M.J. Qin

HgBa2CaCu2Oy superconducting thin films prepared by laser ablation

Abstract HgBa 2 CaCu 2 O y superconducting thin films were prepared on SrTiO 3 (100) and LaAlO 3 (100) substrates using laser ablation and post-Hg-vapor annealing. X-ray diffraction and electromagnetic measurements show that these films are dominated by c -axis-oriented Hg-1212 phase. For the Hg-1212/SrTiO 3 sample, a zero-resitance superconducting transition temperature ( T c0 ) of 110 K and critical current density ( J c ) of up to 10 6 A/cm 2 at 77 K and zero field have been obtained. The Hg-1212/LaAlO 3 sample exhibits slightly lower T c0 ) ( = 100 K) and J c (= 2 × 10 5 A/cm 2 ). Our investigations suggest that by using superconducting HgBaCaCuO target mercury can be incorporated into the films easily during post annealing.

Characterization of Hg-Ba-Ca-Cu-O 1223 superconductor

Abstract The HgBaCaCuO 1223 superconductors have been characterized in this work. XRD and SEM analyses indicate that Hg-Ba2Ca2Cu3O8+δ is the major superconducting phase in the shape of a bar or plate in the samples. The transition temperature Tc of one sample is 129 K by resitivity, AC and DC susceptibility measurements. Magnetic measurements including the magnetic critical current density and magnetization relaxations indicate that a strong thermally activated flux motion also exists in the Hg-based system of high-temperature superconductors.

AC response of high temperature superconductors in the flux creep region

Abstract By numerically solving the flux creep equation, we have investigated the ac response of a high temperature superconducting slab in the flux creep region. We have shown that in the vortex glass state, where the U ( j ) relationship is a highly nolinear one, U ( j ) = ( U 00 U ( H , T )/ μ )[( j c / j ) μ − 1], the straight-line-approximation provides a good description of the flux density profile inside the slab except when both μ and U 00 / k B T c are small enough. We have also shown that in the vortex liquid state where the activation energy U depends linearly on the current density j , the straight-line-approximation breaks down when U 00 / k B T c U 00 / k B T c > 500.

Logarithmic U(j) relationship in melt-textured growth YBa2Cu3O6+x by AC susceptibility measurements

Complex AC susceptibility measurements have been performed systematically on a melt-textured growth YBa2Cu2O6+x sample. The activation energy has been determined to be U(T, H, j) = U0[1 − (T/Tx)2]32(H0/H)n ln(j0/j), with n=0.46 and n=0.59 for H∥ab and H∥c, respectively. The expressions U(H)∼H−0.5 and U(j)∼ln(j0/j) obtained by the AC technique in this work agreee basically with those observed in YBa2Cu3O7 samples by transport or magnetic measurements, which may result from vortex pinning by the planar defects in the sample. We have also shown that the limited time window in conventional magnetic-relaxation measurements can be extended to much smaller values by the AC technique, which is in the range of 4 × 10−5 s−3 × 10−3 s in this work.

Dependence of flux-creep activation energy upon current density in HgBa2Ca2Cu3Oy

Abstract We have investigated the magnetic relaxation on a HgBa 2 Ca 2 Cu 3 O y superconductor over the range 4.2–60 K. Using a linear relation between U ( J ) and kT ln(1 + t / t 0 ), we extracted the effective activation energy and the time-window constant t 0 . A nonlinear dependence of U eff on J , U eff ∝ J − μ , was found. μ is about 0.98 in the temperature range 4.2–60 K, close to the value of 7 9 in the regime of flux creep in large vortex bundles predicted by the three-dimensional collective-creep theory. The time scale t 0 depends exponentially on the initial conditions. The relationship between U ( J ) and the magnetic relaxation curve is discussed. With logarithmic accuracy one can construct the U ( J ) curve simply using several long-time relaxation curves at different temperatures.

Frequency dependence of ac susceptibility in high temperature superconductors

Abstract By numerically solving the flux creep equation, we have investigated the ac susceptibilities of a superconducting slab, for the situation where the activation energy U depends explicitly on current density j as U ( j ) = ( U 0/μ )(( j c / j ) μ − 1). The calculated fundamental response (χ″ 1 ( T ) versus T curves) and third harmonic response (|χ 3 (T)| versus T curves) exhibit a strong dependence of the peak height and the peak position on frequency which compare well with the experimental data reported. The extensively used critical state model breaks down in explaining these results and therefore magnetic relaxation effects must be taken into account.

Low field irreversibility line in (Hg0.66Pb0.34)Ba2Ca2Cu3O8+δ by third harmonic a.c. susceptibility measurements

Abstract First-harmonic χ1 and third-harmonic χ3 a.c. susceptibility measurements have been carried out on a Hg0.66Pb0.34Ba2Ca2Cu2O8+δ sample. The frequency dependence of the onset temperature of χ3 can be described well by T on (f) = C(2πf) 1 ((z − 1)V) + T g . Extrapolating to f = 0, we derived the vortex glass transition temperature Tg, and established the irreversibility line (IL) for the Pb-doped Hg-1223 sample. In the low-field region (H ⩽ 0.1 T), the power law H irr = 9.0(1 − T irr T c ) 1.95 fits the data rather well; however, deviation from the power law is observed in the higher-field region, indicating the transition in dimensionality from 3D to 2D vortices.

Resistance relaxation resulting from current diffusion in superconducting thin films

The current diffusion process in superconducting thin films was investigated numerically. The attention was concentrated on the influence of current diffusion on experimental results of transport measurements. It was found that the voltage and the resistance of the film sample in transport measurements were time dependent. The calculated relaxation curves of resistance differed obviously from those observed in magnetic relaxation measurements. Transport measurements on a GdBawCu3O7 thin film were carried out to examine our numerical investigation. Good agreement between the experimental results and our calculation was obtained

Current and magnetic field distribution of disk-shaped superconducting film

Abstract When the applied field B → a =μ 0 H → a parallel to the rotation axis of a disk-shaped superconducting thin film with radius R and thickness d in a cylindrical coordinate system, and the self-field effects of the screening current B → ′=B z ′ z +B ρ ′ ρ ( z ‖ B → a ⊥ ρ ) are considered, an effective method using finite-element analysis and matrix operation, to obtain the field dependent critical current function J c ( B ) from magnetic moment hysteresis loop m ( B a ) in a self-consistent way, is presented. Then also in a self-consistent way, the current density J ( ρ ) and field B z ( ρ )= B a + B z ′( ρ ) distributions can be obtained, regardless whether the film being fully penetrated by the applied field or not. The effects of the radial stray field B ρ ( ρ , z ) on the current cannot be neglected when B a ≈0. Due to the weak self-field effects, or intense demagnetic effects of the film compared with the long cylinder-shaped superconductor, between the average values of field 〈 B z 〉 and magnetization 〈 M 〉 there is a quantitative relation 〈 B z 〉= μ 0 [ H a +(0.8 d / R )〈 M 〉] for a disk-shaped film and a qualitative relation 〈B z 〉=μ 0 [H a +〈M〉/ 1+(2R/d) 2 ] for an arbitrary cylinder-shaped superconductor under the Bean model approximation.

Paramagnetism and scaling behavior of volume flux pinning force density in a GdBa2Cu3O6+x thin film

Magnetic measurements have been performed on a GdBa2Cu3O6+x superconducting thin film. The paramagnetism carried by rare earth Gd3+ ions in the film tilts the magnetic hysteresis loop and broadens the width of the magnetic hysteresis Delta M, then the magnetization critical current density and the volume flux pinning force density based on the Bean critical state model deviate from intrinsic values. Therefore, in order to get useful information on the pinning mechanism, correction for the paramagnetism is essential. And after correction for the paramagnetism, a scaling law of the volume flux pinning force density is obtained as f(b)=(b/4)(0.5)(1-b/4)(1.5), based on which the possible pinning mechanism in the film is discussed. In the end, deviation from the scaling law at high fields is interpreted by the collective pinning theory