Experimental analysis of resonant tunneling transit time using high-frequency characteristics of resonant tunneling transistors

We systematically estimate the resonant tunneling transit time from the high-frequency characteristics of resonant tunneling transistors. It is found that the transit time across an InGaAs/InAlAs resonant tunneling structure is more than one order of magnitude shorter than that for GaAs/AlAs with the same barrier layer thickness. In addition, the obtained times agree reasonably well with calculated phase time. It is probably the elastic resonance width and not the inelastic scattering effect that mainly determines the resonant tunneling transit time.     

Resonant tunneling and spreading of wave packets

When a wave packet passes through a tunneling structure, the packet shape changes due to interaction with the potential of the structure. Simultaneously the pulse spreads because it contains waves that have different phase velocities and the tunneling is a long-term process. The effect of the spreading on the tunneling pulse shape is investigated analytically for packets with energy width much less than the width of the resonance. It is shown that whereas the tunneling process is a prolonged one, under certain conditions the effect of the spreading is negligible so that the changes of the packet shape are governed mainly by the resonant tunneling process.     

Quantum phase interference and spin-parity in Mn12 single-molecule magnets

Magnetization measurements of Mn12 molecular nanomagnets with spin ground states of S=10 and S=19/2 show resonance tunneling at avoided energy level crossings. The observed oscillations of the tunnel probability as a function of the magnetic field applied along the hard anisotropy axis are due to topological quantum phase interference of two tunnel paths of opposite windings. Spin-parity dependent tunneling is established by comparing the quantum phase interference of integer and half-integer spin systems.     

Stationary phase method and delay times for relativistic and non-relativistic tunneling particles

The stationary phase method is frequently adopted for calculating tunneling phase times of analytically-continuous Gaussian or infinite-bandwidth step pulses which collide with a potential barrier. This report deals with the basic concepts on deducing transit times for quantum scattering: the stationary phase method and its relation with delay times for relativistic and non-relativistic tunneling particles. After reexamining the above-barrier diffusion problem, we notice that the applicability of this method is constrained by several subtleties in deriving the phase time that describes the localization of scattered wave packets. Using a recently developed procedure - multiple wave packet decomposition - for some specifical colliding configurations, we demonstrate that the analytical difficulties arising when the stationary phase method is applied for obtaining phase (traversal) times are all overcome. In this case, we also investigate the general relation between phase times and dwell times for quantum tunneling/scattering. Considering a symmetrical collision of two identical wave packets with an one-dimensional barrier, we demonstrate that these two distinct transit time definitions are explicitly connected. The traversal times are obtained for a symmetrized (two identical bosons) and an antisymmetrized (two identical fermions) quantum colliding configuration. Multiple wave packet decomposition shows us that the phase time (group delay) describes the exact position of the scattered particles and, in addition to the exact relation with the dwell time, leads to correct conceptual understanding of both transit time definitions. At last, we extend the non-relativistic formalism to the solutions for the tunneling zone of a one-dimensional electrostatic potential in the relativistic (Dirac to Klein-Gordon) wave equation where the incoming wave packet exhibits the possibility of being almost totally transmitted through the potential barrier. The conditions for the occurrence of accelerated and, eventually, superluminal tunneling transmission probabilities are all quantified and the problematic superluminal interpretation based on the non-relativistic tunneling dynamics is revisited. Lessons concerning the dynamics of relativistic tunneling and the mathematical structure of its solutions suggest revealing insights into mathematically analogous condensed-matter experiments using electrostatic barriers in single- and bi-layer graphene, for which the accelerated tunneling effect deserves a more careful investigation.     

Larmor spin precession and neutron optics

We consider the neutron-optical phenomena that emerge during the coherent interaction of a neutron with a sample when the neutron spin precesses in a magnetic field. As follows from general considerations, such an interaction gives rise to an extra precession phase, which is added to the Larmor precession phase. This phenomenon can be interpreted as a manifestation of the time delay due to a finite time of the neutron-sample interaction. The Larmor neutron spin precession with a constant frequency serves as a clock for measuring this time delay. We used such a clock to directly measure the difference between the neutron velocity in matter and its vacuum value. We also present the results of the first experiments in which Larmor clocks were used to measure the neutron tunneling time in the resonance of a quasi-bound state and the Bragg diffraction time. Prospects for further applications of the method are discussed.     

Prediction of ferromagnetic correlations in coupled double-level quantum dots

Numerical results for transport properties of two coupled double-level quantum dots (QDs) strongly suggest that under appropriate conditions the dots develop a novel ferromagnetic (FM) correlation at quarter filling (one electron per dot). In the strong coupling regime (Coulomb repulsion larger than electron hopping) and with interdot tunneling larger than tunneling to the leads, an S=1 Kondo resonance develops in the density of states, leading to a peak in the conductance. A qualitative "phase diagram," incorporating the new FM phase, is presented. In addition, the necessary conditions for the FM regime are less restrictive than naively believed, leading to its possible experimental observation in real QDs.     

The asymmetry of the tunneling time in type II semiconductor heterostructures

The tunneling time asymmetry in type II semiconductor heterostructures is related to the phase difference of the reflection coefficients for the two tunneling directions. Analytical expressions and numerical simulations are given for the difference between the left-to-right and right-to-left tunneling times in asymmetric, single and multiple barrier type II heterostructures.     

Quantum tunneling via quantum geometric phase

A new geometric phase is proposed by considering both the energy and momentum conservation, where the corresponding dynamical phases have two parts differently from the conventional calculations for the phase. The results are applied to quantum tunneling process, which is helpful to distinguish the concept about the tunneling time.     

Asymmetric coherent photon tunneling filter in an optical waveguide structure

Based on the well-known electron coherent tunneling phenomenon, by simulation, a photonic tunneling filter fabricated in an optical waveguide is proposed. The Bragg grating structure is applied as the photonic barrier. Two photonic barriers confine a coherent resonance cavity or photon-quantum-well. We report an asymmetric-barrier structure with opposite phase. In this configuration, the central tunneling wavelength is exactly the same as the Bragg wavelength of the grating, independent of the photon-quantum-well dimension. The photon-quantum-well length can be adjusted to make the tunneling peak interval fall to a desired spacing. The photon barrier length is responsible for the filter bandwidth. As an application, an asymmetric grating photon tunneling filter with International Telecommunication Union grid is demonstrated.     

Andreev reflection and tunneling spectrum in a superlattice of metal-superconductor junctions

The tunneling spectrum of an electron and a hole in a superlattice of NS junctions is computed using the BTK approach and the transfer matrix method. It shows sharp resonances at some energies above the superconducting gap. The sharper the resonance is the more layers the superlattice has. We find for the first time a mechanism to balance the incident and outgoing currents on the superlattice by averaging over the phase between the incident electron and the incident hole. This mechanism is more natural and physical than those in literatures.     

一维位势透射系数的计算与谐振隧穿现象的研究

使用具有不同有效质量的一维多阶梯势透射系数的递推计算公式,可以方便地计算任意形状的一维分区位势的透射系数。给出了几种位势的透射系数随入射粒子能量变化的曲线,研究了谐振隧穿现象。对半导体材料GaAlAs/GaAs/GaAlAs的谐振隧穿现象进行了较详细的讨论。计算结果表明,对于两对称方势垒夹一个任意形状势阱的位势,也可能存在谐振隧穿现象。     

On Virtual Phonons,Photons, and Electrons

A macroscopic realization of the peculiar virtual particles is presented. The classical Helmholtz and the Schrödinger equations are differential equations of the same mathematical structure. The solutions with an imaginary wave number are called evanescent modes in the case of elastic and electromagnetic fields. In the case of non-relativistic quantum mechanical fields they are called tunneling solutions. The imaginary wave numbers point to strange consequences: The waves are non-local, they are not observable, and they are described as virtual particles. During the last two decades QED calculations of the solutions with an imaginary wave number have been experimentally confirmed for phonons, photons, and electrons. The experimental proofs of the predictions of non-relativistic quantum mechanics and the Wigner phase time approach for the elastic, electromagnetic and Schrödinger fields will be presented in this article. The results are zero time in the barrier and an interaction time (i.e. a phase shift) at the barrier interfaces. The measured tunneling time scales approximately inversely with the particle energy. Actually, the tunneling time is given only by the barrier boundary interaction time, as zero time is spent inside a barrier.     

N重方势垒结构共振隧道效应研究

本文从理论上研究了N重方势垒结构的共振隧道效应,推导出透射系数及共振隧道条件的解析表达式,结果发现,由于多势垒结构(n≥3)量子阱间的耦合,共振能级不同于量子阱的本征值。此外,由透射系数表示式证实了多势垒结构电子透射谱在共振能级附近为Lorentzian型。所得结果对于分析透射系数随能量的变化关系,估计共振能级以及制造共振隧道器件都具有十分重要的意义。 关键词：     

Measurement of the neutron interaction time with quantum objects

The first experiments aimed at measuring neutron tunneling time in a quasi-bound resonance and Bragg diffraction time were carried out by the Larmor clock method.     

Fermi surface nesting and structural transition on a metal surface: In /Cu(001)

Peierls-type instability and structural phase transition are shown to occur on the surface of a normal metal. An In overlayer on Cu(001) undergoes a reversible transition at approximately 350 K. Scanning tunneling microscopy of the low-temperature, reduced-symmetry phase indicates a strong periodic lattice distortion (PLD). Angle-resolved photoemission of the high-temperature phase reveals that the In-derived surface resonance constitutes a square-shaped, quasi-two-dimensional Fermi surface within the projected bulk Cu bands. The Fermi surface exhibits one-dimensional nesting upon the transition, which is in agreement with the PLD periodicity.     

Photon emission spectroscopy of individual oxide-supported silver clusters in a scanning tunneling microscope

Photon emission spectra of individual alumina-supported silver clusters have been measured for the first time. The light emission stimulated by electron injection from the tip of a scanning tunneling microscope can be assigned to the (1,0) mode of the Mie-plasmon resonance in small silver particles. As cluster sizes decrease, the resonance position shifts to higher energies and the linewidth increases. In the size range examined (1.5-12 nm), intrinsic size effects are discussed as possible origins for the observed size dependence of the Mie resonance.     

强激光场原子隧道电离的研究新进展

光与物质的相互作用一直是科学的主旋律之一.随着超强超短激光技术的快速发展,如今人们可以研究单个原子的内部世界,并调控光与电子的相互作用,从而实现了对原子内电子的超快动力学过程的探索.强激光诱导的原子隧道电离是众多强场物理现象的基石,具有重要的研究意义,也是研究前沿的热点之一.综述了强场原子隧道电离的最新研究进展,基于隧...     

声波在一维声子晶体中共振隧穿的研究

通过从实验和理论方面对声波在一维声子晶体单晶体和被小的共振腔分开的双晶体中传播时发生的隧穿和共振隧穿现象的研究，观察到了声子晶体单晶体在带隙频率范围内发生的隧穿现象，而对于双晶体样品，在带隙频率范围内出现了很强的共振透射峰.共振发生时，实验测得的群时间很大，但是没有共振时，群速度却很快. 关键词： 声波 声子晶体 隧穿 共振     

Phase Time for the Tunneling of Ultracold V-Type Atoms Through a Mazer Cavity

We study the tunneling time of ultracold V-type atoms interacting a high quality microwave cavity. Here atomic coherence is introduced in the system by a strong driving field which couples the two lower states of the three-level atom. It is found that in the presence of coherence, mazer action or the scattering like nature of the interaction may be examined for extended energies of the incident cold atoms. Our results show that position and amplitudes of the peak values of the phase time(traversal time) may be very effectively controlled by the coherent driving field. Further, here we obtained superclassical values of the phase time corresponding to much higher values of the transmission amplitudes of the tunneling atoms which may be advantageous in the possible experimental realization of the superclassical tunneling time of the traversing cold atoms. In addition, we examine a mirror reflection type symmetry in the phase time curve for a judicious choice of the external driving field.     

Quantum tunneling: A general study in multi-dimensional potential barriers with and without dissipative coupling

Quantum tunneling is formulated in terms of the time evolution of a localized state and thus shown to be dependent upon the eigenspectrum of the system Hamiltonian. A number of exactly solvable models with local and non-local double-well potentials are discussed, and it is shown how, for local potentials, other solvable models can be generated by using Gelfand-Levitan and Darboux transformations. Tunneling in multi-dimensional potential barriers is investigated under semi-classical approximation by developing the method of asymptotic expansion of the wave function for large quantum numbers and the WKB approximation for separable systems. General expressions for the imaginary-time tunneling trajectory are obtained in both methods and specific applications are discussed. Approximation schemes for non-separable systems are also presented. A general study of dissipative multi-dimensional tunneling is carried out by using the Gisin equation, the Schrödinger-Langevin equation and the complex potential model. It is shown that, in general, different models of dissipation are not equivalent in the tunneling context. Using these models one can show (a) the existence of critical damping beyond which no tunneling can occur, (b) that tunneling trajectories are dependent on the damping constant and (c) that dissipation may stabilize the excited state rather than the ground state. Finally the tunneling time delay in one-dimensional systems for undamped and for dissipative systems is formulated in terms of the phase shift, and this has been used to show that the effect of damping on the time delay is ignorable.