2017-5-31, KITS, Beijing
Numerical study of electron correlation effects
in spintronic materials
Bo Gu (顾波)
Advanced Science Research Center (ASRC)
Japan Atomic Energy Agency (JAEA)
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Self-introduction
Dec. 1977 born in Hubei, China
Education:
1996.9 – 2000.7 Wuhan University, Bachelor
2000.9 - 2003.7 Peking University, M.S.
（Supervisor: 苏肇冰, Co-supervisors: 向涛, 王孝群，覃绍京）
2004.2 - 2007.1 Graduate University of Chinese Academy of Sciences, Ph.D.
（ Supervisor: 苏刚）
Employment:
2007.5 – 2010.3
2010.4 – 2012.3
2012.4 – 2016.6
2016.7 – now
Tohoku University, Japan, Post-doc
(Supervisor: Sadamichi Maekawa)
Japan Atomic Energy Agency, Post-doc
(Supervisor: Sadamichi Maekawa)
Japan Atomic Energy Agency, Scientist (Permanent Staff)
Japan Atomic Energy Agency, Senior Scientist (Permanent Staff)
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Outline
1. Introduction on diluted magnetic semiconductor (DMS)
2. DMS with wide band gap
3. DMS with narrow band gap
I am sorry to skip the part of spin Hall effect due to time limit.
I am happy to answer questions on spin Hall effect.
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Spintronics
charge (e)
Conventional electronic
devices
electron
spin (μB)
Spintronic devices
Advantage:
Non-volatility (data are retained with power off)
Low electric power consumption
etc…
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Diluted magnetic semiconductors (DMS)
e.g. GaAs
Nonmagnetic semiconductors
(charge)
e.g. (Ga,Mn)As
Diluted magnetic semiconductors
(charge + spin)
Classic DMS (Ga,Mn)As:
(1) Curie Tc ~ 200 K.
(2) p-type (hole) (Ga3+  Mn2+)
Challenges:
(1) High Tc > Room Temperature
(2) p-type (hole) & n-type (electron) (e.g. spin p-n junction)
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A picture of ferromagnetism (FM) in DMS
Bulut PRB 76, 045220 (2007); Tomoda Physica B 404, 1159 (2009)
Host band gap △g = 2 eV
IBS: impurity bound state (split-off state)
~ 0.1 eV
Symmetric model at μ = △g/2
μ
Quantum Monte Carlo (QMC) method
Chemical potential μ = - 6.0 eV (metallic)  RKKY-type
(Ruderman-Kittel-Kasuya-Yoshida)
impurity-impurity
impurity-host
β=1/T
impurity-impurity distance
impurity-host distance
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Chemical potential μ = 0.1 eV (semiconductor)
 long-range FM
IBS: impurity bound state
μ
impurity-host
impurity-impurity
Ferromagnetic
β=1/T
Antiferromagnetic
impurity-impurity distance
Carrier-mediated
ferromagnetism !
impurity-host distance
Impurity
Host
IBS
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Impurity-Impurity
IBS: impurity bound state
~ 0.1 eV
β=1/T
IBS
Chemical potential μ (eV)
μ~
Condition for FM
Bulut PRB 76, 045220 (2007)
 Hartree-Fock approximation
Spatial distribution
of spin of host
Impurity
IBS
Host
effective mass of host
energy level of IBS
M. Ichimura et al, Proceedings of ISQM-Tokyo 2005, p.183-186; cond-mat/0701736.
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A picture of p-type ferromagnetism (FM) in DMS materials
FM in p-type DMS
VB
μ
p-type μ
Strong p-d mixing in VB  IBS near VB
p-type: μ ~
Weak s-d mixing in CB  no IBS near CB
n-type: μ >>
CB
n-type μ
 FM
 No FM
IBS: impurity bound state (split-off state)
The picture for (Ga,Mn)As, (Zn,Mn)O, Mg(O,N), Li(Zn,Mn)P
wide band gap
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DMS: Semiconductor host + Magnetic impurity
Carriers (electrons, holes)
Localized moment
VB: p-orbital; CB: s-orbital
Band structure
Ferromagnetism
Electron correlations
Density functional theory (DFT)
Advantage
of QMC
Quantum Monte Carlo (QMC)
1. Treat electrons correlations correctly.
2. Not rely on separation between spin and charge fluctuations.
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Our method for DMS
Anderson impurity model:
Host band
Mixing
Impurity level
 Density functional theory (DFT)
Coulomb correlations of impurity
 Quantum Monte Carlo (QMC) with Hirsch-Fye algorithm
DFT+QMC: Electron Correlations; Spin and Charge Fluctuations
Chemical potential μ: a free parameter, model p- or n-type carriers.
 Occupation number of impurity
 Magnetic correlations between impurities
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Outline
1. Introduction on diluted magnetic semiconductor (DMS)
2. DMS with wide band gap
3. DMS with narrow band gap
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(Local moment)2
Case 1: (Zn,Mn)O & Impurity bound state (IBS)
wurtzite
zincblende
rocksalt
IBS
IBS
zincblend:
~ 0.1 eV
wurtzite:
~ 0.2 eV
(Shallow IBS)
rocksalt:
~ 1.6 eV
(Deep IBS)
Band gap △g = 3.45 eV
Chemical potential μ (eV)
Gu, Bulut, Maeawa, J. Appl.
Phys. 104, 103906 (2008).
μ
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Ferromagnetic (FM) correlation in (Zn,Mn)O
~0.2 eV
FM in p-type DMS
VB
CB
~0.1 eV
p-type μ
μ~
Condition for FM
~1.6 eV
No FM (Deep IBS)
Hartree-Fock
result:
Mn-Mn distance (alattice)
Gu, Bulut, Maeawa, J. Appl. Phys. 104, 103906 (2008).
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A. Many experiments declare high-temperature ferromagnetism in p-type (Zn,Mn)O
M. Ivill, et al., J. Appl. Phys. 97, 053904 (2005)
K. R. Kittilstved, et al., Nat. Mater. 5, 291 (2006)
J. R. Neal, et al., PRL 96, 197208 (2006)
B. Debate on experiments
Nature of the experimentally
observed ferromagnetic signals
K. Ando, Science 312, 1883 (2006)
Unexpected ferromagnetic
materials (cluster etc.)
Intrinsic (Carrier-mediated)
ferromagnetism
 Our message on (Zn,Mn)O
1. It is possible to have intrinsic ferromagnetism in (Zn,Mn)O.
2. We predict that zincblende structure (stable in thin film) is better than
wurtzite structure (most common phase) in terms of FM.
Gu, Bulut, Maeawa, J. Appl. Phys. 104, 103906 (2008).
Case 2: New generation 111-type DMS
(Ga3+, Mn2+)As
111-type
Li1+x(Zn2+, Mn2+)As
Li1+x(Zn2+, Mn2+)P
Hole & Spin
Hole/Electron Spin
Independently dope charge and spin !
Experiments on 111-type DMS
Li(Zn,Mn)As
Tc ~ 50 K, P-type, Band gap = 1.61 eV
Z. Deng et al, Nat. Commun. 2, 422 (2011)
Li(Zn,Mn)P
Tc ~ 36 K, P-type, Band gap ~ 2 eV
Z. Deng et al, PRB 88, 081203(R) (2013).
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Ferromagnetic (FM) correlations in Li(Zn,Mn)P
 Impurity-Impurity FM coupling
Occupation
number (Mn)
 Impurity bound state (IBS).
~ 0 eV
IBS
Chemical potential μ (eV)
Mn-Mn distance (alattice)
Reasonable p-type μ
FM in p-type DMS
VB
μ
CB
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p-type μ
How to obtain n-type DMS ?
 DMS with wide band gap
IBS: impurity bound state
FM in p-type DMS
VB
CB
μ
p-type μ
n-type μ
 Our idea: DMS with narrow band gap
FM in p & n-type DMS
VB
μ
p-type μ
CB
n-type μ
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Outline
1. Introduction on diluted magnetic semiconductor (DMS)
2. DMS with wide band gap
3. DMS with narrow band gap
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Good materials to check our idea:
122-type DMS: Mn-doped BaZn2As2
FM in p-type (hole)
(Ba2+,K+)(Zn,Mn)2As2
Tc ~ 230 K
K. Zhao et al, Nat. Commun. (2013) ;
Chin. Sci. Bull. (2014).
FM in n-type (electron)
Ba(Zn,Mn,Co)2As2
Tc ~ 80 K
H. Man et al, arXiv.1403.4019 (2014)
BaZn2As2 : Gap = 0.2 eV
 Mn-doped BaZn2As2 and BaZn2Sb2 !
BaZn2As2
(I4/mmm)
Gap = 0.2 eV
BaZn2Sb2
(Pnma)
Gap = 0.2 eV
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Density of state (Mn-3d)
Case 1: Ba(Zn,Mn)2As2
VB
CB
IBS
IBS
IBS
Chemical potential μ (eV)
ARPES: H. Suzuki et al,
PRB 92, 235120 (2015).
p-type μ
n-type μ
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Ferromagnetic (FM) correlation in Ba(Zn,Mn)2As2
P-type
Long-range FM
(larger <M1M2>)
Tc (exp) ~ 230 K
N-type
Long-range FM
(smaller <M1M2>)
Tc (exp) ~ 80 K
Mn-Mn distance (Å)
Gu and Maekawa, PRB 94, 155202 (2016)
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A simple estimation of exchange coupling J12 by <M1M2>
Ferromagnetic
Antiferromagnetic
Gu, Ziman, Maekawa, PRB 79, 024407 (2009)
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Case 2: Ferromagnetic (FM) correlation in Ba(Zn,Mn)2Sb2
P-type
Long-range FM
Mn-Mn distance (Å)
p-type
FM ＜M1M2＞
(2nd n.n.)
FM range
Tc
Mn in BaZn2As2
~ 0.08
~6Å
230 K (Zhao, 2014)
Mn in BaZn2Sb2
~ 0.14
~ 10 Å
> 230 K (expected)
Gu and Maekawa, PRB 94, 155202 (2016)
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Density of state (3d)
Case 3: Cr vs. Mn impurities in BaZn2As2
VB
CB
Impurity bound state (IBS)
Cr: bottom of CB
IBS(Mn) IBS(Cr)
Mn: top of VB
N-type FM:
Cr : more promising
Chemical potential μ (eV)
Impurity level : Ed (Mn2+: d5) < Ed (Cr2+: d4) < EF  IBS(Mn) < IBS (Cr)
AIP Advances 7, 055805 (2017).
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Summary of diluted magnetic semiconductor (DMS)
1. Found ferromagnetic (FM) correlations in
some DMSs with wide band gap. Clarify
the controversial experiments.
FM in p-type DMS
VB
CB
(Zn,Mn)O, Mg(O,N)
Contributed to a new promising direction
Li(Zn,Mn)As, Li(Zn,Mn)P
IBS: impurity
bound state p-type μ
2. Proposed a way to realize p- and ntype DMS: Narrow band gap.
Consistent with exp. in Ba(Zn,Mn)2As2
n-type μ
FM in p & n-type DMS
VB
CB
Predict p- and n-type FM in Ba(Zn,Cr)2As2
Predict high Tc (> 230K) in Ba(Zn,Mn)2Sb2
p-type μ n-type μ
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Outlook & Future
Conduction electrons
Band structure
Density functional theory (DFT)
Localized electrons
Electron correlations
Quantum Monte Carlo (QMC)
DFT + QMC method: Useful in a wide range of materials !
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Acknowledgments (Diluted Magnetic Semiconductor)
Theories:
S. Maekawa (ASRC, JAEA)
N. Bulut (Izmir Inst. Tec.): QMC, (Zn,Mn)O
T. Ziman (Inst. Laue Langevin): Mg(O,N)
J. Ohe (Toho U): (Ga,Mn)As
Experiments:
F. L. Ning（宁凡龙）(Zhejiang U)（浙大）: n-type Ba(Zn,Mn)2As2
C. Q. Jin （靳常青）(IOP, CAS)（物理所）: Li(Zn,Mn)As, Ba(Zn,Mn)2As2
A. Fujimori (U Tokyo): ARPES
Y. J. Uemura (Columbia U): muSR, organization
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Thank you very much !
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