Presentation on theme: "H. Okamura, M. Matsubara, T. Nanba – Kobe Univ."— Presentation transcript:
1 H. Okamura, M. Matsubara, T. Nanba – Kobe Univ. IR absorption due photo-generated carriers in quantum paraelectric SrTiO3H. Okamura, M. Matsubara, T. Nanba – Kobe Univ.K. Tanaka – Kyoto Univ.
2 SrTiO3: “Quantum Paraelectric” Q ~ 35 KMüller 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-excitationPhotocarriers vs lattice
4 Optical properties (Eg = 3.2 eV) LuminescenceAbsorptionStokes shift ~ 0.7 eVLarge energy relaxation before recombinationGrabner, Phys. Rev. 177, 1315 (1969).IL(t) localized quasiparticlesPolarons, self-trapped excitons, etc.
5 Photoconductivity of SrTiO3 EgHigh conductivity when hn > EgKatsu et al., Jpn. J. Appl. Phys. 39 (2000) 2657.Mobile quasiparticles are present.
6 This work: IR absorption due to photocarriers c.b. Ti 3dv.b. O 2phnin・ In-gap stateshnIRIR absorptionhnPL・ Stokes shift (0.7 eV)・ Relaxation processesInformation 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 eVSrTiO3295 KEgTransparent 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)
11 Reflection with UV laser on / off Absorption also observed in the reflection.Not simple Drude responsePhoto-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 couplingSoft mode (Ti)Breathing mode (O)t2geg(3d)1“Large polarons” and “small polarons”Large polarons (extended over many sites): conduction, metastableSmall polarons (at one site) : no conduction, stable
13 Analogy with strongly correlated “dirty metal” oxides Conduction due to hopping and/or tunnelingChemically- doped Mott insulators.“Incoherent peak” in optical spectras(w)wCoherent peak (usual Drude peak)Incoherent peak (mid-IR peak)w0 ≠0Alternative view : binding energy of polaronsMicroscopic 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)