ИСТИНА

Facts are presented evidencing a strong electron–phonon interaction and scaling of a superconducting gap and a critical temperature in doped Bi-2223 and Bi-2212. We have observed a pronounced fine structure in dI/dV- characteristics of Josephson break junctions in Bi-2212 and Bi-2223 which is caused by interaction of АС Josephson current with Raman-active optical phonon modes [1-3]. For Bi-2223 the range of frequencies extended to 25 THz, including the apex oxygen mode OSr with the energy close to 80 meV and two-phonon resonances (OSr + Sr) and (OSr + Cu) with the energies 96 meV and 104 meV respectively. For slightly overdoped and slightly underdoped samples a sharp extra structure in the I(V) – characteristics (CVCs) of Bi-2212 nanocontacts was observed, which we attribute to the presence of the extended van Hove singularity (EVHS) close to the Fermi level [3, 4]. A giant instability in the multibranch CVCs of Bi-2223 stacks of SIS junctions (nanosteps) has been registered at helium temperatures. An observed periodic switching between neighboring branches in multibranch characteristics is probably caused by an emission of 2delta – optical phonons in a process of recombination of nonequilibrium quasiparticles (Krasnov model [5]). The instability has a “resonant” character and evolves when 2delta equals the energy of one of the optical phonon modes. The amplitude of instability dV is “quantized” : dV = (2delta)n, where the integer number n reaches its maximum for the optimally doped samples with the Fermi level being close to the EVHS. The observed giant instability is a direct indication of the phonon pairing mechanism in HTSC [4, 6].