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Presentation to the American Physical Society for the March meeting 2007
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1Electrical Characterization of Superconducting Microbridge
Josephson Junctions with Ferromagnetic Strip
Luis Gmez a, b, Shinichi Kitamura b, Takahiro Kubo b, Haruhisa Kitano b, and Atsutaka Maeda b
a Japan Science and Technology Agencyb The University of Tokyo
2Sponsors and Collaborators
Work supported by JST (Japan Science and Technology Agency)
Through the CREST program (Core Research for Evolutional Science and Technology)
Research area: Creation of Ultrafast, Ultralow-Power, Super-performance Nanodevices and Systems
Project: Single-Flux-Quantum Terahertz Electronics
Collaborators: Prof. Akira Fujimaki (Nagoya University) Prof. Masayoshi Tonouchi (Osaka University) and Prof. Yukio Tanaka (Nagoya University)
3Outline Motivation Important Josephson Junction Parameters for
SFQ Circuits Overview on Microbridges F/S Microbridges Results Summary and Future Work
4These values are seldom achieved:
Motivation
) to(close0T,KmV635.0)(
0T,2/)0(
ccNc
Nc
TTTRI
eRI
NcNc RIRI
mVGHz500
0
For HTS with Tc 100 K
Large Gaps 2=20-60meV
IcRN 10s mV
Ambegaokar and Baratoff, PRL10, 486 (1963)
Fabricate HTS Josephson junctions with high IcRNproducts for SFQ circuits
See for example: 10 mV reported by P. A. Rosenthal et. al., APL 63, 1984
(1993) and 8 mV reported by Y. Divin et. al., Physica C 372-376, 115 (2002)
5Important Josephson Junction Parameters for SFQ circuits
Small spread of individual JJs Ic, RN values Minimal parasitic inductance and capacitance For Phase Mode SFQ logic
used overdamped JJs (C
6Overview on Microbridges
Dolan and Lukens, IEEE Trans. on Magnetics. 1, 581 (1977)
S. Shokhor, Appl. Phys.Lett. 67, 2869 (1995)
R. Y. Chiao et. al., Rev. Phys. Appl. 9,
183 (1974)
A. L. Gudkov et. al., Sov. Tech. Phys.
Lett. 5, 506 (1979)
S. Tolpygo and M. Gurvitch, Appl. Phys. Lett. 69, 3914 (1996)
LTS
HTS
Two metals overlay Magnetically form
Direct E-beam written
SFQ circuit
7F/S MicrobridgesFabrication
La2-xSrxCuO4 via PLD KrF excimer laser (=248nm) 250 mJ, 1 Hz 790C, O2 (10-1 torrs) On LaSrAlO4
EBL, liftoff mask for Fe EBL, Mask for microbridge Wet etch in 0.05% HCl
LSCO
400nm FeLSAO
W=2mts=65nml=400nmtf=60nm
Fe
LSCO
LSAO
Suppression of superconductivity
directly underneath Fe strip
8Results (RTs)
5 10 15 20 25 30 35 40 45 500
5k
10k
15k
20k
25k
30k
35k
Without Fe With Fe, 300nm
2006/09/13: LSCO15% 060811_7.5minC_45sec
Temperature ( K )
A400
B300
C200
D100
NO Fe A&B
NO Fe C&D
14 16 18 20 22 24 26 280
50
100
150
200
250
300
350
400
Without Fe With Fe, 300nm
2006/09/13: LSCO15% 060811_7.5minC_45sec
Temperature ( K )
9Results (IVs)
0 3 6 9 12 150
20
40
60
80
100
120
5.0 K 7.0 K 9.0 K 11. 0K 13.0 K 15.0 K 17.0 K 19.0 K
Voltage ( mV )
14 16 18 20 22 24 26 280
50
100
150
200
250
300
350
400
Without Fe With Fe, 300nm
2006/09/13: LSCO15% 060811_7.5minC_45sec
Temperature ( K )
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 210
10
20
30
40
50
60
IC = IC0 1 - T / Tc2
Temperature
K
-100 -80 -60 -40 -20 0 20 40 60 80 100-1000
-800
-600
-400
-200
0
200
400
600
800
1000
T @ 15.00K IcRN ~ 20mV
Without Fe With Fe, 300nm
2006/09/14: LSCO15% 060811_7.5minC_45sec
Ic ~ 20 AJc ~ 15.4 kA/cm
2
RN ~ 1 kN ~ 65m -cm
Voltage ( mV )
Ic [1V criteria]
10Results (Ic vs. B)
22
0
0 46.0457.20 m
GmG
BlWArea eff
nmmm
WArealeff 2302
46.0 2
-80 -60 -40 -20 0 20 40 60 80-10
-8
-6
-4
-2
0
2
4
6
8
10
@18K
B ( G )
+Ic -Ic For 300nm
wide Fe strip
11
I-Vs + microwaves (Shapiro & Photon induced steps)
-150 -100 -50 0 50 100 150-1m
-800
-600
-400
-200
0
200
400
600
800
1m
Voltage (V)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
T=17.5 K2 GHz
0.0 dBm
Fe, 400nm
2006/10/31: LSCO15%060818_5minC_15sec
-50 -40 -30 -20 -10 0 10 20 30 40 50-800
-600
-400
-200
0
200
400
600
800
Voltage (V)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
T=17.5 K2 GHz
0.0 dBm
Fe, 400nm
2006/10/31: LSCO15%060818_5minC_15sec
V
mVGHz
GHzV AC 4500
201
12Summary and Future work We are fabricating Fe / LSCO microbridge junctions
With promising Josephson characteristics and large IcRN products.
The fabrication method is: Simple; compatible with modern lithographic techniques. Suitable for large integration of Josephson junctions into circuits
Junctions may address all the SFQ requirements Reproducibility of junctions is still an open question
Mechanism for junctions is still an open question Method should work for both HTS and LTS
Future work: Fabrication of SQUIDS and Antennas:
13
End of Presentation
Additional information below
14
Ic vs. B (as a function of temperature)
-80 -60 -40 -20 0 20 40 60 800
2
4
6
8
10
12
14
16
18
20
2006/09/15: LSCO15%060811_7.5minC_45sec
B ( G )
Fe, 300nm 15K 18K 19K
15Ferromagnet/Superconducting microbridges
Sample fabrication (Ferromagnet evaporation)
16Ferromagnet/Superconducting microbridges
Sample fabrication (Etch)
17Ferromagnet/Superconducting microbridges
Top view
18Ferromagnet/Superconducting microbridges
Top view
19Ferromagnet/Superconducting microbridge JJs
Finished sample
20Ferromagnet/Superconducting microbridge JJs
Sample dimensions
21Junctions electrical characterization: Experimental setup
New Cryo-ProbeDC-electrical characterizations as a function of :Temperature range (4.2 K-100 K)
Magnetic field up to (350G)
Microwaves up to (20GHz)
20 individual leads
22R vs. T (Josephson junction signature).A
400B
300
C200
D100
NO Fe A&B
NO Fe C&D
LSCO
400nmLSAO
20 25 30 35 40 45 500
1000
2000
3000
4000
5000
400nm Fe Strip
2006/10/27: LSCO15% 060818_5minC_15sec
Temperature (K)25 26 27 28
0
10
20
400nm Fe Strip
2006/10/27: LSCO15% 060818_5minC_15sec
Temperature (K)
E-beam voltage = 50KV
W=2mtS=65nml=400nmtF=60nm
23Comparison with E-beam damaged JJs
S. Tolpygo et al, Appl. Phys. Lett. 63, 1696 (1993)
E-beam voltage = 120KV
24Photolithography made samples
26 28 30 32 34 36 380
200
400
6002006/08/24: LSCO15% 060613_10min_90sec
Without Fe With Fe
Temperature (K)
26 27 28 29 30 31 320
2
4
6
8
10
122006/08/24: LSCO15% 060613_10min_90sec
Without Fe With Fe
Temperature (K)
26 27 28 29 30 31 321m
10m
100m
1
10
100
1k2006/08/24: LSCO15% 060613_10min_90sec
Without Fe With Fe
Temperature (K)
25
RTs and IVs
-1000 -500 0 500 1000
-3
-2
-1
0
1
2
3
@29.0K
IV (B=0 G) IV (B=400G)
Voltage (mV)
0
100
200
300
400
500
dV/dI (B=0 G) dV/dI (B=400G)
2006/11/23: LSCO20%-061013_10min(A)-400nmFe
28.0 28.5 29.0 29.5 30.0 30.5 31.01E-3
0.01
0.1
1
10
100
2006/11/21:LSCO20%061013_10min(A)_12sec
NoFe 400nmFe
IBias = 1A
Temperature (K)
IcRN ~ 4 mV
26Ic vs. T (Wide vs. narrow junctions)
mlJ Lc
J
4.122
2/1
0
0
For T=15K S. Tolpygo et al, Appl. Phys. Lett. 69, 3914 (1996)
4 6 8 10 12 14 16 18 20 220
10
20
30
40
50
602006/09/14: LSCO15% 060811_7.5minC_45sec
Fe, 300nm
Narrow Junction
Wide Junction
Ic
IC = IC0 ( 1 - T / Tc ) 2
Tc 23.7 K
Temperature (K)
0.000
0.002
0.004
0.006
0.008
0.010
Ic1/2
27Possible explanations for the junction formation (theory)
Ferromagnetic proximity effect in F/S multilayer
V. Pea et al, PRB. 69, 224502 (2004)
Buzdin, RMP. 77, 77935 (2005)LTSHTS
28Possible explanations for the junction formation (theory)
Inverse proximity effect in S near F material
M. A. Sillanpaa et al, Europhys. Lett. 56, 590 (2001)LTS
29PLD growth of LSCO films
Schematic Figure of PLD
KrF Excimer Laser=248nm LSCO
Substrate Holder
250mJ,1Hz
Deposition condition:
79010-1torr
Substrate: LSAO
30
PLD growth parameters
31
Film thickness
32LSCO Structure
H.Sato et al. : Phys. Rev. B61 12447.
Y.Maeno et al. : Physica C173 322.
J. B. Torrance et al. : Phys. Rev. Lett.68 3777.
33LSCO characterization- T X-ray
c-axis length: 13.16 (x=0.15)
12.83 (x=0.20)
34
LSCO thin film properties
M. Suzuki and M. Hikita, PRB 44, 249 (1991)
35R-Ts forLSCO15%060811_7.5minC_45sec
16 18 20 22 24 26 281m
10m
100m
1
10
100
1k
10k
100k
1M
10M
400nm100nm
300nm
200nm2006/09/14:LSCO15%060811_7.5minC_45sec
Fe, 400nm Fe, 300nm No Fe, 300&400 Fe, 200nm Fe, 100nm No Fe, 100&200
Temperature (K)
36
0 10 20 30 40 50 60 70 80 90 1000.01
0.10
1.00
La2-xSrxCuO4 (LSCO)
x = 0.08x = 0.15x = 0.20
Temperature (K)0 50 100 150 200 250 300
10-1
100
101
La2-x
SrxCuO
4 / LaSrAlO
4 Thin Film
Temperature (K)
x = 0.07 (t = 460 nm)x = 0.12 (t = 230 nm)x = 0.14 (t = 270 nm)x = 0.16 (t = 140 nm)x = 0.18 (t = 110 nm)x = 0.20 (t = 90 nm)
T. Ohashi, Master Thesis, The university of Tokyo, pp. 41 (2005)
37
-100 -80 -60 -40 -20 0 20 40 60 80 100-1000
-800
-600
-400
-200
0
200
400
600
800
1000
T @ 15.00K IcRN ~ 20mV
Without Fe With Fe, 300nm
2006/09/14: LSCO15% 060811_7.5minC_45sec
Ic ~ 20 A
Jc ~ 15.4 kA/cm2
RN ~ 1 kN ~ 65m -cm
Voltage ( mV )-100 -50 0 50 100
-250
-200
-150
-100
-50
0
50
100
150
200
250
T @ 15.00K
IcRN ~ 20mV
2006/09/14: LSCO15% 060811_7.5minC_45sec
I
Voltage ( mV )
0
500
1000
1500
2000
dV/dI
1E-4 1E-3 0.01 0.1 1 10 100 100010
100
1k
Without Fe With Fe, 300nm
T @ 15.00K
2006/09/14: LSCO15% 060811_7.5minC_45sec
IC [1V] ~ 18 A
IC [1V] ~ 120 A
Voltage ( mV )0 200 400 600 800 1000
-25
0
25
50
75
100
125
150
175 Without Fe With Fe, 300nm
T @ 15.00K
2006/09/14: LSCO15% 060811_7.5minC_45sec
RN [NoFe](380 +/- 7)
RN [Fe-LineB]
(1670 +/- 20)
Current ( A )