Effect of Bi/Pb ratio and annealing temperature onTc and formation of high-Tc phase in Bi-Pb-Sr-Ca-Cu-O superconductor

We studied the effect of Bi/Pb ratio and annealing temperature onT _c and formation of the high-T _c ; phase in Bi-Pb-Sr-Ca-Cu-O superconductor by the three-step reaction process. The optimum Bi/Pb ratio is about 1.80.3 and the optimum annealing temperature is about 845–855°C. It is found that a variate high-T _c phase existed at the higher annealing temperature. The zero-resistance temperature of the variate high-T _c phase decreased when the annealing temperature increased, although the phase is isostructural with the 110 K phase.     

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.     

Microwave absorption in high-Tc superconductors

We have investigated the 9- and 10-GHz microwave properties of bulk samples of strontium and barium substituted La₂CuO₄ and YBa₂Cu₃O₇ and one YBa₂Cu₃O₇ MBE film on SrTiO₃. Anomalous microwave absorption is seen especially for poorly compacted bulk ceramic samples. Transitions were generally broadened nearT _c and the residual losses were high. The real part of the surface impedance could be fit belowT _c to the empirical equation with an added term linear in temperature. The microwave response of the film was masked by paraelectric absorption and acoustic resonances in the substrate.     

Superconductivity of Ba1 ? xKxBiO3 (0.35 < x < 1) Synthesized by the High Pressure and High Temperature Technique

Ba_{1 – x} K _x BiO₃ (BKBO) samples with 0.35 < x < 1 were synthesized by the high pressure and high temperature technique. XRD analysis showed that the BKBO samples were single phase for the whole range of the potassium doping concentration. The change of superconducting transition temperature, T _c, as well as lattice parameters have been investigated upon doping concentration. As the K doping concentration (x) increases from x = 0.37, T _c decreases from 30.4 K to almost zero at x = 0.74. However, in some BKBO samples without including any barium in the starting composition (x = 1), which is denoted as KBO samples, superconductivity is observed with T _c as high as 9 K with partial substitutions of Bi at the K site. Depending on the synthesis condition of the KBO samples, T _c and lattice parameters were different from sample to sample. Compared with other superconducting bismuthates, the evolution of T _c by potassium doping in the cubic BKBO system is discussed in terms of its electronic band structure.     

t-Model mechanism of high-Tc cuprates

A doublon formalism ofd-p model Hubbard Hamiltonian is studied only with transfer integrals, and without the superexchange term. This gives excellent doping-dependent features ofT _c in high-T _c cuprates.     

Effect of superconducting fluctuations on the nmr relaxation rate of high-Tc superconductors

The effect of superconducting order parameter fluctuations on the nuclear-spin relaxation rate, 1/T ₁, is studied for clean two-dimensional systems by calculating the three Maki-Thomson-type diagrams which represent the lowest-order fluctuation contributions to the transverse susceptibility. For Gaussian fluctuations and for temperatures near the mean field transition temperature,T _c0, we employ a weak-coupling theory in which the pair-fluctuation propagator can also include pair-breaking effects. We also go beyond the Gaussian theory and take into account the interactions between Cooper-pair fluctuations corresponding to the fourth-order Ginzburg-Landau fluctuation terms. We compare our results with previous results in the dirty limit and in 3D. We obtain a pronounced peak in 1/T₁ atT _c and briefly discuss possible reasons why this peak is not observed.     

Specific heat of the high-Tc oxide superconductors

We review the specific heat measurements on La₂CuO₄, La_2–xM_xCuO₄ (M = Ca, Sr, and Ba), YBa₂Cu₃O₇, and the Bi-Ca-Sr-Cu-O and Tl-Ca-Ba-Cu-O systems. Tables of properties derived from the data are presented. Results on RBa₂Cu₃O₇ (R=rare earth elements other than Y) are summarized, as are results on YBa₂(Cu_3–xM_x)O₇ (M=Zn, Cr, Fe, or Ni). The difficulties of analyzing the specific heat data, and specifically the separation of the contributions associated with magnetic impurities, are discussed. It is tentatively concluded that the data nearT _c are consistent with BCS theory, although they show evidence of fluctuation effects. It is also concluded that the low-temperature zero-field data on a majority of the high-T _c oxide superconductors provide evidence of anintrinsic term that is proportional toT, a result that is inconsistent with a gap in the electronic density of states.     

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.     

Superconductivity at differentTc in CdBa2Ca2Cu3Oy

A Cd analogue of the Tl and Hgn =3 series with nominal composition CdBa₂Ca₂Cu₃O_y has been synthesized. The samples were superconducting according to magnetic susceptibility measurements. The critical temperature was 103 or 107 K depending on the preparation conditions. The EDX analysis revealed the presence of Cd-1111, Cd-1121, and Cd-2333 as minor phases. The observed diamagnetic effects were attributed to the differentT _c of these phases.     

Magnetic hysteresis of the zero-resistance critical temperature of high-Tc granular superconductors

We have found a notorious hysteretic behavior in the dependence of the zero-resistance critical temperature obtained through resistivity () versus temperature (T) measurements with applied field (H _e ) in High-T _c granular superconductors. This behavior is explained semi-quantitatively based on the analogy between the present observation and a similar hysteresis found in the field dependence of the transport critical current in these materials.     

Measurements ofHc2(T) in Bi-Sr-Cu-O

H _c2(T) has been measured for thin BSCO films at temperatures down to 65 mK and pulsed fields up to 35 T.H _c2(T) diverged anomalously as the temperature decreased. At the lowest temperature, it was five times that expected for a conventional superconductor.     

Magnetic field enhancement of high-Tc superconducting pancake coils by Mu metal sheets

High permeability ring-shaped mu metal sheets have been used to enhance magnetic fields in the bore of Bi-based high-T _c superconducting coils. The central magnetic fieldB ₀, generated by pancake coils placed between mu metal sheets, was measured at liquid nitrogen temperature 77 K. The increasing rate (IR) of the central magnetic fieldB ₀ for single pancake coils was from 15 to 63% by using a 0.1 mm thick mu metal sheet. IR was decreased when the originalB ₀ value increased. The thickness of mu metal sheets also affected IR. It was found that IR increased as the number of mu metal sheets was increased. In a test double-pancake coil IR reached 101%,B ₀ was enhanced from 902 to 1815 G, when the total thickness of mu metal sheets on the top and bottom surface of the coil was 2.0 mm. These results indicate that the well-designed high-permeability materials can significantly enhance the magnetic fields generated by high-T _c superconducting coils and magnets.     

High-Tc superconductor in an a.c. field

The behavior of high-T _c oxides in an alternating field is studied. The short coherence length and the layered structure require an unconventional microscopic analysis. The complex conductivity(, k) is evaluated. The frequency and temperature dependences of the impedance are obtained (in the region of small frequencies). The effect of impurities is discussed. The microwave properties of the proximity system S_h-N (S_h and N are high-T _c and normal films, respectively) are discussed.     

Theab-plane optical conductivity of high-Tc superconductors

There is anisotropy in theab-plane optical properties of the high-temperature superconductors, both in the normal state and in the superconducting state. In both states, two components appear in the optical conductivity: a free carrier part and a midinfrared component. BelowT _c , the free carriers form the superconducting condensate. In YBa₂Cu₃O_7–, the anisotropy of the penetration depth shows that the chains contribute strongly to this superfluid. In Bi₂Sr₂CaCu₂O₈, where chains are absent, there is stillab plane anisotropy. BelowT _c a finite absorption parallelb remains at frequencies as small as 20 meV. This anisotropy could be due to anisotropy either of the superconducting gap or the midinfrared component.     

Measurements of the surface resistance of high-Tc superconductors at high RF fields

We report measurements of the r.f. surface resistance of a wide variety of high-T _c superconductors in bulk form and as films on silver substrates. Several apparatuses have been constructed and used for measurements at frequencies from 0.15 to 40 GHz and r.f. surface magnetic fields as high as 640 G. In every case in which the field dependence of the surface resistance was measured, the surface resistance increased monotonically with field amplitude through a transition region characterized by a strong field dependence. It then saturated at high field at a value of a few percent of the normal-state surface resistance just aboveT _c . In the presence of this field dependence, the frequency dependence of the surface resistance changed from quadratic to less than linear.     

Low-field magnetic properties of high-TcGdxY1?xBa2Cu3Oy superconductor

The magnetization of Gd_0.85Y_0.15Ba₂Cu₃O _y high-T _c superconductor was measured at low fields (<100 Oe) and at a constant temperature of 77 K. The magnetization curves deviate from linearity at 25 Oe, causing the destruction of the weak couplings of the superconducting grains. The hysteresis due to granular superconductivity was also observed.     

Preparation of YBa2Cu3O7–x high-Tcceramic superconductors from melt

Fabrication of high-T _c ceramic superconductor in the system Y₂O₃-BaO-CuO by melting a mixture of component oxides has been investigated. The compositions of the resulting specimens and the effects of heat treatment have been investigated. It was determined that molten material was composed of phases including BaCuO₂, CuO, Y₂O₃, and Y₂BaCuO₅. A subsequent heat treatment in air produced a nominal amount of the high-T _c phase, while heat treatment in an O₂ atmosphere resulted in a significantly large percentage of the superconducting phase.     

Nanostripes in GdBa2Cu3Ox high-Tc superconductors

High-T_c superconductors with light rare earth (LRE) elements instead of Y exhibit nanoscale stripe structures on the surface as observed by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) scans. Within the GdBa₂Cu₃O_x (GdBCO) system exhibiting relatively high critical current densities, nanoclusters arranged in a stripe-like fashion are observed in undoped material, while adding of nanoparticles (ZnO₂, ZrO₂) leads to the formation of nanostripes as observed in other LRE superconductors. The nanostripes in doped GdBCO exhibit periodicties between 20 and 50 nm and corresponding step heights of 0.3–0.8 nm. Using polarized light microscopy and electron backscatter diffraction (EBSD) analysis, we determined the direction of the nanostripes with respect to the known twin structure.     

Upper limit ofTc for the copper oxides

The value of the critical temperature of the cuprates correlates with the doping level and is affected by the interplay of two competing factors: (1) the increase in carrier concentration, and (2) the pair-breaking effect of magnetic impurities. An analysis of the temperature dependence of the critical field leads to the conclusion that magnetic impurities are present even in a sample with the maximum observed value ofT _c.A new parameter, intrinsicT _c (T _cintr), which is its value in the absence of magnetic impurities, is introduced. The maximum value ofT _cintr, which corresponds to the maximum doping level, appears to be similar for different cuprates and to be equal to 160–170 K. This is the upper limit ofT _c in the cuprates.     

Two Pseudogap Behavior in La2?xSrxCuO4: Thermoelectric Power at High Temperature

Thermoelectric power (TEP) of high-T _C superconductors has been investigated in a wide range of temperature (T _C < T < 700 K) for La_2–x Sr _x CuO₄. TEP of La_2–x Sr _x CuO₄ shows different temperature dependences in three temperature regions. In the low-temperature region, a positive broad TEP peak is observed near T _p, which shifts to lower temperature upon doping. As temperature increases, TEP decreases linearly at intermediate temperature. In the high-temperature region, TEP deviates from the linear temperature dependence at a certain temperature, T _h showing a saturation behavior. As the doping concentration increases, the characteristic temperatures, T _C, T _p, and T _h, show systematic changes. In comparison with pseudogap temperature estimated from other experiments, the large pseudogap behavior in TEP at high temperature has been discussed and distinguished from the small pseudogap observed at lower temperature. A possibility of bound pairs formation in the normal state opening the pseudogap at high temperature is discussed briefly. The coexistence of bound pairs and the normal independent carriers for T _C < T < T_h could be the origin of the intrinsic inhomogeneity.