Resistive studies of the pseudogap in YBCO films with consideration of the transition from BCS to Bose–Einstein condensation

Abstract

A new approach is proposed for analysis of the excess conductivity σ′(T) arising below a characteristic temperature T*⪢Tc in YBa2Cu3O7−y (YBCO) films with different oxygen concentrations. The approach is based on the idea that σ′(T) is formed as a result of the formation at T⩽T* of local pairs (tightly bound bosons) obeying Bose–Einstein statistics in the temperature interval Tm<T<T*. At temperatures Tcmf<T<Tm the pairs obey BCS theory (Tcmf is the critical temperature separating the phase transition region from the region of critical fluctuations). Thus in Y123 systems a transition from Bose–Einstein condensation to condensation of the BCS type occurs with decreasing temperature. An equation in which the dynamics of formation of the tightly bound bosons is taken into account is proposed which gives a good description of the temperature dependence σ′(T) and in which the parameter Δ*, identified with the pseudogap in high-Tc superconductors, is contained in explicit form. The temperature dependence Δ*(T) is obtained for all the films studied.

Specific temperature dependence of pseudogap in YBa2Cu3O7−δ nanolayers

“…8), σ ′−1 ∼ exp(ε) [76]. This exponential dependence turned out to be the common feature of the HTSCs [9,74,76,77]. As a result ln(σ ′−1 ) is a linear function of ε with a slope α * =1.7, which determines parameter ε * c0 = 1/α * =0.59 (insert in Fig.…”

“…In cuprates, ∆ * is referred to as a pseudogap parameter which is mostly due to the local pair formation, as mentioned above. Thus, ∆ * (T ) has to reflect the peculiarities of the LPs interaction along with the decrease of temperature from T * down to T c [9,74,75]. In YBCO-PrBCO compound, ∆ * is assumed to be due to both local pair formation and magnetic interaction.…”

“…where A 4 is a numerical factor which has the meaning of the C-factor in FLC theory [9,21,59,65,74] and σ ′ (T ) is the experimentally measured excess conductivity over the whole temperature range from T * down to T all other samples studied. From our point of view the result has to confirm the conclusion by means of Eq.…”

“…(10) across the whole temperature interval up to T * and fit it to experiment in the range of 3D AL fluctuations near T c (Fig. 8, dashed curve 1), where lnσ ′ (lnε) is a linear function of the reduced temperature ε with a slope λ = −1/2 [9,74]. As can be seen in the figure, the fit with A 4 = 35 is very good, suggesting the perfect structural quality of the studied YBCO nanolayers.…”

“…From our point of view the result has to confirm the conclusion by means of Eq. (11), ∆ * (T ) has to properly reflects the properties of the pseudogap [9,74].…”