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IR absorption due photo- generated carriers in quantum paraelectric SrTiO 3 H. Okamura, M. Matsubara, T. Nanba – Kobe Univ. K. Tanaka – Kyoto Univ.

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Presentation on theme: "IR absorption due photo- generated carriers in quantum paraelectric SrTiO 3 H. Okamura, M. Matsubara, T. Nanba – Kobe Univ. K. Tanaka – Kyoto Univ."— Presentation transcript:

1 IR absorption due photo- generated carriers in quantum paraelectric SrTiO 3 H. Okamura, M. Matsubara, T. Nanba – Kobe Univ. K. Tanaka – Kyoto Univ.

2 SrTiO 3 : Quantum Paraelectric ~ 35 K, but paraelectric down to 0.3 K. Quantum (zero-point) fluctuations of Ti ~ K Müller et al, PRB (79).

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

4 Optical properties (E g = 3.2 eV) Luminescence I L (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

5 Photoconductivity of SrTiO 3 Mobile quasiparticles are present. Katsu et al., Jpn. J. Appl. Phys. 39 (2000) EgEg High conductivity when h > E g

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

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

8 R( ) and ) : 4 meV - 35 eV SrTiO K Transparent region EgEg

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

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 based on polarons Strong e-ph coupling Soft mode (Ti) Breathing mode (O) K. Nasu, Phys. Rev. B 67, (2003). t 2g egeg (3d) 1 t 2g egeg 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 Coherent peak (usual Drude peak) Alternative view : binding energy of polarons Microscopic models needed. Incoherent peak (mid-IR peak) 0

14 Summary IR absorption in SrTiO 3 under photo- excitations above E g – 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 SrTiO 3 (IR, photoemission, and XAS)

15 Kramers-Kronig analysis 3 phonon modes. SrTiO K

16 Soft phonons vs temperature Detailed temperature dependence of the softening

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


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