Flux dynamics of YBCO/Ag composite

DC magnetization measurements have been carried out on bulk YBCO/Ag composites with silver content up to 20wt per cent. DC fields in the range 0·5 mT to 200 mT have been used to investigate the inter- and intragranular properties at 77K. The AC susceptibility as a function of temperature at different AC fields (0·026–0·30 mT) has also been studied. Under small DC fields (≈ 4 mT), depending on the Ag content andH _max, the M-H loop shows a complicated behaviour. This behaviour can be explained on the basis of effect of strong field dependence of transport critical current, grain size and intragrain critical current densityJ _cgm on low-field M-H loop. The estimation of intergranular critical current densityJ _cjm from these loops does not remain a simple function of ΔM/d. The AC susceptibility measurements show a small increase inJ _c(T) with silver content under low AC fields only, consistent with the transportJ _c data; beyond thatJ _c(T) decreases. This improvement inJ _c(T) and transportJ _c with silver can be ascribed to the improved coupling between grains but not to the pinning. Also at higher field (H _max>20 mT) the addition of Ag decreases the intragrain critical current density. The upper critical field of intergranular regionH _c2j and lower critical field of intragrain regionH _c1g also decrease with silver content.     

The Activation Energy <Emphasis Type="Italic">U</Emphasis>(<Emphasis Type="Italic">T</Emphasis>,<Emphasis Type="Italic">H</Emphasis>) in Y-based Superconductors

Based on the Arrhenius equation, a method to calculate the activation energy from the resistance transition is proposed for high temperature superconductors. This method is applied to the Y-based superconductors. The activation energy is found to be U(T,H)∼(1−T/T _c )^4.8(H/H ₀)^−3.8 of YBCO crystal, and U(T,H)∼(1−T/T _c )^3.3(H/H ₀)^−2.2 of Er doped MTG YBCO crystal, respectively. With the obtained activation energy U(T,H), the lower part of the experimental curve ρ(T,H) and its derivative can be reproduced.      

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.     

Effect of magnetic iron impurity on the superconducting properties of an amorphous Nb50Zr35Si15 alloy

The effect of magnetic iron impurity on the superconducting properties of amorphous Nb₅₀Zr_35−x Si₁₅Fe _x (x⩽4 at %) alloys was examined. Doping with an iron impurity resulted in a linear depression ofT _c andH _c2(T) and a decrease in andρ _n after reaching a maximum value at 0.5 to 1.0 at % iron. The observed decrease was about 35% forT _c, 85% forH _c2 at 2.0 K, 16% for and 21% forρ _n. Although the decrease in occurs through the decrease inρ _n as expected from the GLAG theory, the depression inT _c caused by magnetic impurity could not be explained in terms of the GLAG theory which is applicable to Nb-Zr-Si amorphous alloys without magnetic impurity, but was interpreted as arising from the pair-breaking effect in the superconducting nature due to magnetic scattering. However, the pair-breaking effect was found to be smaller by about one-tenth for the present amorphous superconductors than for crystalline superconductors, indicating the high stability of the superconductivity of the Nb-Zr-Si-Fe alloys against the magnetic scattering arising from the magnetic impurity. The reduced magnetic field at which the reduced fluxoid pinning force exhibits a maximum value increased with iron concentration, indicative of an enhancement of fluxoid pinning force. The enhancement in fluxoid pinning force was interpreted as arising from the increase in compositional, electronic and/or magnetic fluctuations by the dope of iron impurity.     

Role of Coexistence of Superconductivity and Paramagnetism in Magnetostriction of High-T c Superconductors

Total magnetostriction in the superconducting state for high T _c superconductors has been separated into critical state and paramagnetic components in terms of a H(x) dependent magnetic flux density. We show that the paramagnetic part is χ(2+χ)〈H(x)²〉, where χ is paramagnetic susceptibility. We have reproduced successfully ΔL/L−H _a curves measured by de la Fuente et al. (Phys. C 244:214, [1995]), in which they clearly observed coexistence of superconductivity and paramagnetism, employing the concepts presented in this work.      

Superconducting gap function and upper critical field in the presence of inhomogeneous magnetic order

Rare earth ternary superconductors are known to exhibit oscillatory magnetic orders below their superconducting transition temperatures. The study of behaviour of superconducting electrons in an inhomogeneous magnetic field is therefore important for such systems. We report here the results of our theoretical study of superconducting gap function Δ(T) and upper critical fieldH _c2 (T). The results are applied to analyse and explain the variation of Δ(T) andH _c2 (T) in case of NdRh₄B₄.     

Dielectric Properties of Na<Subscript>0.7</Subscript>CoO<Subscript>2</Subscript> and of the Superconducting Na<Subscript>0.3</Subscript>CoO<Subscript>2</Subscript>·1.3H<Subscript>2</Subscript>O

We present dielectric properties of ceramic anhydrous Na_0.7CoO₂ and the superconducting Na_0.3CoO₂·1.3H₂O materials. The presence of water which induces superconductivity also may increase the dielectric constant (ε) of the hydrated material. This is consistent with the predicted relationship between the highε and the enhancement ofT _c in highT _c superconductors. The anhydrous sample is porous and the transport is due to some percolation via the pores. The porosity is much higher for the hydrated material and the transport is ionic inside bulk water.     

Electrical properties of Gd-modified Pb(SnTi)O3 ferroelectric ceramics

The Polycrystalline samples of (Pb_1−x Gd _x )(Sn_0.45Ti_0.55)_1−x/4O₃ (PGST) (x = 0, 0.05, 0.07, 0.1) were prepared using a standard mixed-oxide method. The XRD and SEM studies of the samples reveal that PGST samples have tetragonal symmetry with uniform grain distribution. Measurements of dielectric constant, dielectric loss and ac electrical conductivity were made in a wide temperature (300–650 K) and frequency (100 Hz to 1 MHz) range. These materials undergo a ferroelectric–paraelectric phase transition at 569, 582, 598, and 603 K for x = 0, 0.05, 0.07, 0.1, respectively. The ferroelectric property of some samples at room temperature was confirmed through the study of hysteresis loops. The Cole–Cole plots (ε′′ vs. ε′) at 125 and 150 °C form semicircular arcs with the center lying underneath the abscissa, deviated from the ideal Debye relaxation model to some extent. In all the compounds, the slope of ln σ_ac ~ 1/T shows a distinct variation for temperature below 450, 450 K to T _c and for T > T _c. The room temperature ac conductivity at 10 kHz lies in the range of 10⁻⁵–10⁻⁶ (Ω⁻¹ m⁻¹).     

Non-resonant RF and microwave response: A novel technique for the characterization of superconducting materials

When examined using continuous wave electron paramagnetic resonance and nuclear magnetic resonance spectrometers, the highT _c superconductors give rise to intense, low field, ‘non-resonant’ absorption signals in the superconducting state. This phenomenon can be used as a highly sensitive, contactless technique for the detection and characterization of superconductivity even in samples containing only minute amounts of the superconducting phase. Further, it can also be applied to the determination of material parameters of interest such asJ _c andH _c2 in addition to being a powerful way of distinguishing between weak-link superconductivity and bulk superconductivity. The details of these aspects are discussed.     

Composition dependence of transition temperature in some ceramic superconductors

The composition dependence of transition temperature in some ceramic superconductors (La_2−x (Ba, Sr) _x CuO₄) was studied by modifying our earlier approach and developing a Fourier-transformed effective potential which involves the effect of two-dimensional (2D) acoustic plasmons. This potential was used to obtain the pairing (electron-electron attraction) parameter (λ), the averaged Coulomb repulsive parameter (μ*) and the cut-off 2D acoustic plasmon frequency (ω _c) required to compute the superconducting transition temperature (T _c) from the strong coupling theory. The variations ofT _c with compositions (x) obtained for La_2−x (Ba, Sr) _x CuO₄ show reasonably good agreement with experimental data.     

AC susceptibility studies on Bi-Sr-Ca-Cu-O system

We report the low field a.c. susceptibility data χ′(T, H _rms), χ″(T, H _rms) for the newly prepared superconducting system Bi _x Sr₄Ca₂Cu₄O _y (x=0·5, 1·0 and 1·5) containing Bi at concentrations lower than that in the conventional Bi-based system. Our experimental results are discussed in the light of existing theories.     

The Upper Critical Field <Emphasis Type="Italic">H</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis>2</Subscript> in Advanced Superconductors with Anisotropic Energy Spectrum

A brief review of works on the microscopic theory of determining the upper critical field in two-band isotropic and anisotropic superconductors is given. The research is based on a set of the Ginzburg–Landau equations for the order parameters in a magnetic field that are studied in terms of the classical approach to a superconducting system in a magnetic field. Two inequivalent energy bands with different topology of Fermi surface cavities overlapping on the Fermi surface are discussed. The cases of the direction of the external magnetic field [(H)\vec]//\vec{H}// the (ab) plane and [(H)\vec]//\vec{H}// the crystallographic c axis are studied. The equations for determining H _c2(ab) and H _c2(c) for a pure superconductor and a superconductor doped with electrons and holes are derived. The analytical solutions to these equations in the vicinity of the superconducting transition temperature (T _c −T≪T _c ) and in the vicinity of zero (T≪T _c ) are found. The temperature and impurity dependences of the upper critical fields H _c2(ab) and H _c2(c), as well as the anisotropy coefficient γ _H, are studied. The resulting theory is applied to determine the dependences of the above magnetic characteristics of intermetallic compound MgB₂. The theory agrees qualitatively with experimental data.     

Unusual behavior of superconductors near the tricritical Lifshitz point

The properties of superconductors near the tricritical Lifshitz(TL) point are studied. Unusual behavior ofH _c2(T) near the TL point in three-dimensional (3D) systems is found, as well as an oscillatory character ofH _c2(T) in quasi-2D systems. The oscillatory character of the critical currentj _c of the S-S-S junction as a function of thickness and temperature is also studied. Predictions are compared to experimental data in such quasi-2D systems as T_aS₂(pyridine).     

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.     

Dielectric properties of electron irradiated PbZrO<Subscript>3</Subscript> thin films

The present paper deals with the study of the effects of electron (8 MeV) irradiation on the dielectric and ferroelectric properties of PbZrO₃ thin films grown by sol-gel technique. The films were (0.62 μm thick) subjected to electron irradiation using Microtron accelerator (delivered dose 80, 100, 120 kGy). The films were well crystallized prior to and after electron irradiation. However, local amorphization was observed after irradiation. There is an appreciable change in the dielectric constant after irradiation with different delivered doses. The dielectric loss showed significant frequency dispersion for both unirradiated and electron irradiated films. T _c was found to shift towards higher temperature with increasing delivered dose. The effect of radiation induced increase of ɛ′(T) is related to an internal bias field, which is caused by radiation induced charges trapped at grain boundaries. The double butterfly loop is retained even after electron irradiation to the different delivered doses. The broader hysteresis loop seems to be related to radiation induced charges causing an enhanced space charge polarization. Radiation-induced oxygen vacancies do not change the general shape of the AFE hysteresis loop but they increase P _s of the hysteresis at the electric field forced AFE to FE phase transition. We attribute the changes in the dielectric properties to the structural defects such as oxygen vacancies and radiation induced charges. The shift in T _C, increase in dielectric constant, broader hysteresis loop, and increase in P _r can be related to radiation induced charges causing space charge polarization. Double butterfly and hysteresis loops were retained indicative of AFE nature of the films.     

Scaling of the Temperature Dependent Resistivity and Hall Effect in Ba(Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)As<Subscript>2</Subscript>

We show that the temperature-dependent resistivity ρ(T), Hall number n _H(T) and the cotangent of the Hall angle cot θ _H(T) of Ba(Fe_1−x Co _x )As₂ (x=0.0–0.2) can be scaled using a recently proposed model-independent scaling method (Luo et al. in Phys. Rev. B 77:014529, 2008). The zero field normal-state resistivity above T _c can be reproduced by the expresion r(T) = r₀ +cTexp(- \frac2\varDelta T )\rho(T) = \rho_{0} +cT\exp(- \frac{2\varDelta }{T} ) and scaled using the energy scale Δ, c and the residual resistivity ρ ₀ as scaling parameters. The scaling parameters have been calculated and the compositional variation of 2Δ and ρ ₀ has been determined. The 2Δ(x) dependence show almost linear decreasing in underdoped regime, minimum corresponding to the T _c maximum and increasing in overdoped regime. The latter is different from that reported for cuprates. The existence of a universal metallic ρ(T) curve which, however, is restricted for the underdoped compounds to temperatures above a structural and antiferromagnetic transition is interpreted as an indication of a single mechanism which dominates the scattering of the charge carriers in Ba(Fe_1−x Co _x )As₂ (x=0.0–0.2).     

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.     

Effects of Higher Hopping Terms on the Stripe States in the Two-Dimensional Hubbard Model

Using the variational Monte Carlo method for the two-dimensional t-t′-t″-U Hubbard model, we discuss the ground state of the underdoped region on the high-T _c cuprates. We found that, while stripe states with the periodicity consistent with experiments for La_{2− x} Sr _x CuO₄ are stabilized in the case of t′/t < 0, the positive t′/t makes the stripe state unstable with the lowest energy state being the commensurate AF state, which is consistent with experiments on the electron doping system such as Nd_{2− x} Ce _x CuO₄. We also show that the stripe state is sensitive to the value of t″/t > 0. These results indicate that the nesting condition is a critical factor to the stripe instability.     

Superconducting transition temperatures in the Ru-1232 system, RuSr2(Gd1? x Ln x Ce1.8Sr0.2)Cu2O z (Ln = Sm, Dy, and Ho)

We have investigated effects of the lanthanide element Ln and the composition changes on the superconducting transition temperatureT _c in the Ru-1232 system, RuSr₂(Gd_1−x Ln _x Ce_1.8Sr_0.2)Cu₂O _z (Ln = Sm, Dy, and Ho). At first, in the case of the samples with Ln = Sm among almost the single 1232 phase samples, the values of the superconducting onset temperatureT _co are almost the same forx=0.00−0.15, and each of the lattice parametersa andc is almost constant. While, in each of the cases of the samples with Ln = Dy and Ho, the sample withx=0.05 shows the maximum values for both the superconducting onset temperatureT _co and the zero resistivity temperatureT _cz. Especially for the sample with Ln = Dy, the values ofT _co andT _cz are 18.5 and 6.5 K, respectively. These are higher than those of the mother sample of RuSr₂(GdCe_1.8Sr_0.2)Cu₂O _z . Moreover, from variations ofT _co, lattice parameters ofa andc in the RuSr₂(Gd_1−x Dy _x Ce_1.8Sr_0.2)Cu₂O _z system as a function of Dy contentx, the relationship between the superconducting transition temperature and the lattice parameters in the present system are investigated.     

Study of doping effects on transition temperature of La2 −x (Ca,Na, K) x CuO4 superconductors

We have investigated doping effects on the transition temperature (T _c ) of La_{2 −x} M _x CuO₄ (M=Ca, Na and K) by incorporating the effects of two-dimensional (2D) acoustic plasmons in the framework of the strong coupling theory. The contributions from 2D acoustic plasmon mechanism toT _c have been obtained from a Fourier-transformed effective potential, which has been earlier found to be successful in predicting the composition dependence ofT _c in La_{2 −x} (Ba, Sr) _x CuO₄. The results obtained by us on the variation of transition temperature with composition (x) in La_2−x M _x CuO₄ superconductors are in reasonably good agreement with the available experimental data. This success has led to the conclusion that 2D acoustic plasmons are adequate to explain the pairing mechanism and the variation ofT _c with composition (x) in cuprate superconductors.