Influence of sintering time and quenching on the formation of highT c phase

Influence of sintering time and quenching in Bi_{2 −x} Pb _x Ca₂Sr₂Cu₃O _y (x=0.0, 0.1, 0.2, 0.25, 0.3 and 0.4) samples have been studied by resistance and XRD measurements. In samples sintered at 850°C for 4 days,T _c(0) increases with Pb concentration.T _c(0) increased from 81 K forx=0.0 to 109 K inx=0.30 sample and then decreased. Increasing the sintering time to 10 days decreased theT _c Quenching further decreased theT _c(0). From X-ray diffraction patterns, the intensity peaks of low and highT _c phases have been measured. The addition of Pb promotes highT _c-phase. Sintering time, slow cooling and rapid quenching studies show that there is an optimum sintering time and cooling rate to produce a highT _c-phase.     

Preparation and degradation behaviour of Bi-cuprate superconductors

The role of preparation conditions and the effect of addition of Pb in Bi-Sr-Ca-Cu-O (BSCCO) superconductor, on theT _c,J _c and grain orientation have been studied. Calcination at a temperature higher than the melting point of Bi₂O₃ after the prereaction at 800°C leads to formation ofc-axis oriented nearly single phase material. The presence of lead yields a nearly highT _c phase (2223) exhibiting a maximum transition temperatureT _c=110 K and ΔT _c=2 K. The addition of lead and grain orientation together result in an increase inJ _c by nearly two orders of magnitude. The degradation behaviour was studied in terms of changes inT _c,J _c and structure on exposure to atmosphere up to 250 days. Samples with 0.6 Pb were found to be superconducting withT _c=97 K even after 250 days. The highT _c phase was stable against degradation. Our studies indicate that the degradation of leaded BSCCO is a surface phenomenon rather than bulk phenomenon.     

Preparation of RuSr2GdCu2O8 Compounds with Enhanced Superconducting Transition Temperature

The RuSr₂GdCu₂O₈ (Ru1212) crystal with the highest superconducting transition temperature T _c(end)=54.0 K (T _c(onset)=65.0 K) has been obtained from the nominal composition of Ru:Sr:Gd:Cu=0.9:2.0:1.0:2.1. The prepared compounds are not in a single Ru1212 phase and contain a small amount of Sr₂GdRuO₆ and SrRuO₃ impurity phases. Possible correlation between the high-T _c phase and the impurity phase is discussed.     

Preparation, Structure, and Properties of (Bi,Pb)2Sr2Ca2Cu3O10 + δ Ceramics with Si3N4 Additions

The microstructure, phase composition, texture, and superconducting properties (T _c, T _c, j _c(T), and R(T) at H= 0 and 5 mT) of (Bi,Pb)₂Sr₂Ca₂Cu₃O_{10 +} ceramics (sintering at 840°C for 36 h) with ultrafine Si₃N₄ additions (0.05–0.2 wt %) are studied. The introduction of 0.05–0.1 wt % Si₃N₄ is shown to reduce the width of the superconducting transition by 2–3 K and to raise the critical current density at temperatures below 95 K.     

Impact of Gd Doping on Morphology and Superconductivity of NbN Sputtered Thin Films

We report the effect of Gd inclusion in the NbN superconductor thin films. The films are deposited on single crystalline Silicon (100) by DC reactive sputtering technique, i.e., deposition of Nb and Gd in presence of reactive N₂ gas. The fabricated relatively thick films (400 nm) are crystallized in cubic structure. These films are characterized for their morphology, elemental analysis, and roughness by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDAX), and Atomic Force Microscopy (AFM) respectively. The optimized film (maximum T _c ) is achieved with gas ratio of Ar:N₂ (80:20) for both pristine and Gd-doped films. The optimized NbN film possesses T _c (R=0) in zero and 140 kOe fields are at 14.8 K and 8.8 K, respectively. The Gd-doped NbN film showed T _c (R=0) in zero and 130 kOe fields at 11.2 K and 6.8 K, respectively. The upper critical field H _c2(0) of the studied superconducting films is calculated from the magneto-transport [R(T)H] measurements using GL equations. It is found that Gd doping deteriorated the superconducting performance of NbN.     

Studies on Li-doped Ca0·85Sr0·15CuO2: the prototype of cuprate family of oxide superconductors

The layer-type compound, Ca_0·85Sr_0·15CuO₂, which is the prototype of the cuprate family of highT _c oxide superconductors is an electronic insulator, and has a low ordered moment at 300 K indicating that Cu²⁺ ions are highly correlated. Doping with lithium (up tox=0·15) decreases the room temperature resistivity drastically (from 10³ ohm·cm to 0·6 ohm·cm) but metallic behaviour is not induced below 300 K. The magnetic moment on Cu²⁺ remains low but our studies did not show any antiferromagnetic ordering below 300 K in any of the Li-doped phases.     

Preparation and characterization of non-random YBa2Cu3O7-Ag composite

Silver addition to the highT _c superconductor, YBa₂Cu₃O₇ (YBCO) is known to improve its mechanical properties and critical current (J _c) characteristics. We have synthesized the superconductor-metal composite YBCO-Ag by an electroless process at room temperature (using sucrose or hydrazine hydrate in an alkaline medium). The latter method produces a non-random coating whereby each grain of YBCO is coated uniformly by silver. The intrinsic highT _c of 90 K of YBCO can be recovered by compaction of the composite and subsequent heat treatment (900°C and 600°C) in oxygen.     

HighT c of liquid-quenched Bi1.6Pb0.4Sr2Ga2Cu3Ox

AlthoughT _c cannot be found for a liquid-quenched Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_x glassy sample, a highT _c is found after annealing for 24 h at 1100 K. The maximum offset temperature of the superconducting transition is 113.3 K at 2.2 × 10^–2mAmm^–2. The maximumT _{c off} is larger than that (the maximumT _{c off} is 103.4 K at 2.0 × 10^–2 mAmm^–2) of sintered specimens before liquid quenching.     

Pressure Effect on the Coexistence of Magnetism and Superconductivity in Ho1−x Dy x Ni2B2C

We have measured the transport properties of Ho_(1–x)Dy _x Ni₂B₂C (x=0.1, 0.4) single crystals to study the effects of high pressure conditions on the superconductivity and magnetism. The pressure affects the pair-breaking interaction by changing the magnetic exchange integral J _sf. In the case of x=0.1, the superconducting transition temperature T _c is higher than the Néel temperature T _N and decreases as the pressure increases. On the other hand, for x=0.4, when have lower T _N than T _c, T _c does not change with increasing pressure. These results are due to the effect of the pressure on J _sf.     

Sintering period dependence of composition and superconducting properties of (BiPb)SrCaCu,1.8Ox system

The material system (Bi_0.7Pb_0.3)Sr₁ Ca₁ Cu_1.8O _x forms, at 840° C, two major phases having a highT _c (100K) and a lowT _c (70 K) both of which consist of platelets and a non-superconducting minor phase which has a rod-like shape and is isolated by the major phases. As the sintering period increases, the amount of highT _c phase increases accompanying the decrease in lowT _c phase while the amount of the non-superconducting phase is independent of sintering period, resulting in a superconductor withT _c of 100 K. Changes in compositions of each phase also occur during sintering due to evaporation of bismuth and lead.     

Kondo effect and impurity interactions in the resistivity of diluteZnMn alloys

The electrical resistivity of diluteZnMn alloys (c=1.7–2400 ppm Mn) has been investigated in the temperature range from 0.05 to 14 K. For the most dilute sample, single-impurity Kondo behavior is found, well described by the Hamann formula withT _K=0.9 K,S=3/2. ForT<50 mK, aT ² law with _R =0.3 K is expected. In the dilute limit the Kondo slope is –(1/c)d()/d(logT)=3.7±0.2 µ-cm/at % dec. In the more highly concentrated alloys, the slope decreases with increasing c and the lnT-like variation of the Kondo resistivity roughly terminates near a temperatureT _W(c),T _W being related to the average Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction strength between the Mn impurities. For 20 ppm c 1000 ppm, the resistivity slightly decreases at low temperatures and a broad resistivity maximum is observed atT _m (c), withT _m c ^0.7. ForT 2 _m, the resistivity dependence is linear inT, and for the most concentrated alloy aT ^3/2 orT ² dependence is measured at the lowest temperatures attainable. The investigation of the transition temperatureT _c (c) to superconductivity ofZnMn results in a critical concentrationc _cr=18 ppm Mn. The concentration dependence ofT _c below 0.3 K suggests the presence of the Kondo effect, although impurity interactions may also influenceT _c in this temperature range.Supported by Deutsche Forschungsgemeinschaft.     

Low Temperature Heat Capacity of Layered Superconductors SrNi2Ge2 and SrPd2Ge2

Low-temperature heat capacity C(T) of the weakly electron-correlated SrNi₂Ge₂ 122-layer compound undergoes a superconducting transition with onset at 1.4 K and a bulk T _c =0.75 K, where heat-capacity jump ratio ΔC(T _c )/γT _c =0.88–1.05. A small average superconducting energy gap E _g (ave)=2.21 kT _c =0.14 meV is derived for this multi-gap superconductor. Similar results for isostructural SrPd₂Ge₂ include T _c (onset)=3.5 K, bulk T _c of 2.92 K, ΔC(T _c )/γT _c =0.70 and E _g (ave)=2.54 kT _c =0.64 meV. The higher T _c onset could be associated with stoichiometric 1:2:2 grains in the polycrystalline samples. In addition, deviations of E _g /kT _c from the BCS ratio of 3.5 suggest that, just like their iron-based counterpart, these 122-layer germanides may also exhibit an unconventional, fully-opened multi-gap s-wave superconductivity.     

Effect of Impurities on the Specific Heat of the Spin-Triplet Superconductor Sr2RuO4

We have investigated the specific heat of single crystals of the spin-triplet superconductor Sr ₂ RuO ₄ (T _c0 = 1.50 K) with various T _c . We focus on the T _c dependence of the specific heat jump at T _c and of the residual specific heat coefficient. The observed behavior is not quantitatively consistent with a model with a simple isotropic gap. We argue that inclusions of both the in-plane anisotropy of the gap and the dependence of the gap on different Fermi-surface sheets are needed to resolve this discrepancy.     

Upper critical field of Bi1·5-Pb0·5-Ca2-Sr2-Cu3-O x superconductor

The highT _c superconductor of nominal composition Bi_1·5-Pb_0·5-Ca₂-Sr₂-Cu₃-O _x was prepared by solid-state reaction method. The upper critical fieldH _c2 of the material with a zero-resistance transition at 110K was investigated by observing variations in resistance with temperature down to 90 K and with a magnetic field up to7T. The slope ofH _c2 with temperature was about −0·41T/K for zero resistance transition and −6·86T/K for onset of superconductivity.H _c2 values at 0K were estimated to be 31·3T and 563T for zero-resistance transition and onset of superconductivity respectively.     

CaCl2-H2O in the supercritical and two-phase ranges

Critical temperatures (T _c ) and densities (_c) for aqueous CaCl₂ solutions were measured using two different optical techniques, Measurements ofT _c were made using sealed silica capillaries containing 0,3, 0,5, 1,0, and 2,0 molal CaCl₂(aq)) solutions. TheT _c values from these measurements are 661, 666, 678, and 738 K, respectively. Critical temperatures were also determined from measured homogenization temperatures and the observed mode of homogenization (i.e.. to the liquid or vapor phase or by fading of the meniscus) in synthetic fluid inclusions. Critical temperatures determined by the second method for 1.0, 2.0, and 3.0 molal CaCl₂(aq) solutions are 680, 739, and 838 K. respectively. TheT _c for 4.0 molal CaCl₂(aq) was found to be in excess of 933 K, The capillary-tube measurements yield approximate values of the critical density _c, but do not give the critical pressureP _c Approximate values ofT _c ,. are available, however, from other sources.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder. Colorado, U.S.A.     

Detailed Physical Characterizations of Cu-rich and Ru-poor (Ru0.9Cu0.1)Sr2YCu2O7.9

The optimized nominal composition, (Ru_0.9Cu_0.1) Sr₂YCu₂O_7.9 sample, has been prepared through high-pressure and high-temperature solid-state densification method. The obtained material has been studied by X-ray (laboratory) diffraction powder technique, magnetization and detailed magneto-transport measurements. The title compound indicates bulk magneto-superconducting properties under field strengths of H=10, 100, 500 and 1000 Oe. It shows diamagnetic transition at T _d=54, 38, 20 and 8 K for H=10, 100, 500 and 1000 Oe, respectively, in the zero-field-cooled susceptibility measurements. The high-field (H=5 and 10 kOe) molar susceptibility measurements show sharp ferromagnetic transition at ∼150 K with reduced molar susceptibility values. The various field dependence of magnetization, M(H), isotherm curves recorded at constant temperatures (5, 10, 25, 50, 100 and 150 K) indicate ferromagnetic saturation, whereas the MH curves measured at 200 and 300 K conditions reveal the paramagnetic state of the compound. Though the sample showed onset transition temperature, T_c^onsetT_{\mathrm{c}}^{\mathrm{onset}}, at ∼34 K under different field strengths (H=0, 10, 30, 50, 70 and 90 kOe), no T_c^R=0T_{\mathrm{c}}^{R=0} is seen down to 2 K. Even under relatively low applied field (ΔH=10 kOe) the title compound shows large negative magnetoresistance (MR) of about 68% at 2 K and increases with increasing the field strength up to ΔH=90 kOe (MR=77% at 2 K). This value is amazing and probably higher than other 1212 type ruthenocuprates. The title compound which shows little negative MR (about 1%) in the high temperature regions (125–300 K) is not affected much by different field strengths. Among the different fixed temperature MR(H) isotherms, the MR(H) curve measured at 5 K shows maximum negative MR of about 47% at 90 kOe compared to other four (T=50, 100, 200 and 300 K) MR(H) curves.     

T c enhancement in La2−x Sr x CuO4 under anisotropic pressure

We have measured _c (T) for La _2–x Sr _x CuO ₄ (LSCO) with x=0.15 under anisotropic pressures. The c-axis compression, which decreases _c (T), reduces T _c drastically, whereas the c-axis stretch, which increases _c (T), enhances T _c from 38K at the ambient pressure to 51.6K at 8GPa. The T _c = 51.6K is the highest record so far reported for LSCO. The variation of T _c is well scaled with variations of _c . We deduce that the enhancement of T _c in LSCO is mainly caused by the suppression of interlayer coupling, though two dimensionality is generally known as a destructive factor for conventional superconductivity.     

Upper critical field of high-T c oxides: Evidence for 2e Bose liquid

We measure and theoretically describe the upper critical field, H_c ²(T), of BSCCO — 2212 crystals with T_c > 92K by the use of the out-of-plane resistivity in magnetic fields (H ab) up to 15T. The empirical procedure is proposed allowing us to extrapolate the H_c ²(T) curve up to H_c ² 230T and t/t _c 0.35 which is independent of the choice of the R/R_N ratio. We found that H_c2(T) does not follow the conventional theory with or without fluctuations and the magnetic scattering, but is consistent with the prediction based on the Bose-Einstein condensation of charged bosons formed aboveT _c . Our results together with several other measurements of H _c 2 and with the heat capacity data provide an evidence for the possibility of 2 _e Base liquid ground state of high-T _c oxides.We highly appreciate the enlightening discussions with Sir Nevill Mott, P. Edwards, J. Cooper, N. Hussey, J. Loram, A. Mackenzie, and K. Ziebeck.     

Preparation of epitaxial YBa2Cu3O7−x films by magnetron sputtering

Thin films of highT _c superconductor YBa₂Cu₃O_7−x were obtained by magnetron sputtering. MgO, YSZ, YSH and Al₂O₃ single crystals were used as substrates. Epitaxial films with tetragonal structure havingT _c 55–60 K grow at substrate temperaturesT _s between 930 K and 980 K. Orientation of the films in thisT _s range was (100) and (001) for (100) MgO substrate, (111) and (001) for (1012) Al₂O₃ and (111) YSH and (113) or (103) on (110) YSZ and (111) YSH. Single crystalline films with orthorhombic structure and (001) orientation were grown on all the substrates whenT _s exceeded 980 K. They haveT _c>80 K.     

Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La2−x Ce x CuO4−y and Pr2−x Ce x CuO4−y

From measurements of the magnetic penetration depth, (T), from 1.6 K to T _c in films of electron-doped cuprates La_2–x Ce _x CuO_4–y and Pr_2–x Ce _x CuO_4–y we obtain the normalized density of states, N _s(E) at T=0 by using a simple model. In this framework, the flat behavior of ^–2(T) at low T implies N _s(E) is small, possibly gapped, at low energies. The upward curvature in ^–2(T) near T _c seen in overdoped films implies that superfluid comes from an anomalously small energy band within about 3k _B T _c of the Fermi surface.