Superconducting Properties of Carbon and Zn Double-doped Mg1?xZnx(B1?xCx)2

A series of polycrystalline samples of Mg_1−x Zn _x (B_1−x C _x )₂ (x=0.00, 0.02, 0.04, 0.06, 0.08, and 0.10) were synthesized by a conventional solid-state reaction method under a background pressure about 10⁻³ Pa. Phase identification, crystal structure and superconducting transition temperature (T _c) were studied by means of X-ray diffraction (XRD) and resistivity measurements. The results indicated that the lattice parameters a and c show no clear trend in their changes with increasing doping level, and it turned out that the dopants had a marked effect on the crystal-lattice parameters and changed the crystal structure of the samples. The T _c for Mg_1−x Zn _x (B_1−x C _x )₂ decreased with C and Zn doping, but the rate of decrease was slower than single C-doped. We propose that the suppression of T _c by doping originates largely from the structural change.     

Comparison of Superconductivity Parameters of (Cu1?x Tl x )Ba2Ca4Cu5O14?δ Superconductor Synthesized at Different Temperatures

The superconductivity parameters of (Cu_1−x Tl _x )Ba₂Ca₄Cu₅O_14−δ (CuTl-1245) samples synthesized at different temperatures has been compared. Almost all the superconductivity parameters studied in this research work are suppressed with the increase of synthesis temperature beyond 890 ^○C, which may be due to defects induced by the volatility of some constituents such as thallium, oxygen deficiencies, and multiphase in the final compound. The fluctuation induced conductivity (FIC) analysis has shown a decrease in the cross-over temperature (T _o) as well as the mean-field critical temperature (T_c^mf)T_{\mathrm{c}}^{\mathrm{mf}}) and the shift of three- dimensional (3D) Aslamasov–Larkin (AL) regions to the lower temperature with the increase of synthesis temperature beyond 890 ^○C. A direct correlation between (T _o), the zero temperature coherence length {ξ _c (0)}, the interlayer coupling strength (J), mean-field critical temperature (T_c^mf)T_{\mathrm{c}}^{\mathrm{mf}}) as well as carrier concentration has also been observed.     

Extraction of Critical Current Density and Flux Creep Exponent in the Magnetic Superconductor Ru-1212 Using the ac Magnetic Susceptibility Measurements

The ac susceptibility data was employed to extract the temperature dependence of the critical current density, J _c(T), as well as the variation of flux-creep exponent n(T,H _ac) with temperature and ac field amplitude in bulk samples of polycrystalline magnetic superconductor RuSr₂GdCu₂O₈ (Ru-1212). The critical state models and the collective flux-creep approximation model were successfully accounted to describe such behavior below the transition temperature. The calculated values of n(T,H) are well fitted to a power law of the following form: n(T,H)=n ₀(H)T ^−s(H), where s is field dependent exponent whose values varied from −2.4, −1.01 for field amplitudes ranging from 0.5 G and 3.8 G. The power law describing the frequency dependence of χ′ is found to be consistent with the results of the current-dependent effective activation energy of the form U(J)=U ₀ln (J _c/J). Additionally, the dependence of the current density is found to scale according as: J _c(T)=J _c0(1−T/T _c) ⁿ , where the exponent n values varied from 1.05 to 1.25. Such dependence is an indication of intergrain coupling that could be ascribed in terms of superconductor–insulator–superconductor junctions. The derived temperature dependence of J _c(T) is in good agreement with the data obtained from the measurements using the traditional “loss-maximum” approach. Furthermore, the flux-creep effect increased with increasing both ac fields and temperatures except at about 15–25 K below the onset of T _c, where a slowing down of the flux creep was observed.     

Fabrication and magnetoresistivit of ex-situ processed MgB<Subscript>2</Subscript>/Fe monofilament tapes without any intermediate annealing

We have fabricated MgB₂/Fe monofilament wires and tapes by a powder-in tube (PIT) technique, using an ex-situ process without any intermediate annealing. MgB₂/Fe monofilament tapes were annealed at 650–1,050°C for 60 min and 950°C for 30–240 min. We have investigated the effect of annealing temperatures and times on the formation of MgB₂ phase, activation energy, temperature dependence of irreversibility field H _irr(T) and upper critical field H _c2(T), transition temperature (T _c), lattice parameters (a and c), full width at half maximum, crystallinity, resistivity, residual resistivity ratio, active cross-sectional area fraction and critical current densities. We observed that the activation energies of the MgB₂/Fe monofilament samples increased with increasing annealing temperature up to 950°C and with increasing annealing time up to 60 min while it decreased with increasing magnetic field. For the MgB₂/Fe monofilament tape, the slope of the H _c2–T and H _irr–T curves decreased with increasing annealing temperature from 850 to 950°C as well as with increasing annealing time from 30 to 60 min. The transport and microstructure investigations show that T _c, J _c and microstructure properties are remarkably enhanced with increasing annealing temperature. The highest value of critical current density is obtained for the sample annealed at 950°C for 60 min. The J _c and T _c^offset values of the sample annealed at 950°C for 60 min were found to be 260.43 A/cm² at 20 and 38.1 K, respectively.     

Thermal response of a fluid near its critical point:3He atT>T c

The local density response is studied in a simple fluid near the liquid-vapor critical point, subjected to temperature oscillations of its container. This investigation provides a new approach in the study of the adiabatic energy transfer (“piston effect”) in the fluid. The density response functionZ _F (ω, ε,z) is calculated for³He in the absence of stratification, where ω is the angular frequency, ε=(T−T _c )/T _c the reduced temperature,T _c =3.316 K the critical temperature, andz the vertical position in the container. Experiments are described where the density is measured by two superposed capacitive sensors in a cell of 3.5 mm height, and where the temperature oscillation frequencyf=ω/2π is varied between 10⁻⁴ and 2 Hz. Over the experimental range 5×10⁻⁴<ε<5×10⁻² there is in general reasonable agreement between predictions and experiments. The systematic departures might be accounted for by deviations from 1D geometry, which were not included in the calculations. Over the frequency and reduced temperature ranges, the damping effect from the critical bulk viscosity is predicted to be too small to be detectable. The observed effect of the stratification and its frequency dependence inZ _F are briefly discussed. In the appendix, the predicted critical acoustic attenuation from the bulk viscosity is compared with published data, the effect from finite thermal conductivity of the fluid container plates and also the corrections toZ _F for the effects of the cell sidewalls are calculated. F. Pobell     

Theoretical Study of Specific Heat,Density of States,Free Energy,and Critical Field of High Temperature Cuprate Superconductors Based on Intra and Interlayer Interactions

The role of attractive interlayer and intralayer interactions in layered high T _c cuprate superconductors have been investigated using a one-band two layer tight binding Hamiltonian. Self-consistent equations for the superconducting order parameter (Δ) and critical temperature (T _c ) are derived using double time Green’s functions and equation of motion method. The expression for excitonic type correlation (γ _c ), specific heat, density of states, free energy, and critical field are obtained. The interlayer interactions play an important role in the enhancement of T _c in layered high T _c cuprates. The oxygen isotope effect is also analyzed. The agreement between theoretical and experimental results for the system YBa_2−x La _x Cu₃O₇ (0≤x≤0.5) is quite satisfactory.      

The Activation Energy <Emphasis Type="Italic">U</Emphasis>(<Emphasis Type="Italic">T</Emphasis>,<Emphasis Type="Italic">H</Emphasis>) in Y-based Superconductors

Based on the Arrhenius equation, a method to calculate the activation energy from the resistance transition is proposed for high temperature superconductors. This method is applied to the Y-based superconductors. The activation energy is found to be U(T,H)∼(1−T/T _c )^4.8(H/H ₀)^−3.8 of YBCO crystal, and U(T,H)∼(1−T/T _c )^3.3(H/H ₀)^−2.2 of Er doped MTG YBCO crystal, respectively. With the obtained activation energy U(T,H), the lower part of the experimental curve ρ(T,H) and its derivative can be reproduced.      

Influence of transition metals substitution on the superconductor Bi4Ca3Sr3Cu4O y

The results of our investigation on the specimens Bi₄Ca₃Sr₃Cu_4−x T _xO _y (T=Fe, Co, Ni,x⩽0.5) synthesized in air are presented. Fe and Co substituents result in the formation of Bi₂Sr₂CuO _y -type of phase, with considerable depression ofT _c of the main phase. However, Ni is completely soluble with Cu in this concentration range without significant depression of superconducting transition temperature (T _c). This difference in the solubility behaviour of Fe and Co on the one hand and Ni on the other is explained taking into account ionic charge and coordination number mismatches.     

The Upper Critical Field <Emphasis Type="Italic">H</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis>2</Subscript> in Advanced Superconductors with Anisotropic Energy Spectrum

A brief review of works on the microscopic theory of determining the upper critical field in two-band isotropic and anisotropic superconductors is given. The research is based on a set of the Ginzburg–Landau equations for the order parameters in a magnetic field that are studied in terms of the classical approach to a superconducting system in a magnetic field. Two inequivalent energy bands with different topology of Fermi surface cavities overlapping on the Fermi surface are discussed. The cases of the direction of the external magnetic field [(H)\vec]//\vec{H}// the (ab) plane and [(H)\vec]//\vec{H}// the crystallographic c axis are studied. The equations for determining H _c2(ab) and H _c2(c) for a pure superconductor and a superconductor doped with electrons and holes are derived. The analytical solutions to these equations in the vicinity of the superconducting transition temperature (T _c −T≪T _c ) and in the vicinity of zero (T≪T _c ) are found. The temperature and impurity dependences of the upper critical fields H _c2(ab) and H _c2(c), as well as the anisotropy coefficient γ _H, are studied. The resulting theory is applied to determine the dependences of the above magnetic characteristics of intermetallic compound MgB₂. The theory agrees qualitatively with experimental data.     

Temperature-dependent integral exciton absorption in semiconducting InP crystals

The temperature dependence of the fundamental absorption edge in free-standing “epitaxial” InP layers has been experimentally studied. The integral exciton absorption coefficient K(T) exhibits an increase at low temperatures, which is explained in terms of the exciton-polariton mechanism of light transfer in semi-conductor crystals with spatial dispersion. A critical temperature (T _c = 200 K), above which the integral absorption becomes constant, has been experimentally determined, and the corresponding critical decay parameter (Γ_c = 0.341 meV), longitudinal-transverse splitting (ħω_LT = 0.175 meV), and oscillator strength of the exciton transition (β = 0.237 × 10⁻⁴) have been calculated. The temperature dependence of the true dissipative decay has been determined.     

Specific heat and enthalpy of lattice disordering of LaF3 superionic crystals

Specific heat (C _p ) data obtained near the phase transition of LaF₃ are used to estimate the activation enthalpy for anion disordering, H _d. At the critical point T _c = 263 K, the concentration of disordered fluoride ions is n _c = 2.86 × 10²⁰ g⁻¹. In the dielectric phase of LaF₃ (T < T _c), H _d is 0.24–0.26 eV. In the superionic phase (T ≥ T _c), where the concentration of disordered fluoride ions exceeds n _c, the enthalpy of disordering drops to H _d ≃ 0.04 eV.     

Role of Mobile Charge Carriers and Fluctuation Induced Conductivity in (Cu0.5Tl0.5?xKx)Ba2Ca3Cu1Zn3O12?δ Superconductor

The main role played by mobile carriers and fluctuation induced conductivity (FIC) in (Cu_0.5Tl_0.5−x K _x )Ba₂Ca₃Cu₁Zn₃O_12−δ (x=0, 0.25) superconductor samples are analyzed in the light of the Aslamazov–Larken (AL) theory. The enhancement of three-dimensional (3D) conductivity and grain size is achieved with K-doping at Tl sites in the (Cu_0.5Tl_0.5−x K _x )Ba₂O_4−δ charge reservoir layer, which are further improved by post-annealing the samples in oxygen. The improvement of these factors causes the increase in cross-over temperature (T _o), zero resistivity critical temperature {T _c(R=0)} and T _c(onset) of diamagnetism to their higher values. We have also evaluated the exponents (λ _2D and λ _3D), zero temperature coherence lengths ξ _c(0), and interlayer coupling (J) for these samples and tried to correlate them to the superconductivity order parameters.     

Thermodynamic Properties of Methanol in the Critical and Supercritical Regions

The isochoric heat capacity of pure methanol in the temperature range from 482 to 533 K, at near-critical densities between 274.87 and 331.59 kg· m⁻³, has been measured by using a high-temperature and high-pressure nearly constant volume adiabatic calorimeter. The measurements were performed in the single- and two-phase regions including along the coexistence curve. Uncertainties of the isochoric heat capacity measurements are estimated to be within 2%. The single- and two-phase isochoric heat capacities, temperatures, and densities at saturation were extracted from experimental data for each measured isochore. The critical temperature (T_c = 512.78±0.02K) and the critical density (ρ_c = 277.49±2 kg · m⁻³) for pure methanol were derived from the isochoric heat-capacity measurements by using the well-established method of quasi-static thermograms. The results of the C_VVT measurements together with recent new experimental PVT data for pure methanol were used to develop a thermodynamically self-consistent Helmholtz free-energy parametric crossover model, CREOS97-04. The accuracy of the crossover model was confirmed by a comprehensive comparison with available experimental data for pure methanol and values calculated with various multiparameter equations of state and correlations. In the critical and supercritical regions at 0.98T_c≤ T ≤ 1.5T_c and in the density range 0.35ρ_c ≤ ρ leq 1.65 ρ_c, CREOS97-04 represents all available experimental thermodynamic data for pure methanol to within their experimental uncertainties.     

Effect of non-stoichiometry and Ce-doping on the tetragonal superconducting phase CaBaLaCu3O7−δ

We have synthesized CaBaLaCu₃O_7−δ and related compounds Ca_0.9BaLaCu₃O_7−δ , Ca₁₁ BaLaCu₃O_7−δ and Ca_0.9Ce_0.1 BaLaCu₃O_7−δ in order to investigate the effect of doping and non-stoichiometry at Ca-sites on the superconducting behaviour of tetragonal superconductor CaBaLaCu₃O_7−δ . The superconducting transition temperature (T _c) of Ca_0.9 BaLaCu₃O_7−δ is the same as that of pure CaBaLaCu₃O_7−δ (67 ± 1 K). Calcium excess compound Ca_1.1 BaLaCu₃O_7−δ showed a drop inT _c to 72 K while the cerium-substituted compound Ca_0.9 Ce_0.1 BaLaCu₃ O_7−δ showed a drop inT _c to 43 K. The results are explained on the basis of difference in valency between the host and the dopant ion.     

NMR Study Under Pressure on the La1111 Pnictides,LaFeAsO<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>F<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

The relationship between antiferromagnetic (AF) fluctuation and superconductivity was investigated in the La1111 series, LaFeAsO_1−x F _x (x=0.05, 0.08, and 0.14) by examining nuclear relaxation rates (1/T ₁) at both ambient pressure and 3.0 GPa. Although the AF fluctuation is enhanced by applying pressure in the underdoped regime (x=0.05, and 0.08), the increase in critical transition temperature (T _c) is small, whereas T _c increases remarkably in the overdoped regime (x=0.14) where the AF fluctuation is absent, suggesting that high T _c above 40 K originates not from the AF fluctuation but from the density of states at the electron pocket.     

Optimum Synthesis Temperature of (Cu1?xTlx)Ba2Ca3Cu4O12?δ Superconductor

The effect of synthesis temperature on the superconducting properties of (Cu_1−x Tl _x )Ba₂Ca₃Cu₄O_12−δ (CuTl-1234) samples has been explored. Almost all the superconducting parameters studied in this research work are observed to be suppressed with the increase of synthesis temperature beyond 880 °C, which may be due to impurities caused by the volatility of some constituents such as thallium and oxygen deficiencies as well in the final compound. The Fluctuation Induced Conductivity (FIC) analysis has shown a decrease in the cross-over temperature (T ₀) and the shift of three-dimensional (3D) Aslamasov–Larkin (AL) regions to the lower temperature with the increase of synthesis temperature beyond 880 °C. A direct correlation between the cross-over temperature (T ₀), the zero temperature coherence length {ξ _c (0)}, the zero resistivity critical temperature {T _c (R=0)} as well as carrier concentration has also been observed.     

Composition dependence of transition temperature in some ceramic superconductors

The composition dependence of transition temperature in some ceramic superconductors (La_2−x (Ba, Sr) _x CuO₄) was studied by modifying our earlier approach and developing a Fourier-transformed effective potential which involves the effect of two-dimensional (2D) acoustic plasmons. This potential was used to obtain the pairing (electron-electron attraction) parameter (λ), the averaged Coulomb repulsive parameter (μ*) and the cut-off 2D acoustic plasmon frequency (ω _c) required to compute the superconducting transition temperature (T _c) from the strong coupling theory. The variations ofT _c with compositions (x) obtained for La_2−x (Ba, Sr) _x CuO₄ show reasonably good agreement with experimental data.     

How Grain-Boundaries Influence the Intergranular Critical Current Density of Cu1−x Tl x Ba2Ca3Cu4O12−δ Superconductor Thin Films?

The insulating and metallic behavior of the grain-boundary weak links has been studied in thallium rich and the samples with small amount of thallium in the charge reservoir layer of Cu_1−x Tl _x Ba₂Ca₃Cu₄O_12−δ superconductor thin films. The influence of the nature of grain boundaries on the inter-granular critical current density (J _c) has also been investigated. From the power law dependence of H _ac∼(1−T _p/T _c) ⁿ , it was observed that n=1 gives a best fit for the J _c of thallium rich samples and n=2 provides a best fit for the J _c of the samples with small amount of thallium. The polycrystalline thin film samples showing the power law dependence of J _c as n=1 make superconductor-insulator-superconductor (SIS) type while the samples with n=2 follow superconductor-normal metal-superconductor (SNS) types of Josephson junctions. The insulating grain boundaries decrease the inter-granular Josephson coupling and hence the transport properties are suppressed.      

Electronic Energy Levels in High-Temperature Superconductors

Parent materials of high-temperature superconductors (HTSC) need to be doped to become superconducting. The optimum doping for maximum critical transition temperature T _c has been analyzed for more than 20 materials. Assuming a uniform doping distribution the distance x between doped unit cells—projected into the CuO₂ plane for cuprates—shows a strong linear correlation to the inverse of T _c in the form (2x)²=m ₁1/T _c with a slope of m ₁=(2.786±0.029)×10⁻¹⁵ m² K. The mercury cuprate homologous series HgBa₂Ca _n−1Cu _n O_2n+2+δ with n=1,2,3 has been used to demonstrate the procedure deriving the doping distance x from the optimum doping value δ.     

Fluctuation Induced Conductivity in Hg-Doped (Cu0.5Tl0.5?x Hg x )Ba2Ca3 Cu 4O12?δ Superconductor

The fluctuation induced conductivity (FIC) analysis on resistivity data of the Hg-doped (Cu_0.5Tl_0.5−x Hg _x )Ba₂Ca₃Cu₄O_12−δ (x=0, 0.15, 0.25, 0.35) superconductor has been carried out in the light of the Aslamazov–Larken (AL) theory. The superconducting properties have been improved with Hg-doping (up to x=0.25) at Tl sites in the Cu_0.5Tl_0.5Ba₂O_4−y charge reservoir layer. These samples are also post-annealed in oxygen to improve intergrain connectivity and to optimize the carriers’ density in CuO₂ planes. The results of FIC analysis have shown three-dimensional (3D) and two-dimensional (2D) fluctuations in the order parameters. It has been observed that there is an increase in the cross-over temperature (T _o) as well as in zero resistivity critical temperature {T _c(0)} with Hg-doping. These properties have been further improved after post-annealing in oxygen, which may be due to improved inter-grain coupling and optimization of mobile carriers in CuO₂ planes. A direct correlation between T _o, T _c(0) as well as zero temperature coherence length ξ _c(0) has been observed.