Magnetism and Nanoscale Electronic Properties in

Transition Metal Oxides

B. Büchner

Institute for Solid State Research, IFW Dresden

Institute for Solid State Physics, TU Dresden

Cover: In these scanning tunneling microscope images of a copper oxide superconductor known as Na-CCOC, the topographical map (blue) shows the location of individual atoms on the cleaved surface. The differential conductance map (red) in the same field of view shows that the electronic states are arranged in checkerboard-like spatial patterns. As explained in the story on page 24, similar patterns have been found in other copper oxide superconductors. (Image courtesy of Séamus Davis at Cornell University and Hidenori Takagi at the University of Tokyo. Prepared by Curry Taylor.)

Charge Order in Cuprate Superconductors

Bloch's theorem

Eigenfunction of a particle (electron) in a periodic potential is a product of a plane wave and a periodic Bloch function unk(r)that has the same periodicity as the potential!!

Bloch wave equipotential in silicon lattice

http://en.wikipedia.org/wiki/Bloch_wave or any textbook on condensed matter physics

nk (r) ikr unk(r)

Bloch's theorem for electrons in periodic potential (crystals)

Symmetry of the lattice determines spatial variation of the electronic properties!!

Charge order in transition metal oxides

• Spectacular properties• High temperature superconductivity• Colossal magnetoresistance• Novel collective electronic excitations• Intimately coupled degrees of freedom• Complex phase diagrams • ...

• Technical applications• Problems due to intrinsic inhomogeneity • Self organized electronic nanostructures• ...

• Challenge for solid state physics• Theory: extremely complicated• Materials science

(intrinsic versus extrinsic inhomogeneity)• Interpretation of data • New experimental techniques required

(spatially res. spectroscopy, local probes)• ... J. Zaanen, Nature

’’Theory’’’’Experiment’’

STM on Bi2212 superconductorsS.H. Pan et al. Nature (2000)K.M. Lang et al. Nature (2002)K. McElroy et. al. Nature (2003)etc.

Competing interactions in doped magnets Inhomogeneous charge and spin density

half-doped manganitesCE-Phase, van den Brink, PRL 1999

100 150 200 250 300 3500,5

1,0

1,5 La2CuO4

Antiferromagnet

magnetic susceptibility

TN = 312K

(H = 1 Tesla)

La2CuO4

(

10 -4

emu/

mol

e)Temperature (K)

1

10

100

1000

Insulator

resistivity

(

cm)

Electronic Correlations and Antiferromagnetism

2d-antiferromagnet, J/kB ~ 1500K

Coulomb repulsion UInsulator

Hybridisation t Antiferromagnet

La2CuO4

Magnetism and Superconductivity: Cuprates

antiferromagnetic insulator

Antiferromagnetism and Charge Mobility

static defects (Zn2+,S=0)

Doping charge carriers

0.0 0.1 0.2 0.30

50

100

150

200

250

300

3D-AFM

La2Cu1-zZnzO4

La1.8Eu0.2Cu1-zZnzO4

TN (

K)

Zn content (z)

0,00 0,01 0,02 0,030

50

100

150

200

250

300

3D-AFM TN (

K)

La2-xSrxCuO4

Sr content (x)

M. Hücker et al., PRB’ 99, M. Hücker and B.B., PRB 06

Antiferromagnetism:reduced hole mobilityHole motion:suppressed AFM

Antiferromagnetism:reduced hole mobilityHole motion:suppressed AFM

0 100 200

1E-3

0.01

0.1

1

TN=120Kx=0.018

TN=27K

TN=6K

TN=7K

LTT LTO

x=0.12

x=0.08

x=0.04

La1.8-xEu0.2SrxCuO4

Res

istiv

ity (

cm

)

Temperature (K)TN increases in metallic compounds

Cuprates with static stripes

Antiferromagnetism and Charge Mobility

0 100 200

1E-3

0.01

0.1

1

TN=120Kx=0.018

TN=27K

TN=6K

TN=7K

LTT LTO

x=0.12

x=0.08

x=0.04

La1.8-xEu0.2SrxCuO4

Res

istiv

ity (

cm

)

Temperature (K)TN increases in metallic compounds

Cuprates with static stripes

charge

Spin

st

ripe

charge

charge

Spin

st

ripe

am

ag = 8 a

Antiferromagnetism and Charge Mobility

Stripe model

Stripe model

charge

Spin

st

ripe

charge

charge

Spin

st

ripe

am

ag = 8 a

Stripes: Compromise between hole motion and AFM?

Antiferromagnetism and Charge Mobility

Stripes in Cuprate Superconductors

Emery et al.

Tranquada et al. Nature 95

Static Stripes in (La,Nd)7/8Sr1/8CuO4

spin period

charge period

Tranquada et al. Nature04

Theory and further experiments:Hayden et al.Hinkov, Keimer et al. Vojta et alUhrig et al.Seibold et al. ...

Stripes and/or neutron resonance

Charge order in transition metal oxides

• Spectacular properties• High temperature superconductivity• Colossal magnetoresistance• Novel collective electronic excitations• Intimately coupled degrees of freedom• Complex phase diagrams • ...

• Technical applications• Problems due to intrinsic inhomogeneity • Self organized electronic nanostructures• ...

• Challenge for solid state physics• Theory: extremely complicated• Materials science

(intrinsic versus extrinsic inhomogeneity)• Interpretation of data • New experimental techniques required

(spatially res. spectroscopy, local probes)• ... J. Zaanen, Nature

’’Theory’’’’Experiment’’

STM on Bi2212 superconductorsS.H. Pan et al. Nature (2000)K.M. Lang et al. Nature (2002)K. McElroy et. al. Nature (2003)etc.

Competing interactions in doped magnets Inhomogeneous charge and spin density

half-doped manganitesCE-Phase, van den Brink, PRL 1999

Our 300mK - STM

Density of States (DOS)

dampingsystem

stainless steel barrel

He/³He cryostat27” stroke

manipulator

analysischamber

preparationchamber

0.5m

dampingsystem

stainless steel barrel

He/³He cryostat27” stroke

manipulator

analysischamber

preparationchamber

0.5m

Hänke et al., PRL 2013

Topography + DOS of LiFeAs

„New“ 300mK - STM

Local DOS maps

6K

Resonant soft x-ray scattering

Scattering process

core level

occupied states

unoccupied states

DOS

E•Elastic process:initial state=final state

•Element specific:different edges likeO K-edge, TM L2,3-edge, RE M-edges

•Sensitive to charges, magnetism and orbitals

•Polarization dependence:Excitation of different intermediate states

Intermediate state

O K edge in doped cuprates

UHB

CB

O1s

Romberg, Fink et al., PRB ‘90

C.T. Chen et al., PRL ‘91

EELSCB

UHB

XAS

2x -x

UHBCB

Stripe order in (La,Eu)7/8Sr1/8CuO4

Photon energy dependence

4a

5

4

3

2

1

0

Inte

grat

ed in

tens

ity (a

.u.)

934933932931930929928927Photon energy (eV)

12

10

8

6

4

TEY (a.u)

Cu L3

3

2

1

0

Inte

grat

ed in

tens

ity (a

.u.)

532531530529528527526525Photon energy (eV)

8

6

4

2

TEY (a.u)

O K(0.25,0,0.75)

PRB (2009), PRB (2011)

H. Klauss et al, PRL 2004; H. Grafe et al., PRL 2007; J. Fink et al. PRB (2011)

0.05 0.10 0.15 0.20 0.250

50

100

150

200

250

300

Tc

TN

Sr content (x)

TLT

c

SC

HTT

AFM

AFM

LTT

La1.8-xEu0.2SrxCuO4

LTO

Phase diagrams and stripe order in (La,Eu)7/8Sr1/8CuO4

Stripes or density waves

Science 2012

Nesting and CDW/SDW order

Korshounov & Eremin, Phys. Rev. B 78, 140509(R) (2008)

“Perfect” nesting of electron- and hole-like Fermi pockets

Enhancement of instabilities CDW and/or SDW order

Doping: “Off-tuning” of nesting Suppression of SDW

Stripes or density waves

Science 2012

PRL 2013

Stripes or density waves

RSXS

0.1

0.2

0.3

(r.l.u.)

Incommensurate order of charge and spin

Neutron data: Yamada et al. PRB 1998

• wave vector of charge order equal to 2of spin order of fluctuating stripes

• concentration dependence of is not compatible with nesting scenario

Charge order in half doped manganites

see e.g. P. Reutler J. Cryst. Growth (2002)

Single layer manganite: La1-xSrxMnO4

x = 1.5: half-doped 50% Mn3+ and 50% Mn4+

CE type charge/orbital/spin order

PRL 2009

ARPES on charge ordering manganites

CE type order: Nesting scenario

Mn3+ Mn4+ spin

k space real space

Mn4+

t2g

eg

t2g

eg

Mn3+

Double exchange in manganites

CE type order: Local picture

Mn3+ Mn4+ spin

free ion free ion

Double exchange in manganites

Mn4+

t2g

eg

t2g

eg

Mn3+

free ion free ion

Orbital Polaron Ordering in Manganites

FM metallic propertieshole doping: FM interactions (DE) +destabilization of cooperative Jahn-Teller distortions

Metallic materials tendency to ferromagnetism

but:

120 140 160 180 200 220 240 260 2800

4

8

0.0

0.2

0.4

0.6

Temperature (Kelvin)

(

cm

)

M (

B/M

n )

265 270 2757.0x10-3

8.0x10-3

9.0x10-3

Magneti-zation

resistivity

La7/8Sr1/8MnO3

low temperatures:

coexistence of ferromagnetism

and insulating behavior

Ferromagnetic Insulating Manganites

AFM insulating FM metallic

FMI phase: contradicts DE model Orbital degrees of freedom ???

Uhlenbruck et. al., PRL 98T. Niemoeller et. al. EPJ B99Klingeler et. al., PRB 02Geck et. al., PRB 01Geck et. al., PRB 04Geck et al. PRL 05Geck et al. NJP 06

0.00 0.05 0.10 0.15 0.20 0.25

200

400

FMM

orthorhombic O*

triclinic

mono-clinic

rhombohedral

orthorhombic O'

FMMFMIcanted AFI

TJT

TCO

TC

Tem

pera

ture

[K]

Strontium doping x

Phase Diagram of La1-xSrxMnO3

• Resonance at the Mn K-edge

• -scattering

• I ~ sin²

antiferro-orbital order

Fluores-cence

(300)

200 K

300 K

40

60

80

100

120

140

160

6530 6540 6550 6560 6570 6580 6590 6600

6530 6540 6550 6560 6570 6580 6590 6600

0

20

40

60

80

100-100 -50 0 50 100 150 200 250

0

25

50

75

100

Inte

nsity

@12

0mA

[cps

]

Inte

nsity

@12

0mA

[cp

s]

Energy [eV]

Integrated Intensity [a.u]

[deg.]

Resonant X-Ray Scattering on La7/8Sr1/8MnO3

z=0

z=0.25

z=0.75

z=0.5

a

b

c

Orbital-Polaron

Orbital Polaron Lattice in La7/8Sr1/8MnO3

Co3+/Co4+

La3+/Sr2+

O2-

Phase diagram ofLa1-xSrxCoO3

S=2HS S=0

LS S=1 IS

eg

t2g

Co3+ (3d6)

Co4+ (3d5)

La1-xSrxCoO3: Rich diversity of electronic phases

distorted cubicperovskitestructure

S=5/2HS S=1/2

LS S=3/2 IS

eg

t2g

Sr, x

Wu and Leighton, Phys. Rev. B 67 (2003) 174408

Co3+/Co4+

La3+/Sr2+

O2-

Phase diagram ofLa1-xSrxCoO3

S=2HS S=0

LS S=1 IS

eg

t2g

Co3+ (3d6)

Co4+ (3d5)

distorted cubicperovskitestructure

S=5/2HS S=1/2

LS S=3/2 IS

eg

t2g

Sr, x

Wu and Leighton, Phys. Rev. B 67 (2003) 174408

La1-xSrxCoO3: Rich diversity of electronic phases

0 2 4 6 8 10 120.00

0.01

0.02

0.03

0.04

La0.998Sr0.002CoO3

LaCoO3

M (

C

o)H (T)

T = 2 K

M = 15Bper

doped hole

Huge magnetic response at low T at a tiny Sr doping

Cannot be due to individual Co3+/4+ in any spin state

S = 1/2

Sz = -1/2

Sz = 1/2

E = gBH = h

H

ES

R in

tens

ity

ESR

H

Radical centers, weakly interacting half-integer spin species etc.(chemistry, biology, semiconductor physics)

No restriction on minimal ESR frequency and field

ESR: “Easy case”: isolated spin ½ systems

H H

E

• Quantum spin magnets on the basis of complex oxides

• molecular magnets

• unconventional superconductors,• heavy fermion and Kondo metals

ESR

“Difficult case”: Correlated spin systems

High-Field ESR (IFW: 18T) at high frequencies (IFW: 1.2 THz)

ESR on LCO+0.2% Sr at Low T: Spin Clusters

0 5 10 15

35

40

45

50

55

60

65

T= 4K

Peak 5Peak 4

265,5GHz

155GHz

237GHzPeak 3Peak 2

26.9GHz50GHz

74GHz136GHz

552GHz

455GHz380GHz

285GHz

210GHz

Atte

nuat

ion

(arb

.u.)

B (T)

175GHz

Peak 1 Peak 6 Peak 7

0 2 4 6 8 10 120

100

200

300

400

500

Peak 1Peak 2Peak 3Peak 4

g4=3.18

Peak 5Peak 6Peak 7

g5=2.66

T=4K

285GHz

g7=2.07g6=3.66

g1=18.29

g2=11.24

B (T)

g3=3.55

0 2 4 6 8 10 12

B|| [001]

Peak 3

Peak 2Peak 1

B

A

B (T)

F=384GHz

Atte

nuat

ion

(arb

. u.)

50 K

35 K

20 K

4 KPeak 4Peak 5

Gap170 GHz

~0,71 meV

LCO+0.2% Sr: High Field ESR

Multiple gapped excitationswith large g-factors

Big spin multiplicitySubstantial spin-orbit coupling

Response of a big spin cluster!

0.2 % Sr

9 10 11 12 13

= 360 GHz

ES

R in

tens

ity (a

rb. u

.)

Field (T)

5 K

10 K

20 K

30 K

80 K

100 K

130 K

DPPHmarker

Experiment

1 2 3 4 5 6

0.43 T

ESR: Energy structure of the S = 4 multiplet resolved

g = 2.17; D = -/7 = - 0.27 KNegative anisotropy D

bistable Sz = ± 4ground state !

Transfer of spectral

weight, D < 0

J2

J1

J2

J1

J2

J1

J2

J1

all FM !

Ni(II)4-complex with bistable ground state

S=4 Effective Hamiltonian

eg

t2g

eg

t2g

Co3+, IS Co4+, LS

eg

t2g

eg

t2g

Co3+, ISCo4+, LS Co4+, LSCo3+, IS

Hole-induced spin state polaron due to double exchange

The hole dynamically distributedover the cluster

Resonant state due to double exchangeD. Louca and J. L. Sarrao, Phys. Rev. Lett. 91,

155501 (2003).

0 2 4 6 8 10 120

100

200

300

400

500

Magnetic field (T)

Freq

uenc

y (G

Hz)

0 1 2 3 4

-2.2

-2.0

-1.8

-1.6

-1.4

-1.2

-1/2

Magnetic field (T)

Ener

gy (T

Hz)

3/2

-3/2

1/2

-1/2

0 2 4 6 8 10 12

-4

-2

0

2

4

Ener

gy (T

Hz)

Magnetic field (T)

S = 1 x 6 + 1/2 = 13/2g = 2.6

D = 101 GHzT = 4 Kθ = 500

-1/2 1/2-1/2 -3/2

-1/2 3/2

B

θ

cubicθ = 55o [001]

θ = 0oθ = 110o

Model: Energy spectrum of the spin states

H =μBB·g·S + S·D·S

0 2 4 6 8 10 120

100

200

300

400

500

Magnetic field (T)

Freq

uenc

y (G

Hz)

-1/2 1/2-1/2 -3/2

-1/2 3/2

0 2 4 6 8 10 12

-4

-2

0

2

4

Magnetic field (T)

Ener

gy (T

Hz)

S = 13/2g = 2.6

D = 101 GHzT = 4Kθ = 1000

0 1 2 3 4

-2.2

-2.0

-1.8

-1.6

-1.4

-1.2

Magnetic field (T)

Ener

gy (T

Hz)

3/2-3/2

1/2

-1/2

B

θ

cubicθ = 55o [001]

θ = 0oθ = 110o

H =μBB·g·S + S·D·S

Model: Energy spectrum of the spin states

-1.5 -1.0 -0.5 0.0

0

100

200

300

400

0.4

0.3

B (

T)

T=2 K

H=0.4T

H=0.3T

H=0.2T

Neu

tron

cou

nts

Energy transfer (meV)

H=0T

0.20.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

-2.0

-1.9

-1.8

-1.7

-1.6

-1.5

-1.4

-1.3

-1.2

0.66 meV

1.01 meV

Magnetic field (T)En

ergy

(TH

z)

0.79 meV

-1/2

1/2

-3/23/2

Inelastic neutron scatteringin magnetic field

Model: Energy spectrum of the spin states

x

Wu, Leighton, PRB (2003)

ESR on lightly doped LaCoO3: Conclusions

• Small Sr/Ca doping of LaCoO3 yields extraordinary large magnetic moments

• Spectroscopic investigationsevidence the occurrence of big spin clusters

• Spectrum of the spin states well understood

• Hole doping and not structural distortion is essential

• Nucleation of a spin state polaron (septamer !)

• doping yields intrinsic magnetic inhomogeneities in LaCoO3

• spin polaron is a building block of SG and FM phases

Orbital Polarons

Manganites and Cobaltates:• Partial delocalization of a doped hole by changing the adjacent orbital states• Ferromagnetic nanoclusters due to (local) double exchange interaction

Cobaltates• Change of spin state (IS instead of LS)• Isolated clusters in a non-magnetic background

Manganites• Orientation of orbitals changes (spin state always HS)• Polarons coupled to AFM background complicated magnetic state

Nanoscale Electronic Order in Transition Metal Oxides

i. Spin and Charge Stripes in two-dimensional CuO planes

Observing charge order by resonant soft X-ray scattering

Charge and spin stripes due to mobile holes in afm background

Stripes due to local spins and/or CDW due to nesting?

ii. Charge and Orbital Polaron Ordering in Manganites

CE type order: Local double exchange and/or nesting instability?

Ferromagnetic insulating phase due to orbital polarons

iii. Spin State Polaron in lightly doped Cobaltates

Doped holes in non-magnetic cobaltates

Ferromagnetic nanoclusters due to spin state polarons

THANKS TOJ. Geck, A. Alfonsov, V. Kataev, S. Bahr IFW DresdenR. Klingeler, P. Reutler, U. Ammerahl, T. Hänke, C. Hess S. Borisenko, V. Zabolotnyy, D. Inosov, D.S. Evtushinsky, J. Fink

H. Klauss et al. TU Dresden

E. Schierle, E. Weschke HMIA. Podlesnyak, M. Russina

D. Hawthorn, G. Sawatzky UBC Vancouver

D. I. Khomskii U Köln

A. Revcolevschi et al. U Paris-Sud

M. v. Zimmermann Hasylab

M. Hücker Brookhaven

E. Vavilova Kazan