Phonon bottleneck in graphene-based Josephson junctions at millikelvin temperatures.

We examine the nature of the transitions between the normal and superconducting branches in superconductor-graphene-superconductor Josephson junctions. We attribute the hysteresis between the switching (superconducting to normal) and retrapping (normal to superconducting) transitions to electron overheating. In particular, we demonstrate that the retrapping current corresponds to the critical current at an elevated temperature, where the heating is caused by the retrapping current itself. The superconducting gap in the leads suppresses the hot electron outflow, allowing us to further study electron thermalization by phonons at low temperatures (T≲1 K). The relationship between the applied power and the electron temperature was found to be P∝T3, which we argue is consistent with cooling due to electron-phonon interactions.

Universal features of counting statistics of thermal and quantum phase slips in nanosize superconducting circuits.

We perform measurements of phase-slip-induced switching current events on different types of superconducting weak links and systematically study statistical properties of the switching current distributions. We employ two types of devices in which a weak link is formed either by a superconducting nanowire or by a graphene flake subject to proximity effect. We demonstrate that independently of the nature of the weak link, higher moments of the distribution take universal values. In particular, the third moment (skewness) of the distribution is close to -1 both in thermal and quantum regimes. The fourth moment (kurtosis) also takes a universal value close to 5. The discovered universality of skewness and kurtosis is confirmed by an analytical model. Our numerical analysis shows that introduction of extraneous noise into the system leads to significant deviations from the universal values. We suggest using the discovered universality of higher moments as a robust tool for checking against undesirable effects on noise in various types of measurements.

Distribution of supercurrent switching in graphene under the proximity effect.

We study the stochastic nature of switching current in hysteretic current-voltage characteristics of superconductor-graphene-superconductor junctions. We find that the dispersion of the switching current distribution scales with temperature as σ(I) proportional to T(α(G)) with α(G) as low as 1/3. This observation is in sharp contrast to the known Josephson junction behavior where σ(I) proportional to T(α(J)) with α(J)=2/3. We propose an explanation using a generalized version of Kurkijärvi's theory for the flux stability in rf-SQUID and attribute this anomalous effect to the temperature dependence of the critical current which persists down to low temperatures.

Phase diffusion in graphene-based Josephson junctions.

We report on graphene-based Josephson junctions with contacts made from lead. The high transition temperature of this superconductor allows us to observe the supercurrent branch at temperatures up to ∼2 K, at which point we can detect a small, but nonzero, resistance. We attribute this resistance to the phase diffusion mechanism, which has not been yet identified in graphene. By measuring the resistance as a function of temperature and gate voltage, we can further characterize the nature of the electromagnetic environment and dissipation in our samples.

Phase-coherent transport in graphene quantum billiards.

As an emergent electronic material and model system for condensed-matter physics, graphene and its electrical transport properties have become a subject of intense focus. By performing low-temperature transport spectroscopy on single-layer and bilayer graphene, we observe ballistic propagation and quantum interference of multiply reflected waves of charges from normal electrodes and multiple Andreev reflections from superconducting electrodes, thereby realizing quantum billiards in which scattering only occurs at the boundaries. In contrast to the conductivity of conventional two-dimensional materials, graphene's conductivity at the Dirac point is geometry-dependent because of conduction via evanescent modes, approaching the theoretical value 4e(2)/pih (where e is the electron charge and h is Planck's constant) only for short and wide devices. These distinctive transport properties have important implications for understanding chaotic quantum systems and implementing nanoelectronic devices, such as ballistic transistors.