High-frequency response and the possibilities of frequency-tunable narrow-band terahertz amplification in resonant tunneling nanostructures

The characteristics of the high-frequency response of single- and double-well resonant tunneling structures in a dc electric field are investigated on the basis of the numerical solution of a time-dependent Schrödinger equation with open boundary conditions. The frequency dependence of the real part of high frequency conductivity (high-frequency response) in In_0.53Ga_0.47As/AlAs/InP structures is analyzed in detail for various values of the dc voltage V _dc in the negative differential resistance (NDR) region. It is shown that double-well three-barrier structures are promising for the design of terahertz-band oscillators. The presence of two resonant states with close energies in such structures leads to a resonant (in frequency) response whose frequency is determined by the energy difference between these levels and can be controlled by varying the parameters of the structure. It is shown that, in principle, such structures admit narrow-band amplification, tuning of the amplification frequency, and a fine control of the amplification (oscillation) frequency in a wide range of terahertz frequencies by varying a dc electric voltage applied to the structure. Starting from a certain width of the central intermediate barrier in double-well structures, one can observe a collapse of resonances, where the structure behaves like a single-well system. This phenomenon imposes a lower limit on the oscillation frequency in three-barrier resonant tunneling structures.     

The effect of an external electric field on the optical properties of a quantum molecule with a resonance D(-)-state

The average binding energy and the level width for the resonant D^(-)-state in a quantum molecule have been calculated in the presence of an external electric field. The calculations were performed in the zeroradius potential model with allowance for the tunneling decay of the resonant state. The external electric field is shown to stimulate the decay of resonant D^(-)-states under conditions of dissipative tunneling. It was found that the curve of the probability of photoionization of the D^(-)-center as a function of the external electric field strength has two peaks that are connected with a change in the symmetry of the double-well oscillator potential of the quantum molecule and with the transformation (caused by the electric field) of envelope wave functions, respectively.     

Nonresonant transparency channels of a two-barrier nanosystem in an electromagnetic field with an arbitrary strength

The theory of the quasistationary spectrum of the system of electrons interacting with a high-frequency electromagnetic field with an arbitrary strength in a two-barrier resonant tunneling nanostructure has been proposed on the basis of the exact solution of the total one-dimensional Schrödinger equation. It has been shown that nonresonant transparency channels in the planar two-barrier resonant tunneling structure appear owing to the superposition of a certain quasistationary state with first field resonance harmonics of both nearest states; this superposition is due to the interaction of electrons with the electromagnetic field.     

Dynamics of electrons in gradient nanostructures (exactly solvable model)

A flexible multi-parameter exactly solvable model of potential profile, containing an arbitrary number of continuous smoothly shaped barriers and wells, both equal or unequal, characterized by finite values and continuous profiles of the potential and of its gradient, is presented. We demonstrate an influence of both gradient and curvature of these potentials on the electron transport and spectra of symmetric and asymmetric double-well (DW) potentials. The use of this model is simplified due to one to one correspondence between the algorithms of calculation of the transmittance of convex barriers and energy spectra of concave wells. We have shown that the resonant contrast between maximum and minimum in over-barrier reflectivity of curvilinear barrier exceeds significantly the analogous effect for rectangular barrier with the same height and width. Reflectionless tunneling of electrons below the bottom of gradient nanostructures forming concave potential barriers is considered. The analogy between dynamics of electrons in gradient fields and gradient optics of heterogeneous photonic barriers is illustrated.     

Macroscopic resonant tunneling in the presence of low frequency noise

We develop a theory of macroscopic resonant tunneling of flux in a double-well potential in the presence of realistic flux noise with a significant low-frequency component. The rate of incoherent flux tunneling between the wells exhibits resonant peaks, the shape and position of which reflect qualitative features of the noise, and can thus serve as a diagnostic tool for studying the low-frequency flux noise in SQUID qubits. We show, in particular, that the noise-induced renormalization of the first resonant peak provides direct information on the temperature of the noise source and the strength of its quantum component.     

High quantum efficiency of intersubband transitions in coherent tunneling of electrons through asymmetric double-barrier structures

A converging perturbation series that can be summed analytically has been obtained for intersubband transitions of electrons coherently tunneling through the middle of a dimensionally quantized level in an asymmetric double-barrier structure in a high-frequency terahertz electric field. The possibility of a substantial increase in tunneling current accompanied by either absorption or emission of a photon has been demonstrated. The quantum efficiency of radiative transitions between dimensionally quantized levels can be up to 66%. Zh. éksp. Teor. Fiz. 112, 237–245 (July 1997)     

Resonance effect of a magnetic quantum ring on the conductance of a two-dimensional sample

The resonant tunneling of electrons through a magnetic quantum ring in a two-dimensional sample and the effect of this resonant tunneling on the conductivity of the sample are studied in this work. The ballistic conductance of the sample is calculated for several values of the ratio (inner radius)/(outer radius) of the ring. The limit of a conductance plateau is recovered for approximately equal radii and previous results for the conductance of a magnetic quantum dot are recovered in the case of an approximately zero inner radius.     

Electrostatic correction to the square-potential model of nanostructures significantly affects current resonances

Depletion of carriers in nanostructures generates an electrostatic correction to the square potential model. This correction changes the tunneling probability and consequently affects current resonances. We analyze this effect numerically for the double-barrier quantum well (DBQW) at various temperatures and carrier densities, and conclude that the electrostatic correction significantly affects the resonant voltage, the current at resonance, the peak-to-valley ratio, and may even cause disappearance of the lowest resonance.     

High-frequency response of two-well nanostructures

A consistent analytical theory is developed for coherent resonant electron tunnelling in a two-well nanostructure in the presence of a weak electromagnetic field. Simple expressions derived for the transmission coefficient and linear response of the two-well nanostructure make it possible to clarify the physics of processes and to express the gain as a function of the structure parameters. It is shown that the high-frequency response of the two-well nanostructure considerably exceeds the response of a one-well structure (resonance-tunnel diode) and that the application of a constant electric field makes it possible to tune the oscillation frequency and to increase the gain. It is concluded that two-well nanostructures can be used in designing terahertz oscillators. It is shown that, in contrast to a resonance-tunnel diode, interference of electrons between the wells and radiative “laser-type” transition play a decisive role in such structures.     

Effect of frequency dependent electron-electron interaction on resonant tunneling

In electron resonant tunneling through a double barrier structure, we show that dynamical electron-electron interactions in the resonant well can give rise to additional tunneling satellites due to collective electronic excitations. We present a first principle treatment for frequency-dependent electron-electron interactions in the resonant tunneling problem. The result confirms the previously proposed plasmon assisted resonant tunneling mechanism. We also find that the particle-hole excitation has very little effect on resonant tunneling. Our result can be applied to study the effects of various electronic excitations on the resonant tunneling of electrons.     

Conductance of a quantum ring with spin-orbit interaction in the presence of an impurity

The conductance of a quantum ring has been calculated on the basis of the tunneling Hamiltonian in the quasiballistic regime of the motion of electrons with allowance for the spin-orbit interaction. The effect of the scattering of electrons by a single short-range interacting impurity in the quantum ring on the tunneling electron current is analyzed. Two types of impurities, spinless and paramagnetic, are considered. The conductance symmetry is discussed for various electron-spin orientations with respect to change in the sign of the magnetic flux through the quantum ring.     

Spin-orbit coupling and anisotropy of spin splitting in quantum dots

In lateral quantum dots, the combined effect of both Dresselhaus and Bychkov-Rashba spin-orbit coupling is equivalent to an effective magnetic field +/- B(SO) which has the opposite sign for s(z)= +/- 1/2 spin electrons. When the external magnetic field is perpendicular to the planar structure, the field B(SO) generates an additional splitting for electron states as compared to the spin splitting in the in-plane field orientation. The anisotropy of spin splitting has been measured and then analyzed in terms of spin-orbit coupling in several AlGaAs/GaAs quantum dots by means of resonant tunneling spectroscopy. From the measured values and sign of the anisotropy we are able to determine the dominating spin-orbit coupling mechanism.     

半导体量子电子和光电子器件

本文阐述了半导体异质结构电子的量子特性 ,如电子波输运、库仑阻塞效应等。介绍了几种新颖、典型的量子电子器件和量子光电子器件的物理模型和基本原理。这些器件包括了单电子晶体管、共振隧穿二极管、高电子迁移率晶体管、δ掺杂场效应晶体管、量子点元胞自动机、量子阱红外探测器、埋沟异质结半导体激光器、量子级联激光器等。给出了作者在半导体量子器件物理方面的最新研究结果。     

双量子阱中光子辅助电子自旋隧穿

采用单电子有效质量近似理论, Floquet理论和传递矩阵方法, 对包含时间周期场的双量子阱中单电子的自旋隧穿特性进行了研究, 对InP/InAs半导体材料进行了数值计算. 重点研究了Rashba型和Dresselhaus型自旋轨道耦合、量子阱结构以及偏压对电子隧穿的影响. 这些结果可以为设计和调控半导体自旋电子器件提供一定的理论依据. 关键词： 光子辅助隧穿 隧穿概率 量子阱     

Fano-type spin-flip mesoscopic transport through an Aharonov-Bohm interferometer

We have investigated the mesoscopic transport properties of a quantum dot embedded Aharonov-Bohm (AB) interferometer applied with a rotating magnetic field. The spin-flip effect is induced by the rotating magnetic field, and the tunneling current is sensitive to the spin-flip effect. The spin-flipped electrons tunneling from the direct channel and the resonant channel interfere with each other to form spin-polarized tunneling current components. The non-resonant tunneling (direct transmission) strength and the AB phase φ play important roles. When the non-resonant tunneling (background transmission) exists, the spin and charge currents form asymmetric peaks and valleys, which exhibit Fano-type line shapes by varying the source-drain bias voltage, or gate voltage. The AB oscillations of the spin and charge currents exhibit distinct dependence on the magnetic flux and direct tunneling strength.     

Controlled dephasing of a quantum dot: from coherent to sequential tunneling

Resonant tunneling through two identical potential barriers renders them transparent, as particle trajectories interfere coherently. Here we realize resonant tunneling in a quantum dot (QD), and show that detection of electron trajectories renders the dot nearly insulating. Measurements were made in the integer quantum Hall regime, with the tunneling electrons in an inner edge channel coupled to detector electrons in a neighboring outer channel, which was partitioned. Quantitative analysis indicates that just a few detector electrons completely dephase the QD.     

Macroscopic tunnel splittings in superconducting phase qubits

Prototype Josephson-junction based qubit coherence times are too short for quantum computing. Recent experiments probing superconducting phase qubits have revealed previously unseen fine splittings in the transition energy spectra. These splittings have been attributed to new microscopic degrees of freedom (microresonators), a previously unknown source of decoherence. We show that macroscopic resonant tunneling in the extremely asymmetric double-well potential of the phase qubit can have observational consequences that are strikingly similar to the observed data.     

有效质量法调控原子玻色-爱因斯坦凝聚体的双阱动力学

操控原子玻色-爱因斯坦凝聚体在双势阱中的动力学通常是通过改变势阱深度来实现,本文提出了一种基于调节原子有效质量的控制方案,可以在不改变双阱势的前提下操控凝聚体的双阱动力学.利用双模近似,本文解析地导出了超冷原子在双阱势中的隧穿强度和相互作用强度对有效质量的依赖关系,并基于平均场近似数值模拟了在有效质量调节下的凝聚体动力学演化,展示了隧穿振荡和自束缚等典型的双阱动力学行为.此外,本文的研究还发现,借助负有效质量效应,这一方案甚至可以等效地实现对负散射长度原子凝聚体双阱动力学行为的操控.     

Competition between carrier recombination and tunneling in quantum dots and rings under the action of electric fields

In the presence of a stationary electric field applied in the growth direction tunneling of electrons out of the quantum dots can take place. This mechanism competes with the quantum confined Stark effect (QCSE) that produces an increase of the exciton lifetime by increasing the electric field, mainly due to a net decrease of the electron–hole wavefunction overlap. The electric field range where QCSE dominates over tunneling will be mainly determined by the size of the nanostructure along the vertical direction (height), as demonstrated in this work.     

RESONANT TUNNELING THROUGH COUPLED DOUBLE-QUANTUM-WELL

A double-well resonant tunneling structure has been investigated carefully using the nonequilibrium Green's function method. We find that in the transmission probability two maxima appear even when the two levels have the same energy. This characteristic is at-tributed to the resonant tunneliug through mixed quasibound states. The tunneling current formula through this system under a dc voltage has been derived exactly. Three different cases are considered and several novel properties are found, which manifest coherent charac-teristics of the tunneling process.