Photogalvanic effect in asymmetric quantum wells and superlattices

We have investigated the influence of the final states of bound-to-continuum transitions within the conduction band of asymmetric quantum well structures on the photocurrent. This influence manifests itself by an energy-dependent oscillation of the current direction. We observe pronounced oscillations at zero bias voltage in a double quantum well structure, induced by an asymmetric excitation into continuum states with positive and negative momentum, i.e. by a photogalvanic effect (PGE). If this effect is superimposed on an asymmetric backrelaxation, similar oscillations are observed in the spectrum when the latter asymmetry is compensated by an external electric field. Theoretically, we find a strong relation between the PGE and a quantum interference effect occurring in the continuum.     

Quasibound states in graphene quantum-dot nanostructures generated by concentric potential barrier rings

We study the quasibound states in a graphene quantum-dot structure generated by the single-, double-, and triple-barrier electrostatic potentials. It is shown that the strongest quasibound states are mainly determined by the innermost barrier. Specifically, the positions of the quasibound states are determined by the barrier height, the number of the quasibound states is determined by the quantum-dot radius and the angular momentum, and the localization degree of the quasibound states is influenced by the width of the innermost barrier, as well as the outside barriers. Furthermore, according to the study on the double- and triple-barrier quantum dots, we find that an effective way to generate more quasibound states with even larger energy level spacings is to design a quantum dot defined by many concentric barriers with larger barrier-height differences. Last, we extend our results into the quantum dot of many barriers, which gives a complete picture about the formation of the quasibound states in the kind of graphene quantum dot created by many concentric potential barrier rings.     

Magnetotransport in a graphene monolayer with two tunable magnetic barriers

The magnetotransport property for a monolayer graphene with two turnable magnetic barriers has been investigated by the transfer-matrix method. We show that the parameters of barrier height, width, and interval between two barriers affect the electron wave decaying length, which determine the conductance with parallel or antiparallel magnetization configuration, and consequently the tunneling magnetoresistance (TMR) for the system. Interestingly, a graphene attached by two barriers with different heights can produce a resonant TMR peak at low energy region one order of magnitude larger than that for the system with two same height barriers because that the asymmetry of magnetic barriers block the electron transmission in the case of antiparallel magnetization configuration. The results obtained here may be useful in understanding of electron tunneling in graphene and in designing of graphene-based nanodevices.     

Quantum friction controlled by plasmons between graphene sheets

We theoretically study quantum friction between two infinite graphene sheets, which is controlled by plasmons excited at the interfaces of graphenes and dielectrics. In near-field regime, quantum friction can be enhanced due to the coupling of plasmons between two graphene sheets. Dependences of friction coefficient on distance, chemical potential of graphene, temperature of environment, and dielectric constant of substrate have been investigated in detail. Friction coefficient can be increased by increasing temperature or dielectric constants of substrates, and can be reduced by increasing distance or chemical potential.     

Entrance channels and alpha decay half-lives of the heaviest elements

The barriers standing against the formation of superheavy elements and their consecutive decay have been determined in the quasimolecular shape path within a Generalized Liquid Drop Model including the proximity effects between nucleons in a neck, the mass and charge asymmetry, a precise nuclear radius and the shell effects given by the Droplet Model. For moderately asymmetric reactions double-hump potential barriers stand and fast fission of compact shapes in the outer well is possible. Very asymmetric reactions lead to one hump barriers which can be passed only with a high energy relatively to the superheavy element energy. Then, only the emission of several neutrons or an particle can allow to reach an eventual ground state. For almost symmetric heavy-ion reactions, there is no more external well and the inner barrier is higher than the outer one. Predictions for partial decay half-lives are given.     

Enhanced spin polarization in an asymmetric magnetic graphene superlattice

The authors investigate the spin-resolved transport through an asymmetrical magnetic graphene superlattice (MGS) consisting of the periodic barriers with abnormal one in height. To quantitatively depict the asymmetrical MGS, an asymmetry factor has been introduced to measure the height change of the abnormal barrier. It is shown that the spin filter effect is strongly enhanced by the barrier asymmetry both in the Klein and the classical tunneling regimes. In the presence of abnormal barrier, the conductance with certain spin direction is suppressed with respect to different tunneling regimes, and thus high spin polarization with opposite sign can be achieved.     

A study of GaInP/GaAs interfaces: metallurgical coupling of successive quantum wells

We present a study of GaInP/GaAs interfaces by means of photoluminescence (PL) of multi quantum wells (MQW), embedded in GaInP, or asymmetric structures having an AlGaAs barrier GaInP/GaAs/AlGaAs. The PL energies of quantum wells were compared with calculations based on the transfer matrix envelope function approximation, well suited for asymmetric structures. GaInP/GaAs/AlGaAs MQW structures (GaInP grown first) are in reasonably good agreement with calculations. Reverse ones, AlGaAs/GaAs/GaInP, present a lower PL energy than calculated. But the agreement with theory is recovered on single quantum well samples, or in MQW when the GaInP thickness is increased up to 100 nm. We interpret this phenomenon as a diffusion of arsenic atoms from the next GaAs well through the GaInP barrier. Arsenic atoms exchange with phosphorus atoms at the GaInP-on-GaAs interface of the former well, leading to a small gap strained InGaAs region responsible for the lowering of PL energies.     

Depolarization of white light generated by ultrashort laser pulses in optical media

We report measurements of the extinction ratio (ER) of white light generated upon irradiation of BK7 glass by ultrashort (36 fs) laser pulses with incident power approximately 10(3) times larger than the critical power for self-focusing. At low incident powers, the continuum is symmetric about the incident laser wavelength; at high powers it becomes broader and distinctly asymmetric towards the blue side. We observe that ER degrades by 100-fold after the onset of multiphoton-induced free-electron generation (at incident intensity approximately 2 TW cm-2), which also corresponds to the onset of asymmetry in white-light spectra.     

Effects of the entrance channel mass asymmetry in fusion reactions

The symmetric and asymmetric fusion reaction systems forming the same compound nuclei 26Al,30Si,38Ar and 170Hf are investigated with the frame of improved isospin dependent quantum molecular dynamics model.The entrance channel mass asymmetry dependence of compound nucleus formation is found by analyzing the shell correction energies,the Coulomb barriers and the fusion cross sections.The calculated fusion cross sections agree quantitatively with the experimental data.The results indicate that compound nucleus formation is favorable for the systems with larger mass asymmetry because of the smaller Coulomb contribution to the fusion barrier.     

Guided modes in asymmetric graphene waveguides

An asymmetric quantum well in graphene can act as a slab waveguide for electron waves in a manner analogous to the electromagnetic waves in dielectrics. Guided modes and the probability current density are analyzed in the graphene electron waveguide induced by asymmetric electrostatic potential. The modes in an asymmetric graphene waveguide include guided modes, “cover modes”, “substrate modes” and “radiation modes”. The conditions for a guided mode are quantified. It is found that the fundamental mode is absent when both the Klein tunneling and classical motion are present. The confinement of electrons for lower order mode is stronger than for higher order mode. We hope that these characteristics in asymmetric graphene waveguide can provide potential applications in graphene-based waveguide devices.     

Energy and effective mass of a polaron in asymmetric semiconductor quantum well structures

In this paper, the correct electron extended states wave functions and the density of states in asymmetric single quantum wells (QWs) are given for the first time, we put right mistakes from some previous papers of some other authors. Within the framework of the secondorder perturbation theory, the ground-state polaron binding energy and effective mass correction in asymmetric single QWs are studied including the full energy specturm, i.e., the discrete energy levels in the well and the continuum energy spectrum above the barrier, and all possible optical-phonon modes. The effects of the finite electronic confinement potential and the subband nonparabolicity are considered. The relative importance of the different phonon modes is investigated. Our results show that the polaron energy and effective mass are sensitive to the asymmetry of the structure and have a close relation to the interface phonon dispersion. When well width and one side barrier height of asymmetric QWs are fixed and identical with those of symmetric QW, the polaron binding energy and effective mass in asymmetric QWs are always less than those in symmetric QW. It is necessary to include the continuum energy spectrum as intermediate states in the study of polaron effects in QWs in order to obtain the correct results. The subband non-parabolicity has little influence on the polaron effects. The polaron energies given in this paper are excellent agreement with our variational results.     

Barrier height and negative tunnel magnetoresistance

Based on the free-electron approximation method proposed by Slonczewski, we substitute the finite magnetic zone by a semi-infinite magnet. On this basis, the relationship between the tunnel magnetoresistance (TMR) and the barrier height of magnetic tunnel junction (MTJ) is studied. We find the TMR at small bias is always positive for various barrier heights when the MTJ has a symmetric configuration and the negative TMR can be observed when MTJ is with lower barrier height in the asymmetric condition.     

Intense laser field effect on the interband absorption in differently shaped near-surface quantum wells

The 1S-exciton properties and interband absorption spectra in differently shaped near-surface quantum wells (NSQWs) with symmetrical/asymmetrical barriers, under intense laser field, are investigated taking into account the correct dressing effect for the confinement potential and electrostatic interaction between carriers and their image-charges. We found that: i) the 1S-exciton binding energy is significantly reduced by the laser intensity in InGaAs NSQWs of different asymmetrical shape; ii) the red-shift of the absorption peak induced by the asymmetry diminution or by increasing cap layer thickness can be effectively compensated using the blue-shift caused by enhancing laser parameter. Therefore, the optical properties of the differently shaped NSQWs could be tuned by proper tailoring of the heterostructure parameters (well shape, barrier asymmetry) and/or dielectric mismatch as well as by varying the laser field intensity.     

Intersubband transitions in an asymmetric double quantum well

The intersubband optical absorption in an asymmetric double quantum well for different barrier widths and the right well widths are theoretically calculated within the framework of effective mass approximation. The results obtained show that the intersubband transitions and the energy levels in an asymmetric double quantum well can be importantly modified and controlled by the barrier width and the well width. The sensitivity to the barrier and well widths of the absorption coefficient can be used in various optical semiconductor device applications.     

石墨烯过渡层对金属/SiC接触肖特基势垒调控的第一性原理研究

由于SiC禁带宽度大,在金属/SiC接触界面难以形成较低的势垒,制备良好的欧姆接触是目前SiC器件研制中的关键技术难题,因此,研究如何降低金属/SiC接触界面的肖特基势垒高度(SBH)非常重要.本文基于密度泛函理论的第一性原理赝势平面波方法,结合平均静电势和局域态密度计算方法,研究了石墨烯作为过渡层对不同金属(Ag,Ti,Cu,Pd,Ni,Pt)/SiC接触的SBH的影响.计算结果表明,单层石墨烯可使金属/SiC接触的SBH降低;当石墨烯为2层时,SBH进一步降低且Ni,Ti接触体系的SBH呈现负值,说明接触界面形成了良好的欧姆接触;当石墨烯层数继续增加,SBH不再有明显变化.通过分析接触界面的差分电荷密度以及局域态密度,SBH降低的机理可能主要是石墨烯C原子饱和了SiC表面的悬挂键并降低了金属诱生能隙态对界面的影响,并且接触界面的石墨烯及其与金属相互作用形成的混合相具有较低的功函数.此外,SiC/石墨烯界面形成的电偶极层也可能有助于势垒降低.     

Beating oscillation and Fano resonance in the laser assisted electron transmission through graphene δ-function magnetic barriers

We investigate theoretically the transmission of electrons through a pair of δ-function magnetic barriers in graphene in presence of external monochromatic, linearly polarized and CW laser field. The transmission coefficients are calculated in the framework of non-perturbative Floquet theory using the transfer matrix method. It is noted that the usual Fabry–Perot oscillations in transmission through the graphene magnetic barriers with larger inter barrier separation takes the shape of beating oscillations in presence of the external laser field. The laser assisted transmission spectra are also found to exhibit the characteristic Fano resonances (FR) for smaller values of the inter barrier separation. The appearance of the perfect node in the beating oscillation and the asymmetric Fano line shape can be controlled by varying the intensity of the laser field. The above features could provide some useful and potential information about the light - matter interactions and may be utilized in the graphene based optoelectronic device applications.     

石墨烯过渡层对金属/SiC接触肖特基势垒调控的第一性原理研究

由于SiC禁带宽度大,在金属/SiC接触界面难以形成较低的势垒,制备良好的欧姆接触是目前SiC器件研制中的关键技术难题,因此,研究如何降低金属/SiC接触界面的肖特基势垒高度(SBH)非常重要.本文基于密度泛函理论的第一性原理赝势平面波方法,结合平均静电势和局域态密度计算方法,研究了石墨烯作为过渡层对不同金属(Ag,Ti,Cu,Pd,Ni,Pt)/SiC接触的SBH的影响.计算结果表明,单层石墨烯可使金属/SiC接触的SBH降低;当石墨烯为2层时,SBH进一步降低且Ni,Ti接触体系的SBH呈现负值,说明接触界面形成了良好的欧姆接触;当石墨烯层数继续增加,SBH不再有明显变化.通过分析接触界面的差分电荷密度以及局域态密度,SBH降低的机理可能主要是石墨烯C原子饱和了SiC表面的悬挂键并降低了金属诱生能隙态对界面的影响,并且接触界面的石墨烯及其与金属相互作用形成的混合相具有较低的功函数.此外,SiC/石墨烯界面形成的电偶极层也可能有助于势垒降低.     

Size quantization in planar graphene-based heterostructures: Pseudospin splitting,interface states,and excitons

A planar quantum-well device made of a gapless graphene nanoribbon with edges in contact with gapped graphene sheets is examined. The size-quantization spectrum of charge carriers in an asymmetric quantum well is shown to exhibit a pseudospin splitting. Interface states of a new type arise from the crossing of dispersion curves of gapless and gapped graphene materials. The exciton spectrum is calculated for a planar graphene quantum well. The effect of an external electric field on the exciton spectrum is analyzed.     

Vertical transport, transmission coefficients, and dwell time in asymmetric quantum well structures

The transport of hot electrons in the Al_xGa_1−xAs barriers above the wells in a multiple quantum well (MQW) structure is investigated. The structures that are studied are asymmetric quantum well infrared detectors. The transport of the hot electrons normal to the layers is strongly dependent on both voltage and well shape. It is suggested that the key parameter which affects the transport properties is the dwell time of the electrons in the continuum, above the well region. This is most readily seen in asymmetric MQW structures, in which the dwell time under an applied bias depends very strongly on bias polarity. Calculations of electron transmission coefficient and dwell time show that the electron mean free path in asymmetric wells is much larger in positive bias than in a negative one. Employing this model, we achieve a very good fit to experimental data.     

一种特殊的非对称量子阱中的电光效应

运用密度矩阵方法推导出了特殊非对称量子阱中电光系数的解析表达式,并以典型的GaAs/AlGaAs非对称量子阱为例进行了数字计算.计算结果表明,量子阱的非对称性随着参数a的增大而增强,随着参数V₀的增大而减小.电光系数的最大值也随着参数a的增大而增大,随着参数V₀的增大而减小,表明电光系数将随着量子阱非对称性的增大而增大.在取不同的参数a和不同的参数V₀时,电光系数和入射光子能量的关系分别被绘制成曲线图.在图中分别有三个不同的峰,而且系统的非对称性越大,峰值就越大.随着量子阱非对称性的增大,曲线中的峰向能量低的方向移动.另外,在这种量子阱中得到了比较大的电光系数,大约在10^-6m/V量级.随着近来纳米制作技术的进步,使得在实验上制作这种特殊非对称量子阱并得到较好的非线性材料成为可能.