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Wannier Function Based First Principles Method for Disordered
SystemsTom Berlijn, Wei Ku
PhDpostdoc
CMSN network
Collaborators
Wei Ku Dmitri
VoljaChi-Cheng
Lee
Chai-Hui
LinWei-Guo
Yin
Limin
Wang
William
Garber
Peter
HirschfeldYan
Wang
Theory
Theory
Andrivo
Rusydi
Tun Seng
Herng
Dong
Chen Qi
Experiment
Ding
Jun
Outline
• Introduction: Super Cell Approximation
• Method : Effective Hamiltonian (Wannier function) [1]
• Application 1: eg’ pockets of NaxCoO2 [1]
• Application 2: oxygen vacancies in Zn1-xCuxO1-y[2]
[1] T. Berlijn, D. Volja, W. Ku, PRL 106 077005 (2011)
[2] T. S. Herng, D.-C. Qi, T. Berlijn, W. Ku, A. Rusydi et al, PRL 105, 207201 (2010)
Goal: configurationally averaged spectral function of disordered systems from first principles
mean field vs super cell
<A(k,w) > = S A(k,w)iconfig i
Mean Field1
VCA: Virtual Crystal Approximation
CPA: Coherent Potential Approximation
no scattering
non-local physics missing1) A. Gonis, “Green functions for ordered and disordered systems” (1992)
Vvirtual crystal = (1-x) VA + x VB
Approach: super cell approximation
problem
band folding
computational expense
solution
unfolding[1]
effective Hamiltonian[1] W. Ku , T. Berlijn, and C.-C. Lee, PRL 104 216401 (2010)
<A(k,w) > ≈ 1/N ( A1(k,w) + … + AN(k,w) )
H = H0 + Si D(i) + Si,j D(i,j) + …
D(i) = H(i) - H0
D(i,j) = H(i,j) - D(i) - D(j) - H0
higher order moments
Concept: Linearityundoped linear 2-bodydisordered
1) Density Functional Theory
undoped
(normal cell)
1 impurity
(per super cell)
two DFT Calculations
Influence impurity
2) Wannier transfomation
DFT 1 impurity
2 Wannier transformations
2 Tight Binding Hamiltonians
undoped 1 impurity
DFT undoped
En
erg
yE
ner
gy
k
K
Hdft0 Hdft
(i)
|rn> = e-ik•r Unj(k) |kj>kj
3) Linear Superposition
Influence 1 impurity:
D(i) = Hdft(i) - Hdft
0
effective Hamiltonian N impurities:
Heff(1,…,N) = Hdft
0 + SiD(i)
Effective HamiltonianDFTE
ne
rgy
(e
V)
Test : NaxCoO2
66 Wanier Functions
1 diagonalization
2019 LAPW’s + 164 LO’s
self consistency
Co-eg
Co-ag
Co-eg’
O-p
-8
-4
0
4
8
-8
-4
0
4
8
En
erg
y (
eV
)
Effective HamiltonianDFT
Test : Zn1-xCuxO (rock salt)
spin spin
spin spin
-8
-4
0
4
8
-8
-4
0
4
8
-8
-4
0
4
8
-8
-4
0
4
8
-8
-4
0
4
8
-8
-4
0
4
8
En
erg
y (
eV
)
Effective HamiltonianDFT
Test : Zn1-xCuxO (rock salt)
spin spin
spin spin
Why NaxCoO2?High Thermoelectric
Power1
Unconventional Super Conductivity2 ?
1) I. Terasaki et al, PRB 56 R12 685 (1997)
2) K. Takada et al, nature 422 53 (2003)
Intercalation: NaxCoO2
Q) Does Na disorder destroy eg’ pockets3 ?
1) D.J. Singh, PRB 20, 13397 (2000)
2) D. Qian et al, PRL 97 186405 (2006)
3) David J. Singh et al, PRL 97, 016404-1 (2006)
LDA1 ARPES2
eg’
ag
En
erg
y (
eV
)
Co-ag
-0.20.00.2
-0.20.00.2
-4.2-4.0-3.8
0.20.1
Co-eg’
O-p
A) Na disorder does not destroy eg’ 1
En
erg
y (
eV
)non-local physics :
k-dependent broadening
0.2
0.1
0.0
-0.1H A G K
Co-ag
Co-eg’
k-dependent self energy S(k,w)?
non-local physics : short range order
Na(1) above Co
Na(2) above Co-hole
Simple rule: Na(1) can
not sit next to Na(2):
non-local physics : short range order
0.2
0.1
0.0
-0.1H A G K 0.0 0.2 0.4 0.6
A(k,w) A(k0,w) @ k0=G
Ene
rgy
(eV
)
X=0.30
0.2
0.1
0.0
-0.1H A G K 0.0 0.2 0.4 0.6
A(k,w) A(k0,w) @ k0=GE
nerg
y (e
V)
X=0.70
0.3 0.2 0.1 0.0-0.1-0.2
0.3 0.2 0.1 0.0-0.1-0.2
SRO suppresses ag broadening?
Na(1) islandNa(2) island
Application 2:
oxygen vacancies in Zn1-xCuxO1-y
T. S. Herng, D.-C. Qi, T. Berlijn, W. Ku, A. Rusydi et al, PRL 105, 207201 (2010)
Experiment
First principles simulation
Microscopic picture
• Film growth & charactarizationDr. T. S. Herng (NUS)Prof. Ding Jun (NUS)
• Beamline scientistsDr. Gao Xingyu (NUS)Dr. Yu Xiaojiang (SSLS)Cecilia Sanchez-Hanke
(NSLS)
• XAS & XMCDDr. Qi Dongchen (NUS)Prof. A. Rusydi (NUS)
Tom BerlijnWei Ku
three representative films
1. ZnO
2. Cu:ZnO O-rich (2% Cu)
3. Cu:ZnO O-poor (2% Cu & ~1% oxygen vacancies VO)
SQUID
ZnO @ 300K
ZnO:Cu @ 300K (O-rich)
ZnO:Cu @ 5K (O-poor)
ZnO:Cu @ 300K (O-poor)
Observation: oxygen vacancy induce FM @ 300K in Cu:ZnO
NB: 2 Cu-d9 + Vo = 2 Cu-d10
Q: what is the role of oxygen vacancies?
Q What is the influence of the oxygen vacancies?
oxygen vacancy = attractive potential + 2 electrons
Q ) Where do the electrons go? A) one-particle spectral function <A(k,w)>
of Zn1-xCuxO1-y with attractive potential VO but without its donated electrons
Zn Cu↓ Cu↑ O VO
configurationally averaged spectral function
<A(k,w) > ≈ 1/10 ( A1(k,w) + … + A10(k,w) )
configuration 1 configuration 10
≈1/10 +….+
spectral function <A(k,w)>
conduction bandZn-4s
Cu-3d↑
valence band O-2p Cu-3d↓
VO
Cu-3d x 40
<A↑(k,w)> <DOS↑(w)> <DOS↓(w)> <A↓(k,w)>
Ene
rgy
(eV
)
Q) Where do the electrons go?
A) Cu upper Hubbard level(leaving VO empty)
Ene
rgy
(eV
)
<A↑(k,w)> <DOS↑(w)>
Cu-3d↑
VO
Zn-4s
O-2p
e- Cu-3d x 40
Ene
rgy
(eV
)
Cu-3d x 40
<A↑(k,w>
Cu-3d↑VO
Zn-4s
O-2p
Oxygen vacancy states |VO> are big
k-space real-space
FWHM ≈GM/5 oxygen vacancy
wavefunction <x|Vo>
radius ≈ 2.5 a
Ene
rgy
(eV
)
Cu-3d x 40
<A↑(k,w>
Cu-3d↑VO
Zn-4s
O-2p
But why are oxygen vacancy states |VO> so big?
k-space real-space
Attractive potential
|Vo> ≈ ½ (|Zn1-s> +|Zn2-s> +|Zn3-s> +|Zn4-s>)
Oxygen vacancy = attractive potential + 2 electrons
Attractive potential in the 4 neighboring Zn
Zn
O
1. Electrons go into |Cu-d↑>2. Oxygen vacancy states |VO> are big
First principles results
Microscopic picture
no vacancies with vacancies
Conclusion: oxygen vacancies mediate the Cu moments
VOCu d 9 Cu d 10
Outline
• Introduction: Super Cell Approximation
• Method : Effective Hamiltonian (Wannier function) [1]
• Application 1: eg’ pockets of NaxCoO2 [1]
• Application 2: oxygen vacancies in Zn1-xCuxO1-y[2]
[1] T. Berlijn, D. Volja, W. Ku, PRL 106 077005 (2011)
[2] T. S. Herng, D.-C. Qi, T. Berlijn, W. Ku, A. Rusydi et al, PRL 105, 207201 (2010)