1/f critical current noise in short ballistic graphene Josephson junctions

Abstract

Short ballistic graphene Josephson junctions sustain superconducting current with a non-sinusoidal current-phase relation up to a critical current threshold. The current-phase relation, arising from proximitized superconductivity, is gate-voltage tunable and exhibits peculiar skewness observed in high quality graphene superconductors heterostructures with clean interfaces. These properties make graphene Josephson junctions promising sensitive quantum probes of microscopic fluctuations underlying transport in two-dimensions. We show that the power spectrum of the critical current fluctuations has a characteristic 1/f dependence on frequency, f , probing two points and higher correlations of carrier density fluctuations of the graphene channel induced by carrier traps in the nearby substrate. Tunability with the Fermi level, close to and far from the charge neutrality point, and temperature dependence of the noise amplitude are clear fingerprints of the underlying materialinherent processes. Our results suggest a roadmap for the analysis of decoherence sources in the implementation of coherent devices by hybrid nanostructures.

Low-frequency critical current noise in graphene Josephson junctions in the open-circuit gate voltage limit

“…This expression has the same form obtained in the fixed gatevoltage limit in Ref. [19]. The only difference is that in the open-circuit regime ε T = e 2 /(SC Q ) enters the noise amplitude instead of ε Q = e 2 /[S(C Q +C g )], where C g = r /(4πd) is the geometric capacitance ( r is the relative dielectric constant of the substrate).…”

“…The singular temperature behaviour of the noise amplitude is a striking consequence of the open-circuit limit. In the fixed gate voltage regime instead the energy scale ε Q tends to the constant value ε Q ≈ e 2 /(SC g ) for low temperatures and the noise amplitude depends linearly on T [19].…”

“…to proximitized superconductivity of the normal metal forming the junction, fluctuations of carrier density in the graphene insert induce fluctuations of Andreev levels, resulting in noise in the critical current of the ballistic GJJ [19].…”

“…In Ref. [19] the fixed-V G regime was considered under the assumption of instantaneous equilibration of carriers in graphene after each tunneling process. In an equivalent circuit description, in the present manuscript we address the infinite external resistance limit of the bias circuit, whereas the zero resistance limit was considered in Ref.…”

“…In an equivalent circuit description, in the present manuscript we address the infinite external resistance limit of the bias circuit, whereas the zero resistance limit was considered in Ref. [19]. In the following, we will refer to these two regimes as the "open-circuit" and "fixed gate voltage" operating conditions, respectively.…”