Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic...
Transcript of Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic...
![Page 1: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/1.jpg)
Photoemission and the electronic structure ofmagnetic oxides
Dan DessauUniversity of Colorado, Boulder
Duane [email protected]
![Page 2: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/2.jpg)
Einstein’s Photoelectric effectXPS, UPS, ARPES
sample
analyzer
e
-
h
n
e-
hn
occupied DOS
BE
0D.O.S.
![Page 3: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/3.jpg)
Expt
LDA
![Page 4: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/4.jpg)
![Page 5: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/5.jpg)
LDA
Experiment
Inte
nsity
Valence Bands Copper 2p core levels
![Page 6: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/6.jpg)
remove one
Correlated vs. uncorrelated systems
Static Dynamic
Si, Ge, Cu, etc.Band theory
Works even though 1023
electrons!
Correlated systemsMany-body theories
Dynamic interactions --> Novel and rich physics and materials systems.
![Page 7: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/7.jpg)
Crystal structure of the manganitesCubic perovskite
manganite (ie. La0.6Sr0.4MnO3)
Monolayer perovskiteManganite
(i.e. La0.6Sr1.4Mn2O4)
Bilayer perovskitemanganite
(ie. La1.2Sr1.8Mn2O7)
MnO6octahedra
rock-salt layer
cation(Sr or La)
monolayer
bilayer
ab
c
100 200 300 4000
n=1
n=2
n=∞ cubic
10 2
10 4
10 0
10 -2
10 -4
x=0.4
T c
T c
Temperature (K)
resis
tivity
(W-c
m)
![Page 8: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/8.jpg)
Bilayer manganite - La2-2xSr1+2xMn2O7
cb
a }}
}(La,Sr)MnO3
(La,Sr)O
(La,Sr)MnO3
Cleavage Plane
0.3 0.4 0.5 0.6 0.7
50
100
150
200
250
FM type A-AFI
PI
x
TN/TC (K)
CMR
Doping level x
(x=0.4)
resis
tivity
(W-c
m)
Temperature (K)
Y. M
orito
mo
et a
l.
C.D.
Lin
g e
t al.
![Page 9: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/9.jpg)
Evidence for half-metallicity
• Strong spin polarization in La0.67Sr0.33MnO3 epi-films:
J.H. Park et al., PRL 81, 1953 (1998), Nature 392, 794 (1998).
T = 40K
![Page 10: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/10.jpg)
Double Exchange TheoryKinetic energy gain for ferromagnetic alignment
Susc
eptib
ility
(em
u/m
ol)
ParaFerro
Temperature (K)
Ferromagnetism and Metallicity go together.
H
LaMnO3
La1-xSr xMnO 3 (T<<T c)
La1-xSrxMnO3 (T>T c)
La1-xSrxMnO3 (T>Tc) H°0
eg
t2g
Mn3+ 3d4
large t~
small t~
large t~
![Page 11: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/11.jpg)
Double Exchange TheoryKinetic energy gain for ferromagnetic alignment
Change in conductivity going across Ferro-Para transition: ~ 30%Real materials - many orders of magnitude effect! Why?
Ferro
E FD(E)Para
EJ~0.7 W oW o
Change in t --> change in WCan study with ARPES
q
t = tocos(q/2)~
t2g electrons core-like
S=3/2
eg electron itinerant t = to
~Ferro Para
t ~.707 to~
![Page 12: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/12.jpg)
e-
e-
a1)
a2)
e-
e-
b1)
b2)
Transport without polarons Transport with polarons
To help manganite problem, polaronic effectmust be stronger above Tc than below Tc
Feedback effect necessary - Polaronic effectcooperates with Double-exchange and/or otherphenomena (charge ordering).
Polarons and conductivity
negativemagneto-resistance
Temperature (K)Re
sistiv
ity (o
hm-c
m) H=0T
H=1T
H=3TH=5T
H=7T
H=2T
10-1
100
10-2
1000 200 300
![Page 13: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/13.jpg)
Temperature dependent pseudogaps in CMROxides
Thanks to: X.J. Zhou, P. Bogdanov, Z. Hussain ALS BerkeleyK. Altmann, SRC Madison
Support from the DOE and NSF
Y.D. Chuang et al., Science 292,1509 (2001)
ARPES experimentsYi-De Chuang, Zhe Sun, Adam Gromko, Alexei Fedorov,
Fraser Douglas, Max Bunce, D.S.D.University of Colorado
SamplesT. Kimura, Y. Tokura University of TokyoJohn Mitchell Argonne Nat’l Labs
![Page 14: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/14.jpg)
Questions• How to explain very poor conductivity
– In the “metallic” state of the manganites?– M-I transition into the high T insulating state? (--> CMR)
• Possible mechanisms.•Polarons? Large scattering? Charge/orbital ordering?•Temperature dependent pseudogap (includes some of above).
negativemagneto-resistance
Temperature (K)
Res
istiv
ity (o
hm-c
m) H=0T
H=1T
H=3TH=5T
H=7T
H=2T
10-1
100
10-2
1000 200 300
Very poor metal Insulator
![Page 15: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/15.jpg)
binding energy
emission angle
MDC
EDC
Angle Resolved Photoemission (ARPES)
E vs k relation
EDC: Energy Distribution Curve (ARPESintensity versus binding energy)
MDC: Momentum Distribution Curve(ARPES intensity versus momentum)
e-
hn
occupied DOS
BE
0D.O.S.
EF
Arbi
trary
Uni
t
Binding Energy
Fermi Surface crossing point
k
kF
spectral weight loss due toFermi function cutoff
![Page 16: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/16.jpg)
ARPES on La1.2Sr1.8Mn2O7 (n=2, x=0.4)(Low T Ferromagnetic state)
Integrated spectral weight over (0.2eV,-0.2eV) window
LSDA Fermi surface topology(N. Hamada unpublished)
3d x2-y2 hole pockets
3d 3z2-r2 electron pocket
Y.D. Chuang et al., Science 292, 1509 (2001)
![Page 17: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/17.jpg)
Angle-scanned (unsymmetrized) Fermi Surface bilayer manganite x=0.4 (Low T Ferromagnetic state)
![Page 18: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/18.jpg)
LDA+U dispers
ion relatio
n
ARPES on bilayer x=0.4 samples - Low T (20K) ferro state
Binding energy (eV)
EDCs
Binding energy (eV)
emiss
ion
angl
e (de
gree
)MDCs
-4-2
246
0EF
-0.2eV
Fermi surface topology
Ekx
ky
25
20
15
10
420-2-4
Double Lorentzianfit to MDCs
![Page 19: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/19.jpg)
Binding energy (eV)
emiss
ion
angl
e (d
egre
e)
MDCs
-4-2
246
0EF
-0.2eV
Binding energy (eV)
EDCs
ARPES on bilayer x=0.4 samples - Low T (20K) ferro state
-1.5
-1.0
-0.5
0.0
(0,0) (p,0) (p,p)
bind
ing
ener
gy (e
V )
LDA dispersionMDC centroidEDC centroid
25
20
15
10
420-2-4
Double Lorentzianfit to MDCs
![Page 20: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/20.jpg)
Double Exchange TheoryKinetic energy gain for ferromagnetic alignment
Change in conductivity going across Ferro-Para transition: ~ 30%Real materials - many orders of magnitude effect! Why?
Ferro
E FD(E)Para
EJ~0.7 W oW o
Change in t --> change in WCan study with ARPES
q
t = tocos(q/2)~
t2g electrons core-like
S=3/2
eg electron itinerant t = to
~Ferro Para
t ~.707 to~
![Page 21: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/21.jpg)
hv=22.4 eV 200K spectra taken first
-2 -1.5 -1 -0.5 0
0/0
2/0
4/0
6/0
10/0
14/0
18/0
22/0
(0/0) (p/0)
(p,p)
-2 -1.5 -1 -0.5 0
22/0
22/2
22/4
22/6
22/8
22/10
(0/0) (p/0)
(p,p)
50K Ferro200K Para
Energy Relative to EF (eV)
.06 eV
Temperature dependence of (LaSr)Mn2O7 x=.4 Tc ~ 130K
Bandwidth change : .06 eV/1.5 eV = 4%. Much less than the DE prediction of 30%.==> DE relevant but not key effect.
T. Saitoh et al., PRB (2000)
![Page 22: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/22.jpg)
fitting results
-4
-2
0
2
4
-400 -300 -200 -100 0
1.5
1.0
0.5
0.0
Binding energy (meV)
emission angle (degree)em
issio
n an
gle
(deg
ree)
centroid of peak 1
centroid of peak 2 width
Extraction of the key transport parameters - low T ferrometal state
1)Fermi velocity (from band dispersion near EF) vF ~ 0.038 c
2)Band mass (fitting dispersion with parabola) m ~0.27 me
3)Number of carriers (measurement of the FS volume) n ~ 3.4*1021 holes/cm3
4)Momentum width (HWHM) Dk ~1.2o ~ 0.09 p/a ~ 0.07 A-1 (original)4b)Mean free path (lower limit) l=1/ Dk ~ 14 A ~7 times the Mn-O bond length4c) Mean free time between scattering (lower limit) t=l/vF ~1.24 fs
rARPES = 1/s = m* / n e2t ~ 1.3*10-4 W-cm (Drude)Real value r0 ~ 2*10-3 W-cm (more resistive)
Key ingredient not considered in the Drude calculation: pseudogap. Presence of the pseudogapcan effectively remove carriers from conduction process thus increasing the resistivity
-400 -300 -200 -100 0
weight of peak 10
Wei
ght expected behavior
Binding energy (meV)
Pseudogap
(loss of w
eight)
![Page 23: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/23.jpg)
Temperature dependence of the pseudogap
Pseudogap correlateswith and may drivethe M-I transition.
Temperature (K)
0.4
0.3
0.2
0.1
0.0
1401201008060
60
50
40
30
20
10
0
Tc
wei
ght
Strong pseudogapInsulator
Weak pseudogapPoor metal
weight (-1.7eV,0.3eV)
weight (-0.2eV,0.05eV)
Energy spectra averaged across the entire slice
50
40
30
20
10
0
Ave
rage
-1.5 -1.0 -0.5 0.0
5
4
3
2
1
0
Ave
rage
-.2 -.15 -.1 -.05 0 .05
inte
nsity
inte
nsity
50K 80K100K110K120K130K 150K
Energy (eV)
10
5
0
-5
-10
50-5
Bind
ing
Ener
gy (e
V)
analyzer angle (degrees)
![Page 24: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/24.jpg)
The pseudogap affects all families of the manganites - some very strongly(single layer) and some less so (pseudocubic). Resistivity varies in kind.
Dimensionality dependence of near-EF weight
![Page 25: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/25.jpg)
Optical Conductivity of layered manganite
T. Ishikawa et al., Phys. Rev B 57, R8079 (1998)
• Spectral weight transfer with temperature over large energy scale• Gapped low energy spectral weight (no Drude peak)
![Page 26: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/26.jpg)
Loss of near-EF weight (pseudogap)Simple superposition of metallic and insulating regimes?
-400 -300 -200 -100 0
0
Spec
tral W
eigh
t
Binding energy (meV)
Spec
tral W
eigh
t(d
ensit
y of
sta
tes)
-400 -300 -200 -100 0
metal
0
Binding energy (meV)
insulator Experiment
(Need many many types of regions, all with different size gaps)- May consider fluctuations in space and time to get this.
Also, FS volume matches total # carriers expected by chemical doping(no regions with extra # holes, regions with fewer # holes).
![Page 27: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/27.jpg)
d 3x2 -r
2Mn3+
Mn3+
Mn4+
Mn3+
Mn4+
Mn3+Mn3+
Mn3+Mn3+
FM
AF
Real-space Charge Density Waves (CDW’s) and CDW gaps
2201.75,2.25,0
2.25,1.75,0110
200
020
160K
D.Argyriou et al., Argonne/APS
Extra periodicity induced by CDWobservable in diffraction experiments as
weak superlattice spots at (1/4,1/4,0).
Commensurate/static “CE” ordering
Example: LaSr2Mn2O7 (x=0.5) commensurately doped insulator
G
E(k)k
2∆
G
System gains energy when gap iscentered at EF (commensurate
doping levels).
![Page 28: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/28.jpg)
k-space driven CDW’s and Fermi Surface nesting
Entire measured FS of La1.2Sr1.8Mn2O7 is gapped due to large parallel segments(nearly perfect nesting).
Q
E(k)k
2∆
G
We can guarantee that CDW gap is centered at EF in ak-space driven CDW --> gain extra energy.
If many parts of the Fermi Surface connect (nestingcondition) the instability is greatly enhanced.
char
ge d
ensi
ty
Real space atom position
Charge density wave incommensurate with lattice.Short range in space. Fluctuates in time. Elusive!
![Page 29: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/29.jpg)
Real space picture of the k-space driven CDW fluctuations
• Fermi-Surface-driven CDW cooperates withthe Jahn-Teller effect to distort MnO6octahedra. --> increased energy scale of gap.
• Elastic strain mediates the correlations.• Incommensurability with the lattice --> order
is short ranged in real space
x ~ 6a
ac
Mn La/SrO
O3aO1O2
O3b
B. Campbell et al, (Phys. Rev. B 2001)
From an analysis of the intensity of 108 superlatticereflections (only observed in the high temperature
paramagnetic state)Temperature Dependence
DiffractionI(0.3,0,1)
ARPESWeight at EF
0 50 100 150 200 250 300(K)
Tc
Temperature
Inte
nsity
Weak PseudogapPoor metal
Fluctuating CDW(not observed in diffraction!)
Strong PseudogapInsulator
Semi-static CDW
![Page 30: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/30.jpg)
Sharpness of the energy gap
E k2∆ 2∆
E
DOS
CDW with long range order
E kWith short range order
E
DOS
2∆E
DOS
Including polaronic (Jahn-Teller) energy scale
Looks like lower figure even in low T ferromag state!Polarons and/or charge order at all Temps (with varying strengths).
![Page 31: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/31.jpg)
Temperature dependence of the pseudogap
Pseudogap correlateswith and may drivethe M-I transition.
Temperature (K)
0.4
0.3
0.2
0.1
0.0
1401201008060
60
50
40
30
20
10
0
Tc
wei
ght
Strong pseudogapInsulator
Weak pseudogapPoor metal
weight (-1.7eV,0.3eV)
weight (-0.2eV,0.05eV)
Energy spectra averaged across the entire slice
50
40
30
20
10
0
Ave
rage
-1.5 -1.0 -0.5 0.0
5
4
3
2
1
0
Ave
rage
-.2 -.15 -.1 -.05 0 .05
inte
nsity
inte
nsity
50K 80K100K110K120K130K 150K
Energy (eV)
10
5
0
-5
-10
50-5
Bind
ing
Ener
gy (e
V)
analyzer angle (degrees)
![Page 32: Photoemission and the electronic structure of magnetic oxides · Photoemission and the electronic structure of magnetic oxides Dan Dessau University of Colorado, Boulder Duane F625](https://reader033.fdocuments.in/reader033/viewer/2022060223/5f07e58b7e708231d41f4d11/html5/thumbnails/32.jpg)
electronic
(0.3,0,1)orbital stripe
(k-space)
Fermi surfacetopology
k-space
charge/orbitalmodulation
Jahn-Teller distortionsof various magnitude
r-space
+CDW (short range)
pseudogap
ARPES
XRD,EXAFS
Double Exchange
CMR effect
lattice
chargeorbital
spin
Competition and cooperation