Phase diffusion in intrinsic Josephson junction arrays

1
Phase diffusion in intrinsic Josephson junction
arrays and low temperature measurements in a FIB
microscope
J.C. Fenton, P.A. Warburton University College
London, Torrington Place, London WC1E 7JE, UK M.
Korsah, C.R.M. Grovenor University of Oxford,
Parks Road, Oxford OX1 3PH, UK
2
Intrinsic Josephson junctions in Tl-2212
Tl2Ba2CaCu2O8 (Tl-2212) is a highly
anisotropic cuprate superconductor Intrinsic
Josephson junctions (IJJs) form between adjacent
CuO2 double-layers We fabricate devices using
photolithography, Ar-ion milling and FIB
milling. Current flow ? layers (highlighted
region) ?measure a series array of IJJs McCumber
parameter ßc 4?eIcCR2/h characterises damping
for resistively and capacitively shunted
junctions (RCSJs) Hysteresic IV
characteristics as expected for underdamped
RCSJs The number of branches corresponds
closely to the number of CuO2 double-layers in
the stack
3
Phase diffusion
for overdamped junctions with EJkT there is a
finite voltage below the switching current (left
figures) thermal fluctuations cause the phase to
start to slip and then retrap phase
diffusion (see top right figure) ?finite
measured (time-averaged) voltage phase diffusion
is observed in IJJs in Tl-2212 5 (bottom left
figure). ?overdamping at ?p

4
Effect of impedance of junction environment

• Different features for switching out of 0 state
  
• compared with subsequent switches
• -rounding for switch from 0 state
• -apparent noise in 0 state
• Our interpretation
• The high-frequency shunting impedance is
• much larger once any junction in the stack
• is in its resistive state
• 0 state. no junctions resistive. impedance
  100?
• -overdamped dynamics
• -phase diffusion causes rounding
• -Additional periodic noise causes
• periodic increase in effective temperature
• phase-diffusion voltage increases 1
• (b) 1 state. one junction resistive. impedance
  2k?

A0.8mm x 0.2mm
R2kW
Z100W
Z100W
(a)
(b)
5
Temperature dependence of Isw for different
branches
1.2
from 0 state A2mm x 2mm
Top figure compares junctions with similar
damping, but different EJ/kT for A gt1?m2, Isw(T)
follows approximately the usual
Ambegaokar-Baratoff (A-B) form (shown by the
solid line). for small junctions Isw(T) out of
the 0 state departs from the usual A-B form
2. For junctions in the a sub-micron
sample, with increasing temperature Isw from the
0 state decreases faster than subsequent
Isw. subsequent Isw(T) approximately follow the
A-B form phase diffusion causes different Isw(T)
for switching from 0 state (as in the small
junctions in the top figure)
1
0.8
0.6
normalised Isw
0.4
from 0 state A0.5mm x 0.75mm
0.2
0
0
20
40
60
80
100
T (K)
A0.6mm x 0.1mm
6
Liquid-He-cooled sample stage in a FIB microscope
In-situ monitoring of IV characteristics of IJJs
inside a FIB/SEM chamber to Tlt20K. This makes
possible real-time IV measurements on sub-mm2
IJJs during FIB milling.
The stage is cooled by a flow of liquid helium
through tubes wrapped around the stage
block. Wiring connections allow four-point IV
measurements Stage drift when cold is lt50 nm per
minute.
7
In-situ measurements during FIB milling
low bias
These two-point IV measurements demonstrate the
capability of our system for making measurements
during FIB milling 4. They show measurements
before and after successive narrowing of a track
containing A0.3mm2 junctions in this case as a
result of fortuitous arrangement of film defects
rather than lateral milling Low-bias
measurements (top figure) -1st Isw ?0 due to
narrowing Higher-bias measurements (bottom
figure) -higher Isw values decrease by a
smaller fraction -high-bias conductivity
decreases
higher bias
A-before milling B,C,D after successively increa
sing milling times
8
Summary Thermally activated phase-diffusion in
small IJJs occurs only when no junctions are in
the resistive state Subsequent junctions behave
as underdamped junctions (no phase
diffusion) due to increased shunting impedance
caused by resistive junctions In-situ
measurements during milling are made possible
using a liquid-helium cooled sample stage in a
FIB microscope References 1 J.C. Fenton, M.
Korsah, C.R.M. Grovenor, P.A. Warburton,
submitted to M2S proceedings. 2 P.A. Warburton
et al., J. Appl. Phys. 95 4941 (2004). 3 Yu.M.
Ivanchenko, L.A. Zilberman, Sov. Phys. JETP 28
1272 (1969). 4 J.C. Fenton, M. Korsah, C.R.M.
Grovenor, P.A. Warburton, to appear in JPCS
(2006). 5 P.A. Warburton et al., Phys. Rev. B
67 184513 (2003).