A.c. conductivity in AsF5 doped polyphenylacetylene (PPA)

The a.c. conductivity of semiconducting cis-cisoid polyphenylacetylene (PPA) pellets has been studied at temperatures between 230 and 290 K and frequencies, ?, from 37 to 10⁵ Hz. The a.c. conductivity (σ_a.c.) is found to be strongly temperature dependent. σ_a.c. is proportional to ?^s with s varying from 0.45 to 0.75 as temperature is raised from 230 to 290 K. Both frequency dispersion and strong temperature dependence of σ_a.c. are best explained by the mechanism of hopping conduction in the band-tails.     

Dc and Ac conductivity of La2CuO4+δ

The complex conductivity of La₂CuO_4+δ has been investigated for frequencies 20 Hz≤ν≤4 GHz and temperatures 1.5K≤T≤450 K. Two single crystals with δ≈0 and δ≈0.02 were investigated, using dc (four-probe), reflectometric and contact-free techniques. At high temperatures the dc conductivity is thermally activated with low values of the activation energy. For low temperatures Mott's variable range hopping dominates. The real and imaginary parts of the ac conductivity follow a power-law dependence σ～v ^s, typical for charge transport by hopping processes. A careful analysis of the temperature dependence of the ac conductivity and of the frequency exponents has been performed. It is not possible to explain all aspects of the ac conductivity in La₂CuO_4+δ by standart hopping models. However, the observed minimum in the temperature dependence of the frequency exponents strongly suggests tunneling of large polarons as dominant transport process.     

Investigation of a.c. conductivity measurements in a-Se80Te20 and a-Se80Te10M10 (M = Cd, in, Sb) alloys using correlated barrier hopping model

The a.c. conductivity of a-Se₈₀Te₂₀ and a-Se₈₀Te₁₀M₁₀ (M = Cd, In, Sb) alloys has been investigated as a function of temperature in the range from 280 to 330 K and frequency in the range from 10² to 10⁴ Hz. The experimental results indicate that a.c. conductivity σ_ac is proportional to ω^s where s < 1 and decreases with increasing temperature. The results obtained are discussed in terms of the correlated barrier hopping (CBH) model. An agreement between experimental and theoretical results suggests that the a.c. conductivity behavior of a-Se₈₀Te₂₀ and a-Se₈₀Te₁₀M₁₀ (M = Cd, In, Sb) system can be successfully explained by CBH model. The contribution of single polaron and bipolaron hopping to a.c. conductivity in present alloys is also studied.     

AC conductivity and dielectric behavior of bulk Furfurylidenemalononitrile

AC conductivity and dielectric behavior for bulk Furfurylidenemalononitrile have been studied over a temperature range (293–333 K) and frequency range (50–5×10⁶ Hz). The frequency dependence of ac conductivity, σ_ac, has been investigated by the universal power law, σ_ac(ω)=Aω^s. The variation of the frequency exponent (s) with temperature was analyzed in terms of different conduction mechanisms, and it was found that the correlated barrier hopping (CBH) model is the predominant conduction mechanism. The temperature dependence of σ_ac(ω) showed a linear increase with the increase in temperature at different frequencies. The ac activation energy was determined at different frequencies. Dielectric data were analyzed using complex permittivity and complex electric modulus for bulk Furfurylidenemalononitrile at various temperatures.     

Impedance spectroscopic studies of an organic semiconductor device incorporating a thin film of highly doped ZnPc with MoO3

We use experimental results of low signal impedance spectroscopy to investigate the conduction mechanism in organic semiconductor, zinc phthalocyanine (ZnPc). The first 10 nm, of a total of 150 nm thermally deposited ZnPc, was doped with molybdenum oxide (MoO₃) by co-evaporation to obtain a 20% doping concentration. The ac electrical parameters were measured at room temperature in the dc bias and frequency ranges of 0–5 V and 100 Hz–0.1 MHz, respectively. The variation of bulk resistance with applied bias presents a clear indication of space charge limited conduction in the fabricated device. The experimental results show a strong frequency dependence of capacitance and loss tangent at low frequencies and high applied bias, while at higher frequencies and low applied bias a weak dependence is observed. Moreover, the ac conductivity shows a strong dependence on frequency and is found to vary as ω^s with the index s≤1.15 suggesting a dominant hopping mechanism of conduction.     

Electrical conduction and dielectric properties of vanadium phosphate glasses doped with lithium

AC conductivity and dielectric studies on vanadium phosphate glasses doped with lithium have been carried out in the frequency range 0.2-100 kHz and temperature range 290-493 K. The frequency dependence of the conductivity at higher frequencies in glasses obeys a power relationship, σ_ac=Aω^s. The obtained values of the power s lie in the range 0.5≤s≤1 for both undoped and doped with low lithium content which confirms the electron hopping between V⁴⁺ and V⁵⁺ ions. For doped glasses with high lithium content, the values of s≤0.5 which confirm the domination of ionic conductivity. The study of frequency dependence of both dielectric constant and dielectric loss showed a decrease with increasing frequency while they increase with increasing temperature. The results have been explained on the basis of frequency assistance of electron hopping besides the ionic polarization of the glasses. The bulk conductivity increases with increasing temperature whereas decreases with increasing lithium content which means a reduction of the V⁵⁺.     

Frequency dependance of the conductivity in trigonal selenium single crystals

An analysis of the frequency dependences of a barrier-limited conductivity model is presented. Theoretical results are in good accordance with previous experimental results. At low temperature the model predicts the same variation σ (ω) ∝ ω^s with s < 1 as a hopping conduction process.     

Frequency dependent magnetoconductivity and conductivity study in Ni-dispersed silica nano-composite produced by sol-gel technique

The low frequency (20 Hz to 1 MHz) ac conductivity and magnetoconductivity behaviour of ceramic nanocomposite (Ni-SiO₂) at low temperature down to 77 K are reported. The frequency dependent conductivity followed the power law, σ(ω) ∝ ω ^s . The fractional exponent s is a function of temperature and was found to increase with increasing temperature. This type of variation may be attributed to small polaron hopping. A peak present in the loss tangent indicates the presence of a Debye relaxation process. The magnetoconductivity of the samples is positive, which strongly depends on frequency. A firm theoretical explanation of frequency dependent magnetoconductivity is still lacking.     

Structure and dielectric behavior of TlSbS2

A comparison of structure and dielectric properties of TlSbS₂ thin films, deposited in different thicknesses (400–4100 Å) by thermal evaporation of TlSbS₂ crystals that were grown by the Stockbarger–Bridgman technique and the bulk material properties of TlSbS₂ are presented. Dielectric constant ε ₁ and dielectric loss ε ₂ have been calculated by measuring capacitance and dielectric loss factor in the frequency range 20 Hz–10 KHz and in the temperature range 273–433 K. It is observed that at 1 kHz frequency and 293 K temperature the dielectric constant of TlSbS₂ thin films is ε ₁=1.8–6 and the dielectric loss of TlSbS₂ thin films is ε ₂=0.5–3 depending on film thickness. In the given intervals, both of dielectric constant and dielectric loss decrease with frequency, but increase with temperature. The maximum barrier height W _m is calculated from the dielectric measurements. The values of W _m for TlSbS₂ films and bulk are obtained as 0.56 eV and 0.62 eV at room temperature, respectively. The obtained values agree with those proposed by the theory of hopping over the potential barrier. The temperature variation of ac conductivity can be reasonably interpreted in terms of the correlated barrier hopping model since it obeys the ω ^s law with a temperature dependent s (s<1) and going down as the temperature is increased. The temperature coefficient of capacitance (TCC) and permittivity (TCP) are evaluated for both thin films and bulk material of TlSbS₂.     

AC conductivity and dielectric properties of bulk tungsten trioxide (WO3)

AC conductivity and dielectric properties of tungsten trioxide (WO₃) in a pellet form were studied in the frequency range from 42 Hz to 5 MHz with a variation of temperature in the range from 303 K to 463 K. AC conductivity, σ_ac(ω) was found to be a function of ω^s where ω is the angular frequency and s is the frequency exponent. The values of s were found to be less than unity and decrease with increasing temperature, which supports the correlated barrier hopping mechanism (CBH) as the dominant mechanism for the conduction in WO₃. The dielectric constant (ε′) and dielectric loss (ε″) were measured. The Cole–Cole diagram determined complex impedance for different temperatures.     

Magnetic and electric studies of a new Co(II) perovskite-like material

Structural phase transitions in the perovskite-like material [(CH₄)₁₂(NH₃)₂]CoCl₄ have been observed using differential thermal scanning. The material shows an order-disorder transition at T ₁ = 396 ± 5 K with entropy, (ΔS ₁) = 12.8 J/mole/K. A "chain melting" transition with a major endothermic peak at T ₂ = 337 ± 3 K and a minor one at T ′ = 316 ± 2 K, has total entropy ΔS = 28 J/mole/K. At low temperatures, the transitions at T ₃ = 288 ± 3 K and at T ₄ = 188 ± 3 K, have entropies of ΔS ₃ = 14.4 J/mole/K and ΔS ₄ = 2.6 J/mole/K respectively. AC magnetic susceptibility in the temperature range 78-290 K, in a magnetic field of 160 A/m and at a frequency of 320 Hz is presented. The results indicate changes in symmetry at 188 K. Dielectric permittivity has been studied as a function of temperature in the range 300-430 K and frequency range (60 Hz-100 kHz), confirming the observed transitions. The dielectric permittivity reflects rotational and conformational transition for the material. The variation of the real part of the conductivity with temperature is thermally activated with different activation energies in the range of ionic hopping. The temperature dependence of the dc conductivity and that of the ions hopping rate have indicated that the concentration of mobile ions is independent of temperature. The dependence of the conductivity on frequency follows the universal power law, <artwork name="GPHT31040ei1"> in the temperature range 340 K<T<390 K. Values 0 <s ₁ <1 dominate at low frequency and correspond to translational hopping motion and values 1<s ₂<2 dominate at high frequencies and correspond to well localized hopping and/or reorientational motion. For T > 396 K, the AC conductivity was fitted to <artwork name="GPHT31040ei2"> with 0<s<1. Comparison with the corresponding Cu-containing material is discussed.     

Conduction mechanism and the dielectric relaxation process of a-Se75Te25−xGax (x=0, 5, 10 and 15 at wt%) chalcogenide glasses

Se₇₅Te_25−xGa_x (x=0, 5, 10 and 15 at wt%) chalcogenide compositions were prepared by the well known melt quenching technique. Thin films with different thicknesses in the range (185–630 nm) of the obtained compositions were deposited by thermal evaporation technique. X-ray diffraction patterns indicate that the amorphous nature of the obtained films. The ac conductivity and the dielectric properties of the studied films have been investigated in the frequency range (10²–10⁵ Hz) and in the temperature range (293–333 K). The ac conductivity was found to obey the power low ω^s where s≤1 independent of film thickness. The temperature dependence of both ac conductivity and the exponent s can be well interpreted by the correlated barrier hopping (CBH) model. The experimental results of the dielectric constant ε₁ and dielectric loss ε₂ are frequency and temperature dependent. The maximum barrier height W_m calculated from the results of the dielectric loss according to the Guintini equation, and agrees with that proposed by the theory of hopping of charge carriers over a potential barrier as suggested by Elliott for chalcogenide glasses. The density of localized state was estimated for the studied film compositions. The variation of the studied properties with Ga content was also investigated. The correlation between the ac conduction and the dielectric properties were verified.     

Dc and Ac conductivity of La2CuO4+δ

The complex conductivity of La₂CuO_4+δ has been investigated for frequencies 20 Hz≤ν≤4 GHz and temperatures 1.5K≤T≤450 K. Two single crystals with δ≈0 and δ≈0.02 were investigated, using dc (four-probe), reflectometric and contact-free techniques. At high temperatures the dc conductivity is thermally activated with low values of the activation energy. For low temperatures Mott's variable range hopping dominates. The real and imaginary parts of the ac conductivity follow a power-law dependence σ∼v ^s, typical for charge transport by hopping processes. A careful analysis of the temperature dependence of the ac conductivity and of the frequency exponents has been performed. It is not possible to explain all aspects of the ac conductivity in La₂CuO_4+δ by standart hopping models. However, the observed minimum in the temperature dependence of the frequency exponents strongly suggests tunneling of large polarons as dominant transport process.     

Dielectric relaxations,magnetodielectric and magnetoelectric interactions in Bi0.6La0.4MnO3 ceramics

Complex permittivity ε*/ε₀ = ε′/ε₀–iε″/ε₀ of the bismuth–lanthanum manganite Bi_0.6La_0.4MnO₃ ceramics has been measured in the temperature range of 10–220 K at frequencies f = 20–10⁶ Hz and magnetic inductions B = 0–0.846 T. At a temperature of 80 K, the spectra ε′/ε₀(t) and ε″/ε₀(t) demonstrate the dielectric relaxation that is a superposition of contributions of several relaxation processes, each of which is dominant in its frequency range: I (f < 10³ Hz, II (10³ < f < 10⁵ Hz), and III (10⁵ < f < 10⁶ Hz). In the range of 10–120 K, anomalous behavior of ε′/ε₀(T) and ε″/ε₀(T) is observed near the temperature of the transition from the paramagnetic to ferromagnetic phase and is due to the Anderson localization of charge carrier on a spin disorder.     

Transport properties and conduction mechanisms in CuFe2O4 and Cu1−xZnxGa0.3Fe1.7O4 compounds

Single-phase structure of CuFe₂O₄ and Cu_1−xZn_xGa_0.3Fe_1.7O₄; with (0.0≤x≤0.5) are synthesized. Electrical conductivity measurements as a function of temperature are carried out in the frequency range (10²-10⁵ Hz) for the prepared samples. The obtained results of these materials reveal a metallic-like behavior in the low range of frequency. At high frequency regime metallic-to-semiconductor transition has been observed as the compositional parameter x increases. Metallic-like behavior is accompanied with samples having low Zn content, where cation-cation [Cu-Cu] interaction is major at the octahedral B-sites and semiconductor behavior is associated with compounds having high Zn content, where cation-anion-cation [Fe-O-Fe] interaction is most predominant at B-sites in these spinel oxides. All studied compositions exhibit a transition with change in the slope of conductivity versus temperature curve. This transition temperature is found to decrease linearly with increasing Zn concentration x. The relation of the universal exponent s with temperature indicates the presence of two hopping conduction mechanisms; the correlated barrier hopping CBH at low Zn content x≤0.2 and small polaron (SP) at Zn content x≥0.3.     

Electrical properties and conductivity mechanism of LiCuFe2(VO4)3

This paper reports conduction mechanism in LiCuFe₂(VO₄)₃ over a wide range of temperatures (300 to 712 K) and frequencies (209 Hz to 5 MHz). The DC conductivity of the material is thermally activated with activation energy about 0.66 eV. In LiCuFe₂(VO₄)₃, the electrical conductivity is probably due to the hopping of alkali lithium ion along the channel [001]. Temperature dependence of AC conductivity is studied at different frequencies. Frequency exponent s is found to decrease with increase in temperature. The results have been explained on the basis of correlated barrier hopping (CBH) model. Numerical calculations agree well with experimental results. The results show that the frequency and temperature-dependent behavior of AC conductivity of the studied materials are predominantly due to single polaron hopping.     

Investigation of electrical properties (ac and dc) of organic zinc phthalocyanine,ZnPc, semiconductor thin films

We report measurements on the electrical properties of thermally evaporated zinc phthalocyanine, ZnPc, semiconductor thin films. Aluminum and gold metal electrodes were used and both proved to act as ohmic contacts. A relative permittivity, ε_r, of 1.56 was estimated from the dependence of capacitance on film thickness. The room temperature current density–voltage measurements indicated an ohmic conduction at low voltages, while a space–charge-limited conduction at higher voltages. An average value of a thermally generated hole concentration of the order 10¹³ m⁻³ was estimated at room temperature.The ac conductivity, capacitance and loss tangent were measured over a wide range of temperature (from 170 to 430 K) and frequency (between 0.1 and 20 kHz). The ac conductivity of ZnPc films was observed to be proportional to ω^s, where ω is the angular frequency, and the index s is a temperature and frequency-dependent constant. At low temperatures and for higher frequencies the ac conduction was due to hopping. The capacitance, as well as the loss tangent, was found to be dependent on both temperature and frequency, but was constant for all frequencies at low temperatures. Such dependences were accounted for the equivalent-circuit model consisting of inherent capacitance in parallel with a temperature dependent resistive element.     

Effect of impurity (Sb and Ag) incorporation on the a.c. conductivity and dielectric properties of a-Se70Te30 glassy alloy

The effect of additives (Sb and Ag) on a.c. conductivity and dielectric properties of Se₇₀Te₃₀ glassy alloy at temperature range 300-350 K and frequency range 1 kHz-5 MHz has been studied. Experimental results indicate that a.c. conductivity and dielectric parameters depend on temperature, frequency and the impurity incorporated in Se-Te glassy system. The a.c. conductivity in the aforesaid frequency range is found to obey the ω^s law. A strong dependence of a.c. conductivity and exponent s in the entire temperature and frequency range contradicts quantum-mechanical tunneling (QMT) model and can be interpreted in terms of the correlated barrier hopping (CBH) model. The temperature and frequency dependence of the dielectric parameters are also studied and it is found that the results agrees by the theory of hopping of charge carriers over potential barrier as suggested by Elliott in chalcogenide glasses. The change in the dielectric parameters with the opposite influence of the replacement of Te by Sb on the one hand, and by Ag, on the other hand is being correlated by the nature of covalent character of the studied composition and with the change in density of defect states.     

Influence of polarization and iron content on the transport properties of praseodymium–barium manganite

Polarization and iron effects on the electrical properties of Pr_0.67Ba_0.33Mn_1−xFe_xO₃ have been studied using impedance measurements. When iron is introduced, the insulator–metal transition (MI), observed in free compound, disappears and destroying such transition needs an iron concentration less than 5%. We also found that electrical conductance decreases when increasing Fe content. Such results are attributed to the decrease of Mn³⁺/Mn⁴⁺ ratio. Also, they are ascribed to the high probability of encountering Fe³⁺–O–Fe³⁺ and Mn³⁺–O–Fe³⁺ interactions, which greatly weakens the influence of Mn³⁺–O–Mn⁴⁺ interactions. The AC conductivity studies indicate that different types of hopping are involved. The contribution of hopping mechanism is confirmed by the temperature dependence of the frequency exponent ‘s’. Conductivity analysis shows that small polaron hopping (SPH) and variable range hopping (VRH) models are present in the conduction process. For small iron concentrations (x<0.1), we found that activation energy (E_a) does not changes significantly. Such result is in good agreement with the literature. But, for high iron concentrations (x>0.1), we found that E_a depend strongly in Fe content. We also found in this work that DC-bias does not affect the conduction process but proves its thermal activation. The variation of the conductance with polarization is a proof of an electro-resistance effect.     

Electrical conduction and dielectric studies of ZnO pellets

A series of Zinc Oxide pellets sintered at different temperatures was studied by means of dielectric spectroscopy in the wide frequency range of 1–10⁶ Hz and temperature interval from −100 °C to 30 °C. Electrical conductivity was analysed using Jonsher's universal power law, and the values of s were found to decrease with the increase in temperature, which agrees well with the correlation barrier hopping (CBH) model.