Small Polaron Absorption Research Articles

Coloration of tungsten oxide electrochromic thin films at high bias potentials and low intercalation levels

The optical absorption of amorphous tungsten oxide thin films was studied at low intercalation levels. The electronic density of states was obtained by an electrochemical method and bias potential regions were assigned to the conduction band and gap states according to the experimentally estimated conduction band edge. Differences between the coloration in the conduction band and gap states were observed when comparing the experimental results to a theoretical site-saturation model that considers electronic transitions between localized tungsten sites. The model could reproduce the optical response due to conduction band states. However, it underestimated the rate of change of the optical absorption coefficient with intercalation level for gap states. This discrepancy is discussed in the context of small-polaron optical absorption.

Pulse-induced transient blue absorption related with long-lived excitonic states in iron-doped lithium niobate

Transient absorption is studied in Fe-doped lithium niobate single crystals with the goal to control and probe a blue absorption feature related with excitonic states bound to Fe$_\textrm {Li}$Li defect centers. The exciton absorption is deduced from the comparison of ns-pump, supercontinuum-probe spectra obtained in crystals with different Fe-concentration and Fe$^{2+/3+}_\textrm {Li}$Li2+/3+-ratio, at different pulse peak and photon energies as well as by signal separation taking well-known small polaron absorption bands into account. As a result, a broad-band absorption feature is deduced being characterized by an absorption cross-section of up to $\sigma ^\textrm {max}(2.85$σmax(2.85 eV$) = (4\pm 2)\cdot 10^{-22}$)=(4±2)⋅10−22 m$^{2}$2. The band peaks at about 2.85 eV and can be reconstructed by the sum of two Gaussians centered at 2.2 eV (width $\approx 0.5$≈0.5 eV) and 2.9 eV (width $\approx 0.4$≈0.4 eV), respectively. The appropriate build-up and decay properties strongly depend on the crystals’ composition as well as the incident pulse parameters. All findings are comprehensively analyzed and discussed within the model of $\textrm {Fe}^{2+}_{\textrm {Li}}-\textrm {O}^{-}-\textrm {V}_{\textrm {Li}}$FeLi2+−O−−VLi excitonic states.

Investigation on the optical properties of annealed Mn-Co-Ni-O thin films by spectroscopic ellipsometry

Mn-Co-Ni-O thin films have been widely used for their excellent negative temperature coefficient features, but investigations about tailoring their structural and optical properties via a post-annealing (PA) process are still rare in recent years. In this Letter, we derived the visible to near-infrared optical constants of annealed Mn1.56Co0.96Ni0.48O4 (MCNO) films, and we elucidate the absorption mechanism of MCNO films by investigating the microstructure and cation distribution properties. We found that proper PA temperature will result in improved crystallinity, balanced distribution of Mn3+ and Mn4+ ions, and larger small polaron absorption. We also revealed that it is mainly the small polaron hopping, together with the charge transfer (CT) transition process involving Co2+ and O2− cations, which contributes to the absorption at 750 to 1100 nm. The process of Mn3+ occupying the octahedral sites reduces the concentration of Co2+ and the absorption peak at 1.3 eV. This Letter will improve the mechanism study of small polaron hopping and CT absorption for MCNO films, which is important to the development of optoelectronic devices based on transition metal oxides.

Wide range dielectric and infrared spectroscopy of (Nb+In) co-doped rutile ceramics

The dielectric response of ceramics of co-doped rutile $\mathrm{T}{\mathrm{i}}_{1\text{\ensuremath{-}}x}{(\mathrm{N}{\mathrm{b}}_{0.5}\mathrm{I}{\mathrm{n}}_{0.5})}_{x}{\mathrm{O}}_{2}$ has been measured via a combination of impedance, high-frequency coaxial, terahertz transmission, and infrared reflectivity spectroscopies spanning 15 decades of frequency between 0.1 Hz and 240 THz. It is argued that the colossal dielectric permittivity reported by Hu et al. [Hu et al., Nat. Mater. 12, 821 (2013)] has the same explanation as in the original colossal permittivity material $\mathrm{CaC}{\mathrm{u}}_{3}\mathrm{Ti}{\mathrm{O}}_{4}{\mathrm{O}}_{12}$ (CCTO), namely, a combination of the internal barrier layer capacitor (IBLC) and the surface barrier layer capacitor (SBLC) effects. The IBLC effect is caused by a microstructure consisting of insulating grain boundaries (thickness \ensuremath{\sim}1 nm and conductivity of $\ensuremath{\sim}{10}^{\ensuremath{-}6}\phantom{\rule{0.16em}{0ex}}\mathrm{S}/\mathrm{cm})$ surrounding interior regions of bulk conductivity approximately equal to ${10}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}\mathrm{S}/\mathrm{cm}$. The SBLC effect is the result of a depletion layer adjacent to the contacts approximately 100 nm thick with conductivity of $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\phantom{\rule{0.16em}{0ex}}\mathrm{S}/\mathrm{cm}$. The SBLC and IBLC effects give rise to dielectric relaxations in low-frequency and radiofrequency regions, respectively. The temperature dependence of both relaxation rates measured down to 20 K is thermally activated. No separate absorption process has been observed that could be linked to giant defect dipoles. Infrared spectroscopy has revealed four weak defect-related modes at frequencies close to the Raman mode frequencies of rutile, activated in the infrared through a symmetry-breaking process. Co-doping produces a significant loss in the partial spectral weight between 75 and $350\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ associated with the lowest transverse optical mode, which softens on cooling. The loss in its spectral weight corresponds to the gain in near-infrared spectral weight, previously assigned to small polaron absorption.

Picosecond near-to-mid-infrared absorption of pulse-injected small polarons in magnesium doped lithium niobate

Femtosecond-pulse-induced (Epump = 2.5 eV) picosecond infrared absorption is studied in the spectral region between 0.30 eV and 1.05 eV in LiNbO3:Mg. We find a non-instantaneous mid-infrared absorption peak in the time domain up to 1 ps and a broad-band, long-lived absorption (maximum at 0.85 eV, width ≈ 0.5 eV), for t > 1 ps. The modelling succeeds by considering small NbNb4+ electron polaron formation along the sequence: (i) two-photon injection of hot electron-hole pairs at Nb-O-octahedra, (ii) dissociation and electron cooling by electron-phonon-scattering, and (iii) electron self-localization by strong electron-phonon-coupling.

Scanning nonlinear absorption in lithium niobate over the time regime of small polaron formation

Nonlinear absorption is studied in presence of small polaron formation in lithium niobate using the z-scan technique and ultrashort laser pulses with pulse durations of 70 – 1,000 fs. A model for the analysis of the transmission loss as a function of pulse duration is introduced that considers (i) the individual contributions of two-photon and small polaron absorption, (ii) the small polaron formation time and (iii) an offset time between the optical excitation of free carriers by two-photon absorption and the appearance of small polarons. It is shown that the model allows for the analysis of the experimentally determined z-scan data with high precision over the entire range of pulse durations using a two-photon absorption coefficient of β = (5.6 ± 0.8) mm/GW. A significant contribution by small polaron absorption to the nonlinear absorption is uncovered for pulse durations exceeding the characteristic small polaron formation time of ≈ 100fs. It can be concluded that the small polaron formation time is as short as (70 – 110) fs and the appearance of small polaron formation is delayed with respect to two-photon absorption by an offset of about 80 fs.

Intense optical absorption of defects created in Er3+-diffused layer in MgO (5 mol%)-doped LiNbO3crystal by local Er3+diffusion under Li-poor atmosphere

Abstract

Charge dynamics in quasi-one dimensional β-Sr1/6V2O5

Polarized infrared reflectivity was measured as a function of temperature on a quasi-one dimensional β-Sr0.17V2O5 single crystal. Along the conduction direction, the optical conductivity exhibits a weak electronic background in the far infrared and a large mid infrared band. Sum rule analysis shows that both far infrared and midinfrared bands arise from 3d vanadium electrons. As temperature is decreased, the mid infrared band slightly narrows and redshifts, but does not exhibit drastic change. In contrast, the spectral weight grows at low frequencies and peaks appear in the phonon energy range. In the transverse direction, β-Sr0.17V2O5 shows an insulator-like behaviour at all temperatures. The optical response is compared with the iso-electronic and iso-structural compound β-Na0.33V2O5. Although spectra are very similar in both compounds at room temperature, their temperature dependence differs. However, some general trends indicate that charge carriers are of the same nature in both compounds. The results suggest that electron- phonon coupling is relevant to explain the optical properties of β-Sr0.17V2O5. The optical response along the conducting direction is assigned to an adiabatic small polaron absorption, in intermediate or strong electron-phonon coupling regime. In this framework, the optical conductivity is in qualitative agreement with recent Dynamical Mean Field Theory results over a wide temperature range.

Thickness-dependent optical properties of La0.7Sr0.3MnO3 thin films

We report on a systematic study of the thickness dependence of the optical properties of La0.7Sr0.3MnO3 thin films epitaxially grown on a LaAlO3 substrate. The x-ray powder-diffraction data indicate that the c-axis lattice constant is enhanced with decreasing the film thickness due to the compressive strain in the film plane produced by lattice mismatch. Magnetization curves show a decrease of the Curie temperature (TC) for decreasing thickness of films. Optical reflectance and transmittance measurements provide evidence that the position of Mn-O stretching mode shifts toward low frequency and the energy of the charge-transfer transition between O 2p and Mn 3d states increases with the decrease of film thickness. Most importantly, an analysis of the small-polaron absorption in the mid-infrared region shows that the polaron binding energy increases with decreasing the film thickness, suggesting that the strain dependence of TC mainly results from the strain-induced electron–phonon coupling.

Optical absorption edge evolution of vanadium pentoxide films during lithium intercalation

The influence of lithium intercalation on the optical absorption of vanadium pentoxide films is reported in this paper. Thin films were prepared by thermal evaporation followed by annealing in air. Lithium was inserted electrochemically so that Li x V 2O 5 (0 ≤ x ≤ 0.54) was obtained. The pristine and lithium inserted films were characterized by X-ray diffraction (XRD), optical spectroscopy, and ellipsometry. The optical absorption coefficient was calculated from measured normal transmittance and reflectance spectra. XRD analysis has shown that the as-grown V 2O 5 films were amorphous, becoming microcrystal after annealing at 350 °C. The optical absorption spectra of all the samples exhibited two distinct regions of behavior: at high-energy where ( αhν) 1/2 varied linearly with the photonic energy hν, and a low-energy tail. The blue shift of the optical absorption edge is attributed to the Fermi level going into the conduction band after lithium insertion, and the relation between the change of the optical gap and the inserted electron concentration is interpreted in terms of a Burstein–Moss shift. The results confirm that the anodic electrochromism of V 2O 5 thin films is due to the blue shift of the absorption edge, and the near-infrared cathodic electrochromism arises from absorption of small polarons in V 2O 5.

Changes in atomic and electronic structures of amorphous WO 3 films due to electrochemical ion insertion

The structural changes in amorphous WO 3 films were investigated both on the atomic and electronic levels, and the experimental findings were interpreted using molecular orbital calculations. Electrochemical fast intercalation resulted in the splitting of a peak in the valence band region of the X-ray photoelectron spectrum. This splitting could be attributed to the formation of non-bridging oxygen. Decomposition of WO 6 units into WO 4 units could also be inferred from the data. This decomposition was, however, not responsible for the split of the photoelectron peak. From the population analyses it was found that the average bond strength decreased due to the intercalation, while select WO bonds increased in strength. It was expected that these changes in the chemical bonding character lead to localization of electrons and distortion of WO 6 units, which was consistent with the theoretical interpretations of electrochromism, the intervalence charge transfer model and the small polaron absorption theory.

Nonvolatile two-color holography in Mn-doped near-stoichiometric lithium niobate

Nonvolatile holograms are recorded in near-stoichiometric LiNbO3 doped with 8 ppm of Mn by use of a two-color holographic recording method. An IR laser with wavelength of 778 nm is used for writing and a UV laser with wavelength of 350 nm is used for gating. Dependences of two-color sensitivity and M/♯ on gating and writing intensities are presented. Significant improvements of sensitivity, M/♯, and dark decay are obtained compared with reduced near-stoichiometric LiNbO3. The obtained high two-color sensitivity of 0.21 cm/J with a gating intensity of 1.5 W/cm2 is attributed to large light-induced absorption of small polarons.

Optical studies of a layered manganiteLa1.2Sr1.8Mn2O7:Polaron correlation effect

Optical conductivity spectra of a cleaved ab plane of ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ exhibit a small polaron absorption band in the midinfrared region over all temperatures. With decreasing temperature $(T)$ up to Curie temperature ${(T}_{C}),$ the small polaron band moves to a higher frequency, resulting in a finite gaplike feature. However, it shifts to a lower frequency below ${T}_{C}.$ With decreasing T, a stretching phonon mode hardens above ${T}_{C}$ and softens below ${T}_{C}.$ These concurring changes of lattice and electronic structure indicate that short-range polaron correlation exists above ${T}_{C}$ but disappears with the magnetic ordering.

Submillimeter and infrared spectroscopy on La0.85Sr0.15MnO3

The optical conductivity of La0.85Sr0.15MnO3 single crystals was studied by means of submillimeter and infrared spectroscopy for frequencies \(\) cm-1 and temperatures 10 K < T <300 K. The submillimeter conductivity follows the temperature dependence of the dc-data. The phonon spectrum of La0.85Sr0.15MnO3 changes considerably below \(\) K revealing a structural phase transition induced by charge or orbital order. At T=10 K a number of phonon modes can be identified in addition to the room-temperature spectrum. The optical conductivity (\(\)) in the mid-infrared reveals the characteristics of small polaron absorption. Below the magnetic ordering temperature the polaron binding energy is highly reduced, but the onset of charge order interrupts the formation of free charge carriers with a Drude-like behavior. The frequency and temperature dependence of \(\) in this regime qualitatively resembles the small polaron predictions by Millis et al. (Phys. Rev. B 54, 5405 (1996)).

The phase diagram and optical properties of La 1− xSr xMnO 3 for x⩽0.2

We report on detailed susceptibility, magnetization and magnetoresistance studies in La 1− x Sr x MnO 3 for x⩽0.2. We arrive at a phase diagram which shows a ferromagnetic and insulating ground state for 0.1⩽ x⩽0.15, followed by a canted antiferromagnetic phase at higher temperatures. The ferromagnetic transition is strongly coupled to a structural transition from a Jahn–Teller distorted phase to a pseudocubic orthorhombic phase, and is accompanied by the appearance of large positive magnetoresistance effects. We interpret the rich phase diagram close to x= 1 8 in terms of orbital ordering, due to an interplay of superexchange interactions and Jahn–Teller distortions. In addition, we report on the temperature dependence of phonon spectra and on optical conductivities. The temperature dependence of bending and stretching modes clearly indicate the structural and magnetic-phase transitions. The optical conductivities are explained in terms of small polaron absorption.

On the polaron-mechanism in iron-bearing trioctahedral phyllosilicates: an investigation of the electrical and optical properties

The electrical conductivity in the temperature range from 298 to 873 K and the optical absorption in the spectral range from 4000 to 20000 cm-1 (at T = 298 K) of four natural samples of trioctahedral phyllosilicates with varying iron contents have been investigated. It is observed that the values of anisotropy systematically increase with increasing iron content. The increase in conductivity for the electrical field (E) polarized parallel to the (001) plane is explained by using a two dimensional percolation model with conducting paths related to the Fe ions in the octahedral layer of the structure. The conductivity perpendicular to the (001) plane is suggested to be due to defects (conducting bridges between the layers). The E ∥ (001) polarized absorption spectra exhibit a strong increase in the absorption with increasing wavenumbers with features at 9000, 12000 and 14000 cm-1. The E⊥ (001) polarized spectra show similar line profiles with, however, lower intensities and without the 14000 cm-1 feature. The analysis of the spectra is based on the model of small polaron absorption in disordered systems. The result shows that the polaron absorption is always the dominant contribution in the near infrared spectral range. A polaron stabilization energy of about 4800 cm-1 (0.6 eV) is deduced and is related to the energetical separation between Fe2+ and Fe3+ states. The 9000, 12000 and 14000 cm-1 features are superimposed to the polaron absorption.

Optical absorption of free small polarons at high temperatures.

The real part of high-frequency electrical conduction, or the optical absorption, of free small polarons at high temperatures is calculated. The spectrum exhibits a slightly asymmetric Gaussian shape. Comparing with the previous theories, the peak position shifts toward lower energies and there exists an additional broadening of the line shape due to the distribution of localized energy levels. The results also show that the temperature dependence of the dc conductivity is more complicated than previously indicated. A quantitative agreement between theory and experiment is given. Application of this theory to bound small polarons is discussed.

Bipolaron absorption in Ba 1 − xK xBiO 3 and Ba 0.6K 0.4 − xCs xBiO 3

We present new results on the optical absorption of the family of compounds K x Ba 1 − x BiO 3 (KBB) and Cs x K 0.4 − x Ba 0.6BiO 3 (CKBB) (0 < x < 0.4) using powder methods. All cubic compounds of the KBB system ( x ⩾ 0.3, T c ≈ 30 K) possess a broad near- infrared (NIR) absorption band, which appears to evolve on increasing the K dopant content with respect to the noncubic compounds ( x < 0.3), which show no T c. For the series of solid solution with Cs substitution for K (i.e. no change in the nominal charge carrier concentration), we observe a strong increase of the absorption in the visible range with increasing Cs content, simultaneous to the gradual decrease of T c. The NIR absorption is analysed using the theory for small polaron absorption in disordered media. The results indicate the formation of intersite bipolarons. Characteristic differences in the absorption behaviour with the high- T c family Bi 2Sr 2Ca 1 − x Y x Cu 2O 8 + δ (BSCC) are outlined.

Optical properties of large and small polarons and bipolarons.

In systems that are two and three dimensional electronically, a large polaron and a small polaron are distinct types of quasiparticles. The type of polaron formed depends on which electron-lattice interaction is of primary importance. A large polaron forms when the electron-lattice interaction due to the long-range Coulombic interactions between an electronic carrier and a solid's ions are of paramount importance. Competing effects then determine the radius of a large polaron. By contrast, a small polaron can form when a short-range electron-lattice interaction, such as the deformation-potential interaction, is dominant. A small polaron forms as its self-trapped carrier shrinks without limit until it is confined to a single site. Fundamental differences between large and small polarons produce optical spectra with distinguishing features. The absorption due to photoionization of a large polaron depends on products of the matrix elements for exciting a carrier from its self-trapped states to a free-carrier state and the density of these free-carrier states. These matrix elements fall sharply with increasing free-carrier wave vector k when kRg1, where R is the large polaron's radius. A large polaron's photoionization produces a temperature-independent absorption band. This band is asymmetric with the absorption intensity on the high-energy side of the peak exceeding that on the low-energy side of the peak.By contrast, the small-polaron absorption arises as the self-trapped carrier is induced to transfer from its well-localized state to a localized state at an adjacent site. Phonon broadening of these local electronic energy levels produces the widths of these absorption bands. Small-polaron absorption bands are asymmetric with the absorption intensity below the peak energy exceeding that above the peak energy. With rising temperature the phonon broadening of the local electronic energy levels progressively broadens these absorption bands. In addition, the motion of a polaron in response to an ac field can produce a (Drude-like) free-carrier absorption. A large polaron's free-carrier absorption occurs at frequencies below the characteristic phonon frequency. By contrast, if the narrow bands that characterize small polarons did not result in their localization, their coherent motion would produce a free-carrier absorption that is restricted to frequencies far below the phonon frequency. The optical spectra of large and small bipolarons are similar to those for large and small polarons, respectively. Finally, carrier-induced absorption bands observed in semiconducting and superconducting cuprates are compared with the expectations of large- and small-polaronic absorptions. The high-frequency absorption bands are consistent with the existence of large-polaronic carriers. However, taken together, the free-carrier absorptions and the dc transport in the superconductors depart from expectations of independent polaronic carriers. It is suggested that if the carriers in the cuprates are polaronic, their transport in the superconductors' normal states is collective.

Temperature dependence of the mid-infrared absorption in La 1.84Sr 0.16NiO 4+δ

The reflectance of a La 1.84Sr 0.16Ni 4+δ crystal has been measured between 115 cm −1 and 5500 cm −1 with the electric-field vector in the ab-plane for several temperatures between 80 and 300 K. On the basis of the Kramers-Kronig derived optical conductivity data we show that it is unlikely that small-polaron absorption can account for the mid-infrared absorption, as had been previously suggested. A doped-semiconductor model is also considered.