Superconducting fluctuations and the Nernst effect in high-T_csuperconductors

Abstract

The comprehensive ρ(T) measurements and the consequent resistivity curvature mapping (RCM) on Y 0.7 Ca 0.3 Ba 2 Cu 3 O 7−δ thin films (doping levels p = 0.08-0.21) elucidate a phase diagram for the whole doping range. This phase diagram further strengthens a view that the 'normal' phase in hole-doped cuprates should be divided into a strong superconducting (SC) fluctuation phase and the 'real' normal phase in which there is no significant influence of SC. The temperature of superconducting fluctuations T f as a function of p was calculated using the Ginzburg-Landau model for layered superconductors. Comparisons between T f and the Nernst temperature establish the origin of the Nernst effect as SC fluctuations. Some of the details in ρ(T) cannot be fully understood by the existing models and call for a more sophisticated theory of carrier dynamics in cuprates.

Doping Dependencies of Onset Temperatures for the Pseudogap and Superconductive Fluctuation in Bi2Sr2CaCu2O8+δ, Studied from Both In-Plane and Out-of-Plane Magnetoresistance Measurements

“…This observation agrees with many other studies. 10,17,[40][41][42] Furthermore, T * was recently shown 43) to exist even for the heavily Zn-doped non-superconducting YBa 2 (Cu 1−x Zn x ) 3 O y by using the c axis optical spectra. As pointed out in Ref.…”

Strong thermal fluctuations in cuprate superconductors in magnetic fields aboveTc

“…Assuming that the Nernst effect is primarily due to thermal fluctuations, the whole pseudogap region would be associated with preformed Cooper pairs and become a precursor of the superconducting state. The finding motivated additional experiments on the Nernst effect in various HTSCs [12], as well as renewed study of thermal fluctuations in the temperature region between T c and T * by other probes: electric [13][14][15] and thermal conductivity [16] and diamagnetism [17]. The main goal was to try to quantify the superconducting fluctuation effects, so they can be either directly linked or separated from the pseudogap physics.…”

The conductivity and the magnetization around Tc in optimally-doped YBa$_2$Cu$_3$O$_{7-δ}$ revisited: quantitative analysis in terms of fluctuating superconducting pairs

“…Note first that the own nature of this anomalous Nernst effect is still at present a debated issue. [1,55,80,100,153] In fact, the discrepancies manifest already in the procedures proposed to separate the superconducting effects from the normal state (background) behavior: Well above T c , some authors attribute the decrease of the Nernst coefficient, often observed in different cuprate superconductors, to the normal state behavior. Then, the superconducting effects are extracted by using a baseline background that eliminates that decrease.…”

“…In fact, in spite of earlier detailed measurements and analyses, aspects so central within these approaches as the possible presence of indirect fluctuation effects, the multilayering influence and the estimations of the corresponding effective periodicity lengths, the role of the chemical disorder and of the corresponding T c -inhomogeneities, the behavior at high reduced temperatures and magnetic fields, or the physical origin of the corresponding cutoffs, are still controversial. [93][94][95][96][97][98][99][100][101] In this paper, the experimental uncertainties commented above are strongly mitigated by measuring the rounding behavior of the in-plane electrical conductivity, the magnetoconductivity and the magnetization around T c in high quality OPT Y-123 samples. These measurements extend up to 300 K in temperature and up to 9 T in applied magnetic field amplitudes, that when analyzed on the grounds of the GL approaches cover both their low magnetic field limit (Schmidt regime) and moderate fields (or Prange) regime.…”