Thermoelectric power of Ba(Fe1−x Co x )2As2 (0 ≤ x ≤ 0.05) and Ba(Fe1−x Rh x )2As2 (0 ≤ x ≤ 0.171)

Temperature-dependent, in-plane, thermoelectric power data are presented for single crystals of Ba(Fe_1?x Co _x )₂As₂ (0?≤?x?≤?0.05) and Ba(Fe_1?x Rh _x )₂As₂ (0?≤?x?≤?0.171). Given that previous thermoelectric power and angle resolved photoemission spectroscopy studies of Ba(Fe_1?x Co _x )₂As₂ delineated a rather large Co-concentration range for Lifshitz transitions to occur, and the underdoped side of the phase diagram is poorly explored, new measurements of thermoelectric power on tightly spaced concentrations of Co, 0?≤?x?≤?0.05, were carried out. The data suggest evidence of a Lifshitz transition, but instead of a discontinuous jump in thermoelectric power in the range 0?≤?x?≤?0.05, a more gradual evolution in the S(T) plots as x is increased was observed. The thermoelectric power data of Ba(Fe_1?x Rh _x )₂As₂ show very similar behavior to that of Co substituted BaFe₂As₂. The previously outlined T–x phase diagrams for both systems are further confirmed by these thermoelectric power data.     

Transport properties and the anisotropy of Ba1 − x K x Fe2As2 single crystals in normal and superconducting states

The transport and superconducting properties of Ba_{1 ? x} K _x Fe₂As₂ single crystals with T _c ≈ 31 K were studied. Both in-plane and out-of-plane resistivity was measured by a modified Montgomery method. The in-plane resistivity is almost the same for all studied samples, unlike the out-of-plane resistivity, which differs considerably. We have found that the resistivity anisotropy γ = ρ _c /ρ _ab is almost independent of temperature and lies in the range 10–30 for the studied samples. This indicates the extrinsic nature of high out-of-plane resistivity, which may be due to the presence of flat defects along Fe-As layers in the samples. This statement is supported by comparatively small effective mass anisotropy, obtained from the upper critical field measurements, and from the observation of the so-called “Friedel transition,” which indicates the existence of some disorder in the samples in the c-direction.     

Critical properties of superconducting Ba1−xKxFe2As2

Magnetisation and magneto-resistance measurements have been carried out on superconducting Ba_1?xK_xFe₂As₂ samples with x = 0.40 and 0.50. From high field magnetization hysteresis measurements carried out in fields up to 16 T at 4.2 K and 20 K, the critical current density has been evaluated using the Bean critical state model. The J_C determined from the high field data is >10⁴ A/cm² at 4.2 K and 5 T. The superconducting transitions were also measured resistively in increasing applied magnetic fields up to 12 T. From the variation of the T_C onset with applied field, dH_C2/dT at T_C was obtained to be ?7.708 T/K and ?5.57 T/K in the samples with x = 0.40 and 0.50.     

Effect of disorder on the transport properties of the high-T c superconductor Nd2− x CexCuO4+δ

The temperature dependences ρ_ab(T) of Nd_2?x Ce_xCuO_4+δ single crystals with 0≤x≤0.20 are studied and analyzed on the basis of the concepts in the theory of disordered 2D systems. The results are compared with the data obtained for other copper-oxide HTSC. It is found that a transition to the superconducting state in the optimal doping region 0.14≤x≤0.18 occurs only in crystals with a fairly small degree of disorder (k _Fl≥2, where l is the mean free path). This transition is compatible with the weak 2D-localization mode as long as the localization radius is longer than the characteristic size of a Cooper pair. The superconducting transition temperature in the optimal doping region increases monotonically with the parameter k _Fl characterizing the degree of disorder in the crystal. The degradation of superconducting properties upon a further increase in the doping level (x>0.18) is apparently associated with a transition from 2D to 3D conductivity in the single crystal.     

Phase-incoherent superconducting pairs in the normal state of Ba(Fe(1-x)Co(x))₂As₂

The normal state properties of the recently discovered ferropnictide superconductors might hold the key to understanding their exotic superconductivity. Using point-contact spectroscopy we show that Andreev reflection between an epitaxial thin film of Ba(Fe(0.92)Co(0.08))?As? and a silver tip can be seen in the normal state of the film up to temperature T～1.3T(c), where T(c) is the critical temperature of the superconductor. Andreev reflection far above T(c) can be understood only when superconducting pairs arising from strong fluctuation of the phase of the complex superconducting order parameter exist in the normal state. Our results provide spectroscopic evidence of phase-incoherent superconducting pairs in the normal state of the ferropnictide superconductors.     

Single crystal growth and superconductivity of Ca(Fe1−x Co x )2As2

We report the single crystal growth of Ca(Fe_1?x Co _x )₂As₂ (0?≤?x?≤?0.082) from Sn flux. The temperature–composition phase diagram is mapped out based on the magnetic susceptibility and electrical transport measurements. The phase diagram of Ca(Fe_1?x Co _x )₂As₂ is qualitatively different from those of Sr and Ba; this could be due to both the charge doping and structural tuning effects associated with Co substitution.     

A semimetal model of the normal state magnetic susceptibility and transport properties of Ba(Fe1−xCox)2As2

A simple two-band 3D model of a semimetal is constructed to determine which normal state features of the Ba(Fe_1?xCo_x)₂As₂ superconductors can be qualitatively understood within this framework. The model is able to account in a semiquantitative fashion for the measured magnetic susceptibility, Hall, and Seebeck data, and the low temperature Sommerfeld coefficient for 0 < x < 0.3 with only three parameters for all x. The purpose of the model is not to fit the data but to provide a simple starting point for thinking about the physics of these interesting materials. Although many of the static magnetic properties, such as the increase of the magnetic susceptibility with temperature, are reproduced by the model, none of the spin-fluctuation dynamics are addressed. A general conclusion from the model is that the magnetic susceptibility of most semimetals should increase with temperatures.     

Effect of process parameters on the properties of plasma-sprayed superconducting Y1Ba2Cu3O7−x coatings

A study on the effect of powder, spray and coating parameters on superconducting properties of plasma-sprayed Y₁Ba₂Cu₃O_7–x coatings has been carried out. Optimised spray parameters to yield strong, adherent and dense coatings on many substrates including AISI 304 SS and alumina have been obtained. Coatings show superconductivity after oxygen annealing at 950°C for an hour. Coatings thinner than 100 m do not show zero resistivity down to 77 K due to weak links between granular superconductors. Coatings prepared with fine powder have superior superconducting properties, viz. higherT _c andJ _c values than coarse powder coatings. The choice of substrate material strongly affects theJ _c values.     

Effect of addition of planetary milled Gd-211 on the microstructures and superconducting properties of air-processed single grain Gd–Ba–Cu–O/Ag bulk superconductors

Planetary milling technique has been a very promising way to obtain bulk superconductors with very high critical current density, J_c, albeit a detail characterisation of milled secondary phase precursor powders in particular has not been reported to date. Hence we report systematic studies of the effect of addition of planetary milled Gd₂BaCuO₅ (Gd-211) on the final microstructures and superconducting properties of air-processed Gd–Ba–Cu–O/Ag bulk samples. Average size of Gd-211 precursor particles, which were planetary milled with 1.0 mm ZrO₂ beads, has been observed to decrease significantly from 1.03 μm to 0.52 μm with increasing milling duration. Besides the size distribution of milled Gd-211 was narrow compared to that of the reference powder. A small amount of Zr was detected, however, in the milled Gd-211 powder by the inductively coupled plasma-optical emission spectroscopy (ICP-OES) and its content was increased with increasing milling period, which led to an inhomogeneous bulk microstructure. Significantly, the average size of Gd-211 particles milled for 45 min has been observed to decrease from 0.73 μm to 0.48 μm without severe contamination of Zr when the diameter of the beads were reduced from 1.0 mm to 0.3 mm. Trapped magnetic field of single grain Gd–Ba–Cu–O/Ag bulk sample with 32 mm in diameter prepared from almost Zr free Gd-211 fine particles recorded over 1.5 T at 77 K, which was almost 1.3 times greater than that of the reference sample. Nevertheless the repulsive force of both bulk samples showed around 57 N at a gap of zero between the sample surface and SmCo₅ permanent magnet.     

Effect of Co 2 + content on the magnetic properties of Co x Fe 3 − x O 4 /SiO 2 nanocomposites

Non-stoichiometric Co_xFe_3???xO₄/SiO₂ (x = 0.8, 0.9, 1.0, 1.1) nanocomposites have been prepared by sol-gel method. The structure, morphology and magnetic properties of the obtained samples were characterized by X-ray diffraction, transmission electron microscopy, vibrating sample magnetometer and Mössbauer spectroscopy at room temperature. As the Co^2?+? content increases, the average particle size of the spherical Co_xFe_3???xO₄ in the samples decreases and the lattice constants increases. The hyperfine fields for both A- and B-site decrease, while the fraction of Co^2?+? occupying the A-site increases. Magnetization measurements show the saturation magnetization and coercivity of Co_xFe_3???xO₄/SiO₂ decrease with increasing Co^2?+? content. The decrease in magnetization results from the weakened A-B interactions between Fe^3?+?, and the change in coercivity can be related to the variation of Co^2?+? at B-site and the decreasing particle size.     

Effects of the spin–orbit coupling on magnetic and superconducting properties in iron-based superconductors

We analyze the magnetic properties through two-orbital Hubbard model with the spin–orbit coupling (SOC) interaction in the iron-based superconductors. With the help of the Ising approximation for the Hund’s coupling between the itinerant electrons and the localized spins, we give a self-consistent account of the various magnetic orders observed in pnictides and the pairing symmetry. We also calculate the local density of states (LDOS) of the vortex state when a magnetic field is applied. The LDOS without SOC shows no resonant peak at the vortex core center in the superconducting state, while it shows an obvious resonant peak when SOC is applied.     

Effect of Dy-Co on physical and magnetic properties of X-type hexaferrites (Ba2−xDyxCu2Fe28−yCoyO46)

Sol–gel method is used to make a sequence of Barium based “X-type hexagonal ferrite (X-HF)” Ba_2−xDy_xCu₂Fe_28 −yCo_yO₄₆. “X-type hexagonal ferrites” with concentrations of “x = 0.0, 0.02, 0.06, 0.1 and y = 0.0, 0.1, 0.3, 0.5” are taken and the substitution impact of trivalent Dy³⁺ and divalent Co²⁺ is observed on the physical and magnetic properties of X-HFs. The XRD result, the refinement of which is accomplished using CelRef software validates the existence of pure single phase in these ferrites. Morphological structure of the crystal grains is calculated using electron microscopy and it is found that the grain has varying size in the range of 0.75–1.001 mm. FTIR analysis is done with and without the sintering process to examine the changes relevant to the structure and the chemistry of the material and the phases existed in the material. Thermogravimetric analyzer is used to measure the TGA and DSC quantities. All FTIR, DSC, and TGA results show that they are in good harmony with the results outcomes from XRD. “Vibrating sample magnetometer (VSM)” is used to quantify the magnetic properties of the sample under observation. It is observed that with an increase in the concentration of Dy-Co, M_r (Remanence) value decreases this could be reasoned by spin canting effect. The value of coercivity (H_c) changes from 317 to 158 G which follows the inverse relation between grain size and coercivity. The future use of the material may be in the microwave absorption devices.     

Effects of neutron irradiation on superconducting properties of orthorhombic Y–Ba–Cu oxides

Abstract

We have studied the effects of fast neutron (E>0.1 MeV) irradiation at reactor (～ 360 K) and low (～ 20 K) temperatures on the superconducting properties of polycrystalline orthorhombic YBa₂Cu₃O_7?y . Measurements were made on the superconducting critical temperature T_c , critical current J_c , Meissner effect and magnetic field dependence of J_c . The T_c drops by an irradiation at reactor temperature and J_c increases with increasing fluence. On the other hand with the irradiation at low temperature, T_c rises and J_c increases. Results of observation of Meissner effect and the magnetic field dependence of J_c are consistent with the behavior of T_c and J_c .     

Doping – Dependent irreversible magnetic properties of Ba(Fe1−xCox)2As2 single crystals

We discuss the irreversible magnetic properties of self-flux grown Ba(Fe_1?xCo_x)₂As₂ single crystals for a wide range of concentrations covering the whole phase diagram from the underdoped to the overdoped regime, x = 0.038, 0.047, 0.058, 0.071, 0.074, 0.10, 0.106 and 0.118. Samples were characterized by a magneto-optical method and show excellent spatial uniformity of the superconducting state down to at least the micrometer scale. The in-plane properties are isotropic, as expected for the tetragonal symmetry, and the overall behavior closely follows classical Bean model of the critical state. The field-dependent magnetization exhibits second peak at a temperature and doping – dependent magnetic field, H_p(T, x). The evolution of this fishtail feature with doping is discussed. In particular we find that H_p, measured at the same reduced temperature for different x, is a unique monotonic function of the superconducting transition temperature, T_c(x), across all dopings. Magnetic relaxation is time-logarithmic and unusually fast. Similar to cuprates, there is an apparent crossover from collective elastic to plastic flux creep above H_p. At high fields, the field dependence of the relaxation rate becomes doping independent. We discuss our results in the framework of the weak collective pinning and show that vortex physics in iron-based pnictide crystals is much closer to high-T_c cuprates than to conventional s-wave (including MgB₂) superconductors.     

Characterization and magnetic properties of electrospun Co1−x Zn x Fe2O4 nanofibers

By the electrospinning and calcination techniques, we have prepared uniform nanofibers of Co_1−x Zn _x Fe₂O₄ (0.0≤x≤0.5) ferrites with diameters of 110–130 nm. The Co_1−x Zn _x Fe₂O₄ nanofibers are single-phase spinels and the lattice constant with Zn content deviates from the Vegard’s law for these Co_1−x Zn _x Fe₂O₄ nanofibers. The Co_1−x Zn _x Fe₂O₄ nanocrystal grains by which are built nanofibers increase with calcination temperature. Variations of coercivity and saturation magnetization with calcination temperature can be explained in terms of the grain-size (D) effect. The coercivity (H _c) of Co_0.5Zn_0.5Fe₂O₄ nanofibers varies as D ^0.65 and basically follows the predicted D ^2/3 dependence based on the random anisotropy model in a D range below the single-domain size around 40 nm. The saturation magnetization of Co_1−x Zn _x Fe₂O₄ nanofibers initially increases with increasing Zn content, reaches a maximum value at x=0.3 and then decreases with further increase of Zn content, while the coercivity exhibits a continuous reduction with the increase of Zn content.     

Thermal expansion and Grüneisen parameters of Ba(Fe(1-x)Co(x))2As2: a thermodynamic quest for quantum criticality

Thermal expansion data are used to study the uniaxial pressure dependence of the electronic-magnetic entropy of Ba(Fe(1-x)Co(x))2As2. Uniaxial pressure is found to be proportional to doping and, thus, also an appropriate tuning parameter in this system. Many of the features predicted to occur for a pressure-tuned quantum critical system, in which superconductivity is an emergent phase hiding the critical point, are observed. The electronic-magnetic Grüneisen parameters associated with the spin-density wave and superconducting transitions further demonstrate an intimate connection between both ordering phenomena.     

Magnetic properties of U (Fe x Al1−x )2 and U(Fe y Ni1−y )2 compounds

The results of magnetic measurements and ferromagnetic resonance studies performed on U(Fe _x Al_1–x )₂ and U(Fe _y Ni_1–y )₂ compounds over a large temperature range are reported. The saturation magnetization decreases nearly linearly when substituting Fe by Al or Ni. In the composition range x<0.84 and y<0.81, the compounds are Pauli paramagnets, except in the region with y0.10. For UNi₂ two types of magnetic behaviours are shown. This compound can be both a ferromagnet withT _c =23.5 K and a Pauli paramagnet, depending on the crystal structure. Above the Curie temperatures, the reciprocal susceptibility for the compounds with x>0.84 and y>0.81 obeys a temperature dependence of the formX=X _o+C(T-) ^–1. The effective iron moments decrease when substituting iron by nickel or aluminium. The ferromagnetic resonance measurements show that theg values are not composition-dependent. A linear variation of the mean iron magnetization with the exchange field is observed. Finally, the magnetic behaviour of iron in these compounds is analysed.     

Band gap control and photoluminescence properties of Ba(Co2x Ti1−x )O3 thin films prepared by Sol–gel method

This paper reports on the preparation, characterization and optical properties of transparent Ba(Co_2x Ti_1?x )O₃ (0 ≤ x ≤ 0.06) thin films prepared by sol–gel method and deposited on fused quartz substrate by spin-coating technique. Their formation is confirmed by X-ray diffraction patterns, energy dispersive X-ray spectrometry and Fourier transformed infrared measurements. Hitherto unreported near-band-gap photoluminescence in ultraviolet, at 378 nm (3.28 eV), of exciton origin is observed which remains unaffected with change in excitation wavelength from 320 to 350 nm. A weak defect emission appears in green region. For larger excitation wavelength, i.e., 488 nm, emission arising from localized states again occurs in green region but with lower energy. The occurrence of efficient violet–blue PL emission is related to ‘direct’ band gap and shallow levels with high optical band gap values. Analysis of band gap variation with dopant concentration, determined using Tauc’s plot assuming them both of ‘direct’ and ‘indirect’ nature, also indicates the ‘direct’ nature. Co⁺² ions as dopants promote a decrease of band gap of films linearly. Scanning electron micrographs show the granular and flakes-like surface growth. Atomic force microscopy images show the presence of ribbon-like nanostructured grains throughout the surface of the films which is smooth with small values of surface roughness.