Correlation between Si-SiO2 heterojunction and Fowler-Nordheim conduction mechanism after soft breakdown in ultrathin oxides

A stress-induced defect band model is proposed to investigate the Fowler-Nordheim tunneling characteristics of ultrathin gate oxides after soft breakdown. Soft breakdown occurs when the average distance between stress-induced defects locally reaches a critical value to overlap the bound electron wavefunction on adjacent defects and to form a defect band. This model shows that an n⁺-poly-Si/N-SiO₂/p-Si heterojunction structure is formed between electrodes at a local area after a soft breakdown in the ultrathin SiO₂ and the soft breakdown current can be described in terms of the Fowler-Nordheim tunneling process with a barrier height of ∼1 eV.     

Low-temperature thermodynamic properties of a one-dimensional disordered electron lattice system

A new method of low-temperature thermodynamic calculation of a one-dimensional generalized Wigner crystal on a disordered host-lattice is proposed. This method is based on the system statistical sum presentation in terms of modified transfer-matrixes. A gapless structure of the elementary excitations spectrum of the system under consideration is found.     

Photo control of transport properties in a disordered wire: Average conductance,conductance statistics,and time-reversal symmetry

In this paper, we study the full conductance statistics of a disordered 1D wire under the application of light. We develop the transfer matrix method for periodically driven systems to analyze the conductance of a large system with small frequency of light, where coherent photon absorptions play an important role to determine not only the average but also the shape of conductance distributions. The average conductance under the application of light results from the competition between dynamic localization and effective dimension increase, and shows non-monotonic behavior as a function of driving amplitude. On the other hand, the shape of conductance distribution displays a crossover phenomena in the intermediate disorder strength; the application of light dramatically changes the distribution from log-normal to normal distributions. Furthermore, we propose that conductance of disordered systems can be controlled by engineering the shape, frequency and amplitude of light. Change of the shape of driving field controls the time-reversals symmetry and the disordered system shows analogous behavior as negative magneto-resistance known in static weak localization. A small change of frequency and amplitude of light leads to a large change of conductance, displaying giant opto-response. Our work advances the perspective to control the mean as well as the full conductance statistics by coherently driving disordered systems.     

The dynamics of excitons and trions in resonant tunneling diodes

The aim of this work is to study the dynamic formation and dissociation of trions and excitons in double barrier resonant tunneling diodes. We propose a system of rate equations that takes into account the formation, dissociation and annihilation of these complexes inside the quantum well. From the solutions of the coupled equations, we are able to study the modulation of excitons and trions formation in the device as a function of the applied bias. The results of our model agree qualitatively with the experiments showing the viability of these rate equations system to study the dynamics of complex systems.     

Coherent backscattering and localization in a self-attracting random walk model

Intensity propagation of waves in dilute 2D and 3D disordered systems is well described by a random walk path-model. In strongly scattering media, however, this model is not quite correct because of interference effects like coherent backscattering. In this letter, coherent backscattering is taken into account by a modified, self-attracting random walk. Straightforward simulations of this model essentially reproduce the results of current theories on “non-classical” transport behavior, i.e. Anderson localization in 1D and 2D for any amount of disorder and a phase transition from weak to strong localization in 3D. However, in the strongly scattering regime corrections are necessary to account for the finite number of light modes due to their non-vanishing lateral extention. Within our model this correction leads to the observation that strong localization does not take place. Received 17 September 2001     

Spin-dependent electron transport in a type II GaInAsSb/p-InAs heterojunction doped with Mn in quantized magnetic fields

We report a study of spin-related magnetotransport properties of a type II broken-gap heterostructure formed by InAs substrate bulky doped with Mn and δ-Mn-doped GaInAsSb epilayer. Planar and vertical quantum magnetotransport in a 2D-electron-hole system at the single type II broken-gap InAs/GaInAsSb heterointerface was investigated in high magnetic fields under the quantum Hall regime up to 15 T at low temperature (T=1.5 K). The I-V characteristics near the dielectric phase boundary show the step-like behavior that corresponds to the quantum conductance in a disordered 2D structure through the extended edge states of the nearest Landau level closest to the Fermi level. The value of these steps is determined by the orientation of the 2D-electron spin at the Landau level and the magnetic moment of Mn in the δ-layer.     

Influence of spin-glass-like phase on magnetic and electrical transport properties of reactive sputtered Al1−xFexN films

Nanocrystalline Al_1−xFe_xN films were fabricated using the reactive sputtering method. A large amount of spin-glass-like moments are in the films. With the decrease of temperature, the films turn from the spin-glass-like behavior to ferromagnetism. At low temperatures, the saturation magnetization increases and the coercivity decreases with the increase of x. The coercivity increases significantly below 50 K due to the pinning effect of the frozen disordered spin-glass-like moments. All of the films are semiconducting. The low-temperature transport mechanism turns from tunneling to hopping as x increases. Magnetoresistance (MR) shows weak saturation trend with the applied field because of the hard alignment of the frozen moments. Meanwhile, MR follows the relation of log |MR|=a+bT−1, and the spin polarization satisfies P(T)=P₀e−βTα, related with the disordered spin-glass-like moments.     

Electron localization induced by grain boundary disorder in the normal state of high-Tc superconductors

We have studied the effects of superconducting grain boundary disorder on the normal state transport properties of cuprate films. Dip-coated granular YBa₂Cu₃O_7−y (YBaCuO) thick films on polycrystalline MgO substrates were synthesized and networked grains were systematically made less disordered in order to probe the crossover from strong to weak inter-grain disorder. Grain boundary passivation was achieved by metallic inclusions of different forms. We have shown that the normal state of samples exhibit a semiconducting behavior and changes to ‘metallic’ with sharper transitions to the superconducting state as we reduce grain-interfaces disorder, i.e. increase metallic inclusion content. On the basis of electron localization mechanisms, the normal state conductivity is thus shown to undergo a dimensional crossover from 3D to 2D in the frame of the variable-range hopping (VRH) regime. The transition threshold was found to depend on the form of metallic inclusions.     

Optical Bloch oscillation and resonant Zener tunneling in one-dimensional quasi-period structures containing single negative materials

We studied the optical Bloch oscillation and resonant Zener tunneling in macroscopic quasi-period structures of alternatively stratified single negative and dielectric slabs. By a decrease in the thicknesses of the dielectric slabs, the electronic potential of crystals subjected to external dc electric fields is mimicked and the optical Wannier-Stark ladder (WSL) is realized. Both scattering states and the time-resolved transmission of a short pulse are provided to show the existence of the optical analogue of electronic Bloch oscillation. At a critical gradient, the resonant photon Zener tunneling is demonstrated both from the amplitude and the time delay in the transmitted signal of a short pulse.     

Magnetic properties of large-area one-dimensional WO2 and MoO2 nanorods

Large-area one-dimensional (1D) monoclinic WO₂ and MoO₂ nanorods in the space group P2₁/c were synthesized by reactive thermal evaporation. The as-synthesized 1D WO₂ and MoO₂ nanorods become soft magnetic materials at 10 K, implying that structural or magnetic transitions occur. There are large differences in saturation magnetization, the coercive field, and remanence between the 1D WO₂ and MoO₂ nanorods, although both 1D nanorods have a similar shape.     

Single event photon statistics characterization of a single photon source in an imperfect detection system

The single event photon statistics measurement of a single photon source based on the Hanbury-Brown-Twiss (HBT) configuration with an imperfect detection system is studied. It is shown that the imperfect detectors, the imperfect beam-splitter and the unbalanced linear propagation efficiencies will reduce the single event Mandel Q parameter.     

Frequency dependence of the photonic noise spectrum in an absorbing or amplifying diffusive medium

A theory is presented for the frequency dependence of the power spectrum of photon current fluctuations originating from a disordered medium. Both the cases of an absorbing medium (“grey body”) and of an amplifying medium (“random laser”) are considered in a waveguide geometry. The semiclassical approach (based on a Boltzmann-Langevin equation) is shown to be in complete agreement with a fully quantum mechanical theory, provided that the effects of wave localization can be neglected. The width of the peak in the power spectrum around zero frequency is much smaller than the inverse coherence time, characteristic for black-body radiation. Simple expressions for the shape of this peak are obtained, in the absorbing case, for waveguide lengths large compared to the absorption length, and, in the amplifying case, close to the laser threshold. Received 8 August 2000     

Light localization signatures in backscattering from periodic disordered media

The backscattering line shape is analytically predicted for thick disordered medium films where, remarkably, the medium configuration is periodic along the direction perpendicular to the incident light. A blunt triangular peak is found to emerge on the sharp top. The phenomenon roots in the coexistence of quasi-1D localization and 2D extended states.     

Structure due to indirect exciton in photocurrent spectrum of homoepitaxial diamond film

Photocurrent spectrum in homoepitaxal diamond film formed by chemical vapor deposition has been measured in the photon energy range 5-6 eV of the vicinity of the indirect band gap. It has been seen that the line shape of the spectrum in the photon energy range lower than 5.6 eV agrees with the fundamental absorption edge spectrum in natural diamond. Structures in the spectrum have been explained from the optical transition due to indirect excitons assisted by TO phonons and indirect band-to-band transition.     

Simple approach to the mesoscopic open electron resonator: quantum current oscillations

The open electron resonator, described by Duncan et al. [D.S. Duncan, M.A. Topinka, R.M. Westervelt, K.D. Maranowski, A.C. Gossard, Phys. Rev. B 64 (2001) 033310. [1]], is a mesoscopic device that has attracted considerable attention due to its remarkable behaviour (conductance oscillations), which has been explained by detailed theories based on the behaviour of electrons at the top of the Fermi sea. In this work, we study the resonator using the simple quantum quantum electrical circuit approach, developed recently by Li and Chen [Y.Q. Li, B. Chen, Phys. Rev. B 53 (1996) 4027. [2]]. With this approach, and considering a very simple capacitor-like model of the system, we are able to theoretically reproduce the observed conductance oscillations. A very remarkable feature of the simple theory developed here is the fact that the predictions depend mostly on very general facts, namely, the discrete nature of electric charge and quantum mechanics; other detailed features of the systems described enter as parameters of the system, such as capacities and inductances.     

Localized exciton dynamics in AlInGaN alloy

Carrier recombination dynamics in AlInGaN alloy has been studied by photoluminescence (PL) and time-resolved PL (TRPL) at various temperatures. The fast red-shift of PL peak energy is observed and well fitted by a physical model considering the thermal activation and transfer processes. This result provides evidence for the exciton localization in the quantum dot (QD)-like potentials in our AlInGaN alloy. The TRPL signals are found to be described by a stretched exponential function of exp[(−t/τ)^β], indicating the presence of a significant disorder in the material. The disorder is attributed to a randomly distributed QDs or clusters caused by indium fluctuations. By studying the dependence of the dispersive exponent β on temperature and emission energy, we suggest that the exciton hopping dominate the diffusion of carriers localized in the disordered QDs. Furthermore, the localized states are found to have 0D density of states up to 250 K, since the radiative lifetime remains almost unchanged with increasing temperature.     

Exact solution to the problem of nonlinear pulse propagation through random layered media and its connection with number triangles

An energy pulse refers to a spatially compact energy bundle. In nonlinear pulse propagation, the nonlinearity of the relevant dynamical equations could lead to pulse propagation that is nondispersive or weakly dispersive in space and time. Nonlinear pulse propagation through layered media with widely varying pulse transmission properties is not wave-like and a problem of broad interest in many areas such as optics, geophysics, atmospheric physics and ocean sciences. We study nonlinear pulse propagation through a semi-infinite sequence of layers where the layers can have arbitrary energy transmission properties. By assuming that the layers are rigid, we are able to develop exact expressions for the backscattered energy received at the surface layer. The present study is likely to be relevant in the context of energy transport through soil and similar complex media. Our study reveals a surprising connection between the problem of pulse propagation and the number patterns in the well known Pascal’s and Catalan’s triangles and hence provides an analytic benchmark in a challenging problem of broad interest. We close with comments on the relationship between this study and the vast body of literature on the problem of wave localization in disordered systems.     

Magnetic field effects and renormalization of the long-range Coulomb interaction in carbon nanotubes

We develop two theoretical approaches for dealing with the low-energy effects of the repulsive interaction in one-dimensional electron systems. Renormalization Group methods allow us to study the low-energy behavior of the unscreened interaction between currents of well-defined chirality in a strictly one-dimensional electron system. A dimensional regularization approach is useful, when dealing with the low-energy effects of the long-range Coulomb interaction. This method allows us to avoid the infrared singularities arising from the long-range Coulomb interaction at D = 1. We can also compare these approaches with the Luttinger model, to analyze the effects of the short-range term in the interaction. Thanks to these methods, we are able to discuss the effects of a strong magnetic field B in quasi one-dimensional electron systems, by focusing our attention on Carbon Nanotubes. Our results imply a variation with B in the value of the critical exponent α for the tunneling density of states, which is in fair agreement with that observed in a recent transport experiment involving carbon nanotubes. The dimensional regularization allows us to predict the disappearance of the Luttinger liquid, when the magnetic field increases, with the formation of a chiral liquid with α = 0.     

Light localization signatures in backscattering from periodic disordered media

The backscattering line shape is analytically predicted for thick disordered medium films where, remarkably, the medium configuration is periodic along the direction perpendicular to the incident light. A blunt triangular peak is found to emerge on the sharp top. The phenomenon roots in the coexistence of quasi-1D localization and 2D extended states. The text was submitted by the author in English.     

Nature of the electronic states in a chain of potential wells in presence of an electric field

A Kronig-Penney model with a constant electric field is used to study the transmission properties of a non-interacting electron one-dimensional (1D) ordered and disordered systems with uniformly distributed negative strengths of δ-function potentials (wells). In ordered systems we examine the origin of the jumps of the transmission coefficient and the short-range localization (occurring before the first jump) observed previously. For disordered wells, we also examine the phase-diagram in the energy-disorder plane. The short-range localization is observed as a peak in the inverse participation ratio and as a minimum in the localization length. We found that the two distinctive behaviours correspond to a negative differential resistance and to a resonance at particular points corresponding to the edges of the Brillouin zones. Further discussions of these behaviours are included.