Origins of anomalies in the temperature dependences of specific heat and superfluid density in doped high-Tc cuprates: Signatures of Bose-liquid superconductivity.

We show that the temperature-dependent superconducting order parameter and related superconducting properties (in particular, the temperature dependences of specific heat, superfluid density and related London penetration depth) of high- T c cuprates are fundamentally different from those of conventional superconductors and cannot be understood within the existing theories based on the Bardeen–Cooper–Schrieffer (BCS)-type condensation of weakly bound Cooper pairs into a superfluid Fermi-liquid and on the usual Bose–Einstein condensation (BEC) of bosonic Cooper pairs. We examine the validity of an alternative approach to the unconventional superconductivity in high- T c cuprates and establish that these materials exhibiting a λ -like superconducting transition at the critical temperature T c are similar to the superfluid 4He and are also superfluid Bose systems. We argue that the doped high- T c cuprates, from underdoped to overdoped regime, are unconventional (bosonic) superconductors and the tightly bound (polaronic) Cooper pairs in these polar materials behave like composite bosons just like 4He atoms and condense into a Bose superfluid at T c . We identify the superconducting order parameter Δ SC in underdoped and optimally doped cuprates as the coherence parameter Δ B of bosonic Cooper pairs, which appears just below T c and has a kink-like temperature dependence near the characteristic temperature T c ∗ < T c . We find that the λ -like specific heat anomaly in high- T c cuprates near T c predicted by the theory of Bose-liquid superconductivity is similar to that observed both in superfluid 4He near T λ and in Hg-based cuprate superconductor HgBa2Ca2Cu3O8 near T c ≃ 1 3 2 K. We demonstrate that in high- T c superconductor YBa2Cu3 O 6. 9 7 the superfluid density ρ s exhibits distinctly different temperature dependences in the temperature ranges 0 ≤ T ≤ T c ∗ and T c ∗ < T < T c . In this superconductor, a pronounced anomaly in ρ s (T) exists near T c ∗ ≃ 0. 4 T c and the temperature dependence of λ L 2 (0) / λ L 2 (T) below T c ∗ deviates downwards from the high-temperature behavior. Our results for the normalized superfluid density ρ s (T) / ρ s (0) = λ L 2 (0) / λ L 2 (T) are in good agreement with the experimental data on the temperature dependence of λ L 2 (0) / λ L 2 (T) in YBa2Cu3 O 6. 9 7 . The anomalous temperature dependences of specific heat and superfluid density observed in Hg- and Y-based high- T c superconductors are clear signatures of Bose-liquid superconductivity. [ABSTRACT FROM AUTHOR]