V. I. Sokolenko, V. A. Frolov
Electron-topological transition in copper-oxide high-T_C superconductors before superconducting transition

It is well known the conductivity of high-temperature superconductors (HTSCs) with T_C ~100 K (YBaCuO, BiSrCaCuO, etc.) is provided at T~300 K by hole (h) fermions [1]. It is also known the superconducting transition in such cuprates is accomplished by means of the Cooper pairing, while the fluctuating Cooper pairs with charge -2e exist even at T=T_C+(~30 K) [2]. Hence it inevitably follows in the interval  T_C<T<300 K the hole Fermi surface (FS) of these HTSCs transforms into an electron one as a result of a topological transformation (the Lifshitz transition (LT) [3]. There is one of the central questions in the problem of the pseudogap state [1] of copper-oxide high-T_C superconductors:  how and at what temperatures this transformation occurs.

To evidence the charge carrier conversion the Hall effect is used usually. As for the BiSrCaCuO and YBaCuO, their Hall coefficients (R_H) have several features in the temperature range T_C…300 K [4,5]. The most significant of them is observed before the T_C in the region of fluctuation conductivity and can be interpreted as a manifestation of a scale hole-electron (h-e) conversion in a system of charge carriers, i.e. as the LT. However, this point of view is not universally accepted. As for the data on the transformation of the FS obtained by the ARPES (Angle Resolved Photoemission Spectroscopy) method [7], they, like [4,5], support several rearrangements of the FS, including those occurring near T_C.

Meanwhile, it is the possibility to evidence the h-e conversion in a hole HTSC (the last condition is sure), which does not require either electric or magnetic fields to create the Hall potential difference. The technique developed by us [7,8] is based on the phenomenon of rearrangement of the spectrum of charge carriers in the near-surface layer of a hole HTSC being in contact with a normal metal (Me). This phenomenon is a consequence of the annihilation of "aboriginal" hole fermions in the HTSC/Me interface with electrons penetrated from Me. The essence of this technique is the registration of changes in the resistance of the HTSC/Me interface r_C, which is characterized by a small number of hole carriers. The appearance of the temperature singularities of r_C and the sign of r_C variation (δr_C make it possible to obtain an idea of the character of the changes in the system of charge carriers of the HTSC array.

The dependences r_C of the Bi(Pb)SrCaCuO/Pb and YBaCuO/In interfaces have been studied and anomalies near the temperature of the pseudogap opening and before the superconducting transition have been observed. We are shown that in Bi(Pb)SrCaCuO and YBaCuO, when the temperature T=T_C+(~10 K) is reached, that do not concerns to fluctuating Cooper pairs condensation. So, there is due to changing the topology of the FS. As a result, significant piece of FS becomes electronic. The most probable reason for the topological transition is the achievement of the temperature of the 2D-3D crossover (the temperature of the three- dimensionality of HTSC), which is a consequence of a modification in the electronic subsystem that leads to a change in the interaction mechanisms of the fluctuation Cooper pairs [9, 10].