Upload
others
View
5
Download
0
Embed Size (px)
Citation preview
HTc Josephson nano-junctions :physics and applications
Collège de France 2008
Jérôme Lesueur
Phasme team : www.lpem.espci.fr/phasme
Nicolas Bergeal
Martin Sirena
Thomas Wolf
Giancarlo Faini
Rozenn Bernard, Javier Briatico, Denis Crété
Nicolas BergealMartin SirenaThomas WolfAlexandre Zimmers
Pascal Febvre
Denis Crété
Sophie Djordjevic
Robin Cantor
RSFQ
Etalon du Volt
THz
SQUID
Quantum circuits ?
No quantification of the EM field (cf M. Devoret’s lecture)
No quantum states manipulation (cf O. Buisson’s seminar)
1st
2d
!
| 0"+ | 1"
2
Phase properties and superconducting circuits
Phase rigidity of the condensate
!
" =0"
i#
e
!
V( t)dt0
T
" =0
#
!
d" = 0#
dx2−y2
++-
-kx
ky
Intrinsic phase structure in High Tc SC
Josephson nano-junctions
π-junctions
High speed applications
Analog to Digital converters
Communication systems
PetaFLOPS computer …
Superconductive electronics
Superconductors Semiconductors
CMOS transistorJosephson Junction
Aluminium
Aluminium
AlOxSiOx
Silicon1-2 nm
1-2 nm
Quantum circuit
Dream or reality ?
An example : a digital frequency divider
770 GHz ; 1.5 µWTechnology : Nb/Al/AlOx/Nb @4.2 K
Outline
1. Superconductive electronics
3. Physics of High Tc nanojunctions
Dynamics of Josephson Junctions
Rapid Single Flux Quanta logic
Actual RSFQ devices
Proximity effect
Quasi-classical diffusive approach
D-wave order parameter symmetry
π-junctions and RSFQ devices
4. Conclusions
2. High Tc Josephson nanoJunctionsIon irradiation of High Tc Superconductors
Making Nanojunctions
Major characteristics of the Nanojunctions
A few applications
Outline
1. Superconductive electronics
3. Physics of High Tc nanojunctions
Dynamics of Josephson Junctions
Rapid Single Flux Quanta logic
Actual RSFQ devices
Proximity effect
Quasi-classical diffusive approach
D-wave order parameter symmetry
π-junctions and RSFQ devices
4. Conclusions
2. High Tc Josephson nanoJunctionsIon irradiation of High Tc Superconductors
Making Nanojunctions
Major characteristics of the Nanojunctions
A few applications
Josephson Junctions
ψ2=|ψ2 | eiϕ2ψ1=|ψ1 | eiϕ1
SuperconductorSuperconductor Barrier
Josephson Junctions Josephson equations
!
I = Icsin(")
!
" ="1#"
2
Barrier : insulator, normal metal, constriction …
Applications : superconductive electronics
• Photon detection : 1 junction
• SQUID for magnetometry, voltmetry … : 2 junctions
• Rapid Single Flux Quantum (RSFQ) logic : from 10 to millions junctions !
483 597,9 GHz/V
Josephson Energy
!
JE = 0"
cI
2#(1$ cos(% ))
!
"#
"t=2eV
h=2$V
0%
Modeling a Josephson Junction
RC Shunted Junction Model
Josephson Junction : a non-linear inductance
!
I (t) =JI (t)+ RI (t)+ CI (t)
!
V( t) = 0"2#
$%( t)
$t
!
V( t) =JL"
JI ( t)
"t
!
JL = 0"
2#cI cos$( t)
!
I (t) =cI sin("( t))+
V(t)
JR+
JC#V( t)
#t
!
I( t)
cI= sin("(t))+ J0L
JR
#"(t)
#t+
J0L JC
2
# "(t)
#t
The mechanical analogy
Normalized equation
!
J0E"
"#(1$ cos(# )$ i# )+
2
h
2e
%
& '
(
) * 1
JR
"#
"t+
2
h
2e
%
& '
(
) * JC
2
" #2
"t= 0
!
i( t) = I(t)
cI
!
"U +#$x
$t+M
2
$ x
2
$t= 0
!
JE =h
cI2e(1" cos(# ))
« Washboard » potential : phase difference motion
« damping » « mass»
Dynamics of a Josephson Junction
!
I( t)
cI= sin("(t))+ J0L
JR
#"(t)
#t+
J0L JC
2
# "(t)
#t
Characteristic times
!
p" = 02#$ JC
cI
!
LR" = 0#
2$cI JR
!
RC" =JR JC
Mc Cumber parameter
!
C" = RC#
LR#=2$
CI JC2
JR
0%
!
sin"#i+$"
$%+
c&
2$ "
2$%
=0!
" =t
LR"
underdamped
overdamped
!
C" # 1
!
C" >>1
R & C smallNo tunnel
Voltage and current across a current biased JJ
Ibias Current biased overdamped Josephson Junction
!
LR" = 0#
2$cI JR
!
T =2"
LR#2
i $1τLR
!
V =1
TV(t)dt
0
T
" = 0#T
t/τLR0 5 10 15
0
1
2
3
4
5
V(t)
/IcR
J
Quantized pulses
Timescale τLR≈ps
Vmax≈2IcRJ ≈ mV
Energy≈IcΦ0 ≈ aJ
Rapid Single Flux Quanta logic
Likarev’s basic idea (1985) : dynamical logic
Transmission line Transmission line
State 0 State 1IcRJΦ0
Current biased overdamped JJ (Ib < Ic)
Josephson transmission line
Inductance (RF SQUID)
!
L" =
2LCI
0#
Dynamics of an RF SQUID
!
L" =
2LCI
0#
Fluxoid quantification
!
L" <<1
!
L" >>1
!
"
0"
= ext"
0"
# L$2%
sin 2%"
0"
&
'
( (
)
*
+ +
Non-hysteretic
Hysteretic
Storing a pulse
Storing and manipulating pulses
Φ0 < LIc < 2Φ0
Manipulating pulses
RSFQ circuits : JJ and inductances
Φ0 < LIc Φ0 > LIc
Advantages of RSFQ logic
Robust pulse along the way
IcRJΦ0
Φ0
Intrisinc speed : THz frequency
!
LR" = 0#
2$cI JR
Ic ≈ 0.1 - 1 mA
RJ ≈ 1 - 10 Ω1/τLR ≈ 1-10 THz
Low power consumption
Energy ≈ IcΦ0 f ≈ 0.1 - 1 THz P ≈ 0.1 - 1 nW
N ≈ 1O6 gates P ≈ 0.1 - 1 mW
Overdamped : no tunnel junctions
A complete logic library
All logic elements possible
NOT
T-NOR
T-AND
Low Tc Micro processor (ISTEC Japan)
• Technology : Nb/Al/AlOx/Nb @4.2 K
• 8000 to 30000 junctions
• Max speed : 770 GHz, sampler
15.5 GHz 1.6 mW
Superconductive electronics : the Holy Graal ?
Important parameters
!
LR" = 0#
2$cI JR
!
C" = RC#
LR#=2$
CI JC2
JR
0%
unity
!
LR" # JC
CI
Robust against thermal fluctuations
!
CI > Bk T
0"
Increasing current density jC
Increasing superconducting gap IcRN
Decreasing the JJ characteristic spread
Today Low Tc
• Temperature 4.2 K
• Current density 1-3 kA/cm2
• SC gap 1 mV
• 5 to 10% spread
• 30000 JJ
Tomorrow High Tc
100 K
20-30 mV
>>10 kA/cm2
?????
High Tc Superconductive electronics … today
HTSc Josephson Junctions technology :
reliability, ageing, integration, cost effective
Complex materials
Multi-component oxides (YBa2Cu3O7)
High temperature processing (700-800°C)
Very sensitive to disorder …
High Tc A/D convertor (ISTEC Japan)
• Technology : YBCO @77.7 K
• 10 to 15 junctions
Outline
1. Superconductive electronics
3. Physics of High Tc nanojunctions
Dynamics of Josephson Junctions
Rapid Single Flux Quanta logic
Actual RSFQ devices
Proximity effect
Quasi-classical diffusive approach
D-wave order parameter symmetry
π-junctions and RSFQ devices
4. Conclusions
2. High Tc Josephson nanoJunctionsIon irradiation of High Tc Superconductors
Making Nanojunctions
Major characteristics of the Nanojunctions
A few applications
« Standard » High Tc Josephson junctions
Complex materials …
Grain boundary junctions
Alternative technology Ion irradiation
Ramp junctions
Special substrates
Design constraints
Lack of reproducibility
Kahlmann et al & Katz et al APL’98
High Tc cuprate superconductors
Y
Cu
Ba
O
O
bc
High Tc superconductors
Hole-doped in the Cu02 plane
YBa2Cu3O6+x
Tc=90K Order parameter symmetry : dx2-y2
++
-
-kx
ky
kX
ky
Anistropic 2D Fermi Surface
Disorder in High Tc Superconductors
Defect in dx2-y2 superconductor depairing
Abrikosov-Gorkov
YBa2Cu3O7
dx2 −y2
++
-
-
k
k’
kx
ky
Local control of the disorder
Nanoscale engineering (ξN)
30K<T<90K Super/Normal/Super Josephson junction
Substrate
YBCO Tc=90K30K
Proximity effect based Josephson Junctions
SC Normal SC
Phase coherence through normal metal : Josephson coupling
Diffusive case
!
"N(x) #"
N(0
+)e
$x
%N
De Gennes : Rev Mod Physics 1964
!
Ic
= I0 1"T
Tc
#
$ %
&
' (
2
l /)N
sinh(l /)N)
!
"N
=hD
2#kBT
usiv
A few 10 nm
Towards the insulator
Defect in dx2-y2 superconductor depairing
Low defect concentration : Tc decreases
Lesueur et al (1995)
Abrikosov-Gorkov
YBa2Cu3O7
dx2−y2
++
-
-
k
k’
kx
ky
High dose : towards the insulator
YCu
BaO
O
bc
Fabrication of High Tc nano-Junctions
Control the defect density through ion irradiation two-steps strategy :
Abrikosov-Gorkov
YBa2Cu3O7
J. Lesueur et al (1995)
# 1
# 1 : drawing channels : (1015 at/cm2)
# 2
# 2 : creating nanoJunctions : (1013 at/cm2)
Substrate
Film SCInsulator SC channel
Junction
Resist
Au
Oxygen 100 keV
Tailoring at a nanoscale
First step : channel in a YBa2Cu3O7 film (thickness = 1500Å)
Channel
Embedded
Oxygen 100 keV
Second step : nanojunction through a 20 nm slit
30K<T<90K jonction Super/Normal/Super
Oxygen 100 keV
Substrate
Superconductor
Resist
Substrate
SubstrateSubstrateSubstrate
Superconductor
Au
Ins.
Fabrication of nanojunctions
20nm slit
Hall cross
Width 1 to 5µm
Irradiationthrough Au mask
Au IBE removed Irradiationthrough Resist mask
N. Bergeal et al, APL 2005
JAP 2007
R(T) measurements
25
20
15
10
5
0
R (!
)
10090807060504030
Température (K)
Tc
Tc’ Tj
Josephson
regime
L=5µm, E=100keV Φ=6.1013at/cm2
L=5µm, E=100keV
Φ=1.5, 3, 4.5, 6.1013at/cm2
ψ2=|ψ2 | eiϕ2ψ1=|ψ1 | eiϕ1
SupraSupra
3.0
2.5
2.0
1.5
1.0
0.5
0.0
R(!
)
908070605040
Température (K)
Tc’<T<Tj Josephson regime
T<Tc’ <Tj Flux flow regime
Extension of the Josephson regime
ΔT=7K → ΔT=20K
High operating T
I(V) characteristics
Φ=3.1013at/cm2
RSJ behavior
Resistance Rn ranging from 200mΩ to a few Ω
Flux flow behavior
T < Tc’TJ > T > Tc’
70 K72.5 K
Overdamped JJ
Fraunhofer pattern Ic(B)
Phase control by the vector potential
30
25
20
15
10
5
0
Ic (µA
)
-15 -10 -5 0 5 10 15B (Gauss)
73,35 K
Sinc fit
!
Ic
= I0sin("# #0)
"# #0
Φ0
Josephson length λJ
w = 5 µm
λJ = 2 µmλJ = 3 µmλJ = 4 µm
!
I = Icsin(")
!
" ="1#"
2
∇ϕ = Cste JS + 2πφ0-1 A
Shapiro Steps
Phase controlled by microwaves
Almost ideal Bessel functions
N. Bergeal, JAP 2007
!
"#
"t=2eV
h
483 597,9 GHz/V
2000
1500
1000
500
0
Ic (µA
)
7876747270
T (K)
3 1013
at/cm2
500
400
300
200
100
0
Ic (µA
)
85.084.584.083.583.082.5
T (K)
1.5 1013
at/cm2
2000
1500
1000
500
0
Ic (µA
)
72686460
T (K)
4.5 1013
at/cm2
1400
1200
1000
800
600
400
200
0
Ic (µA
)
5550454035
T (K)
6 1013
at/cm2
Ic(T) measurements
!
Ic = I0 1"T
Tj
#
$ % %
&
' ( (
2
l /)Nsinh(l /)N )
De Gennes-Werthamermodel of proximity effect
1.5 1013 at/cm23 1013 at/cm2
6 1013 at/cm24.5 1013 at/cm2
IcRn ~ a few 100 µV
Major characteristics and reproducibility
N. Bergeal, APL 2005,JAP 2007
High Jc
Reproducible JJ
Clues for reproducibility and reliability
Embedded junctions
No annealing of the junctions
In-situ Gold protected YBCO films
All dry process
Low dispersion in IcRn (<10%)
Self-shunted junctions
High values of Jc ( a few 104 A/cm2)
Excellent thermal cycling and aging
N. Bergeal et al, APL 2005
THz detectors
Voltage standard
SQUIDs
RSFQ : T flip-flop
DC SQUIDs with nanojunctions
10µm* 10µm
W ~ 5µm
L = 32 pH
ϕ0 forB~0.08 G
Geometry #1
40
35
30
25
20
15
I c(µA
)
0.20.10.0-0.1-0.2
B (Gaus)
expérience
I=IcIcos(2!"/"
0)I +C
Ic(B) modulation
Cosinus fit with a period 0.08 G
Geometry #2
6µm* 6µm
W ~ 2µm
L = 17 pH
ϕ0 forB~0.3 G
N. Bergeal et al, APL 2006
SQUID modulations
20
15
10
5
0
V(µV)
-0.2 0.0 0.2
B(Gauss)
I=25µA
I=20µA
I=30µA
I=35µA
50
40
30
20
10
V(µ
V)
-1.0 -0.5 0.0 0.5 1.0
B (Gauss)
I=18µA
I=25µA
I=30µA
Modulation of I -> modulation of V
Sensitivity : dV/dΦ = 60 µV/Φ0
Temperature = 77.7 K
Geometry #1 Geometry #2
SQUID characteristics
3 1013 at/cm2
T= 77 K
6 1013 at/cm2
T= 51 K
Fraunhofer + SQUID modulation
Noise < 100pV/√Hz (at 100 Hz)
Sensitivity ≈ qq 10-4 Φ0
Excellent cycling and aging
Networks
N. Bergeal, APL 2006 ; APL 2007
SQUID for « Mr SQUID »
RSFQ T Flip-Flop
Design D. Crété (Thales)
Kim et al (2002) 100 GHz @ 12K
f
f/2
IN
0.24m
A
1.2pH
0.9pH
0.15mA
IC = 150u
0.9pH
Ic=
150u
2.1pH
IC = 300u4.3pH
7.1pH
IC=350uA
IC = 300u
0.27mA
Ic=250u
A
1.0pH
OUT
JTL BIAS
Ib1 = 0.25 mA
JTL
TFF BIAS
Ic3 =Ic4 0.38 mA
=
Ib3= 0.28 mA
Ic7 = Ic6 = 0.32 mA
Ic1=Ic20.18 mA
=
Ib4 = 0.12 mA
Ib2 = 0.22 mA
L = 7.1pH
GND
OUT
IN
A
IC=350uA
GND
A
A
f/2
Outline
1. Superconductive electronics
3. Physics of High Tc nanojunctions
Dynamics of Josephson Junctions
Rapid Single Flux Quanta logic
Actual RSFQ devices
Proximity effect
Quasi-classical diffusive approach
D-wave order parameter symmetry
π-junctions and RSFQ devices
4. Conclusions
2. High Tc Josephson nanoJunctionsIon irradiation of High Tc Superconductors
Making Nanojunctions
Major characteristics of the Nanojunctions
A few applications
A bit of physics …
Tc , resistivity r(T) vs defects
Damage profile (SRIM)
?
Reduced size ?
« No-interface » Josephson Junctions
SC Normal SC
!
"N
=hD
2#kBT
Strength of the coupling
Low interface resistance
Fermi Velocities match
Beyond De Gennes’s approximation
No true interface
Self-consistent calculation of the local gap
Calculating the Josephson critical temperature
Calculating Ic(T) for all temperatures
Quasi-classical approach of the proximity effect
ωn=πkBT(2n+1) Matsubara frequencies
!
"(x) = #S2$K
BT sin%
n
&n
'!
tan"N
=#
$N
Usadel equations parametrized in θ
!
hD(x)
2
" 2#n
"x 2$%
nsin#
n+ &(x)cos#
n$'
AB(x)sin#
ncos#
n= 0
Self-consistant gap equation
Homogeneous SC
G=cosθ
F=sinθ
Limits conditions
Pairs
Quasiparticles
Depairing ΓAB
Δ(x) profil along the junctionIn the limit of vanishing current
Computing the Josephson coupling temperature
80
60
40
20
0
T (
K)
86420
dose* 1013
at/cm2
Régime normal
Tj
T'c
Régime Josephson
Régime flux-flow
Flux-flowregime
Normalregime
JosephsonregimeTc’
TJ
1.0
0.8
0.6
0.4
0.2
0.0
!/!
0
-400 -200 0 200 400
x (nm)
89K
88K
87K
86K
84K
82K
80K
74K
60K
40K
N. Bergeal et al, submitted to PRL
TJ
Computing the critical current density
Usadel equations parametrized in θ
Homogeneous SC
G=cosθ
F=sinθ Pairs
Quasiparticles
Depairing ΓAB
!
hD(x)
2
" 2#n
"x 2$%
nsin#
n+ &(x)cos#
n$'
AB(x)sin#
ncos#
n
$hD(x)
2
"(
"x
)
* +
,
- .
2
sin(#)cos(#) = 0
!
hD(x)
2
" 2#n
"x 2$%
nsin#
n+ &(x)cos#
n$'
AB(x)sin#
ncos#
n= 0
Depairing by supercurrent χ
ωn=πkBT(2n+1) Matsubara frequencies
Supercurrent inside the junction
!
j(x) = "#eNDT$%
$xsin
2&'
(
Comparison with eperiments
Quantitative results for the critical current of our Josephson junctions
Depairing coefficient
Defects profile
With NO adjustable parameters
Irradiations on full films
SRIM simulations
Changing the junctions parameters
Importance of the slit size and irradiation doses
4 different doses
Simulate various
slit size
film thickness
irradiation dose, …
optimize Josephson junctions
Specific temperaturescritical currents
Reduce spread (identifyimportant parameters)
Ic(T)
Temperature (K)
Ic(A
)
3 different slit size
Deeper look in the low temperature regime
Want to reach high critical current lower temperature
Strong anharmonicity develops …
Lower temperatures
2.5
2.0
1.5
1.0
0.5
V(t
)/IcR
n
300025002000150010005000
Time (a.u.)
RSFQ pulses robust against anharmonicity
A d-wave order parameter
1D Quasi-classical equations with an isotropic order parameter
2D symmetry of the d-wave order parameter
!
hD(x)
2
" 2#n
"x 2$%
nsin#
n+ &(x)cos#
n$'
AB(x)sin#
ncos#
n= 0
kx
ky
dx2−y2
++-
-kx
ky
• Anisotropy of the order parameter
• Sign change : Andreev Bound States
• π-junctions and RSFQ circuits
Order parameter anisotropy ?
dx2−y2
++-
-
kk’
kx
ky
Orientations θ = 0 °,10 °,20 °,30 °,40 °,45°
(a,b)
(a,b)
θ D-wave order parameter
Order parameter anisotropy ?
θ = 0°θ = 45°
No effect !!
Diffusion ?
2d order parameter ?
Anisotropic pairing !
Back to Eilenberger equations …
No diffusion approximation
Usadel : l < ξ
Eilenberger : l >< ξ
No wave vector dependence
Confined geometry : full d-wave calculation in a channel
a fe
w ξ
Den
sity
of
stat
es
DOS (E=0)
Andreev Bound State
Andreev Bound State
D-wave SCInsulator|!|
e-
x0 dn
h+e-
h+
k=kf
k=-kf
!1ei"1 !2e
i"2
SC1 SC2Metal
Surface Bound State
ϕ1-ϕ2=π Bound State E=0
Making a π junctionw = a few ξ
Gumann et al APL 2007
Ic>0
Ic<0
Ic=I0sin(Δϕ)
Ic=I0sin(Δϕ+π)
Spontaneous super-current
π SQUID
π junction
0 junctionGround state j=0Flux = 0
Ground state j≠0Flux = φ0/2
LIc > φ0
Free energy
Spontaneous current
π-SQUID in RSFQ circuits
Spontaneous half flux quanta in YBCO/Nb SQUIDs
φ0/2
Numerous bias leads in RSFQ circuits
Ortlepp et al Science 2006
Toggle flip-flop
Comparison with and without π-junctions
Actual device
Outline
1. Superconductive electronics
3. Physics of High Tc nanojunctions
Dynamics of Josephson Junctions
Rapid Single Flux Quanta logic
Actual RSFQ devices
Proximity effect
Quasi-classical diffusive approach
D-wave order parameter symmetry
π-junctions and RSFQ devices
4. Conclusions
2. High Tc Josephson nanoJunctionsIon irradiation of High Tc Superconductors
Making Nanojunctions
Major characteristics of the Nanojunctions
A few applications
Conclusions
RSFQ logic : a promissing technology
High Tc Josephson nano-Junctions : a promissing path
Applications are on their way
Proximity effect based Josephson Junctions
D-wave order parameter :it matters !
Thank you !JnJ : Bergeal & al, APL 87, 102502 (2005),
JAP 102, 083903 (2007)Squids : Bergeal & al, APL 89, 112515 (2006)
APL 90, 136102 (2007)Optimization : J. Lesueur & al,
IEEE Trans Appl Sup 17, 963 (2007)M. Sirena & al, JAP 101, 123925 (2007)M. Sirena & al, APL 91, 142506 (2007)M. Sirena & al, APL 91, 262508 (2007)