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.     

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.     

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.     

Superconductivity of barium-doped Bi2Sr2CaCu2Oy

Sintered ceramic samples of Bi₂Sr_2–x Ba _x CaCu₂O_y with nominal barium fraction 0x0.3 have been prepared by the solid-state reaction method. WDS studies verified that barium enters the superconducting phase. For slowly cooled samples, the midpointT _c of the superconducting transition is significantly increased by barium doping, whereas for quenched samplesT _c is little affected. The increase ofT _c with increasing barium fraction is consistent with a decrease in the hole concentration in the superconducting layers.     

Superconducting Transition Temperatures in the Ru-1232 System, RuSr2(Gd1?xLnxCe1.8Sr0.2)Cu2Oz (Ln = Sm, Dy, and Ho)

We have investigated effects of the lanthanide element Ln and the composition changes on the superconducting transition temperature T _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 temperature T _co are almost the same for x = 0.00–0.15, and each of the lattice parameters a and c is almost constant. While, in each of the cases of the samples with Ln = Dy and Ho, the sample with x = 0.05 shows the maximum values for both the superconducting onset temperature T _co and the zero resistivity temperature T _cz. Especially for the sample with Ln = Dy, the values of T _co and T _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 of T _co, lattice parameters of a and c in the RuSr₂(Gd_{1− x} Dy _x Ce_1.8Sr_0.2)Cu₂O _z system as a function of Dy content x, the relationship between the superconducting transition temperature and the lattice parameters in the present system are investigated.     

Substitution Effect of Ba for Sr on Superconductivity in?the?(Pb0.75P0.25)(Sr2?xBax)(Y0.4Ca0.6)Cu2Oz System

The Pb-based 1212 compounds containing phosphorus were already discovered by us in the (Pb,P)Sr₂(Y,Ca)Cu₂O _z system. Among the almost-single phase samples, a sample with the nominal composition of (Pb_0.75P_0.25)Sr₂(Y_0.4Ca_0.6)Cu₂O _z was a superconductor with the highest T _c of 38 K in the system. Recently, we have been investigating on substitution effect of Ba for Sr on superconductivity in the (Pb_0.75P_0.25)(Sr_2−x Ba _x )(Y_0.4Ca_0.6)Cu₂O _z system. As a result, it is found that the almost–single 1212 phase samples are obtained in the composition area of 0.0≤x≤0.4. In this area, with increase of x, the lattice parameters a,c and the cell volume V are found to be gradually increasing. This finding can be considered to show that larger Ba²⁺ ions certainly occupied the Sr-sites in the sample. However, we obtain such interesting results as that the T _c value is not so enhanced by the substitution in comparison with that of the sample of x=0.0, and that the maximum value of T _c was 40 K for x=0.2 and 0.3 at the best.     

Pressure dependence of Tc in Tl0.2Hg0.8Ba2Ca2Cu3Ox

We report results of our study of the pressure dependence of T_c in Tl_0.2Hg_0.8Ba₂Ca₂Cu₃O_x to 6 GPa in a diamond anvil cell. The sample studied has an ambient T_c of 136.4 K and application of quasihydrostatic pressure raised T_c to 144.4 K at 6 GPa. We will also describe recent modifications of our ac susceptibility apparatus that have improved our sensitivity while simplifying the coil design and the balancing.     

High-pressure Bi(Mg1 ? xZnx)1/2Ti1/2O3 perovskite solid solutions

We report the preparation of Bi(Mg_{1 − x} Zn _x )_1/2Ti_1/2O₃ ceramics at a pressure of 6 GPa and temperatures from 1370 to 1570 K. In the composition ranges 0 < x < 0.5 and 0.8 < x < 1, the ceramics consist of perovskite solid solutions. The solid solutions with x < 0.5 have an orthorhombic structure (sp. gr. Pnnm), and those with x > 0.7 have a tetragonal structure (sp. gr. P4mn). On heating at atmospheric pressure, the solid solutions with 0.1 < x < 0.5 undergo an irreversible orthorhombic-to-rhombohedral (sp. gr. R3c) phase transition, accompanied by heat absorption and an increase in specific volume. The transition temperature decreases with increasing x. The reduced perovskite cell parameter of the solid solutions increases linearly with x. The Curie temperature of the rhombohedral solid solutions exceeds the decomposition temperature of the perovskite phase.     

On the Presence of 1D-Like Stripes in Superconducting Epitaxial La2-xSrxCuO4 thin Films

A large positive magnetoresistance (up to tens of percents) is observed in both underdoped (x < 0.15) and overdoped superconducting La_{2− x}Sr_xCuO₄ (LSCO) epitaxial thin films, at temperatures far above the superconducting critical temperature T _c . In the underdoped regime, this magnetoresistance cannot be described by the Kohler rule and is due to the influence of superconducting fluctuations. On the other hand, in the overdoped regime, the Kohler rule does not seem to be violated. The strong magnetoresistance above T _c can be related to the preformed superconducting pairs existing well above T _c but forming a phase coherent superconducting state below T _c . The observations support the idea of a close relation between the pseudogap and the superconducting gap and provide evidence for the presence of pre-pairs above T _c . Both the observed fluctuations and the observed magnetoresistance are in accordance with the existence of 1D-like stripes. These results are further supported by recent high magnetic field measurements (up to 55 T) of the transverse magnetoconductivity σ _xy, which goes to zero for T→0 K.     

High-pressure dc sputtering of High-Tc BPSCCO thin films withTc(0) above 100 K

High-T _c Bi(Pb)-Sr-Ca-Cu-O thin films have been made on single-crystal MgO substrates using high-pressure dc sputtering technique. X-ray studies confirm the crystallinity and highly oriented structure withc-axis perpendicular to the substrate. By optimizing the annealing schedule the formation of the high-T _c phase is stabilized. The best film exhibited superconducting transition temperature with zero-resistance temperature,T _c(0), as high as 101 K. Temperature dependence ofJ _c indicates the presence of Josephson-type weak links.     

High-Temperature Thermoelectric Power of The Metal Oxides: La2?xSrxCuO4 and Bi1?xSrxMnO3

We have investigated the high-temperature thermoelectric power (TEP) of La_{2− x} Sr _x CuO₄ (0.05 ≤ x ≤ 0.35) and Bi_{1− x} Sr _x MnO₃ (0.5 ≤ x ≤ 0.8) up to 700 K. Based on the TEP results we have discussed the phase transitions on each case. In the case of high-T _C cuprates, La_{2− x} Sr _x CuO₄ (0.05 ≤ x ≤ 0.35), the TEP shows different temperature dependences in three temperature regions. At low temperature, the positive TEP rises showing a broad peak at temperature T _P, which shifts to lower temperature upon Sr doping. Right above T _P, the TEP decreases linearly as temperature increases. At high temperature, TEP deviates from the linear-T dependence at a certain temperature, T _H, showing a saturation behavior. The systematic change of the TEP behavior is discussed in terms of the two-fluids model, which is an intrinsically inhomogeneous state, consisted of bound pairs and independent carriers in the normal state of the high-T _C superconductors. For Bi_{1− x} Sr _x MnO₃ (0.5 ≤ x ≤ 0.8), the negative TEP is almost temperature-independent in the high temperature regime (T _CO < T < 700 K). Near the charge ordering temperature (T _CO), however, TEP suddenly decreases with decrease of temperature, indicating the suppression of carrier mobility with charge ordering transition. As Bi concentration decreases, T _CO shifts to lower temperature from T _CO ∼ 520 K for x = 0.5 to T _CO ∼ 435 K for x = 0.8, which suggests that charge ordering is related to the local lattice distortion due to highly polarizable 6s² character of Bi³⁺ ion. In comparison with the resistivity data, the TEP results have been discussed in terms of the carrier localization accompanied by local lattice distortion.     

The Doping Effect on the Lattices and Electronic Structure in?Superconducting Fe-based Compounds Sr1?xKxFe2As2

The lattices and density of states (DOS) for the novel superconductor Sr_1−x K _x Fe₂As₂ (x=0–1) are calculated based on the density functional theory with the scheme of the linearized augmented plane wave and the improved local orbital (APW+lo). The effects of K-doping on the lattice indicate that the K substitution induces a modification of the FeAs₄ tetrahedron into the regular one gradually. DOS calculations reveale that Fe 3d and As 4p states play a key role to the Fermi surface, the peak at x=0.5 for the total N^l(E _F) DOS at Fermi surface which would be favor of the transition of electrons, and leads to the maximal T _c in Sr_1−x K _x Fe₂As₂ system. This result was considered as a possible origin of the observed T _c behavior in the experiment of Sr_1−x K _x Fe₂As₂ system.     

Effect of high-pressure compaction on the properties of Tl2Ba2CaCu2OyFx (x = 0, 0.1, 0.2) ceramics

Tl₂Ba₂CaCu₂O _y F _x (x = 0, 0.1, 0.2) high-T _c ceramics have been prepared using high-pressure compaction. Their phase composition and electrical properties have been studied, and their crystal structure have been refined by the Rietveld method. The results demonstrate that high-pressure compaction makes it possible to reduce the synthesis temperature of Tl₂Ba₂CaCu₂O _y F _x ceramics from 840 to 825°C and to obtain more structurally perfect materials, with a higher density and narrower superconducting transition (4–7 K). With increasing fluorine content, the carrier (hole) concentration in the Tl₂Ba₂CaCu₂O _y F _x ceramics decreases, leading to changes in their superconducting transition temperature. The Cu-Cu bond distance and CuO₂-Ba interplanar spacing are proposed as key parameters for T _c control. Original Russian Text ? A.I. Akimov, S.A. Lebedev, T.V. Tarasevich, O.V. Ignatenko, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 3, pp. 375–384.     

Nature of the Pseudogap in High-Tc Cuprates: Analysis?of?the?Bulk Magnetic Susceptibility of?La2?xSrxCu1?yZnyO4

The nature of the pseudogap in the quasiparticle spectral density in high-T _c cuprates is a matter of intense debate. In this study we have investigated the effects of Zn substitution on the uniform (q=0\boldsymbol{q}=0, where q\boldsymbol{q} is the wave vector) magnetic susceptibility, χ(T), of La_2−x Sr _x Cu_1−y Zn _y O₄ sintered samples with different hole concentrations, p(≡x), over a wide range of Zn contents (y). Non-magnetic Zn suppressed T _c most effectively and enhanced χ(T) systematically at low temperatures. We have extracted the characteristic pseudogap energy scale, ε _g, from the analysis of χ(T) data. Unlike T _c,ε _g was found to be fairly insensitive to the level of Zn substitution, even when T _c was completely suppressed by Zn. The Zn-induced Curie-like enhancement of the χ(T) was also found to be closely related to the PG energy scale. We discuss the possible implications of these findings in this paper.     

Magnetocaloric effect of (ErxR1−x)Co2 (R = Ho, Dy) for magnetic refrigeration between 20 and 80 K

Two series of RCo₂ compounds, (Er_xHo_1−x)Co₂ and (Er_yDy_1−y)Co₂, were investigated as magnetic refrigerants for hydrogen liquefaction. A large magnetocaloric effect (MCE) was observed just above the Curie temperature (T_c) of these compounds which both undergo a first-order magnetic transition. The isothermal entropy change ΔS and the adiabatic temperature change ΔT_ad of these compounds were larger than those of other Laves-phase materials such as RAl₂ and RNi₂. However, the temperature range of the large MCE for these compounds was limited. It has been shown that T_c increases almost linearly against the de Gennes factor, and can be controlled by changing the concentration of the rare earth elements while maintaining a first-order transition. By measuring the magnetization and heat capacity, we obtained temperature entropy (T–S) diagrams, which are essential for analyzing the magnetic refrigeration cycle. Both series of compounds showed high potential for use in a regenerative thermal cycle, especially as a combination of several compositions to cover a wide temperature range.     

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.     

The Underdoped Region of the Phase Diagram of YBa2Cu3O6+x

Here we present a reviewed phase diagram of the high-T _c superconducting YBa₂Cu₃O_{6+ x} compound, finely mapped in the strongly underdoped region (0 < x < 0.5), from the pure antiferromagnetic state to the superconducting regime. The Neèl and spin freezing temperatures have been measured by μSR experiments while the hole density per Cu atom in the CuO₂ planes has been determined from the resistive T _c and from Seebeck coefficients at 290 K. The phase diagram is discussed in comparison to those of La_{2− x} Sr _x CuO₄ and Y_{1− x} Ca _x Ba₂Cu₃O₆ cuprate systems.     

Lattice instability in the normal state of La2?xSrxCuO4

Novel softening has been found in the transverse elastic constant (C ₁₁–C ₁₂)/2 below 50 K in single-crystalline La_1.86Sr_0.14CuO₄ (LSCO) by high-precision ultrasonic measurements in magnetic fieldsH along thec axis. With decreasing temperature, this lattice softening persists down to the superconducting transition temperatureT _c(H), which is reduced to 14 K by applying fields up to 14 T. BelowT _c(H) the softening turns to rapid hardening. This behavior indicates the presence of lattice instability of the orthorhombic (Bmab) structure in the normal state of LSCO, which disappears in the superconducting state. This is evidence for the intimate interplay between high-T _c superconductivity and the lattice instability in LSCO.     

Investigation of Gd Addition Added on Magnetic and Structural Properties of Bi1.8Pb0.35Sr1.9Ca2.1Cu3GdxOy Superconductors by ac Susceptibility

This study reports the effect of Gd addition on magnetic and structural properties of Bi_1.8Pb_0.35Sr_1.9Ca_2.1Cu₃Gd _x O _y superconductor with x=0, 0.1, 0.2, 0.3, 0.4 and 0.5 by means of ac susceptibility measurements at various ac fields (ranging from 270 to 1352 A/m) and scanning electron microscopy (SEM) images. Critical onset (T_c^on)T_{\mathrm{c}}^{\mathrm{on}}) and loss peak temperatures (T _p) were qualitatively estimated from the ac susceptibility curves. The peak temperature at zero ac-magnetic field (T _p0) and intergrain critical current densities (J _c) were theoretically calculated from the ac susceptibility plots via the critical state models. The results show that peak temperatures and critical current densities were found to decrease with increasing Gd addition. Moreover, using a self-field approximation together with J _c dependence on temperature, the characteristic length (L _c) associated with the pinning force is estimated to be approximately the same as the average grain size (R _g) of the pinning center because of the linear decrease in J _c with increasing temperature. Surface morphology and grain connectivity of the samples were also obtained to degrade with increase in the Gd addition from SEM investigations.     

Crystallographic and low frequency conductivity studies of the spinel systems CuFe2O4 and Cu1?xZnxGa0.1Fe1.9O4; (0.0 ≤ x ≤ 0.5)

Spinel solid solutions of CuFe₂O₄ and Cu_1−xZn_xGa_0.1Fe_1.9O₄ with (0.0 ≤ x ≤ 0.5) are synthesized. Crystallographic phase transformation from tetragonal-to-cubic occurred at x = 0.2. The derived structural parameters manifest that Zn occupies the tetrahedral A-site while Cu and Ga occupy the octahedral B-site and Fe distributes among A- and B-sites. Electrical conductivity measurements of these materials as a function of temperature and frequency revealed semiconducting behavior except CuFe₂O₄ sample, which has a metallic behavior at low frequency and at high frequency, semiconductor-to-metallic transition occurred as temperature increases. The metallic behavior in this sample is attributed to cation-cation interactions at B-site while, the semiconductor behavior in Cu_1−xZn_xGa_0.1Fe_1.9O₄ compounds is due to the cation–anion–cation interactions at the same site in the spinel lattice. All compositions exhibit transition with change in the slope of conductivity versus temperature curve. This transition temperature (T_c) decreases linearly with increasing Zn content x. The relation of the universal exponent s with temperature gives evidence that over large polaron OLP and correlated barrier hopping CBH conduction mechanisms are presented in CuFe₂O₄ and Cu_1−xZn_xGa_0.1Fe_1.9O₄ compounds respectively.