"Sub-Micron
Intrinsic Josephson Junctions:
Bridging Between Underdamped and Overdamped Dynamics"
日時：2009/5/15 15:00-16:00
場所：桂キャンパスA1棟 大会議室(A1-313)
講師：Prof. Paul
A. Warburton
(London Centre for Nanotechnology, University College
London, United Kingdom)
アブストラクト
Since the coherence length in
high-temperature superconductors is of order Angstroms,
the fabrication of high-TC Josephson junctions with
artificially-engineered barriers requires very precise
control over the interface transparency. By contrast,
intrinsic Josephson junctions (IJJs) have no artificially-engineered
barriers and furthermore explicitly take advantage
of this short coherence length. Specifically the
c-axis coherence length is less than the separation
of adajacent cuprate double-planes in Bi2Sr2CaCu2O8
and Tl2Ba2CaCu2O8. Hence adjacent cuprate double-planes
are intrinsically Josephson-coupled. When the bias
current flows perpendicular to the planes, the resulting
current-voltage characteristics resemble those of
an array of underdamped Josephson junctions.
In our experiments we have fabricated sub-micron-scale
intrinsic Josephson junctions using Tl2Ba2CaCu2O8
thin films and Bi2Sr2CaCu2O8 single crystals. In
addition to observing several well-known Josephson
phenomena (including the d.c. and a.c. Josephson
effects; Fraunhofer modulation of the critical
current; switching statistics which confirm the
sinusoidal Josephson current-phase relationship;
and phase diffusion) we have observed a number
of effects related to the fact that the dynamics
of the intrinsic junctions are moderately damped
(Q ~ 8) ? i.e. falling between the well-understood
underdamped and overdamped regimes. This results
in a number of counter-intuitive experimental observations
including a suppression of thermal fluctuations
as the temperature is increased and a shift of
the skewness of the switching current distributions
from negative at low temperatures to positive at
high temperatures. We further show that increased
dissipation increases the supercurrent in the moderate
damping regime. We discuss these results in the
light of measurements of macroscopic quantum tunnelling
rates in intrinsic Josephson junctions.
