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Hu-Jong Lee (POSTECH) Switching of the pair-tunneling state to the resistive state in an underdamped Josephson junction can be described in terms of escaping of a phase particle trapped in a current-tilted washboard potential, either by thermal activation (TA) over the potential barrier or by macroscopic quantum tunneling (MQT) through the potential barrier. The crossover from TA in a high temperature range to MQT in a low temperature range has been confirmed in assorted conventional underdamped tunneling-type Josephson junctions as well as inductively coupled stacks of natural Josephson junctions forming in Bi2Sr2CaCu2O8+x high-Tc cuprate superconductors. In this presentation I will introduce the TA-to-MQT crossover observed in the depinning of Josephson vortices from the pinning potential in naturally stacked Josephson junctions and in the switching of the critical current in graphene-based proximity-coupled Josephson junctions. In stacks of natural Jospehson junctions the pinning potential to the Josephson vortices, which arises from the presence of either disorder in the crystal or the crystal edge, plays the role of the washboard potential to the phase particles of ordinary pair tunneling. In the study we measured the depinning current distribution with varying temperatures. Below the crossover temperature, the distribution width is almost saturated and independent of temperature, which suggests quantum depinning of JV chains out of the pinning potential. In the thermal depinning regime above the crossover temperature observed feature will be discussed in terms of two kinds of pinning sources: the pancake- vortex pinning and the crystal edge pinning. In graphene-based Josephson junctions (GJJs) consisting of monolayer graphene in contact with two superconducting electrodes, the supercurrent is carried by massless Dirac electrons and holes through the graphene layer, while the critical current can be tuned by the application of the gate voltage. Here, I present the MQT nature of the switching current in a GJJ, which exhibits a crossover temperature between the classical to quantum regime. Most importantly, the crossover temperature can be controlled by the gate voltage, implying that the discrete energy level of a phase particle is also gate-tunable. A new class of quantum devices such as a gate-tunable phase qubit may be realized by utilizing the MQT behavior of the GJJs. |