**4. Conclusion**

26 Will-be-set-by-IN-TECH

**Figure 14.** a. Spectral weight W(*EF* ) vs T (blue dots) for OP35K Bi2201. The solid line is the guidance for

W(*EF*) (Fig. 14a) obtained from the ARPES experiments performed on the same sample. In this way, the results of ARPES experiments reported in Ref. [80] are believed to confirm our

Also plotted in Fig 14b is the normalized spectral gap (*SG*(*T*)) (red dots) equals to the energy of the spectral peaks of EDCs measured by ARPES [80]. Important in this case is that *SG*(*T*) smoothly evolves through both *Tpair* and *Tc*. The fact is believed to confirm assumed in the LP model the local pair existence above *Tpair*. Despite the evident similarity there are, however, at least two differences between the curves shown in Fig. 14b. First, there is no direct correlation between the *SG*(*T*) and the *W*(*EF*)(*T*) (Fig. 14 a, b). Why the maximum of *SG*(*T*) is shifted toward low temperatures compared to *Tpair*, has yet to be understood. The second difference is the absolute value of the SG compared to the pseudogap. The spectral gap has *SGmax* ≈ 40 meV and *SG*(*Tc*) ≈ 38 meV [80]. It gives 2*SG*(*Tc*)/*kBTc* ≈ 26 which is apparently too

2Δ∗(*Tc*)/*kBTc* ≈ 6.4 which is a common value for the Bi compounds [124] with respect to

conclusion as for existence of the local pairs in HTS's, at least in Bi2201 compounds.

*max* (green dots) and spectral gap SG/*SGmax* (red dots) [[80]] as

*max* ≈ 16.5 meV and Δ∗(*Tc*) ≈ 6.96 meV, respectively. It gives

eyes only [[80]]. b. Pseudogap Δ∗(*T*)/Δ∗

high. The PG values are Δ∗

the functions of temperature for the same sample.

The Chapter presents a detailed consideration of the LP model developed to study the PG in HTS's. In accordance with the model the local pairs have to be the most likely candidate for the PG formation. At high temperatures (*Tpair* < *T* ≤ *T*∗) we believe the local pairs to be in the form of SBB which satisfy the BEC theory (non-SC part of a PG). Below *Tpair* the local pairs have to change their state from the SBB into fluctuating Cooper pairs which satisfy the BCS theory (SC part of a PG). Thus, with decrease of temperature there must be a transition from BEC to BCS state [2, 27]. The possibility of such a transition is considered to be one of the basic physical principals of the high-*Tc* superconductivity. The transition was predicted theoretically in Ref. [23, 24, 74] and experimentally observed in our experiments [2, 20, 27, 95].

A key test for our consideration is the comparison of the Δ∗(*T*), calculated within the LP model, with the temperature dependence of the loss of the spectral weight close to the Fermi level *W*(*EF*)(*T*), measured by ARPES for the same sample [80]. The resulting Δ∗(*T*) is found to be in a good agreement with the W(*EF*)(*T*) obtained for OP35K Bi2201 (Fig. 14). It allows us to explain reasonably the W(*EF*)(*T*) dependence, both above and below *Tpair*, in terms of local pairs.

The obtained results are also in agreement with the conclusions of Ref's. [16, 90, 91] as for SC and non-SC parts of the PG in Bi systems. Besides, formation of the local pairs is also believed to explain the rise of the polar Kerr effect and response of the time-resolved reflectivity, both observed for Bi systems just below *T*∗ [90]. While, the Nernst effect [16], which is likely due to the SC properties of the local pairs, is observed only below *Tpair*, or below *Tmax* in terms of our model. All the facts have to support the local pair existence in HTS's at *T* ≤ *T*∗, Thus, we may conclude, that on the basis of the developed LP model the self-consistent picture of the PG formation in HTS's is obtained. At the same time the issue concerning the pairing mechanism in HTS's still remains controversial [22].
