Superconducting gap symmetry in the superconductor BaFe1.9Ni0.1As2
We report on the Andreev spectroscopy and specific heat of high-quality single crystals of BaFe1.9Ni0.1As2.
The intrinsic multiple Andreev reflection spectroscopy reveals two anisotropic superconducting gaps L ≈
3.2−4.5 meV, S ≈ 1.2−1.6 meV (the ranges correspond to the minimum and maximum value of the coupling
energy in the kxky plane). The 25%−30% anisotropy shows the absence of nodes in the superconducting gaps.
Using a two-band model with s-wave-like gaps L ≈ 3.2 meV and S ≈ 1.6 meV, the temperature dependence of the electronic specific heat can be well described. A linear magnetic field dependence of the low-temperature specific heat offers further support of s-wave type of the order parameter. We find that a d-wave or single-gap BCS theory under the weak-coupling approach cannot describe our experiments.
Iron-based superconductors, just after their discovery in 2008, have become a subject of great interest for the scientific community and occupy one of the leading places among the most topical subjects in contemporary solid-state physics. The present development of investigations of iron-containing superconductors can be compared perhaps with the great efforts to study properties of cuprate high-temperature superconductors (HTSCs) in the first years after their discovery. At present more than one hundred Fe-based superconductors of different compositions have been found. These compounds represent quite a new class of superconducting materials with crystal lattice containing ions of 3d metals (Fe, Co, Ni) well known as ferromagnetic metals. Therefore, a priori, other mechanism of superconducting pairing in Fe-based superconductors, different from the traditional electron±phonon coupling, cannot be ruled out. A characteristic feature of all iron-containing superconduc- tors is the presence in their crystal structure of FeAs layers in the case of pnictides or FeSe layers in the case of chalcogenides. At present, the maximum critical temperature Tc of the superconducting transition in Fe-based super- conductors reaches 56 K (in a Gd1-xThxFeAsO compound), which is inferior only to Tc of cuprate HTSCs. This circumstance undoubtedly makes it possible to place ironbased superconductors in the class of HTSCs.
We report on successful fabrication of superconducting FeSe wire using hot gas extrusion (HGE) ex-situ PIT (powder in tube) method. Length of the obtained wire was about 60cm with cross-sectional superconducting core area nearly 2.5x 10^-3 cm^2. For the wire sample we observed superconducting transition temperature, T_onset = 11 K, about 1.2K lower compared to the preliminary prepared FeSe powder. Heat treatment in argon atmosphere at 350C resulted in transition width decrease from \Delta T(10% - 90%) = 1.75K in sample without heat treatment down to \Delta T = 0.9 K in annealed samples. Estimated derivative of the upper critical eld as a function of temperature of the sample annealed during 72h in argon atmosphere at 350C is dHc2/dT =2.9 T/K. Applying WHH theory to our data allows to define Hc2(0K) = 0.69Tc x(dHc2/dT )= 19.8T. The untreated wire shows critical current density, Jc = 75 A/cm2 at T=4.0 K in zero field. Increasing annealing time up to 72 hours at 350C in argon atmosphere gives rise to Jc increase of about 60% approaching 120 A/cm2 at T=4.0K and H=0T . Also Jc measurements were made in magnetic fields up to 9T. Our results show applicability of the HGE PIT method for fabrication of superconducting wires based on FeSe compound. Long-range heat treatment is necessary to improve superconducting properties of the samples.
We report on comparative study of magnetic phase diagram and critical current of the hole- and electron-doped BaFe2As2 single crystals with close values of superconducting critical temperature, Tc, (slightly underdoped Ba0.64K0.36Fe2As2 with Tc=25K and optimally doped BaFe1.9Ni0.1As2 with Tc=20K) obtained from measurements of the temperature dependence of ac-susceptibility and isothermal irreversible magnetization loops, M(H), in magnetic fields parallel to the c-axis of the crystal. From ac-susceptibility measurements we get estimation of a slope of the upper critical eld, Hc2, in dependence on temperature, dHc2/dT =- 4.2T/K for BaFe1:9Ni0:1As2 single crystal and dHc2/dT =-1.75T/K for Ba0.64K0.36Fe2As2 sample that in accordance with Werthamer, Helfand, and Hohenberg (WHH) model gives Hc2(0) = 0.69Tc((dHc2)/dT) = 56T for BaFe1.9Ni0.1As2 sample and lower value of Hc2(0) = 31T for Ba0.64K0.36Fe2As2 crystal. However, obtained from M(H) measurements temperature dependence of the irreversibility field, Hirr(T), for BaFe1.9Ni0.1As2 crystal located below the one for Ba0.64K0.36Fe2As2 crystal. Furthermore, at T=4.2K and higher temperatures our results for critical current density, Jc, calculated from M(H) curves clearly show slower reduction of Jc with increasing field for even underdoped Ba0.64K0.36Fe2As2 sample compared to optimally doped BaFe1.9Ni0.1As2 crystal demonstrating higher capacity of K-doped 122 compounds for production of superconducting cables and wires with high critical current in strong magnetic fields.
The dynamics of a two-component Davydov-Scott (DS) soliton with a small mismatch of the initial location or velocity of the high-frequency (HF) component was investigated within the framework of the Zakharov-type system of two coupled equations for the HF and low-frequency (LF) fields. In this system, the HF field is described by the linear Schrödinger equation with the potential generated by the LF component varying in time and space. The LF component in this system is described by the Korteweg-de Vries equation with a term of quadratic influence of the HF field on the LF field. The frequency of the DS soliton`s component oscillation was found analytically using the balance equation. The perturbed DS soliton was shown to be stable. The analytical results were confirmed by numerical simulations.
Radiation conditions are described for various space regions, radiation-induced effects in spacecraft materials and equipment components are considered and information on theoretical, computational, and experimental methods for studying radiation effects are presented. The peculiarities of radiation effects on nanostructures and some problems related to modeling and radiation testing of such structures are considered.
This volume presents new results in the study and optimization of information transmission models in telecommunication networks using different approaches, mainly based on theiries of queueing systems and queueing networks .
The paper provides a number of proposed draft operational guidelines for technology measurement and includes a number of tentative technology definitions to be used for statistical purposes, principles for identification and classification of potentially growing technology areas, suggestions on the survey strategies and indicators. These are the key components of an internationally harmonized framework for collecting and interpreting technology data that would need to be further developed through a broader consultation process. A summary of definitions of technology already available in OECD manuals and the stocktaking results are provided in the Annex section.