A. Gencer

On the Ginzburg-Landau analysis of the upper critical field Hc2 in MgB2

We study the temperature dependence of the upper critical field Hc2(T) for superconducting magnesium diboride, MgB2 in the vicinity of Tc using a two-band Ginzburg-Landau (G-L) theory. The temperature dependence of Hc2(T) near Tc exhibits a positive curvature. In addition, the calculated temperature dependence and its higher-order derivatives are also shown to be in good agreement with the experimental data. In analogy with the multi-band character of the Eliashberg microscopic theory, the positive curvature of Hc2(T) is described reasonably by solving the two-band G-L theory.

Two-band Ginzburg-Landau theory for the lower critical field Hc1 in MgB2

The temperature dependence of the lower critical field Hc1(T) for the superconducting magnesium diboride, MgB2, is studied in the vicinity of Tc using a two-band Ginzburg–Landau (G–L) theory. The theoretically calculated Hc1(T) near Tc exhibits a negative curvature. The results are shown to be in good agreement with the experimental data. In addition, two-band G–L theory calculations give a temperature dependence of the Ginzburg–Landau parameter κ (T) = λ (T)/ξ (T), which varies little with the temperature in a manner similar to the microscopic single-band BSC theory.

AC susceptibility study of BiPbSrCaCuO(2223) superconductors

The magnetic response of Bi-based superconductors of nominal composition against low AC field amplitudes is studied in terms of fundamental and third harmonic AC susceptibilities to investigate various physical parameters. Optimum conditions of a wet preparation technique have been determined. Both transport and inductive critical current densities have increased considerably on samples subjected to a single intermediate grinding. The magnetic data accompanied by XRD measurements support the theory that the minor superconducting phase 2212 is mostly screened by a dominating matrix of 2223 high- phase.

Degradation of Superconducting Properties in MgB2 by Formation of the MgB4 Phase

Superconducting MgB2 polycrystalline samples have been fabricated under two different conditions in order to determine the effect of MgB4 phase. A series of samples was placed in an α-alumina container closed with a cup and fired under high purity argon gas. The other series of samples was placed in an α-alumina boot without any lid and fired under similar conditions. For the first series of samples, we have found pure MgB2 phase formation and a narrow transition width at 0.4 K. For the second series of samples, significant amount of MgB4 phase were formed and the Tzero was decreased to 27 K. For both the group of samples magnetization hysteresis loops obtained at various temperature range and applied field up to 2 T. The best Jcmag for the first series of samples was 1.9 × 105 A/cm2 at 10 K and 0 T, and for the second series of samples was 0.7 × 104 A/cm2 at 10 K and 0 T.

Thermodynamic Magnetic Field and Specific Heat Jump of a Bulk Superconductor MgB2 Using Two-Band Ginzburg–Landau Theory

Temperature dependence of the thermodynamic magnetic field for the superconducting magnesium diboride, MgB 2 , is studied in the vicinity of T c using a two-band Ginzburg–Landau (G–L) theory. The results are shown to be in good agreement with calculations from experimental data. In addition, the two-band G–L theory gives a smaller specific heat jump, than a single-band G–L theory.

London penetration depth λ(T) in two-band Ginzburg–Landau theory: application to MgB2

Abstract Two-band Ginzburg–Landau (G–L) theory equations were solved analytically to determine the temperature dependence of the London penetration depth λ(T). Its temperature dependence is non-linear near Tc. The strength of the non-linearity is mainly dependent on the interaction coupling between the order parameters of two separate bands. In addition, the inter-gradient interaction as well as the ratio of effective masses of two separate bands was also found to be important in determining its temperature dependence. The solutions were applied to MgB2, which was characterized as a two-band superconductor. The results of the calculations are in good agreement with the experimental data for bulk MgB2.

Harmonic susceptibilities of an alloy of

Low-field harmonic susceptibilities of a alloy have been measured in the temperature range 15-325 K at three different frequencies. The measurements exhibit both frequency and amplitude dependencies. Observed dependencies are compared with the existing theories of linear and nonlinear susceptibilities with reference to short- and long-range interactions. Although the sample exhibits some features similar to the ordinary spin-glass systems at low temperatures, the Curie temperature was found to be larger than expected. However, the sample exhibits some interesting properties in terms of spin frustration below at some frequency and amplitude. The coexistence of spin-glass with ferromagnetic states at low temperatures signifies the nondivergent behaviour of the high-order harmonic susceptibilities in the vicinity of . The critical exponents and suggests that the third harmonic susceptibility is essentially different from the observed divergent behaviour in spin glasses.

Studies on the magnetic properties of martensitic phase in an Fe-Cr-C alloy

Abstracts are not published in this journal

Influence of low magnetic fields on ac losses in bulk superconductor Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10

The fundamental and third harmonic ac susceptibility measurements as a function of temperature (20–120 K), frequency (5 Hz–1 kHz) and ac magnetic field amplitude (1–1600 A m−1) on a bulk Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10 high-Tc superconductor prepared by the ammonium nitrate fusion technique are reported. The height and temperature of the peak in the out-of-phase component of the fundamental susceptibility increase with increasing field amplitude and the measuring frequency employed. In the presence of a superimposed Hdc = 1520 A m−1, an additional small peak appears and the larger peak broadens even at lower ac fields of 20 A m−1. The third harmonic susceptibility exhibits an interesting temperature dependence with increasing ac field amplitude. Experimental results are qualitatively discussed in the framework of the critical state model. The results are in good agreement with the theory at fields lower than the least field needed for full flux penetration, Hp, below which ac losses are essentially due to the hysteretic intergranular coupling. The critical state model fails to explain the data at high temperatures with ac fields larger than Hp, where the ac losses are interfered by the irreversible flux penetration into the grains. This is presumably due to the weakening of the intergrain coupling with magnetic field penetration.

Critical Current and AC Loss of DI-BSCCO Tape Modified by the Deposition of Ferromagnetic Layer on Edges

Modification of local magnetic field with the help of a ferromagnetic layer could be utilized to improve the performance of multifilamentary Bi-2223/Ag tapes. We investigated the case of the horseshoe-shaped cover on the tape edges, characterized by layer thickness and total length. Numerical simulations have been carried out to find the critical currents of tapes with different covers. According to the indications obtained from these calculations, samples with covered edges have been prepared with the primary aim of increasing the self-field critical current. We observed up to 7% enhancement of this quantity. AC loss was also investigated theoretically and experimentally in a case of applied ac transport current or ac magnetic field applied perpendicular to the tape wide surface. Reduction of both the transport loss and the magnetization loss has been observed. Good agreement is obtained between the experimental data and the calculations.