Recent results:

· The model of local pairs has shown that at high temperatures T ≤ T* >> T_c the local pairs exist in the form of strongly bound bosons, obeying the theory of Bose-Einstein condensation (BEC), and transform into the fluctuation Cooper pairs, which obey the BCS theory, when T approaches T_c [1].
· The transition from the BEC to the BCS in various HTSC systems was experimentally observed with decreasing temperature [1].
· It is shown that the temperature dependence of pseudogap PG, Δ*(T), has always demonstrated a maximum. The maximum temperature T_max ≈ 130K for YBa₂Cu₃O_7-y and does not depend on the oxygen concentration in the sample [1].

· For the first time, the fluctuation conductivity (FLC) and PG in the iron-based superconductor SmFeAsO_0.85 with T_c = 55K was studied, taking into account the model of local pairs [2].
· An unusual temperature dependence of the FLC with the enhanced Maki-Thompson fluctuation contribution was found [2, 3], indicating that the conductivity occurs in the layers of Fe-As.
· An unusual temperature dependence of the PG, Δ*(T), with a negative slope at high temperatures [3, 4], was revealed, which is explained by the theory of antiferromagnetic superconductors by Machida-Nokura-Matsubara.

· For the first time, within the model of local pairs the FLC and PG in HoBa₂Cu₃O_7-y single crystals, having a significant intrinsic magnetic moment (μ_eff = 9,7μV), were investigated under the applied pressure up to 5kbar [5].
· The same temperature dependences of the FLC (with the enhanced fluctuation contribution of the Maki-Thompson) and PG (with a negative slope), as in SmFeAsO_0.85, were found confirming the noticeable role of the magnetic subsystem in the formation of both FLC and PG in HoBa₂Cu₃O_7-y [5].

· A good agreement was found between the temperature dependence of the spectral weight W (EF), obtained by ARPES for Bi2201 single crystals with Tc ≈ 35K in the works of other authors, and the pseudogap Δ*(T), calculated within our model of local pairs for the same sample [6]. It is shown that the maximum temperature of the PG, T_max, coincides with the temperature T_pair, below which the W (EF)(T) deviates from linearity.
· The concept of local pairs in high-T_c superconductors was developed, which allows  explain  reasonably the dependence of W (EF) (T), at least in bismuth HTS’s [7].

· For the first time the temperature dependence of the excess current (Iexc) in Andreev contacts Ag-Bi2223 with Tc =110 K has been revealed and thoroughly analyzed at T >> Tc [8]. It was shown that the excess current is clearly observed up to Tpair = 180 K. Importantly, in the temperature interval Tc<T<Tc Tpair the temperature dependence Iexc(T) is shown to be linear. Above 180 K Iexc(T) sharply decreases and becomes zero at T= 210 K. According to the theory, the Iexc observation above Tc directly points to the existence of the local pairs with Δ ≠ 0 in high-Tc superconductors [1]. The presence of such local pairs at T>>Tc just produces excess current due to Andreev reflection process in the superconductor.

· The influence of hydrostatic pressure up to 0.48 GPa on the fluctuation conductivity σ'(T) and pseudogap (PG) Δ*(Т) of slightly doped HoBa2Cu3O7-δ single crystals with Tc ~ 62 K and δ ≈ 0.35 is studied with current passing under an angle 45º to the twin boundaries. It is shown that near Tc the conductivity σ'(T) is well described by the Aslamasov–Larkin and Hikami–Larkin fluctuation theories demonstrating 3D–2D crossover with the increase of temperature. Δ*(Т) displays two representative maxima at Tmax1 ≈ 219 K and Tmax2 ≈ 241 K likely caused by the phase stratification of the single crystal (сгкму 1). Pressure leads to disappearance of these maxima and linear Δ*(Т) with a positive gradient at high temperatures. Essentially, with the removal of pressure the maxima are restored (сгкму 2). The comparison of our results with those obtained for YBa2Cu3O7-δ sheds more light on the role of magnetic subsystem in the high-Tc superconductors [9].


1. Solovjov A.L., Dmitriev V.M. Fluctuation conductivity and pseudogap in high-temperature superconductors YBCO (review) // Low Temp. Phys., 35, № 3, 168-206 (2009).
2. Solovjov A.L, Sidorov S.L., Tarenkov V.Yu, D’yachenko A.I. Fluctuation conductivity and pseudogap in SmFeAsO_1-x // Low Temp. Phys., 35, № 10, 826-828 (2009).
3. Solovjov A.L., Sidorov S.L., Tarenkov V.Yu., D’yachenko A.I. Fluctuation conductivity and pseudogap in optimally doped SmFeAsO_1-x // arXiv: Cond.-mat.1002.2306 (2010).
4. Solovjov A.L., Sidorov S.L., Tarenkov V.Yu., D'yachenko A.I., Svetlov V.N., Stepanov V.B., Agafonov A.B. Possibility of local pair existence in optimally doped SmFeAsO_1-x in the pseudogap regime // Low Temp. Phys., 37, № 7, 557 (2011).
5. Solovjov A.L., Tkachenko M.A., Vovk R.V., Obolenskii M.A. Behavior of the fluctuation conductivity and the pseudogap in the lightly doped HoBa₂Cu₃O_7-δ single crystals under pressure // Low Temp. Phys., 37, № 9-10, 840 (2011).
6. Solovjov A.L., Tkachenko M.A. Pseudogap and local pairs in high-T_c cuprate superconductors // arXiv: Cond-mat.supr-con. 1112.3812v1 (2011).
7. Solovjov A.L. Pseudogap and local pairs in high-Tc superconductors. Superconductors - Materials, Proerties and Applications. Chapter 7 // Rijeka: InTech, 2012, pp. 137-170.
8. Dyachenko A.I., Tarenkov V.Yu., Sidorov S.L., Varyukhin V.N., and Soloviev A.L. An excess current in Bi2223–Ag contacts above the critical temperature // Low Temp. Phys., 39, № 4, 323-328 (2013).
9. Solovjov A.L., Tkachenko M.A., Vovk R.V., Chroneos A. Fluctuation conductivity and pseudogap in HoBa₂Cu₃O_7−δ single crystals under pressure with transport current flowing under an angle 45 to the twin boundaries // "Physica C. Superconductivity and its Applications", 501, 1, 24-31 (2014).