Excess Conductivity Analysis of (Cu0.5Tl0.5)-1223 Substituted by Pr and La

Series of superconducting samples of type Cu_0.5Tl_0,5 Ba₂Ca_2?y R _y Cu₃O_10??? , where R=Pr and La with 0??y??0.20, were prepared in a sealed quartz tube via a single-step solid-state reaction technique. The prepared samples were characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), and electron dispersive X-ray (EDX). XRD studies indicated that the tetragonal structure of (Cu_0.5Tl_0.5)-1223 phase does not change by Pr or La-substitutions whereas the lattice parameters a and c do. The elemental compositions analysis, determined from EDX, indicated that both Pr and La were successfully introduced into the microstructure of (Cu_0.5Tl_0.5)-1223 phase. The electrical resistivity ??(T) was measured as a function of temperature using conventional dc four-probe technique. The fluctuation conductivity ????, above the superconducting transition temperature T _c, was analyzed as a function of temperature using Aslamazov and Larkin model. It exhibits four different fluctuation regions namely critical (cr), three-dimensional (3D), two-dimensional (2D), and short-wave (sw). The zero-temperature coherence length, effective layer thickness of the two-dimensional system and inter-layer coupling strength were estimated as a function of the substitution-content y. Furthermore, the thermodynamics critical field, lower critical magnetic field, upper critical magnetic field, critical current density and Fermi energy were calculated from the Ginzburg number. The data indicated that both Pr and La-substitutions have quite similar behaviors.     

Influence of Electron–Phonon Interaction on Superconducting State Parameters of MgB2

A three-square well model is employed for the three interactions namely, electron–acoustic phonon, electron–optical phonon, and Coulomb in the calculation of superconducting transition temperature (T _c) for layered structure MgB₂. The analytical solutions for the energy gap equation allow us to understand the relative interplay of these interactions. The values of the coupling strength and of the Coulomb interaction parameter indicate that the test material is in the intermediate coupling regime. The superconducting transition temperature of MgB₂ is estimated as 41 K for λ_ac ≈ 0.3, λ_op ≈ 0.1, and μ* ≈ 0.07. We suggest from these results that both the acoustic and optical phonons within the framework of a three-square well scheme consistently explains the effective electron–electron interaction leading to superconductivity in layered structure MgB₂.     

Predictions of Multiband s?? Strong-Coupling Eliashberg Theory Compared to Experimental Data in Iron Pnictides

The experimental critical temperatures and the gap values of the superconducting materials LaFeAsO_1?x F _x , SmFeAsO_1?x F _x and Ba_1?x K _x Fe₂As₂, which are exponent of the most important families (1111 and 122) of iron pnictides, can be reproduced by an effective s-wave three-band Eliashberg model in which the dominant coupling is in the interband channel and the order parameter is nodeless but undergoes a sign reversal between hole and electron bands (s?? symmetry). In particular, high values of the electron-boson coupling constants and small typical boson energies (in agreement with neutron diffraction experiments) are necessary to reproduce the exact T _c, the experimental gap values and their temperature dependence. The same parameters allow fitting the experimental temperature dependence of the upper critical field and predicting the presence of characteristic structures related to the electron?Cboson coupling in the Andreev-reflection spectra.     

Thermometry with a nearly temperature independent sensitivity using a normal-superconducting tunnel diode biased close to the superconducting gap

We analyze the temperature dependence of the tunneling current in a normal-insulator–superconductor (NIS) tunnel junction and find that such a junction can serve as a primary thermometer below the superconducting critical temperature T_c with a nearly temperature independent sensitivity, provided the junction is suitably biased close to the superconducting gap. Deviations from expected behavior when the superconducting electrode does not follow most simple s-wave BCS density of states, and possible applications are briefly discussed.     

Superconductivity in Hydrogen-rich Material: GeH<Subscript>4</Subscript>

The electronic properties, lattice dynamics, and electron–phonon coupling of the Cmmm phase of GeH₄ have been studied by first-principle calculations using density functional perturbation theory. The electronic band structure shows the Cmmm phase metallic nature. It is found strong electron phonon interaction, and the superconducting critical temperature, predicted by Allen–Dynes modified McMillan equation, is about 40 K at 20 GPa.     

Theory of <Emphasis Type="Italic">d</Emphasis>-Wave High Temperature Superconductivity in the Cuprates Involving Non-linear Lattice Modes

The transition mechanism in high temperature cuprate superconductors is an outstanding puzzle. A previous suggestion on the role of non-linear local lattice instability modes on the microscopic pairing mechanism in high temperature cuprate superconductors (Lee, J. Supercond. Nov. Magn. 23(3), 333; 2009) is re-examined to provide a viable mechanism for superconductivity in these cuprates via an unusual lattice vibration in which an electron is predominantly interacting with a non-linear Q ₂ mode of the oxygen clusters in the CuO₂ planes. It is shown that the interaction has explicit d-wave symmetry and leads to an indirect coupling of d-wave symmetry between electrons. As a follow-up of Lee (J. Supercond. Nov. Magn. 23(3), 333; 2009), in this paper, we report detailed derivation of the superconducting gap equation and numerical solutions for the transition temperature as inherently integrated into the so-called extended Hubbard model (EHM). A unique feature in the EHM is that the transition temperature has an inherent k-dependence. In addition, superconducting gap solutions are restrained to specific regions in the first Brillouin zone (1BZ). It is very feasible to expect that the EHM naturally inherits a huge parameter space in which experimentally measured results, such as the well-known superconducting dome and the phase diagram from electronic Raman scattering (Sacuto et al., Rep. Prog. Phys. 76(2), 022502; 2013) can be accommodated. The EHM model hence offers a viable venue to search for or confirm any signature in k-point-sensitive experimental measurements.     

Multicomponent Order Parameter in the YBa2Cu3O7−δ

We have analyzed the temperature dependencies of the superfluid density in YBa₂Cu₃O₇ along a- and b- crystallographic axes using the multicomponent order parameter for the superconducting gap. Estimated values of the gap components for the d-wave and the isotropic s-wave are Δ _d =29 meV and Δ _s =5 meV, correspondingly. Band structure parameters were taken accordingly ARPES and neutron scattering data.     

The description of superconductivity in terms of dielectric response function

A critical temperatureT _c of a superconducting transition is calculated for a rather general form of the electron-electron interaction. It is shown that even if both the energy and momentum dependence of the interaction is included, the equation determiningT _c coincides formally with the corresponding equation of the BCS theory. The kernel of this equation is a smooth real function of its variables; it is expressed through ρ(k, E), the spectral density of the inverse dielectric function of the system. The expression forT _c is written in terms of ρ(k, E); this enables us to analyze the dependence of the critical temperature on the properties of the metal in a normal state. Some simple models illustrating the results are considered, and a discussion of the limits onT _c is given.     

Excess Conductivity Studies in Zn0.95Mn0.05O and ZnO Added YBa2Cu3O y Superconductors

We report electrical conductivity fluctuation analyses on YBa₂Cu₃O _y (denoted as YBCO) granular samples added with nanosize ZnMnO (ZnMnO for brevity) and ZnO (30 nm) particles. Nanoparticles are added to the precursor powders during the final sintering cycle of a two-step preparation process. Phase analysis by X-ray diffraction and granular structure examination by transmission electron microscopy (TEM) were carried out. When ZnMnO and ZnO are added to the YBCO, the orthorhombic structure is maintained. TEM and energy dispersive X-ray spectroscopy analysis show the presence of inhomogeneities embedded in the superconducting matrix. The temperature dependence of electrical resistivity in zero magnetic field has been measured on free, 1 wt.% ZnMnO and 1 wt.% ZnO added samples and the effect of microscopic inhomogeneities in the paraconductivity region has been reported. Data about the dimensionality of the thermodynamic fluctuation are obtained by analyzing the excess of conductivity ???? as a function of the reduced temperature $\varepsilon =\ln(\frac{T}{T_{c}^{mf}}-1)$ on the basis of the Aslamazov?CLarkin theory. In the mean-field region a crossover from 3D to 2D was observed for each sample. 1D behavior of fluctuation conductivity was found at high temperatures (above the 2D regime) for nanoparticle added samples. The zero-temperature coherence length, the effective layer thickness of the two-dimensional system, the wire cross-sectional area for one-dimensional system, critical magnetic fields and critical current density are estimated. Superconducting parameters are affected by the nanoparticle additions.     

Non-ohmic Electrical Transport Properties Above the Critical Temperature in Optimally and Underdoped Superconducting YBa2Cu3O6+x

The resistivity and the Hall effect as a function of current density and temperature in the normal state of very thin films of YBa₂Cu₃O_6+x are measured with a fast pulsed-current technique. The in-plane longitudinal and transverse (Hall) conductivities are reduced in intense current densities up to several $\mathrm{MA\,cm}^{-2}$ . In optimally-doped samples, this non-ohmic effect is limited to temperatures below 100?K. In a moderately underdoped sample, however, the non-linearity extends to the temperature range from T _c =53?K to ??150?K. The non-linear part of the Hall conductivity does not scale with a critical exponent, thus excluding classical amplitude fluctuations of the superconducting order parameter as possible origin.     

Phase Diagram and Upper Critical Field of Homogeneously Disordered Epitaxial 3-Dimensional NbN Films

We report the evolution of superconducting properties with disorder, in 3-dimensional homogeneously disordered epitaxial NbN thin films. The effective disorder in NbN is controlled from moderately clean limit down to Anderson metal?Cinsulator transition by changing the deposition conditions. We propose a phase diagram for NbN in temperature-disorder plane. With increasing disorder, we observe that as k _F l??1 the superconducting transition temperature (T _c ) and normal state conductivity in the limit T??0 (?? ₀) go to zero. The phase diagram shows that in homogeneously disordered 3-D NbN films, the metal?Cinsulator transition and the superconductor?Cinsulator transition occur at a single quantum critical point, k _F l??1.     

Investigation of the Phase Diagrams of Small Superconducting Squares of Al

An isolated superconductor with its size comparable to or smaller than its zero temperature superconducting coherence length should not host Abrikosov vortices, but may in principle still feature spatial variation in the superconducting order parameter. We report low-temperature electrical transport measurements on simply connected filled Al squares prepared by e-beam lithography featuring a lateral sample size ranging from 126 nm to 616 nm, with their zero-temperature superconducting coherence lengths, ξ(0), ranging from ～88–111 nm. We found that phase diagrams for these samples in the magnetic field and temperature space exhibit the expected features for vortex admission in large devices, but no vortex admission when the size becomes comparable to ξ(0). In the smallest samples we observe steps in the phase diagram that may be attributable to the effects of the measurement leads.     

On the high-temperature superconductivity of Srx La 2−xCuO4−δ

The superconducting properties of Sr_xLa_2?xCuO_4?δ are analyzed by employing the model of interaction between charge carriers and the oxygen-displacive modes of the lattice. The role played by the hole concentration, as related to the Sr content x and oxygen deficiencyδ, is thoroughly emphasized. The critical temperature, the ratio of twice the superconducting gap to the critical temperature, and the isotope shift are estimated as functions of the hole concentration and compared with the experimental data.     

Superconducting Properties of NdO<Subscript>0.5</Subscript><Emphasis Type="Bold">F</Emphasis><Subscript>0.5</Subscript>BiS<Subscript>2</Subscript> Single Crystals

We report on superconducting properties of high-quality single crystals of F-substituted NdOBiS₂ using low-temperature magnetization and transport measurements. Using the mixture of CsCl and KCl as the flux, we have synthesized our single crystals. This compound exhibits bulk superconductivity with a transition temperature of about T _c～4.6 K. The critical current density J _c as a function of temperature has been derived and decreases with the increasing temperature. We construct the phase diagram H _c2(T). The zero-temperature value for \(H_{\mathrm {c2}}^{B\parallel c}\) for value for \(T_{c}^{90~\%}\) and \(T_{c}^{0~\%}\) is estimated to be approximately 2.17 and 1.72 T respectively by using Werthamer-Helfand-Hohenberg model.     

Unusual Commensurability Effect in Superconducting Sn Films with Triangular Lattice of Microholes

The interaction of the magnetic field with superconducting Sn perforated films prepared by electron beam lithography was studied. Shift of the peaks in the critical current I _c from integral multiples of the matching field was observed. An approach based on existence of the excited vortex lattice states is proposed to explain the effect. Some energetically unfavorable vortex states are stable in the structures made of superconducting films with strong interaction of the vortices with defect in the films. By using the approach it is possible to explain the unusual matching effect, and other results obtained in the present paper.     

Determination of Superconducting Parameters of GdBa2Cu3O7?δ Added with Nanosized Ferrite CoFe2O4 from Excess Conductivity Analysis

Superconductor samples of the type (CoFe₂O₄) _x GdBa₂Cu₃O_7??? , 0.0??x??0.1?wt.%, were synthesized by the conventional solid-state reaction technique and were characterized using X-ray powder diffraction (XRD) and scanning electron microscope (SEM). XRD analysis indicated that the orthorhombic structure of Gd-123 is not affected by nanosized ferrite CoFe₂O₄ addition, whereas the volume fraction of Gd-123 increased up to x=0.01?wt.%. Excess conductivity analysis of the investigated samples was analyzed as a function of temperature using the Aslamazov and Larkin (AL) model. It exhibited four different fluctuation regions, namely critical (cr), three-dimensional (3D), two-dimensional (2D), and short-wave (sw). The zero-temperature coherence length along c-axis, effective layer thickness of the two-dimensional system, and inter-layer coupling strength were estimated as functions of nanosized ferrite CoFe₂O₄ concentration. In addition, the thermodynamics, lower and upper critical magnetic fields, and critical current density were calculated from the Ginzburg number. It was found that the low concentration of nanosized ferrite CoFe₂O₄ addition up to x=0.01?wt.% improved the physical properties of Gd-123, while for x>0.01?wt.%, these properties were deteriorated.     

Coexistence of Superconductivity and Spin Density Wave (SDW) in Ferropnictide Ba<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>K<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript>

This work focuses on the theoretical investigation of the coexistence of superconductivity and spin density wave (SDW) in ferropnictide Ba_1?x K _x Fe₂As₂. By developing a model Hamiltonian for the system and by using quantum field theory Green’s function formalism, we have obtained mathematical expressions for superconducting transition temperature (T _C), spin density wave transition temperature (T _sdw), superconductivity order parameter (Δ_Sc), and spin density wave order parameter (Δ_sdw). By employing the experimental and theoretical values of the parameters in the obtained expressions, phase diagrams of superconducting transition temperature (T _C) versus superconducting order parameter (Δ_Sc) and spin density wave transition temperature (T _sdw), versus spin density wave order parameter (Δ_sdw) have been plotted. By combining the two phase diagrams, we have demonstrated the possible coexistence of superconductivity and spin density wave (SDW) in ferropnictide Ba_1?x K _x Fe₂As₂.     

Dynamical Structural Instabilities in La1.9Sr0.1CuO4 Under Intense Laser Photoexcitation

The femtosecond timescale photoinduced reflectivity response in La_1.9Sr_0.1CuO₄ under intense laser photoexcitation is reported as a function of intensity and temperature. We observe saturation of the response, which corresponds to the destruction of both superconducting gap and the pseudogap. We find that the saturation thresholds scale approximately as ?? ², where ??=?? _S for the superconducting gap and ??=?? _P for the pseudogap where ?? _P /?? _S ??10. The pseudogap destruction threshold correlates with the onset of a structural phase transition measured with time-resolved electron diffraction.     

Phase Fluctuations of s-Wave Superconductors on a Lattice

Based on an attractive U Hubbard model on a lattice with up to second neighbor hopping we derive an effective Hamiltonian for phase fluctuations. The superconducting gap is assumed to have s-wave symmetry. The effective Hamiltonian we finally arrive at is of the extended XY type. While it correctly reduces to a simple XY in the continuum limit, in the general case, it contains higher neighbor interaction in spin space. An important feature of our Hamiltonian is that it gives a much larger fluctuation region between the Berezinskii–Kosterlitz–Thouless transition temperature identified with T _c for superconducting and the mean field transition temperature identified with the pseudogap temperature.