II. Applications of the Josephson Effect
II. Applications of the Josephson Effect
II. Applications of the Josephson Effect
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R. Gross, A. Marx, and F. Deppe © Wal<strong>the</strong>r-Meißner-Institut (2001 - 2013)<br />
4.1.3 Operation and Performance <strong>of</strong> dc-SQUIDs<br />
for given S V (f): maximize H and L:<br />
1. bias current: I ¸ I c : largest modulation <strong>of</strong> hVi(F ext )<br />
2. flux bias: close to (2n+1) F 0 /4<br />
3. junction critical current: coupling energy E J À k B T<br />
simulations show: I c ¸ 5¢I th ´ 5¢2¼k B T/F 0<br />
@ 4.2 K: I c ¸ 1 ¹A<br />
4. loop inductance: should be large<br />
but: <strong>the</strong>rmal noise flux (k B TL) 1/2 ¿ F 0<br />
define <strong>the</strong>rmal inductance: L th I th = F 0 /2<br />
simulations: L · 0.2¢L th<br />
@ 4.2 K: L · 1 nH<br />
define:<br />
5. screening parameter: ¯L = 2I c L/© 0 < 1 no hysteresis in hVi(F ext )<br />
small L, but large area choose ¯L » 1<br />
small I c ≈ 1 ¹A @ 4.2 K L · 1 nH<br />
6. Stewart-McCumber parameter: ¯c < 1 no hysteresis in IVC (external shunt)<br />
choose ¯c ' 1 for large voltage output<br />
AS-Chap. 4 - 20
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