Direct current hopping conductance in one-dimensional diagonal disordered systems

Based on a tight-binding disordered model describing a single electron band, we establish a direct current (dc) electronic hopping transport conductance model of one-dimensional diagonal disordered systems, and also derive a dc conductance formula. By calculating the dc conductivity, the relationships between electric field and conductivity and between temperature and conductivity are analysed, and the role played by the degree of disorder in electronic transport is studied. The results indicate the conductivity of systems decreasing with the increase of the degree of disorder, characteristics of negative differential dependence of resistance on temperature at low temperatures in diagonal disordered systems, and the conductivity of systems decreasing with the increase of electric field, featuring the non-Ohm's law conductivity.     

Characteristics of alternating current hopping conductivity in DNA sequences

This paper presents a model to describe alternating current (AC) conductivity of DNA sequences, in which DNA is considered as a one-dimensional (1D) disordered system, and electrons transport via hopping between localized states. It finds that AC conductivity in DNA sequences increases as the frequency of the external electric field rises, and it takes the form of σ_ac (ω)～ω ²\ln ²(1/ω). Also AC conductivity of DNA sequences increases with the increase of temperature, this phenomenon presents characteristics of weak temperature-dependence. Meanwhile, the AC conductivity in an off-diagonally correlated case is much larger than that in the uncorrelated case of the Anderson limit in low temperatures, which indicates that the off-diagonal correlations in DNA sequences have a great effect on the AC conductivity, while at high temperature the off-diagonal correlations no longer play a vital role in electric transport. In addition, the proportion of nucleotide pairs p also plays an important role in AC electron transport of DNA sequences. For p < 0.5, the conductivity of DNA sequence decreases with the increase of p, while for p ≥ 0.5, the conductivity increases with the increase of p.     

Itinerant electron model and conductance of DNA

DNA (Deoxyribonucleic acid) has recently caught the attention of chemists and physicists. A major reason for this interest is DNA’s potential use in nanoelectronic devices, both as a template for assembling nanocircuits and as an element of such circuits. However, the electronic properties of the DNA molecule remain very controversial. Charge-transfer reactions and conductivity measurements show a large variety of possible electronic behavior, ranging from Anderson and band-gap insulators to effective molecular wires and induced superconductors. In this review article, we summarize the wide-ranging experimental and theoretical results of charge transport in DNA. An itinerant electron model is suggested and the effect of the density of itinerant electrons on the conductivity of DNA is studied. Calculations show that a DNA molecule may show conductivity from insulating to metallic, which explains the controversial and profuse electric characteristics of DNA to some extent.      

Density of states and extent of wave function: two crucial factors for small polaron hopping conductivity in 1D

Abstract

We introduce a theoretical model to scrutinize the conductivity of small polarons in 1D disordered systems, focusing on two crucial – as will be demonstrated – factors: the density of states and the spatial extent of the electronic wave function. The investigation is performed for any temperature up to 300 K and under electric field of arbitrary strength up to the polaron dissociation limit. To accomplish this task, we combine analytical work with numerical calculations.     

一维二元非对角关联无序体系交流跳跃电导特性

在单电子紧束缚无序模型基础上，建立了一维二元非对角关联无序体系电子跳跃输运交流电导模型，并推导了其交流电导公式，通过计算其交流电导率，探讨了格点能量无序度、格点原子组分、非对角关联及温度、外场对体系交流跳跃电导的影响.计算结果表明，一维二元非对角关联无序体系的交流电导率随格点能量无序度的增大而减小.同时，体系中两种原子的组分的变化实际代表着体系成分无序程度的变化，因而对其交流电导率的影响很大，表现为随A类原子含量p的增加而先减小后增大.当引入非对角关联时，体系出现退局域化现象，电子波函数由局 关键词： 二元无序体系 交流跳跃电导 格点能量无序度 非对角关联     

准一维多链无序体系跳跃电导特性

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     

一维无序体系的交流跳跃导电

建立了electronphononfield(EPF)电子隧穿电导模型,推导了一维无序体系新的交流电导公式.通过计算具有20000—65000个格点的无序体系的交流电导率,分析了交流电导率与温度及外场频率的关系,讨论了无序度对交流电导的影响.计算结果表明,无序体系的交流电导率随外场频率的增加而近似线性的增大;无序体系在低温区出现了负微分电阻特性,电导率随温度的升高而增大,在高温区电导率随温度的升高而减小;无序度对无序体系的交流电导影响明显:在低温区,无序度越小,体系的电导率越大;在高温区,适当增大无序度, 关键词： 无序体系 电子隧穿 跳跃电导     

Scanning tunneling spectrosopy study of DNA conductivity

We used STM to study the conductivity of 32 nucleotide long DNA molecules chemically attached to a gold surface. Two oligonucleotides containing all four base types namely G, A, C, T, one single stranded and one double helical, all showed conductance data significantly higher than DNA containing only T and A that were either single stranded d(T32) or double helical d(T32).d(A32) in confirmation. Within each sequence group, the conductivity of the double helical form was always higher than that of the single strand. We discuss the impact of structure, particular base stacking and affinity to the phase transition.      

Statistical properties of Lyapunov exponents and of quantum conductance of random systems in the regime of hopping transport

The statistics of the zero-temperature conductance and the Lyapunov exponents of one-, two- and three-dimensional disordered systems in the regime of strong localization is studied numerically. In one dimension, the origin of the universality of the moments of the conductance is explained. The relation between the most probable value of the conductance and its configurational average is discussed. The relative fluctuations of the conductance (and of the resistance) are shown to grow exponentially with the system length. In higher dimensions the conductance is almost entirely determined by the smallest of the Lyapunov exponents. The statistics of the conductance is therefore the same as in the one dimensional case. A model is proposed for the treatment of the fluctuations in hopping transport at finite temperatures. An exponential dependence of the relative fluctuations of the conductance/resistance on the temperature is predicted, log (δg/g) ∞ T^?a with α = 1/(d+1). It is concluded that the presently available experimental data on the temperature dependence of the conductance fluctuations in the hopping regime can be understood by replacing the system size in the zerotemperature result for the fluctuations of the conductance by the hopping length.     

一维二元非对角关联无序体系跳跃电导特性

在单电子紧束缚无序模型基础上，建立了一维二元关联无序体系电子跳跃输运直流电导模型，并推导了其直流电导公式，通过计算其直流电导率，探讨了格点能量无序度、非对角关联及温度、外场对体系跳跃电导的影响.计算结果表明，一维二元无序体系的直流电导率随着格点能量无序度的增大而减小；当引入非对角关联时，体系出现退局域化现象，从而使体系的直流电导率增大；温度对体系的电子输运的影响表现为体系的直流电导率随温度的升高而增大；在外加电场的调制下，体系的直流电导率在强场区随电场强度增加而增长很快，呈现出非欧姆定律特性，但在弱场区外场的作用不明显. 关键词： 二元无序体系 跳跃电导 格点能量无序度 非对角关联     

Humidity Effects on Conductivity of DNA Molecules

We present a model related to the humidity to describe the conductivity of homogeneous DNA molecule,where the hydration of phosphate group and bases are taken into account. The calculated results show the oscillation feature of dⅠ/dⅤ-Ⅴ curves and the semiconductor behavior of DNA. With the relative humidity increasing, the voltage gap becomes narrow and the maximum of conductance increases nonlinearly. The conductivity of DNA approaches to stabilization when the relative humidity reaches a certain value. These results are in agreement with experimental measurements.     

Humidity Effects on Conductivity of DNA Molecules

We present a model related to the humidity to describe the conductivity of homogeneous DNA molecule, where the hydration of phosphate group and bases are taken into account. The calculated results show the oscillation feature of dI/dV-V curves and the semiconductor behavior of DNA. With the relative humidity increasing, the voltage gap becomes narrow and the maximum of conductance increases nonlinearly. The conductivity of DNA approaches to stabilization when the relative humidity reaches a certain value. These results are in agreement with experimental measurements.     

共轭高分子链中电荷迁移的热效应

从理论上研究了共轭高聚物链中在电场作用下极化子运动的热效应.基于SSH模型以及通过绝热动力学演化的方法,模拟了共轭高聚物链中极化子在电场作用下从链左端向右端运动的过程.晶格受到的热扰动作用假设为通过局域的晶格范围内原子位移的随机涨落来实现.结果发现,晶格中的局域热涨落对于运动中的极化子而言等效于一个势垒.势垒高度由高分子中受到热扰动的区域的范围大小以及该区域与其周围环境的温差来决定.当分子中存在热吸收不均匀的现象时,链内极化子迁移率在低电场范围内随电场的变化遵循对数曲线变化规律.     

畸变对hopping电导的影响：Thue Morse纳米结构模型

发展实空间重正化群方法，研究了一维非周期Thue Morse纳米结构链的hopping电导率.计算表明Thue Morse纳米结构体系的晶粒种类、晶粒尺寸对hopping电导有显著的调制作用，界面结构和晶格畸变对hopping电导也有不同程度的影响.从无序度对hopping电导的影响来看，Thue Morse纳米结构链的无序度介于Fibonacci链和周期链之间. 关键词： Thue Morse纳米结构链 重正化群方法 hopping电导 尺寸效应 界面效应     

Effects of photoexcitation on the current transport mechanism in amorphous indium selenide thin films

The effect of photoexcitation on the current transport mechanism in amorphous indium selenide thin films was studied by means of dark and illuminated conductivity measurements as a function of temperature. Analysis of the dark electrical conductivity in the temperature range 110–320 K reveals behaviour characteristic of carriers excited to the conduction band and thermally assisted variable-range hopping (VRH) at the Fermi level above 280 K and below 220 K, respectively. In the temperature range 220–280 K, a mixed conduction mechanism was observed. A conductivity activation energy of ～300 meV (above 280 K), a density of localised states (evaluated assuming a localisation length of 5 Å) of 1.08 × 10²¹ cm^?3 eV^?1, an average hopping distance of 20.03 Å (at 120 K) and an average hopping energy of 27.64 meV have been determined from the dark electrical measurements. When the sample was exposed to illumination at a specific excitation flux and energy, the values of the conductivity activation energy, the average hopping energy and the average hopping range were significantly decreased. On the other hand, the density of localised states near the Fermi level increased when the light flux was increased. Such behaviour was attributed to a reversible Fermi level shift on photoexcitation.     

Study of alternating current conduction mechanism in polypyrrole-magnesium ferrite hybrid nanocomposite through correlated barrier hopping model

In the present work, both polypyrrole (PPy) and optimized polypyrrole–magnesium ferrite (PPy-MgFe₂O₄) hybrid nanocomposite were synthesized separately by simple oxidative chemical polymerization method and then structurally characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The FTIR spectrum of the composite showed the presence of characteristic absorption bands of both PPy and MgFe₂O₄ in the composite confirming interfacial interaction of PPy with MgFe₂O₄. That this interaction is not affected by crystalline behaviour of predominant MgFe₂O₄ particles but that MgFe₂O₄ has embedded in PPy matrix was confirmed by XRD studies. Agglomerated granular spherical morphology of the composite was confirmed by SEM studies. Decrease in AC conductivity of the composite as compared to PPy due to the formation of interfacial heterojunction barrier between p-type PPy and n-type MgFe₂O₄ was confirmed experimentally and well supported theoretically by calculating binding energy, hopping distance and density of states at Fermi level of PPy and the composite as per CBH model.     

New results on the melting thermodynamics of a circular DNA chain

We investigate the impact of supercoil period and nonzero supercoil formation energy on the thermal denaturation of a circular DNA. Our analysis is based on a recently proposed generalization of the Poland-Scheraga model that allows the DNA melting to be studied for plasmids with circular topology, where denaturation is accompanied by formation of supercoils. We find that the previously obtained first-order melting transition persists under the generalization discussed. The dependence of the size of the order-parameter jump at the transition point and the associated melting temperature are obtained analytically.     

Electron transport through asymmetric DNA molecules

We investigate quantum mechanical electron transport along the long axis of the DNA molecule using an effective tight-binding model. The overall contour plot of transmission, the current-voltage characteristics, and the differential conductance are examined for the variation of backbone onsite energy, the energy-dependent hopping strength, and the contact coupling between the leads and the DNA molecule. It is shown that as backbone asymmetry increases, the merging and collapse of the two mini-bands take place and an extra resonance peak in the transmission appears. In addition, we present the modulation of voltage threshold in the current-voltage curves and a double-peak structure in the differential conductance due to the disappearance of the merged mini-band. Finally, in the Coulomb blockade regime of asymmetric contact coupling, a distinct and under-unity resonance in the transmission appears due to the interference effects between the DNA molecular bands and the electronic structure of the leads at the DNA-lead interface.     

Theoretical study of conductance in stretched monatomic nanowires

Recent experiments showed that the last, single channel conductance step in monatomic gold contacts exhibits significant fluctuations as a function of stretching. From simulations of a stretched gold nanowire linked to deformable tips, we determine the distribution of the bond lengths between atoms forming the nanocontact and analyze its influence on the electronic conductance within a simplified single channel approach. We show that the inhomogeneous distribution of bond lengths can explain the occurrence and the 5% magnitude of conductance fluctuations below the quantum conductance unit g₀=2e²/h.