Magnetic properties of pure type II/1 superconductors

A detailed theoretical analysis of the magnetic properties of pure type II/1 superconductors at T 0°K is presented.     

Ac loss minimum in type II superconductors

Theoretical predictions with respect to a minimum in the ac losses in type II superconductors are compared with experimental results on Nb-50% Ti and Nb-10% Mo. Extensions of the theory are suggested to explain the deviations.     

Low-field peak effect in type II superconductors

The combined effect of a surface (edge) barrier and volume pinning on the dependence of critical current I _c on the magnetic field (I⊥H ₀) in bulk type II superconductors is investigated. In low magnetic fields, there is a portion of the curve I _c(H ₀) where I _c grows with H ₀, causing a nontrivial peak effect in this field range. Such behavior is explained by the combined effect of a surface (edge) barrier and volume pinning, the latter being rather sensitive to the transport current density distribution in a superconductor.     

Force free fields in type II superconductors

It seems possible that, under certain field and current conditions, the critical current of a type II superconductor is not determined simply by application of a Bean-London model, but by the joint use of the latter and a second equation.     

Magnetic flux distribution in type II superconductors with large demagnetization and a high edge barrier

The trapped flux distribution in thin wafers of both polycrystalline and granular superconductors having large demagnetization and edge barriers of different heights is measured by means of polarized neutrons. It is shown that the nature of the critical state in polycrystalline wafers, unlike that in a ceramic wafer, is not described by the Bean model. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 12, 771–775 (25 December 1999)     

Muonic study of type II superconductors

A theory of μSR method is developed for uniaxial anisotropic high-T _c superconductors. In two extreme cases ofH _ext‖c andH _ext⊥c analytical formulas are obtained which makes it possible to determine from the position of van Hove singularities in the Fourier-spectrum of muonium polarization, the type and parameters of the vortex lattice, such as λ _ab (λ _c ). In caseH _ext≲H _c2 we obtained the shape of the Fourier-spectrum in analytical form and the simple method of determining the Ginsburg-Landau parameter κ. Convenient expressions for numerical calculations are obtained for the arbitrary orientation of the external field, and an algorithm is provided to compute the mean fieldB, the vortex lattice parameters and the bulk field distribution in anisotropic superconductor. The Fourier spectrum of polarization based on these calculations can be used to independently check the validity of the high-T _c parameters determination for “appropriate” orientations.     

Vortex breaking and cutting in type II superconductors

In technological superconductors, the Lorentz force on the flux vortices is opposed by inhomogeneous pinning and so the critical current may be controlled by a combination of vortex entanglement, cutting, and cross-joining. To understand the roles of these processes we report measurements of structures in which a weak pinning layer is sandwiched between two strongly pinning leads. Quantitative modeling of the results demonstrates that in such systems the critical current is limited by the deformation of individual vortices and not by subsequent cross-joining processes.     

Vortex liquid crystals in anisotropic type II superconductors

In an isotropic type II superconductor in a moderate magnetic field, the transition to the normal state occurs by vortex lattice melting. In certain anisotropic cases, the vortices acquire elongated cross sections and interactions. Systems of anisotropic, interacting constituents generally exhibit liquid crystalline phases. We examine the possibility of a two step melting in homogeneous type II superconductors with anisotropic superfluid stiffness from a vortex lattice into first a vortex smectic and then a vortex nematic at high temperature and magnetic field. We find that fluctuations of the ordered phase favor an instability to an intermediate smectic-A in the absence of intrinsic pinning.     

Magnetic superconductors

Simultaneous occurence of magnetism and superconductivity in Mo₅SnGa_0.5S₆ has been reported by Alekseevskii, Dobrovol'skii and Tsebro. X-ray studies show that this results from two phase mixtures of Mo₃Sn_0.5S₄ and Mo₂Ga_0.5S₄ which have previously been reported to be superconducting and ferromagnetic respectively.     

Magnetic superconductors

The old problem of the coexistence of superconductivity and magnetism has obtained new interest since the discovery of the unusual magnetic and superconducting properties of some ternary compounds. In this report I want to discuss the influence of antiferromagnetic order on the superconducting state and, in particular, will compare different pairing approximations which have been proposed in the literature. Furthermore I will investigate the influence of fluctuations on the stability of the periodic spin structure and other superconducting phases at the onset of ferromagnetic order.     

The structure of single vortices in type II superconductors

This paper reports the first results of a program for the investigation of the detailed electronic and magnetic structure of type II superconductors. The structure of a single flux bundle or vortex in an infinite superconducting specimen is investigated, assuming weak coupling and a spherical Fermi surface of the material. The equations governing the gap parameterΔ(r) and the vector potentialA(r) are suitably scaled and simplified using the symmetry of the system. These equations can be solved numerically only; for their solution the asymptotics ofΔ(r) andA(r) far from the center of the vortex are required. These asymptotics are derived here by properly linearising the equations; they are found to be of the typer ^?1/2 exp (?r/rin0) wherer ₀ is determined by temperature and material parameters. Implicit equations forr ₀ are derived and discussed. We find that for materials withκ-values less than 1.7 (e.g. pure niobium)r ₀ becomes complex at low temperatures; i.e. the deviation ofΔ(r) andA(r) from their values at infinity is a damped oscillation as function ofr.     

Flux vortices and transport currents in type II superconductors

This article is concerned with the mechanisms by which type II super-conductors can carry currents. The equilibrium properties of the vortex lattice are described and the generalized driving force in gradients of temperature and field is derived using irreversible thermodynamics. This leads to expressions for thermal cross effects which can include pinning forces.

The field distributions which occur in a range of situations are derived and a number of useful solutions of the critical state given. In particular, the distribution in a longitudinal field is obtained, and the conditions under which force-free configurations can break down by the cutting of vortices discussed.

The effects of lattice rigidity on the summation of pinning forces is considered and it is shown that a summation based on statistical arguments uses the same approximations and leads to the same results as a dissipation argument. Theoretical expressions are derived for the vortex pinning interaction to a number of different metallurgical defects. The theoretical models are compared critically with experimental measurements of pinning forces and other related phenomena, such as flux creep, low amplitude vortex oscillations and vortex lattice defect effects. Finally, the implications for technological materials are assessed.     

ESR of Nd+3 in type II superconductors

The resonance properties of the Γ₆ ground state of Nd⁺³ in the type II Superconductors CeRu₂, ThRu₂ and LaRu₂ are reported. A significant change of the g value and lineshape were observed for CeRu₂:Nd upon going from the normal state to the superconducting state. The data in the superconducting state is discussed in terms of conduction electron's pairing mechanism and inhomogeneous field distribution mechanism in the superconducting state. A preliminary lineshape analysis is not completely consistent with previous theoretical predictions and experiments.     

Flux vortices and transport currents in type II superconductors

This article is concerned with the mechanisms by which type II superconductors can carry currents. The equilibrium properties of the vortex lattice are described and the generalized driving force in gradients of temperature and field is derived using irreversible thermodynamics. This leads to expressions for thermal cross effects which can include pinning forces. The field distributions which occur in a range of situations are derived and a number of useful solutions of the critical state given. In particular, the distribution in a longitudinal field is obtained, and the conditions under which force-free configurations can break down by the cutting of vortices discussed. The effects of lattice rigidity on the summation of pinning forces is considered and it is shown that a summation based on statistical arguments uses the same approximations and leads to the same results as a dissipation argument. Theoretical expressions are derived for the vortex pinning interaction to a number of different metallurgical defects. The theoretical models are compared critically with experimental measurements of pinning forces and other related phenomena, such as flux creep, low amplitude vortex oscillations and vortex lattice defect effects. Finally, the implications for technological materials are assessed.     

On the electrodynamics of the type II superconductors

Using the two fluid model, modified Londons equations have been obtained for the type II superconductors in the Schubnikov state (H _c1<H<H _c2). The effect of the normal electrons which form the cores of the flux vortices have been included. These equations can be reduced to the Londons equations under suitable assumptions. The electric field inside the superconductor and the field equations have also been obtained.     

Flux flow noise spectra of type II superconductors

Measurement of the flux flow noise have been made in foil samples of Pb₈₀In₂₀. The results obtained cannot be explained by previous models which assume direct transit of fluxoids across sample. However, using a model based on the assumption that the flux motion is interrupted by pinning centers it is possible to obtain a good fit to the measured noise curves. Good agreement is also obtained between the calculated and measured critical frequencies using this model. These results may also help explain certain features of noise measurements reported by other workers.