Measurements of the surface resistance of high-T c superconductors at high RF fields

We report measurements of the r.f. surface resistance of a wide variety of high-T _c superconductors in bulk form and as films on silver substrates. Several apparatuses have been constructed and used for measurements at frequencies from 0.15 to 40 GHz and r.f. surface magnetic fields as high as 640 G. In every case in which the field dependence of the surface resistance was measured, the surface resistance increased monotonically with field amplitude through a transition region characterized by a strong field dependence. It then saturated at high field at a value of a few percent of the normal-state surface resistance just aboveT _c . In the presence of this field dependence, the frequency dependence of the surface resistance changed from quadratic to less than linear.     

Irreversible change of surface impedance of high-Tc thin films due to D.C. magnetic field commutation

The magnetic field dependence of the surface impedance (resistance and reactance) of high-T _c thin films is found employing measurements of the quality factor and the frequency of the parallel plate resonator in a dc magnetic field up to 500 Oe at 10 GHz.c-Oriented YBa₂Cu₃O₇ thin films are examined. Enhancement of surface resistance and inductance with increase of magnetic field is observed. The effect of irreversible increase of surface impedance as compared to its initial values after a cycle of magnetic field commutation is found. A qualitative explanation of the observed effects based on the picture of magnetic vortex penetration and accumulation in the film due to strong pinning is presented.     

Magnetic hysteresis of the zero-resistance critical temperature of high-Tc granular superconductors

We have found a notorious hysteretic behavior in the dependence of the zero-resistance critical temperature obtained through resistivity () versus temperature (T) measurements with applied field (H _e ) in High-T _c granular superconductors. This behavior is explained semi-quantitatively based on the analogy between the present observation and a similar hysteresis found in the field dependence of the transport critical current in these materials.     

Microwave Properties of In Situ Grown MgB2 Superconducting Thin Films

The electrodynamic response at 20 GHz of c-axis oriented MgB₂ superconducting thin films is reported. Mg-rich Mg-B precursor samples were grown on MgO and Al₂O₃ single crystal substrates by a d.c. planar magnetron sputtering technique, and subsequently annealed in situ at 800°C for 10 min in a In-sealed Nb box in the presence of saturated Mg vapor. The films were characterized by a variety of structural and electronic techniques including XRD, EDS, STM-AFM analyses, and transport measurements. The dependence of the surface impedance from temperature and radiofrequency (r.f.) field amplitude was measured via a dielectric resonator technique. Temperature data clearly confirm the s-wave nature of the newly discovered superconductor, even if the value of the energy gap is smaller than BCS prediction. An effective two-band model can be applied to quantitatively explain the experimental results. In spite of previous reports claiming the absence of weak link behavior in MgB₂, the power dependence show that granularity governs the performance of these films in the microwave region.     

High-pressure dc sputtering of High-Tc BPSCCO thin films withTc(0) above 100 K

High-T _c Bi(Pb)-Sr-Ca-Cu-O thin films have been made on single-crystal MgO substrates using high-pressure dc sputtering technique. X-ray studies confirm the crystallinity and highly oriented structure withc-axis perpendicular to the substrate. By optimizing the annealing schedule the formation of the high-T _c phase is stabilized. The best film exhibited superconducting transition temperature with zero-resistance temperature,T _c(0), as high as 101 K. Temperature dependence ofJ _c indicates the presence of Josephson-type weak links.     

The Edge Effect of a Microstrip Resonator on the Field Dependence of the Surface Resistance of a High-Tc Superconducting Thin Film

A new behavior of the field dependence of microwave surface resistance (R _s), which was observed on a microstrip resonator and may be caused by the edge of the center strip, is reported in this paper for epitaxial high-T _c superconducting (HTSC) thin films. The exhibited behavior is that R _s remains almost unchanged below a certain rf magnetic field H _rf, and then increases abruptly at this field, after which it increases in proportion to H _rf. To explain the behavior, the morphology of the microstrip resonator was examined by atomic force microscopy (AFM), which showed that the edge of the resonator was damaged in some regions because of the acid etching. If the damaged edge is considered as a weakened granular superconductor, the observed R _s behavior could be explained well in terms of the high-frequency critical state model. This implies that the edge condition should be considered in studying the field dependence of R _s when the planar resonator technique is used.     

Superconductivity of Ba1 ? xKxBiO3 (0.35 < x < 1) Synthesized by the High Pressure and High Temperature Technique

Ba_{1 – x} K _x BiO₃ (BKBO) samples with 0.35 < x < 1 were synthesized by the high pressure and high temperature technique. XRD analysis showed that the BKBO samples were single phase for the whole range of the potassium doping concentration. The change of superconducting transition temperature, T _c, as well as lattice parameters have been investigated upon doping concentration. As the K doping concentration (x) increases from x = 0.37, T _c decreases from 30.4 K to almost zero at x = 0.74. However, in some BKBO samples without including any barium in the starting composition (x = 1), which is denoted as KBO samples, superconductivity is observed with T _c as high as 9 K with partial substitutions of Bi at the K site. Depending on the synthesis condition of the KBO samples, T _c and lattice parameters were different from sample to sample. Compared with other superconducting bismuthates, the evolution of T _c by potassium doping in the cubic BKBO system is discussed in terms of its electronic band structure.     

A New Method for the Estimation of Hc2 Anisotropy in Polycrystalline MgB2 Samples

By using isothermal magnetization measurements in polycrystalline MgB₂ samples, we estimate the H ^c _c2 in the interval [0, T _c]. By combining these measurements to the estimated H ^ab _c2 from the onset of the diamagnetic transition in isofield and isothermal magnetic measurements, an estimation of the anisotropy parameter can be achieved. The H ^c _c2 values coming from high quality polycrystalline samples agrees nicely to those obtained on single crystals. Our results show a temperature variation of the (T ) = H ^ab _c2/H^c2 with (T _c) 3.     

Thermodynamic and Magnetic Properties of Superconductors with Anisotropic Energy Spectrum,MgB<Subscript>2</Subscript>

The behavior of the superconducting transition temperature T _c and that of the jump of electron heat capacity (C _S −C _N )/C _N of the compound MgB₂ at T=T _c with substitution of boron and magnesium atoms by other atoms from the periodic table of the elements, corresponding to introduction of additional electrons or holes in this compound are researched. The microscopic superconductivity theory in MgB₂ systems in the magnetic field parallel to the crystallographic axis (H ‖ c) is built. The magnitude of the upper critical field H _c2 is determined and its temperature dependence in two-band systems with different and identical topologies of Fermi surface cavities of the corresponding bands is studied. The obtained results and their comparisons with the experimental data demonstrate that all kinds of anomalies of the physical properties of the compound MgB₂ are effectively described by the two-band model.     

The Influence of Bi Doping on the In Situ Growth of TlBa2Ca2Cu3O9 High-Tc Films

The in situ process—laser ablation in combination with thermal evaporation of Tl₂O—has turned out to be a preparation method for single-phase and epitaxial TlBa₂Ca₂Cu₃O₉ (1223) thin films with T _c values up to 109 K. It was found by several groups that a partial substitution of Tl by Bi simplifies the phase development of the 1223 compound in the usual two-step process. We have investigated the influence of the Bi doping on the in situ growth. X-ray measurements show that the films consisted mainly of the 1223 compound. In 300-nm thin films there was no evidence of a Bi amount in the crystal structure, but thinner films (80 nm) show a small amount of Bi. We concluded that Bi doping supports the phase development of the 1223 compound only in an early stage of the film growth. The Bi-doped films have higher T _c values up to 114 K, higher j _c values up to 6 × 10⁵ A/cm² (77 K, 0 T), and lower surface resistances of 56 m (77 K, 87 GHz) than the undoped films.     

Field, temperature, and angle dependence of the critical current density in Bi2Sr2CaCu2O10/Ag ribbons

Current-voltage characteristics of high-critical-current Bi₂Sr₂CaCu₂O₁₀/Ag ribbons were measured using both transport and magnetization techniques. The slope of these curves changes with magnetic field and temperature in a way very similar to the observedj _c (H, T) behavior. This correspondence between the critical current and the slope of theI–V characteristics can be explained within the thermally activated flux creep framework. The dependence ofj _c on the angle between field and ribbon is compared to the existing intrinsic anisotropy models.     

Upper limit ofTc for the copper oxides

The value of the critical temperature of the cuprates correlates with the doping level and is affected by the interplay of two competing factors: (1) the increase in carrier concentration, and (2) the pair-breaking effect of magnetic impurities. An analysis of the temperature dependence of the critical field leads to the conclusion that magnetic impurities are present even in a sample with the maximum observed value ofT _c.A new parameter, intrinsicT _c (T _cintr), which is its value in the absence of magnetic impurities, is introduced. The maximum value ofT _cintr, which corresponds to the maximum doping level, appears to be similar for different cuprates and to be equal to 160–170 K. This is the upper limit ofT _c in the cuprates.     

The Jc Limit of Multifilamentary Ag/Bi-2223 Tapes

To improve on present critical current (J _c) performance, multifilamentary Ag/Bi-2223 tapes with a large range of reduction rates were manufactured. The relative core mass density D was calculated, dependent on the measured geometric dimensions of the tapes. Experimental results, D vs. J _c, D vs. maximum pinning force density F _max , and D vs. irreversible magnetic field B _irr, are quantitatively formatted. In particular, the magnetic field dependence of J _c is critically dependent on its core density. If the core density increases by 10%, J _c of the tapes in this experiment is enhanced by as much as 100%. Therefore, in the present state of the technological process for manufacturing Ag/Bi-2223 tape, increasing the core density is clearly a significant strategy in improving the electronic and magnetic properties of the tapes and enhancing the capacity for carrying current at high magnetic fields. The limit of the bulk self-field-J _c can be calculated by the relationships of J _c vs. D. The limit is estimated to be on the order of 200 kA/cm² for multifilamentary Bi-2223 tapes, which was supported by magneto-optical (MO) magnetization measurements results. It is a hard task to approach this limit with the present state of the art in manufacturing Ag/Bi-2223 tape, and it is the time to suggest some new ideals for Bi-2223 tapes to promote large-scale applications.     

Current controlled high Tc superconducting switch

When a current is applied above the critical current l_c of a superconductor, the material is in its normal state and has a finite resistance. Below l_c the material becomes a superconductor with zero resistance. Switching between these two states can be achieved by modulating a current through the sample. Various high T_c superconducting (HTS) line structures have been made. In the normal state these structures are ordinary resistors with resistances ranging from 10 μ to 100 kμ. The critical currents are in the range 10 μA–100 mA. Switching behaviour has been observed in a simple divider circuit using the HTS lines at 77 K. Applications of the current controlled HTS switch to digital and logic circuits are discussed.     

High-Pressure Studies on Superconductivity in LaFeAsO1−x F x and SmFeAsO1−x F x

The pressure dependence on the superconducting transition temperature (T _c ) was investigated for the iron-based superconductors LaFeAsO_1−x F _x and SmFeAsO_1−x F _x . The T _c ’s increase largely for LaFeAsO_1−x F _x with a small increase of pressure, while a sharp decrease of T _c was observed for SmFeAsO_1−x F _x . The electrical resistivity measurements reveal pressure-induced superconductivity for undoped LaFeAsO and SmFeAsO. These pressure effects seem to be related to an anisotropic decrease of the lattice constants under high pressure from the x-ray diffraction measurements up to 10 GPa for the LaFeAsO_1−x F _x system.     

Extraction of Critical Current Density and Flux Creep Exponent in the Magnetic Superconductor Ru-1212 Using the ac Magnetic Susceptibility Measurements

The ac susceptibility data was employed to extract the temperature dependence of the critical current density, J _c(T), as well as the variation of flux-creep exponent n(T,H _ac) with temperature and ac field amplitude in bulk samples of polycrystalline magnetic superconductor RuSr₂GdCu₂O₈ (Ru-1212). The critical state models and the collective flux-creep approximation model were successfully accounted to describe such behavior below the transition temperature. The calculated values of n(T,H) are well fitted to a power law of the following form: n(T,H)=n ₀(H)T ^−s(H), where s is field dependent exponent whose values varied from −2.4, −1.01 for field amplitudes ranging from 0.5 G and 3.8 G. The power law describing the frequency dependence of χ′ is found to be consistent with the results of the current-dependent effective activation energy of the form U(J)=U ₀ln (J _c/J). Additionally, the dependence of the current density is found to scale according as: J _c(T)=J _c0(1−T/T _c) ⁿ , where the exponent n values varied from 1.05 to 1.25. Such dependence is an indication of intergrain coupling that could be ascribed in terms of superconductor–insulator–superconductor junctions. The derived temperature dependence of J _c(T) is in good agreement with the data obtained from the measurements using the traditional “loss-maximum” approach. Furthermore, the flux-creep effect increased with increasing both ac fields and temperatures except at about 15–25 K below the onset of T _c, where a slowing down of the flux creep was observed.     

Transformation of the YBa2Cu3Ox superconducting phase to an insulator-nonsuperconducting phase

Three YBa₂Cu₃O _x samples (A, B, and C) were prepared under similar conditions. The starting materials were Y₂O₃ (99.99%), CuO (99.999%), and BaCO₃ (99.999%). All the specimens demonstrated a Meissner effect at liquid-nitrogen temperature. From temperature-dependent resistivity measurements, theT _c for the three samples was found to be aroundT _c =93 K, in agreement with other reports [1].     

Superconductivity in epitaxial Bi2Sr2CuO6/Bi2Sr2CaCu2O8 superlattices: The superconducting behavior of ultrathin cuprate slabs

Utilizing atomic layer-by-layer molecular beam epitaxy (ALL-MBE), we have synthesized a series of high-quality superlattices in which ultrathin slabs (one-half unit cell thick) of the high-T _c superconductor Bi₂Sr₂CaCu₂O₈ alternate with up to five such layers of the low-T _c Bi₂Sr₂Cu₁O₆ phase. In all these superlattices we foundT _c to be essentially equal to that of the high-T _c Bi₂Sr₂CaCu₂O₈ phase itself, which indicates that this cuprate is a 2D superconductor insofar as the interslab coupling plays at best a secondary role. Furthermore, it is demonstrated thatT _c need not be reduced at heterostructure interfaces.     

Influence of sintering time and quenching on the formation of highT c phase

Influence of sintering time and quenching in Bi_{2 −x} Pb _x Ca₂Sr₂Cu₃O _y (x=0.0, 0.1, 0.2, 0.25, 0.3 and 0.4) samples have been studied by resistance and XRD measurements. In samples sintered at 850°C for 4 days,T _c(0) increases with Pb concentration.T _c(0) increased from 81 K forx=0.0 to 109 K inx=0.30 sample and then decreased. Increasing the sintering time to 10 days decreased theT _c Quenching further decreased theT _c(0). From X-ray diffraction patterns, the intensity peaks of low and highT _c phases have been measured. The addition of Pb promotes highT _c-phase. Sintering time, slow cooling and rapid quenching studies show that there is an optimum sintering time and cooling rate to produce a highT _c-phase.