Paraconductivity in layered cuprates behaves as if due to pairing of nearly free quasiparticles

We revisit the Aslamazov-Larkin theory of paraconductivity in two dimensions to identify its universal and its specific model-dependent features. We show that both the numerical prefactor and the temperature dependence of the experimental paraconductivity in underdoped ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ provide evidence that the paired carriers behave as if they were nearly free fermionic quasiparticles. This conclusion is strengthened by the analysis of paraconductivity data in the presence of a finite magnetic field. In this case we show that there exists a temperature range above $T_c$ where the superconducting fluctuations are purely Gaussian and their destruction is determined by a rather low magnetic field scale $H_{c2}^G\left(T=0\right)$.

Figure 1

Typical diagram for the Baym-Kadanoff functional (a) and $\mathcal{T}$-matrix propagator of Gaussian fluctuations (b) adopted in this paper. Dashed and solid lines represent, respectively, the pairing interaction and the fermion propagator (see text).

Figure 2

Diagrams of the current-current response functions generated from the Baym-Kadanoff functional of Fig. 1a: (a) DOS correction, (b) Maki-Thompson vertex correction, and [(c1) and (c2)] AL contributions. The full circle with a thin line represents a current-vertex insertion (see text).

Figure 3

(Color online) Comparison between the theoretical Gaussian paraconductivity $\delta \sigma \left(T,H\right)$, Eq. (9) (lines), and the experimental data (symbols) taking an interlayer distance $d=6.6\text{Å}$ [data at $H=0\text{T}$ (black dots) should be compared with the theoretical result, Eq. (6) (straight solid line)]. $H=1\text{T}$ (solid line and circles), 5 T (dotted-dashed line and diamonds), and 14 T (dashed line and squares). Inset: Gaussian critical temperatures vs $H$ and estimated $H_{c2}^G\left(T=0\right)$ (see text). The line is a guide for the eyes.