Thermodynamic properties of R32 + R134a and R125 + R32 mixtures in and beyond the critical region

A parametric crossover equation of state for pure fluids is adapted to binary mixtures. This equation incorporates scaling laws asymptotically close to the critical point and is transformed into a regular classical expansion far away from the critical point. An isomorphic generalization of the law of corresponding states is applied to the prediction of thermodynamic properties and the phase behavior of binary mixtures over a wide region around the locus of vapor-liquid critical points. A comparison is made with experimental data for pure R32, R 125 and R 134a, and for R32 + R 134a and R 125 + R32 binary mixtures. The equation of state yields a good representation of thermodynamic property data in the range of temperatures 0.8T_c(x) ≤ T ≤ 1.5T_c(x) and densities 0.35 ?_c(x) ≤ ? ≤ 1.65?_c(x).     

Electronic Damping of Dislocations and Kinetic Phenomena in Superconductors Under Plastic Deformation

Interaction of a moving screw dislocation with conduction electrons is considered in the conventional pure superconductors. At temperaturesT≪T _c , both “slow” dislocations are considered, which interact only with thermally excited quasiparticles, and “fast” dislocations which also break the Cooper pairs. Near the critical temperature, two limiting cases are considered depending on the relation between Meissner penetration depth and anomalous skin-layer depth for the dislocation-induced electromagnetic field. Power dissipation dependence on temperature and dislocation velocity is obtained. Due to low field intensity in the short-wave part of spectrum, the superconducting state is shown not to be destructed by a moving dislocation in a broad range of temperatures. Non-equilibrium states of electronic system created by the dislocation field are analyzed in the paper. With this purpose, Eliashberg’s kinetic equation for a multi-mode excitation source is used. Dislocation field is shown to reduce the order parameter whenT≪T _c , or stimulate superconductivity when(T−T _c )/T _c≪ 1. Damping reduction due to stimulation effect is discussed. Power dissipation dependence on the dislocations concentration in non-equilibrium state is obtained.     

Thermally activated dissipation mechanism in Sn doped CuTl-1223 superconductors

We have carried out magneto-resistance measurements of Sn-doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y S _y O_10−δ (y = 0.5, 1.0, 1.5) superconductors and from there studied the effect of Sn doping on the thermally activated dissipation mechanism. A systematic decrease in T _c(R = 0) and shift of T _c (onset) towards lower temperature is typical feature of Sn-doped samples with the increased strength of external magnetic field. In these samples pronounced broadening of resistive transitions has been observed on the application of external magnetic field. The activation energy of carriers obtained from the Arrhenius plots of log (ρ) versus 1000/T have been found to decrease with the increase in the applied external field as well as increased Sn doping concentration. From log (ρ) versus U _o/T plots we have found that for lower concentration of Sn the thermally activated dissipation can be explained in terms of flux creep. On the other hand for higher concentration of Sn, flux flow mechanism of energy dissipation seems to be dominant.     

11B NMR Study of Pure and Lightly Carbon-Doped MgB2 Superconductors

We report a¹¹B NMR line shape and spin-lattice relaxation rate (1/(T ₁ T)) study of pure and lightly carbon-doped MgB_2−x C _x forx=0, 0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe. We show that while pure MgB₂ exhibits the magnetic field distribution from superposition of the normal and the Abrikosov state, slight replacement of boron with carbon unveils the magnetic field distribution of the pure Abrikosov state. This indicates a considerable increase ofH _c ^2/c with carbon doping with respect to pure MgB₂. The spin-lattice relaxation rate 1/(T ₁ T) demonstrates clearly the presence of a coherence peak right belowT _c in pure MgB₂, followed by a typical BCS decrease on cooling. However, at temperatures lower than ≈10 K strong deviation from the BCS behavior is observed, probably from residual contribution of the vortex dynamics. In the carbon-doped systems both the coherence peak and the BCS temperature dependence of 1/(T ₁ T) weaken, an effect attributed to the gradual shrinking of the σ hole cylinders of the Fermi surface with electron doping.     

11B NMR Study of Pure and Lightly Carbon-Doped MgB2 Superconductors

We report a ¹¹B NMR line shape and spin-lattice relaxation rate (1/(T₁T))(1/(T_1T)) study of pure and lightly carbon-doped MgB_2-xC_x_{2-x}{\rm C}_x for x = 0,0.02x = 0,0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe. We show that while pure MgB₂_2 exhibits the magnetic field distribution from superposition of the normal and the Abrikosov state, slight replacement of boron with carbon unveils the magnetic field distribution of the pure Abrikosov state. This indicates a considerable increase of H^c_c2H^c_{c2} with carbon doping with respect to pure MgB₂_2. The spin-lattice relaxation rate 1/(T₁T)1/(T_1T) demonstrates clearly the presence of a coherence peak right below T_cT_{\rm c} in pure MgB₂_2, followed by a typical BCS decrease on cooling. However, at temperatures lower than ?\approx 10 K strong deviation from the BCS behavior is observed, probably from residual contribution of the vortex dynamics. In the carbon-doped systems both the coherence peak and the BCS temperature dependence of 1/(T₁T)1/(T_1T) weaken, an effect attributed to the gradual shrinking of the σ hole cylinders of the Fermi surface with electron doping.     

Temperature dependence of thermal conductivity of vanadium substituted BPSCCO system between 10 and 150 K

Thermal conductivity of a set of (Bi_0.8Pb_0.2−y V _y )₂Sr₂Ca₂Cu₃O_10+δ (0 =y ≤ 0.05) pellets in the temperature range between 10 and 150 K is reported. Vanadium substitution influences strongly the magnitude of thermal conductivity (λ,) over the entire temperature range. But the nature of λ(T) dependence remains similar to that generally observed for HTSCs. The electronic contribution to the total λ in the normal state is estimated to be ∼ 25%. We have attempted to examine our data, assuming the role of both electrons and phonons in the origin of the λ(T) behaviour belowT _c. Observed temperature variation of λ(T) for the present set of samples could be explained very well assuming this electron + approach. Some of the microscopic quantities estimated from the best-fit parameters give reasonable values.     

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.     

Extended Domain of Existence for PSCs in Superconductors

By means of nanosecond pulse techniques, we have studied the current-induced dissipation in one-dimensional superconducting bridges, namely, metallic and high-T _c films. It is well known that narrow strips dissipate through phase-slip centers (PSC) close to T _c, or hot spots (HS) at low T, rather than by flux flow. When driven by step pulses of current, PSC give stable voltages, while HS produce a voltage linearly changing with time. By using two-step pulses of current, we have studied the decay of a HS into another HS, or a PSC, or into a zero-resistance state. It was thus found possible to reach the PSC state at arbitrary low temperatures.     

Effect of Oxygen Concentration on Superconducting Properties of Rubidium Tungsten Bronzes Rb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>WO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

Superconducting transition temperature (T _c) as a function of oxygen concentration for hexagonal rubidium tungsten bronzes Rb _x WO _y with 2.80 ≤ y ≤ 3.07 and x = 0.19, 0.23, and 0.27 has been systematically investigated. Three regions corresponding to T _c < 2 K (denoted as superconductivity suppressed region), T _c∼ 3 K (superconductivity uniform region) and T _c > 3 K (superconductivity enhanced region) were identified in T _c–y phase diagram for Rb_0.19WO _y and Rb_0.23WO _y . No superconductivity enhanced region was observed for Rb_0.27WO _y . The superconductivity suppressed region shifts toward higher oxygen content as rubidium concentration increases. The local ordering of the intercalated rubidium atoms might be responsible for the intriguing T _c–y phase diagram of Rb _x WO _y .     

Effect of copper content on glass formation and superconductivity in the Bi-Pb-Sr-Ca-Cu-O system

High-T _c superconducting ceramics of formula Bi_0.8Pb_0.2SrCaCu _x O _y (x=1.5, 1.8 and 2.0) were prepared by using the melt-quenching method, and the effect of copper content on glass formation and superconductivity was examined. It was found that the composition withx=1.5 had a tendency to form a glass and Bi₂(Sr, Ca)₂CuO_y crystals tended to precipitate easily during the rapid quenching of melts in the compositions withx=1.8 and 2.0. It was found from the temperature dependence of a.c. complex susceptibility that the intergrain coupling of superconducting crystals in the samples obtained was weak, but the weak coupling was improved by increasing the annealing time. The superconducting glass-ceramics Bi_0.8Pb_0.2SrCaCu_1.5O_y exhibited superconductivity with aT _c (zero) of 106 K and aJ _c of 250 Acm^–2.     

Role of barium addition on the properties of bismuth-based superconductors

The effect of the addition of barium on the zero resistance temperature,T _c, of Bi_1.6Pb_0.4Sr_1.6Ca₂Ba _x Cu₄O _y ,x= 0.4, 0.5, 0.6, 0.8 and 1.6 was studied. The added barium had the effect of raisingT _c to a higher temperature region, although too much barium gave rise to semiconducting resistance temperature behaviour. X-ray diffraction analysis showed that as the barium concentration,x, increased from 0.4 to 0.8, a decrease in the lowT _c phase and peaks due to CuO and BaBiO₃ appeared, whereas an increase in the peaks due to the highT _c phase and BaCuO₂ were seen. Critical current densities were also measured in zero field at 77 K.     

Pseudogap Analysis of Normal State Transport Behavior of 11 and 1111 Fe-Based Superconductors

In this paper we analyze transport data of different families of Fe-based superconductors, within a pseudogap framework. The Fe(Te, Se) samples exhibit s-shaped resistivity ρ(T) curves, while SmFeAs(O, F) ones present a departure from their high-temperature linear behavior. In either case, a characteristic temperature can be identified. These temperatures correspond to those at which abrupt changes in the temperature behavior of Hall resistance and of Seebeck coefficient occur, suggesting that they are signatures of pseudogaps opening in the density of states. The direct correlation between these characteristic temperatures and the superconducting transition temperatures suggests that the pseudogap and the superconducting state originate from the same mechanism. Scaling procedures of resistivity curves confirm such proportionality. On the other hand, excess Fe content in Fe(Te, Se) samples affects the pseudogap temperature much more strongly than the superconducting T _c . Finally, we find out that the pseudogap in the 1111 family is almost insensitive to disorder, as in high-T _c cuprates.     

Effect of synthesis conditions on the phase composition and critical current density of Bi1.6Pb0.4Sr2−x KxCa2Cu3F0.8Oy

Effects of the temperature-time schedule on the phase composition, critical current density j _c, critical temperature T _c, and electric resistance in the normal state (ρ) of the samples of Bi_1.6Pb_0.4Sr_2−x K_xCa₂Cu₃F_0.8O_y (x=0 or 0.02) compositions were studied by X-ray diffraction analysis and by measuring resistances and current-voltage characteristics of the samples. The values of j _c=of 583.6 A/cm² (at 77.4 K) and T _c(R=0)=107.1 K were obtained in the samples with x=0.02 upon partial fusion. In order to obtain high j _c values, the samples must be synthesized (sintered) prior to the fusion stage.     

The Grüneisen Parameter for Aluminum in the Temperature Range 0.3–1.1 K

Available critical field data as a function of temperature and pressure are used to calculate the electronic contribution to the Grüneisen parameter for pure aluminum in the superconducting state. Large negative values for the parameter are found in the range of temperatures of interest starting from the value ^e _s (T=T _c)=–10.7±0.8. This fact could be taken into account as a possible explanation for the cosmic ray large signals detected at a rate higher than expected by the gravitational wave antenna NAUTILUS when operated in superconducting regime.     

Comparative Studies of 2212 Type Superconductors In The TL-HG-BA-SR-CA-CU-O System

Superconductors in the Tl _1.3 Hg _0.7 Ba _2-x Sr _x+y Ca _1-y Cu ₂ O ₇₊ system (sp.gr. I4/mmm) were prepared through standard powder reaction in closed vessels. The transition temperatures (T _c ) for the different stoichiometries spanned from 100 to 45 K. Both cell parameters (a and c) increased in proportion to T _c . These changes were mainly related to the distribution of size differing atoms at two sites in the structure and not to the oxidation state of Cu. The presence of Ba seemed favorable when comparing composition and T _c .     

Equation of State and Thermodynamic Properties of Pure D2O and D2O + H2O Mixtures in and Beyond the Critical Region

A parametric crossover model is adapted to represent the thermodynamic properties of pure D₂O in the extended critical region. The crossover equation of state for D₂O incorporates scaling laws asymptotically close to the critical point and is transformed into a regular classical expansion far from the critical point. An isomorphic generalization of the law of corresponding states is applied to the prediction of thermodynamic properties and the phase behavior of D₂O + H₂O mixtures over a wide region around the locus of vapor-liquid critical points. A comparison is made with experimental data for pure D₂O and for the D₂O + H₂O mixture. The equation of state yields a good representation of thermodynamic property data in the range of temperatures 0.8T _c(x)T1.5T _c(x) and densities 0.35_c(x)1.65_c(x).     

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.     

Proximity effect tunneling into virtual bound state alloys

The effects of a narrow virtual bound state formed by transition metal impurities dissolved in the normal layer of a superconducting proximity effect sandwich are studied. Using standard renormalization techniques, we obtain the changes in the transition temperatures and the jumps in the specific heat atT _c as a function of the thickness of the normal layer, of the widths of the virtual bound states, and of the impurity concentrations. It is seen that narrow virtual bound states lead to decreases in the transition temperatures, while broad virtual bound states do not. It is further seen that the narrow virtual bound state causes the reduced specific heat jump atT _c to deviate from the BCS behavior expected of the pure sandwich.     

Transport characteristics related with microstructure of (Bi,Pb)-Sr-Ca-Cu-O superconductor prepared by the sol-gel method

Differences in the transport characteristics of Bi₂Sr₂Ca₂Cu₃O _y (high-T _c phase) superconducting ceramics produced by two-step heat-treatment (calcination and reheating at various temperatures) of metal acetate-derived gels are discussed in relation to their microstructure. Variation of the volume fraction increase rate of the high-T _c phase, which depends on the calcination temperature, results in much difference in the transport characteristics of the resultant samples. Superconducting bodies possessing a higher T _c(end) and critical current density can be obtained at a lower volume fraction increase-rate of the high-T _c phase. It is considered that the origin of the above results is an improvement of the weak-link structure.     

Some Remarks on Vortex Matter in High-T c Superconductors

We show that some experimentally observed features of vortex matter in high-T _c superconductors may be interpreted in simpler ways than it is usually done. In particular, we consider magnetic flux creep at low temperatures as well as the irreversibility line in the H–T phase diagram. We also discuss a new approach to the analysis of the equilibrium magnetization in the mixed state of type-II superconductors and we suggest an alternative configuration for the mixed state in magnetic fields close to the upper critical field.