6.5 K, respectively. These are higher than those of the mother sample of RuSr₂(GdCe_1.8Sr_0.2)Cu₂O _z . Moreover, from variations ofT _co, lattice parameters ofa andc in the RuSr₂(Gd_1−x Dy _x Ce_1.8Sr_0.2)Cu₂O _z system as a function of Dy contentx, the relationship between the superconducting transition temperature and the lattice parameters in the present system are investigated.     

Superconductivity in MgB2: Phonon modes and influence of carbon doping

Following a brief overview, results of our investigations on phonon modes in MgB₂, and superconducting transition in carbon doped MgB₂ are presented. The superconducting transition temperature in MgB_{2 x}C_x as obtained from susceptibility and resistivity measurements is observed to decrease systematically from 39-4 K forx = 0 to 26 K forx = 0.5. It is shown the changes in lattice volume, as obtained from x-ray diffraction measurements, can account only partially for the observed decrease inT _c . The observed variation ofT _c with carbon content is seen to correlate with the Debye temperatures, obtained from an analysis of the resistivity data. Investigation of the phonon modes in MgB₂, through infrared absorption measurements indicate three modes at 410,475 and 560 cm^-1. The former two are associated with the infrared active modes, and the third component is associated with the Raman mode, that gets activated due to disorder. A study of the temperature dependence of these modes indicates no changes across the superconducting transition. The mode at 560 cm^-1 shows a significant hardening and a corresponding decrease in linewidth, with the lowering of temperature, that can been accounted in terms of anharmonicity.     

Effect of Cd Addition on Superconducting Fluctuations and Mechanical Properties of Bi1.82Pb0.36Sr2Ca2Cd x Cu3O y System

A study was made of the effect of Cd additions on the superconducting and mechanical properties of Bi_1.82Pb_0.36Sr₂Ca₂ Cd_xCu₃O_y (x = 0.0, 0.15, 0.25, 0.35 and 0.55). Characterization of the Cd-samples using XRD, DTA, and SEM techniques, has confirmed that remarkably formation of low-T _c phase (2212) by the addition of Cd up to 0.35. High-resolution electrical resistivity ρ (T) data on the composition of Cd = 0.35 have been taken for investigating critically the superconducting fluctuations. Using the Aslamazov and Larkin (AL) and Lawrence and Doniach (LD) models of excess conductivity. Excess conductivity analysis shows that this composition (Cd = 0.35) is 2D in the temperature range 137.8–163.7 K and a 3D one below 137.8 K. Thus, a crossover from 2D to 3D is observed at 137.8 K. Sample microhardness and density are greatly improved by Cd-additions (0.35). This trend is probably due to the intercalation of cadmium between superconducting grains in compositions may provide a plastic-flow region that allows relaxation of undesirable stresses resulting from the grain anisotropy of superconductors.     

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.     

Transport properties of textured Bi2Sr2CaCu2O8+y, thin films

Superconducting films of the high-T _c compound Bi₂Sr₂CaCu₂O_8+y , have been grown on (111)-oriented gadolinium gallium garnet substrates by a liquid-phase technique. The films show a very high degree of preferential orientation with thec-axis perpendicular to the substrates. The onset of the resistive transition was 85 K while zero resistance was obtained at 78 K. Results concerning the critical current properties of the films are described. Measurements of the paraconductivity effects on the electrical resistivity above the superconducting transition due to thermodynamic fluctuations are also reported.Supported by Ansaldo S.p.A Divisione Ansaldo Ricerche, Via Corso Perrone 25, I-16100 Genova, Italy.     

Study of the Influence of MgO Nano-Oxide Addition on the Electrical and Mechanical Properties of (Cu<Subscript>0.25</Subscript>Tl<Subscript>0.75</Subscript>)-1234 Superconducting Phase

The effect of nano-size MgO (40–60 nm) addition on the physical properties of polycrystalline (Cu_0.25Tl_0.75)-1234 was studied. The MgO-concentration (x) varied from 0.0 to 1.0 wt.% of the sample’s total mass. X-ray analysis, granular structure examination, energy dispersive X-ray spectroscopy, electrical resistivity, critical current density at different applied magnetic fields and Vickers microhardness measurements were carried out to study the performance of these samples. Phase examination by X-ray diffraction indicated that MgO-addition enhanced the (Cu_0.25Tl_0.75)-1234 phase formation till x=0.6 wt.%. Granular investigation by scanning electron microscope reveled that both number and size of voids decreased as x increased from 0.0 to 0.6 wt.%. These results were supported by the sample’s porosity calculation. The superconducting transition temperature and critical current density were found to have optimum values at x=0.6 wt.%. Furthermore, the Vickers microhardness increased till x=0.6 after that it decreased with further increase in x.     

Stabilization of the 108 K superconducting phase in the Bi0.7Pb0.3SrCaCu1.8Oy system

Samples having the nominal composition Bi_0.7Pb_0.3SrCaCu_1.8O _y , were prepared by a solid-state reaction and characterized by energy-dispersive x-ray microanalysis, resistivity measurements, and magnetically modulated microwave absorption (MAMMA). Longer annealing times resulted in more homogeneous samples having a single superconducting transition to zero resistance at 108 and 105 K at 0.1 and 1 mA, respectively. The MAMMA measurements clearly show differences in sample homogeneity with different annealing times.     

Observation of X-band SQUID signals in a High-Tc superconducting film device

A microwave superconducting magnetometer is described in which a microstrip resonator is coupled to a two-hole high-T _c thin-film SQUID device. Both the microstrip circuit and the thin-film SQUID were fabricated by photolithography techniques. The YBCO thin film was deposited on single-crystal substrate of yttria-stabilized zirconia [YSZ(100)] by an ion beam sputtering technique producing a superconducting transition measured at a critical temperature ofT _c =92 K to within T 3 K. Non linear oscillatory behavior was observed in the microstrip resonator when inductively coupled to the SQUID. This nonlinear behavior yielded a microwave device in which the reflected microwave power varied with applied DC magnetic flux.     

Thermoelectric Power of Deoxygenated Bi1.6Pb0.4Sr2Ca2Cu3O10 + δ Sintered Superconducting Pellets

The thermal variation of the electrical resistivity and thermoelectric power of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_{10 +} pellets subjected to various degrees of deoxygenation is reported. The temperature dependence of the electrical resistivities of deoxygenated samples displays gradual transformation from metallic-like to semiconductor-like features in the normal state. All the samples however, show superconducting transition, but increasing deoxygenation depresses T _C0 from 102 to 45 K. Gross features of the temperature variation of thermoelectric power observed in properly oxygenated (Bi, Pb)-2223 cuprates are retained in all the deoxygenated samples. Our results on electrical resistivity and thermoelectric power in the normal state have been found to be consistent with a two-band model.     

Surface Superconducting State of Heavy-Fermion Superconductor CeIrIn5 Probed by CeIrIn5-Ag Junctions

The CeIrIn₅-Ag junctions of about 2×10^–9 cm^–2 area have been made using microfabrication techniques, and the surface superconducting state of CeIrIn₅, which has two characteristic temperatures T ₀ and T _c, has been investigated, where T ₀ and T _c are the transition temperature to zero-resistivity state and the bulk, thermodynamic transition temperature, respectively. The temperature, below which superconducting anomalies are observed, varies from junction to junction, and yet it is always well above T _c=0.4 K. This result, together with no indication of transition at T _c, suggests that at least the surface of CeIrIn₅ is in the superconducting state above T _c. The data on the critical current I _c in superconducting anomalies point to the possibility to define a local transition temperature for each junction.     

Evaluation of superconducting ceramic thick films produced from a large bulk specimen of YBa2Cu3Ox

This experimental study concerns the production and evaluation of YBa₂Cu₃O _x superconducting thick films produced by slicing these films from a large bulk specimen. The resistivity/temperature behavior of these films was found to be equivalent to, or better than, the resistivity/temperature behavior of identically prepared bulk specimens made from the same material. Practical applications involving the use of this technique are probably most likely in hybrid microelectronic devices. Applications involving larger-scale electronic circuits and other superconducting devices are also possible.     

Superconductivity at 103 K in CdBa2(Ca0.7Y0.3)Cu2Oy

A cadmium analogue of the mercury system with nominal composition CdBa₂(Ca_1–xY_x)Cu₂O_y has been synthesized. Thex=0 samples contain about 12 vol.% of the 1212 phase but are not superconducting. Thex=0.3 samples are superconducting atT _on = 103 K. The EDX analysis of 18 microcrystals shows a broad cationic distribution of the different components. The observed broad superconducting transition is attributed to the variousT _c of the different microcrystals.