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H. Okamura, M. Matsubara, T. Nanba – Kobe Univ.

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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 SrTiO3
Eg 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 !

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