1. Pathways to new magnetic semiconductors and half metals
G.A.Sawatzky
Department of Physics and Astronomy, University of British Columbia, Canada

2. 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

3. 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

5. Some ( Nano) ways to dramatically change properties
Electronic reconstruction of polar surfaces
2. Interface engineering
3. Controlled Defects and symmetry
4. Large Hund’s rule coupling of O,N
ALL BASED ON SURFACES OR THIN FILMS and
MULTILAYERS

7. Novel Nanoscale Phenomena in Transition-Metal Oxides7
Ionic Oxide Polar Surfaces
Stabilization of polar surfaces by epitaxy
Correlated Electron System Surfaces
Kinks and steps stabilized by epitaxy
NiO (100) D Metallic steps
Superconducting Copper oxides
Applications: Novel SC; QuBits Sr O -1 +2 -2 +1
< 10 ML
LaMnO3 eg
t2g
Mn3+ 3d 4
Strained 2D Layers
Positive and negative pressure
Applications: CMR; M-I Transition; Orbital Ordering
Transparent insulator ½ metallic FM
Applications: Spintronics; CMR
Electronic Structure of Interfaces
Metal-Insulator interface: gap suppression
Applications: Molecular Electronics;
Fuel Cells; Thermal Barrier Coatings
Artificial Molecules Embedded into a Material
Ca, Mg, Sr, Ni vacancies or O-N substitution in oxides
New class of magnetic materials by ‘‘low-T’’ MBE growth
Applications: Spintronics; Novel Magnets J O N

9. Introduction Rock-salt structure Band insulator Ca : [Ar](4s)2-4 -2 2 4 6 8 10 12 L  X W K
Energy (eV)
Ca : [Ar](4s)2
O : [He](2s)2(2p)4

10. Correlated Electrons in a Solid
U :
dn dn  dn-1 dn+1
Cu (d9)
O (p6)
Δ :
p6 dn  p5 dn+1
U = EITM – EATM - Epol
If Δ < (W+w)/2  Self doped metal
Δ = EIO – EATM - Epol + δEM
EI ionization energy EA electron affinity energy EM Madelung energy
EM is strongly reduced at surfaces
Prop. to coordination no. ΔS<< ΔB
J.Hubbard, Proc. Roy. Soc. London A 276, 238 (1963)
ZSA, PRL 55, 418 (1985)

11. Neutral (110) surfaces of NiO
Slab of 7 NiO layers
LSDA+U: U=8eV J=0.9eV
Band gap at the surface decreases from 3 eV to 1.2 eV
Step edges could be 1D strongly correlated metals

12. POLAR SURFACES ELECTRONIC RECONSTRUCTION
For review see Noguera J.Phys. Condens
Matter 12 (2000) R367)

13. 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
ΔV=58 Volt per double layer!

14. The surface atoms are electron or hole doped!!!
Can also atomically reconstruct
Or strongly charge an overlayer (2D gas)
Demonstrated above is ELECTRONIC RECONSTRUCTION

15. LSDA Band Structure of CaO (111) Slab terminated with Ca and O
-10 -5 5 10 Γ K M A L H
Energy (eV)
-10 -5 5 10 Γ K M A L H
Ca 4s
Spin Up
Spin Down
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

16. ELECTRONIC RECONSTRUCTION
Transfer one electron from O layer to Sr layer Sr O -1 +2 -2 +1
< 10 ML

17. 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 al;Phys. Rev. Lett. 89, (2002)

18. Cation Vacancy Total Density of States
Partial Density of States projected on Ca-vacancy site

19. Anion Vacancy Total Density of States
Partial Density of States projected on O-vacancy site

20. Definition of hopping parameters
Cluster model
Definition of hopping parameters
tpp=1/2(tpp-tpp)
t’pp=1/2(tpp+tpp)

21. Exact diagonalization results
Single-particle picture
Three lowest states for two particles
HOLES in anion orbitals and
ELECTRONS in cation orbitals.
ELECTRONS in cation orbitals and
HOLES in anion orbitals.
Solid symbols are for triplet state

22. Example of two particles in U= limit1 2
Triplet
Singlet
“+” for singlet; “-” for triplet
Energy level diagram for holes (t>0)
-2t -t t 2t
Triplet
Singlet
MOLECULAR HUND’S RULE
HUGE STABILIZATION OF S=1

23. 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 Hund’s rule coupling with energetic determined by kinetic energy and symmetry considerations rather than exchange interactions.

24. 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)

25. Anion substitution
Replace O with N

26. 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

27. 3
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

28. DFT Study of Anion Substitution in SrO

29. 25% Nitrogen

30. How to make N substituted Oxides
with out nitroxide formation ?
First work by Hibma”s 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

31. 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.

32. X-ray Absorption Spectra of SrO1-xNx Films
Nitrogen K-edge
Oxygen 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.

33. 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.

34. 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 :

36. Molecular Orbital Structure is conserved !
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 !

37. EF
+ Bending
Egap ~ 1eV
Depends on Orientation!
Orientation changes the gap at interface !
Orientation disorder is really bad !!

38. Molecules Si, Ge, GaAs Band width ~ 0.5 eV >10 eV Exciton B.E.
~20 meV
Polarons
ћ 0 ~ W ( ~ >1) —
Electr. – Electr.
UW
U<<W
Magnetism
Yes (T-S~0.5eV) No
Cond. Gap
Egap  W
Egap << W

39. 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.

40. 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 Hund’s 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.