Scaling of the Temperature Dependent Resistivity and Hall Effect in Ba(Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)As<Subscript>2</Subscript>

We show that the temperature-dependent resistivity ρ(T), Hall number n _H(T) and the cotangent of the Hall angle cot θ _H(T) of Ba(Fe_1−x Co _x )As₂ (x=0.0–0.2) can be scaled using a recently proposed model-independent scaling method (Luo et al. in Phys. Rev. B 77:014529, 2008). The zero field normal-state resistivity above T _c can be reproduced by the expresion r(T) = r₀ +cTexp(- \frac2\varDelta T )\rho(T) = \rho_{0} +cT\exp(- \frac{2\varDelta }{T} ) and scaled using the energy scale Δ, c and the residual resistivity ρ ₀ as scaling parameters. The scaling parameters have been calculated and the compositional variation of 2Δ and ρ ₀ has been determined. The 2Δ(x) dependence show almost linear decreasing in underdoped regime, minimum corresponding to the T _c maximum and increasing in overdoped regime. The latter is different from that reported for cuprates. The existence of a universal metallic ρ(T) curve which, however, is restricted for the underdoped compounds to temperatures above a structural and antiferromagnetic transition is interpreted as an indication of a single mechanism which dominates the scattering of the charge carriers in Ba(Fe_1−x Co _x )As₂ (x=0.0–0.2).     

Low-temperature magnetoresistance of biaxially strained 24-nm-thick La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films

The lateral unit cell parameter in nanodimensional La_0.67Ca_0.33MnO₃ (LCMO) films grown on (001)-oriented LaAlO₃ substrates is significantly (approximately 4%) smaller than the value measured along the normal to the substrate plane. At T < 140 K, the temperature dependence of the resistivity ρ of LCMO films follows the relation ρ − ρ (T = 4.2 K) ≈ρ₂(H)T ^4.5, where ρ₂ is independent of the temperature but decreases with increasing magnetic field H. It is shown that this decrease is related both to a decay of the spin waves in ferromagnetic domains and to the transformation of antiferromagnetic phase inclusions into ferromagnetic ones.     

Experimental Study of the Critical Behavior of the Isochoric Heat Capacity of Aqueous Ammonia Mixture

The isochoric heat capacity of a NH₃ + H₂O (0.2607 mole fraction of ammonia) mixture has been measured in the near- and supercritical regions. Measurements were made in the single- and two-phase regions including the coexistence curve using a high-temperature, high-pressure, nearly constant-volume adiabatic calorimeter. Measurements were made along 38 liquid and vapor isochores in the range from 120.03 kg · m⁻³ to 671.23 kg · m⁻³ and at temperatures from 478 K to 634 K and at pressures up to 28 MPa. Temperatures at the liquid–gas phase transition curve, T _S(ρ), for each measured density (isochore) and the critical parameters (T _C and ρ _C) for the 0.2607 NH₃ + 0.7393  H₂O mixture were obtained using the quasi-static thermograms technique. The expanded uncertainty of the heat-capacity measurements at the 95 % confidence level with a coverage factor of k = 2 is estimated to be 2 % to 3 % in the near-critical and supercritical regions, 1.0 % to 1.5 % in the liquid phase, and 3 % to 4 % in the vapor phase. Uncertainties of the density, temperature, and concentration measurements are estimated to be 0.06 %, 15mK, and 5×10⁻⁵ mole fraction, respectively. An unusual behavior of the isochoric heat capacity of the mixture was found near the maxcondetherm point (in the retrograde region). The value of the Krichevskii parameter was calculated using the critical properties data for the mixture and vapor-pressure data for the pure solvent (H₂O). The derived value of the Krichevskii parameter was used to analyze the critical behavior of the strong (C _P , K _T ) and weakly (C _V ) singular properties in terms of the principle of isomorphism of critical phenomena in binary mixtures. The values of the characteristic parameters (K ₁, K ₂), temperatures (τ ₁, τ ₂), and the characteristic density differences (Δρ ₁, Δρ ₂) were calculated for the NH₃ + H₂O mixture by using the critical-curve data.     

Quench-condensed indium-hydrogen films. II. Changes in electric and superconducting characteristics near the metal-insulator transition

Electrical and superconducting properties of indium films condensed in a H₂ atmosphere (pressurep _{H 2}=6×10⁻⁶ to 1.4×10⁻⁴ Torr) onto a substrate cooled with liquid helium are investigated. As hydrogen content is increased, a continuous increase in residual resistivity ρ* is observed, permitting systematic study of the resistance vs. temperature dependenceR(T) and the superconducting transition temperatureT _c on approaching the metal-insulator transition (MIT). With regard to ρ*, four regimes of conductivity can be observed: (1) conductivity with a positive temperature resistance coefficient (TRC), (2) conductivity with a small, constant, negative TRC, (3) conductivity under weak localization with ΔR (T) ∼ln T or type corrections, (4) hopping conductivity.T _c rises continuously with ρ* and reaches its peak (∼5.2K) in the second regime. A further increase of ρ* leads to a decrease ofT _c and complete suppression of superconductivity. The experimental dependenceR(T) is compared with theory. TheT _c variation on approaching the MIT and the relation between Mooij's rule and the superconducting properties are discussed.     

Electric resistance and magnetoresistance of 30-nm-Thick La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films elastically compressed in the (110) plane

We have studied the structure, electric resistance, and magnetoresistance of 30-nm-thick (110)La_0.67Ca_0.33MnO₃ (LCMO) films grown by laser deposition on (001)-oriented LaAlO₃ substrates. The unit cell parameters a and b (along the [100] and [010]LCMO axes, respectively) of these manganite films are significantly (by ∼1.2%) increased as compared to the corresponding values in the pseudocubic unit cell of bulk stoichiometric LCMO crystals. At T < 150 K, the temperature dependence of the resistivity of LCMO films is well described by the relation ρ = ρ₁ + ρ₂ (H) T ^4.5. The value of ρ 2 decreases with increasing magnetic field and is close to the analogous coefficient for manganite films grown on substrates with small lattice misfit.     

Van der Waals chain-molecule fluid in self-consistent field approximation: Some thermal properties

The van der Waals equation for a monomer is used to derive the equation of state for a fluid consisting of chain molecules of equal length. The evolution of a part of the diagram of state pertaining to liquid-vapor equilibrium is treated for the case of an increase in the number of links in a molecule. The dependences on the number of links n are found for the following properties of polymer fluid: the critical temperatureT _c , the critical pressurep _c , the critical density p_c, the critical compressibilityz _c , the temperature of normal boiling, the Riedel parameter of similarity a, the acentric factor Ω, and the enthalpy of vapor formation δH_V, A comparison with the experimental data forn-alkanes and 1-alkanols reveals that the obtained dependences reflect qualitatively correctly the variation of the above-identified properties with an increase of the number of links in a molecule. For long chains(n ≪ 1), the scaling dependences are determined for the properties of a chain-molecule fluid:T _c ∽n ^−1/2,p _c ∽n ^−3/2ρ_c∽n ^−1/2,z _c∽n ⁻¹,α∽n, ω∽n ^2/3,ΔH _ν∽n.     

Comment on ‘Resistivity Minimum Behavior and Weak Magnetic Disorder Characteristics in La<Subscript>2/3</Subscript>Ca<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript> Manganites’

In a recent paper (Chen et al. in J. Supercond. Nov. Magn. 22:465, 2009) Zhenping Chen et al. reported measurements of the electrical resistivity (ρ) and magnetization versus temperature (T) and magnetic field (H) on polycrystalline manganite compound La_2/3Ca_1/3MnO₃. In particular, the ρ(T) dependence shows a shallow minimum at T _min∼20 K. Application of the external H up to 8 T leads to marked flattening of such minimum and to notable lowering of the T _min on ρ(T,H) curves. The authors attributed this low-temperature anomaly to competitive quantum Kondo effect and three-dimensional electron–electron interaction, using the results of numerical fittings of ρ versus T and H dependences in the interval of about 5–30 K. It is suggested in the comment that such a claim seems to be doubtful due to the fundamental inapplicability of this approach for analysis of low-temperature conductivity in polycrystalline manganites. It is noted that a natural cause for both the low-temperature minimum on ρ(T) curve and its evolution upon magnetic field application, is the well-known grain boundary effect. The numerous uncertainties and obscurities are also characteristic for the paper being commented upon.     

Calculation of the Critical Field and the Nernst Effect in Cuprate Superconductors

A recent experiment which measured the Nernst effect, the diamagnetic signal and the H _c2 field was interpreted as a support to the scenario which the pseudogap has the pair condensate without long-range phase coherence. We present here calculations which qualitatively reproduces the onset of the Nernst signal temperature T _ν (ρ) and T _c (ρ) as function of the doping level ρ. Together with our previous calculations to the magnetization and H _c2, we conclude that the phase separation scenario supports also the new Nernst effect experiments. This work has been partially supported by CAPES and CNPq.     

On the α-relaxation in bulk polyoxymethylene

Values quoted forΔH _α, the activation energy of the high temperatureα-relaxation in polyoxymethylene (POM), range from 20 to 92 kcal mol⁻¹. This paper seeks to rationalize the discrepancy by remeasuringΔH _α using time-temperature superposition of torsional creep and dynamic compliances for a POM specimen annealed at 160° C. Superposition of loss compliance curvesJ″ (ω, T) is possible over the range 20 to 120° C but creep compliance curvesJ(t, T) fail to superpose above about 70° C. The creep anomaly is explained in terms of the McCrum-Morris reduction equations in which the unrelaxed complianceJ _U ^T increases with temperature more rapidly than the relaxed complianceJ _R ^T . The activation energyΔH _α has a constant value of 21±1 kcal mol⁻¹ below about 70° C. Above about 70° C,ΔH _α increases steadily up to 33±2 kcal mol⁻¹ at 120° C.     

Glass transition and fragility of telluro-vanadate glasses containing antimony oxide

The activation energy (ΔH ^*) of the glass transition and the heating-rate dependence of the glass transition temperature (T _g) of V₂O₅–Sb₂O₃–TeO₂ glasses were determined using differential scanning calorimetry technique. Non-isothermal measurements were performed at different heating rates φ (=3, 6, 9, 10, 13 K/min). The heating rate dependence of T _g was used to investigate the applicability of different theoretical models describing the glass transition. The application of Moynihan and Kissinger et al. models to the present data led to different values of (ΔH ^*) at each different heating-rate regions. This behavior was attributed to the strong heating rate dependence of the activation energy of the process. The fragility parameter (m = ΔH ^*/RT _g) were ≲90, suggesting that these glasses may be classified as strong glasses. The viscosity, η, calculated at a few selected temperatures near the glass transition region increased with increasing Sb₂O₃ content at any given temperature, which is also expected. Also the compositional dependence of T _g and ΔH ^* was investigated.     

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.      

Study of galvanomagnetic effects in Bi-Sr-Ca-Cu-O films

Galvanomagnetic effect has been studied at 77 K on Bi-Sr-Ca-Cu-O films as a function of d.c. bias current. These films were prepared by spray pyrolysis and screen-printing techniques, withT _c (R=0) 80 K and 100 K respectively. Magnetic field dependence ofI _c of sprayed-film showed very small hysteresis whereas screen-printed film showed greater hysteresis. Application of small magnetic field on these films destroyed the zero-resistance state and a finite resistance was developed. The slope of the resistance vs magnetic field curveΔR/ΔH depended on the current flowing through the sample. To increaseΔR/ΔH, meander-shape pattern was prepared on the sprayed film. In generalΔR/ΔH increased by a factor of 10³ after sample patterning. The possibility of using these films as a sensitive magnetic field sensor is discussed.     

Normal and Superconducting State Properties of Doped CePt3Si

We report on X-ray powder diffraction, magnetic susceptibility, electrical resistivity, and specific heat measurements on samples of Ce_1-xLa_xPt₃Si and CePt₃Si_1+δ in order to better understand this unusual heavy Fermion superconductor’s normal and superconducting state properties. By suppressing the antiferromagnetic transition, T_N = 2.2 K, using La-doping on the Ce-site in CePt₃Si, we find that in Ce_1-xLa_xPt₃Si, x ≥ 0.8, ρ, χ, and C/T show non-Fermi liquid temperature dependences. C/T ~ T^α with α ~ 1 for the temperature range of 0.3–3 K while low-temperature (2–20 K) susceptibility data follow 1/χ−1/χ₀ = aT^η behavior with η = 0.36. In addition, χ and C/T exhibit single-ion behavior in this composition range (x ≥ 0.8), i.e. the measured values expressed per Ce-mole are independent of x. Via doping the Si-site with a small excess—2–4—of Si, we present specific heat data for the bulk superconducting transition at T_c ~ 0.8 K showing that this excess sharpens the transition, obviating the need for annealing. Measurements of the X-ray diffraction patterns, ρ, χ, and C/T of these CePt₃Si_1+δ samples, when compared to similar measurements on annealed CePt₃Si samples, indicate that the microscopic mechanism for the strengthening and sharpening of ΔC(T_c) with Si-excess may be similar to that responsible in the annealed material.     

History effects in low-field magnetoresistance of BPSSCO polycrystals

Low-field (H<40 G) magnetoresistance measurements have been made on Bi_1·6Pb_0·4Sr₂Ca₂Cu₃O₁₀ polycrystals at several temperatures between 80 and 105 K. Considerable hysteresis in ρ(H) is found in a zero-field-cooled sample when the applied field is increased from 0 to a maximum value and then lowered back to 0 at all temperatures. The observation of hysteresis is taken as an evidence for field trapping in the grains. We show that the hysteresis in ρ(H) occurs for applied fields much lower than that at whichdρ(H)/dH exhibits a discontinuity. In addition, we find that when the applied magnetic field (H _a) is lowered from a maximum field, the effective intergranular field,H _eff, becomes zero forH _c>0, which gives rise to a minimum in ρ(H).     

Induced of zeeman coherence in an ensemble of cesium atoms interacting with a two-frequency (microwave+rf) magnetic field

Interference between Zeeman states corresponding to the forbidden magnetic-dipole transition ΔF=0, Δm _F=2 is reported in connection with the simultaneous interaction of 6² S _1/2 cesium atoms with a resonant microwave field and an rf field which varies at twice the Larmor frequency and is directed perpendicular to a static magnetic field H ₀=0.2 Oe. Pis’ma Zh. Tekh. Fiz. 24, 89–93 (July 26, 1998)     

Sound Velocity Measurements in the Low and the High Field Phases of the Nuclear-Ordered bcc Solid 3He in Magnetic Fields

We have measured the temperature and magnetic-field dependences of the sound velocity for one longitudinal and two transverse waves in the low field phase (LFP) and the high field phase (HFP) of nuclear spin ordered bcc solid ³He crystals with a single magnetic domain along the melting curve. From sound velocity measurements for various crystal orientations as a function of the sound propagation direction, we determined the elastic stiffness constants, c _ij (T,B). In the LFP with tetragonal symmetry for the nuclear spin structure, we extracted six nuclear spin elastic stiffness constants Δc _ij ^ℓ (T,0.06 T) from the temperature dependence of the sound velocity at 0.06 T and Δc _ij ^ℓ (0.5 mK,B) from the magnetic-field dependence of sound velocity at 0.5 mK. In the HFP with cubic symmetry for the nuclear spin structure, we extracted three Δc _ij ^h (T,0.50 T) at 0.50 T and Δc _ij ^h (0.5 mK,B) at 0.5 mK. At the first-order magnetic phase transition from the LFP to the HFP at the lower critical field B _c1, large jumps in sound velocities were observed for various crystal directions and we extracted three . Using the thermodynamic relation between Δc _ij and the change in the internal energy for the exchange interaction in this system, ΔU _ex(T,B), Δc _ij are related to the generalized second-order Grüneisen constants Γ _ij ^X ≡∂ ²ln X/∂ ε _i ∂ ε _j as Δc _ij (T,B)=Γ _ij ^X ΔU _ex(T,B), where X represents some physical quantity which depends on the molar volume and ε _j is the j-th component of a strain tensor. In the LFP, the Δc _ij ^ℓ (T,0.06 T) were proportional to T ⁴, and Δc _ij ^ℓ (0.5 mK,B) were proportional to B ². We extracted for the spin wave velocity in the LFP, s ^ℓ , from Δc _ij ^ℓ (T,0.06 T) and for the inverse susceptibility, 1/χ ^ℓ from Δc _ij ^ℓ (0.5 mK,B). In the HFP, Δc _ij ^h (T,0.50 T) were proportional to T ⁴ and Δc _ij ^h (0.5 mK,ΔB) were proportional to ΔB(≡B−B _c1). We obtained for the spin wave velocity in the HFP, s ^h , from Δc _ij ^h (T,0.50 T) and for B _c1 from Δc _ij ^h (0.5 mK,ΔB). The values obtained for and were compared with the Multiple Spin Exchange model (MSE) with three parameters by using analytic expressions for s ^ℓ and χ ^ℓ . The three-parameter MSE does not agree with the observed Δc _ij in the LFP.      

Experimental Study of the Isochoric Heat Capacity of Diethyl Ether (DEE) in the Critical and Supercritical Regions

Two- and one-phase liquid and vapor isochoric heat capacities (C _{V ρ} T relationship) of diethyl ether (DEE) in the critical and supercritical regions have been measured with a high-temperature and high-pressure nearly constant-volume adiabatic calorimeter. The measurements were carried out in the temperature range from 347 K to 575 K for 12 liquid and 5 vapor densities from 212.6 kg·m⁻³ to 534.6 kg·m⁻³. The expanded uncertainties (coverage factor k =  2, two-standard deviation estimate) for values of the heat capacity were 2% to 3% in the near-critical region, 1.0% to 1.5% for the liquid isochores, and 3% to 4% for the vapor isochores. The uncertainties of density (ρ) and temperature (T) measurements were 0.02% and 15 mK, respectively. The values of the internal energy, U(T, V), and second temperature derivative of pressure, (∂² P/∂T ²) _ρ , were derived using the measured C _V data near the critical point. The critical anomaly of the measured C _V and derived values of U(T, V) and (∂² P/∂T ²) _ρ in the critical and supercritical regions were interpreted in terms of the scaling theory of critical phenomena. The asymptotic critical amplitudes (A₀⁺ and A₀^- ){({A_0^+} {\rm and} {A_0^- )}} of the scaling power laws along the critical isochore for one- and two-phase C _V were calculated from the measured values of C _V . Experimentally derived values of the critical amplitude ratio for C_V (A₀⁺ /A₀^- = 0.521){C_{V} \left({A_0^+ /A_0^- = 0.521}\right)} are in good agreement with the values predicted by scaling theory. The measured C _V data for DEE were analyzed to study the behavior of loci of isothermal and isochoric C _V maxima and minima in the critical and supercritical regions.     

Thermodynamic properties of NaZr2(PO4)3

The heat capacity of crystalline NaZr₂(PO₄)₃ was measured between 7 and 340 K by adiabatic calorimetry. The results were used to calculate the thermodynamic functionsC _p ⁰ ,H ⁰(T) -H ⁰(0),S ⁰(T), andG ⁰(T) -H ⁰(0) in the range 0-340 K. The absolute entropy was found to be S⁰NaZr₂(PO₄)₃, cr, 298.15 K) = 327.1 ±1.0 J/(mol K), and the standard entropy of formation Δ_fS⁰(NaZr₂(PO₄)₃, cr, 298.15 K) = -1101±1 J/(mol K). Solution calorimetry was used to determine the standard enthalpy of formation, Δ_f H ⁰(NaZr₂(PO₄)₃, cr, 298.15 K) = -5236 ±5 kJ/mol. By combining the data obtained by the two techniques, the standard Gibbs energy of formation was determined to be Δ_fG⁰(NaZr₂(PO₄)₃, cr, 298.15 K) = -4908 ±5 kJ/mol.     

Superconducting gap function and upper critical field in the presence of inhomogeneous magnetic order

Rare earth ternary superconductors are known to exhibit oscillatory magnetic orders below their superconducting transition temperatures. The study of behaviour of superconducting electrons in an inhomogeneous magnetic field is therefore important for such systems. We report here the results of our theoretical study of superconducting gap function Δ(T) and upper critical fieldH _c2 (T). The results are applied to analyse and explain the variation of Δ(T) andH _c2 (T) in case of NdRh₄B₄.     

Thermodynamic properties of NaZr<Subscript>2</Subscript>(AsO<Subscript>4</Subscript>)<Subscript>3</Subscript>

The heat capacity C _p ⁰ of crystalline NaZr₂(AsO₄)₃ has been measured in the range 7–650 K using precision adiabatic calorimetry and differential scanning calorimetry. The experimental data have been used to calculate the standard thermodynamic functions of the arsenate: C _p ⁰, enthalpy H ⁰(T) − H ⁰(0), entropy S ⁰(T), and Gibbs function G ⁰(T) − H ⁰(0) from T → 0 to 650 K. The standard entropy of its formation from elements is Δ_f S ⁰(NaZr₂(AsO₄)₃, cr, 298.15 K) = −1087 ± 1 J/(mol K).