Ultrasonic attenuation in the mixed state of V-42 at % Ti

The ultrasonic attenuation in the mixed state of V-42 at % Ti was measured at T = 4.14K, using shear and longitudinal waves with frequencies 10–130 MHz, in magnetic fields up to 100 kG. The results are compared to a phenomenological model in which the mixed-state resistivity is used as an effective resistivity in the Alpher-Rubin theory. For our V-42 at % Ti, pinning forces are weak, so that the mixed-state attenuation is particularly sensitive to the dc flux-flow resistivity. For magnetic fields 40–60 kG, the attenuation is in quantitative agreement with a model employing the dc flux-flow resistivity of Kim et al., % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaacqaHbpGCdaWgaa% WcbaacbaGaa8NzaaqabaGccqGH9aqpcqaHbpGCdaWgaaWcbaGaa8NB% aaqabaGccaWFibGaai4laiaa-HeadaqhaaWcbaGaam4yaiaaikdaae% aacaGGQaaaaOGaaiikaiaaicdacaGGPaaaaa!400D!\[\rho _f = \rho _n H/H_{c2}^* (0)\]. For high frequencies, where pinning may be neglected, the magnetic field dependence of the attenuation for HH_c2 is in better agreement with the use of the flux-flow resistivity calculated from the time-dependent Ginzburg-Landau equation.Research supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Conselho Nacional de Desenvolvimento Cientifico e Tecnológico (CNPq).Supported by Comissão de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).     

Thermomagnetic phenomena in the mixed state of HTSC: Anisotropy and effect of pinning

We extend our model of the flux-flow thermomagnetic transport coefficients of superconductors [A. V. Samoilov, A. A. Yurgens, and N. V. Zavaritsky,Phys. Rev. B 46, 6643 (1992)] to the pinning region. Using a method due to Vinokur, Geshkenbein, Feigel'man, and Blatter, we show that if the vortex dynamics in disorder-dominated,N/gr _xx andS/ _xx (whereN is the Nernst coefficient,S is the thermopower, and _xx is the longitudinal resistivity) do not depend on the pinning strength. The theoretical consideration is illustrated by our experimental results on the high-temperature superconductors.     

Phase-slip resistivity in high-temperature superconductors: a manifestation of intrinsic nano-granularity

Several critical experiments have shown that the so-called background magnetoresistance which occurs with no Lorentz force activation is isotropic in thea-b plane. This isotropic resistivity is linked to paradoxical behavior of vortices which cannot be changed by pinning, a result which may imply intrinsic pinning. Nevertheless, a number of flux-flow observations indicate that vortices may also behave as if they were unpinned. An explanation is proposed which states that this behavior arises as a consequence of defects in oxygen ordering in the Cu-O planes of HTSC crystals; this explanation involves segmentation of the Cu-O planes, and Josephson coupling between adjacent segments. The effective junction areas are seen to be very small, with dimensions characteristic of the size of the unit cell. Thermal motion of nano-scale weak links resolves the pinning paradox by inducing phase-slip resistive dissipation. The resulting phase-slip resistivity is shown to have axial symmetry, that is, the resistivity is independent of the angle between the current and the applied field, with both in thea-b plane.     

The effect of Sm substitution on properties of Bi1.6Pb0.4Sr2Ca2? x Sm x Cu3O y superconductors

The effect of the partial substitution of Ca by Sm in the Bi-2223 superconducting samples have been investigated in terms of X-ray diffraction (XRD), EDXRF (Energy Dispersive X-ray Fluorescent), magnetoresistivity, critical temperature, transport critical current density, and ac susceptibility measurements. The samples were prepared by the conventional solid-state reaction method. XRD patterns are used to calculate lattice parameters and phase ratio of the Bi-2223 samples. The volume fraction was determined from the intensities of Bi-2223 and Bi-2212 peaks. The room temperature XRD patterns of the samples showed the presence of Bi-2223 phase decreases with increasing the Sm content. We estimated the transition temperature of the samples from the resistivity versus temperature measurements in dc magnetic fields up to 0.6 T. We observed that transition temperature, T _c , and transport critical current density, , depend on the Sm substitution. They both decrease with increasing the Sm substitution. We extracted the peak temperature, T _p , and the pinning force density from our previous ac susceptibility measurements. The pinning force density decreased with increasing the Sm content. The possible reasons for the observed decreases in critical temperature and critical current density due to Sm substitution were discussed.     

Twin-boundary pinning and flux flow in zone-melt-textured YBa2Cu3O7-δ crystals

Using a microwave (13 GHz) technique which is sensitive to field-induced changes in the surface resistance, we have observed twin-boundary pinning in zone-melt-textured crystals of YBa₂Cu₃O₇-δ. With the rotation of a static 1.9-T field applied in thea-b plane, the surface resistanceR _s exhibits minima at 90° intervals. The twin-boundary minima are shown to be connected to the orientation of the crystalline axes of the sample. The resistance minima are observed superimposed on a flux-flow response over the temperature range ofT _c to the lowest temperature attempted. Substantial flux mobility is observed to the lowest temperatures. The angular dependence of the flux-flow response at low temperatures is consistent with the “brick” model for local intergranular conductivity, which provides for an in-plane isotropic component.     

Ion irradiation effects in EuBa2Cu3Oy thin film

The effect of He ion irradiation on the pinning potential in EuBa₂Cu₃O _y , thin film was investigated by measuring the temperature dependence of resistivity in magnetic fields. The pinning potential decreased as the ion fluence increased. A slower decrease of pinning potential was observed in higher magnetic field in the fluence region <3.5×10¹⁵ cm^–2.     

Mixed-State Hall Effect in HgBa2CaCu2O6+δ Thin Films with Columnar Defects

The mixed state Hall effect has been studied in high quality HgBa₂CaCu₂O_6+δ thin films while the pinning strength is modified by the irraddition of columnar defects. It has been shown that the pinning strength can be significantly improved by columnar defects. The sign reversal in Hall resistivity is found to be nearly independent of pinning while the scaling behavior between Hall resistivity (ρ_xy) and longitudinal resistivity (ρ_xx) is influenced by pinning. The exponent β in the scaling law β _xy ～ β _xy ^β decrease from 1.8 ± 0.1 to 1.5 ± 0.1 as pinning is enhanced. This results suggest that the sign reversal and the scaling are unrelated, and pinning is not the mechanism of the sign reversal.     

Magneto-transport Properties and Thermally Activated Flux Flow in Ba(Fe 0 . 9 1 Co 0 . 0 9 ) 2 As 2 Superconductor

Thermally assisted flux flow (TAFF) based on magneto-resistivity and ac susceptibility measurements is studied in a Ba(Fe_0.91Co_0.09)₂As₂(T _c=25.3 K) sample in magnetic fields up to 18 T. In addition to the upper critical field μ ₀ H _c2 and the coherence length ξ(0), the flux flow activation energy U(T,H) has also been determined. The resistive transition width is proportional to μ ₀ H, in contrast to Tinkham’s theoretical prediction. By applying Fisher’s model, the glass melting transition temperature T _g, which occurs in the upper TAFF state and not in the zero resistivity vortex solid regime, is calculated. The onset of TAFF temperature and the crossover temperature T _x from TAFF to flux flow are determined. By contrasting the ac susceptibility data with the resistivity data, considerable flux penetration appears even in the zero resistivity state, in addition to ac losses. The H-T phase diagram is drawn and shows weak pinning regime as the field approaches μ ₀ H _c2, and the strength of the weak pinning decreases to 0 with increasing magnetic field from 0 to 18 T.     

Microwave Properties of Nb/PdNi/Nb Trilayers

We combine wideband (1–20 GHz) Corbino Disk and Dielectric Resonator (8.2 GHz) techniques to study the microwave properties in Nb/PdNi/Nb trilayers, grown by UHV dc magnetron sputtering, composed by Nb layers of nominal thickness d _S =15 nm, and a ferromagnetic PdNi layer of thicknesses d _F =1, 2, 8 and 9 nm. We focus on the vortex state. Magnetic fields up to H _c2 were applied. The microwave resistivity at fixed H/H _c2 increases with d _F , eventually exceeding the Bardeen–Stephen flux-flow value. Vortex pinning is very weak at large d _F .     

AC Response of Cu0.5Tl0.5Ba2Ca2Cu3?xSnxO10?δ Superconductor

We have studied the ac response of Sn doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−x Sn _x O_10−δ superconductor samples from their ac-susceptibility measurements under different magnitudes of ac magnetic fields; H _ac=0.4, 4, 16 A/m. The samples with x=0.5 and 1.0 have shown strong flux pinning and intergrain coupling. However, the sample with Sn doping of x=1.5 has shown very poor flux pinning characteristics.     

Field Dependent Microwave Resistivity in YBa2Cu3O7−δ

We investigate the effects of directional pinning due to the layered structure and to columnar defects on the microwave response in YBa₂Cu₃O_7– films. We present measurements of the field-induced microwave resistivity at 48 GHz and 21 GHz taken in various relative orientations between the microwave currents, dc magnetic field and (a, b) planes. From measurements taken in the Lorentz-force-free configuration we experimentally show the relevance of the magnetic field induced increase of quasiparticle (qp) density. We identify the vortex motion contribution, and extract the vortex parameters. We estimate a pinning frequency of order 30 GHz when the field is aligned to the (a, b) plane. Secondly, we show that the introduction of columnar defects gives rise to a strong pinning along the columns, detectable even at 48 GHz. The pinning frequency appears to be of the same magnitude than for pinning by the layered structure.     

Investigation of Gd Addition Added on Magnetic and Structural Properties of Bi1.8Pb0.35Sr1.9Ca2.1Cu3GdxOy Superconductors by ac Susceptibility

This study reports the effect of Gd addition on magnetic and structural properties of Bi_1.8Pb_0.35Sr_1.9Ca_2.1Cu₃Gd _x O _y superconductor with x=0, 0.1, 0.2, 0.3, 0.4 and 0.5 by means of ac susceptibility measurements at various ac fields (ranging from 270 to 1352 A/m) and scanning electron microscopy (SEM) images. Critical onset (T_c^on)T_{\mathrm{c}}^{\mathrm{on}}) and loss peak temperatures (T _p) were qualitatively estimated from the ac susceptibility curves. The peak temperature at zero ac-magnetic field (T _p0) and intergrain critical current densities (J _c) were theoretically calculated from the ac susceptibility plots via the critical state models. The results show that peak temperatures and critical current densities were found to decrease with increasing Gd addition. Moreover, using a self-field approximation together with J _c dependence on temperature, the characteristic length (L _c) associated with the pinning force is estimated to be approximately the same as the average grain size (R _g) of the pinning center because of the linear decrease in J _c with increasing temperature. Surface morphology and grain connectivity of the samples were also obtained to degrade with increase in the Gd addition from SEM investigations.     

Nano Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Induced Fluxoid Jumps and Low Field Enhanced Critical Current Density in MgB<Subscript>2</Subscript> Superconductor

Nano particle of Fe₃O₄ (nFe₃O₄) up to 6 at% were doped in the superconducting MgB₂ samples. Despite the strong ferromagnetic nature of Fe₃O₄, both the ac susceptibility and the resistivity measurements show that up to 4 at% of Fe₃O₄, T _c =38 K is not changed, whereas for 6% T _c decreases by 6 K. This indicates that a low concentration of Fe does not substitute either the Mg or B sites and probably occupies the intergrain spaces. For 0.5% doped Fe₃O₄, an increase in J _c with respect to the pure MgB₂ samples is observed in the lower field and temperature regions (H<2 T and 20 K) indicating an enhanced flux pinning and the magnetic activation, i.e., the interaction between the magnetic dipole of Fe ion and the vortices is weak in comparison to the effective pinning potential. Whereas, at H>2 T, J _c of the doped samples is always less than that of MgB₂, and the activation is dominant in comparison with the effective pinning potential provided by the doping. Flux jumps are observed in lower T and H regions for the samples doped up to 1% nFe₃O₄ only. Magnetization plots of higher Fe content samples exhibited clear paramagnetic background. Mossbauer measurements for the higher (4, 6 at%) nFe₃O₄ doped MgB₂ samples show that at RT, the hyperfine field for both samples is ∼100 kOe and ∼120 kOe at 90 K. This means that the nFe₃O₄ particles decompose and form possibly an intermetallic Fe-B phase in the matrix.     

Flux Flow and Fluctuation Resistivity at 48 GHz in High-T c Superconductors

We present measurements of the real part of the microwave resistivity at 48 GHz in YBa ₂ Cu ₃ O _7– and Bi ₂ Sr ₂ CaCu ₂ O _8+x films, as a function of the temperature and of the static magnetic field. In zero field, the resistivity above the critical temperature is well described by the frequency-dependent fluctuational excess conductivity. In a static magnetic field and below T _c , the resistivity in YBa ₂ Cu ₃ O _7– can be fully described by a simple flux-flow model, while in Bi ₂ Sr ₂ CaCu ₂ O _8+x this model applies only partially. We discuss possible alternative models.     

Study of History Effect of Vortex Matter by AC Susceptibility

We have observed numerically the history effect, based on bulk flux pinning framework, in the susceptibility (T) (acs), experimentally measured in different samples. The dependencies of history effect in (T) on frequency, ac and dc fields, and bulk pinning strength are calculated. Our calculation matches well the behavior of in 2H–NbSe₂, showing the bulk model works well. It is also shown that bulk pinning and surface pinning will result in a history effect of very different characteristics. In addition, the numerical result shows that a peak in critical current density, which is usually accompanied by the vortex melting transition, may not be observed as a dip in acs in some cases, which is a dynamic response of a sample to applied as field and is dependent on ac field frequency and amplitude.     

Anisotropy of flux flow in layered superconductor 2H-NbSe2−x S x

Magnetic-field dependence of the resistivity for various currents in the layered superconductor 2H-NbSe_1.8S_0.2 was investigated. The flux flow was observed for various angles between the magnetic-field direction and the crystal layers near zero-field critical temperature. Anisotropy of the pinning force and the viscosity coefficient was found. The peak effect was observed under a magnetic field perpendicular to the layers, while it was not observed under a magnetic field parallel to the layers. A strong pinning force was found acting only on flux lines parallel to the layers. These results can be explained by the pinning mechanism due to the stacking faults.     

Vortex Motion and Quasiparticle Resistivity in MgB<Subscript>2</Subscript> at Microwave Frequencies

The electrical transport in superconductors at nonzero frequencies is affected by the normal and superfiuid fractions, as well as moving vortices, resulting in intricate expressions for the complex resistivity. In MgB₂, additional complications arise from the two-band nature of this material. We present an accurate study of microwave resistivity data measured in MgB₂ thin films by means of the Corbino disk broadband technique between 2 and 20 GHz. We show that a two-fluid model applies well in a relatively large region of the H–T phase diagram. Excellent agreement is found between the derived superfluid parameters (superfluid density, upper critical field) and theoretical predictions. In the same H–T region we isolate and discuss the vortex motion complex resistivity. To this end, we make use of the expressions given by the model by Coffey and Clem (CC). We show that the frequency dependence of the complex vortex resistivity recovers the CC model. However, the temperature and field dependence of the obtained parameters are at odds with the assumptions of the model. We discuss possible explanations of these oddities by considering collective pinning of vortices.     

Studies on structural,dielectric and electrical properties of Dy<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Bi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>FeO<Subscript>3</Subscript> solid solutions

The polycrystalline samples of Dy _x Bi_1−x FeO₃ (x = 1, 0.8, 0.6, 0.4 and 0.2) were synthesized by standard solid state reaction technique. The samples synthesized were characterized by X-ray diffraction (XRD) technique. Further the samples were characterized by infrared (IR) spectroscopic technique. The dielectric measurements were carried out as a function of frequency in the range 100 Hz to 1 MHz at room temperature. Also the dielectric measurements were carried out as a function of temperature at certain fixed frequencies. The ac and dc resistivity measurements were carried out using two probe method. Also temperature dependence of ac and dc resistivity was noted. These measurements suggest polaron conduction in the samples. Finally, the data from XRD, IR, dielectric measurements were correlated.