H. Yasuoka

Material parameters of YBa2Cu3Oy in the Kim–Anderson critical-state model

Abstract The magnetic response of a melt-processed YBa 2 Cu 3 O y under an external ac magnetic field has been investigated in the framework of the Kim–Anderson critical-state model, J c ( B i )= k /( B 0 +| B i |), where k and B 0 are the material parameters, and B i is the local internal magnetic-flux density. The magnetization curves obtained from direct measurements using a superconducting quantum interference device and from the Fourier series of higher-harmonic complex susceptibilities were simulated with the magnetization equations based on the model and the temperature-dependence of k and B 0 were studied. The k ( T ) behavior indicates that at lower temperatures, weak point defects (oxygen vacancies) play a predominant role as the pinning center, but at higher temperatures below T c , a contribution from the normal-state Y 2 BaCuO 5 particles becomes important. The temperature-dependence of B 0 was found to be B 0 ∝(1− T / T c ) 3/2 , suggesting that B 0 is the quantity closely correlated to the irreversibility field.

Ferromagnetic and superconducting properties of pure RuCe0.5Eu1.5Sr2Cu2O10 samples prepared by polymerizable complex method

Abstract The polymerizable complex method was successfully applied for synthesis of RuCe 0.5 Eu 1.5 Sr 2 Cu 2 O 10 compound. To obtain single-phase samples one should control oxygen partial pressure and prevent Ru loss by conducting synthesis in the sealed volume. Prepared single-phase samples became ferromagnetic below 120 K and did not show bulk superconductivity down to 7 K. The additional annealing in oxygen at 500 °C, P O 2 =10 atm resulted in the R ( T ) dependence of more metallic character, however it did not increase superconducting phase fraction to a significant extent. On the other hand, preparation of Ru x Ce 0.5 Eu 1.5 Sr 2 Cu 2 O 10 samples and studies of their superconducting properties revealed that appearance of superconductivity in the case of Ru-1222 phases might be related to the Ru loss during high temperature synthesis.

Critical currents of ceramic superconductors induced by pulsed magnetic fields

We attempted to measure the critical current density Jc of ceramic superconductors with pulsed magnetic fields. Using a pair of search coils coaxially mounted just below a ring sample, we measured induced signals that were considered to be proportional to the time derivative of the sample magnetization. Four samples were used: YBa2Cu3Oy, YBa2Cu4Oy, Bi2Sr2CaCu2Oy, and Bi1.6Pb0.4Sr2Ca2Cu3Oy. These were all synthesized by the polymerized complex method. It was found that the induced signals from the paired search coils are typically of two types. One shows a monotonic decrease to zero as a function of time. The other has a minimum at a certain time and then approaches zero. The observed signals were simulated in the framework of the Bean model and the Kim–Anderson model for critical current density. Theoretical profiles by these two models reproduce the observed results well and suggest that the two types of induced signals are the same in appropriate limits. We discuss how to evaluate Jc from the observed s...

Lower critical fields of Bi2Sr2CaCu2O8+d and YBa2Cu3O7−d single crystals

Temperature dependence of the lower critical fields Hc1(T) of Bi2Sr2CaCu2O8+d and YBa2Cu3O7−d single crystals has been investigated for both orientations of the applied fields, parallel and perpendicular to the c axis. The magnetization curves of the samples were measured using a superconducting quantum interference device, and Hc1,c(T) (H∥c axis) and Hc1,ab(T) (H⊥c axis) were obtained using the modified Kim–Anderson critical-state model. From the results, we found both Hc1,c(T) and Hc1,ab(T) for a Bi2Sr2CaCu2O8+d single crystal had an anomalous upturn below about 40 K, while a YBa2Cu3O7−d single crystal did not show any anomaly down to about 10 K. Discussion on a remarkable anomaly Hc1,ab(T) at low temperatures is made using the formulas for anisotropic Hc1 in the Ginzburg–Landau theory. The result suggests that the temperature dependence of the coherence length in the ab plane plays an essential role. We may insist that the anomalous upturn of Hc1,ab for a Bi2Sr2CaCu2O8+d single crystal at low temperatu...

Harmonic susceptibilities of polycrystalline (La1-xCax)(Ba1.75-xLa0.25+x)Cu3O7+d synthesized by amorphous metal-complex method

Abstract Single-phase tetragonal (La 1− x Ca x )(Ba 1.75− x La 0.25+ x )Cu 3 O 7+ d polycrystals with the composition range 0.1⩽ x ⩽1.0 were synthesized by the amorphous metal-complex method. The fundamental and the higher-harmonic complex susceptibilities χ n = χ n ′ −i χ n ′′ ( n =1, 2, 3, 5, and 7) were measured by means of Hartshorn bridge. The ac-magnetic field superimposed on the earths field was applied to the specimen. We observed nonzero values of χ n for all n in the temperature range of the superconducting transition. The observed χ n – T curves were analyzed by the Kim–Anderson critical-state model. We found that the pinning force density increases up to x =0.5 and decreases over x =0.5 with increasing the Ca content x , and is proportional to two kinds of the power of the elemental pinning force.

Temperature dependence of the material parameters in the Kim-Anderson critical-state model for a melt-processed YBa2Cu3O7

Abstract The magnetic response of a melt-processed YBa2Cu3O7 was measured to make clear the physical meanings of the parameters k and B0 in the Kim-Anderson critical-state model where the critical current density Jc is assumed to be a function of the local internal magnetic-flux density Bi, Jc(Bi) = k/(B0 + |Bi|). The isothermal magnetization curves of the sample were measured in the wide temperature region below the critical temperature, and the parameters were selected so as to fit the calculated curve to the observed one at each temperature. We found that k and B0 are proportional to (1 − T/Tc)γ, where γ = 2.5 – 3.0 and to (1 −T/Tc)1.5, respectively. We propose that B0 approximately corresponds to the irreversibility magnetic-flux density Birr.

Effects of Ca doping and oxygen ordering in Cu–O chains on the superconducting properties of (CaxLa1−x)(Ba2−x−yLax+y)Cu3O7+d prepared by amorphous metal complex method

Abstract The role of sintering conditions in synthesis of tetragonal superconducting (Ca x La 1− x )(Ba 2− x − y La x + y )Cu 3 O 7+ d ( x =0.0–0.6, y =0.25) polycrystalline samples prepared via an amorphous metal complex method was investigated. The series of samples sintered at 910, 930 and 950 °C in O 2 were characterized by XRD, EDS, Raman spectroscopy and iodometric titration to establish the structural and chemical differences responsible for their different superconducting properties determined from AC complex magnetic susceptibility measurements. We concluded that heat treatment below 930 °C was required to preserve the high compositional homogeneity of the initial precursor powder and obtain sharp superconducting transition for the final sample. For such high quality samples, we also found an interesting feature in the behavior of T c vs. Ca concentration, which has not been reported yet. The T c decreased with increasing Ca content for x T c started to increase with increasing Ca content for above 0.10.

Photoinduced Nanodots in Bi2Sr2CaCu2O8+d

We report a fabrication of high-density nanodots by photodoping in overdoped Bi2Sr2CaCu2O8+d thin film (Tc = 80 K). A scanning near-field optical microscope probe is used to locally excite carrier, and photodoped region is associated with lower Tc phase (Tc = 75 K) via overdoping. Nanoscale characterizations with optical reflectivity reveal that nanodots (30-nm diameter) are regularly distributed in 50-nm step. The resultant films with photoinduced nanodots enhance Jc, a situation being similar to strong pinning effects observed in films modified by either ion irradiation or sputtered nanoparticles. These results suggest that photoinduced nanodots with lower Tc act as effective pinning centers.

Hole density of (La1−xCax)(Ba1.75−xLa0.25+x)Cu3O7+d synthesized by amorphous metal-complex method

Abstract We measured the resistivity and the optical reflectivity spectra, and performed the coulometric titration to examine the average hole density in tetragonal (La 1− x Ca x )(Ba 1.75− x La 0.25+ x )Cu 3 O 7+ d superconductors. High-purity polycrystalline samples were synthesized for Ca cation content 0⩽ x ⩽1 by the amorphous metal-complex method. With increasing x T c increases until x = 0.5–0.6, then decreases over x =0.6. Temperature dependence of the resistivity in 0.4⩽ x ⩽0.9 shows a linear relation like a metallic behavior. The resistivity at room temperature decreases until x =0.5, and increases over x =0.5. The x dependence of the optical conductivity within the low energy region is consistent with the results of electrical dc conductivity. The results of the plasma frequency and the coulometric titration also show the tendency that the hole density increases until x =0.5 and decreases over x =0.5 with increasing x .

Superconducting condensate density of (La1−xCax)(Ba1.75−xLa0.25+x)Cu3O7+δ polycrystals

Abstract The superconducting condensate density n s of single-phase tetragonal (La 1− x Ca x )(Ba 1.75− x La 0.25+ x )Cu 3 O 7+ δ polycrystals was examined by the magnetic measurements. The samples were synthesized with the composition range 0.1⩽ x ⩽1.0 by the amorphous metal-complex method. The magnetization curves were measured by a SQUID magnetometer. The lower critical field H c1 was obtained from the magnetization curve using the modified Kim–Anderson critical-state model, in which H c1 was taken into consideration. We estimated n s at 0 K from the temperature dependence of H c1 . The n s value increases until x =0.5 and decreases over x =0.5 as x increases. Besides, n s was found to be linear to the hole density. The present relation between the transition temperature T c and n s is compared with the results for the underdoped samples by means of muon spin relaxation.