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A Josephson junction is the simplest system in which the
dissipative phase transition (DPT), predicted for various systems, can be
obserevd. The physical origin of this transition is the suppression of
macroscopic quantum tunneling of the phase by the interaction with
dissipative quantum-mechanical environment, described by the Caldeira-Leggett
model. Suppression of tunneling in a Josephson junction changes the
character of conductivity by enhancing it essentially. Hence, this
transition is often called a superconductor-insulator transition (SIT).
VI-curves of resistively shunted single Josephson junctions with different
capacitances and tunneling resistances are found to display a crossover at
which a resistance bump (negative second derivative) appears at zero-bias.
The crossover corresponds to the dissipative phase transition
(superconductor-insulator transition) at which macroscopic quantum tunneling
delocalizes the Josephson phase and destroys superconductivity.
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 Our
measured phase diagram, displayed by the solid curve, confirms the concept
of the dissipative phase transition, but it is essentially different from
the original theoretical one (dashed line), being determined by the accuracy
of voltage measurements and by thermal fluctuations |
