1. H. Okamura, M. Matsubara, T. Nanba – Kobe Univ.
IR absorption due photo-generated carriers in quantum paraelectric SrTiO3
H. Okamura, M. Matsubara, T. Nanba – Kobe Univ.
K. Tanaka – Kyoto Univ.

2. SrTiO3: “Quantum Paraelectric”
Q ~ 35 K
Müller et al, PRB (’79).
Q ~ 35 K, but paraelectric down to 0.3 K.
Quantum (zero-point) fluctuations of Ti

3. Photo-induced enhancement of dielectric constant
Hasegawa et al., JPSJ 72, 41 (‘03)
Takesada et al., JPSJ 72, 37 (‘03)
e enhanced by UV photo-excitation
Photocarriers vs lattice

4. Optical properties (Eg = 3.2 eV)
Luminescence
Absorption
Stokes shift ~ 0.7 eV
Large energy relaxation before recombination
Grabner, Phys. Rev. 177, 1315 (1969).
IL(t)  localized quasiparticles
Polarons, self-trapped excitons, etc.

5. Photoconductivity of SrTiO3Eg
High conductivity when hn > Eg
Katsu et al., Jpn. J. Appl. Phys. 39 (2000) 2657.
Mobile quasiparticles are present.

6. This work: IR absorption due to photocarriers
c.b. Ti 3d
v.b. O 2p
hnin
・ In-gap states
hnIR
IR absorption
hnPL
・ Stokes shift (0.7 eV)
・ Relaxation processes
Information about photocarriers and in-gap states

7. Experimental STO single crystals Near-normal incidence, R(w) and T(w).
Frequency-doubled Ti:Sapphire laser.
IR beam line BL43IR at SPring-8.

8. R(w) and s(w) : 4 meV - 35 eV
SrTiO3295 K Eg
Transparent region

9. Transmission with UV laser on / off
(Photo-induced decrease in the IR transmission)
Two broad absorption bands (in-gap states).
Stronger absorption at lower T. 　　　　　　　　(similar to luminescence)

10. Excitation Power Dependence

11. Reflection with UV laser on / off
Absorption also observed in the reflection.
Not simple Drude response
Photo-induced mid-IR absorption band

12. Model for photo-enhancement of e based on polarons
K. Nasu, Phys. Rev. B 67, (2003).
Strong e-ph coupling
Soft mode (Ti)
Breathing mode (O)
t2g eg
(3d)1
“Large polarons” and “small polarons”
Large polarons (extended over many sites): conduction, metastable
Small polarons (at one site) : no conduction, stable

13. Analogy with strongly correlated “dirty metal” oxides
Conduction due to hopping and/or tunneling
Chemically- doped Mott insulators.
“Incoherent peak” in optical spectra
s(w) w
Coherent peak (usual Drude peak)
Incoherent peak (mid-IR peak)
w0 ≠0
Alternative view : binding energy of polarons
Microscopic models needed.

14. Summary IR absorption in SrTiO3 under photo-excitations 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)

15. Kramers-Kronig analysis
SrTiO3295 K
3 phonon modes.

16. Soft phonons vs temperature
Detailed temperature dependence of the softening

17. Temperature dependence?
Lower-frequency data needed !! (in progress).
QW !