Superconductor In Magnetic Field Research Articles

Mixed state in high temperature superconductors

In high T c superconductors the macroscopic mixed state (MACMS), corresponding to a mixture of macroscopic large supeconducting and normal domains, may be realized in magnetic field H. On the H-T phase diagram MACMS lies between high temperature normal state and low temperature Abrikosov vortex state. The irreversibility line corresponds in this case to the topological cross-over from MACMS with isolated singly-connected superconducting “droplets”, surrounded by the normal phase, to be sponge-like MACMS with multiply-connected superconducting “body”, surrounding isolated normal-state domains, which finally decay into Abrikosov vortices, as temperature goes down. The irreversibility appears due to the change of connectivity caused by the transition from superconducting droplets to superconducting sponge. The possibility of the multiple ф 0 normal cores existence has been demonstrated. Related experimental data on resistivity, magnetization, critical currents of high T c superconductors in magnetic field are discussed. The new experiments have been proposed to check predictions of the MACMS model.

Computer simulation of a two-dimensional type-II superconductor in a magnetic field

The authors study the time-dependent behaviour of type-II superconductors in an external magnetic field. Based on the time-dependent Ginzburg-Landau equation, simulations for several situations are performed in tow dimensions in order that they might discuss the dynamical behaviour of magnetic flux structures.

Theoretical explanation of broad resistive transition in high temperature superconductors in magnetic field

Experimental data on the broad resistive transitions in high temperature superconductors (HTSC) in strong magnetic fields, obtained with high quality samples, are compared with results of the renormalized fluctuation theory. The theory is extended to lower temperature by a partial summation method called SPH. The layer structure is kept throughout the calculation. The quantitative agreement is obtained over a wide range of temperature in both geometries I//H and I⊥H. It is remarkable that the theory can explain the observed low temperature behavior of ρ ⊥ suggestive of the flux flow mechanism and its absence in ρ // .

Broad resistive transition under magnetic field in high-temperature superconductors

A significantly broad resistive transition has been observed in high-temperature superconductors in magnetic fields, and its mechanism has been extensively discussed. A similar broad transition was observed by French et al. in 1967 even for low-temperature superconductors, where a variation in the magnetic field dependence of the resistance was measured for various current densities. The resistive properties in low-temperature superconductors are mainly determined by the fluxoid motion. Hence, this suggests that the resistive properties of high-temperature superconductors are similarly determined. In this paper, the broad resistive transition in high-temperature superconductors expected from the simple flux flow is estimated. The fluxoid motion in the longitudinal field geometry where the Lorentz force does not work is also discussed.

The theory of the fracture of a superconductor in a magnetic field

The stress-strain state of a superconductor in a static magnetic field is investigated from the point of view of the possibility of fracture. Only one force factor is taken into account, namely, the interaction between the field and the surface currents generated by the magnetic field (the Meissner effect /1/). When there are stress concentrators present (corner points, microcracks, inclusions etc.) comparatively weak magnetic fields, for which the specimen does not lose its property of ideal superconductivity, may turn out to be dangerous /1–3/. However, the formulation of the problem remains correct when the superconductor transforms into the normal phase (or simply for a normal conductor) in a variable intense magnetic field under skin-effect conditions and in a quasistatic mechanical state. In this case t 1 ⪢ t 2 is the condition of quasistatics, where t 1 and t 2 are the characteristic times of variation of the magnetic field and the range of wave deformation (volume or shape) of the characteristic dimension of the specimen. Moreover, when there are many factors present, this makes the problem a multiparametric one and extremely complicated to analyse, a preliminary investigation of the effect of each of these factors separately is advisable. The properties of the solutions of plane problems are analysed in detail, in particular, using the examples of regions of canonical forms: a corner, a cylinder, and a specimen with a cut in the form of an ellipse. The solution of a spatial problem for a sphere is given.

Resistive Transition Broadening of Metal‐Oxide Superconductors in Magnetic Fields

Mesure et calcul de la transition supraconductrice dans les composes Bi 4 Sr 3 Ca 3 Cu 4 O x

Cosmic Strings: Topological Fossils of the Hot Big Bang

Was the physics of the early universe like the physics of a rotating bucket of superfluid helium? Condensed matter physicists know that phase transitions can form exotic topological objects, such as quantized vortices in superfluid helium or vortex lines of magnetic field in superconductors. In recent years cosmologists have been exploring whether analogous phenomena occurred in the early universe. Cosmic strings, remnants of an ultrahightemperature phase transition at 1029 K, may have seeded galaxy formation and could have given rise to other effects observable in the universe today.

The measurement of critical currents of high T c superconductors in magnetic fields

Sample geometries suitable for I c testing of high T c material in magnetic fields are described. We discuss the problems of resistive contacts, estimating their possible effect on critical current and describe our attemps to reduce contact resistance or minimise its effect.

Hysteresis of microwave absorption of hard superconductors in magnetic field

Field- and temperature-dependent peculiarities of the microwave power loss in hard superconductors (Nb, NbSe 2, V 3Si, La 1.8Sr 0.2CuO 4, YBa 2Cu 3O 7 etc.) were discovered and investigated. The qualitative interpretation suggested allows to connect the low field mw-absorption to the peculiarity appearing near H c1 and hysteresis of mw-absorption — to the pinning of the Abrikosov's flux-lines.

On the Resistive State of High Temperature Superconductors in Magnetic Field

AbstractExperimental studies of the resistive state of high temperature superconductors in the mixed state were carried out on thin film samples of YBa2Cu3O7‐y and Bi2Sr2CaCu2Os+y. Precise angular dependence measurements on Yba2Cu3O7‐y epitaxial film have revealed not only the critical field anisotropy with respect to the a, b, and c‐axes but also a feature associated with twin boundaries. Dependence on the angle between the transport current and the magnetic field is investigated and discussed in terms of flux dynamics in highly anisotropic layered superconductors. Peculiar behavior of the Hall effect in the resistive state is reported.

Absence of current direction dependence of the resistive state of high temperature superconductors in magnetic fields

Resistive transition of Bi 2 Sr 2 CaCu 2 O 8+ y film in magnetic fields is studied in the light of the currently proposed flux creep model. Measurements with precise angular alignment have clarified that the resistive transition does not depend on the relative direction between magnetic field and transport current. This calls for a significant revision of the Lorentz-force-driven flux creep picture. A fluctuation induced phase-slip phenomenon as an alternative model is also discussed.

Resistivity of High Temperature Superconductors in Magnetic Field

Temperature dependence of resistivity observed in high temperature superconductors such as YBCO under a magnetic field is shown to be in marked disagreement with the standard theory of fluctuation enhanced conductivity. By estimating relevant energies, we suggest that vortex pairs or rings can be important fluctuations in these superconductors and lead to melting of the flux lattice near H c2 . A mechanism due to phase slippage at grain boundaries is also discussed.

Josephson-junction model of critical current in granular Y1Ba2Cu3O7- delta superconductors.

We calculate the transport critical-current density in a granular superconductor in magnetic fields below about 5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}3}$ T. The field dependence in this region is assumed to be controlled by intragranular or intergranular Josephson junctions. Various model calculations are fitted to transport critical-current data on bulk ${\mathrm{Y}}_{1}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ ceramic superconductors, whose average grain size somewhat exceeds 10 \ensuremath{\mu}m. The results yield an average junction cross-sectional area (thickness x length) of 4-6 \ensuremath{\mu}${\mathrm{m}}^{2}$. If the junctions are at the grain boundaries, a London penetration depth of about 150-300 nm is inferred, consistent with other estimates. We conclude that Josephson junctions are limiting the transport critical current in these samples and that they lie at the grain boundaries. The parameters of the fit are not consistent with Josephson junctions at twinning boundaries.

Generalized Ginzburg-Landau functional for highT c-superconductors in magnetic field in the critical region

Using a generalized Ginzburg-Landau functional, which phenomenologically describes the critical region of a phase transition, we discuss some thermodynamic properties of high T c superconductors for which the critical region is experimentally accessible. For instance, the Little-Parks experiment could indicate entry in the critical region. Some advantages of this approach for superconductors in magnetic field w.r.t. simple scaling theory is discussed also.

Crystal Site Occupancy of Alloying Elements in Polycrystalline Nb3Sn Superconductors By Electron Channeling

Many of the crystallographic problems encountered in materials science require techniques that can be used on small volumes of materials, e.g., in VLSI semiconductors, thin films, and the composite materials used in high magnetic field superconductors. In recent years several sensitive and accurate microcrystallographic techniques, based on diffraction of 100-keV electrons, have been developed. For example, convergent beam electron diffraction is a powerful technique to determine the symmetry of microcrystals and electron channeling techniques for the location of dilute elements in the crystal unit cell.We shall here deal with the application of the electron channeling technique called ALCHEMI (Atom Location by Channeling Enhanced Microanalysis) to study composite materials by determining the crystal sites of small amounts of alloying elements in Nb3Sn made by the bronze process. This is a problem of considerable interest in conjunction with the development of high magnetic field superconductors, because small amounts of certain alloying elements increase the upper critical magnetic field significantly.

High critical magnetic field superconductor La3S4

Abstract We report electrical conductivity, specific heat and magnetization measurements on La3S4 single crystals. The results show, that La3S4 is a strong coupling superconductor with a BCS coherence length of 132 A. This extremely low value makes La3S4 an intrinsic high critical magnetic field superconductor with a Landau-Ginsburg parameter κ0 of 20. For the temperature gradient of the upper critical magnetic field at the transition temperature we find values up to 35 kG/K.

低掃引速度磁界中におけるflux jumpの検討

低掃引速度磁界中におけるflux jumpの検討

Studies on flux jump of superconductor in magnetic field at slow sweep rate

Studies on flux jump of superconductor in magnetic field at slow sweep rate

Nuclear spin-lattice relaxation in small superconductors in magnetic field

Abstract The magnetic field dependence of nuclear spin-lattice relaxation time in zero-dimensional superconductors of Al is measured. The Zeeman effect of conduction electron is observed. The order parameter fluctuation and the discreteness of one-electron levels are discussed.

Spin splitting of the density of electronic states of thin superconductors in magnetic field

The spin splitting of the density of states of a thin superconducting film in a parallel magnetic field was investigated. The analytical expression for the density of states in the limit case of impurity parameters of theory β ∼ α2 ⪡ 1 is obtained for different energetical intervals. The expression for the energy gap ωg is also given.