Generation of 3rd and 5th harmonics in a thin superconducting film by temperature oscillations and isothermal nonlinear current response

Abstract

The generation of harmonics of the voltage response is considered when an AC current is applied through a superconducting film above Tc. It is shown that almost at all temperatures the mechanism of the temperature oscillations created by the AC current and the temperature dependence of the resistance dominates over the isothermal nonlinear electric conductivity. Only in a narrow critical region close to Tc the latter is essential for the generation of the harmonics. A detailed investigation of harmonics generation provides an accurate method for measuring the thermal boundary conductance between the film and the insulating substrate. The critical behaviour of the third harmonic will give a new method for the determination of the lifetime of metastable Cooper pairs above Tc. The comparison of the calculated fifth harmonics of the voltage with the experiment is proposed as an important test for the applicability of the employed theoretical models.

Observation of a sharp lambda peak in the third harmonic voltage response of high-T $ \mathsf {_c}$ superconductor thin films

“…As was mentioned in previous papers [7], [19], the current has to be much smaller than a maximal value I G :…”

“…The goal of this processing is to calculate the first and the second derivatives of the resistance with respect to temperature, which we need to estimate the thermal conductance G, as described in ref. [19]. Since our cryogenic system induces strong temperature noise, temperature data were smoothed over time, using a Savitzky-Golay filter [22], which performs a local polynomial regression to determine the smoothed value for each data point.…”

“…4. These results were used to calculate the thermal conductance G between the strip and the heat sink, as described in [19] and to evaluate the contribution of the thermal effect in the peak observed in the third harmonic voltage V 3ω , which will be described in the next section. Figure 6 gives the temperature dependence of the third harmonic response of the narrowest strip patterned on the YBCO film to the current exci-tation with amplitude I 0 equal to 5.65 mA and a modulation frequency of 1024 Hz.…”

“…Following the theoretical works [18], [19], we have adopted a fitting procedure which takes into account this parasite current. In any case, even with these difficulties, it is important to note that the third harmonic signal presented in this paper follows qualitatively the expected lambda-shaped behaviour predicted in [19]. Figure 10 presents the first attempt to connect theory and experiment.…”

Higher harmonics of ac voltage response in narrow strips of YBa2Cu3O7 thin films: evidence for strong thermal fluctuations

“…Recently, special attention has been paid to the role of non-ohmic harmonics in thermodynamic and transport properties of high-temperature superconductors (HTS) [4][5][6][7][8] and in a contactless inductive method to measure critical currents in superconducting films [9][10][11], which complements the nonlinear ac susceptibility first harmonic method to measure critical currents [12]. Moreover, some interesting theoretical predictions have been made regarding third harmonics generation based on the anisotropic scattering rate of normal charge carriers [13,14] as well as produced by electric field modified fluctuation effects in the electrical conductivity [15] or in the thermal response of the film to an harmonic current [16].…”