Resolving the effects of frequency-dependent damping and quantum phase diffusion in YBa2Cu3O7−

Abstract

We report on the study of the phase dynamics of high critical temperature superconductor Josephson junctions. We realized YBa2Cu3O7−x (YBCO) grain boundary (GB) biepitaxial junctions in the submicron scale, using low loss substrates, and analyzed their dissipation by comparing the transport measurements with Monte Carlo simulations. The behavior of the junctions can be fitted using a model based on two quality factors, which results in a frequency dependent damping. Moreover, our devices can be designed to have Josephson energy of the order of the Coulomb energy. In this unusual energy range, phase delocalization strongly influences the device's dynamics, promoting the transition to a quantum phase diffusion regime. We study the signatures of such a transition by combining the outcomes of Monte Carlo simulations with the analysis of the device's parameters, the critical current and the temperature behavior of the low voltage resistance R0.

What happens in Josephson junctions at high critical current densities

“…A comparative study of the phase dynamics of biepitaxial JJs on STO and LSAT substrates [25][26][27]51], confirms the effects of the stray capacitance of the STO substrate [58][59][60]. These experiments [25,51] use SCD measurements for a more sophisticated estimate of the effective C and have given a more quantitative account of the effects of nonequilibrium heating mechanisms in high-J c junctions [52]. The relation between C and J c might be more subtle and more questionable at high-J c values.…”

“…HTS JJs are fundamental reference systems despite their complexity, because they span a wide range of junction parameters, as for instance J c , the specific resistance R n A, where A is the cross section, and because their characteristic energies can be also scaled over three orders of magnitude. Figure 1, panels (h) and (i), refer to YBCO biepitaxial GB junctions with J c of 65 and 5 A/cm 2 , respectively [25]. The I-V curves are highly hysteretic, with a difference between the critical and the retrapping current up to 70% at 300 mK.…”

“…The voltage state involving steady motion of the phase is instead dominated by the higher quality factor Q. Therefore, the system will exhibit a different damping depending on the voltage (frequency) range [25].…”

“…Interestingly, hysteresis and phase diffusion coexist in the I-V curves. In order to account for this coexistence, a modified RSJ model is required, including a frequency dependent damping [16,25,28,29].…”

“…In all the three rows, the panel on the left reports the temperature dependence of nonhysteretic I-V characteristics, while the central panel shows the case of hysteretic I-V curves, respectively. In panel (g) the I-V curves refer to ferromagnetic NbN-GdN-NbN spin filter junctions with different values of J c [21], and finally panel (h) and panel (i) show the I-V characteristics of YBCO biepitaxial GB junctions with J c of 65 A/cm 2 and 5 A/cm 2 , respectively [25]. The black line in panel (i) indicates the finite slope R 0 of the supercurrent branch (see the text).…”