Article

# Search for Cooper-pair fluctuations in severely underdoped Y1-deltaCadeltaBa2Cu3O6+x films( delta=0 and 0.2)

Physical Review B (Impact Factor: 3.66). 01/2006; 74. DOI: 10.1103/PhysRevB.74.012502

Source: arXiv

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**ABSTRACT:**In this paper we study the BCS Bose Einstein condensation (BEC) crossover scenario within the superconducting state, using a T-matrix approach which yields the ground state proposed by Leggett. Here we extend this ground state analysis to finite temperatures T and interpret the resulting physics. We find two types of bosonic-like excitations of the system: long lived, incoherent pair excitations and collective modes of the superconducting order parameter, which have different dynamics. Using a gauge invariant formalism, this paper addresses their contrasting behavior as a function of T and superconducting coupling constant g. At a more physical level, our paper emphasizes how, at finite T, BCS-BEC approaches introduce an important parameter \Delta^2_{pg} = \Delta^2 - \Delta_{sc}^2 into the description of superconductivity. This parameter is governed by the pair excitations and is associated with particle-hole asymmetry effects which are important for sufficiently large g. In the fermionic regime, \Delta_{pg}^2 represents the difference between the square of the excitation gap \Delta^2 and that of the superconducting order parameter \Delta_{sc}^2. The parameter \Delta_{pg}^2, which is necessarily zero in the BCS (mean field) limit increases monotonically with the strength of the attractive interaction g. It follows that there is a significant physical distinction between this BCS-BEC crossover approach (in which g is the essential variable which determines \Delta_{pg}) and the widely discussed (Coulomb-modulated) phase fluctuation scenario in which the plasma frequency is the tuning parameter.Physical review. B, Condensed matter 07/1999; · 3.77 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**In this paper we compare two broad classes of theories for the pseudogap in cuprate superconductors. The comparison in made in reference to measurements of the superfluid density, $\rho_s(T,x)$, in $YBa_2CuO_{7- \delta}$ films having a wide range of stoichiometries, $\delta$, or, hole doping, $x$. The theoretical challenge raised by these (and previous) data is to understand why the T-dependence of $\rho_s(T,x)$ is insensitive to the fermionic excitation gap $\Delta(T,x)$, which opens in the normal state and persists into the superconducting state, when presumably $\rho_s(T)$ is governed, at least in part, by fermionic excitations. Indeed, $\rho_s(T,x)$ seems to have a BCS-like dependence on $T_c(x)$, which, although not unexpected, is not straightforward to understand in pseudogapped superconductors where $T_c(x)$ and the excitation gap have little in common. Here, we contrast "extrinsic" and "intrinsic" theoretical approaches to the pseudogap and argue that the former (for example, associated with a competing order parameter) exhibits more obvious departures from BCS-like $T$ dependences in $\rho_s(T)$ than approaches which associate the pseudogap with the superconductivity itself. Examples of the latter are Fermi liquid based schemes as well as a pair fluctuation mean field theory. Thus far, the measured behavior of the superfluid density appears to argue against an extrinsic interpretation of the pseudogap, and supports instead its intrinsic origin. Comment: 9 pages, 8 figures; text rewritten in the PRB formatPhysical Review B 05/2002; · 3.66 Impact Factor -
##### Article: Pairing fluctuation theory of high $T_c$ superconductivity in the presence of nonmagnetic impurities

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**ABSTRACT:**The pseudogap phenomena in the cuprate superconductors requires a theory beyond the mean field BCS level. A natural candidate is to include strong pairing fluctuations, and treat the two-particle and single particle Green's functions self-consistently. At the same time, impurities are present in even the cleanest samples of the cuprates. Some impurity effects can help reveal whether the pseudogap has a superconducting origin and thus test various theories. Here we extend the pairing fluctuation theory for a clean system [Phys. Rev. Lett. 81, 4708 (1998)] to the case with nonmagnetic impurities. Both the pairing and the impurity $T$ matrices are included and treated self-consistently. We obtain a set of three equations for the chemical potential $\mu$, $T_c$, the excitation gap $\Delta(T_c)$ at $T_c$, or $\mu$, the order parameter $\Delta_{sc}$, and the pseudogap $\Delta_{pg}$ at temperature $T<T_c$, and study the effects of impurity scattering on the density of states, $T_c$ and the order parameter, and the pseudogap. Both $T_c$ and the order parameter as well as the total excitation gap are suppressed, whereas the pseudogap is not for given $T\le T_c$. Born scatterers are about twice as effective as unitary scatterers in suppressing $T_c$ and the gap. In the strong pseudogap regime, pair excitations contribute a new $T^{3/2}$ term to the low $T$ superfluid density. The initial rapid drop of the zero $T$ superfluid density in the unitary limit as a function of impurity concentration $n_i$ also agrees with experiment.Physical review. B, Condensed matter 03/2002; · 3.77 Impact Factor

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