共查询到20条相似文献,搜索用时 46 毫秒
1.
在单电子紧束缚近似下,建立了准一维多链无序体系直流、交流电子跳跃输运模型,通过计算探讨了无序模式、维度效应、温度及外场对其直流、交流电导率的影响.计算结果表明:准一维多链无序体系的直流、交流电导率随着格点能量无序度的增大而减小,非对角无序具有增强体系电子输运能力的作用.随着链数的增加,体系的直流、交流电导率增大,但格点能量无序度较小时,维度效应的影响不明显.在对角无序情况下准一维多链无序体系的交流电导率随温度的升高而增大,而在非对角无序模式下却随温度的升高而减小,但对于直流情况,体系的直流电导率随温度的升 相似文献
2.
Studies of dielectric relaxation and ac conductivity have been made on three samples of sodium tungsten phosphate glasses over a temperature range of 77–420 K. Complex relative permitivity data have been analyzed using dielectric modulus approach. Conductivity relaxation frequency increases with the increase of temperature. Activation energy for conductivity relaxation has also been evaluated. Measured ac conductivity (σm(ω)) has been found to be higher than σdc at low temperatures whereas at high temperature σm(ω) becomes equal to σdc at all frequencies. The ac conductivity obeys the relation σac(ω)=Aω S over a considerable range of low temperatures. Values of exponent S are nearly equal to unity at about 78 K and the values decrease non-linearly with the increase of temperature. Values of the number density of states at Fermi level (N(E F)) have been evaluated at 80 K assuming values of electron wave function decay constant α to be 0.5 (Å)?1. Values of N(E F) have the order 1020 which are well within the range suggested for localized states. Present values of N(E F) are smaller than those for tungsten phosphate glasses. 相似文献
3.
The response of charge-density-wave systems to the joint application of ac and dc electric fields is investigated within the phenomenological model proposed by Tua and Zawadowski. The model is solved numerically using a finite number Ns of segments. Even if overdamped motion is assumed, the dc I–V characteristic shows harmonic as well as subharmonic steps with a devil's staircase structure, in contrast with the classical (single-particle) model with sinusoidal potential. The devil's staircase appears to be complete but no fractal dimension can be inferred. The structure of the staircase depends on NS and vanishes in the thermodynamic limit NS → ∞. The relevance of these results to the available experimental data is discussed. 相似文献
4.
R.E. Thorne W.G. Lyons J.H. Miller J. Richard J.R. Tucker 《Solid State Communications》1984,50(9):833-836
The charge-density wave contribution to the dc and the small-signal ac conductivity of orthorhombic TaS3 has been completely characterized for a single crystal. The ac conductivity is found to be independent of dc bias below threshold. The tunneling theory of charge-density wave depinning successfully predicts the real and imaginary parts of the ac conductivity as functions of both frequency and applied dc bias, using only a fit to the dc I–V data and one adjustable parameter. 相似文献
5.
The frequency (1-10 kHz) and temperature (80-350 K) dependences of the ac conductivity and dielectric constant of the V2O5-MnO-TeO2 system, containing two transition-metal ions, have been measured. The dc conductivity dc measured in the high-temperature range (200-450 K) decreases with addition of the oxide MnO. This is considered to be due to the formation of bonds such as V--O--Mn and Mn--O--Mn in the glass. The conductivity arises mainly from polaron hopping between V4+M and V5+ ions. It is found that a mechanism of adiabatic small-polaron hopping is the most appropriate conduction model for these glasses. This is in sharp contrast with the behaviour of the Mn-free V2O5-TeO2 glass, in which non-adiabatic hopping takes place. High-temperature conductivity data satisfy Mott's small-polaron hopping model and also a model proposed by Schnakenberg in 1968. A power-law behaviour ( ac = Aωs , with s < 1) is well exhibited by the ac conductivity σac data of these glasses. Analysis of dielectric data indicates a Debye-type relaxation behaviour with a distribution of relaxation times. The MnO-concentration-dependent σac data follow an overlapping large-polaron tunnelling model over the entire range of temperatures studied. The estimated model parameters are reasonable and consistent with changes in composition. 相似文献
6.
P. Lunkenheimer G. Knebel A. Pimenov G. A. Emel’chenko A. Loidl 《Zeitschrift für Physik B Condensed Matter》1995,99(4):507-516
The complex conductivity of La2CuO4+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 La2CuO4+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. 相似文献
7.
The dielectric relaxation and ac conduction of CaCu3Ti4O12 (CCTO) ceramics were investigated at different temperatures under a dc bias. The dc bias gives rise to space charge accumulation, i.e. an electrode response, resulting in the significant increase of dielectric permittivity and dielectric loss tangent. Two Debye-like relaxations, arising from electrode and grain boundary responses, are present at low frequency with an increase of the dc bias. The electrode and grain boundary relaxations are distinguished according to the impedance spectroscopy and the frequency-dependent ac conductivity. The relaxation times of electrode and grain boundary relaxation are 0.955 ms and 0.026 ms, respectively, with a dc bias of 10 V at 328 K. 相似文献
8.
本文分析了量子化极限情况下MOS反型层二维电子气(2DEG)定域态电子电导率的频率特性。主要内容:(1)用费密分布函数随频率变化导出了在2DEG两种主要导电过程的频率特性。即向迁移率边激发导电过程电导率σME(ω)和可变程跳跃导电过程电导率σVRH(ω)。发现:σME(ω)和σVRH(ω)是一个复数。(2)说明了σME(ω)有较长时间常数τn,对应一个低频过程。σVRH(ω)有较短时间常数,对应一个高频过程。(3)理论与实验作了比较,拟合结果表明,在实验条件下,向迁移率边Ec激发时间常数τn 3.6×10-6。
关键词: 相似文献
9.
On crystalline silicon specimens with a nonuniform carrier concentration distribution produced by an optical method, a dispersion of the effective transverse conductivity σ ⊥ eff (ω) is observed near the frequency ω≈ωc=τ ⊥ ?1 ≡ε/4πσ ⊥ eff . At ω<ωc, an anomalous transverse effective conductivity is observed: σ ⊥ eff (ω) is greater than the transverse conductivity of a homogeneous specimen σ ⊥ h (ω) (in the frequency range studied in the experiment σ ⊥ h (ω) = const). Near ω≈ωc, the conductivity σ ⊥ eff decreases, and, at ω>ωc it coincides with σ ⊥ h . 相似文献
10.
《Current Applied Physics》2015,15(2):64-70
Dielectric properties, ac conductivity and thermal characteristics of terbium fumarate heptahydrate crystals grown by gel diffusion method have been carried out. The real part of dielectric constant, dielectric loss and ac conductivity of the material have been measured as a function of temperature and frequency of the applied electric field. Dielectric constant, dielectric loss and ac conductivity of the title compound were systematically investigated, showing a hump at about 85 °C, which could be attributed to water molecules in the crystal boundary. The dielectric anomaly exhibited by the material has been correlated with its thermal behaviour. The ac conductivity of the material obeys the Jonscher's power law relation; σ(ω) = σo + Aωs, with the temperature dependent power exponent s < 1. The ac conductivity of the compound has been found to increase with the increase in frequency. The material is suggested to show protonic conduction. The non-isothermal kinetics was used to evaluate the activation energy for the dehydration step of thermal decomposition of terbium fumarate heptahydrate by using the Coats–Redfern integral method. 相似文献
11.
P.G. Bruce 《Solid State Ionics》1985,15(3):247-251
The ac conductivity of an ionically conducting glass, 0.4Ca(NO3)2:0.6KNO3, is reexamined. The data is seen to exhibit a Jonscher, power-law dependence on frequency; however, two power-law expressions are required to describe the ac response σ=G0+A1ωn1+A2ωn2. An explanation is advanced involving the formation of clusters within the glass on cooling. 相似文献
12.
Electrical conductivity and dielectric measurements of ([C2H10N2]CdCl2(SCN)2)n were carried out from 200 Hz–5 MHz over a temperature range of 307–352 K. The frequency dependence of electrical data have been analyzed in two frameworks: the electrical modulus formalism with the Kohlrausch-Williams-Watts (KWW) stretched exponential function and the electrical conductivity by using the Jonscher’s power law σ′Tot(ω,T) = σDC(T) + A(T)ωs(T) in the frequency domain. The conduction mechanism is attributed to the nonoverlapping small polaron tunneling (NSPT) model. Furthermore, the dielectric data have also been analyzed in modulus and polarizability formalisms. The close values of activation energies obtained from the conductivity, the relaxation process, the electric modulus, and the complex polarizability data confirm that the transport is through ion-hopping mechanism. 相似文献
13.
Anisotropic dielectric behavior of the superionic conductor lithium nitride (Li3N) is reported for low temperature where the dc ionic conductivity can be neglected (T<17oK). For E ⊥ c the static dielectric constant followed a Langevin-Debye (ε ∝ 1/T) type law from 170 to 80K which changed to a temperature independent value below 40K. Debye relaxations were measured between 10 and 50K in the frequency range 10Hz to 1MHz. The observed properties are discussed in terms of a locally restricted ionic motion in shallow potentials. Different types of potentials are considered and an overdamped oscillator model is proposed. 相似文献
14.
We investigate the frequency dependent conductivity σ(ω) for ω in the vicinity of the jump rate Γ0. The most general model based on hopping diffusion predicts a σ(ω) consistent with experiment for β-Al2O3 but qualitatively at odds for Ag+ and Cu+ conductors. We consider the coupling of the transition rate to an external variable such as a long lived, low frequency lattice mode and show that this mechanism can give structure in σ(ω) of the form observed in Ag+ and Cu+ conductors. 相似文献
15.
The optical conductivity of PdMn0.7Fe0.3, Pd2AuFe, and GdCu alloys with different degrees of homogeneity has been measured. Possible reasons for a low-frequency (E < 1 eV) anomalous maximum in the σ(ω) curves are discussed. The deviation from the Drude behavior of σ(ω) is assigned to the presence of small “metallic” inclusions in the high-resistivity matrix of these alloys. 相似文献
16.
Frequency dependence of the real part of the conductivity σ1(ω) in the region of the transition from almost linear (s < 1) to quadratic (s ≈ 2) can indicate a change in the conduction mechanism (the transition from the variable-range to the fixed-range hopping with increasing frequency); in this case, the sharpness of the change in the slope of the frequency characteristic is related to the dependence of the preexponential factor of the resonance integral on the intercenter distance in the pair. The frequency dependence of the imaginary part of the conductivity σ2(ω) has no kink in the vicinity of the transition frequency ωcr, remaining almost linear. A large dielectric loss angle |cotγ| = |σ2|/σ1 can indicate that the imaginary part of the conductivity at ω < ωcr is defined by the larger zero-phonon contribution in σ2res the region of weak variation in the loss angle γ(ω), which significantly exceeds the relaxation contribution σ2res. 相似文献
17.
《Physics letters. A》2020,384(3):126073
Hall effect measurements conventionally rely on the use of dc magnetic fields. For electronic devices made of ultrathin semiconducting materials, such as molybdenum disulfide (MoS2), the dc Hall effect measurements have practical difficulties. Here, we report the results of the Hall effect measurements using ac magnetic fields and a lock-in detection of the Hall voltage for field effect transistors with ultrathin MoS2 channels. The ac Hall effect measurements have some advantages over the dc measurements. The carrier concentration and the Hall mobility were estimated as a function of gate voltage from the results of the ac Hall effect measurements. They used a magnetic field strength that was lower by two orders of magnitude than those used in prior studies on MoS2 devices, which relied on dc magnetic fields. 相似文献
18.
The a.c. conductivity σ(ω) of ionic materials takes the form, σ(ω) = σ(0) + Aωn. The carrier hopping rate, ωp, is obtained from the new expression σ(0) = A ωpn and the carrier concentration is estimated from σ(0). The contribution of creation and migration terms to the activation energy for conduction may be determined from the thermal activation of σ(0) and ωp and the corresponding entropy terms quantified. Data have been analyzed for four widely different ionic materials: single crystal Na β-alumina, polycrystalline Li4SiO4, Ag7l4AsO4 glass and Ca(NO3)2/KNO3 glass and melt. For each, the carrier concentration and hopping rates have been obtained. 相似文献
19.
We study transport in Weyl semimetals with N isotropic Weyl nodes in the presence of Coulomb interactions or disorder at temperature T. In the interacting clean limit, we determine the conductivity σ(ω,T) by solving a quantum Boltzmann equation within a "leading log" approximation and find it to be proportional to T, up to logarithmic factors arising from the flow of couplings. In the noninteracting disordered case, we compute the Kubo conductivity and show that it behaves differently for ω < T and ω > T: in the former regime we recover a previous result, of a finite dc conductivity and a Drude width vanishing as NT(2); in the latter, we find that σ(ω,T) vanishes linearly with ω with a leading term as T → 0 equal to the clean, free-fermion result: σ(0)((N))(ω,T = 0) = Ne(2)/12h|ω|/v(F). We compare our results to transport data on Y(2)Ir(2)O(7) and comment on the possible relevance to recent experiments on Eu(2)Ir(2)O(7). 相似文献
20.
Imran Murtaza Ibrahim Qazi Khasan S. Karimov Muhammad H. Sayyad 《Physica B: Condensed Matter》2011,406(6-7):1238-1241
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 ωs with the index s≤1.15 suggesting a dominant hopping mechanism of conduction. 相似文献