G.A.Sawatzky

Pathways to new magnetic semiconductors and half metals

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

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

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

Some ( Nano) ways to dramatically change properties

1. Electronic reconstruction of polar surfaces2. Interface engineering 3. Controlled Defects and symmetry4. Large Hund’s rule coupling of O,N

ALL BASED ON SURFACES OR THIN FILMS and MULTILAYERS

Novel Nanoscale Phenomena in Transition-Metal Oxides

Ionic Oxide Polar Surfaces

Stabilization of polar surfaces by epitaxy

Transparent insulator ½ metallic FM

Applications: Spintronics; CMR

SrO O

-1+2-2

Sr SrO

+1 +2-2

< 10 ML

Artificial Molecules Embedded into a MaterialCa, Mg, Sr, Ni vacancies or O-N substitution in oxides

New class of magnetic materials by ‘‘low-T’’ MBE growthApplications: Spintronics; Novel Magnets

JO N

LaMnO3

eg

t2g

Mn3+ 3d 4

Strained 2D Layers Positive and negative pressure

Applications: CMR; M-I Transition; Orbital Ordering

Correlated Electron System Surfaces

Kinks and steps stabilized by epitaxy

NiO (100) 1D Metallic stepsSuperconducting Copper oxides

Applications: Novel SC; QuBits

Electronic Structure of InterfacesMetal-Insulator interface: gap suppression

Applications: Molecular Electronics;Fuel Cells; Thermal Barrier Coatings

7

Introduction• Rock-salt structure • Band insulator

Ca : [Ar](4s)2

O : [He](2s)2(2p)4

-4

-2

0

2

4

6

8

10

12

L X W L K

Ene

rgy

(eV

)

Correlated Electrons in a Solid

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

If Δ < (W+w)/2 Self doped metal

dn dn dn-1 dn+1U :

p6 dn p5 dn+1Δ :

U = EITM – EA

TM - Epol

Δ = EIO – EA

TM - Epol + δEM

EI ionization energyEA electron affinity energyEM Madelung energy

Cu (d9)

O (p6)

EM is strongly reduced at surfacesProp. to coordination no. ΔS<< ΔB

Neutral (110) surfaces of NiO

-10 -8 -6 -4 -2 0 2 4 60

4

8 "Bulk"

Energy (eV)

0

4

8 1st layer below

0

4

8 Surface O 2p + N 3d

DO

S (

stat

es/e

V c

ell)

0

40

80

Total O 2p

LSDA+U: U=8eV J=0.9eVSlab of 7 NiO layers

Band gap at the surface decreases from 3 eV to 1.2 eVStep edges could be 1D strongly correlated metals

POLAR SURFACES

For review see Noguera J.Phys. Condens Matter 12 (2000) R367)

ELECTRONIC RECONSTRUCTION

Polar (111) Surfaces of MgO

2-

2+

Finite slab of charged planes

ΔV=58 Volt per double layer!

2- 2+

NiO,MnO,EuO,CaO,SrO,MnS,EuS,-----

Will reconstruct!!Unless we terminate itproperly

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

LSDA Band Structure of CaO (111) Slab terminated with Ca and O

-10

-5

0

5

10

Γ K M Γ A L H A

Ene

rgy

(eV

)

-10

-5

0

5

10

Γ K M Γ A L H A

Spin Up Spin Down

Bulk material is an insulator

Ca 4s

O 2p

The O terminated surface is a half metallic ferromagnet

NOTICE THE CROSSING OF THE VALENCE AND CONDBANDS . IN VERY THIN LAYERS THEY WILL HYBRIDIZE

SrO O

-1+2-2

Sr SrO

+1 +2-2

< 10 ML

Transfer one electron from O layer to Sr layer

ELECTRONIC RECONSTRUCTION

Defects in ionic insulators leading to Effective imbedded magnetic molecules

Cation vacancies in simple Oxides

Elfimov et al;Phys. Rev. Lett. 89, 216403 (2002)

I think these can only be made in MBEUltra thin film growth

Cation Vacancy

Partial Density of States projected on Ca-vacancy site

Total Density of States

Anion Vacancy

Partial Density of States projected on O-vacancy site

Total Density of States

Definition of hopping

parameters

tpp=1/2(tpp-tpp)

t’pp=1/2(tpp+tpp)

Cluster model

Exact diagonalization results Single-particle picture Three lowest states for two particles

(a) HOLES in anion orbitals and (b) ELECTRONS in cation

orbitals.

(a) ELECTRONS in cation orbitals and

(b) HOLES in anion orbitals.Solid symbols are for triplet state

Example of two particles in U= limit

t t

t

1 1

2 2 2 1

0

0

0

tt

tt

tt

H

),(),( 2121 ss mmxx

2

12

1

),( 21 ss mmTriplet

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

• 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.

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)

Anion substitution Replace O with N

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

3

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

Recall O2 is magnetically ordered!!

DFT Study of Anion Substitution in SrO

25% Nitrogen

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

9 8 7 6 5 4 3 2 1 Ef = 0 -1 -23000

3500

4000

4500

5000

5500

6000

6500

7000

7500

O2p

N2p

Co

un

t Ra

te

Binding Energy (eV)

XPS with MgK-sourceT=293 K

SrO SrO

0.82N

0.18

SrO0.75

N0.25

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.

385 390 395 400 405 410 415 420 425 430 435 440 445

1.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14 A

To

tal Y

ield

(n

orm

aliz

ed

)

Photon Energy (eV)

E||ab; T = 293 K SrO

0.80N

0.20

SrO0.70

N0.30

Ta3N

5

525 530 535 540 545 550 555 560 565 570 575

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

B

528 530 532 534 536

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

Total Yield (nor

malized)

Photon Energy (eV)

To

tal Y

ield

(n

orm

aliz

ed

)Photon Energy (eV)

E||ab; T = 293 K SrO SrO

0.80N

0.20

SrO0.70

N0.30

Nitrogen K-edge Oxygen K-edge

X-ray Absorption Spectra of SrO1-xNx Films

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.

402 401 400 399 398 397 396 395 394 393 392

36000

38000

40000

42000

44000

46000

2.91 eV1.63 eV

Peak 3

Peak 2

Peak 1

Peak1:Peak2:Peak3 = 3.95:1:0.14

Co

un

t Ra

te

Binding Energy (eV)

XPS with MgK-sourceT=293 K

SrO0.75

N0.25

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.

Manipulating Material Properties

How about using Image Charge Screening ?

magnetic : (super) exchange, TC, TN

electrical : (super) conductivity, TC, M-I-T

optical : band gaps

D

eEE II 22

1 20

D

eEE AA 22

1 20

Coulomb energy :

Charge transfer energy :

Band gap :

D

eUU

2

2

0

D

e

2

2

0

D

eEE gg 2

20

U

tJ

2

U

tJ

2

4

q ’ q

2 1R 1

R 2n

a

0

P o t e n t i a l o f a p o i n t c h a r g e i n t h e n e i g h b o u r h o o d o f a d i e l e c t r i c

M a c r o s c o p i c c o n t i n u u m -u n i f o r m

4)( 21 nDD

- s u r f a c e c h a r g e

0)( 21 nEE

lE 41

02 E0 E

0z0z

211

'1

R

q

R

q

qq

)(

)('

12

12

E n e r g y t o c r e a t e a c h a r g e q a t a :

Q

o a

Qqdq

aE

21

21

1

2

12

12

1 42

1

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 !

EF + Bending

Egap ~ 1eV Depends on Orientation!

Orientation changes the gap at interface !

Orientation disorder is really bad !!

Band width ~ 0.5 eV >10 eV

Exciton B.E. ~ 1 eV ~20 meV

Polarons ћ 0 ~ W ( ~ >1) —

Electr. – Electr. UW U<<W

Magnetism Yes (T-S~0.5eV) No

Cond. Gap Egap W Egap << W

Si, Ge, GaAsMolecules

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 affinityWill 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 whichalso are highly polarizable.

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.