Giant Nernst Effect due to Fluctuating Cooper Pairs in Superconductors

Abstract

A theory of the fluctuation-induced Nernst effect is developed for arbitrary magnetic fields and temperatures beyond the upper critical field line in a two-dimensional superconductor. First, we derive a simple phenomenological formula for the Nernst coefficient, which naturally explains the giant Nernst signal due to fluctuating Cooper pairs. The latter is shown to be large even far from the transition and may exceed by orders of magnitude the Fermi liquid terms. We also present a complete microscopic calculation (which includes quantum fluctuations) of the Nernst coefficient and give its asymptotic dependencies in various regions on the phase diagram. It is argued that the magnitude and the behavior of the Nernst signal observed experimentally in disordered superconducting films can be well-understood on the basis of the superconducting fluctuation theory. PACS numbers: 74.40.+k, 74.25.Fy, 72.15.Jf A series of recent experimental studies has revealed an anomalously strong thermomagnetic signal in the normal state of the high-temperature superconductors [1,2,3,4,5,6,7,8,9] and disordered superconducting films [10,11]. In the pioneering experiment [1], Xu et al. observed a sizeable Nernst effect in the La 2−x Sr x CuO 4 compounds up to 130 K, well above the transition temperature, T c . This and further similar experiments on the cuprates have sparked theoretical interest in the thermomagnetic phenomena. Theoretical approaches to the anomalously large Nernst-Ettingshausen effect currently include models based on the proximity to a quantum critical point [12], vortex motion in the pseudogap phase [2,13,14], as well as a superconducting fluctuation scenario [15,16,17]. While the two former theories are specific to the cuprate superconductors, the latter scenario should apply to other more conventional superconducting systems as well. Very recently, a large Nernst coefficient was observed in the normal state of disordered superconducting films [10,11]. These superconducting films are likely to be well-described by the usual BCS model and, hence, the new experimental measurements provide indication that the superconducting fluctuations are likely to be the key to understanding the underlying physics of the giant thermomagnetic response.Various groups have previously calculated the fluctuationinduced Nernst coefficient in the vicinity of the classical transition [15,16,17,18,19]. However, these analyses were limited to the case of very weak magnetic fields and temperatures close to the zero-field transition, when Landau quantization of the fluctuating Cooper pair motion can be neglected. In experiment, however, other parts of the phase diagram (in particular strong fields) are obviously important and how the quantized motion of fluctuating pairs would figure into the thermomagnetic response has remained unclear. In this Letter we clarify this physics, explaining the origin of the giant fluctuation Nernst-Ettingshausen effect, and develop a complete microscopic theory of Gaussian superconducting fluctuations at arbi...

Nernst effect in the electron-doped cuprate superconductorPr2−xCexCuO4: Superconducting fluctuati

“…SUMMARY Our measurements of the Nernst effect in the electrondoped cuprate superconductor PCCO elucidate the nature of superconducting fluctuations above the critical temperature T c . We find that the superconducting Nernst signal N sc is in qualitative and quantitative agreement with the theory of Gaussian fluctuations in dirty 2D superconductors, [18][19][20] at all dopings. Indeed, N sc (T, H) in PCCO behaves as it does in the conventional superconductor Nb x Si 1−x .…”

Fractal superconductivity near localization threshold

“…[108] These experiments were performed recently on InO x samples and show [109] Nernst signal which scales as N (T ) ∝ T −n , with the "Nernst exponent" n ≈ 7.5 at low magnetic fields. Application of the conventional theory [89] of the superconducting fluctuations to the Nernst effect in 2D superconductors [110,111] gives Nernst signal that scales as 1/T ln T at high temperatures. This behavior was indeed observed in conventional superconductor NbSi [112].…”

Flux-induced Nernst effect in a superconducting loop

“…Figure 8 shows the voltage at θ = π as a function of the parameter K defined under eq. (8). We see that |V (π)| decreases as K increases, but this trend seems to saturate.…”

“…Ullah and Dorsey evaluated the magnetothermoelectric coefficients [5,6] within the framework of the timedependent Ginzburg-Landau model (TDGL), ignoring cooperon contribution [7,8,9], and obtained a giant value for the fluctuation Nernst effect above the superconducting transition for weak magnetic fields. Ussishkin et al [10] noticed that the analysis of the Nernst effect should distinguish between transport and magnetization currents.…”