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IR absorption due photo-generated carriers in quantum paraelectric SrTiO 3

IR absorption due photo-generated carriers in quantum paraelectric SrTiO 3. H. Okamura, M. Matsubara, T. Nanba – Kobe Univ. K. Tanaka – Kyoto Univ. e ~ 20000. Q ~ 35 K. SrTiO 3 : “Quantum Paraelectric”. Q ~ 35 K, but paraelectric down to 0.3 K. Quantum (zero-point) fluctuations of Ti.

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IR absorption due photo-generated carriers in quantum paraelectric SrTiO 3

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

  2. e ~ 20000 Q ~ 35 K SrTiO3: “Quantum Paraelectric” • Q ~ 35 K, but paraelectric down to 0.3 K. • Quantum (zero-point) fluctuations of Ti Müller et al, PRB 19 3593 (’79).

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

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

  5. Eg High conductivity when hn > Eg Photoconductivity of SrTiO3 • Mobile quasiparticles are present. Katsu et al., Jpn. J. Appl. Phys. 39 (2000) 2657.

  6. c.b. Ti 3d ・ In-gap states ・ Stokes shift (0.7 eV) ・ Relaxation processes hnin hnPL hnIR v.b. O 2p IR absorption This work: IR absorption due to photocarriers 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. Transparent region Eg R(w) and s(w) : 4 meV - 35 eV SrTiO3295 K

  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. eg eg t2g t2g (3d)1 Model for photo-enhancement of e based on polarons K. Nasu, Phys. Rev. B 67, 174111 (2003). • Strong e-ph coupling • Soft mode (Ti) • Breathing mode (O) • “Large polarons” and “small polarons” • Large polarons (extended over many sites): conduction, metastable • Small polarons (at one site) : no conduction, stable

  13. Incoherent peak (mid-IR peak) s(w) Coherent peak (usual Drude peak) w0≠0 w Analogy with strongly correlated “dirty metal” oxides • Conduction due to hopping and/or tunneling • Chemically- doped Mott insulators. • “Incoherent peak” in optical spectra • 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 • 3 phonon modes. SrTiO3295 K

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

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

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