3He Vapor Pressure near Its Critical Point

Between 0.65 K and 3.2 K, the temperature dependence of the vapor pressure P of ³He is defined by the International Temperature Scale of 1990 (ITS-90). However, the ITS-90 vapor pressure equation was not designed to be consistent with the scaling law required for the second temperature derivative of the vapor pressure in the vicinity of the liquid-vapor critical point. In this paper, two scaling-type equations are used to describe the ³He vapor pressure in the region near the critical point. The first scaling equation contains two unknown coefficients which are obtained by taking as reference the temperature $\bar{T}$ at which the product (T _c ?T)P presents a maximum ( $\bar{T}=2.56736$  K). The second scaling equation contains three unknown coefficients which are obtained by using as references $\bar{T}$ and T _up=3.2 K, the upper value of the ITS-90 interval. In both equations we take for the critical temperature and pressure the values T _c =3.31554 K and P _c =114?632.7 Pa. The proposed equations, specially the second one, are satisfactorily compared with experimental data for P and dP/dT within the temperature range (T _c ?T)/T _c ≤0.065 and with semiempirical data for d ² P/dT ² within the temperature range 0.0001≤(T _c ?T)/T _c ≤0.03.     

On the strain dependence of T c for niobium and lead

Recent measurements of the elastic constants at temperatures close toT _c for niobium and lead would indicate strong individual quadratic strain dependences ?² T _c /?ε _i ε _j , which combined would predict a considerableP ² contribution to the pressure dependences of their superconducting transition temperatures. These predictions are in conflict with the direct experimental observations of the variation ofT _c with pressure to 25kbar, which exhibit no evidence of any significant quadratic component. The implications of this discrepancy are discussed.     

Phase transformations in water-aliphatic alcohol binary systems

The parameters of liquid-vapor phase transitions p _s , ρ _s , T _s and critical points p _c, ρ_c, T _c were determined from the experimental data on the p, ρ, T, x-dependences of aqueous solutions of aliphatic alcohols (methanol, ethanol, n-propanol) that contain 0.2,0.5, and 0.8 mole fractions (x) of ethanol and correspond to single-phase (gas, liquid), two-phase, or subcritical areas. The dependence of the pressure of saturated vapor in solutions on the temperature and density was described by means of the expansion of the compressibility factor Z = p/RTρ _m in powers of the density and temperature along the coexistence curve away from the critical point. The temperature dependence of the density of solutions along the coexistence curve and inside the critical area was fitted using the power functions of parameters ω ～ $\tau ^{\beta _i } $ , τ = (T ? T _c) and ω = (ρ_1,v ? ρ_c)/ρ_c.     

Transport properties of Nd1.85Ce0.15 δ single crystals: The narrow-band model

The anisotropy of the resistivity and thermoelectric power (TEP)S of Nd_1.85Ce_0.15CuO_4?δ single crystal (T _c =17 K) has been investigated. In the temperature rangeT _c <T<300 K the ratioρ _c/ρ_ab≈10⁴ and the dependencesρ _ab (T) andρ _c (T) change from quadratic to linear atT～200 K. The dependencesS _ab (T) andS _c (T) reach a maximum atT>T _c and then decrease almost linearly with increasing temperature, changing sign from positive to negative nearT～ 150 K. The features of the resistivity and TEP temperature dependences (the lawρ∝T ² changing toρ∝T, the change in the sign of S with temperature, and the low TEP anisotropy at largeρ anisotropy) have been interpreted in the framework of the narrow-band model.     

A principle of corresponding states for the viscosity of non-quantum mono- and diatomic condensed gases

The reduced viscosity coefficient η_R for the saturated liquid state of argon, neon, krypton, xenon, oxygen, nitrogen, and fluorine is examined as a function of T_R and as a function of 1/ρ_R ? 1/ρ_oR. The reduction parameters are the critical ones: η_R = η/η_c where η_c is the viscosity of the liquid calculated at the critical point supposing there is no anomalous behaviour; T_R = T/T_c, ρ_R = ρ/ρ_c and ρ_oR = ρ_o/ρ_c. The density ρ_o is the density of the liquid where η = ∞.For the inert gases there is a good η_R(T_R) correspondence. From this fact the viscosity of radon is calculated.Where η_R is plotted as a function of 1/ρ_R ? 1/ρ_oR, a principle of corresponding states is found for the investigated condensed gases. For densities greater than about 2ρ_c the reduced fluidity φ_R = 1.868 (1/ρ_R ? 1/ρ_oR). In this range the overall average deviation between the calculated and the experimental fluidity is 2.6%.     

Electrical and Magnetic Behaviour of PrFeAsO_{\mathbf{0.8}}F_{\mathbf{0.2}} Superconductor

The superconducting and ground state samples of PrFeAsO_0.8F_0.2 and PrFeAsO have been synthesised via the easy and versatile single step solid state reaction route. X-ray and Reitveld refine parameters of the synthesised samples are in good agreement to the earlier reported value of the structure. The ground state of the pristine compound (PrFeAsO) exhibited a metallic-like step in resistivity below 150 K followed by another step at 12 K. The former is associated with the spin density wave (SDW)-like ordering of Fe spins and later to the anomalous magnetic ordering for Pr moments. Both the resistivity anomalies are absent in case of the superconducting PrFeAsO_0.8F_0.2 sample. Detailed high field (up to 12 Tesla) electrical and magnetization measurements are carried out for the superconducting PrFeAsO_0.8F_0.2 sample. The PrFeAsO_0.8F_0.2 exhibited superconducting onset ( $T_{c}^{\mathrm{onset}}$ ) at around 47 K with T _c (ρ=0) at 38 K. Though the $T_{c}^{\mathrm{onset}}$ remains nearly invariant, the T _c (ρ=0) is decreased with applied field, and the same is around 23 K under an applied field of 12 Tesla. The upper critical field (H _c2) is estimated from the Ginzburg–Landau equation (GL) fitting, which is found to be ～182 Tesla. Critical current density (J _c ), being calculated from high field isothermal magnetization (MH) loops with the help of Beans critical state model, is found to be of the order of 10³ A/cm². Summarily, the superconductivity characterization of the single step synthesised PrFeAsO_0.8F_0.2 superconductor is presented.     

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).     

Flow of superfluid3He through micrometer-size channels

Measurements of the critical mass currentJ _c through 0.8-µm-diameter channels in the superfluid phases of³He are reported. Experiments were made at pressures from 0 to 27.4 bar in zero external magnetic field. The pressure difference ΔP along the flow channels is immeasurable within our resolution of ±0.1 µbar for sufficiently low currents in both the A and B phases, implying small or zero dissipation. In the B phase ΔP grows rapidly with increasing current aboveJ _c. At low pressuresJ _c behaves like (1?T/T _cyl)^3/2, whereT _cyl is interpreted as the reduced superfluid transition temperature inside the flow channels;T _cyl/T _c=0.935 atP=0. If the liquid in the channels is in the A phase, the behavior of ΔP vs. the mass currentJ _s depends on the phase A or B outside the channels. During warming a drop of about30% inJ _c is found both atT _BA(cyl) and at;T _BA(cyl) is the reduced B→A transition temperature in the channels. AboveT _BA a second dissipation mechanism, with a smallerd(ΔP)/dJ _s, is observed at lower currents. These features indicate that in the A phase the ends of the channels have an important effect on the flow.     

Effects of Transition-Metal V-Doping on the Structural, Magnetic and Transport Properties in La0.67Sr0.33MnO3 Manganite Oxide

We have investigated the structural, magnetic, and electrical transport properties of a series of ABO₃-type perovskite compounds, La_0.67Sr_0.33Mn_1?x V _x O₃ (0≤x≤0.15). The samples were characterized by X-ray diffraction and data were analyzed using Rietveld refinement technique, it has been concluded that these materials have the rhombohedral structure with $\mathrm{R}\overline{3}\mathrm{C}$ space group. The magnetization and resistivity measurements versus temperature proved that all our samples exhibit a ferromagnetic to paramagnetic transition and a metallic to semiconductor one when the temperature increases. Both the Curie temperature T _C and the resistivity transition temperature T _P of the composites decrease, while the resistance increases as the V content increases. It has been concluded that the electrical conduction mechanism in the metallic regime at low temperatures (T<T _P) can be explained on the basis of grain boundary effects and the single electron-magnon scattering process. Resistivity data were well fitted with the relation ρ=ρ ₀+ρ ₂ T ²+ρ _4.5 T ^4.5, whereas the adiabatic Small Polaron Hopping (SPH) and Variable Range Hopping (VRH) models are found to fit well in the paramagnetic semiconducting regime at the high temperature (T>T _P).     

Fast negative ion thermometer for 3He superfluids

A thermometer based on the temperature dependence of the negative ion mobility in superfluid ³He is described. The device can be used for monitoring the temperature of an open geometry sample of ³He in the range 0.4 ≲ T/T_c ≤ 1. The ion thermometer is most sensitive immediately below T_c, in the Ginzburg-Landau region; above T ≈ 0.95T_c a resolution of 0.1 μK can be achieved. The measuring technique provides an on-line display of the relative mobility, μ(T)/μ(T_c), and the ions probe the temperature of the ³He sample directly, which leads to a short time constant τ < 10 s. The relative mobility versusT/T_c is insensitive to the external magnetic field and the pressure of the sample. Only a low electric field, E ≲ 10 V cm⁻¹, with essentially no homogeneity requirements is needed. Parasitic heating due to the flow of ions is of the order of 1 pW.     

Critical behavior of a charged Bose gas

A fully self-consistent Hartre-Fock theory, using the Coulomb interaction screened by the polarization insertions calculated in the self-consistent random-phase approximation, is applied to thed-dimensional, dense, charged Bose gas at temperatures close to the transition temperatureT _c . The quasiparticle energy spectrum is calculated and shown to behave atT _c like ε(k)=Ak ^σ for smallk, and σ is calculated as a function of the dimensionalityd. The change in transition temperature from that of an ideal gas at the same density, and of the chemical potential are shown to be given by (T _c ?T _c0 )/T _c0 ≈Xr _s ^(d?2)/3 and μ _c ≈Yr _s ^2/3 , wherer _s is the ratio of the interparticle spacing to the Bohr radius. Approximate expressions are given for the coefficientsX andY. The critical exponents are calculated, and the system is shown to obey exact scaling.     

Scaling of the Temperature Dependent Resistivity in 111 Iron-Pnictide Superconductors

In Li_1?x FeAs, Li_1?x FeP, and Na111 (Na_1?δ FeAs, Na_0.9FeAs, NaFe_0.95Co_0.05As, NaFeAs_0.8P_0.2) members of the 111-iron-pnictide superconductor family, the temperature dependent resistivity ρ can be scaled into a single curve described by a scaling function. In particular, the ρ(T) dependences can be reproduced by the expressions $\rho(T) = \rho_{0} + cT\exp( - \frac{2\varDelta }{T})$ and $\rho(T) = \rho_{0} + (a/T)\exp( - \frac{2\varDelta }{T}) + bT$ for Li_1?x FeP and Li_1?x FeAs, Na-111 crystals, respectively. The scaling was performed using the energy scale 2Δ, the parameters a, b, c, and the residual resistivity ρ ₀ as scaling parameters. The existence of a single metallic ρ(T) curve is interpreted as an indication of a few mechanisms in various compounds, which dominates the different scattering of charge carriers in 111-iron-pnictide superconductors studied so far. Thus, the scaling of the normal-state properties seems to be a general feature not only for high-T _c cuprates, but also for the iron-pnictides superconductor family.     

The description of superconductivity in terms of dielectric response function

A critical temperatureT _c of a superconducting transition is calculated for a rather general form of the electron-electron interaction. It is shown that even if both the energy and momentum dependence of the interaction is included, the equation determiningT _c coincides formally with the corresponding equation of the BCS theory. The kernel of this equation is a smooth real function of its variables; it is expressed through ρ(k, E), the spectral density of the inverse dielectric function of the system. The expression forT _c is written in terms of ρ(k, E); this enables us to analyze the dependence of the critical temperature on the properties of the metal in a normal state. Some simple models illustrating the results are considered, and a discussion of the limits onT _c is given.     

A practical vapor pressure equation for helium-3 from 0.01 K to the critical point

A saturation vapor pressure equation, p(T), is an essential component in the ³He state equation currently under development. The state equation is valid over the range 0.01-20 K with pressures from 0 to the melting pressure or 15 MPa. The vapor pressure equation consequently must be valid from 0.01 K to the critical temperature. This paper surveys available ³He critical temperature and pressure measurements, leading to new recommended critical values of 3.3157 K and 114603.91 Pa. The ITS-90 temperature scale is defined by the ³He vapor pressure from 0.65 to 3.2 K. A new vapor pressure equation is developed for the interval from the upper end of the T₉₀ scale to this newly defined critical point, employing a mathematical form in which the second derivative d²p/dT² diverges in agreement with scaling laws at the critical point. Below 0.65 K, an empirical vapor pressure expression is adopted, consistent with a theoretical expression valid in the limit T → 0. These two new components are fitted to be piecewise continuous with the EPT-76 p(T) scale rather than the ITS-90 T(p) scale between 0.65 and 3.2 K. Probable deviations between this vapor pressure scale and PLTS-2000 melting pressure-temperature scale are recognized, but not reconciled.     

Temperature Dependence of the Intergranular Critical Current Density in Uniaxially Pressed Bi1.65Pb0.35Sr2Ca2Cu3O10+δ Samples

We have studied the normal and superconducting transport properties of Bi_1.65Pb_0.35Sr₂Ca₂Cu₃O_10+δ (Bi-2223) ceramic samples. Four samples, from the same batch, were prepared by the solid-state reaction method and pressed uniaxially at different compacting pressures, ranging from 90 to 250 MPa before the last heat treatment. From the temperature dependence of the electrical resistivity, combined with current conduction models for cuprates, we were able to separate contributions arising from both the grain misalignment and microstructural defects. The behavior of the critical current density as a function of temperature at zero applied magnetic field, J _c (T), was fitted to the relationship J _c (T)∝(1?T/T _c ) ⁿ , with n≈ 2 in all samples. We have also investigated the behavior of the product J _c ρ _sr , where ρ _sr is the specific resistance of the grain-boundary. The results were interpreted by considering the relation between these parameters and the grain-boundary angle, θ, with increasing the uniaxial compacting pressure. We have found that the above type of mechanical deformation improves the alignment of the grains. Consequently the samples exhibit an enhance in the intergranular properties, resulting in a decrease of the specific resistance of the grain-boundary and an increase in the critical current density.