SingletTriplet and DoubletDoublet Kondo Effect

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Singlet-Triplet and Doublet-Doublet Kondo Effect
in an Artificial Atom S. Sasaki,1 S. Amaha2,
N. Asakawa2 and S. Tarucha1,2,3   1NTT Basic
Research Laboratories 2Department of Physics,
University of Tokyo 3Mesoscopic Correlation
Project, ERATO, JST E-mail satoshi_at_nttbrl.jp
2
Outline

• Quantum dot is suitable for the study of the
  Kondo effect various parameters tunable
  (gate voltage, magnetic field etc).
  manipulation of spin state is easy
• Advantage of using a vertical quantum dot
  artificial atom
• well defined electron number (down to
  0) and spin state
• Experimental results
• Dot lead coupling G 400meV
• singlet-triplet Kondo for even N
• similar to our previous report (Nature 405
  (2000) 764)
• doublet-doublet Kondo for odd N doublet with
  orbital degeneracy New!

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Electronic states in a circular artificial atom
N and S clearly defined!
2D disk shaped dot
B
8
6
degeneracy
4
2p
2
1s
Harmonic potential
4
Kondo effect in quantum dots
Quantum dot manipulation of spin state via
various parameters (gate voltage, magnetic field
etc.) ? detailed analysis of the Kondo effect
Anti-ferromagnetic spin coupling between dot and
leads
GL
GR
E
m
U
Coherent higher-order tunneling
e0
lead
lead
dot
DOS
Gate voltage fixed at S?0 valley
Increase of S?0 valley conductance
Vd 0
N-1(odd) S1/2
N (even) S0
N1(odd) S1/2
TltTK
dI/dVd
Conductance
dI/dVd
TgtgtTK
logT
0
TK
Gate voltage
Vd
5
Sample structure (leveling technique)
T1.5K
I
Vg
Good pinch-off characteristics
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Fabrication process (leveling technique)
EB-litho.
dry wet etch
AuGe/Ni
PMGI
gate
Ti/Au
AlGaAs barriers
Develop in NMD
AZ5214E
Several seconds in NMD
Ti/Au
to bonding pad
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B-N diagram with large G (Kondo effect)
New Kondo effect expected for Sgt1 ?
Chess-board-like S-T and D-D Kondo
n2
n1
-0.3
Vac3mV (Lock-in) T60mK
Spin flip
MDD
Vg (V)
10mS
N2
N0
-2.4
B (T)
0
6
8
Enhanced Kondo at level crossings
Level crossing (n2)
TKS-TTKD-DgtT gtTKD (gtgtTKT)
m
N4
S0
N3
S1/2
N2
S1
S0
S0
S1/2
N1
S0
N (even)
Doublet-Doublet
B
Orbital degeneracy ? Enhanced Kondo
1/2, 1/2gt
1/2, 1/2gt
S-T Kondo (even N)
1/2,-1/2gt
1/2,-1/2gt
D-D Kondo (odd N)
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S-T and D-D Kondo at orbital crossings
-0.4
2nd Landau level
1st Landau level
Vg (V)
n2
N16
-0.9
1.5
0.2
B (T)
10mS
10
Coulomb diamonds for S-T Kondo
B0
BB0
-0.5
D
D
Vg (V)
Kondo peak _at_Vsd0
T
S
S
D
N15
-0.74
0
0.95
2
1.4
-2
B (T)
Vsd (mV)
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Temperature dependence of S-T and D-D Kondo peak
N16
N15
T. A. Costi and A. C. Hewson, Phil. Mag. B65
(1992) 1165.
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Kondo peak splitting with S-T degeneracy lifting
60mK
S
S
T
Vg
1.4
mS
S-T
Vsd (mV)
0
S
S
T
-1.4
0.95
1.4
B (T)
Coulomb blockade gap
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Kondo peak splitting with D-D degeneracy lifting
60mK
S
T
Vg
D
D
1.4
D-D
Vsd (mV)
0
D
-1.4
1.15
1.35
B (T)
Coulomb gap
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Mysterious behavior at S1/2 ?3/2 degeneracy
dip at Vsd 0
No zero-bias peak in dI/dV??
Conductance
1K
70mK
-1.0
1.0
Vsd (mV)
-0.6
S1/2-3/2 degeneracy
Vsd (mV)
0.6
1.84
2.0
B (T)
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Summary

• Magnetic field induced Kondo effect in a vertical
  quaytum dot
• Kondo effect for even N singlet-triplet
• Kondo effect for odd N doublet-doublet ?New!
• Both give similar Kondo temperature due to
• four-fold degeneracy