A phenomenological theory of phase transitions in high Tc su- perconductors

– A phenomenological theory is suggested to explain the experimentally discovered “paramagnetic peculiarity” and the unconventional change of the energetic state of a layers of high Tc of Y-Ba-Cu-O in the vicinity of the phase transition. The physical conditions are found under which these peculiarities are revealed. It is shown that the suggested theory qualitatively describes the experimental data. Despite the substantial success in the investigations of the high Tc superconductors, the microscopic nature of the superconducting state in these materials has not been understood to date [1]. In recent years promising attempts of experimental investigations of the superconducting phase transition were made in high Tc Y-Ba-Cu-O [2, 3]. In this work a new “paramagnetic” peculiarity and unconventional change in the energetic state of layer of high Tc superconductor in the vicinity of the phase transition is discovered. This success was achieved due to the construction of a highly sensitive magnetometer, which permits to measure very small changes ∆λ of the penetration depth of the magnetic field with frequency of the order several MHz in a sample of a flat high Tc superconductor. The experimental set-up permits to measure changes ∆λ of absolute magnitude of the order ∆λ ∼ 1 − 3 A with relative accuracy ∆λ/λ ∼ 10 [2]. The first of these peculiarities shows an increase of the penetration depth of the order of few micrometers when the temperature is decreased in the vicinity of the phase transition, prior to its decrease from the value of δ of the order of hundreds of micrometers up to the London penetration depth λ of the order of a few micrometer [2]. The second peculiarity is manifested in an increase of the energy of the sample with decreasing temperature prior to the well-known decrease due to the transition to the superconducting state [3]. The aim of this work is to show within the framework of a phenomenological theory of superconductivity that peculiar dependence of the the penetration depth on the temperature can be explained within an approach, which suggests a specific behavior of the Cooper pairs. We will show as well, that the account of the Coulomb interaction in a solid-state plasma, which is composed of normal ions a and normal and superconducting electrons, can explain the observed absorption of the energy of the electromagnetic field during the superconducting phase transition [3]. Section 2 derives an expression for the penetration depth λ and the conditions are obtain under which the maximum of the λ can be at T = 85.4 K, as observed in ref. [2]. In Section