Upper critical field also

Summary. On the basis of phenomenological Ginzburg-Landau approach we investigate the problem of order parameter nucleation in a ferromagnetic superconductor and hybrid superconductor - ferromagnetic (S/F) systems with a domain structure in an applied external magnetic field H. We study the interplay between the superconductivity localized at the domain walls and between the domain walls and show that such interplay determines a peculiar nonlinear temperature dependence of the upper critical field. For hybrid S/F systems we also study the possible oscillatory behavior of the critical temperature TC(H) similar to the Little-Parks effect. 

In the fictive gray system the value of Tc would be on the upper curve in Figure 6 with a value between the values of both parent compounds. However, in the real pseudoquaternary system YxLui xNi2B2C, Tc has a considerably lower value than expected from a linear interpolation, i.e. the gray system. As shown in Figure 60 the concentration dependence of Tc is non-monotonic with a minimum near x = 0.5. A similar behavior was found also for other quantities characterizing the electronic state of the system as the upper critical field H 2 and the parameter a [from Hc2(T) = H 2( 1 - T/Tc)1+ ] which is a measure for the positive curvature of Hc2(T), the residual resistance ratio RRR = pn(300 K)/pn(Tc), where pn(T) is the normal-state resistivity, and the two parameters kn and describing the field... 

Critical current density also depends on magnetic field, decreasing monotonically to zero at the upper critical field Hc2 (16-17). In YBaCuO the slope of Hc2 with temperature is unusually large, of order 2 T/K when field is applied parallel to the predominant conduction planes of the structure (18-19). This implies record values (up to 200 T has been estimated) for the upper critical field at low temperatures and opens up the possibility of very high field magnets. 

As the magnetic field increases, the amount of normal phase also increases relative to the superconducting part. Ultimately, the flux lines are so close together that no superconducting material exists between them, and the solid becomes normal. The field that finally destroys the superconductivity is the upper critical field, Hd-... 

Here, kab and kc are the London penetration depths for a magnetic field in the c-direction and parallel to the afo-plane, respectively. Bc2(c) and Bc2(ab) are approximate upper critical fields aligned in the c-direction and parallel to the afo-plane, respectively and ab are the coherence lengths in c-dtrecfion and in the ab plane, respectively. Considerably different values have been reported for the parameters listed in Table 4.2-28. This is tme in particular for Hq2 and also for which is usually... 

Fig. 17.22. Upper critical field versus t = TJT for different amounts of impurities which are in a singlet ground state with an excited singlet at energy S = Tjq. Also shown for comparison are the corresponding curves for S = 0 (dashed lines). The normalization is such that h(() = H,2(T)( dH,2(T)ldT)T,Tj (from Keller and Fulde. 1973).

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