L. Qiu

Evidence of spin-density-wave transition and enhanced thermoelectric properties in Ca3−xCexCo4O9+δ

We have investigated the effect of Ce doping on the magnetic and low-temperature thermoelectric properties of Ca3Co4O9+δ. A small shoulder occurring at about 23 K in the dχ−1/dT curve indicates an occurrence of spin-density-wave transition, which is confirmed by resistance characteristics. With partial Ce substitution, the thermopower increases while the thermal conductivity decreases. The magnetic measurements give a support to the concentration reduction in Co4+ ions, which results in the enhancement of thermopower. The dimensionless figure of merit ZT=0.016 is achieved at 335 K for Ca2.9Ce0.1Co4O9+δ, demonstrating that heavy atoms Ce doping may promise an effective way for improving thermoelectric properties of Ca3Co4O9+δ system.

The collective flux pinning force in (Sm0.5Eu0.5)Ba2Cu3O7−δ superconductors

We have measured the isothermal magnetization loops of top-seeded melt processed (Sm0.5Eu0.5)Ba2Cu3O7??+10?mol%?(SmEu)BaCuO5+10?wt% Ag crystals in a magnetic field up to 8?T parallel to the c-axis from 50 to 87?K using a vibrating sample magnetometer. All of the isothermal Jc(H) curves show a second peak. The scaling of the Fp?H curves indicates that the flux pinning force Fp comes from the collective pinning of two kinds of pinning centres, Fp = FpH+FpL. One (FpH) is the ?Tc-type pinning centre originating from the RE?Ba and RE?RE substitution responsible for the second peak; the other (FpL) is the normal pinning contributed by the surface and point pins. The Hc2?T diagram implies two critical temperatures. The crossover occurs at about 88.2?K.

Harmonic ac susceptibilities for textured YBa2Cu3O7

Abstract Harmonic ac susceptibilities were measured as a function of temperature ( T ), ac field frequency ( f ) and amplitude ( B ac ), and dc field ( B dc ) for bulk textured YBa 2 Cu 3 O 7 . The higher harmonics consist mainly of odd ones in strong dc fields ( B dc >50 mT) and depend on B dc , B ac and f . It is confirmed that the origin of harmonic generation for the textured sample is thermally activated flux diffusion over barriers U which depend nonlinearly on current density ( j ). The high nonlinear flux diffusion induces a self-organized Bean critical state in which the straight line approximation is a good description of the field profile except that the current density is j ( f ) instead of the critical value j c . Thus the frequency dependent Bean critical state accounts for the origin of harmonic generation and explains the present and other experiments well.

Improved Jc properties and microstructure in Na-doped MTG-YBCO crystals

The effect of Na2C2O4 addition on the microstructure, magnetization and dc transport properties of melted-textured-growth (MTG) YBa2?xNaxCu3Oy + 40 mol% Y2BaCuO5 crystals with x = 0, 0.1 and 0.2 has been investigated. X-ray diffraction patterns reveal that the length of the c-axis increases with increasing the Na-doped concentration and that these crystals are in single phase except for very weak peak of Y211. Composition analyses indicate the evidence of Na+ ion existing in the Na-doped samples and the part Y3+ or Ba2+ ion being replaced by Na+ ion. Magnetization curves at 77 K show an improved field dependence of the critical current density and a second peak around 0.8?1 T for Na-doped YBCO crystals. DC transport measurement gives a shift of the irreversibility line to lower temperatures and a larger anisotropy for the Na-doped YBCO crystal. The possible source of these behaviours were supposed to be connected with the certain inhomogeneous distribution of Na (different x) in MTG YB(Na)CO crystal, which leads to a distribution of different Tc clusters.

Effective activation energy and phase diagram in the Er-doping MTG-YBa2Cu3O7−δ crystal

Abstract The resistivity of melted textured growth (MTG) Y 0.8 Er 0.2 Ba 2 Cu 3 O 7− δ (YErBCO) crystal has been measured as a function of temperature T , applied magnetic field H and the angle θ between the applied magnetic field and ab -plane. Based on the Yeshurun–Malozemoff model [Phys. Rev. Lett. 60 (1988) 2202], we investigated the vortex dynamic behavior from the Arrhenius form ρ ( T , H )= ρ 0 exp(− U 0 / T ) and found that the model can explain the dissipation in the vortex liquid state for the Er-doped YBa 2 Cu 3 O 7− δ (YBCO) crystal. The effective activation energies for H ∥ c in the Er-doped MTG-YBCO are about 2.7×10 3 K (7.5 T) and 5.6×10 4 K (0.5 T) less than that for H ∥ c in pure YBCO. The anisotropy factor γ =10 obtained from the angular dependencies of effective activation energy U ( θ ) and the zero resistance temperature T c0 ( θ ) for Er-doped YBCO crystal are larger those that for YBCO crystal. The field dependence of the effective activation energy U e follows a power law, U e ∝ H − α , with α =1.0 for H ∥ c and for various angles θ at a fixed magnetic field of 4 T. The phase diagram of the Er-doped YBCO crystal is presented and discussed.

The effect of Er-substitution on the superconducting properties of MTG-YBaCuO crystals

The high quality Er-doping Y0.8Er0.2Ba2Cu3O7−δ (YErBCO) crystal is grown by the melted-textured-growth (MTG) method. The microstructure photograph shows that the MTG-YErBCO crystal has an obvious layered structure. Its magnetization loops show that the irreversibility field is decreased by Er-doping for H//c and that there is an anomalous behaviour, a second peak, in the magnetization curve for H//ab. The critical current density along the ab-plane (Jcab(77 K, 0 T) = 4.75 × 104 A cm−2) is higher than that along the c-axis (Jcc(77 K, 0 T) = 470 A cm−2). The field dependence of the irreversibility temperature Tirr follows a power law, H = H0(1 − Tirr(H)/Tirr(0))1.3, with H0 = 550 T for H//ab and 51 T for H//c, respectively. The field dependence of the activation energy Ue shows a different behaviour for H//ab and for H//c, namely a linear relation, U = 49131–3006H, for H//ab and a power law, U ∝ H−0.68, for H//c, respectively. The anisotropy factor γ = 15 was obtained from the angular dependences of the activation energy and the irreversibility temperature. A possible reason for the effect of Er addition on the flux-pinning behaviour has been discussed.

Transport properties in melt-textured-growth-YBa1.9Na0.1Cu3O7−δ crystals

Abstract The temperature dependence of resistivity for a Na-doped melted-textured-growth (MTG)-YBCO (YBa 1.9 Na 0.1 Cu 3 O y +40 mol% Y211) crystal with a second peak in the magnetization curves and a Na-free MTG-YBCO crystal has been measured in the applied magnetic field H up to 7.5 T for H ∥ ab and H ∥ c , respectively. The results show that the field dependence of zero resistance temperature T c0 follows the same power law behavior, namely T c0 (0)− T c0 ( H )∝ H 1/ n with n =1.56 for H ∥ ab and 1.36 for H ∥ c , but the line of H – T for the Na-doped MTG-YBCO crystal shifts to lower temperature. The field dependence of average activation energy shows a power law U ∝ H − m with m =0.55 for H ∥ ab and 0.69 for H ∥ c for the Na-doped MTG-YBCO crystal and with m =0.45 for H ∥ ab and 0.94 for H ∥ c for the Na-free MTG-YBCO crystal, respectively. The temperature dependence of resistivity for a Na-doped MTG-YBCO crystal at constant magnetic fields of 1 and 6 T for various angles θ between the magnetic field H and the CuO 2 plane with H ⊥ I has revealed that the field dependence of zero resistance temperature T c0 can be scaled by T c0 (0)− T c0 ( H , θ )∝(sin 2 θ +cos 2 θ / γ 2 ) 1/2 n with n =1.36 and γ =11 at 1 T and γ =9 at 6 T. However, the angular dependence of average activation energy does not fit by both the effective mass and two-dimensional models. The field dependencies of critical current density in the Na-doped MTG-YBCO crystal have been compared with that in the MTG-YBCO crystal. The effect of Na addition on the flux pinning behavior has been discussed.

Microstructure, magnetization and dc transport properties of MTG-YBa1.8 Na0.2Cu3Oy crystal

The Na-doping MTG-YBCO crystal with the nominal composition YBa1.8Na0.2Cu3Oy + 40?mol% Y211 has been successfully synthesized by the melted-textured-growth (MTG) method with top seed. The microstructure photographs show that the crystal has a good layered structure with a number of twins. There is a second peak in the magnetization curve for the magnetic field parallel to the c-axis and at 77?K. The field of zero resistance temperature Tc0 shows a power law, H = H0 (Tc0(0)-Tc0(H))n with n = 1.52 for H?ab and n = 1.41 for H?c, respectively. The thermally activated flux creep model can scale the dissipative resistivity below the peak temperature Tp. The temperature and field dependences of the effective activation energy can be described by the formula U = U0(1-T/Tp)mH-?, where m = 1.75, ? = 0.75 for H?ab and m = 2.5, ? = 1.52 for H?c, respectively. The effect of Na-doping induced disorder on flux pinning has been discussed.

AC losses of superconductor MgB2

MgB2 bulk samples were synthesized by the usual solid-state reaction method. AC losses of the MgB2 bulk sample were measured using the ac susceptibility system (ACS) in external dc magnetic fields up to 7 T. The dependence of losses of the sample on temperature (T), frequency (f) and magnitude of ac field (Bac) was experimentally obtained. A comparison is made between the properties of losses of MgB2 and that of high-Tc superconductor Ag-Bi2223 and low-Tc superconductor NbTi. It is found that the mechanism of losses in the MgB2 sample may still be the same as that of high-Tc superconductors and can be explained by the flux creep models.

History effect in inhomogeneous superconductors

A model was proposed to account for a different kind of history effect in the transport measurement of a sample with inhomogeneous flux pinning coupled with flux creep. The inhomogeneity of flux pinning was described in terms of alternating weak pinning (lower j c ) and strong pinning regions (higher j c ). The flux creep was characterized by logarithmic barrier. Based on this model, we numerically observed the same clockwise V-I loops as reported in references. Moreover, we predicted behaviors of the V-I loop at different sweeping rates of applied current dI/dt and magnetic fields B a , etc. Electric transport measurement was performed in Ag-sheathed Bi 2 - x Pb x Sr 2 Ca 2 Cu 3 O y tapes immersed in liquid nitrogen with and without magnetic fields. A V-I loop at certain dI/dt and B a was observed. It is found that the area of the loop is more sensitive to dI/dt than to B a , which agrees well with our numerical results.