Polaron Relaxation Research Articles

Dielectric properties of ErFeO3 ceramics over a broad temperature range

Erbium orthoferrite (ErFeO3) ceramics were prepared by the solid state reaction. The dielectric properties of the samples were investigated over a broad temperature (120–850K) in the frequency range ranging from 50Hz to 10MHz. The sample shows three dielectric responses without colossal dielectric constant behavior. The first dielectric response occurring around 200K is a thermally activated relaxation and argued to be a polaron relaxation caused by the hopping motions of charge carriers between Fe2+ and Fe3+. The second response occurring around 550K shows a relaxor-like behavior. It contains two relaxation processes: dipolar-type and Maxwell–Wagner-type relaxations induced by the hopping motions of oxygen vacancies. The third one occurring around 725K also exhibits as a relaxor-like behavior. It was found to be an artificial effect caused by negative capacitance.

Role of charged defects in the photoconductivity of Se95As5 chalcogenide glassy semiconductor with the EuF3 impurity

The energy spectrum of local states associated with charged defects D− and D+ playing a significant role in carrier generation and recombination in the chalcogenide glassy semiconductor system Se95As5 containing EuF3 impurities is proposed based on the study of the temperature dependence of the dark conductivity and steady-state photoconductivity, current-luminance characteristic, and the spectral distribution of the photocurrent. It is shown that EuF3 impurities nonmonotonically change the concentrations of these states. Low concentrations form chemical compounds with selenium and arsenic due to the chemical activity of the rare-earth element and fluorine ions, which result in a decrease in the concentration of initial intrinsic defects. High concentrations, according to the charged-defect model, lead to a decrease in the concentration of D+ centers and an increase in the concentration of D− centers due to the presence of Eu3+ ions. Some parameters of the charged-defect model are estimated, in particular the effective correlation energy Ueff (0.6 eV) and the polaron relaxation energy (W+ = 0.4 eV, W− = 0.45 eV).

Optically Controlled Spin-Flipping of Charge Carriers in Conjugated Polymers

On the basis of the dynamics of polaron relaxation in conjugated polymers, it is found theoretically that the spin of an injected electron, which converts to a negative polaron in a conjugated polymer, is reversed by photoexcitation. This effect of photoinduced spin-flipping in polymers can be used to design a device of an optically controlled spin valve for organic spintronics. Here the spin-flipping is driven by the electric dipole transition rather than the magnetic dipole transition, and it consequently has a much higher efficiency.

Colossal dielectric behavior in TbFeO3 ceramics

TbFeO3(TFO) ceramic samples were prepared via the traditional solid-state reaction route. The low-temperature (100–300 K) dielectric properties of TFO have been systematically investigated in the frequency range from 100 Hz to 5 MHz. Colossal dielectric behavior contributed from two thermally activated dielectric relaxations was found in the sample. The low-temperature relaxation with the activation energy of 0.22 eV was confirmed to be a polaron relaxation caused by electrons hopping between the mixed-valent states of Fe ions. The high-temperature relaxation shows the activation energy of 0.59 eV and can be enhanced by annealing treatment in oxygen and weakened in nitrogen. This relaxation was ascribed to be a Maxwell–Wagner relaxation due to surface-layer effect.

Origin of Colossal Dielectric Behavior In Double Perovskite Ba2CoNbO6

Ba2CoNbO6 ceramic samples were synthesized by solid‐state reaction technique. X‐ray photoemission spectroscopy reveals the coexistence of mixed‐valent structure of Co2+/Co3+ in the sample. Dielectric measurements reveal that the sample shows colossal dielectric behavior. This behavior is contributed by two thermally activated dielectric relaxations: the low‐temperature relaxation is a polaron relaxation resulting from the hopping motions of self‐trapped holes; the high‐temperature relaxation is a Maxwell–Wagner relaxation caused by the grain‐boundary response. Our results indicate a multirelaxation mechanism, i.e., the coupling of the polaronic relaxation and the Maxwell–Wagner relaxations might be a common origin of the colossal dielectric behavior found in different materials.

Polaron relaxation and non-ohmic behavior in CaCu3Ti4O12 ceramics with different cooling methods

Structure, defects, non-ohmic behavior and dielectric properties of CaCu3Ti4O12 (CCTO) ceramics with furnace-cooled and water-quenched methods have been systemically investigated. Cooling methods make an influence on reoxidation process and cause changes of electric properties in grains and at grain boundaries. SEM–EDS provides detailed imaging information about Cu-rich phase on the fracture of CCTO ceramics which is explained by the reoxidation reaction during cooling. Morphologies of the water-quenched sample show very small grains floating in bigger grains; whereas they reveal that many grains are surrounded by an exfoliated sheet of Cu-rich phases in the furnace-cooled sample. The polaron relaxation associated with Ti3+ ions and oxygen vacancies occurs at low temperatures, and more Ti3+ ions are remained in the water-quenched sample. The Schottky barrier height (I–V, non-ohmic behavior) associated with the reoxidation of grain boundaries can be modified by cooling method.

Origin of the colossal dielectric properties in double-perovskite Sr2CoNbO6

Sr2CoNbO6 ceramics were prepared via the solid-state reaction route. The dielectric properties of Sr2CoNbO6 ceramics were investigated as a function of temperature (90–330 K) in the frequency range from 20 Hz to 10 MHz. Our results reveal that the dielectric properties of Sr2CoNbO6 are closely linked with the conductivity of the sample. The conductivity is dominated by hopping localized carriers. The hopping process not only produces considerable conductivity followed the universal dielectric response behavior, but also gives rise to dipolar effects leading to the polaron relaxation in intermediate-temperature range. At low enough temperatures, the hopping process is frozen-in and the low-temperature Maxwell-Wagner relaxation due to frozen carriers featuring the nearly constant loss behavior appears. At higher temperatures, the long-distance transportation of the carriers becomes remarkable. These carriers are easily blocked by various interfaces resulting in space charge therein and the conventional Maxwell-Wagner relaxation in the high-temperature range. Our results demonstrate that a multirelaxation mechanism instead of a single relaxation underlies the colossal dielectric properties of Sr2CoNbO6.

Packing Dependent Electronic Coupling in Single Poly(3-hexylthiophene) H- and J-Aggregate Nanofibers

Nanofibers (NFs) of the prototype conjugated polymer, poly(3-hexylthiophene) (P3HT), displaying H- and J-aggregate character are studied using temperature- and pressure-dependent photoluminescence (PL) spectroscopy. Single J-aggregate NF spectra show a decrease of the 0-0/0-1 vibronic intensity ratio from ~2.0 at 300 K to ~1.3 at 4 K. Temperature-dependent PL line shape parameters (i.e., 0-0 energies and 0-0/0-1 intensity ratios) undergo an abrupt change in the range of ~110-130 K suggesting a change in NF chain packing. Pressure-dependent PL lifetimes also show increased contributions from an instrument-limited decay component which is attributed to greater torsional disorder of the P3HT backbone upon decreasing NF volume. It is proposed that the P3HT alkyl side groups change their packing arrangement from a type I to type II configuration causing a decrease in J-aggregate character (lower intrachain order) in both temperature- and pressure-dependent PL spectra. Chain packing dependent exciton and polaron relaxation and recombination dynamics in NF aggregates are next studied using transient absorption spectroscopy (TAS). TAS data reveal faster polaron recombination dynamics in H-type P3HT NFs indicative of interchain delocalization whereas J-type NFs exhibit delayed recombination suggesting that polarons (in addition to excitons) are more delocalized along individual chains. Both time-resolved and steady-state spectra confirm that excitons and polarons in J-type NFs are predominantly intrachain in nature that can acquire interchain character with small structural (chain packing) perturbations.

Magnetic field tuning of polaron losses in Fe doped BaTiO3 single crystals

Artificial tuning of dielectric parameters can result from interface conductivity in polycrystalline materials. In ferroelectric single crystals, it has already been shown that ferroelectric domain walls can be the source of such artificial coupling. We show here that low-temperature dielectric losses can be tuned by a dc magnetic field. Since such losses were previously ascribed to polaron relaxation we suggest this results from the interaction of hopping polarons with the magnetic field. The fact that this loss alteration has no counterpart in the real part of the dielectric permittivity confirms that no interface is involved in this purely dynamical effect. The contribution of mobile charges hopping among Fe-related centers was confirmed by ESR spectroscopy, showing a maximum intensity at ca T ∼ 40 K.

Polaronic relaxation in Ca2TiMnO6 at low temperatures

Dielectric properties of Ca2TiMnO6 (CTM) were systematically investigated. Our results revealed that (1) the observed relaxation does not feature one clear Arrhenius behavior, (2) the peak intensity of the imaginary part of the complex permittivity can be well expressed by a relation similar to the Fermi–Dirac distribution function, and (3) the colossal dielectric behavior of the sample can be well understood based on the framework of universal dielectric response. These features indicate that the dielectric properties of CTM are related to polaron relaxation.

Impedance spectroscopic characterization of fine-grained magnetoelectric Pb(Zr 0.53Ti 0.47)O 3–(Ni 0.5Zn 0.5)Fe 2O 4 ceramic composites

Temperature dependent impedance spectroscopic analysis of fine-grained magnetoelectric Pb(Zr 0.53Ti 0.47)O 3–(Ni 0.5Zn 0.5)Fe 2O 4 (PZT–NZFO) composites was investigated. Debye-like impedance relaxation peaks were observed at intermediate frequency range. Maxwell–Wagner (MW) relaxation model was used to explain the space charge effect due to heterogeneous PZT and NZFO grain boundary in finer structure. The total resistivity was dominated by the grain boundary resistance due to the blocking effect arisen from the glass phase additive. The small value of conductivity measured in this system suggested the glass additive markedly modified the grain boundary properties. Electric modulus spectra reflected the contributions from two different effects: the large resolved semicircle arc was caused by the grain effect and the small poorly resolved semicircle arc was attributed to the grain boundary. The activation energy calculated from the impedance spectra was consistent with value estimated from the modulus spectra. Investigation on dielectric spectra revealed a polydispersive dielectric relaxation existing in the system, which was also demonstrated in the ac conductivity spectra. Small polaron relaxation and MW-type polarization mechanism were discussed through the analysis on the ac conductivity spectra.

Dimensional crossover of polaron dynamics inNb:SrTiO3/SrTiO3superlattices: Possible mechanism of thermopower enhancement

Using optical spectroscopy, we investigated the electrodynamic properties of $\text{Nb}:{\text{SrTiO}}_{3}/{\text{SrTiO}}_{3}$ superlattices. In these superlattices, a large enhancement of the Seebeck coefficient $(S)$ has been reported with decreasing $\text{Nb}:{\text{SrTiO}}_{3}$ layer thickness [H. Ohta et al., Nature Mater. 6, 129 (2007)]. By analyzing the optical spectra, we found that the polaron plays an important role in determining the electrodynamic properties of the superlattices. With decreasing $\text{Nb}:{\text{SrTiO}}_{3}$ layer thickness from 11 to 1 unit cell, we observed a threefold enhancement of the polaron effective mass and relaxation time. Such increases were attributed to a dimensional crossover of the polaron from three-dimensional to quasi-two-dimensional. Moreover, the modified nature of the polaron at low dimensions enhanced the thermoelectric properties of the oxide superlattice, by simultaneously increasing $S$ and preventing the decrease in carrier mobility. Our results indicate that strong electron-phonon coupling can provide an alternative pathway in searching for efficient thermoelectric materials.

Transient light-induced absorption in periodically poled lithium niobate: Small polaron hopping in the presence of a spatially modulated defect concentration

The study of the transient light-induced absorption in thermally reduced, periodically poled lithium niobate doped with Y uncovers the simultaneous action of the ${\text{Nb}}_{\text{Li}}^{4+}:{\text{Nb}}_{\text{Nb}}^{4+}$ bipolaron and ${\text{O}}^{\ensuremath{-}}$ hole polaron relaxation in the blue and near-infrared spectral range. The findings can be comprehensively explained taking into account a periodic, spatial modulation of the ${\text{Nb}}_{\text{Li}}^{5+}$ antisite defect distribution, which results in a modulation of both concentrations and relaxation lifetimes of optically excited, small bound electron and hole polarons and bipolarons.

Polaron relaxation associated with the localized oxygen vacancies in Ba0.85Sr0.15TiO3 ceramics at high temperatures

The temperature and frequency dependences of dielectric properties of barium strontium titanate [Ba0.85Sr0.15TiO3 (BST15)] ceramics have been investigated. The dielectric relaxation behavior above 180 °C was observed in BST15 ceramics with broad dielectric peaks. The contribution of dc conduction and universal dielectric response to the relaxation has been studied. In contrast to typical ion conductor such as La2Mo2O9, the extent of dielectric dispersion shows the similar temperature dependence characteristic. Polaron theory indicates that the relaxation in BST15 ceramics at high temperatures is associated with the hopping localized oxygen vacancies.

The influence of polaron–phonon interaction on absorption spectra in molecular crystals

We investigate theoretically the possible manifestation of the small-polaron effect in the IR absorption spectrum of the one-dimensional molecular crystals. Phonon-induced relaxation of small-polarons is taken into account explicitly on the microscopic level. It was found that it may have substantial influence on optical properties of molecular crystals. Consequently, the accounting of the polaron relaxation effects on the microscopic level is necessary for proper understanding of the optical spectra of molecular crystals. Their hitherto interpretation in terms of polaron model, based on the phenomenological treatment of the polaron damping, has been critically assessed on the basis of our results.

Polaron relaxation in a quantum dot due to anharmonic coupling within a mean-field approach

We study the electronic relaxation in a quantum dot within the polaron approach by focusing on the reversible anharmonic decay of longitudinal optical (LO) phonons forming the polaron into longitudinal-acoustic (LA) phonons. The coherent coupling between the LO and LA phonons is treated within a mean-field approach. We derive a temperature-dependent interlevel coupling parameter, related to the Gr\"uneisen parameter and the thermal-expansion coefficient, which characterizes an effective decay channel for the electronic (or excitonic) states. Within this theory, we obtain a characteristic anharmonic decay time of 1 ns, 2--3 orders of magnitude longer than previous predictions based on the Fermi's Golden Rule. We suggest that coherent relaxation due to carrier-carrier interaction is an efficient alternative to the (too slow) polaron decay.

Structural effect of fullerene derivative on polaron relaxation and charge transfer in poly(3-hexylthiophene)/fullerene composite

The structural effect of a side chain in a fullerene derivative on electron relaxation and polaronic charge transfer in a poly(3-hexylthiophene)/fullerene composite was studied by 3 cm waveband electron paramagnetic resonance. All the systems with different fullerene derivatives demonstrate a sum spectrum of small localized and mobile polarons whose contributions depend on the structure of the fullerene derivative. Both the spin–lattice and spin–spin relaxation times were determined separately in a wide (77–320 K) temperature region using the steady-state saturation method. It was shown that spin relaxation, susceptibility and dynamics are governed by the dipole–dipole interaction of the polarons as well as by the structure of the side chains in the fullerene derivative. The treatment of the polymer matrix by the fullerene changes its dimensionality and accelerates polaron diffusion along polymer chains.

Polaron relaxation in self-assembled quantum dots: Breakdown of the semiclassical model

We calculate the lifetime of conduction band excited states in self-assembled quantum dots by taking into account LO-phonon-electron interaction and various anharmonic phonon couplings. We show that polaron relaxation cannot be accurately described by a semiclassical model. The contributions of different anharmonic decay channels are shown to depend strongly on the polaron energy. We calculate the energy dependence of the polaron lifetime and compare our results to available experimental measurements of polaron decay time in $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dots.

Dielectric relaxation in A2FeNbO6 (A = Ba, Sr, and Ca) perovskite ceramics

A2FeNbO6 (A = Ba, Sr, and Ca, abbreviated as AFN) ceramics were synthesized by a wolframite method with monoclinic FeNbO4 precursor. X-ray diffraction studies revealed that Sr2FeNbO6 and Ca2FeNbO6 had a orthorhombic structure, while Ba2FeNbO6 had a cubic symmetry. Dielectric properties of the AFN ceramics were characterized in a broad frequency range (1 Hz–10 MHz) at a temperature ranged from −120 to 300 °C. Two dielectric relaxations were observed to follow the Arrhenius law at both low and high temperatures for all samples. Polaron relaxation was proposed to explain the low temperature dielectric relaxations in AFN. The high temperature dielectric relaxations in AFN were a competitive phenomenon between the dielectric relaxation and the electrical conduction of the relaxing species.

Singlet and triplet polaron relaxation in doubly charged self-assembled quantum dots

Polaron relaxation in self-assembled InAs/GaAs quantum dot samples containing 2 electrons per dot is studied using far-infrared, time-resolved pump–probe measurements for transitions between the s-like ground and p-like first excited conduction band states. Spin–flip transitions between singlet and triplet states are observed experimentally in the decay of the absorption bleaching, which shows a clear biexponential dependence. The initial fast decay (∼30 ps) is associated with the singlet polaron decay, while the decay component with the longer time constant (∼5 ns) corresponds to the excited state triplet lifetime. The results are explained by considering the intrinsic Dresselhaus spin–orbit interaction, which induces spin–flip transitions by acoustic phonon emission or phonon anharmonicity. We have calculated the spin–flip decay times, and good agreement is obtained between the experiment and the simulation of the pump–probe signal. Our results demonstrate the importance of spin-mixing effects for intraband energy relaxation in InAs/GaAs quantum dots.