S.A. Aruna

Field-induced thermally-activated dissipation and lock-in transition in a (YBCO)24/(PrBCO)2 multilayer thin film

We have measured the temperature dependence of the resistivity ρ of an epitaxial (YBa2Cu3O7-δ)24/(PrBa2Cu3O7-δ)2 multilayer nanometre film at several magnetic fields parallel to the c-axis and at a constant magnetic field H = 4 T. The measurements were taken for various angles α between the magnetic field and the surface of the film with H in the plane between the measuring current I and the c-axis, and for various angles θ between the magnetic field H and the surface of the film (ab-plane) with H ⊥ I. The resistive broadening depends mainly on the effective applied magnetic field He = H×(sin2θ + cos2θ/γ2)1/2 with respect to the c-axis, and shows a three-dimensional anisotropic behaviour, where γ is the anisotropy parameter. The angular dependences of the peak temperature Tp and the activation energy Ue for angle θ and α>5° are in agreement with the effective mass model, namely Tp(0)-Tp(H,θ) ∝(sin2θ + cos2θ/γ2)1/2n and Ue∝He-m with γ = 10, n = 1.31 and m = 0.87. The dissipative resistivity below Tp for various magnetic fields can be scaled onto a single curve by thermally activated flux motion. When the applied field tilts the CuO2 plane, the angular dependences of Tp and Ue reveal a lock-in transition, showing that the flux lines were trapped in the non-superconducting PrBCO layers.

Temperature dependence of the distribution of the thermally activated energy barriers in Tl2Ba2CaCu2O8 film

The effects of frequency and ac amplitude on ac susceptibility have been measured for a thin Tl2Ba2CaCu2O8 film in the range 100 Hz-100 kHz in magnetic field 0.52 T. A phenomenological equation with an asymmetrical distribution of thermally activated energy barriers has been used to analyse these frequency and amplitude dependences of the ac susceptibility (,hac) in the vicinity of the peak temperature of ´´. We obtain the effective energy barrier U against amplitude hac (current density j): U hac-0.38. This U(j) relationship shows that the flux lines are in the 3D collective creep regime. Therefore, we conclude that the effective energy barrier is in fact an average of the barriers distribution, and the distribution function is a distinguished asymmetrical one in this 3D collective creep regime.

Dip effect in χ′ of ac susceptibility in Bi2Sr2CaCu2O8 single crystal

Abstract We present a direct evidence of the softening of the shear modulus constant c 66 by a measurement of the real part χ ′( T ) of ac susceptibility. A dip in χ ′( T ) for a single crystal of Bi 2 Sr 2 CaCu 2 O 8 has been found at an intermediate field region between 50 and 3000 G. We showed that the dip effect was due to the softening of the shear modulus constant c 66 , equivalent to the peak effect in resistivity measurement. The dependence of the peak temperature T p in χ ″ on magnetic field and current density shows the transition between vortex solid and vortex liquid. Thus, the dip in χ ′( T ) is illustrated as a precursor to vortex lattice melting in this narrow field region, indicative of lattice softening prior to melting. In the region between T d and the melting temperature T m , the flux lines move by a type of plastic deformation, and the pinning energy barrier U has been obtained as: U ∝[1−( J / J c ) ν ] H μ with ν , μ >0.

Current distribution and ac susceptibility response of a thin superconducting disc in an axial field: a theoretical approach

Within the framework of the thermally activated process of the flux line or flux line bundles, and by time integration of the 1D equation of motion of the circulating current density ( ,t ), which is suitable for thin superconducting films (R >>d , ), we present numerical calculations of the current profiles, magnetization hysteresis loops and ac susceptibility n = ´n +i ´´n for n = 1, 3 and 5 of a thin disc immersed in an axial time-dependent external magnetic field Ba (t ) = Bdc +Bac cos(2 t ). Our calculated results are compared with those of the critical state model (CSM) and found to prove the approximate validity of the CSM below the irreversibility field. The differences between our computed results and those of the CSM are also discussed.

Pressure-induced charge distribution effect on superconducting critical temperature of HgBa2Ca2Cu3O8+δ☆

Abstract According to the local density approximation for electronic structure calculations, the doping level of CuO 2 layer does increase under pressure by a relatively small amount, less than 0.02 additional holes per Cu atom at 9.2GPa. Thus, it seems necessary to consider mechanisms other than changes in doping level for the increase of T c under pressure. Based on the data obtained from the powder neutron diffraction experiments up to a pressure of 8.5GPa, we have calculated the valences of Cu and the pressure-induced change of the hole concentration in the inner and outer CuO 2 layers in Hg-1223 samples by using the method of Bond Valence Sums (BVS). The results are rather good and could provide a possible explanation for the large increase of superconducting critical temperature in this material under pressure.

Correspondence between magnetic time relaxation and field sweeping rate dependence of magnetic hysteresis

Abstract The dependence of magnetization in flux creep experiments M(t) and field sweep rate dependence of magnetic hysteresis M(dH/dt) are derived and found to correspond with each other in Taylor expansions of the effective activation energy U eff (J). The agreement of U eff (J) extracted from the magnetic relaxation and magnetic hysteresis loops of a polycrystalline HgBa 2 Ca 2 Cu 3 O 8+δ sample is a strong support of this correspondence.

Dissipation Power of a Disk-Shaped Superconducting Thin Film Along Its Virgin Magnetization Curves

Based on the Bean critical state model j???jc for both longitudinal and transverse geometry, the total dissipation power P along the virgin magnetization curves is calculated. The dissipation power obtained from the corresponding magnetic measurements for a disk-shaped YBa2Cu3O7-? thin film in a perpendicular magnetic field is verified. The virgin magnetization curves for the sample at different temperatures are calculated.

Role of nanometer PrBCO layers in (YBa2Cu3O7-δ)24/(PrBa2Cu3O7-δ)2 multilayer film

The field and angular dependencies of the resistive transition for an epitaxial (YBa2Cu3O7-?)24/(PrBa2Cu3O7-?)2 nanometer multilayer film have been measured at magnetic fields up to 7.5?T parallel and perpendicular to the c-axis and at constant fields of 0.12?T and 4?T for various angles, respectively. The coupling and decoupling between the YBCO layers have been investigated. A special field dependence of activation energy for H?ab, namely, a field independent behaviour in fields smaller than 0.12?T, a logarithmic dependence in medium fields and an exponential behaviour in fields higher than 1?T is observed. The angular dependence of the activation energy reveals that when the parallel field component is less than 0.12?T, the dissipation is only related to the perpendicular field component, which is consistent with Kess two-dimensional (2D) pancake model. Above this field the activation energy depends on the decoupling and coupling between the superconducting YBCO layers.

Critical currents and pinning mechanism of superconducting Nb-50% wt Ti alloys

Abstract The results of transmission electron microscopy examination as well as the critical current measurements at 4.2K under fields of up to Bc 2 for superconducting Nb-50% wt Ti multifilamentary wire optimized with a new method of multiple heat treatment and processing cycles are presented. The pinning mechanism of the optimized samples is discussed, demonstrating the theoretical basis for the optimization process. In order to improve the T c of the Nb-50% wt Ti alloys under high fields, we should control the flux shear motion at high fields besides those of both the fine-scale sub-band and α-Ti precipitates.

Antiferromagnetism and superconductivity in Hg-based cuprate compounds of (Hg1-xPbx)Ba2Ca2Cu3O8+δ

Abstract The phase diagram of the Hg-based compound of (Hg 1−x Pb x )Ba 2 Ca 2 Cu 3 O 8+δ is constructed based on data obtained from experimental measurements of its magnetic susceptibility, neutron scattering results, X-ray photo-electron spectroscopy, and the electron probe microanalyzer (EPMA). The relationship between the transition temperature for the antiferromagnetic phase and the superconducting phase with the doping concentration were traced out in the phase diagram. Due to difficulty in measuring the transition from the insulating to the metallic state in the phase diagram, the phase boundary for these two phases is yet to be determined. From our results, so for this boundary is only a qualitative one.