On the consequences of the uncertainty principle on the superconducting fluctuations well inside the normal state

Abstract

We first argue that the collective behaviour of the Cooper pairs created by thermal fluctuations well above the superconducting transition temperature, T C , is dominated by the uncertainty principle which, in particular, leads to a welldefined temperature, T C , above which the superconducting coherence vanishes. On the grounds of the BCS approach, the corresponding reduced-temperature, ε C ≡ ln(T C /T C ), is estimated to be around 0.55, i.e., above T C ≃ 1.7 T C coherent Cooper pairs cannot exist. The implications of these proposals on the superfluid density are then examined using the Gaussian-Ginzburg-Landau approximation. Then we present new measurements of the thermal fluctuation effects on the electrical conductivity and on the magnetization in different lowand high-T C superconductors with different dopings which are in excellent agreement with these proposals and that demonstrate the universality of ε C . 74.20.-z Theories and models of superconducting state 74.20.De Phenomenological theories (two-fluid, Ginzburg-Landau, etc.) 74.40.+k Fluctuations (noise, chaos, nonequilibrium superconductivity, localization, etc.)

In-plane and transverse superconducting fluctuation diamagnetism in the presence of charge-density waves in2H−NbSe2single crystals

“…This effect has been interpreted in terms of the shrinkage, induced by the magnetic field, of the superconducting wave-function to lengths of the order of the Cooper pairs size perpendicular to the applied field. 17,18 Due to the high H C2 (0) value (∼ 17 T) ∆M /H does not vanish even for the highest magnetic fields used in the experiments.…”

“…16 The lines are the normal-state or background contributions, M B (T ) H , obtained by fitting a Curie-like function [M B (T ) H = a + bT + c/T ] in the temperature region between ∼ 11 K (∼ 1.5T C ) and ∼ 20 K (∼ 2.8T C ). The lower limit corresponds to a temperature where the fluctuation effects are expected to be negligible, 17 while the upper limit is well below T CDW . As it may be already seen in this figure, the effect of the superconducting fluctuations is clearly anisotropic, being more pronounced for H ⊥ ab.…”

Critical fluctuation conductivity in layered superconductors in a strong electric field

“…9,19 Very recently 36 it was suggested that the physical meaning of the "total energy" cut-off follows from the uncertainty principle, which impose a limit to the confinement of the superconducting wave function. It must be however mentioned that in the critical fluctuation region, the two cut-off prescriptions almost coincide quantitatively, due to the low reduced-temperature value ε with respect to the factor c.…”

Comment on “High-field studies of superconducting fluctuations in high-Tccuprates: Evidence for a small gap distinct from the large pseudogap”

“…1 of this Comment. We have used s = c/2, and for the total-energy cutoff parameter [10,11] ε C = 0.5, which is close to the value estimated in the BCS scenario [11]. For the remaining parameter, ξ c (0), we have used the same value as proposed in Ref.…”

“…[9] to [11] for optimally doped YBa 2 Cu 3 O 6+x ). The resulting ∆σ SF (T, 0) is the upper solid curve in Fig.…”