Superconducting Parameters Research Articles

Nonlocal current-field relation and the vortex-state magnetic properties ofYNi2B2C

The equilibrium magnetization M of single crystal ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ superconductor has been studied with magnetic field $H\ensuremath{\parallel}c$ axis. The material is clean, with low electrical resistivity and weak magnetic hysteresis (for $H=10\mathrm{kG},$ the critical current density is less than ${10}^{\ensuremath{-}6}$ of the depairing current density). The magnetization $M(H)$ deviates from conventional London predictions, but is well described by recent nonlocal London theory, with well-behaved superconductive parameters. The temperature dependence of the London penetration depth $\ensuremath{\lambda}(T)$ and other parameters are deduced.

A pseudopotential approach to the superconducting state properties of metallic glass

The superconducting state properties of the metallic glass have been investigated in the BCS - Eliashberg - McMillan framework by extending this theory to the binary metal glasses. Pseudo ions with average properties have been considered to replace both types of ions in the system. Values of the superconducting state parameters, namely electron - phonon coupling strength , Coloumb pseudopotential , transition temperature , isotope effect exponent and interaction strength have been worked out using Ashcroft's potential and the linear potential due to Sharma and Kachhava along with six different forms of dielectric screening. The form factors directly obtained from the screened pseudopotential of Veljkovic and Slavic have also been used to explicitly observe the effect of the dielectric screening on and through . The results obtained established the presence of a superconducting phase in glass.

Magnetic phase diagram of anisotropic superconductor 2H-NbSe 2

We report the magnetic phase diagram of a clean single crystal of 2H-NbSe 2 and present an update on the determination of its intrinsic superconducting parameters.

Synthesis, structure, and properties of the high-temperature superconductorHgBa2CuO4+δ

We report here the attempted synthesis of (${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cu}}_{\mathrm{y}}$)${\mathrm{Ba}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$ (0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.2, 0\ensuremath{\leqslant}y\ensuremath{\leqslant}0.2, y\ensuremath{\leqslant}x) and the results of a detailed characterization of the structural and superconducting properties of the resulting samples. Rietveld refinement of powder x-ray-diffraction data collected on all 15 samples synthesized found no partial Cu occupancy of the Hg site. We therefore conclude that for these samples no solid solution exists and the superconducting phase is simply ${\mathrm{HgBa}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$. Only one interstitial oxygen position O(3) was found, located exactly at the center of the Hg plane. An analysis of the structural refinements has produced a number of correlations between the exact location of the ${\mathrm{Ba}}^{2+}$ cation within the unit cell and a number of important superconducting parameters; these include the occupancy of the O(3) site, the extent of oxidation of the ${\mathrm{CuO}}_{2}$ planes, and the magnetic penetration depth.

Synthesis and superconducting properties of aRb3C60single crystal

A pristine ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ single crystal with larger than millimeter dimensions was prepared by reaction of rubidium vapor with a seed-grown ${\mathrm{C}}_{60}$ single crystal (which exhibited remarkably sharp x-ray-diffraction peaks). A narrow superconducting transition width \ensuremath{\Delta}${\mathrm{T}}_{\mathrm{c}}$ (5--85% diamagnetic shielding) of the ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ single crystal is less than 1 K. The onset of the transition was at a temperature, ${\mathrm{T}}_{\mathrm{c}}$=30.0tK. Measurements of temperature dependence of the zero-field-cooled and field-cooled dc magnetization (using ten applied magnetic fields up to 5.0 T) are reported here. These and magnetization vs field data at different temperatures (ranging from 2 to 29 K) have been used to determine thermodynamic superconducting parameters, including the lower [${\mathrm{H}}_{\mathrm{c}1}$(T)] and upper [${\mathrm{H}}_{\mathrm{c}2}$(T)] critical fields, the Ginsburg-Landau coherence length \ensuremath{\xi}, and the London or Ginzburg-Landau penetration depth, \ensuremath{\lambda}. These are significantly different from those extracted from similar data in the literature for powder specimens. Hysteresis and magnetic relaxation measurements have been used to determine the nominal critical current density ${\mathrm{J}}_{\mathrm{c}}$(H,T) and the effective-pinning potential ${\mathrm{U}}_{\mathrm{eff}}$(J,H), respectively.

Superconducting state parameters of metallic glass Mg70Zn30 using linearized screened pseudopotential

A linearized form for the screened form factors of electron-phonon interaction in the metallic glass Mg70Zn30 is applied for the first time to predict the superconducting state parameters viz. electron-phonon coupling strength (λ), Coulomb pseudopotential (μ*), transition temperature (T c), isotope effect exponent (α) and the interaction strength (N 0 V). Computed results agree with the experimental data available in the literature.

Scanning Tunneling Spectroscopy Sensitive to Layer Structure of BSCCO

Scanning tunneling microscopy images and scanning tunneling spectroscopy characteristics were measured at 4.2 K in liquid helium bath on the cleaved in air α —b surface of Bi2Sr2CaCu2O8 (BSCCO-2212). Electronic densities of states and superconductivity parameters 6 and r evaluated from dl/dV characteristics depend on tip—sample distance s: with shortening of the distance s superconducting gap structure becomes more distinct, i.e. 6 increases and F decreases. We explain this phenomenon as a non-vacuum tunneling, where for longer s tunneling electrons reach only the surface contamination layer on non-metallic BiO top-surface layer, whereas for shorter s tunneling electrons penetrate also deeper lying CuO layers reflecting their superconducting properties. The dependence of a on s is evaluated. This result allows to understand better the non-vacuum scanning tunneling microscopy imaging: by adjusting properly the tip—sample distance one can select suitable local density of states contributing dominantly to the scanning tunneling microscopy images taken on BSCCO.

The analysis of tunneling characteristics measured on Bi 2 Sr 2 CaCu 2 O 8 surface

Cleaved in air a-b surface of Bi2Sr2CaCu2O8 (BSCCO-2212) was measured by means of STM and STS at 4.2 K in liquid hellium bath. From fitting experimental conductivity curves by Dynes' function two superconductivity parameters Δ (gap value) and Γ (smearing parameter) were obtained. The shape of gap structure superimposed on dI/dV characteristics depends on tip-sample distance, what is expressed by the increase of Δ and decrease of Γ with shortening of s. The phenomenon of becoming gap structure more distinct when approaching the tunneling tip to the surface is explained by us as the non-vacuum tunneling, where the surface contamination layer on non-metallic BiO top-surface layer strongly influences the tunneling process. Only for s short enough tunneling electrons penetrate to deeper situated CuO layers and reflect their superconducting behaviour. Non-vacuum STM images are therefore sensitive to the tip-sample distance adjustment. The dependence of gap parameters on lateral position of the tip above the sample can also occur. In such cases STS enables to state which elements of the image belong to the topography of the surface and which to its electron density of states.

Coherence lengths and magnetic penetration depths in YBa2Cu3O7 and YBa2Cu4O8 superconductors

The superconducting state parameters of the high-Tc layered structure YBa2Cu3O7 (Y123) and YBa2Cu4O8 (Y124) systems have been evaluated. The two-dimensional (2D) CuO2 planes as well as one-dimensional (1D) CuO chains are the important features of these systems. The anisotropy of the layered structure is well reflected in the shape of open Fermi surface. The carrier effective mass (m*) and the charge carrier density (nv) are deduced from the Fermi velocity and the Hall effect coefficient, respectively, following the electronic energy band structure studies (EEBS) within the local approximation to the density functional theory. Then the anisotropic magnetic penetration depth (λL) as well as the coherence length (ξ) are estimated. The analysis yields smaller values of (λL) than those revealed from penetration depth measurements in Y124 while the (λL) values appear higher in Y123 when compared with the muon spin relaxation (μSR) and radio frequency surface impedance (RSI) measurements. The deduced values of coherence lengths and its anisotropy are consistent with the reported data. The temperature dependence of the in-plane magnetic penetration depth and the Ginzburg-Landau parameter predicts that they are of a similar nature as those revealed from experiments. The results on lower and upper critical magnetic fields are also presented. It is found, with a simple analysis of the superconducting state, that the parameters of YBCO (124 and 123) based on EEBS are consistent but are not in good agreement with experiments. Furthermore, the transport parameters (m*,nv) as obtained from the Fermi Liquid description are used to determine the in-plane magnetic properties. This technique permits a reasonably good agreement with recently published data. The implications of the above analysis are discussed.

Anisotropic superconducting state parameters of Nd-Ce-CuO and La-Sr-CuO systems

Major superconducting state parameter of high-Tc electron-doped Nd-Ce-CuO and hole-doped La-Sr-CuO systems have been estimated. The analysis is based on the electronic energy band structure (EEBS) within the local density approximation. Anisotropy of the layered structure is well reflected in the shape of open Fermi surface. Determination of the effective mass of the charge carrier from the Fermi velocity using EEBS and estimates of the anisotropic superconducting state parameters, particularly the magnetic penetration depth (λL), lead to smaller values than those observed by muon spin relaxation (μSR) measurements on polycrystalline samples. The coherence length (ξ∥ andξ⊥) is evaluated and appears to be higher. The temperature dependence of the Ginzburg-Landau parameter (K) and along the plane magnetic penetration depth (λ L ∥ ) shows the same nature as those revealed from experiments. Furthermore, results on lower and upper critical magnetic field are also presented. The results based on EEBS studies are consistent but do not agree well with experiments. On the other hand, if the Fermi Liquid approach (FLA) parameters are used for this analysis, the results are in better agreement with those reported earlier. The implications of the above investigations are discussed.

A new metastable superconductor Li 2Na 1N yC 60 prepared from azides

A new metastable superconductor with nominal composition of Li 2Na 1N y C 60 ( y unknown) is prepared by thermal decomposition of lithium azide (LiN 3) and sodium azide (NaN 3). It is suggested that the superconducting phase in this compound is created by intercalation of the neutral spacer such as NaN 3. The supeconductivity is examined by a low-field signal of microwave absorption (LFS) and by SQUID. The critical temperature is estimated to be 12 K from SQUID and 14 K from LFS measurement, and the shielding fraction is over 9% at 5 K. Superconducting parameters, such as critical temperature, upper and lower critical fields, coherence length and penetration depth are evaluated and found to be distinct from those in other fulleride superconductors.

Gravitational Paramagnetism, Diamagnetism and Gravitational Superconductivity

In the weak field approximation to the gravitational field equations, we study gravitational paramagnetism and diamagnetism, the gravitational Meissner effect and gravitational superconductivity. The spontaneous symmetry breaking corresponds to crossing from closed geodesics to open ones, and to the existence of a critical temperature in the frame of a gauge model at finite temperature. In this later case one can obtain expressions giving the dependence of several superconducting parameters on temperature.

Evolution of Superconducting and Normal-State Properties in C60“Polymer” Doped with K

Abstract Abstract: Bulk superconductivity was observed in a new iodine-treated aggregated form of solid C60, believed to be a “polymer”, by doping with alkali metal. Evolution of superconducting (Tc = 17.2 K) and normal-state properties, has been studied as a function of doping with K. The normal-state dc susceptibility exhibited a complex behavior with the increasing K uptake, evolving from an enhanced paramagnetism at low doping level, to a weakly temperature-dependent diamagnetism at the optimum doping level for superconductivity, and ending with a strong temperature-independent diamagnetism in over-doped samples. The measured superconductivity parameters, such as Tc, London penetration depth and the Ginzburg-Landau coherence length, are compared with those of the monomer K3C60. and the main factors determining the differences between the two systems are discussed.

Microscopic theory of single particle and pair condensation of an attracting bose gas as the basis for superfluidity and superconductivity

A consistent and unified microscopic theory of superfluidity and superconductivity is developed on the basis of two-stage Fermi-Bose-liquid (FBL) (in particular case, one-stage Bose-liquid) scenarios. It is shown that these phase transition scenarios is accompanied, as a rule, by the formation of composite bosons (Cooper pair and bipolarons) with their subsequent single particle (SPC) and pair condensation (PC). A brief outline of the modified and generalized BCS-like pairing theory of fermions is presented. In an analogy to that, a detailed boson pairing theory is developed. The SPC and PC features of an attracting 3d- and 2d-BG as a function of the interboson coupling constant in the complete range 0≤T≤T B is studied in detail. It is argued that the coexistence of the order parameters of attracting fermions ΔF and bosons ΔB leads to the superfluidity (in3He) and superconductivity (in superconductors) by two FBL scenarios. One of these scenarios is realized in the so-called fermion superconductors (FSC) and the other in the boson superconductors (BSC) in which the gapless superconductivity is caused by the absence of the gap ΔSF in the excitation spectrum of bosons and not by the presence of point or line nodes of the BCS-like gap ΔF. The new adequate definitions for basic superconducting parameters of FSC and BSC are given. The theory proposed is consistent with the experimental data available.

On the normal and superconducting properties of Ba6C60

From SQUID measurements the authors determine the basic critical magnetic field values for bcc phase Ba{sub 6}C{sub 60} superconductor and hence evaluate the major parameters of superconductivity in this material. A comparative analyses of these parameters observed for the alkaline-earth-doped vs. alkali-metal-doped C60 superconductors is presented. These two systems are found to differ substantially in their superconducting properties: for Ba{sub 6}C{sub 60} the authors find that the upper critical field H{sub c2} {approx} 2 T, and the Ginzburg-Landau parameters k {approx} 15, as compared to the values 50 T and 100, respectively, for superconducting K{sub 3}C{sub 60}. On the other hand, the electronic properties for all the superconducting fullerenes are rather similar with, for example, the Fermi energy E{sub F} {approximately} 0.1 eV.

Thermodynamic parameters of HgBa2Ca2Cu3O8-x from high-field magnetization.

We have measured the temperature dependence of reversible magnetization of grain-aligned ${\mathrm{HgBa}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{8\mathrm{\ensuremath{-}}\mathit{x}}$ high-${\mathit{T}}_{\mathit{c}}$ superconductors with external magnetic field parallel to the c axis. Our data show the crossover of the magnetization below the superconducting onset temperature, which indicates a dominant vortex fluctuation. Except this vortex fluctuation region, reversible magnetization data could be described by using Hao and Clem's model. From the analysis magnetization curve based on this model, the thermodynamic critical field ${\mathit{H}}_{\mathit{c}}$(T) and Ginzburg-Landau parameter \ensuremath{\kappa}=118 were extracted. Also, we obtained the superconducting parameters; the penetration depth ${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$(0)=2060 \AA{}, coherence length ${\ensuremath{\xi}}_{\mathit{a}\mathit{b}}$(0)=18 \AA{}, and the zero-temperature upper critical field ${\mathit{H}}_{\mathit{c}2}$(0)=108 T.

Superconductivity indications of the two-chain Hubbard model due to the two-band effect

Corroborating our assertion worked out in preceding papers that the two-chain Hubbard model has a superconducting (SC) parameter region, we report new results obtained by the Lanczos diagonalization method. The SC region indicated by SC pair correlation functions show the tendency to converge to a considerable definite region in the parameter space with increase of the system size. A certain SC correlation function reveals that the SC order parameters pertinent to the two bands have different signs. The SC pair operators at two rungs keep strong spatial correlation in our SC region in striking contrast to the non-SC region. Calculation of correlation exponent K ϱ allows us to get an SC parameter region which proves to be in a qualitative agreement with the above-mentioned one.

Anisotropic superconducting parameters from reversible magnetization measurements: The dramatic effects of a distribution of T c's in ideal vs modified single-crystal YBa 2Cu 3O 7− δ

We report the effects of a distribution of T c 's on the measurements of the anisotropic superconducting thermodynamic parameters from reversible dc magnetization. We first present new results on a fully-oxygenated YBa 2Cu 3O 7 crystal with H ⊥ c and find that dH c / dT=−211 Oe/K and κ = 280. Then, for H ‖ c, we examine the effects of introducing oxygen defects through an annealing process and find that the same analysis produces dH c / dT = −86 Oe/K and κ∼20. Attempts to use the distribution of T c 's to extract intrinsic, bulk parameters from this broad-transition crystal are discussed. Such analysis, here applied to an ideal system, will be especially relevant for the randomly-doped cuprates.

Correlation of pinning properties with superconducting and microstructural parameters in NbTa thin films

We have used an AC transport technique to measure the force-displacement response of vortices in granular NbTa thin films, Results are presented for the dependence of this behaviour as the applied magnetic field amplitude and its orientation with respect to the columnar microstructure is varied. Analysis of the results allows us to determine the effective gradient of the pinning wells and elastic limit of the displacement of the vortices before depinning. The pinning site density, distribution and strengths are also deduced. The results are seen to correlate well both with the DC transport properties and with independent microstructural investigation of the thin film samples. They are analysed using various theoretical predictions and also compared to similar measurements recently reported on YBCO films.

Magnetic properties of a Tl 0.5Pb 0.5Sr 2CaCu 2O 7 single crystal

High-quality millimeter size single crystals of Tl 0.5Pb 0.5Sr 2CaCu 2O 7 with T c = 82 K have been synthesized for the first time. The regularity of their structure has been checked by using X-ray diffraction techniques and electron microscopy. Magnetic measurements have been performed in the geometry B| c. Fitting the reversible magnetization versus B with the London model yields the temperature dependences of both λ ab and H c2. From these results the following superconducting parameters have been obtained: (- d B c2/d T) Tc = 1.71 T/K, ξ ab (0) = 1.92 nm and λ ab (0) = 173.9 nm. Furthermore, Tl 0.5Pb 0.5Sr 2CaCu 2O 7 single crystals exhibit the highest irreversibility line B ∗( T/ T c) among all the single crystals of thallium cuprates. Moreover a scaling of the hysteresis loops M( B) can be performed in a wide range of temperature. All these data are discussed in connection with the structural parameters of Tl 0.5Pb 0.5Sr 2CaCu 2O 7.