Sadleir, John E.

NASA Technical Reports Server

Magnetic field dependance of 
the critical current in SIN 
bilayer thin films 
Jack Sadleir 
https://ntrs.nasa.gov/search.jsp?R=20130013415 2019-08-31T00:57:43+00:00Z
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L=29I'm, T=165mK 
- . - lIe ..q(B)1 data 
I ..... (B) data 
Operations on 
Ie ..... (B) data, 
B goes to: 
B- 0.012' I. pO. 
___ B - 0.024' Ie ..... 
-c---B 
I O ~~~~~~~~~~~========:r=============+====~~~===1F. =,======~~~~~~~~~~~~-
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-30 . -20 -10 a 1 1\ 10 
It would follow the minimum value of thelirt\BJseclion of\lc(B) , 
20 30 
with 8 = 8tot= 8a+gl line with the line B=8a=gl 
smaller thermal G less bias current less selHielding should be able to take a larger 9 before 
becoming double valued. 
dlcldB - Uxi * Exp[-Uxi] *L2/PhiO 
dlcldB - L3/(xi PhiO) Exp[-Uxi] 
le-L/xi * Exp[-L/xi] 
Ie - L/xi * Exp[-Uxi] 
dB=PhiO/L2 

$elMield trajectories. Larger self-fielding factor g means more sloped lines. 
Offset of lines comes from different field coild values. You see that your Igl can 
be so large that the Ic is double valued (for some values of the applied field 
coil). Therefore it is possible to have ''too much" self-field such that the range of 
positive magnetic feedback operating region is too small or doesn't exist. 
Plot ITES vSBtot o 
o 
o 
• 
Self-field trajectories. Larger self-fielding factor g means more sloped lines. 
Offset of lines comes from different field coild values. You see that your Igl can 
be so large that the Ic is double valued (for some values ofthe applied field 
coil) .. Therefore it is possible to have "too much" self-field such that the range of 
positive magnetic feedback operating region is too small or doesn't exist. 
Plot ITES vs Btot D 
D 
D 
• 
SelMield trajectories. Larger self-fielding factor g means more sloped lines. 
Offset of lines comes from different fieldcoild values. You see that your Igl can 
be so large that the Ic is double valued (for some values of the applied field 
coil). Therefore it is possible to have "too much" self-field such that the range of 
positive magnetic feedback operating region is too small or doesn't exist. . 
Plot ITES VS Btot o 
o 
o ? 
Self-field trajectories. Larger self-fielding factor g means more sloped lines. 
Offset of lines comes from different field coild values. You see that your Igl can 
be so large that the Ic is double valued (for some values of the applied field 
coil). Therefore it is possible to have "too much" self-field such that the range of 
positive magnetic feedback operating region is too small or doesn't exist. 
Plot ITES V~ Btot 
o 
o ? 
Self~field trajectories. Larger self-fielding factor g means more sloped lines. 
Offset of lines comes from different field coild values. You see that your Igl can 
be so large that the Ic is double valued (for some values of the applied field 
coil). Therefore it is possible to have "too much" self-field such that the range of 
positive magnetic feedback operating region is too small or doesn't exist. 
Designs to Reduce Magnetic 
Cross Talk and potentially 
improve performance 
Electromagnetic Cross Talk 
• When the current in pixel #2 changes from absorbing .an X-ray there are 
two types of EM cross talk at pixel #1. 
Change in local DC field value at TES #1. 
• We want this change in DC magnetic field B value at pixel #1 · to be very small 
relative to the Josephson oscillation period of pixel #1. 
• We can reduce this cross talk by having the currentflowingin #2 cancel 
approximately cancel out better making the field like a higher order pole which will 
have B decay much faster with distance. E.g. isolated wire lead B-r1, versus 
microstrip B-r2 
Induced EMF in the circuit loop connected to #1. 
• Reduce the geometric area of the leads connecting TES #1 . 
~{~:kPV 
=.7r6.9 
• Same array pitch 
• Reduced loop area (light blue) reduced EMF cross talk 
Larger neighboring dipole lead separation and slightly 
better current cancelation both leading to reduced DC B 
cross talk 
• Increased lead self field that is large and asymmetric 
- Reduced criIicaJ current at zero applied field 
- Increased crftlcal current asymma!Jy . 
·What loop matters for 
. EMF's? 
·Better current 
cancelation but larger 
footprint 
-=-<rp J+; .. //~/ ~ tL .... -- / f ______ :!=:~ ______ ~
• Opposite ends different but has 
slighly smal.ler footprint 
Design Considerations 
• Small TES footprint: so we can fit many pixels into the densest possible array. 
• What is the TES current distribution 
- uniform or concentrated aUhe edges? 
- Meandering around fingers/stems or not? 
- Depends upon T. RlRn. and design. 
- This impacts whether the current injection and removal geometry will decrease increase a leave 
unchanged the. critical current of the device and with it determine the Ic asymnietry with bias 
direction. 
• Well canceled TES + Lead current distribution 
So small DC B crosstalk. 
• Small loop 
(so small EMF crosstalk) 
• When the current splits want each arm as uniform as possible 
- E.g. we may not want a mircovia on each arm because if nonuniform may split current differently. 
• Is a continuous superconducting loop of lead material ok or will it produce undesired 
effects? 
Existing microslrip 2D lead design. Lead self field can be 
approximated as uniform over a certain range of high T. As loop 
(blue) becomes sma/ler self field is larger. Ie asymmetry is larger and 
Ie at Ba~O is reduced 
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· · ·~. b 
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C" t~ · fT" . i --., ~ -~ 
+1 . ~ f " · · r,-( . . 
--+ \ii.,..-D-·-·---·t-------' ~ . .-..- .. . 
-I 
Potential issue with simpliest "3D 
lead" concept. 
(+) still increases space to pullout 
leaves 
(+) canreduce DC B cross talk. 
(-) concept drawn below shows . 
increased induced EMF crOSs talk 
beCause iarger loop area and 
potential stronger coupling between 
neighboring pixels. 
All have: 
. I. small loop pick-up area. 
1. small B on neighboring pixels 
2D, self-field--=.:~_;;; ./)/;/ · ~.. . ' /~/' 
reduces Ie at . X!/ ..../ ... ;/ / . __ . ~-a:- /
Ba=O and . .~------r:{,.-------- . ,/ . 
evenBa 
maximized Ie 
3D. Self-field 
increases Ie. 
Fingers. 
3D. Self-field 
increases Ie. 
· ~~;~>Fn '~. 
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Magnetic Field Dependence of the Critical Current in S/N Bilayer Thin Films

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