Large Polaron Hopping Research Articles

High-temperature dielectric response in pulsed laser deposited Bi1.5Zn1.0Nb1.5O7 thin films

Cubic pyrochlore Bi1.5Zn1.0Nb1.5O7 thin films were deposited by pulsed laser ablation on Pt(200)/SiO2/Si at 500, 550, 600, and 650 °C. The thin films with (222) preferred orientation were found to grow at 650 °C with better crystallinity which was established by the lowest full-width half maxima of ∼0.38. The dielectric response of the thin films grown at 650 °C have been characterized within a temperature range of 270–650 K and a frequency window of 0.1–100 kHz. The dielectric dispersion in the thin films shows a Maxwell–Wagner type relaxation with two different kinds of response confirmed by temperature dependent Nyquist plots. The ac conduction of the films showed a varied behavior in two different frequency regions. The power law exponent values of more than 1 at high frequency are explained by a jump-relaxation-model. The possibility of grain boundary related large polaronic hopping, due to two different power law exponents and transformation of double to single response in Nyquist plots at high temperature, has been excluded. The “attempt jump frequency” obtained from temperature dependent tangent loss and real part of dielectric constants, has been found to lie in the range of their lattice vibronic frequencies (1012–1013 Hz). The activation energy arising from a large polaronic hopping due to trapped charge at low frequency region has been calculated from the ac conduction behavior. The range of activation energies (0.26–0.59 eV) suggests that the polaronic hopping at low frequency is mostly due to oxygen vacancies.

Transport and ordering of polarons in CER manganites PrCaMnO

The temperature-dependent resistivity and the colossal resistance effect induced by strong electric fields of thesmall-band Pr1−xCaxMnO3 (PCMO) manganites are analysed with respect to the influence of the Ca doping,post-annealing, the prehistory of the electric stimulation, and the physical dimensions ofthe sample. Despite the phase separation between charge and orbital ordered anddisordered phases, PCMO reveals the properties of a homogeneous medium with aconductivity governed by the hopping of small polarons if the electric field is not toostrong. In contrast, high electric fields induce a structural transition which gives rise to aglassy behaviour in the transient regime. In the low resistance state the small activationenergy of charge carrier hopping implies a transition to large polaron hopping.

Electrical Conductivity, Thermoelectric Power, and Optical Properties of Organo-Soluble Polyaniline Organic Semiconductor

The electrical conductivity, thermoelectric power (TEP), and optical properties of organo-soluble polyaniline doped with HCl have been investigated. The electrical conductivity and TEP of the sample increase with increasing temperature. The electrical conductivity and TEP results of the polymer suggest that it is a p-type semiconductor. The fundamental absorption edge in the polymer is formed by the direct allowed transitions, and the optical band gap value was found to be 2.79 eV. The absorption spectra for an acidic solution of the polymer indicate two new absorption bands, which are due to polaron formation. The polaron bands are responsible for the conductivity of the polymer. The TEP results indicate that the conductivity mechanism of the polymer is controlled by the large polaron hopping model.

Large polaron hopping under high electric fields

Current voltage characteristics of organic light emitting diodes cannot be understood without the knowledge of the field dependent mobility in the organic material. This issue is addressed in the present paper within the framework of a large polaron hopping theory under high electric fields. We find that the hopping rate is very sensitive to the conjugation length. At short conjugation lengths the mobility is essentially activated as a function of the temperature and the field , like in small polaron hopping. At long conjugation lengths, it exhibits a Fowler Nordheim like dependence with the field, and a very weak temperature dependence at high fields.

Electrical properties of semiconducting (1−x)(90V2O5–10P2O5)+xBaTiO3 glass and glass–ceramics

Homogeneous (1−x)(90V2O5–10P2O5)+x(BaTiO3) glasses (abbreviated as VPBT) are formed with lower concentrations (x) of BaTiO3 (x<0.3). For x>0.3 microcrystallization of BaTiO3 appears during glass formation (corresponding samples are termed as glass–ceramics). All of these oxide glasses show semiconducting behavior. Electrical conductivity data measured between 80 and 450 K follow the adiabatic small polaron hopping conduction mechanism for glasses with lower concentrations (x<0.2), but for higher concentrations of BaTiO3 (x>0.2) the said conduction mechanism is changed to the nonadiabatic regime. Low temperature (T≤150 K) conductivity data of these glass samples can be well fitted with the variable range hopping model. At higher temperatures, Mott’s model of phonon assisted small polaron hopping between nearest neighbors is consistent with the conductivity data. The polaron hopping model of Schnakenberg can predict the conductivity data in the entire temperature range of measurement. However, the percolation model, valid for the similar V2O5–Bi2O3+SrTiO3 glass (denoted by VBBT), is not consistent with the experimental results of VPBT glass samples. Again the frequency dependent ac conductivity data of the VPBT glasses follow overlapping large polaron hopping model, while those of the VBBT glasses satisfy the correlated barrier hopping model.