G.A.Sawatzky

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Pathways to new magnetic semiconductors and half
metals

• G.A.Sawatzky

Department of Physics and Astronomy, University
of British Columbia, Canada
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collaborators

• I.Elfimov UBC
• S.Yunoki Trieste
• P.Steeneken Philips
• H. Tjeng Cologne
• A.Damascelli UBC
• K.Shen UBC
• D.Hawthorn UBC
• N.Ingle UBCb

• T.Hibma Groningen
• P.Abbamonte Illinois
• A.Rushdy Hamburg

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Nanostructuring can dramatically alter physical
properties

• Bad for conventional devices based on
  semiconductors
• Interfaces may dominate the properties

• May be good for otherwise boring materials
• Change a transparent non magnetic insulator into
  a half metallic ferromagnet

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Some ( Nano) ways to dramatically change
properties

• Electronic reconstruction of polar surfaces
• 2. Interface engineering
• 3. Controlled Defects and symmetry
• 4. Large Hunds rule coupling of O,N

ALL BASED ON SURFACES OR THIN FILMS and
MULTILAYERS
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Novel Nanoscale Phenomena in Transition-Metal
Oxides
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Introduction

• Rock-salt structure

• Band insulator

Ca Ar(4s)2
O He(2s)2(2p)4
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Correlated Electrons in a Solid
U
dn dn ? dn-1 dn1
Cu (d9)
O (p6)
?
p6 dn ? p5 dn1
U EITM EATM - Epol
If ? lt (Ww)/2 ? Self doped metal
? EIO EATM - Epol dEM
EM is strongly reduced at surfaces Prop. to
coordination no. ?Sltlt ?B
EI ionization energyEA electron affinity
energyEM Madelung energy

• J.Hubbard, Proc. Roy. Soc. London A 276, 238
  (1963)
• ZSA, PRL 55, 418 (1985)

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Neutral (110) surfaces of NiO
LSDAU U8eV J0.9eV
Slab of 7 NiO layers
Band gap at the surface decreases from 3 eV to
1.2 eV Step edges could be 1D strongly correlated
metals
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ELECTRONIC RECONSTRUCTION
POLAR SURFACES
For review see Noguera J.Phys. Condens Matter 12
(2000) R367)
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Polar (111) Surfaces of MgO
Finite slab of charged planes
NiO,MnO,EuO,CaO,SrO, MnS,EuS,-----
2
2-
2
2-
Will reconstruct!! Unless we terminate it properly
?V58 Volt per double layer!
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• The surface atoms are electron or hole doped!!!
• Can also atomically reconstruct
• Or strongly charge an overlayer (2D gas)
• Demonstrated above is ELECTRONIC RECONSTRUCTION

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LSDA Band Structure of CaO (111) Slab terminated
with Ca and O
Ca 4s
Spin Down
Spin Up
O 2p
Bulk material is an insulator
The O terminated surface is a half metallic
ferromagnet
NOTICE THE CROSSING OF THE VALENCE AND COND BANDS
. IN VERY THIN LAYERS THEY WILL HYBRIDIZE
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ELECTRONIC RECONSTRUCTION
Transfer one electron from O layer to Sr layer
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Defects in ionic insulators leading to Effective
imbedded magnetic molecules
Cation vacancies in simple Oxides
I think these can only be made in MBE Ultra thin
film growth
Elfimov et alPhys. Rev. Lett. 89, 216403 (2002)
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Cation Vacancy
Total Density of States
Partial Density of States projected on Ca-vacancy
site
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Anion Vacancy
Total Density of States
Partial Density of States projected on O-vacancy
site
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Definition of hopping parameters
Cluster model
tpp1/2(tpp?-tpp?)
tpp1/2(tpp?tpp?)
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Exact diagonalization results
Three lowest states for two particles
Single-particle picture

1. HOLES in anion orbitals and
2. ELECTRONS in cation orbitals.

• ELECTRONS in cation orbitals and
• HOLES in anion orbitals.
• Solid symbols are for triplet state

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Example of two particles in U? limit
for singlet - for triplet
Energy level diagram for holes (tgt0)
MOLECULAR HUNDS RULE HUGE STABILIZATION OF S1
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• Point structural defects in crystals such as
  vacancies can indeed confine the compensating
  charges in molecular orbitals formed by atomic
  orbitals on the nearest neighbours.
• Under certain conditions local magnetic moments
  will be formed due to a kind of molecular Hunds
  rule coupling with energetic determined by
  kinetic energy and symmetry considerations rather
  than exchange interactions.

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Strange magnetic materials

• This could be the origin of the high Tc materials
  such as Co in TiO2, or ZnO, or in oxides of non
  magnetic materials like HfO2
• Prelier et al Phys Cond. Matter 15,R1583 (2003)
• Venkatesan et al Nature 430, 630 ( 2004)

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Anion substitution Replace O with N
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N substitution for O in simple non magnetic
Oxides Use N Hunds rule coupling Use impurity
band resulting from N spanning the fermi energy
This again seems only possible in MBE thin films
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Recall O2 is magnetically ordered!!

• Hunds rule coupling of O 2p or N 2p is as large
  as Mn!!!
• All we need is
• Holes in O 2p or N 2p
• Small band widths ( large lattice constant)
• Prevent dimerization and Nitroxide formation

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DFT Study of Anion Substitution in SrO
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25 Nitrogen
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How to make N substituted Oxides with out
nitroxide formation ? First work by Hibmas
group in Groningen on Fe Oxides MBE WITH NO2
Rather than O2 Low temperature (350C) use the
high surface Diffusion
RBS and ion channeling show substitutional N
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XPS Valence Band Spectra of SrO1-xNx Films
In agreement with the results of band structure
calculations the N 2p peak is found to be about 2
eV lower in binding energy relative to the
position of O 2p peak. Relative change in the
intensities of these two peaks upon doping
indicates that the growth process is indeed a
process of substitution. This is also supported
by RHEED and LEED data.
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X-ray Absorption Spectra of SrO1-xNx Films
Oxygen K-edge
Nitrogen K-edge
Nitrogen and Oxygen K-edges spectra both show
pre-edge peak resulted from the presence of a
hole in the Nitrogen 2p states. Note that neither
Ta3N5 nor SrO has this particularity in their
K-edge spectra.
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N 1s core-level XPS spectra of SrO0.75N0.25
Low binding energy double peak structure is due
to the interaction of core hole with
charge-compensating hole in the Nitrogen 2p
orbitals. Peak1 and peak2 are triplet and singlet
states, respectively.
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Manipulating Material Properties

• magnetic (super) exchange, TC, TN
• electrical (super) conductivity, TC, M-I-T
• optical band gaps

How about using Image Charge Screening ?

• Coulomb energy
• Charge transfer energy
• Band gap

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Combined photoemission (solid lines) and inverse
photoemission (dots with solid lines as guide to
the eye) spectra of the C60 monolayer on Ag(111)
(upper panel) and the surface layer of solid C60
(lower panel). Also included are the
photoemission spectra (dashed lines) of the fully
doped C60 (K6C60) monolayer on Ag(111) and the
surface layer of solid K6C60.

• Band gap is reduced !
• Molecular Orbital Structure is conserved !

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Bending
EF
?Egap 1eV
Depends on Orientation!
Orientation changes the gap at interface
! Orientation disorder is really bad !!
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Si, Ge, GaAs
Molecules
Band width 0.5 eV gt10 eV
Exciton B.E. 1 eV 20 meV
Polarons ? ?0 W (? gt1)
Electr. Electr. U?W UltltW
Magnetism Yes (T-S0.5eV) No
Cond. Gap Egap ? W Egap ltlt W
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The influence of external polarizable media
For band width small compared to the response
time of the polarizable medium we should treat
the quasi particles as dressed electronic
polarons The band gap Ionization potential
electron affinity Will be strongly reduced at the
interface. This can amount to a gap closing of
more than 1.5 eV for a molecular system on a
metal or on Si or GaAs which also are highly
polarizable.
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Summary

• Reduced dimensionality enforces correlations and
  can drastically change the properties of
  material, surface gap reduction
• Substitution of Oxygen for Nitrogen is very
  promising path to a new class of magnetic
  materials
• Under certain conditions local magnetic moments
  can be formed due to a kind of molecular Hunds
  rule coupling with energetic determined by
  kinetic energy and symmetry considerations rather
  than exchange interactions.
• Strong charge transfers can result at interfaces
  of dissimilar materials and especially for
  crystallographic orientations involving polar
  surfaces.
• Organic molecular systems will exhibit strong
  band gap reduction at interfaces. Both electrons
  and holes will be attracted to that interface.