Abstract
We have studied the temperature dependence of escape phenomena in various underdamped Josephson junctions (JJs). The junctions had different Josephson coupling energies EJ which were relatively small, but larger than the charging energy EC. Upon increasing the temperature T, we first observe the usual cross-over between macroscopic quantum tunnelling and thermally activated (TA) behaviour at temperatures kBT ∼ ℏωp, where ωp is the plasma frequency of the junction. Increasing T further, the width of the switching current distribution has, counterintuitively, a non-monotonic temperature dependence. This can be explained by the novel cross-over from TA behaviour to underdamped phase diffusion. We show that this cross-over is expected to occur at temperatures such that kBT ∼ EJ(1 − 4/πQ)3/2, where Q is the quality factor of the junction at the plasma frequency, in agreement with experiment. Our findings can be compared with detailed model calculations which take into account dissipation and level quantization in a metastable well.Particular attention is paid to the sample with the smallest EJ, which shows extensive phase diffusion even at the lowest temperatures. This sample consists of a dc-SQUID and a single JJ close to each other, such that the SQUID acts as a tunable inductive protection for the single junction from fluctuations of a dissipative environment. By varying the flux through the dc-SQUID, we present, for the first time, experimental evidence of the escape of a JJ from the phase diffusion regime to the free running state in a tunable environment. We also show that in the zero voltage state the losses mainly occur at frequencies near the plasma resonance.
Highlights
DEUTSCHE PHYSIKALISCHE GESELLSCHAFT diffusion regime to the free running state in a tunable environment
If we assume that the two junctions in the dc-SQUID are identical, we can infer that the corresponding critical currents at the lowest temperature are Ic( ) = 200, 128 and 55 nA, at / 0 = 0, 0.28 and 0.41 respectively
The parameters of the measured samples were in the intermediate range, where the simple escape from a single metastable state, and phase diffusion both play a role
Summary
JJ is a weak link (e.g. tunnel junction) between two superconducting electrodes. It can be characterized by its coupling energy. The dynamics of a JJ can be described by the RSCJ model presented in figure 1(b) In this model the junction capacitance and dissipative resistor are in parallel with an ideal JJ. This leads to the model where a fictive phase particle of mass m = h 2/8EC, whose position is given by the difference of the superconducting phase of the junction, φ, resides in a tilted cosine potential. The I–V characteristic of the junction is hysteretic: with increasing current, the voltage will jump abruptly from zero to V ≈ 2 BCS/e at I = Isw < Ic. Here BCS is the superconducting gap. In the case of Q < 1 the I–V characteristic is non-hysteretic and the voltage increases continuously
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
