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as predicted by the mean-field theory and whose M=0 intercept at différent fields détermines the "diamagnetic Ha." The results show the mean-field Unear behavior of Ha(T) in YBaCuO near Tc with slopes dHJdT of 2 and 10 T/K for H||c and H i c , respectively, implying the low-T cohérence lengths £ab— 16 Â,fc=3 Â. Thèse smaU values, compared to the much larger pénétration depths referred to above, confirm the strongly type II Umit for the high température superconductors. They are also Ukely to be smaller than the transport mean free path £, which would put thèse materials into the so-called superconducting "clean" Umit,19,20 at least for current I1| ab. This is confirmed by the observation of similar Ha's on crystals Experiments on the high température of différent normal-state resistivity;1718 superconductors hâve confirmed many in the "dirty" Umit, Ha would hâve inof thèse features. For example, magcreased Unearly with normal-state resisnetic particle déposition revealed the tivity or inverse mean free path. A vortices, their quantization according to 18 also explains the paired-electron flux quantum 0= récent theoretical analysis 17 18 21 the observed variation ' ' in dM/dT hclle, and the formation of array s in the with field, which earUer appeared to be ab-plane, which range from hexagonal in disagreement with the mean-field to almost completely amorphous.6-8 The prédiction, and leads to a small réduclower critical field has been determined tion in the deduced dHJdJ'. by detecting the onset of vortex pénétration into the material as one crosses from Conventional superconductors in the the Meissner to the Abrikosov phase.9-13 Abrikosov phase are usually characterMean-field linear-temperature behavior ized by strong irréversible magnetic appears in a région above about Tc/2. phenomena, which hâve been interThe deduced YBaCuO London mag- preted in the context of the so-called netic pénétration depths, extrapojated Bean or critical-state model.22 The présto zéro température, are Xab=l,400 Â ence of defects or pinning in the mateand Xc=4,500-7,000 Â (for supercur- rial permits a critical flux gradient to be rents flowing parallel to or perpendicu- sustained, which, through Maxwell's lar to the ab planes, respectively). In the équation Vx H = 4ir//c, can be related to régime below Tc/2 an anomalous increase a bulk critical current density Jc. Apin Hcihas been reported.1113 Since H d = proximately rectangular hystérésis loops