Photoinduced IR absorption in WO3: determination of the polaron binding energy

Photoinduced IR absorption measurements are reported on WO3. A photoinduced midinfrared small polaron peak centered at 4800 cm−1 (0.59 eV) was observed. The data were analyzed in the framework of the photon-assisted small-polaron hopping theory and briefly compared to previously published infrared absorption measurements in WO3 and photoinduced IR absorption measurements in high Tc cuprates. PACS. 71.38.-k Polarons and electron-phonon interactions – 78.30.Hv Other nonmetallic inorganics – 78.30.-j Infrared and Raman spectra Recent indications of possible surface superconductivity at 91 K in sodium tungsten bronze NaxWO3 [1,2] has enhanced experimental interest in the idea of hightemperature bipolaronic superconductivity, which had led to the discovery of cuprate high temperature superconductors [3]. Measurements of polaron properties in this material may therefore be important for reaching a deeper understanding of the phenomenon. Polarons and bipolarons in tungsten bronzes were studied in past with electron spin resonance and optical spectroscopy [4–6]. An optical absorption peak found at 0.71 eV in tungsten trioxide WO3 was attributed to the photon-assisted hopping of small polarons [4]. It was also shown that bipolarons present in oxygen deficient WO3 appear to dissociate under light illumination forming single polarons [5]. In cuprates [7–11] and manganites [12] measurements of the photoinduced (PI) infrared absorption have been shown to be a useful tool for investigating polaronic carriers, especially in the range of weak doping where PI absorption spectra are interpreted in terms of the photonassisted hopping of small polarons [12,13]. It is therefore natural to extend measurements of the PI absorption also to WO3 especially because the superconducting Tc is the largest at low carrier doping [1], much closer to carrier densities achieved by photoexcitation than in cuprates. A ventron 99.7%-WO3−x powder sample (of unspecified oxygen deficiency x), greenish in appearance, was ground and mixed with KBr powder in 0.1–0.2 wt.% ratio and pressed into 12 mm diameter pellets. The pellets were mounted in a He-flow cryostat equipped with optical windows. Special care was taken that the KBr pellet a e-mail: [email protected] was in a good thermal contact with the sample holder. PI spectra were measured at 25 K using an Ar-ion-laser photoexcitation at 514.5 nm (hν = 2.41 eV) with an optical fluence of ∼500 mW/cm. To minimize heating effects due to laser light absorption in the sample and instrumental drift, the PI spectra were taken by alternating one sample scan (with the excitation laser on) and one reference scan (laser off) approximately every two seconds. At the same temperature a thermal-difference (TD) transmittance change was also measured without laser excitation by first measuring a reference spectrum and then increasing the sample holder temperature by 2 K and measuring a sample spectrum. To remove the effects of any instrumental drift, the same procedure was then inverted to measure the sample spectrum 2K above the given temperature first. The thermal difference (TD) transmittance change was then obtained by averaging both spectra. The room temperature IR transmittance spectrum shown in Figure 1 is typical for an insulating material. There are two major phonon absorption peaks at 371 cm−1 and 839 cm−1. The high frequency peak has a narrow shoulder at 779 cm−1. The transmittance monotonically decreases towards higher frequencies mainly due to the increasing scattering in the KBr pellet at shorter wavelengths. A PI transmittance spectrum at 25 K is shown in Figure 2b. Comparison of the PI spectrum with the TD spectrum in Figure 2a clearly shows that the PI spectrum is not caused by laser heating of the pellet. The PI spectrum is characterized by a relatively narrow structure in the phonon-frequency region and a broad absorption centered at 4800 cm−1 (0.59 eV). The structure in the phonon part shown in the inset in Figure 2 consists of a bleaching 326 The European Physical Journal B Fig. 1. Infrared transmittance of WO3−x powder dispersed in a KBr pellet relative to a pure KBr pellet at room temperature. The inset shows infrared transmittance in the phononfrequency region. peak at 840 cm−1 and a weaker PI absorption peak at 929 cm−1. As previously in high Tc superconductors [13] and manganites [12] we fit the PI transmittance spectrum using the theoretical expression for optical absorption due to the small polaron hopping in the Holstein model [14–17], given in a compact form by Emin [17]. Assuming that −∆TPI/T is proportional to the absorption coefficient [7], the PI transmittance is given by: −∆TPI T ∝ α ∝ 1 ~ω exp ( − (2Epol − ~ω) 2