グラフェンにおけるスピン伝導・ 超伝導近接効果
DESCRIPTION
091127 「グラフェン・グラファイトとその周辺の物理」研究会(筑波大). グラフェンにおけるスピン伝導・ 超伝導近接効果. Akinobu Kanda University of Tsukuba, Japan. Collaborators - PowerPoint PPT PresentationTRANSCRIPT
グラフェンにおけるスピン伝導・超伝導近接効果
091127 「グラフェン・グラファイトとその周辺の物理」研究会(筑波大)
Akinobu KandaUniversity of Tsukuba, Japan
Collaborators
U. Tsukuba H. Goto, S. Tanaka, H. Tomori, Y. OotukaMANA, NIMS K. Tsukagoshi, H. MiyazakiAkita U. M. HayashiNara Women’s U. H. YoshiokaSupported by CREST project.
Outline
• Brief introduction to graphene
• Spin transport in multilayer graphene
• Cooper-pair transport in single and multilayer graphene
Specialty of multilayer graphene
Allotropes of graphite
From Wikipedia
3D diamond, graphite amorphous carbon (no crystalline structure)
1D carbon nanotubes
0D fullerenes (C60, C70 ...)
2D (graphene)
Graphene is a material that should NOT exist!Thermodynamically unstable (Landau, Peierls, 1935, 1937)
In 2004, graphene was discovered by Geim’s group.
Atom displacements due to thermal fluctuation is comparable to interatomic distance at any temperature.
Obtained by mechanical cleavage from bulk graphite. High crystal quality, as a metastable state
Electronic structure of graphene
Linear dispersion at K and K’ points.
Charge carriers behave as massless Dirac fermions, described by Dirac eq.
シュレディンガー方程式parabolicな分散関係
FkvE
Conventional metals and semiconductors have parabolic dispersion relation, ruled by Schoedinger eq.
Electrons and holes correspond to electrons and positrons, having charge conjugation symmetry in quantum electrodynamics (QED).
Relativistic effects in graphene
Klein paradox (propagation of relativistic particles
through a barrier)O. Klein, Z. Phys 53,157 (1929); 41, 407 (1927)
Geim & Kim, Scientific American, April, 2008
Relativistic Josephson effect Superconducting proximity effect
Graphene as a nanoelectronics material
– Electric field effect– High mobility – Band gap possible– Stable under ambient conditions– Easy to microfabricate (O2 plasma etc
hing)– Abundance of resource
K. S. Novoselov et al., Science 306 (2004) 666.
Also good for spintronicsSmall spin-orbit interactionSmall hyperfine interaction
Long spin relaxation length
Multilayer graphene (MLG)
single layer graphene bilayer
bulk graphite
band overlap ~ 40meV
Thickness
Multilayer graphenethickness: 1-10 nm(interlayer distance = 0.34 nm)
Electric field effect Screening of gate electric field
semimetal
interlayer screening length SC ~ 1.2 nm (3.5 layers) (Miyazaki et al., APEX 2008)
Spin transport in multi-layer graphene
FM/MLG/FM sample
Co2
Co1
Cr/Au
Cr/Au4 m
optical microscope image
Scotch tape method
Micromechanical cleavage (Scotch tape method) (Geim, Kim, Scientific American (April, 2008))
Graphene was found in 2004 by Novoselov, Geim et al. (Manchester).
Scotch tape method
Micromechanical cleavage (Scotch tape method) (Geim, Kim, Scientific American (April, 2008), 日経サイエンス( 2008年 7月))
Graphene was found in 2004 by Novoselov, Geim et al. (Manchester).
Scotch tape method
Micromechanical cleavage (Scotch tape method) (Geim & Kim, Scientific American (April, 2008), 日経サイエンス( 2008年 7月))
Repeat cleavage
Graphene was found in 2004 by Novoselov, Geim et al. (Manchester).
Scotch tape method
Micromechanical cleavage (Scotch tape method) (Geim, Kim, Scientific American (April, 2008), 日経サイエンス( 2008年 7月))
Si Substrate with 300 nm of SiO2
Graphene was found in 2004 by Novoselov, Geim et al. (Manchester).
Scotch tape methodGraphene was found in 2004 by Novoselov, Geim et al. (Manchester).
Micromechanical cleavage (Scotch tape method) (Geim, Kim, Scientific American (April, 2008), 日経サイエンス( 2008年 7月))
Under optical microscope
Scotch tape methodGraphene was found in 2004 by Novoselov, Geim et al. (Manchester).
Micromechanical cleavage (Scotch tape method) (Geim, Kim, Scientific American (April, 2008), 日経サイエンス( 2008年 7月))
Optical microscope image
No need for MOCVD...
FM/MLG/FM sample
Co2
Co1
Cr/Au
Cr/Au4 m
optical microscope image
1 m
Co1: 200 nm
Co2: 330 nmL = 290 nm
SEM image
I
V–+
H
thickness ~ 2.5 nm (AFM)(4 - 5 layers)
AFM image
substrate UGF
Highly doped Si substrate is used as a back gate.
F. J. Jedema et al. Nature 416, 713 (2002)Nonlocal measurement
V
I
FM/MLG/FM sample
Co2
Co1
Cr/Au
Cr/Au4 m
optical microscope image
1 m
Co1: 200 nm
Co2: 330 nmL = 290 nm
SEM image
I
V–+
H
thickness ~ 2.5 nm (AFM)(4 - 5 layers)
AFM image
substrate UGF
Highly doped Si substrate is used as a back gate.
F. J. Jedema et al. Nature 416, 713 (2002)Nonlocal measurement
Ferro1 Ferro2Parallel alignment of magnetization
positive voltage
V
I
FM/MLG/FM sample
Co2
Co1
Cr/Au
Cr/Au4 m
optical microscope image
1 m
Co1: 200 nm
Co2: 330 nmL = 290 nm
SEM image
I
V–+
H
thickness ~ 2.5 nm (AFM)(4 - 5 layers)
AFM image
substrate UGF
Highly doped Si substrate is used as a back gate.
F. J. Jedema et al. Nature 416, 713 (2002)Nonlocal measurement
Ferro1 Ferro2Parallel alignment of magnetization
positive voltage
Antiparallel alignment of magnetization negative voltage
V
I
Nonlocal measurement
RP ~ -RAP > 0
Rs
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
-2000 -1000 0 1000 2000
V/I
(
)
H (Oe)
Vg = 0 V
RP
RAP
0.35
0.40
0.45
0.50
0.55
Rs (
)
50
100
150
200
-100 -50 0 50 100
R (
)
Vg (V)
R: 4-terminal resistance of MLG
Rs: spin signal4K
Rs: spin accumulation signal (spin signal)
Nonlocal measurement
RP ~ -RAP > 0
Rs
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
-2000 -1000 0 1000 2000
V/I
(
)
H (Oe)
Vg = 0 V
RP
RAP
0.35
0.40
0.45
0.50
0.55
Rs (
)
50
100
150
200
-100 -50 0 50 100
R (
)
Vg (V)
R: 4-terminal resistance of MLG
Rs: spin signal4K
Rs: spin accumulation signal (spin signal)
Nonlocal measurement
RP ~ -RAP > 0
Rs
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
-2000 -1000 0 1000 2000
V/I
(
)
H (Oe)
Vg = 0 V
RP
RAP
0
0.1
0.2
0.3
0.4
0.5
0.6
0 50 100 150 200 250
Vg > V
n
Vg < V
n
Rs (
)
R ()
Spin signal is a linearly decreasing function of resistance.
4K
Quite different from conventional spin signals
Rs: spin accumulation signal (spin signal)
General expression for spin signalTakahashi and Maekawa, PRB 67, 052409 (2003)
N
N
L
N
F
FN
i
Ji
N
F
F
F
N
i
J
J
iLNS
eRR
pRR
P
RR
p
pRR
P
P
eRR
/222
2
1
22
2
1/
1
2
1
21
114
RN
RN RN
RN RN
RF
RFRi
Ri
PJ: interfacial current polarizationpF: current polarization of F1 and F2L: separation of F1 and F2
General expression for spin signalsTakahashi and Maekawa, PRB 67, 052409 (2003)
N
N
L
N
F
FN
i
Ji
N
F
F
F
N
i
J
J
iLNS
eRR
pRR
P
RR
p
pRR
P
P
eRR
/222
2
1
22
2
1/
1
2
1
21
114
RN
RN RN
RN RN
RF
RFRi
Ri
Two limiting cases are well studied.Tunnel junctions
Co/Al2O3/Al
NLNJS eRPR /2 RN
Jedema et al., Nature 416, 713 (2002).
R1,R2 >> RN >> RF
General expression for spin signalsTakahashi and Maekawa, PRB 67, 052409 (2003)
N
N
L
N
F
FN
i
Ji
N
F
F
F
N
i
J
J
iLNS
eRR
pRR
P
RR
p
pRR
P
P
eRR
/222
2
1
22
2
1/
1
2
1
21
114
RN
RN RN
RN RN
RF
RFRi
Ri
Two limiting cases are well studied.Tunnel junctions
Co/Al2O3/Al
NLNJS eRPR /2 RN
Py/Cu
Transparent junctions
NL
L
N
FN
F
FS Re
e
R
RR
p
pR
N
N 1
1)1(
4/2
/2
22
2
RN RN RN
Jedema et al., Nature 416, 713 (2002). Jedema et al., Nature 410, 345 (2001).
R1,R2 >> RN >> RF RN >> RF >> R1,R2
General expression for spin signalTakahashi and Maekawa, PRB 67, 052409 (2003)
N
N
L
N
F
FN
i
Ji
N
F
F
F
N
i
J
J
iLNS
eRR
pRR
P
RR
p
pRR
P
P
eRR
/222
2
1
22
2
1/
1
2
1
21
114
RN
RN RN
RN RN
RF
RFRi
Ri
Two limiting cases are well studied.Tunnel junctions
NL
L
N
FN
F
FS Re
e
R
RR
p
pR
N
N 1
1)1(
4/2
/2
22
2
Co/Al2O3/Al
NLNJS eRPR /2 RN
Py/Cu
Transparent junctions
RN RN RN
Jedema et al., Nature 416, 713 (2002). Jedema et al., Nature 410, 345 (2001).
R1,R2 >> RN >> RF RN >> RF >> R1,R2
Intermediate interface
RN >> R1,R2 >> RF
NS bRaR
General expression for spin signalTakahashi and Maekawa, PRB 67, 052409 (2003)
N
N
L
N
F
FN
i
Ji
N
F
F
F
N
i
J
J
iLNS
eRR
pRR
P
RR
p
pRR
P
P
eRR
/222
2
1
22
2
1/
1
2
1
21
114
RN
RN RN
RN RN
RF
RFRi
Ri
1 2 1 22 2 2
( ) 2
1 (1 )N
N
R R R R RR R
P P
only under the following condition,
Interface resistance: R1+R2 = 540 (c.f. 490 from independent estimation)
Current polarization: PJ = 0.047 (c.f. PJ ~ 0.1 in Co/graphene[*])
From the fitting and condition (2),
(1)
. (2)
221 2 1 2
2 21 2 1 2
22
1 ( )s
R R R RPR P R
P R R R R
Rs R
Linearly decreasing asymptotic form
Fitting parameters take reasonable values, justifying the fit to eq. (1).
[*] Tombros et al. Nature 448, 571 (2007).
General expression for spin signalTakahashi and Maekawa, PRB 67, 052409 (2003)
N
N
L
N
F
FN
i
Ji
N
F
F
F
N
i
J
J
iLNS
eRR
pRR
P
RR
p
pRR
P
P
eRR
/222
2
1
22
2
1/
1
2
1
21
114
RN
RN RN
RN RN
RF
RFRi
Ri
1 2 1 22 2 2
( ) 2
1 (1 )N
N
R R R R RR R
P P
only under the following condition,
Interface resistance: R1+R2 = 540 (c.f. 490 from independent estimation)
Current polarization: PJ = 0.047 (c.f. PJ ~ 0.1 in Co/graphene[*])
From the fitting and condition (2),
(1)
. (2)
221 2 1 2
2 21 2 1 2
22
1 ( )s
R R R RPR P R
P R R R R R
Linearly decreasing asymptotic form
Spin relaxation length: N >> 8 m RN >> R1,R2 >> RFIntermediate interfaceLonger than N of SLG, Al, and Cu.
Rs
1. Nearly perfect crystal free of structural defects
2. Origins of scattering
Long spin relaxation length in MLG
J. H. Chen et al. Nature Nanotech. (2008)
SLG on SiO2
graphite
MLG
charged impurities
1. Nearly perfect crystal free of structural defects
2. Origins of scattering
Long spin relaxation length in MLG
SiO2 layer
charge impurities, phonon
(multilayer) graphene
contaminant adsorbed molecules
modulation of carrier densitySC
SC: interlayer screening length SC ~ 1.2 nm (3.5 layers) (Miyazaki et al., APEX 2008)
Distance from contaminant and adsorbed molecules becomes larger.Ripple becomes smaller.
J. H. Chen et al. Nature Nanotech. (2008)
SLG on SiO2
graphite
MLG
Smaller scattering Longer spin relaxation length
c.f. N = 1.5 - 2 m in SLG
Tombros et al. Nature 448, 571 (2007).
charged impurities
C1C4
Contact resistance in thick MLG devices
SC
thickness: 5 nm
c1 ( L = 180 nm)c2 ( L = 290 nm)c3 ( L = 380 nm)c4 ( L = 490 nm)
c1 c2 c3 c4
Ni
contact resistance
Contact resistance in thick MLG devices
thickness: 5 nm
c1 ( L = 180 nm)c2 ( L = 290 nm)c3 ( L = 380 nm)c4 ( L = 490 nm)
c1 c2 c3 c4
SC
Ni
contact resistance
C1C4
Contact resistance in thick MLG devices
thickness: 5 nm
c1 ( L = 180 nm)c2 ( L = 290 nm)c3 ( L = 380 nm)c4 ( L = 490 nm)
c1 c2 c3 c4
SC
Ni
contact resistance
C1C4
Contact resistance in thick MLG devices
thickness: 5 nm
c1 ( L = 180 nm)c2 ( L = 290 nm)c3 ( L = 380 nm)c4 ( L = 490 nm)
c1 c2 c3 c4
SC
Ni
contact resistance
C1C4Gate-controllable intrinsic contact
resistance in thick MLG
Layered structure Screening of gate electric field
Contact resistance in thick MLG devices
c1 c2 c3 c4
Ni
SC
Gate-controllable intrinsic contact resistance in thick MLG
Layered structure Screening of gate electric field
Contact resistance in thick MLG devices
c1 c2 c3 c4
Ni
SC
Gate-controllable intrinsic contact resistance in thick MLG
Layered structure Screening of gate electric field
)(2,1 gintrinsicc
contactc VRRR
Rccontact can be reduced.
Contact resistance in thick MLG devices
contact resistance
C1C4
thickness: 5 nm
c1 ( L = 180 nm)c2 ( L = 290 nm)c3 ( L = 380 nm)c4 ( L = 490 nm)
c1 c2 c3 c4
Ni
slope: graphene resistance
)()( ggraphenegintrinsicc VRVR
If one can sufficiently reduce Rccontact,
.constR
R
N
i
Rccontact
Contact resistance and spin signalTakahashi and Maekawa, PRB 67, 052409 (2003)
Transparent junctions
N
N
L
N
F
FN
i
Ji
N
F
F
F
N
i
J
J
iLNS
eRR
pRR
P
RR
p
pRR
P
P
eRR
/222
2
1
22
2
1/
1
2
1
21
114
RN
RN RN
RN RN
RF
RFRi
Ri
NL
L
N
FN
F
FS Re
e
R
RR
p
pR
N
N 1
1)1(
4/2
/2
22
2
RN RN RN
RN >> RF >> R1,R2
Tunnel junctions
NL
NJS ReRPR N /2RN
R1,R2 >> RN >> RF
RN
Transparent junctions (Rccontact) with MLG,
NS RR
(RF ~ 1m)
.constR
R
N
i
46.48
46.49
46.50
-1500-1000 -500 0 500 1000 1500
Vg = 0 V
Magnetic field (Oe)
Sample for local measurement
_I
V+
Thickness: 9 nm
parallel – small R
MLG
antiparallel – large R
Spin valve effect
R
H
Gate voltage dependence
37.71
37.72
37.73V
g = 80 V
46.48
46.49
46.50V
g = 0 V
29.84
29.85
29.86
-1500-1000 -500 0 500 1000 1500
Vg = -80 V
Magnetic Field (Oe)
spin induced magnetoresistance (SIMR)
Oe1200
Oe0)()( dHHRHR
4K
Gate voltage dependence
37.71
37.72
37.73V
g = 80 V
46.48
46.49
46.50V
g = 0 V
29.84
29.85
29.86
-1500-1000 -500 0 500 1000 1500
Vg = -80 V
Magnetic Field (Oe)
spin induced magnetoresistance (SIMR)
Oe1200
Oe0)()( dHHRHR
4K
Might indicate Rs proportional to RN?
Contact resistance and spin signalTakahashi and Maekawa, PRB 67, 052409 (2003)
Transparent junctions
N
N
L
N
F
FN
i
Ji
N
F
F
F
N
i
J
J
iLNS
eRR
pRR
P
RR
p
pRR
P
P
eRR
/222
2
1
22
2
1/
1
2
1
21
114
RN
RN RN
RN RN
RF
RFRi
Ri
NL
L
N
FN
F
FS Re
e
R
RR
p
pR
N
N 1
1)1(
4/2
/2
22
2
RN RN RN
RN >> RF >> R1,R2
Tunnel junctions
NL
NJS ReRPR N /2RN
R1,R2 >> RN >> RF
RN
Transparent junctions (Rccontact) with MLG,
NS RR
(RF ~ 1m)
.constR
R
N
i
Gate controllable
Cooper pair transport in single and multi-layer graphene
Why Cooper-pairs in graphene?
Beenakker, Rev. Mod. Phys. 80, 1337 (2008).
relativity superconductivity
Single layer graphene (SLG)
Injection of Cooper-pairs by proximity effect
Andreev reflection
Intraband A. R.
Interband A. R.
Why Cooper-pairs in graphene?Multilayer graphene (MLG)
semimetal
Usual proximity effect
Large gate electric field effect (-1012cm-2 < n < 10-12cm-2)Never obtained in other SNS systems
S/graphene/S junctions
super-conductor
super-conductor
graphene
Mechanical exfoliation of kish graphite followed by e-beam lithography and metal deposition.
Electrode: Pd(5 nm)/Al(100 nm) or Ti(5 nm)/Al(100 nm)/Ti(5 nm)
Gap of electrodes d ≈ 0.2 - 0.6 m
Doped Si is used as a back gate.
graphene
Josephson effect in SLG
IV characteristics
-3000 -2000 -1000 0 1000 2000 3000-0.2
-0.1
0.0
0.1
0.2
V (
mV
)
I (nA)
-75V -50V -25V 8V
VgT=200mK, B=0.00mT
sweep
Gate voltage dependence
Magnetic field dependence
B = 0
gap: d = 0.22 m
Temperature dependence of critical supercurrent
gap: d = 0.22 m
Vg =-75 V
-50V
75V
50V-25V
25V 0V 8V
Conventional theory for Ic(T)
Long junctions (d >> N)
))/(exp(
)/exp(
0
TT
dIc N
15.0
Nl
Dirty limit:Nl
Clean limit:
Conventional theory for Ic(T)
Long junctions (d >> N)
Short junctions (d << N)
ballistic, ideal interfacediffusive, ideal interface
))/(exp(
)/exp(
0
TT
dIc N
Nl
Dirty limit:Nl
(l: mean free path)15.0
Clean limit:
Two kinds of Kulik-Omel’yanchuk theory
Conventional theory for Ic(T)
Long junctions (d >> N)
Short junctions (d << N)
ballistic, ideal interfacediffusive, ideal interface
))/(exp(
)/exp(
0
TT
dIc N
15.0
Ambegaokar-Baratoff result
Two kinds of Kulik-Omel’yanchuk theory
Nl
Dirty limit:Nl
Clean limit:
Temperature dependence of critical supercurrent
gap: L = 0.22 m
Vg =-75 V
-50V
75V
50V-25V
25V 0V 8V
Temperature dependence of critical supercurrent
KO1 theory(short junctiondirty limit:l << d << N)
I. O. Kulick and A. N. Omel'yanchuk, JETP Lett. 21, 96 (1975).
Vg =-75 V
-50V
75V
50V-25V
25V 0V 8V
Temperature dependence of critical supercurrent
KO1 theory(short junctiondirty limit:l << d << N)
I. O. Kulick and A. N. Omel'yanchuk, JETP Lett. 21, 96 (1975).
Ballistic junction is needed for relativistic Josephson effect!
-75 -50 -25 0 25 50 750
500
1000
1500
2000
Ic(T
=0
) (n
A)
Vg (V)
Ic
-75 -50 -25 0 25 50 750.0
0.2
0.4
0.6
0.8
1.0
Tc
(K)
Vg (V)
Tc
Injection of Cooper pairs into graphene
Ic(T=0)
Tc
Vg =-75 V
-50V
75V
50V-25V
25V 0V 8V
Never seen in other SNS systems
0
50
100
150
200
250
300
-60 -40 -20 0 20 40 60
mfp
(nm
)
Vgate(V)
Making ballistic junctions
Fke
hl
22
ngF VVk 1214 Vm102.7
mean free pathAngle deposition of metals
Substrate
graphene
Resist mask
50 nm
shorter junctions
cleaner graphene
K.I. Bolotin et al., SSC 146, 351 (2008)
Multilayer graphene
Tc of Pd/Al
Temperature dependence of resistance(Inset: Vg dependence of normal-state resistance)
Current-voltage (I-V) characteristics
Vgp
supercurrent
dV/dI at 0.06 K
hole supercurrent
electron supercurrent
Ic
0.2 K
Critical supercurrent Ic depends on the gate voltage. Ambipolar behavior was observed.
I-V curves do not show hysteresis due to small Rn, in clear contrast to the single layer graphene Josephson junctions.
Electron and hole supercurrents
Relation between Ic and Rn
Tc of Pd/Al
Asymmetry in electron and hole supercurrents
Temperature dependence of resistance(Inset: Vg dependence of normal-state resistance)
Electron and hole supercurrents
Temperature dependence of Ic
))/(exp( 20TTIc
02
2
dT
Icd
Vg=75V
60V
45V
Conventional theory for Ic(T)
Long junctions (d >> N)
Short junctions (d << N)
ballistic, ideal interfacediffusive, ideal interface
))/(exp(
)/exp(
0
TT
dIc N
Nl
Dirty limit:Nl
(l: mean free path)15.0
Clean limit:
02
2
dT
Icd
02
2
dT
Icd
measurement
Ambegaokar-Baratoff result
Two kinds of Kulik-Omel’yanchuk theory
In our measurement, = 2.
Possible origin of exp(T/T0)2 behavior
In thick MLG, when large Vg is applied, the carriers at the bottom of the MLG increases due to the screening of the gate electric field.
Assumption:The number of superconducting layers increases with decreasing temperature.
MLG
SC ~ 3.5 layers
SiO2 (300 nm)
Si (Back gate)
Model for Ic(T) of multilayer graphene
Regard each layer as independent single-layer graphene with different carrier density.
Assumptions
Model for Ic(T) of multilayer graphene
Regard each layer as independent single-layer graphene with different carrier density.
Critical supercurrent of each layer follows the KO1 theory. (Note that the results are almost the same for Ambegaoker-Baratoff or KO2 theory.)
Assumptions
Model for Ic(T) of multilayer graphene
Ngate: gate-induced carrier density
)()( ),(0 nNnTnI gateCC
Regard each layer as independent single-layer graphene with different carrier density.
Critical supercurrent of each layer follows the KO1 theory. (Note that the results are almost the same for Ambegaoker-Baratoff or KO2 theory.)
The onset temperature TC(n), and zero-temperature critical supercurrent IC0(n) of n-th layer becomes infinitesimally small when the carrier density of the layer is small enough:
Assumptions
For example
Numerical result
))/(exp( 20TTIc is reproduced in a wide temperature range.
A, B, C... : Onset of supercurrent in 1st, 2nd, 3rd... layers
Message
• Multilayer graphene is also an attractive material!
Screening of gate electric field leads to– Large spin relaxation length– gate-dependent contact resistance
Good for spintronics– Large modulation of supercurrent
Good for superconducting transistors