scanning tunneling spectrosopy of single magnetic adatoms and complexes at surfaces
DESCRIPTION
Scanning Tunneling Spectrosopy of single magnetic adatoms and complexes at surfaces. Peter Wahl Max-Planck-Institute for Solid State Research Stuttgart. STS of adsorbates Spin detection via the Kondo effect Scaling behavior of single Kondo impurities Chemical analysis by STM - PowerPoint PPT PresentationTRANSCRIPT
Scanning Tunneling Spectrosopy of single magnetic adatoms and
complexes at surfaces
Peter WahlMax-Planck-Institute for Solid State Research
Stuttgart
1. STS of adsorbates
2. Spin detection via the Kondo effect
3. Scaling behavior of single Kondo impurities
4. Chemical analysis by STM
5. The Kondo effect of molecules
Experimental
•Low temperature STM operating at 4K•Up to 5T magnetic field•UHV sample preparation•In-situ sample transfer
E F
UE F
E
E
E F
E
E F
UE F
E
E
Scanning Tunneling Spectroscopy
• Energy Resolution governed by the temperature of the tip• dI/dV~LDOS for U<<Φ
E F
E
• Spectra contain contributions from sample and tip
Background subtraction
-100 -50 0 50 100
0.0
0.2
0.4
0.6
0.8
Bias (mV)
LDO
S (
a.u.
)
6.5
7.0
-100 -50 0 50 100
Bias (mV)
dI/d
V (
a.u.
) on
off
Example: CO/Cu(100)
E F
UE F
E
E
Spin detection by STS
• Spin-polarized STS
• Spin-Flip Spectroscopy
• Spin detection via the Kondo effect
The Kondo Effect
26.7
26.6
26.5
26.4
26.8
26.9
1 2 3 4 5
R (
norm
aliz
ed)
T (K )
1 W.J. de Haas, J. de Boer and G.J. van den Berg, Physica 1, 1115 (1934)
2 J. Kondo, Phys. Rev. 169, 437 (1968)
A.C. Hewson, The Kondo Problem to Heavy Fermions (1993)
1934: Resistivity minimum in dilute magnetic alloys1
1968: Kondos explanation by spin-flip scattering2
The spin of magnetic impurities is screened by the conduction electrons.
The Kondo Effect
1 P.W. Anderson, Phys. Rev. 124, 41 (1961)
02
1
KB
JeTk
Anderson Model1
- 1
0
1
2
U
2d
22
~k
TB
Kd+U
Ene
rgy
(eV
)LD O S (a.u.)
e- Impurity
J+,-
Jz
The Renaissance of the Kondo Effect
V. Madhavan et al., Science 280, 567 (1998)J. Li et al., Phys. Rev. Lett. 80, 2893 (1998)
STS on Co/Au(111)
5 nm
-60 -40 -20 0 20 40 60
dI/d
V (
a. u
.)
Bias (mV)Vds (mV)
Vg
0 1.0-1.0
D. Goldhaber-Gordon et al., Nature 391, 156 (1998)S. M. Cronenwett et al., Science 281, 540 (1998)
1998: Single spin in a quantum dot
STS on Cobalt Adatoms
Phys. Rev. B 65, 121406 (2002)
Friedel oscillationsin surface state LDOS
-40 -20 0 20 40
1.0
1.1
1.2
1.3
1.4
dI/d
V (
a.u.
)
Bias (mV)
width = 2 kBTK10 nm
Co/Ag(111)
Lineshape
M. Plihal and J.W. Gadzuk, Phys. Rev. B 63, 085404 (2001)
indirectdirect
-5 0 5
q=100
q=1
q=0
Fano lineshape
1
)(
d
d2
2
x
qx
V
I
0x
STS on Cobalt Adatoms
What determines TK ?
Substrate TK (K) εK (meV)
Cu(111)1 54 1.8
2 53
Cu(100)1 88 -1.3
Ag(111)3 92 3.1
Ag(100)4 41 2.0
Au(111)5 76
6 75 6.5
1 Phys. Rev. Lett. 88, 096804 (2002); 2 H.C. Manoharan, C.P. Lutz, and D.M. Eigler, Nature 403, 512 (2000);
3 Phys. Rev. B 65, 121406 (2002); 4 Phys. Rev. Lett. 93, 176603 (2004);
5 N. Knorr, PhD Thesis, Lausanne (2002); 6 V. Madhavan et al., Science 280, 567 (1998)
Monolayer systems
Kondo effect is dominated by the local environment.
-40 -20 0 20 408
9
10
dI/d
V (a
.u.)
Bias (mV)
TK=92 K
Co/1 ML Ag/Cu(111)Co/Cu(111)
-40 -20 0 20 40
7
8
dI/d
V (
a.u.
)
Bias (mV)
TK=54 K
Co/Ag(111)
-40 -20 0 20 40
10
11
dI/d
V (a
.u.)
Bias (mV)
TK=92 K
Model
U 2.8eV
Δ 0.2eV2
1
2
3
2KB 2
dd nn
U
eU
Tk
O. Ujsaghy et al., Phys. Rev. Lett. 85, 2557 (2000)
nd ~ overlap between adatom and substrate orbitals
Cenn d
a
d
NN
λd 1Åcoordination
distance to nearest neighbor
extent of d-orbital
a
nNN=3 nNN=4
dd UUe
UTk
2
KB 2A.C. Hewson, Cambridge University Press, Cambridge (1993)
Model
• excellent agreement with experimental data
Test of the model:
Position of the resonance
0.8 0.9 1.0 1.1 1.2-202468
occupation nd
K (
me
V)
40
50
60
70
80
90
1000.8 0.9 1.0 1.1 1.2
Co/Ag(111)
Co/Cu(111) Co/Ag(100)
Co/Cu(100)
Co/Au(111)
occupation nd
TK (
K)
Phys. Rev. Lett. 93, 176603 (2004)
The Kondo Effect of Molecules
1 http://www.webelements.com/webelements/elements/text/Co/key.html
Ref. 1
Can we tune the spin by chemistry ?
Preparation
2 nm
U=-0.2V, I=2nA
(110)
(110)
Preparation of Co(CO)n/Cu(100):
1. Deposition of Cobalt at ~150K (Θ~0.005ML)
2. Exposition to ~0.1L CO
3. Annealing to 260-320 K
DFT calculations
Cu
Co
CO
Image size: 1 nm2
Calculation
U=-0.7V, I=2nA
STM image
(110)
(110)
Collaboration with A.P. Seitsonen, University of Zurich
-3 -2 -1
0
5
10
dI/d
V (
a.u
.)
Bias (V)
Breaking ofCo-CO bonds
voltage sweepSTS taken in open feedback mode with stabilization atU=-0.8V, I=0.6nA.
5Å
U=-3V, I=0.6nA
STM induced chemical reaction
molecules are Co(CO)n
2 nm
5 Å
-100 -50 0 50 100
6.0
6.5
dI/d
V (a
.u.)
Bias (mV)
Kondo feature2
Cobalt adatom
2 Phys. Rev. Lett. 88, 096804 (2002)
Chemical Identification
vibrational features1
CO molecule
1 L.J. Lauhon and W. Ho, Phys. Rev. B 60, R8525 (1999)
-100 -50 0 50 100
-1
0
1
dI2 /d
V2 (
a.u.
)
Bias (mV)
Partial Dissociation(110)
(110)
Co(CO)2
Co
Co(CO)4
Rotation of Dicarbonyl(110)
(110)
Co(CO)2
1.56 1.58 1.60 1.62 1.64
-0.1
0.0
0.1
I (
nA)
Time (s)
tip
Cobaltcarbonyls on Cu(100)(110)
(110)
Co(CO)3 Co(CO)4Co Co(CO)2
-100 -50 0 50 100
dI/d
V (
a.u.
)
Bias (mV)-100 -50 0 50 100
dI/d
V (
a.u.
)
Bias (mV)-100 -50 0 50 100
dI/d
V (
a.u.
)
Bias (mV)
TK=88 K TK=170 K TK=283 K
-100 -50 0 50 100
dI/d
V (
a.u.
)
Bias (mV)
TK=165 K
Irontetracarbonyl
5 nm
Preparation as for cobalt …
Fe(CO)4
(Fe(CO)3)2
(Fe(CO)2)2
5 Å
(110)
(110)
-100 -50 0 50 100
dI/d
V (
a.u.
)Bias (mV)
TK142K
-75 -50 -25 0 25 50 75
dI/d
V (
a.u.
)
Bias (mV)
Copperdicarbonyl
Preparation as for cobalt …
Cu(CO)2
5 Å
no Kondo feature !
(110)
(110)
Spin tuning by ligands
0 1 2 3 4
100
200
300
K
ondo
tem
pera
ture
TK (
K)
# of ligands
02
1
KB
JeTk
A.C. Hewson, Cambridge University Press, Cambridge (1993)
dd UJ
110
Spin Mapping
Spatial mapping of the Kondo resonance
-100 -50 0 50 100Bias (mV)
Topography U=0.6V, I=2nA dI/dV (2mV)-dI/dV(-60mV)
5Å 5Å
(110)
(110)
Spin Mapping
5Å 5Å
(Co(CO)2)2 (Co(CO)3)2
-100 -50 0 50 100
dI/d
V (
a.u.
)
Bias (mV)
TK176±13K
-100 -50 0 50 100
dI/d
V (
a.u.
)
Bias (mV)
TK138±21K
(110)
(110)
Spin Mapping
(Co(CO)2)2 (Co(CO)3)2
(110)
(110)
Interaction between Impurities
J
I
?
1 nm
Preparation of cobalt nanostructurestip-induced dissociation
(Co(CO)3)2
-20 -10 0 10 20
0
50
100
150
Hei
ght (
pm)
distance x (Å)
6.4Å
Interaction between Impurities
-100 -50 0 50 100
dI/d
V (
a.u.
)
Bias (mV)
5.12Å TK180K
-100 -50 0 50 100
dI/d
V (
a.u.
)
Bias (mV)
5.72ÅTK100K
-100 -50 0 50 100
dI/d
V (
a.u.
)
Bias (mV)2.56Å
No Kondo
-100 -50 0 50 100
dI/d
V (
a.u.
)
Bias (mV)
TK=88 K
Interaction between Impurities
-100 -50 0 50 100dI
/dV
(a.
u.)
Bias (mV)
T*78±13K
TK368±37K
1D Kondo chain !
Arrays of Magnetic Impurities – 2D
M.A. Lingenfelder et al., Chem. Eur. J. 10, 1913 (2004)
Fe
TPA
Coupling between Fe atoms ? Kondo Effect ?2D Kondo lattice ?
Inelastic Spin Flip Spectroscopy
A. Heinrich et al., Science 306, 466 (2004)
• Spin is locked at kBT<gµBH• Spin flip can be excited for U>gµBH
H
insulating layer
metal
Magnetic Adatom(Mn)
(NiAl(110))
(Al2O3)
ESR-STM
1. Y. Manassen, R.J. Hamers, J.E. Demuth and A.J. Castellano Jr., Phys. Rev. Lett. 62, 2531 (1989)2. C. Durkan and M.E. Welland, Appl. Phys. Lett. 80, 458 (2002)
Spin is fluctuating at kBT>gµBH
H Larmor precession L=gµBH
Detection of noise with L when the tip is placed on top of the atom.
BDPA/HOPG
10 nm
ESR-STM @ 210G
Ref. 2
Conclusions
1. The Kondo effect can be exploited to study the coupling of a single spin.
2. Chemical analysis by STM & Modification of magnetic properties by ligands
3. Spatial mapping of the Kondo resonance with submolecular resolution.
4. Interaction between Impurities.
Acknowledgments
MPI Stuttgart:• L. Diekhöner (now University of Aalborg)• G. Wittich• L. Vitali• M.A. Schneider• K. Kern
Theory:• A.P. Seitsonen (DFT)• O. Gunnarsson, J. Merino, H. Kroha (Kondo)