MPI Kernphysik, Heidelberg Humboldt Univ. Berlin Ruhr-Univ. Bochum Univ. Hamburg
IR absorption due photo- generated carriers in quantum paraelectric SrTiO 3 H. Okamura, M....
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Transcript of IR absorption due photo- generated carriers in quantum paraelectric SrTiO 3 H. Okamura, M....
IR absorption due photo-generIR absorption due photo-generated carriers in quantum paraated carriers in quantum para
electric SrTiOelectric SrTiO33
H. Okamura, M. Matsubara, T. Nanba – Kobe Univ.
K. Tanaka – Kyoto Univ.
SrTiOSrTiO33: “Quantum Paraelectric”: “Quantum Paraelectric”
~ 35 K, but paraelectric down to 0.3 K. Quantum (zero-point) fluctuations of Ti
~ 20000
35 KMüller et al, PRB 19 3593 (’79).
Photo-induced enhancement of Photo-induced enhancement of dielectric constantdielectric constant
enhanced by UV photo-excitation Photocarriers vs lattice
Hasegawa et al., JPSJ 72, 41 (‘03) Takesada et al., JPSJ 72, 37 (‘03)
Optical properties (EOptical properties (Eg g = 3.2 eV)= 3.2 eV)
Luminescence
IL(t) localized quasiparticles Polarons, self-trapped excitons, etc.
Absorption
Grabner, Phys. Rev. 177, 1315 (1969).
Stokes shift ~ 0.7 eV
Large energy relaxation before recombination
Photoconductivity of SrTiOPhotoconductivity of SrTiO33
Mobile quasiparticles are present.
Katsu et al., Jpn. J. Appl. Phys. 39 (2000) 2657.
Eg
High conductivity when h > Eg
This work: This work: IR absorption due to photocarriersIR absorption due to photocarriers
c.b. Ti 3d
v.b. O 2p
hin
Information about photocarriers and in-gap states
hIR
IR absorption
hPL
・ Stokes shift (0.7 eV)
・ Relaxation processes
・ In-gap states
ExperimentalExperimental
STO single crystals
Near-normal incidence, R() and T().
Frequency-doubled Ti:Sapphire laser.
IR beam line BL43IR at SPring-8.
0
0.5
1.0
0.01 0.1 1 100
2
4
6
Photon Energy (eV)
Re
flect
ivity
Co
nd
uct
ivity
(1
03
-1cm
-1)
RR(() and ) and ) : 4 meV - 35 eV) : 4 meV - 35 eV
SrTiO3
295 K
Transparent region
Eg
Transmission with UV laser on / offTransmission with UV laser on / off
Two broad absorption bands (in-gap states). Stronger absorption at lower T.
(similar to luminescence)
0
0.5
1.0
0 0.5 1.0 1.5
SrTiO3laser = 365 nm (h = 3.4 eV)Plaser = 0.22 W/cm-2
8 K
80 K
Photon Energy (eV)
- T
/ T
(
10
-2)
(Photo-induced decrease in the IR transmission)
Excitation Power DependenceExcitation Power Dependence
0
1
2
0 0.2 0.4 0.6 0.8 1.0 1.2
SrTiO3 T = 8 Klaser = 365 nm (3.40 eV)
Laser power [W/cm2]
1.00.64
0.32
0.16
0.08
0.03
Photon Energy (eV)
-T
/ T
[1
0-2
]
Reflection with UV laser on / offReflection with UV laser on / off
Absorption also observed in the reflection. Not simple Drude response Photo-induced mid-IR absorption band
0
0.005
0.010
0.015
0.020
0 0.2 0.4 0.6 0.8 1.0
Transmission
Reflection
(laser) = 365 nmPower =0.9 W / cm2
T = 8 K
Photon Energy (eV)
Ph
oto
-ind
uce
d a
bso
rptio
n
Model for photo-enhancement oModel for photo-enhancement of f based on polarons based on polarons
Strong e-ph coupling Soft mode (Ti) Breathing mode (O)
K. Nasu, Phys. Rev. B 67, 174111 (2003).
t2g
eg
(3d)1
t2g
eg
“Large polarons” and “small polarons” Large polarons (extended over many sites): conduc
tion, metastable Small polarons (at one site) : no conduction, stable
Analogy with strongly correlated Analogy with strongly correlated “dirty metal” oxides“dirty metal” oxides
Conduction due to hopping and/or tunneling Chemically- doped Mott insulators. “Incoherent peak” in optical spectra
Coherent peak (usual Drude peak)
Alternative view : binding energy of polarons Microscopic models needed.
Incoherent peak (mid-IR peak)
0≠0
SummarySummaryIR absorption in SrTiO3 under photo-excit
ations above Eg
– Two broad absorption bands (mIR-visible)
– Incoherent carrier dynamics (“generalized Drude response”) and/or real trapping states.
– Microscopic origin ? large / small polarons ?
– Similar to those observed for n- and p-type SrTiO3 (IR, photoemission, and XAS)
Kramers-Kronig analysisKramers-Kronig analysis
3 phonon modes.
-500
0
500
1000
200 400 600
Im]
Re[]
x10
0
500
1000
1500
0 200 400 600
Wavenumber (cm-1)
[
-1cm
-1] SrTiO3
295 K
Soft phonons vs temperatureSoft phonons vs temperature
Detailed temperature dependence of the softening
0.85
0.95
20 40 60 80 100 120
8 K40 K80 K120 K160 K200 K240 K295 K
80 K
8 K
295 K
Wavenumber (cm-1)
Re
flect
ivity
0
10
20
0 100 200 300
Temperature (K)P
ea
k W
idth
(cm
-1)
540
542
544
546
0 50 100 150 200 250 300
172
173
174
175
0 50 100 150 200 250 300
Pe
ak
po
sitio
n (
cm-1
)
020406080
0 50 100 150 200 250 300
Temperature (K)
Temperature dependenceTemperature dependence
W !
?
Lower-frequency data needed !! (in progress).
?
?
?