Transcript Semifluxon dynamics in artificial Josephson 0

Experiments with
semifluxon generator
Edward Goldobin
University of Tübingen, Germany
M. Paramonov, M. Fominsky, V. Koshelets
IRE RAS, Moscow
[Logo of МОН 1.5 here]
Where is Tübingen?
Tübingen:
• small university town on r. Neckar
• population ~80000
• 30 km south from Stuttgart
-- a capital of Baden-Württemberg
University:
• students ~28000
• university is 530 years old !!!
• two faculties of theology ;-)
• strong medicine
• phys., chem., math are small
Tübingen
Our Group (~30 people):
• 2 Profs: R. Kleiner, D. Koelle
• 1 Assistant Prof.
• 4 Post Docs
• ~12 PhD students
• ~10 Diploma students
Josephson junction
I
S
I
S
 1  n s e i
1
 2  ns ei
 1  ns e
i 1
 2  ns ei
Josephson phase:   2  1
+other terms
2
I
S
I
S
Conventional JJ. (0-junction)
Unconventional JJ (p-junction)
2
jc=1–5000 A/cm2, (Nb-AlOx-Nb)
supercurrent
1.0
0.5
phase
0.0
0
-0.5
-1.0
1
2
3
4
5
6
Long Josephson 0-p junction
0
p
x
1D model
YBCO-Nb ramp zigzags
E. Goldobin, R. Straub,
D. Dönitz,
D. Koelle, R. Kleiner,
H. Hilgenkamp (2003).
LTSEM image of supercurrent
 H.-J. Smilde at al. PRL 88, 57004 (2002)
SFS/SIFS junctions
 V. Ryazanov et al. PRL 86, 2427 (2001)
 T. Kontos et al. PRL 89, 137007 (2002)
sine-Gordon equation for 0-p LJJ
— dimensionless damping
— dimensionless field
— the first critical field
Phase discontinuity points!
 Goldobin et al., PRB 66, 100508 (2002)
Semifluxon=vortex carrying F0/2
p

lu
xo
n
1.0
supercurrent
2
0.5
(x)
p
1.5
i -f
2
(b)
se
m
magnetic field
flu
xo
n
2.0
(a)
n
xo
flu
phase
p
1.0
(x)
x(x)
-f l
mi
se
0.5
on
ux
(c)
0.0
-0.5
n
xo
flu
-1.0
0.0
2
-3
-2
-1
0
1
coordinate x
2
3
-3
-2
-1
0
1
2
3
-3
-2
-1
0
1
2
3
coordinate x
coordinate x
Pinned, two degenerate states  and 
 Xu et al., PRB 51, 11958 (1995)
 Goldobin et al., PRB 66, 100508 (2002)
Mechanical analog:pendula chain
(x)
(x)
Semifluxons observation
SQUID microscopy on
YBCO-Nb ramp zigzag LJJs
 H. Hilgenkamp et al. Nature 422, 50 (2003).
Artificial 0-k junctions

A. Ustinov, Appl. Phys. Lett. 80, 3153 (2000).
Goldobin et al., Phys. Rev. Lett. 92, 057005 (2002)
Nb LJJ with two injectors
lJ  30 m (jc  100 A/cm2)
Calibration of injectors
1.0
critical current c
0.6
0.4
0.2
0.0
0.25
0.20
0.15
Iinj = +0.3 mA
critical current Ic (mA)
0.30
0.8
Iinj = +13.5 mA
Experimental
Iinj = –12.8 mA
Numerical
0.10
0.05
0.00
-2p
-p
0
+p
phase discontinuity k
+2p
-40
-30
-20
-10
0
10
20
30
40
current through injectors Iinj (mA)
Goldobin et al., PRL 92, 057005 (2004)
Switching on the current...

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
at zero bias we have a static pinned semifluxon.
bias current pulls semifluxons, just like fluxons.
but semifluxons are pinned --> deformation
at bias=2/p Ic0.64 Ic switching to the R-state
 Goldobin et al., PRB 67, 224515 (2003).
Overcritical bias:oscillator
2.0
L=40lJ
L=20lJ
L=10lJ
0.5
0.0
0
1
2
3
4
5
normalized voltage u
6
7
Frequency depends on:
bias current, damping, length
two outputs shifted by 180°
more stable than flux-flow due
to semifluxon pinning
magnetic field
1.0
voltage
bias current
1.5
 N. Lazarides, PRB 69, 212501 (2004).
Semifluxon -- half-integer ZFS
magnetic field (a.u.)
 Finite length --> image technique:
 1 real semifluxon + 2 anti-semifluxons (images)
 Bias current  Force  SF hopping.
1
0
-1
-3
-2
-1
0
1
coordinate x
Goldobin et al., Phys. Rev. B 67, 224515 (2003).
2
3
Half integer ZFS (full IVC)
bias current, mA
0,5
0,4
0,3
semi-integer ZFS
integer ZFS
0,2
0,1
0,0
0,0
0,5
1,0
1,5
2,0
2,5
3,0
voltage across LJJ, mV
Goldobin et al., PRL 92, 057005 (2004)
bias current (A)
80
60
resonant
structures
40
integer ZFS
Im
semi-integer ZFS
Half integer ZFS
20
0
0
100
200
300
400
voltage across LJJ (V)
Goldobin et al., PRL 92, 057005 (2004)
Experiments last week in IRE
Layout
L = 16..32 m
dx=dw=1, 1.5, 2 m
Calibration: Ic(ICL) & Ic(Iinj)
I-V characteristic of generator
SIS detector
Emission lines
Different bias points along the semifluxon step
Autonomous linewidth ~1.2 … 10 MHz
And now with PLL ;-)
with PLL the spectral ratio is up to 97%
Summary & Problems
Summary
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
many samples work (both generator and detector)
we observe expected operation:
a semifluxon step, SIS pumping
we have measured autonomous linewidth
..and linewidth with PLL
Outlook (problems & todos):
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
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maximum Ic in Ic(H) and Ic(Iinj) are not equal
in high freq. setup strange shunt resistance of ~3 Ohm
in high freq. setup strange dep. of voltale on Iinj.
Repeat again with remaining 2 samples and try to make new ones.
Compare linewidth with inj with the one with CL.
Compare with ZFS
not all LO freq were good