Electrical conductance properties for magnetic tunnel junctions with MgO barriers

We measured inelastic electron tunneling (IET) spectra and conductance for MgO tunneling magnetoresistance (TMR) films to obtain information on the ferromagnetic/barrier layer interface. The IET spectra showed the difference between amorphous and crystalline structures in the barrier. In the magnetic tunnel junction (MTJ) with a crystalline barrier the IET spectra indicated an Mg-O phonon peak at a low bias voltage by measurement with a parallel magnetization configuration. On the other hand, no peak was observed in the MTJ with an amorphous barrier.     

Spectral properties of linear chain with one-point dynamical disorder

We have investigated the one-dimensional model of an exciton moving along a (finite or semi-infinite) chain of sites with the local energy at one specific site being randomly modulated. The modulation is described by the Markoff stochastic process and we do not invoke the white-noise assumption. For this model, we give an approximation-free calculation of the density of states, the projected density of states and the optical absorption spectrum. The exact solution is found for a broad family of stochastic processes which bridges (and includes) the dichotomic process and the Gaussian one. The results are discussed in various physical limits such as whitenoise limit, weak-perturbation limit and weak-tunneling limit. Particularly, in the slow-modulation (or static) limit, our model represents a chain with a static distribution of energy at the distinguished impurity site and the spectral characteristics reveal a nontrivial dependence on the width of this energy distribution.     

Thermally correlated frozen-in disorder in a linear magnetic chain

Exact solutions for the thermodynamic functions of a spin-diluted Ising chain are obtained for the situation, where the magnetic impurities are randomly frozen in positions determined by a previous annealing process. The effect of correlations between impurity locations that are present in such a system is investigated. It is found that the Brout free energy correctly describes the physics of the system even in those limits, where correlations between impurity locations are of infinite range. Quenched disorder proper is recovered as the limit of infinite annealing temperature.     

Transport through cavities with tunnel barriers: a semiclassical analysis

We study the influence of a tunnel barrier on the quantum transport through a circular cavity. Our analysis in terms of classical trajectories shows that the semiclassical approaches developed for ballistic transport can be adapted to deal with the case where tunneling is present. Peaks in the Fourier transform of the energy-dependent transmission and reflection spectra exhibit a nonmonotonic behaviour as a function of the barrier height in the quantum mechanical numerical calculations. Semiclassical analysis provides a simple qualitative explanation of this behaviour, as well as a quantitative agreement with the exact calculations. The experimental relevance of the classical trajectories in mesoscopic and microwave systems is discussed. Received: 23 October 1997 / Received in final form and Accepted: 11 March 1998     

Laser interaction with the end atoms of a linear chain

The effects of laser radiation on a one-dimensional linear system are examined using classical mechanics. Laser-induced desorption of oxygen from silicon and hydrogen from lead are studied in detail.     

Current induced resistance change of magnetic tunnel junctions with ultra-thin MgO tunnel barriers

Ultra-thin magnetic tunnel junctions with low resistive MgO tunnel barriers are prepared to examine their stability under large current stress. The devices show magnetoresistance ratios of up to 110% and an area resistance product of down to . If a large current is applied, a reversible resistance change is observed, which can be attributed to two different processes during stressing and one relaxation process afterwards. Here, we analyze the time dependence of the resistance and use a simple model to explain the observed behavior. The explanation is further supported by numerical fits to the data in order to quantify the timescales of the involved phenomena.     

Delocalization in one-dimensional tight-binding models with fractal disorder

In the present work, we investigated the correlation-induced localization-delocalization transition in the one-dimensional tight-binding model with fractal disorder. We obtained a phase transition diagram from localized to extended states based on the normalized localization length by controlling the correlation and the disorder strength of the potential. In addition, the transition of the diffusive property of wavepacket dynamics is shown around the critical point.     

On the oscillatory structure in MIM tunnel conductance

Oscillations in the MIM conductance derivative versus bias voltage due to impurity levels in the insulator are analysed by means of a stationary resonance model recently proposed by the authors. Current sensitivity and oscillation shape are determined, emphasizing their temperature dependence. From these results and taking into account the oscillation asymmetry, the position of the impurity level is obtained. The results are in agreement with experimental data on Al-Al₂O₃-Pd structures.     

Imaging fractal conductance fluctuations and scarred wave functions in a quantum billiard

We present scanning-probe images and magnetic-field plots which reveal fractal conductance fluctuations in a quantum billiard. The quantum billiard is drawn and tuned using erasable electrostatic lithography, where the scanning probe draws patterns of surface charge in the same environment used for measurements. A periodicity in magnetic field, which is observed in both the images and plots, suggests the presence of classical orbits. Subsequent high-pass filtered high-resolution images resemble the predicted probability density of scarred wave functions, which describe the classical orbits.     

Study of MgO tunnel barriers with conducting atomic force microscopy

The local topographic and conducting properties of ultra-thin MgO films with polycrystalline Ru and amorphous CoFeB as lower electrodes were investigated. The local conductance and topography of the ultra-thin MgO films (from 0.5 to 1.0 nm) were simultaneously measured with a modified conducting atomic force microscope (CAFM). The imaging force was carefully chosen in order to avoid structural damages in the insulating layers. The promising results include the decrease of the density of hotspots with large conductance with the thickness of MgO and show that an insulating barrier is obtained at 1 nm thickness.     

硫酸铜电解沉积的分形与电导率研究

用电化学的方法,制备出具有分形结构的金属铜样品,计算了不同电解电压下铜样品的分形维数,并求出其电导率.发现随着电解电压的增大,分形结构的维数趋于增大,而铜样品的电导率减小.     

Distribution of fusion barriers

Heavy ion fusion cross sections and compound nucleus average spin values obtained from distribution of fusion barriers are discussed. Various shapes of distribution functions are studied using a truncated Gaussian distribution function (TGD). It is shown that fusion cross section and average spin values are less sensitive to different parametrization of TGD function, whereas the second derivative of the product of energy and fusion cross sections (w.r.t. energy), obtained from the corresponding TGD functions are significantly different depending on the shape of the barrier distribution function. It is also shown byχ ² analysis of fusion cross section data that some systems favour a narrow Gaussian distribution function whereas others, for which the vibrational and rotational collective states are less important, favour a flat barrier distribution. A physical interpretation of the dynamical process that gives rise to different barrier distribution is given in the framework of microscopic coupled channel calculations.     

Fractional conductance quantization in multiwalled carbon nanotubes with disorder

A close relationship is established for describing fractional conductance quantization in multiwalled carbon nanotubes with disorder in the framework of the particle-in-a-box model. Our results agree well with experimental data in the context of nanotubes with defects.     

Magnetic properties of a linear chain antiferromagnet

Magnetic properties of one dimensional antiferromagnets in presence of an axial and rhombic crystal field have been studied using correlated effective field theory. In this theory long range order is assumed and considering Heisenberg type exchange interaction, the susceptibilities of Cs₂CrCl₅·4H₂O measured in different directions are explained.     

Dynamical properties of the Haldane chain with bond disorder

By using Lanczos exact diagonalization and quantum Monte Carlo combined with stochastic analytic continuation, we study the dynamical properties of the S = 1 antiferromagnetic Heisenberg chain with different strengths of bond disorder. In the weak disorder region, we find weakly coupled bonds which can induce additional low-energy excitation below the one-magnon mode. As the disorder increases, the average Haldane gap closes at δ_∆ ~ 0.5 with more and more low-energy excitations coming out. After the critical disorder strength δ_c ~ 1, the system reaches a random-singlet phase with prominent sharp peak at ω = 0 and broad continuum at ω > 0 of the dynamic spin structure factor. In addition, we analyze the distribution of random spin domains and numerically find three kinds of domains hosting effective spin-1/2 quanta or spin-1 sites in between. These “spins” can form the weakly coupled longrange singlets due to quantum fluctuation which contribute to the sharp peak at ω = 0.     

The spin dynamics of the random transverse Ising chain with a double-Gaussian disorder

The dynamical properties of one-dimensional random transverse Ising model （RTIM） with a double-Gaussian disorder is investigated by the recursion method. Based on the first twelve recurrences derived analytically, the spin autocorrelation function （SAF） and associated spectral density at high temperature were obtained numerically. Our results indicate that when the standard deviation σg （or OrB） of the exchange couplings Ji （or the random transverse fields Bi） is small, no long-time tail appears in the SAE The spin system undergoes a crossover from a central-peak behavior to a collectivemode behavior, which is the dynamical characteristics of RTIM with the bimodal disorder. However, when σJ （or σB） is large enough, the system exhibits similar dynamics behaviors to those of the RTIM with the Gaussian disorder, i.e., the system exhibits an enhanced central-peak behavior for large σJ or a disordered behavior for large σB. In this instance, SAFs exhibit a similar long-time tail, i.e., C（t） ~ t ^-2 for large t. Similar properties are obtained when Ji （or Bi） satisfy the double-exponential distribution or the double-uniform distribution. Besides, when both the standard deviations and the mean values of the exchange couplings are small, the effects of the Gaussian random bonds may drive the system undergo two crossovers from a triplet state to a doublet state, and then to a collective-mode state.     

Minima in conductance characteristics of MIS-Schottky tunnel junctions

Theoretical expression for tunnel current of MIS-Schottky tunnel junction, based on the WKB analytical formula for the tunnelling probability, is presented. The dependence of the position of the main minimum in conductance characteristic on the semiconductor Fermi degeneracy is calculated numerically and compared with our experimental data in Au-SiO₂-pSi tunnel junctions.The authors would like to thank Dr. Z. Sroubek for helpful comments.