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PHOTO-POLARONS AND RELATED PHENOMENA IN QUANTUM PARAELECTRIC PEROVSKITE-LIKE OXIDES

Dipartimento di Fisica “A. Volta” UniPv – Pavia - Italy. PHOTO-POLARONS AND RELATED PHENOMENA IN QUANTUM PARAELECTRIC PEROVSKITE-LIKE OXIDES V.A. Trepakov 1,2 , A. Gubaev 3 , S.E. Kapphan 3 , P. Galinetto 4 , F. Rossella 4 , P.A. Markovin 2 , P.P. Syrnikov 2 and L. Jastrabik 1.

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PHOTO-POLARONS AND RELATED PHENOMENA IN QUANTUM PARAELECTRIC PEROVSKITE-LIKE OXIDES

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  1. Dipartimento di Fisica “A. Volta” UniPv – Pavia - Italy PHOTO-POLARONS AND RELATED PHENOMENA IN QUANTUM PARAELECTRIC PEROVSKITE-LIKE OXIDES V.A. Trepakov1,2, A. Gubaev3, S.E. Kapphan3, P. Galinetto4, F. Rossella4, P.A. Markovin2, P.P. Syrnikov2 and L. Jastrabik1 1Institute of Physics AS CR, Prague, Czech Republic 2A.F. Ioffe Physical-Technical Institute, Russia 3 FB Physik, University of Osnabrueck, Germany 4 Physics Dept. “A. Volta”, University of Pavia, Italy Spb- Russia

  2. Milano 2006, July 13th P. Galinetto DFAV Uni of Pavia ITALY Outline Introduction: Charge carriers doping effects; role and nature of photo-carriers localized states; “Photo­induced structural transitions”. “Giant low-T photo-dielectricity” in Incipient Ferroelectrics • Main Part • UV-light induced IR absorption in SrTiO3 and KTaO3 • Dilute KTN :UV-light induced IR absorption • Nb4+ photo-polarons in diluted KTN • Photoconductivity and visible luminescence ….conclusions

  3. Charge carriers doping effects; role and nature of photo-carriers localized states Milano 2006, July 13th P. Galinetto DFAV Uni of Pavia ITALY • Phys. Rev. 137, A1575–A1582 (1965)Some Transport Properties of Oxygen-Deficient Single-Crystal Potassium Tantalate (KTaO3) • S. H. Wemple • Phys. Rev. 180, 547–556 (1969)Electron Scattering in Perovskite-Oxide Ferroelectric Semiconductors • S. H. Wemple, M. DiDomenico, Jr., and A. Jayaraman The photo charge carriers dinamics, the localization processes and the nature of the carriers localized states at low T in incipient FE are ”old” problems …but there is a lack of optical experiments and reliable data on high purity controlled crystals in order to elucidate these phenomena. Recently... • Hasegawa T., Shiraii M., Tanaka K.: Localizing nature of photo-excited states in SrTiO3. J. Lumin. 87-89, 1217-1219 (2000) • Hasegawa, K. Tanaka K.: Photo-induced polaron states in strontium titanate. J. of Lumin. 94-95, 15-18 (2001) • Kapphan S.E., Gubaev A.I., Vikhnin V.S.: UV light-induced absorption in pure KTaO3 at low temperature. Phys. Stat. Sol. (c) 2, 128-131 (2005) • Laguta V., Glinchuk M., Bykov I., Cremoma A., Galinetto P., Giulotto E., Jastrabik L., Rosa J.: Light-induced defects in KTaO3. J. Appl. Phys. 93, 6956-6054 (2003) • Maiwald M., Schirmer O.F.: O- dynamic Jahn-Teller polarons in KTaO3. Europhys. Lett. 64, 776-782 (2003)

  4. Charge carriers doping effects; role and nature of photo-carriers localized states Kapphan S.E et al. Phys. Stat. Sol. (c) 2, 128-131 (2005) Milano 2006, July 13th P. Galinetto DFAV Uni of Pavia ITALY the broad absorption bands peaking at 1.23 eV, 1.53 eV and 2.36 eV have been reported for reduced KTO samples at LT. Additional UV-illumination in the band-gap region at LT gave rise to broad light induced absorption bands in the same 1-2 eV region, more pronounced for as grown and oxidized specimens. This IR light-induced effect has been tentatively attributed to different sorts of the O- hole polaronic centers, which where revealed in recent EPR experiments. Maiwald M., Schirmer O.F.: O- dynamic Jahn-Teller polarons in KTaO3. Europhys. Lett. 64, 776-782 (2003) Laguta et al.: Light-induced defects in KTaO3. J. Appl. Phys. 93, 6956-6054 (2003)

  5. Charge carriers doping effects; role and nature of photo-carriers localized states O- reorienting hole polaron with dipole moment O- role in electronic photoconductivity enhancement STO, KTO, STO:Ca, KTO:Li

  6. Photo-Induced Phase Transitions Photoinduced LOCAL and GLOBAL Phase Transitions (PhIPT) Schematic illustration of photoinduced phase transitions: - hidden multi-stability of the ground state - proliferation of optical induced states Relaxation of optical excited states results in collective ordering motions of large number of atoms and electrons, appearance of macroscopic domains, new structural and electronic orders M. Ueta, H. Kanzaki, K. Hanamura, Y. Kobayashi, E. Toyozawa, Excitonic proceses in solids (1986); K. Nasu, Relaxation of excited states and PhIPT (1997)

  7. Milano 2006, July 13th P. Galinetto DFAV Uni of Pavia ITALY Photo-Induced Phase Transitions Ferroelectrics: cooperative interactions electron-dipole PhIPT are facilitated in systems with the hidden multi-stability, and quasi/degenerated ground state, close to stability limit, e.g., in the system close to structural phase transitions (PT). STO, KTO at the low-T, nearly quantum mechanical stability limit, are namely such materials.

  8. Giant low-T photo-dielectricity in Incipient Ferroelectrics • Recent observations of giant UV light induced: • enhancement of the dielectric constant (“critical behavior” or “giant photo-dielectricity”), • unconventional Debye-type dispersion in STO and KTO • have been tentatively connected with possible PhIPT.

  9. Giant low-T photo-dielectricity” in Incipient Ferroelectrics Observations of gigantic PhI dielectricity in STO, KTO but not in SrTi18O3 allowed to propose that this phenomenon is inherent to QPE in the region of dominating quantum statistic but not in FE, although this effect was found later in incipient ferroelectric CaTiO3 too. • T. Hasegava, K. Tanaka, ICL 2002 • Hasegava, S. Mouri, Y. Yamada, K. Tanaka, J. Phys. Soc. Jpn. 72 (2003) 41. • I. Katayama, Y. Ichikawa, K. Tanaka, Phys. Rev. B 67 (2003) 100102(R). • M. Takesada, T. Yagi, M. Itoh, S-y Koshihara, J. Phys. Soc. Jpn. 72 (2003) 37. • T. Ishikawa, M. Itoh, M. Kurita, H. Shimoda, M. Takesada, T. Yagi, S. Koshihara, Ferroelectrics 298 (2004) 141. • M. Takesada, T. Yagi, M. Itoh, T. Ishikawa, S. Koshihara, Ferroelectrics 298 (2004) 317.

  10. The mechanisms suggested: • Space charge effects – no, due to absence in STO-18 and dc-dependence. • FE transition with photo-induced polar clusters. • Photoinduced insulator-to-metal transition – no, weak light intensity • Important role of Quantum Paraelectricity due to effect absence in STO-18 Role and nature of photo-carriers localized states + Photo­induced structural transitions” + “Giant low-T photo-dielectricity” in Incipient Ferroelectrics Carrier’s localization is essential for the comprehension of the gigantic PhI dielectric effectsand PIPD

  11. Milano 2006, July 13th P. Galinetto DFAV Uni of Pavia ITALY Polaronsand their role in structural transformations, dielectric and optical properties in ABO3 perovskites are actual and intensively discussed problems nowadays. Whereas hole O- polarons have already been established in KTO [1,2] BaTiO3 [3] LiNbO3 [4] Electronic polarons were not proven experimentally neither in STO nor in KTO Small electronic Nb4+ polarons are known for LNO [4] and nonisovalent Nb doping plays a crucial role in the Ti3+ Jahn-Teller electronic polaron formation for BTO [5] 1. V. Laguta et al. J. Appl. Phys. 93, 6956 (2003). 2. M. Maiwald, O. F. Schirmer, Europhys. Lett. 64, 776782 (2003). 3. F. Schirmer et al. Phys. Stat. Sol. (c) 2, 124 (2005) 4. O. Schirmer, D. von der Linde, Appl. Phys. Lett. 33, 35 (1978). 5 S. Lenjer et al. Phys. Rev. B 66, 165106-1 (2002).

  12. Milano 2006, July 13th P. Galinetto DFAV Uni of Pavia ITALY • Studies of polaronic problem for KTN are practically absent, although the PhI Nb4+ polaronic concept has been used as the possible origin of laser-induced transient gratings in four-wave-mixing experiments for KTN • Liu H., Powell R.C., Boatner L.A., . J. Appl. Phys. 70, 20-28 (1991) • Liu H., Powell R.C., Boatner L.A., Phys. Rev. B 44, 2461-2469 (1991) • But KTN is very suitable object to study polaronic problems and their role in structural transformations in ABO3 perovskites. • We believed to get evidences of Nb4+ polarons in KTN • in the UV light-induced NIR optical absorption • in PC vs T • in photo-luminescence

  13. The transparent colorless high purity KTN single crystals with Nb content x = 0, 0.004, 0.007, 0.012, 0.07 were grown and used; Samples were prepared as optically polished parallelepipeds oriented along <100> cubic axes. The specimens’ conductivity at RT was in the order of 10-13-1cm-1 EXPERIMENTAL DETAILS ABS in IR-Vis Range Fourier spectrometer (Bruker IFS120HR, 200-32000 cm-1), with L-He cryostat (Leybold, 1.3–600 K) and XBO high pressure lamp with band pass filter (300–400 nm) for PhI Abs. PC measurements: sandwich Ag/KTN/Ag structures (Ag electro­des) in the temperature region 15 – 300 K using an electctrometer (Keithley Model 6517) and XBO lamp light filtered by monochromator. The current dc- driving field was about 70 kV/m. PL spectra were measured by a photon counting system with SPEX1402 double grating monochromator with a cooled RCA31034 GaAs photomultiplier and closed cycle cryostat (Leybold), XBO 300W/HTP lamp and SPEX Minimate monochromator or/and the variable bandpass filters as the excitation source.

  14. RESULTS Milano 2006, July 13th P. Galinetto DFAV Uni of Pavia ITALY All KTN and KTO samples at RT display a) fundamental absorption in the UV (> 3.25 eV for KTN and >3.5 eV for KTO b) a weak absorption in the FIR region (< 0.2 eV  6) at ~ 3500 cm-1 due to OH centers incorporated during crystal growth.

  15. No new absorption bands were observed at RT under interband UV-light excitation. BUT AT LOW TEMPERATURES: KTaO3 and SrTiO3 only absorption related with O- photo-hole polarons

  16. KTN-0.4 and KTN-0.7. Nb concentration below the QMDL xcr = 0.008. Both compositions are cubic paraelectric down to lowest T. UV-light induced NIR absorption in KTO, KTN-0.4 and KTN-0.7 • KTN-0.4- PhI absorption is very close to that for nominally pure KTO. • The rather pronounced band peaking at ~ 9200 cm-1 (1.14 eV) - O- hole polaron; • a very faint band in the ~ 5000 – 6000 cm-1 region. • KTN-0.7 - Emergence of a wide light-induced NIR absorption band at 5600 cm-1 • (0.69 eV) with half-width ~ 0.5 eV at 1.3 K

  17. KTN-0.7UV-light induces a wide NIR absorption band peaking at ~ 5600 cm-1 (0.69 eV) with half-width ~ 0/5 eV at 1.3 K. UV light-induced IR absorption increases and saturates approx. 60 s after UV light switching on and does not decay in the dark at least for hour. • The position of absorption maximum agrees well with electronic polarons energies in: • theory: 0.76 eV for Nb4+ [Eglitis, Kotomin, Borstel 2005] • in TiO2(0.82eV)and BaTiO3 (0.6 eV) [Firsov 1975, Bursian 1973 ] • 0.8 eV in KNbO3 [pulsed electronic beam excitationGrigor’eva, Millers, SSC 2004] • Large bandwidth evidences strong electronic-lattice coupling. • We suggest that this absorption is connected with Nb4+ electronic polarons

  18. KTN-1.2: displays only one cubic-rhombohedral FE PT at ~20 K. UV light-induced absorption increases with Nb concentration and absorption maximum shifts closer to energies that for Nb4+ polarons in KNbO3.

  19. The UV light-induced absorption in the paraelectric cubic phase: • increases with T up to ~ 40 K, • afterward decreases, and • practically vanishes at ~ 80 K

  20. KTN-7: Three C-T-Rhombic-Rhombohedral Fe PT take place • Character of the UV light-induced NIR absorption appears to be very similar. • PhI absorption maximum locates at 6171 cm-1 (1.62, 0.76 eV) for 1.3 K, which can be • related with changes in the lattice constant due to Nb doping and correlation effects • A similar behavior is shown in the T -dependence of the UV light-induced absorption for • different Nb concentrations

  21. The optical absorption spectra can be described by the conventional model of polaron formation at Nb ions in close Nb pairs. Configurations curves for two-site model Absorption maximum corresponds to the “interband” optical Frank-Condon O  O’ transition at ~ 0.7 eV and polaronic activation energy ¼E~175 meV (~ 2000 K). At low characteristic energy of LO phonons width of polaronic absorption band This gives ~ 0.5 – 0.7 eV

  22. Milano 2006, July 13th P. Galinetto DFAV Uni of Pavia ITALY The presence of Nb pairs is not a strong assumption. KTN-0.4: the quantity of Nb pairs is small. UV light-induced absorption only faintly pronounced as a weak shoulder in the 5000 – 6000 cm-1 region. KTN-0.7the concentration of Nb pairs increases, as does the respective electron polaron absorption. KTN-1.2: The light-induced absorption increases appreciably. But further on, for KTN-7 this increasing appears to be rather weak. This fact can be understood if in the polaron optical absorption only pairs of neighboring Nb ions play the main role and it is not necessary to form infinite clusters of the nearest neighbor Nb ions.

  23. However, absence of the infinite Nb clustersand Nb4+ polarons formation should strongly decrease the static conductance and photoconductivity at low temperatures. • Nb- admixture depress PhC at all temperatures in respect to KTaO3 • PhC is depressed strongly at lowest T. • PhC temperature maximum appears to be in the T-region, where UV-light • induced IR absorption decreases strongly.

  24. Milano 2006, July 13th P. Galinetto DFAV Uni of Pavia ITALY Estimation of the traps depth KTN – 1.2 Good agreement with activation energy for UV-induced IR absorption Estimation of the decay-time constant

  25. These are reasonable arguments for assumption of: • Nb-pair polarons formation; • strong charge localization at the lowest temperatures just in the dominant quantum fluctuations region. The emission intensity smoothly decays with temperature practically vanishing at ~100 K. This suggestion is supported also by: dielectric permittivity enhancement (9.46 MHz) under UV light; strong increasing of the VIS luminescence decay time just below the T above which UV- light induced absorption decreases (KTO -  ~ ms, in KTN-1.2 increases below 40 K up to 10 s at 10 K, QFR) This coincidence strongly supports the suggestion that the localization and following release of the photo-induced charge carriers occurs mainly by Nb constituting.

  26. Milano 2006, July 13th P. Galinetto DFAV Uni of Pavia ITALY • Temperature dependence modeling. • Low temperatures: • The light-induced absorption increases with T. • The nature of this effect is not clear and needs further studies. Theory is not developed. • It only can be assumed that photo-carriers generation, transport, trap transformation • processes and changing in oscillator strength can play here a decisive role. High temperatures: The strong decay above 40 K yields from fitting an estimate of the value for the optical absorption thermal quenching activation energy ~ 36 meV. for PC quenching a trap depth ~ 28 meV

  27. CONCLUSIONS: • We report on detailed and systematic stuides of the low-temperature UV-light induced NIR abs, PhC and VIS-PL in KTaO3 and SrTiO3 high polarizable perovskite like oxides as well as in series of KTa1-xNbxO3 single crystals (x=0, 0.004, 0.007, 0.012, 0.07) in which depending on Nb concetration behaviour of the incipent ferroelectrics and low-T ferroelectrics can be realized (modelled). UV-induced NIR ABS in KTO and STO  localized optical hole polarons UV-induced NIR ABS in dilute KTN  Results are considered as evidence for the formation of the small electronic Nb4+ polarons at low temperatures: • a) No photochromic effect was found at RT in all Nb containing species as well as in nominally pure KTaO3. • b) The emergence of the UV-light induced wide absorption bands in the NIR region with maxima at 0.69–0.8 eV at low temperatures is treated s a manifestation of the localization of photo-induced electrons and the formation of Nb4+ small electron polarons in close-neighbor Nb-Nb pair centers. • c) Position and width of the light-induced NIR absorption bands is in good agreement with theory of small polarons.

  28. Nb doping suppresses the photoconductivity in KTN at low temperatures. This agrees well with the close-neighbors Nb4+-Nb5+ pair centers model, considering Nb5+ ions as effective traps for photo-electrons at low T. The effective trapping of charge carries is supported also by the, characteristic for ABO3 oxides, “green” photoluminescencestudy. The strong increasing of the luminescence decay time from millisecond up to about 10 seconds was found in KTN. In the medium temperature region UV-light induced NIR absorption begins to decrease and then vanishes (~ at 40 K and 80 K respectively for KTN with x=0.012), due to the presence of a quenching level or thermal activation of the electron from the Nb4+ polaron into CB. The results evidence localization of UV excited charges carriers as electronic polarons (Nb4+) at the low T for diluted KTN.

  29. Subjects under inquiry (in progress): • Photo-EPR • UV-light induced effects in Raman light scattering; • UV-light induced effects in thermo-optical properties; • Dielectric spectroscopy: low-f and THz (THz: TOs or plasma frequency?)

  30. Obtained low-T optical hole and electronic polarons formation can lead to strong increasing of measurable dielectric permittivity (giant dielectriciry) under UV-light action at low T, as it was reported by Japanies. But this effects is rather atributed to polaronic migration contribution of excited charge carriers (polarons) than ferroelectric phase transition. At least our UV-ligh Raman experiments gave no evidences of UV light effect on soft TO mode or appearance of UV-light induced polar region.

  31. UV-light induced Raman light scattering: KTN-0.4 Thank you for your attention

  32. Milano 2006, July 13th P. Galinetto DFAV Uni of Pavia ITALY Giant low-T photo-dielectricity” in Incipient Ferroelectrics Photocarriers are promptly charge separated and localized as small polarons T Hasegava, M. Shirai, K. Tanaka, J. Lumn.87-82 (2000) 12. Small polarons accompanied by strong lattice distortions are origin of PhI dipoles and PhIPD PhIPD interaction [J(n) <0]. Finite lifetime Prevents PhIPD from long-range ordering Schematic picture of PhIPD. Photocarriers recombines radiatively with mean~ 1ms and PhIPD vanish.

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