Measurement of the current-phase relation in Josephson junction rhombi chains

Abstract

We present low-temperature transport measurements in one-dimensional Josephson junctions rhombi chains. We have measured the current-phase relation of a chain of eight rhombi. The junctions are either in the classical phase regime with the Josephson energy much larger than the charging energy, ${E}_{J}⪢{E}_{C}$, or in the quantum phase regime where ${E}_{J}/{E}_{C}\ensuremath{\approx}2$. In the strong Josephson coupling regime (${E}_{J}⪢{E}_{C}⪢{k}_{B}T$) we observe a sawtoothlike supercurrent as a function of the phase difference over the chain. The period of the supercurrent oscillations changes abruptly from one flux quantum ${\ensuremath{\Phi}}_{0}$ to half the flux quantum ${\ensuremath{\Phi}}_{0}/2$ as the rhombi are tuned in the vicinity of full frustration. The main observed features can be understood from the complex energy ground state of the chain. For ${E}_{J}/{E}_{C}\ensuremath{\approx}2$, we do observe a dramatic suppression and rounding of the switching current dependence which we found to be consistent with the model developed by Matveev et al. [Phys. Rev. Lett. 89, 096802 (2002)] for long Josephson junctions chains.