Magnetoresistance,Irreversibility Fields,and Critical Currents of Superconducting 2G Tape

The magnetoresistance, irreversibility fields, and critical current density were studied for a commercial 2G tape at the two relative orientations of magnetic field and superconductor plane. The critical temperatures of this tape of T _{c50 %} = 91.5 K and T _c0 = 90 K and the width of superconducting transition of ΔT = 1.5 K were obtained. The widths of the transition from the normal to the superconducting state do not increase at the applied magnetic field up to 90 kOe and do not depend on the orientation of the magnetic field with respect to the tape plane. The irreversibility field values were obtained and successfully fitted as a function of temperature using the formula: \(H_{\text {irr}} =H_{\text {irr0}} \left ({1-\frac {T}{T_{\text {c0}} }} \right )^{n}\). The irreversibility fields show an anisotropy, and at the liquid nitrogen temperature, they reach H _irr = 430 kOe and H _irr = 106 kOe for the parallel and perpendicular directions, respectively. The anisotropy ratio amounts to γ = 4 at 77 K and is small in comparison with other high-temperature superconducting materials. The critical current density of this tape was found to be of the order of 10⁶ A cm^?2 at 77 K in the self-magnetic field.     

Quantum-critical Pairing with Varying Exponents

We analyze the onset temperature T _p for the pairing in cuprate superconductors at small doping, when tendency towards antiferromagnetism is strong. We consider the model of Moon and Sachdev (MS), which assumes that electron and hole pockets survive in a paramagnetic phase. Within this model, the pairing between fermions is mediated by a gauge boson, whose propagator remains massless in a paramagnet. We relate the MS model to a generic γ-model of quantum-critical pairing with the pairing kernel $\lambda(\Omega_{n})\propto1/\Omega^{\gamma}_{n}$ . We show that, over some range of parameters, the MS model is equivalent to γ=1/3-model (λ(Ω)∝Ω^?1/3). We find, however, that the parameter range where this analogy works is bounded on both ends. At larger deviations from a magnetic phase, the MS model becomes equivalent to γ model with varying γ>1/3, whose value depends on the distance to a magnetic transition and approaches γ=1 deep in a paramagnetic phase. Very near the transition, the MS model becomes equivalent to γ model with varying γ<1/3. Right at the magnetic QCP, the MS model is equivalent to the model with λ(Ω _n )∝log?Ω _n , which is the model for color superconductivity. Using this analogy, we verify the formula for T _c derived for color superconductivity.     

Kapitza conductance of the gadolinium gallium garnet,Gd3Ga5O12 between 1.4 K and 2.1 K

The Kapitza conductance at the interface between Gd₃Ga₅O₁₂ and saturated helium ll has been measured under three applied magnetic fields. The results are represented by the expression h_ko = 0.067 T_b^3.15 for a very small temperature difference ΔT at the interface and no applied magnetic field. The Kapitza conductance tends to decrease with increasing applied magnetic field. In addition, the results for large ΔT are described briefly.     

Field Dependence of Magnetocaloric Properties in La 0.6Pr 0.4Fe 10.7Co 0.8Si 1.5

In this paper, the field dependence of magnetocaloric properties of La _0.6Pr _0.4Fe _10.7Co _0.8Si _1.5 with second-order phase transition material is studied using a phenomenological model. The model parameters were determined from the magnetization data adjustment and were used to give better fits to magnetic transition and to calculate the magnetocaloric thermodynamic quantities. The entropy curves have been observed to behave as an asymmetrical broadening of ΔS _M peak with increasing magnetic field. For larger fields, the peak shifts to higher temperatures, while the overall shape of the curve broadens over a wide temperature range. The values of maximum magnetic entropy change, full width at half maximum, and relative cooling power, at several magnetic field variations, were calculated. The maximum magnetic entropy changes of 3.957(5) and 14.197(4) J kg ^?1 K ^?1 and the relative cooling power (RCP) values of 95.420(3) and 392.729(2) J kg ^?1 are obtained for 1 and 5 T, respectively. The theoretical calculations are compared with the available experimental data. The critical exponents associated with ferromagnetic transition have been determined from magnetocaloric effect (MCE) methods. By using the field dependence of ΔS _{M max}≈a(μ ₀ H) ⁿ and the distance (T _peak?T _c)≈b(μ ₀ H)^1/Δ, we have investigated the critical behavior of La _0.6Pr _0.4Fe _10.7Co _0.8Si _1.5. From the analysis of the relationship between the local exponent n and the gap exponent Δ, we have calculated other exponents: β, γ, and δ. The large MCE, relatively high RCP, high magnetization, and low cost jointly make the present compound a promising candidate for magnetic refrigerant near room temperature.     

Evolution of Superconducting Properties of LiFeAs Single Crystals Doped with Magnetic or Nonmagnetic Impurities

The effect of magnetic Co²⁺ and nonmagnetic Ga³⁺ impurities on the crystal structure and superconducting properties of LiFeAs single crystals has been investigated. A large T _c decrease of about 4.8 K/at% is observed in Ga-doped LiFeAs. This rate is higher than that of the material doped with magnetic Co impurities (～3.7 K/at%). The greater T _c suppression in the Ga case is likely due to the pair breaking associated with the significant changes in the crystal structure of the doped material. The increase of the critical current densities in intermediate magnetic fields (H⊥?ab) indicates that a very small amount of Ga (0.5 at%) acts as an effective pinning site for flux pinning enhancement in the material. The analysis of the temperature and field dependencies of the magnetic relaxation is consistent with the collective pinning model for the Co-doped material, while the magnetic relaxation measurements combined with the peak position of the critical current density in the B–T phase diagram of Ga-doped LiFeAs suggest an elastic–plastic transition of the vortex lattice at higher temperatures and fields.     

Magnetic Specific Heat of Heavily p-Type Doped (Zn,Mn)Te:P

The magnetic contribution to the specific heat of bulk crystals of Zn_1?x Mn _x Te ( x = 0.03) heavily (up to 10¹⁹ cm^?3) p-type doped with P is studied over the temperature range 0.5–15 K and magnetic field range 0–3 T. The magnetic specific heat observed at zero magnetic field indicates that a substantial part of the magnetic ions has the degeneracy of their magnetic ground state lifted by d–d and p–d exchange interactions. The effect increases for doped and annealed samples with higher concentration of conducting holes. We have also carried out a theoretical analysis that takes into account the contributions due to small magnetic clusters, single magnetic ions in crystal field of distorted crystal lattice, and low energy excitations of the p–d exchange-coupled system of local moments and carriers.     

Disorder Tuned Superconductor Insulator Transition in La2−x (Sr/Ce) x CuO4 & NbN Superconducting Thin Films

In studying the magneto-resistivity ρ(B) in high pulsed magnetic fields up to 55 T, it is often observed that the ρ(B) curves for different temperatures in the vicinity of the critical temperature cross at the same field value, B=B _CP . We show how the crossing field changes as a function of the hole (or electron) doping x in La_2?x (Sr/Ce) _x CuO₄. The resistivity ρ and the magnetic field B at different temperatures may be scaled as R/R _CP and |B?B _CP|/T ^γ , respectively. This kind of scaling resulted in a conventional critical exponent γ=1/ν z=1.35 (ν z～0.74) in our La_2?x (Sr/Ce) _x CuO₄ thin films, and a much higher value of γ=4.35 (ν z～0.23) in the case of our disordered NbN films.     

Dependence of energy confinement time on toroidal magnetic field in the TUMAN-3M tokamak

The influence of toroidal magnetic field on the energy confinement time ??_E in the ohmic H-mode has been studied in the TUMAN-3M tokamak with low magnetic field. The experiments were performed at a toroidal magnetic field of B _T = 0.68?1.0 T, which is about twice as large as that (0.25?C0.5 T) studied in analogous experiments on the NSTX and MAST spherical tokamaks. The results are indicative of a strong dependence of the energy confinement time on toroidal magnetic field: ??_E ?? B _T ^0.75?C0.8 . This scaling is much stronger than that projected for the ITER (??_{E_IPB98} ?? B _T ^0.15 ), while being somewhat weaker than the scalings observed on the NSTX and MAST devices. The stronger (as compared to the ITER scaling) dependence of ??_E on B _T observed in these experiments should be taken into account in designing thermonuclear facilities with small aspect ratios and toroidal magnetic fields??in particular, fusion neutron sources.     

Magnetization of a Current-Carrying Superconducting Disk with B-Dependent Critical Current Density

In the frame work of the critical state model (CSM), the magnetic response of a thin type-II superconducting disk that carries a radial transport current and is subjected to an applied magnetic field have been studied. To this end, we have studied the process of the magnetic flux-penetration. For a disk initially containing no magnetic flux but carrying a radial current, when a perpendicular magnetic field is applied, magnetic flux-penetration occurs in three stages: (1) the magnetic flux gradually penetrates from the edges of the disk until an instability occurs, (2) there is a rapid inflow of magnetic flux into the disk’s central region, which becomes resistive, and (3) magnetic flux continues to enter the disk, while persistent azimuthal currents flow in an outer annular region where the net current density is equal to J _c . Also the behavior of a current-carrying disk subjected to an AC magnetic field is calculated. The magnetic flux, the current profiles and the magnetization hysteresis loops are calculated for several commonly used J _c (B) dependences. Finally, the results of the applications of the local field-dependent of the critical current density J _c (B) are compared with those obtained from the Bean model.     

Magnetic, magnetocapacitance and dielectric properties of Cr doped bismuth ferrite nanoceramics

BiFe_1−xCr_xO₃ (x = 0, 0.04, 0.06 and 0.08) nanoceramics were prepared by sol-gel method. Nanoceramics were calcined at 450 °C. Calcined powders were leached in diluted nitric acid to get single phase. TEM analysis shows the particle size to be ∼80 nm. Thermogravimetric analysis of as prepared powder indicates that the single phase is formed at around 450 °C. Magnetization was found to increase as the concentration of Cr was increased. Dielectric constant and dielectric loss were found to decrease with increase in frequency for all the compositions. Magnetocapacitance was found to increase with magnetic field. For BiFe_1−xCr_xO₃ (x = 0.04, 0.06 and 0.08) nanoceramics, the change of dielectric constant induced by magnetic field may be well approximated by Δ?/? = γM², here, γ (magnetoelectric interaction) is small and positive. A linear fit gave the value of γ of ∼18.4 × 10⁻², 12.3 × 10⁻² and 3.3 × 10⁻² for BiFe_1−xCr_xO₃ (x = 0.04, 0.06 and 0.08) nanoceramics, respectively.     

The Influence of Lithium Halides on the Superconducting Properties of YB2Cu3O7−x

The effects of addition to YBa₂Cu₃O_7?x of lithium halides (YBa₂Cu₃O_7?x (LiF) _y , and YBa₂Cu₃O_7?x (LiCl) _y ) on the structural, electric, magnetic, and transport properties are analyzed. Both structural and superconducting properties depend weakly on the lithium content up to y= 0.10. The critical temperature keeps on a value well above 91 K for a small amount of lithium halide (reaching 93.48K. for y= 0.02 in YBa₂Cu₃O_7?x (LiF) _y and 91.30 K in YBa₂Cu₃O_7?x (LiCl) _y ), but for higher concentration of Li it rapidly decreases (81.68K for y= 0.20). The same behavior is exhibited in the lower intragranular critical field. The intragranular critical current density depends on the magnetic field as a power law:j _c∝B ^?α, with a lithium-concentration-dependent α. The voltage–current characteristics follow a law typical for granular superconductors, V∝(I?I _c(B,T)) ^n(B,T). The dependence of the intergranular critical current, I _c, and of the exponent, n, on temperature, magnetic field, and concentration is analyzed.     

Critical Behavior Near the Paramagnetic to Ferromagnetic Phase Transition Temperature in Sr<Subscript>1.5</Subscript>Nd<Subscript>0.5</Subscript>MnO<Subscript>4</Subscript> Compound

The magnetic properties and the critical behavior in Sr_1.5Nd_0.5MnO₄ have been investigated by magnetization measurements. The magnetic data indicate that the compound exhibits a second-order phase transition. The estimated critical exponents derived from the magnetic data using various techniques such as modified Arrott plot, Kouvel–Fisher method, and critical magnetization isotherms M (T _C, H). The critical exponent values for this compound was found to match well with those predicted for the mean-field model (δ = 2.212 ± 0.124, γ = 0.975 ± 0.018, and β = 0.502 ± 0.012) at T _C = 228.59 ± 0.17. The critical exponent γ is slightly inferior than predicted from the mean-field model. Such a difference may be due, within the context of the quenched disorder and essentially the presence of the Griffiths phase. The temperature variation in the effective exponent (γ _eff) is similar to those for disordered ferromagnets.     

Magnetic dependence of cyclotron transition absorption in piezoelectric materials based on the quantum transport theory

We investigated theoretically the magnetic field dependence of the quantum optical transition of quasi 2-Dimensional Landau splitting system, in CdS and ZnO. Through the analysis of the current work, we found the increasing properties of the optical Quantum Transition Line Shapes (QTLSs) which show the absorption power and the Quantum Transition Line Widths(QTLWs) with the magnetic-field in CdS and ZnO. We also found that QTLW, γ(B)_total of CdS < γ(B)_total of ZnO in the magnetic field region B < 25 Tesla.     

AC Magnetic Technique to Measure Specific Absorption Rate of Magnetic Nanoparticles

The electrodynamic method is applied to determine the specific absorption rate (SAR) of an assembly of superparamagnetic nanoparticles as a function of frequency and magnetic field amplitude. The home made frequency-adjustable electromagnet is used to create a nearly uniform magnetic field in a core gap of a volume 1×3×3 cm³ in the frequency range f=10–150 kHz and for magnetic field amplitudes up to H ₀=250 Oe. Two oppositely connected pick-up coils are used to record the electromotive force signal (EMF) generated by magnetic nanoparticles. By integrating the EMF signal one can determine the low-frequency hysteresis loops of the assembly and the assembly SAR. Using this method the measurement of SAR has been carried out for magnetite nanoparticles with an average diameter D=25 nm. The electrodynamic method is shown to be capable of measuring a small amount of magnetic nanoparticles, up to 5×10^?5 g, dispersed in a solid matrix. The maximal SAR ～?80 W/g has been obtained for the magnetite nanoparticle assembly investigated.     

Gamma-ray detection via D(γ, n)H neutron counting

The photodisintegration of deuterium (Q = −2.22 MeV) in combination with a neutron counter has been used to detect high-energy γ-rays. The γ-ray source is surrounded by a tank filled with heavy water and the emitted neutrons are counted in 4π geometry with neutron detectors embedded in a graphite moderator. The absolute detection efficiency of a small test system (1.8 1 D₂O) has been determined for E_γ = 2.6–8.1 MeV and found to agree well with calculations (e.g., 8.0 × 10⁻⁵ at E_γ = 4.4 MeV). It is proposed that the system should be useful for the study of capture reactions induced on highly radioactive targets. Improvements in detection efficiency as well as limitations in data analyses are discussed.     

Analysis of Current and Magnetic Distributions in REBCO Superconducting Coated Conductors in Self- and External Fields

The influence of a self-field on the critical current density J _c for a REBCO superconducting tape is presented in this paper. The distributions of the current density and magnetic field are analyzed in the tape under three kinds of conditions, i.e., applying an external magnetic field only, applying a transport current only, and applying a transport current together with an external magnetic field. In the analysis, the two-dimensional Poisson equation for the vector potential is employed. For the convenience of calculation, that the dependence of critical current versus the perpendicular and parallel fields tested from experiment is substituted for the traditional Kim-type or Bean model. The results show that the distributions of the current density and magnetic field in the REBCO tape change for the different frequencies and amplitudes of the transport current I _a and applied magnetic field B _a.     

Magnetic Transport Properties in GdBa2Cu3?x Ru x O7?δ Superconducting Phase

Bulk superconducting samples of type GdBa₂Cu_3?x Ru _x O_7?δ phase, Gd-123, with?x ranging from 0.0 to 0.15 were prepared by the conventional solid-state reaction technique. X-ray powder diffraction (XRD) and the electrical resistivity measurements were performed in order to investigate the effect of Ru⁴⁺ ions substitution on Gd-123 phase. Enhancement of the phase formation and the superconducting transition temperature T _c for GdBa₂Cu_3?x Ru _x O_7?δ phase up to x=0.05 was observed. The effect of magnetic field up to 4.4?kG on the electrical resistivity behavior of the prepared samples was studied to investigate the flux motion of this phase. The derived flux pinning energy?U, based on the thermally activated flux creep TAFC model, decreased with increasing the magnetic field?B. The flux pinning energy followed the exponent behavior as U(B)～B ^?β . The superconducting transition width ΔT increased as the magnetic field increased, showing the scaling relation as ΔT～B ⁿ . Using Ambegaokar and Halperin AH theory, the magnetic field and temperature dependence of U was found to be U(B,T)～ΔTB ^?η , η=β+n. The critical current density J _c (0) enhanced up to x=0.05, beyond which it decreased with further increase in Ru-content.     

Magnetoresistance Study of Y3Ba5Cu8O18 Superconducting Phase Substituted by Nd3+ and Ca2+ Ions

Superconducting samples of type Y_3?x Nd _x Ba_5?x Ca _x Cu₈O₁₈ with 0.0 ≤ x ≤ 0.4 have been prepared via the solid-state reaction technique. The prepared samples were characterized using X-ray powder diffraction (XRD) technique for phase analysis. The elemental content of the prepared samples was determined using particle-induced X-ray emission (PIXE). In addition, the oxygen content of these samples was obtained using non-Rutherford backscattering spectroscopy (RBS) at 3 MeV proton beam. The results indicate that these substitutions do not affect the orthorhombic structure, while they decrease the oxygen content of Y-358 phase. The electrical resistivity of the prepared samples was measured by the conventional four-probe technique from room temperature down to the zero superconducting transition temperature (T ₀). A slight change in the superconducting transition temperature (T _c) is observed for 0 ≤ x ≤ 0.1, and then it decreases linearly with further increase in x. The linear decrease in T _c is attributed mainly to the partial substitution of Ba²⁺ ions by Ca²⁺ ions rather than the partial substitution of Y³⁺ ions by Nd³⁺ ions. The effect of magnetic fields up to 4.44 kG on the electrical resistivity has been studied to investigate the vortex dynamics for the prepared samples. The experimental data, in the second stage of superconducting transition, fit well with the thermally activated flux creep (TAFC) model, and the activation energy U(B) shows a power law dependence on magnetic field as B ^?β . Also, the transition width is related to the magnetic field according to the relation ΔT α B ⁿ . The values of β and n are strongly dependent on the Nd³⁺ and Ca²⁺ ion substitution. The magnetic field and temperature dependence of the activation energy U(B, T) is found to be U(B, T)? ΔT B ^?η , where η = β + n. Furthermore, the critical current density at zero temperature, J _c(0), as a function of the applied field was calculated for all the prepared samples. The result shows an enhancement in J _c(0) of Y-358 phase at x equals 0.4 at different applied fields.     

Magnetic Properties of Mg x Cu1−x Cr2O4 Spinels are Studied by Different Theoretical Methods

The external magnetic field induced the reorientation of magnetization of a ferromagnetic (or antiferromagnetic) treated within the framework of many-body Green’s function theory by considering all components of the magnetization. We present a new method for the calculation of expectation values in terms of the eigenvalues and eigenvectors of the equations of motion matrix for the set of Green’s functions. Magnetization and magnetic susceptibility are investigated when an external magnetic field is applied in (x-z)-plane. The mean field theory is applied to calculate the nearest neighbour and the next-neighbour super-exchange J ₁(Cr?Cr) and J ₂(Cr?(Mg(Cu)?O)?Cr), respectively, for the Mg _x Cu_1?x Cr₂O₄ in the range of 0 ≤ x ≤ 1. The intra-planar and the inter-planar interactions are deduced. The high-temperature series expansions (HTSEs) are derived for the magnetic susceptibility and two-spin correlation functions for a Heisenberg ferromagnetic model on the B-spinel lattice. The calculations are developed in the framework of the random-phase approximation (RPA). The magnetic-phase diagram is deduced. A spin-glass phase is predicted for intermediate range of concentration. The spin glass is obtained. The obtained results are comparable with those obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility (γ) and the correlation lengths (ν) have been deduced. The obtained values are comparable to those of 3D Heisenberg model.     

Switching of the vacuum-arc discharge direct current

Interruption of the direct current of a vacuum-arc discharge (VAD) is analyzed with three methods: the connection of a shunt circuit with a capacitor to the discharge chamber, the overlaying of a pulse nonhomogeneous axisymmetric magnetic field upon the discharge, and the combined action of the magnetic field and the shunt circuit. The probability of interruption of the current I = 500 A as a function of the magnetic field induction is obtained for various shunt capacitances within the range 25 ≤ C ≤ 2500 microfarad (μF). It is shown that the probability of interruption for a given magnetic field induction increases as the capacitance is raised and for the given capacitance it increases as the induction is increased. The magnitude of the magnetic system current I _m to ensure the current interruption with the probability P = 1 is obtained as a function of the shunt capacitance. The impact of the shunt circuit parameters upon the arc current is analyzed. The time-domain plot of the arc current for the shunt capacitor C = 2500 μF was estimated. Its results agree with the experiment. It is shown that the combined use of the magnetic field and the capacitive shunt circuit is an effective method of VAD direct current interruption.