Flux dynamics and low-field magnetic properties of the heavy-fermion superconductor CeCu2Si2

Strong flux creep effects are observed in CeCu₂Si₂ at low magnetic inductions and temperatures in the range 6 mK<T<T _c . Measurements of flux creep in single crystal as well as polycrystalline specimens show that the magnetization in CeCu ₂ Si ₂ decays following a power law time dependence of the typeM(t)t ^– for 1<t<10 ⁵ sec, where the exponent a ranges between 0.015 and 0.06. The activation energy values obtained from the relaxation rates are around 2–3 meV. The flux creep data give clear evidence of the existence of different types of pinning centers in CeCu ₂ Si ₂ : a large number of shallow potential barriers that characterize the vortex dynamics at short times and few strong pinning centers responsible for the long-time relaxation behavior. Quantum tunneling of flux lines through weak pinning centers probably gives rise to the non-thermally activated relaxation observed at the lowest temperatures. The physical origin of weak and strong pinning centers in CeCu ₂ Si ₂ is discussed. From isothermal dc magnetization measurements we estimate the lower critical fieldH _c1 as well as the critical current densityJ _c . TheH _c1 versusT curve shows an anomalous positive curvature for temperatures close toT _c .     

Intrinsic critical current of Ag-clad (Bi,Pb)2Sr2Ca2Cu3Oz tapes

True zero-field critical current densityJ _c of a well-characterized BPSCCO/Ag tape has been determined by means of high-resolution ac susceptibility in the temperature range 77–110 K. The resultant values (30,000 A/cm² at 77 K) agree well with the transportJ _c of the same tape. Because of a very thin BPSCCO, the coreJ _c determined from the imaginary part of the ac susceptibility is nearly the same as the zero field one. AllJ _c 's follow the same (1-T/T _c )ⁿ withn=1.45 dependence.J _c shows an approximateH ^–0.5 field dependence over the explored temperature range. Accordingly, the variations ofJ _c withT andH seem to be determined by the flux creep.     

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.     

Remarkable peak effect in Jc(H, T) of Y–Ba–Cu–O bulk by using infiltration growth (IG) method

This study presented the superior superconductivity of Y–Ba–Cu–O (YBCO) bulk materials grown by using modified infiltration growth (IG) method with a small Sm–Ba–Cu–O crystal as seed. Compare to the bulks grown by melt-textured growth (MTG) method, IG bulk is denser and with finer second phase (Y₂Ba₁Cu₁O₅, Y211). In addition, remarkable peak effect was observed of the IG YBCO bulk material. Peak effect in J_c(H, T) is an distinctive property of RE–Ba–Cu–O (RE = rare earth element, e.g. Sm, Nd) superconductors. However, it is found that peak effect is unusual in Y–Ba–Cu–O material. In this study, magnetism measurements also show peak effect in J_c(H, T), especially at low temperatures. At 50 K, the peak position of J_c(H, T) was about 5 Tesla. J_c value of peak, J_c(peak, 50 K), is comparable to that of self-field, J_c(0, 50 K). On the other hand, it is noted that this effect is spatial-dependent, i.e. it only can be found in particular part of bulk. To interpret the mechanism of peak effect and the spatial-dependence, different microstructure analysis, which includes scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and X-ray absorption spectrum (XAS) were used. The results indicated that the composition is inhomogeneous throughout the bulk. Diffusion of Sm from seed to YBCO bulk was observed, which is believed to occur during high temperature, and thus changed the superconductivity. The inhomogeneity of microstructure was correlated to the spatial-dependent of peak effect. Finally, this study also demonstrates a possible way to introduce peak effect in YBCO materials and thus enhance the applications in high field regions.     

Study of Flux-Creep in Bulk Melt-Textured YBa2Cu3Ox in the Regime of Individual Pinning

Magnetic relaxation, with magnetic fields applied parallel to the c axis, was measured in a high-quality melt-textured YBa₂Cu₃O_x bar. Zero-field-cooled magnetization-versus-time data were obtained within a regime of field and temperatures dominated by single vortex pinning. The activation pinning energy U = U _M was calculated from experimental data and compared to U = U ₀ ln(J _c /J), where U, is a constant, J _c is the critical current density, and J the current density. This expression for U obtains within the individual pinning regime. A logarithmic dependence of U _M on J was found, and the data indicated that a scaling function of U _M , g(T/T _c ) = (1–T/T _c )^1/2, was appropriate for describing the single-vortex pinning regime.     

Strong Vortex Pinning in the Low–Temperature Superconducting Phase of (U1−xThx)Be13

We have found a sharp transition at T _c2 = 350 mK in the vortex creep rate of a single crystal of (U _1–x Th _x )Be ₁₃ with T _c = 523 mK (x = 0.0275). For T T _c2 , no creep of vortices is observed in a time scale of 10⁵ s, while for T _c2 < T < T _c , vortices creep at very high rates (30% of decay from a metastable configuration in the first 10⁵ s at T = 400 mK). The sharp transition occurs at the same temperature at which the second jump in the specific heat appears in these samples. Similar low levels of creep rates have been reported by us in the low–T superconducting phase of UPt ₃.¹     

Measuring Turbidity in a Near-Critical, Liquid–Liquid System: A Precise, Automated Experiment

A ground based (1g) experiment is in progress that measures the turbidity of the density-matched, binary fluid mixture methanol–cyclohexane extremely close to its liquid–liquid critical point. By covering the range of reduced temperatures t (T–T _c)/T _c from 10^–8 to 10^–2, the turbidity measurements should allow the Green–Fisher critical exponent to be determined. This paper reports measurements showing ±0.1 % precision of the transmitted and reference intensities, and ±4K temperature control near the critical temperature of 320 K. Preliminary turbidity data show a nonzero consistent with theoretical predictions. No experiment has precisely determined a value of the critical exponent , yet its value is significant to theorists in critical phenomena. Relatively simple critical phenomena, as in the liquid–liquid system studied here, serve as model systems for more complex behavior near a critical point.     

Reproducibility of transition temperature and critical current density of thin films of YBa2Cu3O7 on (100) LaAlO3

Submicrometer epitaxial films of YBa₂Cu₃O₇(YBCO) on (100) LaAlO₃ were made by coevaporation and furnace annealing. Samples from more than a dozen runs are used in this study. The zero resistance transition temperature (T _c) is high (89 or 90 K) if the film composition is phase pure (Ba/Y=2, Cu/Y=3) or if it is enriched in Ba and Cu. For these compositions the critical current density (J _c) at 77 K has an average value of 2×10⁵ A cm^–2, with a tendency for decreasingJ _c with increasing film thickness (0.2 to 0.8m). Variations inJ _c are not correlated with deviations from ideal stoichiometry. Steeper slopes of the resistance-temperature curves above 100 K and lower values of the room-temperature resistivity are associated with high values ofJ _c. If the film composition is enriched in Y relative to Ba and Cu,T _c decreases by several degrees.     

Stability of supercurrents in cylindrical films of tin

Using pulsed currents of 80-nsec duration the stability of supercurrents in cylindrical films with current return through the center has been investigated. It is found that the voltage along the film rises with timet approximately asV = a[1 – erf(t/2(t – c)) ^1/2] where the time constantt is an exponentially decreasing function ofi(1 + 420/d)/(1 –(T/T _c)⁴). Herei is the current through the sample,d the thickness of the film in angstroms, andT/T _c the reduced temperature. Comparison with older data by Mydosh and Meissner and by Hagedorn indicates that it is indeed the currenti, rather than the current densityJ, which determinest. It is suggested that the instability is of the nature of those investigated by Kramer and by Fink and Presson for currents in a superconducting surface sheath.Supported in part by the National Science Foundation and in part by the Cryogenics Center through a Grant from the DOD Themis Program.     

Magnetic field dependence of the critical current density for the bismuth-based bulk high-Tc superconductors

Three types of bismuth-based bulk samples were prepared through uniaxial pressing at room temperature, hot isostatic pressing (HIP) and drawing and rolling. Transport current properties were characterized in a steady field up to 1.12 T at 77 K (T/T _c=0.75). The Josephson weak-link decoupling fields have been found to be 5 mT for the cold-pressed pellet and 30 mT for the HIPed pellet and the rolled tape. At the decoupling field the transport critical current density,J _c, drops 80% from 124 (OT) to 29 A cm^–2 (5 mT) for the cold-pressed pellet, 80% from 582 (OT) to 126 A cm^–2 (30 mT) for the HIPed sample and 50% from6500 (0 T) to 2850 A cm^–2 (30 mT) for the rolled tape. In the flux flow regime, whereB is perpendicular to thec-axis a modified Kim's modelJ _c=(/B ₀)/[(1+B/B ₀)] ⁿ can be used to describe the field dependence of the critical current density, J_c, in the field range 0.2–1.12 T. The effective upper critical fields were estimated to be 0.98, 1.54 and 1.94 T for the three types of samples, respectively. An adjustable range ofB _c2 for bismuth-based bulk highT _c superconductors is given. Flux shear may operate in these materials. The prediction of this pinning mechanism is yielded from fitting the equation qualitatively. WhenB is parallel to thec-axis, the absence of strongly intragranular flux-pinning is emphasized by the poor flux flow regime for the rolled tape sample.     

Scaling Behavior and Estimation of Activation Energy of Polycrystalline RuSr2GdCu2O8 Superconductor from AC Susceptibility Measurements

In this study, the measured curves of AC susceptibility (ACS) components, (T) and (T), of polycrystalline RuSr₂GdCu₂O₈ (Ru-1212) superconductor were scaled onto a single curve using the peak temperature of its imaginary part (T _p) as the scaling parameter for various AC field amplitudes from 0.5 to 24 G. The dependence of the AC magnetic field amplitude on T _p is scaled as: H _ac (1–T _p/T _c)^2.25. Similarly, the current density J _c, extracted from the AC field amplitude is also scaled as: J _c (1 – T _p/T _c)^2.25. The dependencies of T _p on frequency and AC field amplitude are also investigated and the time parameter t ₀ of the order of 10^–8 s is estimated from the dependence of T _p on frequency. The dependencies of activation energy on temperature, T, and the field amplitude, H _ac, are obtained from the Arrhenius-like semilog plot of frequency () and T _p. Such dependencies on temperature and field amplitude can be described by a scaling law of the form: U(H _ac,T) = U ₀[1 – T/T _p]H _ac ^–0.17.     

Evolution of transient thermal processes in layer counterflow apparatuses and heat exchangers

Results are given of an analytic investigation of transient processes inside counterflow apparatuses and heat exchangers with temperature disturbance in one of the heat carriers at the entry to the apparatus.Notation =(t–t₀)/(T₀–t₀),=(T–t₀)/(T₀ s-t₀) relative temperatures - t, T temperatures of material and gas respectively - t₀, T₀ same for the initial state - Z=[ _Vm₁/c(1–w/w_g)] [–(y₀–y)/w_g] dimensionless time - m₁=1/(1+Bi/) solidity coefficient - B₁=( _FR/) Biot number - _{F V} heat-exchange coefficients referred to 1 m² surface and 1 m³ layer - R depth of heat penetration in a portion - portion heat conductivity coefficient - shape coefficient (=0 for a plate,=1 for a cylinder,=2 for a sphere) - c, C_g heat capacities of material and gas respectively - , _g volumetric masses - w, W_g flow velocities of material and gas - y distance from the point of entry to the heating heat carrier - y₀ heat-exchanger length - Y= _Vm₁y/W_gC_{g g} dimensionless coordinate - m=cw/C_g– _gW_g water equivalent ratio Deceased.Translated from Inzhenerno-Fizicheskii Zhurnal, vol. 20, No. 5, pp. 832–840, May, 1971.     

Activation Energy of Bi2Sr2Ca1−x Er x Cu2O8+d Epitaxial Thin Films

The scaling behavior of the effective activation energy U(T, H) of epitaxial oriented Bi₂Sr₂Ca_1–x Er _x Cu₂O_8+d thin films with 0x0.10 has been studied as function of temperature and magnetic field. It has been found that the activation energy scales as U(H, T)=U ₀(1–t)^m H ⁿ with exponent m=1.20±0.03 and n=–1/2. The field and temperature dependence of the activation energy, as well as its overall magnitude, is consistent with a model in which U(T, H) arises from plastic deformations of viscous flux liquid above the vortex-glass transition temperature.     

Devitrification and vitrification of tellurite glasses

The crystallization of pure tellurite glass during various heating rates was studied. The activation energy for crystallization was 115 × 10²² eV mol^–1. The glass transformation, T _g, starting crystallization, T _x, crystallization, T _c and melting temperatures, T _m, have been reported for binary tellurite glasses of the form (1 – x) TeO₂–xA_nO_m [A_nO_m = MnO₂, Co₃O₄ and MoO₃]. Among many different parameters of the glass forming potential the two-thirds rule, T _g/T _m, the glass stabilization range, T= T _x – T _g, and the glass forming tendency, K _g= (T _c– T _g)/(T _m – T _c), are reported for the first time for tellurite glasses.     

Hot hardness of (Fe,Cr)3C and (Fe,Cr)7C3 carbides

Hot hardness was measured on the primary carbides, (Fe, Cr)₃C and (Fe, Cr)₇C₃, in unidirectionally solidified iron-carbon-chromium hypereutectic alloys with chromium more than 4.8 wt %. The hardness-temperature relation was represented by two Ito-Shishokin formulae,H _v =A(— BT), and thus was drawn by two lines on a semilogarithmic graph. The inflection temperature where the two lines intersected was found at 730 to 860 K for (Fe, Cr)₃C carbide containing 0 to 14 wt % Cr, increasing with an increase in the chromium concentration in the carbide, and at about 910 K for (Fe, Cr)₇C₃ carbide containing 36 to 76 wt % Cr. With increasing chromium concentration in each carbide, the hardness of the carbide increased and the thermal softening coefficients decreased. The effect of chromium on the hardness, the inflection temperature and the thermal softening coefficients was more pronounced for (Fe, Cr)₃C carbide than for (Fe, Cr)₇C₃ carbide. Each of the thermal softening coefficients,B ₁(T<T _t),B ₂(T>T _t), the inflection temperature,T _t, room-temperature hardness,H _v(T _RT), and the hardness atT _t,H _v(T _t), related linearly to the chromium concentration in the carbides, and hence the hot hardness of the carbides could be expressed as functions of temperature and chromium concentration in the carbides. The relationships betweenH _v(T _RT) andH _v(T _t) and between the thermal softening coefficient,B ₂, and the activation energy for creep,Q _c(kJ mol^–1), were represented by the following equations:H _v(T _t)0.7H _v(T _RT),B ₂=1.26/Q _c.     

Thermally Assisted Flux Creep of a Driven Vortex Lattice in Untwinned YBa2Cu3O7−δ Single Crystals

We have studied the driven vortex lattice in untwinned, clean YBa ₂ Cu ₃ O _7– single crystals, showing the first order (melting) transition T _m . At high enough driving currents (j 10 ³ A/cm ² , j j _c ) and temperatures T < T _m , a clear distinction is found between two different behaviours of the moving vortex lattice. The onset of dissipation is characterized by a noisy flux creep with a temperature independent activation energy U(j). At higher temperatures, the creep regime crosses over into a flux flow regime with linear resistivity. Apart from the dip in resistivity at T _m , associated with the peak effect and usually assigned to a softening of the shear modulus, the resistivity of the flowing flux lattice continuously extends into the vortex-liquid.     

New equations for enthalpy of vaporization from the triple point to the critical point

Two new equations are proposed for the enthalpy of vaporization from the triple point to the critical point. One of these equations containing four parameters is exceptionally good for fitting the data. The other equation containing three parameters is quite adequate for fitting the data but it is exceptionally suited for interpolation when the data do not cover the entire range. These equations have been tested using the enthalpy of vaporization of water from the triple point to the critical point and are compared with other equations.Nomenclature T _c Critical temperature, K - T _t Triple point, K - T _x Any particular temperature, K - T _r Reduced temperature - P _r Reduced pressure - R Gas constant - P Vapor pressure - X (T _c–T)/T _c - Y (T _c–T)/T - X _x (T _c–T)/(T _c–T _x) - X _t (T _c–T)/(T _c–T _t) - H _vt Enthalpy of vaporization at the triple point, kJ · mol^–1 - H _vx Enthalpy of vaporization at any temperature x, kJ · mol^–1 - Z _v Compressibilty factor of the saturated vapor - Z ₁ Compressibilty factor of the saturated liquid Relative deviation = 100[H_v(obs)–H_v(cal)]/H_v(obsd) Standard deviation = { [H _v(obs)–H _v(cal)]²/(No. points — No. parameters)}^0.5     

Enhancement of Pinning Role by Nonstoichiometry in YBa2Cu3O7?y Superconductors

Considerably improved flux pinning and critical current density J c values have been achieved in Y-deficient Y-123 superconductors by directional solidification in air. In comparison with the regular Y-123 composition, Y-deficient one also has an orthorhombic structure and Y-123 main crystal phase remains in it. Whereas with the shortage of Y, Y_1–x Ba₂Cu₃O_7–y can be regarded as (YBa₂Cu₃)_1–x O_7–t(Ba₂Cu₃) _x O _t–y or (YBa₂Cu₃O_7–z )( _x ^YO _z–y ), so there may develop several kinds of microstructure defects as pinning sites in the system, such as highly dense, fine-scale, and faultlike defects, as well as localized superstructure, which are able to induce the increasing in flux pinning and J c values in higher external magnetic fields. This kind of simple nonstoichiometric route could lead to a commercial technique for flux-pinning enhancement in Y-123 bulk materials.     

Upper critical field and related properties of superconducting amorphous alloys Zr-Si

The upper critical fieldH _c2 has been measured for the superconducting amorphous alloys Zr_100–x Si _x withx=13, 15, and 17 over a wide temperature range down to 50 mK. Measurements for related properties, such as the depinning current densityJ _p , pinning force densityF _p , and flux flow resistivity _f , reveal that these systems are well characterized as typically soft and dirty type II superconductors with an extremely lowJ _p of 1 A/cm² andF _p of 1.5×10⁴ N/m³ ath (H/H _c2)=0.5 andt (T/T _c )=0.5. The temperature dependence ofH _c2 is well fitted by a curve of the Werthamer-Helfand-Hohenberg theory with Maki parameter =1.7–1.5 and spin-orbit interaction parameter _so=1–2.4. These results indicate that the Pauli spin-paramagnetic effect plays an important role in the low-temperature behavior ofH _c2 in these low-T _c and high-H _c2 amorphous superconductors. Corrections of the electron-phonon coupling (_ep=0.6) to the Pauli spin-paramagnetic limiting field lead to a reduced importance of the spin-orbit interaction effect, with _so less than 1.0. A discussion is given of the structural inhomogeneities in an amorphous phase, which are sensitively reflected in the strength of the fluxoid line pinning interaction (J _p ,F _p ) and are intimately related to the nonintrinsic anomaly, observed in many amorphous alloys, thatH _c2 depends linearly on temperature even at low temperatures, exceeding the theoretically allowedH _c2 values.