Characterization of Fe 3d states in CuFeS2 by X-ray emission

spectroscopy

K. Sato1,2, Y. Harada3, M. Taguchi3, S. Shin3 and A. Fujimori41. Tokyo University of Agriculture and Technology2. Japan Science and Technology Agency3. RIKEN/Spring-84. University of Tokyo

ICTMC-16に向けた研究討論会2008.8.22

Tokyo University of Agriculture and Technology

CONTENTS

• Introduction: Review on experimental studies on the 3d electronic states of Fe in chalcopyrite CuFeS2 and CuAlS2 :Fe

• Overview of X-ray Emission Spectroscopy Why and how XES provides information on d- d transitions buried in the CT absorption

• XES of CuFeS2

• Interpretation of XES results

Introduction• In the following is presented a review on

experimental and theoretical studies on the 3d electronic states of Fe in chalcopyrite CuFeS2 , CuGaS2 :Fe and CuAlS2 :Fe

www.asahi-net.or.jp/~ug7s-ktu/e_odo.htm

http://www.uwgb.edu/dutchs/PETROLGY/Sphal-Chalc%20Structure.HTM

Cells with iron atoms (orange) alternate with cells containing copper (blue).

FeCu

S

Mystery of CuFeS2

• Chalcopyrite, CuFeS2 is a mineral compound with a golden lustre and has been known as a semiconductor with an antiferromagnetic ordering. The local magnetic moment of Fe has been known to be as small as 3.85 μB from neutron scattering experiments[1].

• The result is conflicting with ionic bonding model of CuFeS2 in which high spin Fe3+(3d5) with local moment of 5 μB is expected.

• To elucidate the electronic structure of CuFeS2optical studies has been conducted in single crystals of CuGaS2 :Fe and CuAlS2 :Fe as well as CuFeS2 .

[1] G.Donnay, L.M. Corliss, J.D.H. Donnay, N. Elliot and J.M. Hastings: Symmetry of Magnetic Structures: Magnetic Structure of ChalcopyritePhys. Rev. 112 (1958) 1917-1923.

Optical absorption spectrum in CuFeS2 and CuGaS2 :Fe and CuAlS2 :Fe

• In order to elucidate electronic structures of Fe in CuFeS2 , Teranishi and Sato studied optical spectroscopy in Fe- doped chalcopyrite- type semiconductors, CuAlS2 and CuGaS2 , and found broad and strong absorption band with two peaks around 1.3 eV and 1.9 eV [2]

Abs

orpt

ion

Coe

ffici

ent ×

10-3

(cm

-1)

Photon Energy (eV)

0

1

2

3

4

321

a)

b)

c)

d)

e)

a) CuFeS2 , b) CuAlS2 :Fe0.07 ,c) CuAlS2 :Fe0.006 , d) CuAlS2 :Fe0.0008 ,e) CuAlS2

4

0 21Photon Energy (eV)

2

4

6

8

Abs

orpt

ion

Coe

ffici

ent ×

10-3

(cm

-1)

CuGaS2 :Fe

CuAlS2 :Fe

a) CuGaS2 :Fe0.006 , b) CuGaS2

[2] T. Teranishi and K. Sato:J. Phys. Soc. Jpn. 36 (1974) 1618-1624.

Theoretical Studies on the Energy Band of CuFeS2

• Kambara calculated absorption spectra of CuFeS2 and CuGaS2 :Fe using a model cluster consisting of 17 atoms.

• Energy gap between occupied and unoccupied levels is 1.0eV and 1.5eV in CuGaS2:Fe corresponding to observed absorption band energies.

• If antiferromagnetic configuration is assumed Fe moment is 2.8μB at the center Fe and -3.7μB at the corner Fe. Overlap of 3d orbitals is responsible to the reduction.

T. Kambara: J. Phys. Soc. Jpn. 36 (1974) 1625-1634 CuGaS2 :Fe CuFeS2

Infrared photoluminescence spectra in CuGaS2 :Fe and CuAlS2 :Fe

• Sharp photoluminescence (PL) peak was found around 0.61 eV (CuGaS2 ) and 0.72 eV (CuAlS2 ) . A Zeeman spectrum analysis allowed us to assign the sharp PL lines to the ligand-field transition from an excited state 4T1 to a ground state 6A1 in the 3d5 manifold of Fe3+.

K.Sato and T.Teranishi: Infrared Luminescence of Fe3+ in CuGaS2 and CuAlS2; J. Phys. Soc. Jpn. 37 [2] (1974) 415-422.

Interpretation of IR luminescence spectrum by Ligand-field theory

• The observed IR PL was interpreted as a ligand field transition from 4E manifold derived from 4T1 of 3d5 state in Fe3+, taking into account the Zeeman spectra of the PL line.

• However, extremely reduced values of the Racah parameters were necessary to account for the enrgy position of the IR PL.

Tota

l ele

ctro

n en

ergy

Crystal field strength

High spin

Low spin

S=5/2

S=1/2

上村，菅野，田辺「配位子場理論とその応用」（裳華房）

T. Kambara, K. Suzuki and K. Gondaira, Electronic States of Fe in I-III-VI2 Compounds, JPSJ 39 764-771 (1975)

Molecular orbital + CI calculationMolecular orbital + CI calculation in the Fein the Fe--S4 clusterS4 cluster

FeS4 cluster

CuAl1-x Fex S2CuCu

AlAl

SS

FeFeCT transition

Δ > 0 Δ < 0

HaldaneHaldane--Anderson mechanism for the formation of Anderson mechanism for the formation of localized localized ““dd”” states in states in negtivenegtive--ΔΔ

systems systems

M. Imada, A. Fujimori, Y. Tokura, RMP ‘98

dn

dn+1Ldn-1

dn+1L

dn

dn+1L

p-d hybridization p-d hybridization

E –

Nμ

E –

Nμ

ΔΔ

Opt abs

Charge-transfer state

Opt abs

Δ = Ε(dn+1L) – E(dn): Charge-transfer energy

μ

U < Δ U > Δ

W: bandwidth ~3 eVU : Coulomb energy ~7 eVΔ: Charge-transfer energy

p

3d

3d

ΔU

Mott-Hubbard type insulator Charge-transfer-type insulator

p

3d

3d

UΔ

W

W

Electronic structure of transitionElectronic structure of transition--metal compoundsmetal compounds

ε ε μ

gap ~ U - W gap ~ Δ

- W

A.E. Bocquet et al., PRB ’92M. Imada, A. Fujimori, Y. Tokura, RMP ‘98

Z valence

Chemical trend in chargeChemical trend in charge--transfer energy transfer energy ΔΔ

ligand

LaCu3+O3

SrCo4+O3

SrFe4+O3

SrCo4+O3

SrFe4+O3

High-spin *High-spin

Δ = Ε(dn+1L) – E(dn): Charge-transfer energy

large small

Chemical trend in chargeChemical trend in charge--transfer energy transfer energy ΔΔ

large

small

largesmall

Transition-metal

Non-metal

smalllargevalence：2- - …. + 2+ 3+ 4+ 5+

負の電荷移動エネルギーをもつ磁性半導体カル負の電荷移動エネルギーをもつ磁性半導体カル コパイライト型コパイライト型CuFeSCuFeS22

M.Fujisawa, S.Suga, T.Mizoguchi, A.Fujimori and K.Sato: Electronic Structures of CuFeS2 and CuAl0.9Fe0.1S2 Studied by Electron and Optical Spectroscopies;Phys. Rev. B49 [11] (1994) 7155-7164.

クラスターモデルCI計算

価電子帯光電子スペクトル

クラスターモデルCI計算

Fe内殻光電子スペクトル

μ

U < Δ U > Δ

W: bandwidth ~3 eVU : Coulomb energy ~7 eVΔ: Charge-transfer energy

p

3d

3d

ΔU

Mott-Hubbard type insulator Charge-transfer-type insulator

p

3d

3d

UΔ

W

W

Electronic structure of transitionElectronic structure of transition--metal compoundsmetal compounds

ε ε μ Metal?

gap ~ U - W gap ~ Δ

- W

Indeed, SrFeO3is a metal. However, ….

HaldaneHaldane--Anderson mechanism for the formation of Anderson mechanism for the formation of localized localized ““dd”” states in states in negtivenegtive--ΔΔ

systems systems

M. Imada, A. Fujimori, Y. Tokura, RMP ‘98

dn

dn+1Ldn-1

dn+1L

Δ > 0

dn

dn+1L

p-d hybridization p-d hybridization

Δ < 0

E –

Nμ

E –

Nμ

ΔΔ

Opt abs

Charge-transfer state

Opt abs

Δ = Ε(dn+1L) – E(dn): Charge-transfer energy

large small

Chemical trend in chargeChemical trend in charge--transfer energy transfer energy ΔΔ

large

small

largesmall

Transition-metal

Non-metal

smalllargevalence：2- - …. + 2+ 3+ 4+ 5+

CuFeS2

実際の物質模式図

ZaanenZaanen--SawatzkySawatzky--AllenAllen相図相図

なぜ絶縁体なのか？

CuFeS2

電荷移動エネルギー：Δ原子内クーロンエネルギー：U

Unusually lowUnusually low--energy energy dd--dd optical absorption peakoptical absorption peak

Optical absorption due to Fe3+(d5) ion in chalcopyrite-type I-III-VI2 semiconductor

CuFeS2

Fe0.08%

Fe7%

Fe0.8%

Pure CuAlS2

T. Teranishi and K. Sato, J. Phys. Soc. Jpn. ’74

CuCu

AlAl

SS

FeFe

基底状態

電荷移動励起に隠れた電荷移動励起に隠れたdd--dd遷移を見る遷移を見る ーー共鳴非弾性共鳴非弾性XX線散乱（線散乱（RIXSRIXS））ーー

3dn

2p3dn+1

軟X線

軟X線（蛍光・ラマン）

E

Resonant inelastic x-ray scattering （RIXS)

内殻励起状態

SPring8理研ビームライン

辛グループcf. ヘムタンパク中のFe

RIKEN

hνouthνin

～ f sec (core hole lifetime)

XESXAS

•Applicable to solids ranging from metals to wide gap insulators•Large probing depth•Element specificity

core

valence

X-ray absorption(XAS) and emission(XES) spectroscopy

XAS & XES

•XAS probes unoccupied electronic states•XES probes occupied electronic states

RIKEN

Mount type

Spot size (μm)

Slit width (μm)

Lines/mm & α

(deg.)Radius (mm)

Detector type Resolution(eV)@ Fe 2p edge

BL27SU Flat Field type

7~10 None Valid line 2200 & 87.5

8940 背面照射型

CCD(2k x 2k)

0.6

T. Tokushima Y. Harada, H. Ohashi, Y. Senba, S. Shin, Rev.Sci.Instruments 77 (2006) 063107

SPring-8 BL27SU c3 station

450mm

750mm

Cylindrical VLS Grating

mSlit-less

CCD Detector with super- resolution reconstruction

mm750mm

450mm

RIKEN

Polarization dependence

‘depolarized’ configurationpolarization vector of the incident photon is NOT included in detected photons

depolarized and polarized configurations for the XES measurement

‘polarized’ configurationpolarization vector of the incident photon is included in detected photons

RIKEN

735730725720715710705Photon energy (eV)

Inte

nsity

(arb

.uni

ts) CuFeS2 Fe 2p XAS

TPY calc.

Inte

nsity

(arb

.uni

ts)

735730725720715710705Photon energy (eV)

CuFeS2 Fe 2p XAS

TPY TEY calc.

Experimental results (XAS)

XAS experiment : Y. Mikhlin et al., J. Electron Spectrosc. Relat. Phenom. 142,83 (2005)

Our work

or

L3 L2L3 L2

2p

3d

Initial state Intermediate state Final state

|3d n › |2p5 3d n+1 › |3d n ›

Transition Metal L-edge RIXS

3d

Ene

rgy

Ground state

Initial state Intermediate state Final state

3d

Energy

3d

2p core hole

Coulomb interaction between 2p and 3d

spin-orbit interaction of 2p core level

3d

2p core hole

dd excitation

Elastic peak

dd excitation (3d 5 )

3d

3d

Inte

nsity

(arb

.uni

ts)

715710705700695Emission energy (eV )

CuFeS 2

raw - f luorescence x 0.82 calculat ion (nonpolar ization)

Fe L3 XES horizontal ver tical fluorescence(750eV)

Δ

~ -3 eV

RIXS expt: Y. HaradaCluster-model calc: M. Taguchi

Unraveling hidden Unraveling hidden dd--dd transitions by resonant transitions by resonant inelastic xinelastic x--ray scattering (RIXS)ray scattering (RIXS)

Cluster CI calc

fluorescence

RIXS/Raman spectra

Ela

stic

pea

k

RIKEN

CI model with full multiplet

Approximations

(I) Central atom: Neighboring atom:

Fe 3d5, 3d6

ligand

CI model

Parameters

Ｖ（Γ）：Hybridization

Ｕｄｄ

：

On-site Coulomb interaction

Ｕｄｃ

：Core-hole potential

Δ：Charge transfer energy

Slater Integrals (Racah parameter) are calculated by Hartree-Fock method and are rescaled by 80%

RIKEN

CI model

Fe 3d Fe 2p ligandFe3d - ligand charge transfer

Fe3d on-site Coulomb interaction

Fe 2p-3d core-hole potential

Ground state：linear combination of 3 configurations3dn 3dn+1L 3dn+2L2

RIKEN

Inte

nsity

(arb

.uni

ts)

710705700695Emission energy (eV)

CuFeS2 Fe L3 XES

Cluster CI calc (nonpolarized) Δ ~ -3eV

experiment (depolarized) experiment (polarized)

Ela

stic

pea

k

RIXS/Raman spectra

dd excitation

RIXS expt: Y. HaradaCluster-model calc: M. Taguchi

Experimental results (XES)

Δ

~ -3 eVCluster CI calc

d5: B,C x80%d6: B,C x77%

M. Taguchi

Unraveling hidden Unraveling hidden dd--dd transitions by resonant transitions by resonant inelastic xinelastic x--ray scattering (RIXS)ray scattering (RIXS)

dn-1

dn+1L

Δ < 0

dn

dn+1L

p-d hybridizationE

–N

μ

Δ

Charge-transfer state

Opt abs

Ligand-field and CI cluster-model calcs

Conclusion

• Unusually low energy positions of IR-PL and strong CT absorption in CuAlS2 :Fe can be explained by Haldane-Anderson mechanism.

• CuFeS2 is found to be a negative CT energy insulator from PES and RIXS (XES) studies suggesting that this material is a typical “Haldane-Anderson” crystal.