Superconductor critical magnetic field

Figure 6.10 Temperature dependence of critical magnetic field for a superconductor. Reprinted, by permission, from L. Solymar and D. Walsh, Lectures on the Electrical Properties of Materials, 5th ed., p. 429. Copyright 1993 by Oxford University Press.

Figure 13.16 Magnetization verses applied magnetic field for (a) a type I superconductor and (b) a type II superconductor. For the type I superconductor, the magnetic flux does not penetrate the sample below 9 Cc where the sample is a superconductor. Above rMc, the sample is a normal conductor. For the type II superconductor, the magnetic field starts to penetrate the sample at 3Cc, 1, a magnetic field less than rXc, the thermodynamic critical field. Superconductivity remains in the so-called vortex state between 9 c and Ci2 until WCt2 is attained. At this magnetic field, complete penetration occurs, and the sample becomes a normal conductor.

Figure 13.17 (a) Typical graph of critical magnetic field 3Cc as a function of temperature for a type I superconductor. Magnetic fields greater than 3tc suppress the superconducting transition, (b) Critical magnetic fields for several type I superconductors. 

Because useful currents carried by a Type I superconductor generate magnetic fields it follows that there are also critical current densities Jc, corresponding to the critical applied magnetic fields. In a Type II superconductor the... 

Superconductivity is the absence of resistance to dc conduction this occurs only below a critical temperature Tc, a critical magnetic field (which is a function of T and current density j), and a critical current density j, which is a function of T and H. For alloys, does not exceed 23 -24 K (by contrast, some of the recently discovered ceramic high-Tc cuprate superconductors, such as HgBa2Ca2Cu30j , have Tc values as high as 140 K and can have comparable y c values), Designers of superconducting solenoid magnets... 

The preceding material may be used to characterize the thermodynamics of transitions from the normal to the superconducting state. This transformation takes place for a limited class of materials at a particular temperature Tc, currently below 140 K. For soft superconductors of type I this state is marked by a complete disappearance of electrical resistivity and by the fact that at moderate values the magnetic induction B = M. + AnH vanishes within the bulk of the sample, so that for such materials M = —AnH. However, as the field is increased a critical magnetic field He is reached beyond which the material reverts back to its normal state. In first approximation He depends only on temperature according to the relation... 

The critical magnetic-field strength He, for the second kind of superconductors, to which oxide HTSCs belong, it is the so-called upper critical field Hc2- At H > Hc2, the superconducting state disappears throughout the material bulk. 

The most distinguishing features of superconductive materials are the sudden and complete disappearance of electrical resistance below T, the high critical current density J ), which allows superconductors to conduct with no power loss and the high critical magnetic fields (H 2) in which superconductivity can exist. The relationship among these features is shown in Figure 3,... 

Superconductivity [1.35]. Timgsten is a Type I superconductor with a transition temperature of 0.0154 0.0005 K. The critical magnetic field strength -> 0) is 1.15 0.03Oe. (91.5 A-m ). Impurities only show a minor influence on the transition... 

The following tables include superconductive properties of selected elements, compounds, and alloys. Individual tables are given for thin fUms, elements at high pressures, superconductors with high critical magnetic fields, and high critical temperature superconductors. 

A class of metal cluster superconductors of particular Interest consists of ternary molybdenum chalcogenides, conmonly known as the Chevrel phases (13). These phases were the first type of superconducting ternary system found to have relatively high critical temperatures and exhibited the highest known critical magnetic fields (Hc2 before discovery of the copper oxide... 

In Fig. 1 the transition temperature of the trilayers as a function of dcuNi is reported. A nonmonotonic dependence of Tc is observed. A minimum, in fact, occurs at dcuNi=4 nm, followed by a maximum at dcUNi=5 nm, until Tc decreases again. This behavior was theoretically predicted in [8] as well as experimentally observed in other superconductor/conventional ferromagnet based trilayers [9,10]. The existence of the Ji-phase was also proved in Nb/Cui xNix/Nb junction through critical current measurements [4,11]. In these cases the alloys were very diluted (x=48-52 %, TCurie 20-30 K) and, for this reason, the 0-rc crossover occurred at thicknesses of the order of dCUNi 20 nm. To check if the presence of the Ji-phase is detectable also by a change in the Nb layers coupling, the critical magnetic fields were measured. 

---