Enhancement of Josephson Phase Diffusion by Microwaves

Abstract

We report an experimental and theoretical study of the phase diffusion in small Josephson junctions under microwave irradiation. A peculiar enhancement of the phase diffusion by microwaves is observed. The enhancement manifests itself by a pronounced current peak in the current-voltage characteristics. The voltage position Vtop of the peak increases with the power P of microwave radiation as Vtop ∝ √ P , while its current amplitude weakly decreases with P . As the microwave frequency increases, the peak feature evolves into Shapiro steps with finite slope. Our theoretical analysis taking into account the enhancement of incoherent superconducting current by multi-photon absorption is in good agreement with experimental data. [5,6], to name just a few. The significance of these various effects depends on the ratio of the Josephson energy E J to the charging energy E C and on the thermal fluctuations. Here, we will deal with properties of moderately small Josephson tunnel junctions, with the ratio of E J /E C ≫ 1, and E J ≃ k B T , which corresponds to the phase diffusion regime induced by thermal fluctuations.The Josephson phase diffusion in small junctions has been studied in detail both experimentally [5,7] and theoretically [8,9]. Recently, the phase diffusion regime has been observed also in layered high-temperature superconductors [10]. The characteristic feature of this regime is the absence of pure zero-voltage superconducting state accompanied, however, by a hysteresis in the I-V curve. The phase diffusion behavior is a consequence of frequency dependent damping [5], i.e. the dissipation can be rather weak at zero frequency but it reaches a substan- * Present address: Theoretische Physik III, Ruhr

Incoherent microwave-induced resistive states of small Josephson junctions

“…3). In the presence of microwave radiation we observed a rather unusual low-voltage resistive state [6]. Thus, at large temperature, e.g.…”

“…In particular, we focus on the crossover regime between completely incoherent microwave-assisted phase diffusion [6] and the conventional textbook-like Shapiro steps [9]. Our experimental data are compared with the theoretical analysis, which consistently explains an enhanced phase diffusion branch [6], the zero-crossing branch characterized by a negative resistance, and a fine-structure branching of the resistive state. We also show how various parameters such as the temperature, junction size and frequency of microwave radiation influence the -I V curve.…”

“…when the junction is irradiated by microwaves. In our first paper published few years ago [6] we have studied experimentally and theoretically the enhancement of phase diffusion by microwaves. It has been observed that the application of microwaves leads to a shift of the phase diffusion branch to larger voltages, and the zero-voltage resistance substantially increases.…”

Phase Dynamics of Superconducting Junctions Under Microwave Excitation in Phase Diffusive Regime

“…Koval et al performed experiments on sub-micron Nb/AlO x /Nb junctions and found a smooth and incoherent enhancement of Josephson phase diffusion by microwaves. (Koval, Fistul et al 2004) This enhancement is manifested by a pronounced current peak at the voltage p VP ∝ . Recently experiments on untrasmall Nb/Al/Nb long SNS junctions have found that the critical current increases when the ac frequency is larger than the inverse diffusion time in the normal metal, whereas the retrapping current is strongly modified when the excitation frequency is above the electron-phonon rate in the normal metal.…”

“…In section 2, we will start from a classical picture by considering a Langevin www.intechopen.com Superconductor 312 equation for phase to explore different regimes for phase dynamics. (Koval, Fistul et al 2004) In certain condition the phase demonstrates the mode locking of the internal and external frequencies, namely the phenomenon of Shapiro steps. In section 3, we will focus on the phase diffusive regime and begin with the phase correlation so as to derive the resulting charge tunneling rate.…”