Transit-time frequency quantum beats and THz negative conductance in nanoscale semiconductor heterostructures

A quantum-mechanical calculation of the THz negative conductance of hot ballistic electrons in nanoscale semiconductor heterostructures has been performed. It is shown that quantum beats of the ballistic electron wave function induce an ac current in the structure, which can do negative work on the ac field.     

A variable kinematic Ritz formulation for vibration study of quadrilateral plates with arbitrary thickness

A new variable kinematic Ritz method applied to free vibration analysis of arbitrary quadrilateral thin and thick isotropic plates is presented. Carrera's unified formulation and the versatile pb-2 Ritz method are properly combined to build a powerful yet simple modeling and solution framework. The proposed technique allows to generate arbitrarily accurate Ritz solutions from a large variety of refined two-dimensional plate theories by expanding so-called Ritz fundamental nuclei of the plate mass and stiffness matrices. Theoretical development of the present methodology is described in detail. Convergence and accuracy of the method are examined through several examples on thin, moderately thick, and very thick plates of rectangular, skew, trapezoidal and general quadrilateral shapes, with an arbitrary combination of clamped, free and simply supported edges. Present results are compared with existing three-dimensional solutions from open literature. Maximum and average differences of various higher-order plate theories and three-dimensional results are also presented with the aim of providing useful guidelines on the choice of appropriate plate theory to get a desired accuracy on frequency parameters.     

Natural frequencies of completely free annular and circular plates having polar orthotropy

A simple approximate, yet quite accurate, Ritz method analysis is presented for dealing with vibration of completely free annular plates having polar orthotropic characteristics. It is shown that the method is readily applicable to the determination of approximate frequency values for solid circular plates. The natural frequencies of these plates are obtained for the various orthotropic parameters, and comparison is made with exact values for isotropic cases, showing excellent agreement.     

Free vibration of continuous polar orthotropic annular and circular plates

The free vibration of a polar orthotropic annular plate supported on concentric circles is analyzed by the Ritz method with use of Lagrange multipliers. A trial function for the deflection of the plate is expressed in terms of simple power series, and a frequency equation for the plate is derived by the condition for minimizing the total potential energy with the constraint equations included. In the numerical examples it is also shown that the method can directly yield quite accurate frequency values for a solid circular plate. Natural frequencies of annular and circular plates are calculated for wide ranges of the support location and orthotropic parameters.     

Electronic spectrum and ballistic transport in a helical nanotube

A model of a helical nanotube permitting analytical calculation of the spectrum and wave functions is proposed and studied. It is shown that the specific character of the symmetry of the Hamiltonian results in an electronic spectrum having no band structure despite the periodicity of the potential along the system axis. A two-dimensional ribbon rolled into a helix and a quasi-one-dimensional helix in a magnetic field are considered as limiting cases of the model. It is shown that the presence of additional local minima in the subbands of the electronic spectrum leads to a nonmonotonic dependence of the ballistic conductance of the system on the chemical potential.     

Free vibrations of a rectangular plate of variable thickness elastically restrained against rotation along three edges and free on the fourth edge

A literature search has shown that the title problem has received no treatment.In this paper solutions are presented as obtained (I) by use of the Ritz method with deflection functions which are simple polynomials, and (II) by use of the extended Kantorovich Method. The natural boundary conditions along the free edge are not satisfied in the first case, while they are complied with approximately when using the second approach. The fundamental frequency coefficient is determined and good agreement is shown to exist between the results obtained by the two methods.     

A simple approximate method for the solution of the Helmholtz equation in the case of rectangular,non-homogeneous domains

The dynamic behavior of non-homogeneous continuous media is of interest in many fields, from geophysics to everyday mechanical engineering systems and especially in bioengineering.Discontinuous types of non-homogeneities are considered in this paper and it is shown that the Ritz method is convenient for this complicated type of structural element.     

Terahertz negative conductivity of heterostructure barriers during ballistic transport of hot electrons

A mechanism for achieving the terahertz negative conductivity on transmit time effects during ballistic transport of hot carriers in nanoscale semiconductor heterostructures was theoretically considered. It was shown that selection of the heterostructure potential profile can significantly increase high-frequency negative conductivity.     

Electric flow through an ideal ferromagnet–superconductor junction

It is investigated the possibility of controlling the electric flow through a ferromagnet–superconductor junction by spin polarization, within a simple, ideal model of a perfect ferromagnetic–superconductor junction. The ferromagnetic and superconducting properties as well as the Andreev reflection are briefly reviewed and the electrical resistance of the junction is computed both in the diffusive and ballistic regime for the ferromagnetic sample. It is shown that the resistance of the junction increases with increasing magnetization, including both positive or negative jumps on passing from the ballistic to the diffusive regime.     

Phase separation in binary systems with competing internal and external multiplicative noises

We have considered phase separation processes in binary stochastic systems with thermal diffusion and ballistic mixing representing irradiation influence. Introducing fluctuations of thermal flux and an external source of atom relocations due to ballistic diffusion into dynamics of a globally conserved field, we have shown that there are two competing mechanisms of phase transitions type of “order-disorder”: thermally assisted diffusion and irradiation induced atomic exchange. We have studied dynamics of the structure function at early stages of decomposition. In the framework of the mean field theory we have derived the effective Fokker-Planck equation to describe phase separation processes. It was shown that the ordering processes can be controlled by both regular and stochastic parts of external source influence. A reentrant behavior of a mean field order parameter versus the external noise intensity and fluctuations correlation radius is found.     

Anomalous magnetoresistance of two-dimensional systems in the presence of spin-orbit scattering

A theory of weak localization in two-dimensional semiconductor structures and metal films is developed for spin relaxation by the Elliott-Yafet mechanism. The theory is valid in the entire range of classically weak magnetic fields. It is shown that effects due to spin-orbit interaction substantially modify magnetoresistance in both diffusive and ballistic regimes.     

Conductance of the elliptically shaped quantum wire

The conductance of a ballistic elliptically shaped quantum wire is investigated theoretically. It is shown that the effect of the curvature results in a strongly oscillating dependence of the conductance on the applied bias.     

From ballistic to non-ballistic magnetoresistance in nanocontacts: theory and experiments

This paper reports on a large set of experiments using different materials and samples to explore the ballistic and non-ballistic magnetoresistence in magnetic nanocontacts. Ballistic contacts are obtained with 3-d magnetic crystalline metals and non-ballistic by using permalloy, amorphous metal, ceramic perovskites and Heusler alloys that have large resistivity values. It is shown that while the ballistic nanocontacts can present 200% magnetoresistance, the non-ballistic exhibit a non-noticeable value (<2%) at room temperature. Two criteria are given according to theory and in good agreement with experiments to obtain large magnetoresistance values.     

Composite Surface Integral Transforms in Neutron Therapy of Cancer

Composite surface finite integral transforms are applied to formulate the optimal ballistic property for a temporally tuned multibeam neutron cancer 3D therapy as a single-valued dynamical system. By invoking Pontryagin's maximum principle, with the operation functions of the beams constituting the control vector, it is proved, in an inverse problem formulation, that for every spatial configuration of the neutron beams, there exists an optimal temporal control vector satisfying an a priori system of linear homogeneous Volterra integral equations of the first kind and convolution type. A version of this newly advanced, temporally optimalized, multibeam 3D irradiation therapy, with a linearized ballistic property, is shown to result from a shooting-type solution to a related, semihomogeneous dual system of linear integral equations of the first kind. A criterion for the controllability of this optimization problem has also been established.     

Vibration of a rectangular plate with a central power-law profiled groove by the Rayleigh–Ritz method

The prediction of natural frequencies of rectangular plates where a profiled indentation is present is made using a Rayleigh–Ritz variational energy method. Panels with holes are often found for access cables and access gaps, and it is shown that the application of damping to the profile leads to a more efficient method of reducing vibration than covering a whole rectangular plate, which is advantageous where weight saving is necessary.     

Ballistic/quasi-ballistic transport in nanoscale transistor

The current voltage characteristics of the silicon ballistic MOSFETs are introduced and discussed. They are derived by considering the current capacity through the bottleneck point in the channel, and they provide a simple measure of the performance limit. The performance of experimental nanoscale bulk MOSFETs are compared with the ideal ballistic limit. It was shown that the performance degradation due to carrier scattering amounts to several to several tens percent in recent nanoscale MOSFETs. Quasi-ballistic transport in MOSFETs was also analyzed by a simple approach based on the transmission viewpoint. Channel-length reduction was found to yield consistent improvement of the ballisticity. Considerable performance degradation, however, was still found to persist even in 10-nm MOSFETs. The role of each carrier scattering mechanism is analyzed. It is shown that elastic scattering degrades the performance, but the inelastic energy relaxation improves the performance of the MOSFET.     

Carbon nanotube ballistic thermal conductance and its limits

Calculations of the quantum-mechanical ballistic thermal conductance of single-walled carbon nanotubes, graphene, and graphite are presented, which explain previous experimental results, and directly disprove earlier theoretical calculations. The ballistic thermal conductances are smaller than had been previously thought, whereas the maximum sample lengths in which phonon transport remains ballistic are orders of magnitude larger than previously suggested. Good agreement with previous experiments is obtained, which shows that measured lower bounds to the thermal conductance of multiwalled carbon nanotubes are very close to the upper theoretical bounds for graphite. The bounds shown here draw a line between what is physical and unphysical in any measurements or calculations of carbon nanotube thermal conductance, and constitute a necessary test to their validity.     

A nonlinear transport device with no intrinsic threshold

A ballistic rectifier, based on guidance of carriers by a triangular antidot, is shown to be both experimentally and theoretically capable of operating for weak signals. At T = 4.2 K, we find that the ballistic rectifier works when the input voltage is as low as 0.5 mV, the same order ofk_BT. Based on an extended Landauer–Büttiker formula for nonlinear transport, we show that even when the input signal is much smaller thank_BT, temperature has no obvious influence on the rectification effect.     

一种新型内弹道速度诊断技术

本文首次提出一种新型内弹道速度诊断技术──半导体激光微波多普勒测速技术。在理论分析的基础上,设计并组装了半导体激光微波多普勒测速计实验样机,对测速装置的实验研究表明,这种新颖技术完全可以用来诊断内弹道速度。     

Spatially distributed two-photon excitation fluorescence in scattering media: Experiments and time-resolved Monte Carlo simulations

We report on measurements and Monte Carlo analysis of the spatial distribution of light in two-photon excitation fluorescence (TPEF) microscopy operating through scattering media. The axial profile of TPEF produced by a high numerical aperture microscope objective focusing through a tissue-like optical phantom at depths more than five photon mean-scattering lengths is studied. The measured profile is quantitatively interpreted with time-resolved Monte Carlo simulations that take into account the spatio-temporal distribution of the photons within the turbid medium under a femtosecond excitation regime. It is shown that considering only the ballistic photons leads to an under-estimation of the actual out-of-focus distribution of the fluorescence, whereas it is over-estimated when the ballistic and scattered photons are treated alike. Comparison of the in-focus signal to the overall out-of-focus background clearly pointed out that the signal-to-background ratio of TPEF microscopy images benefits from a shortening of the excitation pulses. Moreover, in homogeneously labeled specimen, the background is shown to overcome the signal at imaging depths of about five to six photon mean-scattering paths.