C.M. Foster

Localized charged excitations in polyaniline: Infrared photoexcitation and protonation studies

Abstract We report the infrared photoinduced activity of polyaniline in the “emeraldine” base form while pumping into the two optical yransitions at 2.0 eV and 4.0 eV, respectively. For optical pumping at energies near 2.0 eV, we found photoinduced infrared active vibrational (IRAV) modes which are essentially identical to those observed subsequent to protonation. With 4.0 eV optical pumping, the IRAV modes are significantly different, demonstrating that a different species is produced when pumping into the π-π ∗ transition. We propose that the optical absorption at 2.0 eV in the emeraldine form is due to an n -π ∗ transition from the non-bonding nitrogen lone-pair to the conduction ( π ∗ ) band. Our analysis, based on the amplitude mode formalism, indicates that the charged photoexcitations and the polarons which are generated through protonation are localized and not highly mobile.

Photoinduced localized charged excitations in polyaniline

Abstract We report the infrared photoinduced absorption spectra of polyaniline in the ‘emeraldine’ base form while pumping into the two optical transitions at 2.0 eV and 4.0 eV, respectively. For optical pumping at energies near 2.0 eV, we found photoinduced infrared-active vibrational (IRAV) modes that are essentially identical to those observed subsequent to protonation. We propose that the optical absorption at 2.0 eV in the emeraldine form is due to an n −π ∗ transition from the non-bonding nitrogen lone-pair to the conduction ( π ∗ band. With 4.0 eV optical pumping, the IRAV modes are significantly different, demonstrating that a different species is produced when pumping into the π−π ∗ transition. Our analysis, based on the amplitude mode formalism, indicates that the charged photoexcitations and the polarons that are generated through protonation are localized and not highly mobile.

Anharmonicity and frequency shift of the apex oxygen O(4) Raman mode in Y1Ba2Cu3O7−δ as a function of doping

Abstract We report the Raman spectra of Y1Ba2Cu3O7−δ with δ=0.1 and δ>0.9, as a function of temperature (up to 200 C). We suggest that the appearance of additional modes in the region of 580–600 cm-1 is due to anharmonicity of the crystal potential of the apex O(4) oxygen vibration in addition to disorder. The anomalous positive frequency shift of the 485 cm-1 mode with doping, as well as the anomalous frequency shift at high temperature are shown to be consistent with the reduction of anisotropy of the crystal potential of the O(4) oxygen caused by the gradual addition of charge in the CuO2 planes. Infrared active vibrational modes at this frequency seen in photoexcitation studies also support this conclusion.

Photogenerated carriers in La2CuO4, YBa2Cu3O7−δ and Tl2Ba2Ca(1 − x)GdxCu2O8: Polarizability-induced pairing of polarons

Abstract Photo-induced absorption measurements have been carried out in the infrared spectral range on Tl2Ba2Ca(1 − x)GdxCu2O8 (x = 0.02), YBa2Cu3O7 − δ(δ = 0.75) and La2CuO4. In all three materials, the observation of infrared active vibrational (IRAV) modes and an associated electronic transition with common intensity and temperature dependence indicate the formation of self-localized polarons as photogenerated carriers. The photo-induced electronic transition deep in the infrared implies a major shift of oscillator strength leading to a large local polarizability in the vicinity of the polaron. The dynamic mass (Md) associated with the distortion around the charge carriers and the associated gap state energy ( h ω el ) are correlated with Tc of the superconducting phase of each material: Tc ∝ 1/Md, and Tc ∝ [1/ωel]β where β ≈ 0.6. We suggest that the enhanced local polarizability may provide an important mechanism for pairing.

Photoinduced self-localized polarons in Tl2Ba2Ca(1-x)GdxCu2O8: A proposal for Van Der Waals pairing

Abstract Photoinduced absorption measurements are reported for the infrared spectral range from 130 to 5000 cm −1 . The observation of infrared activity vibrational (IRAV) modes and an associated electronic transition (at 0.095 eV) with common intensity and temperature dependence indicate the formation of self-localized polarons. Both the dynamic mass (M d ) and the gap state energy (ħω el ) are smaller than in either YBa 2 Cu 3 O 7-δ or La 2 CuO 4 . For all three systems, M d and ω e1 are correlated with T c of the superconducting phase; T c ∝ 1/M d , and T c ∝ [1/ω el ] β where β ≈ 0.6.

Infrared reflection of epitaxial Tl2Ba2CaCu2O8 thin films in the normal and superconducting states

Abstract For T>T c , the frequency-dependent conductivity, σ(ω), is Drude-like in the far-infrared (IR) and indicates a broad absorption in the mid-IR. For T c , R S (ω)=1.0±0.005 for ω −1 . All the normal state Drude oscillator strength shifts to δ(ω), but with no shift or loss of oscillator strength from the mid-IR. No feature in R (ω) or σ(ω) can be assigned to the energy gap. The penetration depth (λ L ) and carrier density in the condensate (N S ) are λ L ≈2000A and N S ≈4.5X10 20 (m ∗ /m) cm −3 (m ∗ and m are the optical effective mass and free-electron mass). The data demonstrate that the quality of these epitaxial Tl 2 Ba 2 CaCu 2 O 8 films is comparable to the best YBa 2 Cu 3 O 7-δ films.

Localization of charged excitations in polyaniline

Abstract Photogenerated polarons in the emeraldine base form of polyaniline have been extensively studied by using the infrared and optical photoexcitation, photoconductivity, and light-induced electron spin resonance (LESR) techniques. The photogenerated polarons in polyaniline are strongly localized and stable for the experimental time scales (≥ 1 msec.) at both 2.0 eV and 4.0 eV optical pumping energies as proved by infrared and LESR studies. The dynamic mass of the charged excitations in polyaniline is found to be two orders of magnitude larger than the electron band mass. Steady state photoconductivity measurements demonstrate that the photogenerated charges are highly immobile for both 2.0 eV and 4.0 eV pumping, consistent with the infrared studies. Significantly different results were obtained in the fast time scale (≤ 800 psec.) photoconductivity measurements. Photoexcitation at 4.0 eV leads to a measured photoconductivity one order of magnitude less than that of other conjugated polymers. The 2.0 eV optical pumping yields a factor of 30 smaller photoconductivity than that of the 4.0 eV pumping for equal number of incident photons. We propose that photogenerated holes are responsible for the transport, and the photogenerated electrons are mainly localized on the benzene ring (as monitored by the softening of the ring deformation mode caused by losing the aromaticity of the benzenoid structure).

Photogeneration of self-localized polarons in YBa2Cu3O7-δ and La2CuO4

Photoinduced infrared absorption measurements of YBa2Cu3O7-δ (δ = 0.75) and La2CuO4 are reported. We have observed photoinduced infrared active vibrational modes and associated phonon bleachings which indicate the formation of a localized structural distortion in the Cu-O plane around the photogenerated carriers. In both materials, an associated electronic transition indicates that this structural distortion causes the formation of a self-localized electronic state in the energy gap. The photoinduced distortion and the associated self-localized gap state demonstrate that the photoexcitations are relatively long-lived polarons (or bipolarons). The dynamic mass associated with the distortion is smaller in YBa2Cu3O7-δ than in La2CuO4 (smaller dynamic mass implies a longer range distortion). Since these features are not observed in the isostructural compound, La2NiO4, we conclude that polaron (or bipolaron) formation plays an important role in the high temperature superconductivity.

Infrared activities in poly(2,5-thienylene vinylene)

Abstract We present a detailed study of the infrared activities of poly(2,5-thienylene vinylene) (PTV) including photoinduced absorption in the energy range from 400 to 4500 cm−1. Upon photoexcitation, we observed the production of infrared active vibrational (IRAV) modes and self-localized electronic states in the π-π ∗ gap. We present a detailed analysis of the IRAV modes which provide a structural fingerprint of the lowest energy charged excitations of PTV. We also present an in-situ infrared study of the conversion of PTV from its polyelectrolyte precursor and an analysis of the infrared modes of PTV.

Polarons in high Tc superconductors: IRAV modes and electronic transitions to gap states as in conducting polymers

Abstract Photoinduced infrared absorption measurements of YBa2Cu3O7−δ (δ=0.75) are reported. We observed photoinduced infrared active vibrational modes, associated phonon bleachings, and an electronic transition from a self-localized gap state. The results indicate a local structural distortion in the CuO plane around the photogenerated charge carriers. Remarkable similarities in photoinduced activity between conducting polymers and high Tc superconductors were found. The photoinduced structural distortion and the associated self-localized gap state demonstrate that the photoexcitations are relatively long-lived polarons or bipolarons. The dynamic mass associated with the distortion is smaller in YBa2Cu3O7−δ than in La2CuO4, which is comparable to those in conducting polymers. Since these characteristic features are not observed in the isostructural compound, La2NiO4, we conclude that polaron (or bipolaron) formation plays an important role in the high temperature superconductivity.