Charge dynamics in CuGeO3: a combined RIXS and EELS study

Charge dynamics in CuGeO3:

a combined RIXS study at O-K and Cu-L3 edges

V. Bisogni, C. Monney, J. MalèK, S.-L. Drechsler, R. Kraus, K.J. Zhou,

V. Strocov, J. van den Brink, B. Büchner, T. Schmitt and J. Geck

FundingFundingFundingFunding:

NGSCES Conference 3-8 July 2011 – Santiago de Compostela

Acknowledgments

V. Bisogni – IFW Dresden NGSCES 2011 1

IFF Dresden:

Jochen Geck

Roberto Kraus

Martin Knupfer

Bernd Büchner

ITF Dresden:

Jiri Malèk

Krzysztoff Wohlfeld

Liviu Hozoi

Roman Kuziak

Satoshi Nishimoto

Stephan-L. Drechsler

Jeroen van den Brink

SLS - PSI Villigen:

Claude Monney

Kejin Zhou

Vladimir Strokov

Thorsten Schmitt

Université Paris-Sud :

Alexandre Revcoleshi FundingFundingFundingFunding:

ICAM-I2CAM : Travel expenses have

been supported by I2CAM trough the

NSF grant number DMR-0844115.(ICAM-I2CAM, 1 Shields Avenue, Davis, CA 95616)

Outline

• Introduction to the case of CuO2 plane

• System under study: CuGeO3

• Experimental method: Resonant Inelastic X-ray Scattering (RIXS)

• Motivations: d-d excitations and Zhang-Rice singlet excitons

• Conclusions


t

U

Correlated electron systems: example of a CuO2-plane

Cu:d9→one hole per Cu

single band Mott-Hubbard (U~8 eV, t~0.4 eV)

-3 eV

∆~5 eV

-5 eV

U~8 eV

LHB UHB

DOS

E

Introduction


t

U


-3 eV

∆~5 eV

-5 eV

U~8 eV

LHB UHB

DOS

E

Multi-band Mott-Hubbard model:

O:2p + TM:3d bands

Introduction


t

U


-3 eV

∆~5 eV

Zhang-Rice Singlet

d9L

ZRS

-5 eV

U~8 eV

LHB UHB

DOS

E

Multi-band Mott-Hubbard model:

O:2p + TM:3d bands

Introduction


t

U


-3 eV

∆~5 eV

ZRS

-5 eV

U~8 eV

LHB UHB

DOS

E

high energy interactions (U,∆,t) are

connected to

low energy excitations:

• ZRS states close to EF

• localized spins→spin dynamicsFurther:

• QP couple to magnons/phonons

• phonons/magnons can influence

high energy scales

Introduction


• model compound of one-dimensional edge-sharing cuprate

• composed by the same unit as the high-Tc cuprates: CuO4 plaquette

• Cu2+: 3d9 S=1/2

• θ = 99°⇨ AFM J1 (~160 K) and AFM J2 (α·J1~ 56 K)*

• Spin-Peierls transition at TSP=14 K and dimerization starting at T<60K

CuGeO3

a

b

c

CuO2 chain

CuO2- chain


*Fabricius et al., PRB 57, 1102 (1998)

Resonant Inelastic X-ray Scattering (RIXS)

RIXS

*RIXS Review: L.J.K. Ament et al., arXiv1009.3630 (2010)


• photon in – photon out

• site and chemical selective

• energy loss: ∆E=hν2-hν1

• momentum transfer Q=ko-ki• in-plane Q projection: q// ∝Q·sinδ

SAXESSwiss Light Source

Politecnico d i Milano&

G. Ghiringhelli et al., Rev. Sci. Instrum. 77, 113108 (2006)

hν1,

hν2,

RIXS


Cu-L3 edge

2p3/2

3d

∆∆∆∆S=0,±1 ∆∆∆∆S=0

1s

2p

O-K edge

• d-d excitations

• magnetic excitations (∆S=0,1)

• charge-transfer excitations

• magnetic excitations (∆S=0)

• d-d excitations

i f

n

q1,hνννν1

q2,hνννν2

ener

gy

loss

tota

l en

erg

y

• intra-site d-d orbital excitations give information on the 3d valence band

• inter-site charge-transfer excitations (Zhang-Rice singlet exciton) give

information on the magnetic correlation at the low energy scale

Motivations

AFM Initial state: d9↓; d

9↑

Final state: d9↓ L ↑; d

10

3z2-r2

x2-y2

xy

xz/yz

3d9

Cu2+

1 hole

In high-Tc cuprate, the Ex2-y2-Ez2 splitis connected to TcPRL 105, 057003 (2010)

∆E

O 2px,y

Cu 3dxy = ZR singlet

exciton


Ligand field

d-d excitations at Cu-L3 edge• d-orbitals have strong spatial anysotropy

→ d-d excitations have strong ε dependence

• single ion model *+ ab-initio quantum chemical calculations**

* M. Moretti Sala et al., NJPhys 13, 043026 (2011);

** L. Hozoi et al., ariXiv:1012.3603 (2010)

δ=-55°

δ=-25°

δ=0°

δ=25°

δ=55°


928 930 932 934

Inte

nsity (

a.u

.)

photon energy (eV)

CuGeO3 XAS

Cu-L3

θ=90°,

T=40 K

H

V

RIXS Energy resolution ∆E=120 meVδ =−55°

ε

δ

δ =55°

εδ

Exy=0 eV

Ex2-y2=1.55 eV ΓΓΓΓx2-y2=0.12 eV

Exz/yz=1.7 eV ΓΓΓΓxz/yz=0.125 eV

Ez2=1.85 eV ΓΓΓΓz2=0.14 eV

6 4 2 0

E3

E1

energy loss (eV)

E2

Spin-charge excitations at O-K edge

hωin

hωout

V

Hq

θθθθ

δδδδ

T=9 K

θ=65°↔ δ=0°

Pol H, q=0


RIXS Energy resolution ∆E=60 meV530 531 532

E1

δ=0° (θ=65°)

Hpol

photon energy (eV)

XAS

E2

E3

5 4 3 2 1 0

Inte

nsity (

a.u

.)

9 K

40 K

75 K

100 K

150 K

200 K

250 K

300 K

energy loss (eV)

Temperature dependence

d-d excitations

No T dependence

charge-transfer excitations

strong T dependence530 531 532

E2 δ=0° (θ=65°)

Hpol

photon energy (eV)

XAS

A: ~ elastic

line

B:

C: Zhang-Rice singlet (ZRS) *

D: ?

* S. Atzkern et al., PRB 64, 075112 (2001); J.Kim et al., PRB 79, 094525 (2009)


,

ZRS excitons T dependence

Peak C at 3.8 eV:

• Zhang-Rice singlet exciton*

• spectral weight depends on the nearest

neighbor (NN) spin-spin correlation

5.0 4.5 4.0 3.5

300 K

250 K

200 K

150 K

100 K

75 K

40 K

9 K

energy loss (eV)

Spe

ctr

al W

eig

ht

(T)-

Spe

ctr

al W

eig

ht

(30

0 K

)∆E=0.8 eV

CD

J1=160K


Peak D at 4.6 eV

• Zhang-Rice object ⇔ same T trend as

peak C

• What kind of ZR exciton?

* S. Atzkern et al., PRB 64, 075112 (2001); J.Kim et al., PRB 79, 094525 (2009)

Low Temperature

High Temperature

O-K RIXS theory (by J. Malèk and S.-L. Drechsler, IFW Dresden)

O 2px

Cu 3dxy

* Y. Mizuno et al., PRB 57, 5326 (1998)

** K. Okada et al., JPJS 76, 123706 (2007)

O 2py

x

y

Temperature dependence:

• RIXS initial states ⇔ eigenstates which

can be thermally polutated

• ∆S=0 at the O-K edge E (

me

V)

0

10

22

GS S=1/2

ES1 S=1/2

ES2 S=3/2


Model:

• cluster of 3 plaquettes with open boundary conditions;

• pd-Hamiltonian* based on 2px and 2py orbitals for O and 3dx2-y2 orbital for Cu;

• RIXS cross-section **:

5 4 3 2 1

GS

ES1

ES2

In

ten

sity (

a.u

.)

energy loss (eV)

CuGeO3

E (

me

V)

0

10

22

GS

ES1

ES2

0 50 100 150 200 250 300

0.0

0.2

0.4

0.6

0.8

1.0 GS

ES1

ES2

Pro

ba

bili

ty

temperature (K)

• weights for T dependence

• fundamental RIXS spectra for CuGeO3


O-K RIXS theory

5.0 4.5 4.0 3.5 3.0

Inte

nsity (

a.u

.)

9 K

40 K

75 K

100 K

150 K

200 K

250 K

300 K

energy loss (eV)

5.0 4.5 4.0 3.5 3.0

Inte

nsity (

a.u

.)

10 K

20 K

50 K

80 K

100 K

150 K

200 K

250 K

300 K

energy loss (eV)

0 50 100 150 200 250 300

0

5

10

Pe

ak A

rea

(a.u

.)

temperaturte (K)

EXPERIMENT

Peak Area*:

C

D

0 50 100 150 200 250 300

0

5

10

Pe

ak A

rea

(a.u

.)

temperaturte (K)

THEORY

Peak Area*:

C

D

C

D

C

D

* After subtraction of the spectrum at 300 K


Temperature

dependence of ZRS

depends on the spin

configuration at the

ground state

O-K RIXS theory

THEORY EXPERIMENT

O 2px

Cu 3dxy

O 2py

x

y

O 2px

Cu 3dxy

O 2py

Two ZRS peaks �

charge-transfer paths through Opx

and Opy

Two ZRS peak�

charge transfer path between

nearest-neighbor (NN) and

next-nearest neighbor (NNN)*

Explanation 1: Explanation 2:

RIXS calculation on a dimer (two-plaquette cluster)


* Y. Matiks et al., PRL 103,187401 (2009)

O-K RIXS theory

• 4.5 eV peak is present only in the trimer

� NNN ZRS exciton

• trimer is the smallest cluster which describes


and

O-K RIXS theory

5 4 3 2 1

in

tensity (

a.u

.)

trimer

dimer

energy loss (eV)

• deep understanding of d-d excitations energy and symmetry in CuGeO3

by Cu-L3 RIXS

• clear evidence of NN and NNN Zhang-Rice singlet excitons enhanced by

O-K RIXS

• the temperature dependence of the ZRS excitons is linked to the local

magnetic correlation at the ground state

• high-energy charge-transfer excitations (~4 eV) are directly connected to

the magnetic phenomena at much lower energy scale (J1~10 meV)

Conclusions



Thanks for your attention

Dresden from the Elbe River

Documents

Charge dynamics in CuGeO3: a combined RIXS and EELS study