Extended paraconductivity regime in underdoped cuprates

Abstract

We reconsider transport experiments in strongly anisotropic superconducting cuprates and we find that universal Aslamazov-Larkin (AL) paraconductivity in two dimensions is surprisingly robust even in the underdoped regime below the pseudogap crossover temperature T * . We also establish that the underlying normal state resistivity in the pseudogap phase is (almost) linear in temperature, with all the deviations being quantitatively accounted by AL paraconductivity. The disappearence of paraconductivity is governed by the disappearence of gaussian pair fluctuations at an energy scale related to T * .PACS numbers: 74.25.Fy, 74.40.+k, 74.20.De Recent transport experiments 1,2,3 in superconducting cuprates have shown that the paraconductivity effects in the normal state close to the critical temperature T c are well described by the following expressions valid in two and three dimensions, respectively,where d is the distance between CuO 2 layers, ξ c0 is the coherence length along the direction perpendicular to the layers, ε ≡ log(T /T c ), and ε 0 ≡ log(T # /T c ). Here T # is a temperature scale which increases with decreasing doping and appears to follow the characteristic crossover temperature T * below which many different experiments in the cuprates detect a pseudogap opening 4 . The above expressions (and the experimental data that they fit well) display two remarkable features. First of all, close to T c , for small values of ε, they reproduce the AslamazovLarkin (AL) form of paraconductivity 5These expressions account well for the fluctuating regime near T c both in optimally and underdoped cuprates, with YBaCuO 6+x (YBCO) displaying three-dimensional (3D) fluctuations, whereas the other more anisotropic compounds (LSCO and BSCCO) have a two-dimensional (2D) behavior. The fact that the paraconductivity in strongly anisotropic (quasi-2D) underdoped cuprates is described by "traditional" AL fluctuations is at odds with the widespread idea that below the pseudogap formation temperature T * particle-particle pairs are formed, which only become phase-coherent at the lower superconducting transition temperature T c . According to this picture, below the temperature of pair formation the fluctuations would be vortex-driven and should display a KosterlizThouless behavior, with exponential temperature dependences. On the contrary, it seems a well-established experimental fact that the superconducting fluctuations in the more 2D-like systems (essentially all, but the YBCO) display AL power-law behaviors in ε 6,7,8,9,10,11 . Remarkably, in D = 2 the AL theory of paraconductivity does not allow for any fitting parameter besides the experimentally well accessible distance between the 2D layers, which translates the 2D conductivity, with dimensions Ω −1 , in a 3D conductivity with dimensions Ω −1 m −1 . Therefore the AL paraconductive behavior observed near T c strikingly shows that the establishment of superconducting phase coherence in these materials is not due to a simple condensation of preformed pairs. This by n...

Effective two-dimensional thickness for the Berezinskii-Kosterlitz-Thouless-like transition in a highly underdopedLa2−xSrxCuO4

“…However, the BKT fluctuation regime only extends up to T c ≈ 6.5 K and, above it, ordinary AL-like Gaussian fluctuations are at play. Finally, we note that some deviations start to occur above T ≃ 10 K. As we discuss in Appendix B, this effect has already been observed in several families of cuprates, 41,42 and it can be interpreted as a signature of a pseudogap state above T c .…”

“…Since in this regime we are already exploring Gaussian fluctuations, such a failure is not correlated with the BKT character of the fluctuations, but it pertains instead to the regime of ordinary Cooper-pairs fluctuations. Interestingly, such behavior has been already observed in several families of cuprates, 41,42 and it has been interpreted theoretically 41,75 as an effect of the pseudogap. Indeed, by phenomenological modelling of the suppression in the electronic density of states characteristic of a preformed pseudogap, one can reproduce a faster decay of the Cooper-pairs correlation length ξ(T ) in Eq.…”

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

“…Since the paraconductivity diverges at T c , uncertainties in the determination of N are rather immaterial for T Ϸ T c and our finding is quite robust. The contribution of Gaussian fluctuations to paraconductivity spreads over a broad temperature range, T − T c ϳ T c , similarly to that found in underdoped BSCCO, 6 where, however, the need to guess the reference normal state made the analysis much less stringent. Rewriting = ͑ 0 / ͒ 2 and assuming 0 ϳ 20 Å, we can estimate the coherence length of the Gaussian fluctuations.…”

“…4 If this were the case, however, the most anisotropic cuprates ͓e.g., Bi 2 Sr 2 CaCu 2 O 8+␦ ͑BSCCO͔͒ should display an exponential temperature dependence in the enhancement of conductivity due to SC fluctuations at temperatures T Ͼ T c ͑the so-called paraconductivity͒ associated with vortical fluctuations, typical of a Kosterlitz-Thouless transition in two dimensions. 5 Instead, it is well documented [6][7][8][9][10][11][12][13][14][15][16][17] that paraconductivity in all the families of cuprates is fully accounted for by the standard AslamazovLarkin ͑AL͒ theory 18,19 based on Gaussian SC fluctuations, with the real and imaginary parts of the SC order parameter ⌬ fluctuating around zero. While YBa 2 Cu 3 O 7−x is less anisotropic and displays the AL behavior characteristic of threedimensional systems, all other compounds, which have a more anisotropic structure, display the standard AL behavior for two-dimensional ͑2D͒ systems.…”

Paraconductivity of underdopedLa2−xSrxCuO4thin-film superconductors using high magne

“…Previous analyses have been made upon the assumption that the resistivity in the normal state remains linear in T for slightly underdoped compounds, as long as no charge carrier localization is present. [21][22][23][24]36 This analysis had allowed us to evidence both an AslamasovLarkin regime in YBCO thin films and a high-temperature power-law conductivity decrease 21 equivalent to a highenergy cutoff 25 whose energy increases with underdoping. A total energy cutoff had also been introduced by other groups.…”