磁性隧道结电子输运特性的研究

在Slonczewsik自由电子理论模型下,研究了两铁磁性金属电极被一平面磁性势垒隔开的磁性隧道结零偏压下的隧穿电导、自旋极化率和隧穿磁阻比率,研究表明隧道结的磁结构对隧穿电导和隧穿磁阻的值有很大的影响,当两磁性电极分子场方向相同,且都与势垒层分子场反平行时,隧穿电导数值达到最大,两者平行时,其数值最小,同时还分析了分子场的相对取向等对磁性隧道结自旋极化电子输运性质的影响。研究结果对自旋电子器件的设计具有一定的指导意义。     

磁性隧道结电子输运特性的研究

在Slonczewsik自由电子理论模型下,研究了两铁磁性金属电极被一平面磁性势垒隔开的磁性隧道结零偏压下的隧穿电导、自旋极化率和隧穿磁阻比率,研究表明隧道结的磁结构对隧穿电导和隧穿磁阻的值有很大的影响,当两磁性电极分子场方向相同,且都与势垒层分子场反平行时,隧穿电导数值达到最大,两者平行时,其数值最小,同时还分析了分子场的相对取向等对磁性隧道结自旋极化电子输运性质的影响.研究结果对自旋电子器件的设计具有一定的指导意义.     

磁性隧道结自旋极化电子的隧穿特性

铁磁金属间通过中间层的自旋极化电子隧穿产生的磁性耦合,在自旋电子器件中有许多潜在的应用.考虑由一平面磁性势垒层隔开的两铁磁性金属电极构成的磁性隧道结,针对中间层形成的矩形势垒,在近自由电子模型的基础上,计算零偏压下的隧穿电导、自旋极化率和隧穿磁阻比率,分析势垒层特性、分子场强弱、分子场相对取向等对隧道结自旋极化电子隧穿特性的影响.计算结果对自旋电子器件的设计具有一定的指导意义.     

分子电子器件简化模型的电子透射谱的计算

基于分子线耦合到电极的构成特点,采用简化的非对称多势垒连续隧穿模型模拟复合分子器件偏压下的电子隧穿过程,推导电子透射谱的解析表达式,同时计算垒宽、垒距、垒高、电子有效质量和所加偏压等参数与透射系数的关系,结果发现：当电子的能量为某些值时,出现明显的共振隧穿,且透射系数对这些参数的变化非常敏感,这表明可以通过适当的控制方式（如改变复合分子组成、构型等）来修改分子电子器件的输运性质. 关键词： 分子器件 非对称势垒模型 电子透射谱 共振隧穿     

自旋轨道耦合玻色-爱因斯坦凝聚体在尖端势垒散射中的Klein隧穿

本文对自旋-轨道耦合的玻色-爱因斯坦凝聚体经过尖端势垒散射的过程进行了数值模拟,发现散射过程中存在Klein隧穿现象.相较于高斯势垒,尖端势垒散射的Klein阻塞及Klein隧穿区域会向较高势垒方向移动.在Klein隧穿区域,透射系数随势垒高度而振荡下降,且振幅随着势垒增高而逐渐减小.最后,分别讨论了原子相互作用对经典透射区域、Klein阻塞区域以及Klein隧穿区域散射过程的影响.     

无质量手性狄拉克费米子的克莱因隧穿

无质量手性粒子在势垒中的隧穿被研究.粒子在势垒中的透射概率被详细计算,手性粒子在势垒中的运动特征也被阐明.本文用简单的物理观点来解释克莱因隧穿,这对深入理解量子力学中势垒隧穿十分有益.     

电子双势垒量子隧穿的散射矩阵方法及其数值模拟

采用散射矩阵的方法研究了电子在由两个方势垒组成的双势垒结构中的隧穿特性.将电子在双势垒中的隧穿过程分为相干输运和非相干输运两部分来研究,相干输运导致了隧穿透射系数随中间层厚度变化产生量子振荡,而非相干输运导致了振荡振幅的衰减.双势垒总的透射系数与势垒高度、入射和出射波矢的匹配性有关,数值计算的结果证实了相关结论.     

Bloch定理在单层石墨烯多势阱结构体系中的应用

石墨烯材料中观测到Klein隧穿效应,也因其特殊的光学、电学等特性而被广泛研究.本文设计了等高的多势垒体系来分析载流子的隧穿效应.由Bloch定理得到周期性势垒结构中的能带结构,分析载流子的状态和共振隧穿区.数值研究结果表明,通过调制势垒、势阱宽度、势垒高度、以及载流子入射能量等参数来获得所需的隧穿概率,并可进一步分析电导特性.     

界面势垒对碳纳米管场发射特性的影响

以界面势垒对碳纳米管(CNT)场发射的影响为研究目的,在硅衬底上引进很薄的二氧化硅层,以二氧化硅层作为绝缘势垒,然后在二氧化硅界面层上直接生长CNT,来研究二氧化硅绝缘势垒层对CNT场发射的影响。场发射结果为:Fowler-Nordheim(F-N)曲线分为两部分,高电场下偏离F-N曲线并趋于饱和。在双势垒模型的基础上,从电场在两势垒上的分布不同及电子在两势垒上的隧穿几率不同,理论上分析了界面势垒对场发射的影响:低电场下电子在界面势垒的隧穿几率大于在表面势垒的隧穿几率,界面势垒对场发射不起阻碍作用,场发射遵守F-N规律;高电场下电子在界面势垒的隧穿几率小于在表面势垒的隧穿几率,场发射偏离F-N规律。理论对实验结果进行了合理的解释。     

含单负介质层受阻全内反射结构的光子隧穿现象研究

利用稳定相位理论得到了光子穿越含单负介质层受阻全内反射结构的隧穿时间以及光子穿透光学势垒后产生的横向位移.分析结果表明，当势垒为单负介质时，光子隧穿可能表现出负的隧穿时间和负的横向位移.隧穿时间和横向位移存在Hartman效应，使得光子隧穿过程具有超光速性质.此外，基于TM波和TE波通过负介电常数介质和负磁导率介质势垒产生的横向位移的方向正好相反，得到了一种有效的区分两类单负介质的方法. 关键词： 光子隧穿 负折射 单负介质 超光速     

An IETS study of a silane coupling agent; the interaction of 3-(trimethoxysilyl)propanethiol with aluminium oxide and silver surfaces

The inelastic electron tunnelling spectrum of a silane coupling agent, 3-(trimethoxysilyl)propanethiol, is presented. Approximately monolayer quantities of this silane are present on the barrier oxide of an aluminium-aluminium oxide-metal tunnelling junction, in which the counter electrode is either lead or silver. It is deduced from the IETS spectra that the alkoxy groups of this silane condense with the surface hydroxyl groups. No interaction with the lead is observed but the silver layer clearly reacts with the thiol group of the silane molecule, since the thiol stretching vibration is absent in the spectra, and probably forms the corresponding silver thiolate. The influence of this reaction on the conformation of the bound silane is discussed.     

Physical manipulation of single-molecule DNA using microbead and its application to analysis of DNA-protein interaction

We carried out an individual DNA manipulation using an optical trapping for a microbead. This manipulation system is based on a fluorescent microscopy equipped with an IR laser. Both ends of linear DNA molecule were labeled with a biotin and a thiol group, respectively. Then the biotinylated end was attached to a microbead, and the other was immobilized on a thiol-linkable glass surface. We controlled the form of an individual DNA molecule by moving the focal point of IR laser, which trapped the microbead. In addition, we applied single-molecule approach to analyze DNA hydrolysis. We also used microchannel for single-molecule observation of DNA hydrolysis. The shortening of DNA in length caused by enzymatic hydrolysis was observed in real-time. The single-molecule DNA manipulation should contribute to elucidate detailed mechanisms of DNA-protein interactions.     

Adsorption dynamics of double-stranded DNA on a graphene oxide surface with both large unoxidized and oxidized regions

The adsorption dynamics of double-stranded DNA (dsDNA) molecules on a graphene oxide (GO) surface are important for applications of DNA/GO functional structures in biosensors, biomedicine and materials science. In this work, molecular dynamics simulations were used to examine the adsorption of different length dsDNA molecules (from 4 bp to 24 bp) on the GO surface. The dsDNA molecules could be adsorbed on the GO surface through the terminal bases and stand on the GO surface. For short dsDNA (4 bp) molecules, the double-helix structure was partially or totally broken and the adsorption dynamics was affected by the structural fluctuation of short dsDNA and the distribution of the oxidized groups on the GO surface. For long dsDNA molecules (from 8 bp to 24 bp) adsorption is stable. By nonlinear fitting of the contact angle between the axis of the dsDNA molecule and the GO surface, we found that a dsDNA molecule adsorbed on a GO surface has the chance of orienting parallel to the GO surface if the length of the dsDNA molecule is longer than 54 bp. We attributed this behavior to the flexibility of dsDNA molecules. With increasing length, the flexibility of dsDNA molecules also increases, and this increasing flexibility gives an adsorbed dsDNA molecule more chance of reaching the GO surface with the free terminal. This work provides a whole picture of adsorption of dsDNA molecules on the GO surface and should be of benefit for the design of DNA/GO based biosensors.     

Quantum perspective on the localized strand separation and cyclization of DNA double helix

We apply Heisenberg model to study local strand separation and cyclization of double-stranded DNA molecule. By mapping the conformational properties of DNA onto the Heisenberg spin system, we show that denaturation can be viewed as a quench-induced quantum phase transition (QPT) at a finite non-zero temperature. These phase transitions lead to the formation of kinks in Heisenberg model that, in turn, correspond to sharp bends in helical axes of double-stranded DNA molecule. In this scenario we study the cyclization of DNA fragments of short length. Our results are in agreement with the reported observation that there is 1 open bp per circle of about 70 bp. Also we have computed the probability of the formation of kinks in circles of various sizes and have observed that beyond 100 bp it decreases sharply which is in agreement with experimental result.     

Electrostatic investigation into the bonding of poly(phenylene) thiols to gold

The advancement of nanotechnology relies on the understanding of electrical connection to individual molecules. Electrostatic surface potential measurements of self-assembled monolayers can provide insight into the structural and electronic properties of molecules attached to surfaces. In this paper we report on the electrostatic potential of poly(phenylene) thiol molecules bound to gold surfaces. Kelvin force microscopy is used to probe self-assembled monolayers of a series of phenyl, biphenyl, and triphenyl thiol molecules. The dipole moments of the isolated molecules have been determined and show similar electronic trends. A difference in polarity between the isolated molecules and the electrostatic surface potential of a monolayer attached to gold reflects the electron transfer on to the bound molecule.     

Hydrophobic Interactions in Donor-Disulphide-Acceptor (DSSA) Probes Looking Beyond Fluorescence Resonance Energy Transfer Theory

Donor –linker –acceptor (DSSA) is a concept in fluorescence chemistry with acceptor being a fluorescent compound (FRET) or quencher. The DSSA probes used to measure thiol levels in vitro and in vivo. The reduction potential of these dyes are in the range of ?0.60 V, much lower than the best thiol reductant reported in literature, the DTT (?0.33 V). DSSA disulphide having an unusually low reduction potential compared to the typical thiol reductants is a puzzle. Secondly, DSSA probes have a cyclized rhodamine ring as acceptor which does not have any spectral overlap with fluorescein, but quenches its absorbance and fluorescence. To understand the structural features of DSSA probes, we have synthesized DSSANa and DSSAOr. The calculated reduction potential of these dyes suggest that DSSA probes have an alternate mechanism from the FRET based quenching, namely hydrophobic interaction or dye to dye quenching. The standard reduction potential change with increasing complexity and steric hindrance of the molecule is small, suggesting that ultra- low Eo’ has no contribution from the disulphide linker and is based on structural interactions between fluorescein and cyclized rhodamine. Our results help to understand the DSSA probe quenching mechanism and provide ways to design fluorescent probes.     

Chemical Raman enhancement of organic adsorbates on metal surfaces

Using first-principles theory and experiments, chemical contributions to surface-enhanced Raman spectroscopy for a well-studied organic molecule, benzene thiol, chemisorbed on planar Au(111) surfaces are explained and quantified. Density functional theory calculations of the static Raman tensor demonstrate a strong mode-dependent modification of benzene thiol Raman spectra by Au substrates. Raman active modes with the largest enhancements result from stronger contributions from Au to their electron-vibron coupling, as quantified through a deformation potential. A straightforward and general analysis is introduced to extract chemical enhancement from experiments for specific vibrational modes; measured values are in excellent agreement with our calculations.     

The Vibrational Spectrum Of Allyl Mercaptan

The vibrational analysis of allyl mercaptan has vener been published. There are two internal rotors, the SH group and the CH₂SH group, in the allyl mercaptan molecule. As in the case of propylene (1), the molecule has only one plane of symmetry (C_s point group) in its most symmetrical configuration (when the thiol group lies in the plane of the vinyl group). However, microwave studies have shown that it exhibits C₁ symmetry (2). The infrared and Raman spectra of allyl mercaptan have been investigated in this study. The assignment of its fundamental modes has been made based on C₁ symmetry of the molecule.     

Rotation of hydrogen molecules during the dissociative adsorption on the Mg(0001) surface: a first-principles study

Using first-principles calculations,we systematically study the potential energy surfaces and dissociation processes of the hydrogen molecule on the Mg(0001) surface.It is found that during the dissociative adsorption process with the minimum energy barrier,the hydrogen molecule first orients perpendicularly,and then rotates tobecome parallel to the surface.It is also found that the orientation of the hydrogen molecule in the transition state is neither perpendicular nor parallel to the surface.Most importantly,we find that the rotation causes a reduction of the calculated dissociation energy barrier for the hydrogen molecule.The underlying electronic mechanism for the rotation of the hydrogen molecule is also discussed in the paper.