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)