Effects of Bi Substitution on Magnetic and Transport Properties of La0.7?x Bi x Ag0.3MnO3 Ceramics

Colossal magnetoresistance, CMR ceramics with starting composition of La_0.7?x Bi _x Ag_0.3MnO₃ (x=0?C0.2) were synthesized using the conventional solid-state synthesis method to investigate the effects of Bi and Ag on their magnetic and electrical transport properties as well as their magnetoresistance behavior. Magnetic susceptibility measurements showed that the La_0.7?x Bi _x Ag_0.3MnO₃ samples with x=0, 0.10 and 0.15 exhibit single paramagnetic to ferromagnetic transition at Curie temperature, T _C , which was observed to decrease from 289.5?K (x=0) to 186.5?K (x=0.15) while the x=0.2 sample showed two magnetic transitions at T _C1 (160.5?K) and T _C2 (214.0?K). Electrical resistivity measurements showed metal?Cinsulator transition behavior for all samples. Bi substitution caused resistivity to increase while metal?Cinsulator transition temperature, T _MI shifted to lower temperature from 252.7?K (x=0) to 136.3?K (x=0.20). The metallic region of the ??(0,T) curve below T _MI for all samples was well fitted to the equation ??=?? _o +?? ₂ T ²+?? _4.5 T ^4.5 indicating a combination of grain or domain boundary, electron?Celectron and electron?Cmagnon scattering mechanism while the insulator region was governed by the Variable Range Hopping (VRH) model at T _MI<T<?? _D /2 and adiabatic small polaronic model (SPH) at T>?? _D /2. The increase of hopping activation energy, E _a for the latter is suggested to be due to possible hybridization between Bi 6s² lone pair and O orbital. Bi³⁺ substitution was also observed to enhance intrinsic MR at the vicinity of T _MI due to increase in DE interaction when external magnetic field was applied. On the other hand, the substitution also caused reduction of extrinsic MR effect at low temperatures, which is suggested to be due to reduction of Mn spin disorder at grain boundaries as a result of the presence of small amount of Ag secondary phase.     

Doping Dependent Quasiparticle Relaxation Dynamics in?SmFeAsO1?x F x Single Crystals: Comparison of?Spin-Density Wave and Superconducting States

We use optical femtosecond spectroscopy to investigate the quasiparticle relaxation and low-energy electronic structure in undoped and near-optimally doped SmFeAsO_1?x F _x iron-pnictide superconductor single crystals (SC). In the undoped SC, a single relaxation process with a divergent-like relaxation time at the spin density wave (SDW) transition is observed. From the relaxation time in the normal state, significantly above T _SDW, the second moment of the Eliashberg function is determined to be ?????? ²????135?meV². Below T _SDW, the temperature dependence of the photoexcited reflectivity transients indicates the appearance of a bottleneck due to opening of a SDW gap with a BCS-like temperature dependence and the characteristic magnitude, 2??_SDW/k _B T _SDW=7??3, at?4.2?K. In the superconducting SC, multiple relaxation processes are present. In addition to the relaxation processes observed in the normal state, a distinct superconducting state relaxation component is observed consistent with the presence of BCS T-dependent gaps.     

Observation of Re-entrant Resistance in NbN/NbO/Co Trilayer

Transport measurements on a niobium nitride (NbN) film covered with a ferromagnetic cobalt layer are reported here. The sample shows a superconducting transition (T _SC) at 6.5?K. In the superconducting state, a dip in resistance is observed at a temperature referred to as T _MIN. Below T _MIN, resistance reappears, with a magnitude of about 1% of the normal-state resistance. The observed resistance is found to decrease on increasing the applied current?(I). A?possible reason for re-entrant resistance might be the vortex dynamics in NbN superconducting layer due to the stray fields created by the Co layer. Further confirmation of the observed behavior is obtained from the plot of resistance versus probe current measured at various temperatures in close vicinity of?T _SC.     

Superconductivity in Itinerant Ferromagnetic Systems

We studied the possible superconducting state in an electronic itinerant ferromagnetic system characterized by a density of states that presents a moderately strong peak that is controlled by a specific parameter a and is positioned near the band edge. Specifically, we investigated the superconducting critical temperature, T _c , and the zero-temperature superconducting gap, ??₀. The analysis is done in a self-consistent way, the BCS mean-field equation being solved together with the electron density equation to trace possible changes in the system??s chemical potential due to the strong correlations between the component electrons. We discussed the density dependence of the superconducting critical temperature and zero-temperature superconducting gap for various values of the control parameter a and of the electron?Celectron attractive interaction. In the zero temperature limit we derive the system??s phase diagram and discuss the possible fermionic and bosonic regimes of the diagram as function of the strength of the attractive interaction.     

Intergrain Effects in the AC Susceptibility of?Polycrystalline LaFeAsO0.94F0.06

The AC susceptibility, ??, at zero DC magnetic field of a polycrystalline sample of LaFeAsO_0.94F_0.06 (T _c ??24?K) has been investigated as a function of the temperature, the amplitude of the AC magnetic field (in the range H _ac =0.003?Oe??4?Oe) and the frequency (in the range f=10?kHz??100?kHz). The ??(T) curve exhibits the typical two-step transition arising from the combined response of superconducting grains and intergranular weak-coupled medium. The intergranular part of ?? strongly depends on both the amplitude and the frequency of the AC driving field, from few Kelvin below T _c down to T=4.2?K. Our results show that, in the investigated sample, the intergrain critical current is not determined by pinning of Josephson vortices but by Josephson critical current across neighboring grains.     

Preparation and Characterization of Nanometer Grained High Temperature Superconducting High-Quality Epitaxial Bi-2223 Thin Films Grown by DC Sputtering

Bi₂Sr₂Ca₂Cu₃O_10+?? HTSC epitaxial thin films with thickness in the order of 6.0?nm were prepared onto (100) aligned SrTiO₃ single-crystal substrates by DC sputtering from stoichiometric targets. As-grown samples were characterized by X-ray diffraction, AC-susceptibility and scanning electron microscopy. X-ray diffraction patterns show that all obtained superconducting thin films were c-axis oriented with a Bi-2223 phase. All reflections (except the substrate ones) can be assigned to the (00l) reflections of the film material (h=k=0, l??0), indicating that the films were grown preferentially with the c-axis normal to the film plane. In order to investigate the crystal quality of these Bi-2223 films, the rocking curves of the (0012) peaks were explored by ??-scans. The rocking curve of the (0012) reflection had a full width at half maximum (FWHM) of 0.30??. This demonstrates that our prepared Bi-2223 thin films have good crystalline quality and high degree of c-axis orientation. The grain size has well known important effects in the magnetic, optical, and electrical properties of metals and alloys. High temperature superconducting thin films, obtained in this work, have nanometer grain size. The mean size of the grains of the samples were determined by X-ray diffraction (XRD) and found to be in the order of 34.8?nm. The superconducting transitions temperature of several Bi-2223 samples is about 103?K. Surface morphology of the films and chemical composition were studied using scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDX).     

Dependence of Superconductivity and Its Weakly Linked Behavior in Bulk LaO1?x F x FeAs on F Doping

Samples of oxypnictide compound LaO_1?x F _x FeAs, with x=0.15 and 0.2 corresponding to over- and highly over-doped compositions, respectively, were prepared by solid-state reaction. We present their characterization by XRD and HRTEM, as well as resistivity ??(T), magnetization M(B) and microwave modulated absorption (MMA) response between 4.2?C300?K and applied fields B=0?C8?T. With change in?x, both the superconducting and magnetic behavior of the samples shows an interesting pattern. The ??magnetic anomaly?? at T??130?K, observed in M(T) for x=0, instead of getting totally suppressed shows a tendency to reappear in x=0.2 sample. Both samples typically show ??(300?K)>2.8×10^?3????cm and critical current density J _c(5?K, 1?T)<2×10⁷?A/m². The superconducting transitions as measured by ??(T) at B=0 are found broad for both x=0.15 and 0.2 samples with transition widths ??2.5 and 6?K, respectively. The slope |dB _c2/dT| (where B _c2 is upper critical field) determined by resistive onsets, for the x=0.15 and 0.2 samples, has values ??7.5 and 3.5?T/K, respectively. The superconducting state characteristics as reflected by ??(T,B), M(T), magnetic J _c(B) and MMA response are typical of the presence of weakly linked inter-grain regions in both the samples. Our HRTEM images of the x=0.15 sample show the presence of high angle (??43^°) grain boundaries, which are well known to limit the J _c in cuprate-based high T _c bulk materials.     

Impact of Gd Doping on Morphology and Superconductivity of NbN Sputtered Thin Films

We report the effect of Gd inclusion in the NbN superconductor thin films. The films are deposited on single crystalline Silicon (100) by DC reactive sputtering technique, i.e., deposition of Nb and Gd in presence of reactive N₂ gas. The fabricated relatively thick films (400 nm) are crystallized in cubic structure. These films are characterized for their morphology, elemental analysis, and roughness by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDAX), and Atomic Force Microscopy (AFM) respectively. The optimized film (maximum T _c ) is achieved with gas ratio of Ar:N₂ (80:20) for both pristine and Gd-doped films. The optimized NbN film possesses T _c (R=0) in zero and 140 kOe fields are at 14.8 K and 8.8 K, respectively. The Gd-doped NbN film showed T _c (R=0) in zero and 130 kOe fields at 11.2 K and 6.8 K, respectively. The upper critical field H _c2(0) of the studied superconducting films is calculated from the magneto-transport [R(T)H] measurements using GL equations. It is found that Gd doping deteriorated the superconducting performance of NbN.     

Vortex Dynamics in YBa2Cu2O7?�� Films and Superlattices: A?Magnetization?Relaxation Study

The relaxation of the irreversible magnetization of optimally doped YBa₂Cu₃O_7??? (YBCO) films and YBa₂Cu₃O_7??? /PrBa₂Cu₃O_7??? (YBCO/PrBCO) superlattices in a magnetic field H oriented along the c axis was investigated over a large temperature T interval. The observed behavior is dominated by the presence of a crossover elastic vortex creep at low T?Cplastic creep at high T generated by the T-dependent current density J of the macroscopic currents induced in the specimen during experiments. Magnetization relaxation in the flux-creep annealing regime indicates the reduction of the creep crossover temperature with decreasing J, which suggests that in static conditions the disordered vortex phase at high H behaves like a plastically pinned vortex assembly, rather than an elastic vortex glass. The often reported scaling of the electric field E?Ctransport current density J characteristics in agreement with an elastic vortex glass?Cvortex fluid transition seems to originate from the creep crossover generated by an increasing J.     

Co-doping effects of Ca and Ce on the Superconducting Properties in Y1?x Ca x (Ba1?y Ce y )2Cu3O7?δ

A series of Y_1?x Ca _x (Ba_1?y Ce _y )₂Cu₃O_7??? (0??x??0.3, 0??y??0.3) polycrystalline superconductor samples were prepared using the solid-state reaction technique. The phase identification, crystal structure, and superconducting transition temperature (T _c ) were studied by means of X-ray diffraction (XRD) and resistivity measurements. The results indicted that the phase of samples changed from orthorhombic phase to tetragonal phase with increasing Ca concentration x and Ce concentration?y, and Ce did not form the superconducting structure. The lattice constants had a little change. The a-axis and c-axis lattice parameters increased. The b-axis lattice parameter decreased. The T _c and resistance had an obvious dropping tendency with increasing Ca and Ce concentrations. The transition width became sharper with the increase of x (=y). We drew a conclusion that the Ce-doping had an effect for strengthening the intergrain connectivity, and it counteracted the weakening effect of Ca-doping which introduced the hole causing a reduction in the interlayer coupling strength.     

Influence of a Finite Tc on Enhanced Superconductivity Near the Metal/Insulator Transition

We adapted the BCS equation for T _c so that it applies to disordered conductors near the metal/insulator transition (MIT). The resulting expression for T _c as a function of the conductivity accounted quite well for several disparate classes of superconducting materials near the MIT [disordered metals, oxide conductors (including high-T _c superconductors), semiconducting materials, organic conductors, and C₆₀]. This function, however, was applicable only to situations for which T _c was zero in the pure limit—which was the case for the data chosen. This paper extends the model to the nonzero case and compares the predictions to the appropriate data.     

Point-Contact Andreev-Reflection Spectroscopy in Fe-Based Superconductors: Multigap Superconductivity and Strong Electron–Boson Interaction

We performed point-contact Andreev-reflection measurements in Ba(Fe_1?x Co _x )₂As₂ single crystals (x=0.1, T _c =24.5?K) and SmFeAsO_1?x F _x polycrystals (x=0.2, T _c =52?K). The spectra indicate the presence of two superconducting gaps with no line nodes on the Fermi surface, but also feature additional structures related to the electron?Cboson interaction (EBI). From the spectra, it is possible to extract the characteristic energy ?? ₀ of the mediating boson. In Co-doped Ba-122, we obtain ?? ₀=12?meV that coincides with the spin-resonance energy observed in neutron-scattering experiments. In Sm-1111, ?? ₀=20?meV fulfils the relation ?? ₀=4.65k _B T _c inferred from neutron-scattering results on other Fe-based superconductors. The strong electron?Cboson coupling may also explain some anomalies in the PCAR conductance curves (e.g., the excess conductance at high energy) which sometimes prevent a good fit of the curves with models based on constant, BCS-like order parameters.     

Preparation and superconductivity of NbN powder by a vapour phase reaction

The formation of superconducting NbN powders by the vapour phase reaction of the NbCl₄-NH₃-H₂ system has been investigated. The properties of NbN powders changed with the reaction temperature, mixing temperature of NbCl₄ and NH₃, and gas composition ratio ([NH₃]/[NbCl₄]). The reaction system gave NbN powders consisting of particles with sizes less than 50 nm. The process of particle formation is discussed. The NbN powders produced had vacancies at both the Nb and N sublattices. The vacancies influenced the superconducting transition temperature (T _c) of NbN, the highestT _c observed being 14.1 K.     

The (k B T C/zJ,k) Phase Diagram for the D/J=1 on?the?Four-Dimensional Blume�CEmery�CGriffiths (BEG) Model

The 4-d spin-1 Ising (BEG) model has been simulated using a cellular automaton (CA) cooling algorithm improved from the 3-d BEG model algorithm for the four-dimensional hypercubic lattices. The ground state diagram (k,d) of the 4-d BEG model has ferromagnetic (F), quadruple (Q) and staggered quadruple (SQ) ordering regions. The simulations have been made in the ?3??k=K/J??1 interval through the d=1 line for L=18 lattice size. The (k _B T _C/zJ,k) phase diagram has been obtained through the d=1 phase line. The results show that the model has a bicritical point (BCP) as in d<4 dimensions. The phase diagram has been separated to the three regions of the staggered quadruple (SQ), the ferromagnetic (F) and the paramagnetic (P) cases.     

Superconducting Properties of Hg0.8Cu0.15Sb0.05Ba2Ca2Cu3O8+δ Ceramic with Controlling Sintering Conditions

Hg_0.8Cu_0.15Sb_0.05Ba₂Ca₂Cu₃O_8+?? High temperature superconducting samples were prepared by using a standard solid-state reaction method with different sintering temperatures of 1073, 1093, 1113, 1133 and 1153?K. The electrical resistivity measurements showed that the sintering conditions have major effects on the transition temperature of the superconducting compound. The samples prepared at 1113 and 1153?K have transition temperatures at zero resistivity T _c(offset)=107,112?K and T _c(onset)=121,128?K respectively. Meanwhile, the sample prepared at 1133 has highest transition temperature T _c(offset)=121?K and T _c(onset)=134?K. However, the samples which were prepared at 1073 and 1093?K showed metallic behavior with respect to temperature, but this could not help us to obtain the values of T _c(off) because they were less than the point of liquid nitrogen. The effect of sintering times on the transition temperature has been investigated by using the samples prepared at 1133 K. The value of T _c(offset) at the sintering times of 16?C24 hours rises from 121?K to 126?K. The XRD data collected from various samples were all polycrystalline. It was found that the increase of sintering temperatures from 1073?C1153?K caused an extremely high amount of decomposition of the low-phase (1212) superconductor and produced high-phase (1223) superconductors. The oxygen content (??), volume fraction V _Ph(1223) and the lattice parameters were changing with the increase of the sintering temperature.     

Quantitative analysis of lattice disorder and superconducting transition in epitaxial YBa2Cu3O6.9 films

We show that the average lattice disorder in YBa₂Cu₃O_6.9 films grown by ion-beam sputtering is homogeneous and can be quantified by introducing the lattice coherence lengthr _c that is extracted from the width of X-ray diffraction rocking curves. The superconducting properties of the films are correlated withr _c ∶T _c decreases with increasing disorder forT _c ?10 nm, while the width of the resistive transition and the normal-state resistivity increase.     

Influence of the Extra Layer on the Transport Properties of NdFeAsO1?0.14F0.14 and FeSe0.88 Superconductors from Magneto Dynamic Analysis

In this letter, the electromagnetic response of the NdFeAsO_1?0.14F_0.14 (T _c =49?K) superconductor system, characterized by FeAs and NdO alternating layers, has been compared with that of FeSe_0.88. We have studied the flux dynamics of these two systems by means of ac multi-harmonic magnetic susceptibility. The analysis shows that although characterized by larger thermal fluctuations due to its higher T _c , NdFeAsO_1?0.14F_0.14 exhibits a stronger pinning force relative to FeSe_0.88. The further Irreversibility Line (IL) analysis also points out that both superconductors have a 3D flux pinning behavior. We associate the stronger pinning force in the NdFeAsO_1?0.14F_0.14 structure to the presence of the extra NdO layer. Different pinning contributions can be associated to the structural stress associated to FeAs superconducting layers and/or to the Nd³⁺ ions magnetic moment (????3.6???_B) contribution on the flux cores. We will also show that these pinning are over imposed to a weak collective contribution due to the dopant F atoms.     

Effect of Annealing Time and Temperature on Microstructural and Superconducting Properties of (Bi,Pb)-2212 Thin Films Produced by Magnetron Reactive Sputtering

This study deals with the role of various annealing time (1?h, 2?h, 4?h, 6?h and 8?h) and temperature (840 and 850?°C) on the microstructural and superconducting properties of thin films with the aid of scanning electron microscopy (SEM), X-Ray analysis (XRD), electron dispersive X-Ray (EDX), resistivity and transport critical current density (J _c ) measurements. The T _c , J _c , variation of transition temperatures, hole-carrier concentration, grain size, phase purity, lattice parameter, surface morphology, element distribution, crystallinity and resistivity (at room temperature) values of the films prepared are compared with each other. Critical transition temperatures (T _c ) of the samples are deduced from the dc resistivity measurement while critical current density values are estimated from the critical current and total cross-sectional area values. It is found that maximum T _c of 79.7?K and J _c of 1520?A/cm² are observed for the film annealed at 840?°C for 6?h as against 54.9?K and 30?A/cm² (minimum values), respectively, for the film annealed at 840?°C for 4?h. Moreover, SEM images indicate that the former has the best crystallinity, grain connectivity and largest grain size. Based on these results, T _c and J _c values of the samples studied are found to depend strongly on the microstructure. Additionally, EDX results show that the elements used for the preparation of all the samples are observed to distribute homogeneously. As for the XRD results, all the samples exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines. According to the refinement of cell parameters done by considering the structural modulation, the largest lattice parameter a and c are obtained for the film annealed at 840?°C for 8?h. To sum up, the aim of the present study is not only to investigate the changes of microstructural and superconducting properties of the samples fabricated in the varied time and temperature but to determine the best ambient for the film fabrication and show the feasibility of obtaining Bi-2212 film with tailored structure, as well.     

Abnormal Behavior of the Angular Dependence of Resistivity in?YBCO?Thin Films

Careful investigation of the angular dependence of resistivity ??(??) (?? is the angle between the magnetic field and the ab-planes) as a function of the temperature within the superconducting transition in an applied magnetic field B up to 1 T for a series of YBa₂Cu₃O_7??? (YBCO) thin films revealed a large variation in the shape and width of the minimum displayed in the vicinity of ??=0??, from a flat to a very sharp behavior. The series of films studied included both optimally doped and underdoped samples of different T _c , critical current density?J _c , film thickness, and preparation techniques. ??(??) measured for B close to ??=0?? (B parallel to ab-planes) for both B??J and B??J (J is the applied current density) showed two classes of samples; class of samples where ??(??) is independent of the direction of B relative to J and the other class where ??(??) depends on the orientation B relative to?J. This unusual unique behavior motivated us to investigate its origin by looking at the scaling of ??(??) as a function of the reduced field. Scaling of ??(??)) with the reduced field B(?? ^?2cos?² ??+sin?²)^1/2 allowed a quantitative determination of the value of ?? (intrinsic anisotropy) which varies between 7 and 400, and is independent of film thickness and J _c . Analysis of the microstructure though XRD of the films studied showed that the anisotropy is related to microstrain of the films.     

Superconducting Single Photon Nanowire Detectors Development for IR and THz Applications

We present our progress in the development of superconducting single-photon detectors (SSPDs) based on meander-shaped nanowires made from few-nm-thick superconducting films. The SSPDs are operated at a temperature of 2–4.2 K (well below T _c ) being biased with a current very close to the nanowire critical current at the operation temperature. To date, the material of choice for SSPDs is niobium nitride (NbN). Developed NbN SSPDs are capable of single photon counting in the range from VIS to mid-IR (up to 6 μm) with a record low dark counts rate and record-high counting rate. The use of a material with a low transition temperature should shift the detectors sensitivity towards longer wavelengths. We present state-of-the art NbN SSPDs as well as the results of our recent approach to expand the developed SSPD technology by the use of superconducting materials with lower T _c , such as molybdenum rhenium (MoRe). MoRe SSPDs first were made and tested; a single photon response was obtained.