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.     

Quasibound states in semiconductor quantum well structures

We present a study on quasibound states in multiple quantum well structures using a finite element model (FEM). The FEM is implemented for solving the effective mass Schrödinger equation in arbitrary layered semiconductor nanostructures with an arbitrary applied potential. The model also includes nonparabolicity effects by using an energy dependent effective mass, where the resulting nonlinear eigenvalue problem was solved using an iterative approach. We focus on quasibound/continuum states above the barrier potential and show that such states can be determined using cyclic boundary conditions. This new method enables the determination of both bound and quasibound states simultaneously, making it more efficient than other methods where different boundary conditions have to be used in extracting the relevant states. Furthermore, the new method lifted the problem of quasibound state divergence commonly seen with many other methods of calculation. Hence enabling accurate determination of dipole matrix elements involving both bound and quasibound states. Such calculations are vital in the design of intersubband optoelectronic devices and reveal the interesting properties of quasibound states above the potential barriers.     

Lifetime analysis of quasibound states in coupled quantum wells

A simple time-dependent model is presented to investigate lifetimes of the quasibound states in coupled quantum wells (CQWs). The transfer matrix approach is employed to discretize the conduction-band profile of the heterostructure and form a dispersion equation whose zeros correspond to the complex eigenenergies. Both the bound and quasibound states are extracted numerically in the complex plane by Newton's method. The lower and higher well subbands are found to have negative and positive energy shift, respectively, as following the no level crossing theorem. Besides, the decay rate of the quasibound state is approximately proportional to the absolute energy shift. The quasibound states, which have larger energy shift, have shorter lifetime and decay more quickly. Furthermore, the differences in lifetime between the quasibound states in CQWs can be easily realized as all the wave functions are specially adjusted to form the relative probability density distributions.     

Electron Conductance and Lifetimes in a Ballistic Electron Waveguide

Ballistic electron waveguides are open quantum systems that can be formed at very low temperatures at a GaAs/AlGaAs interface. Dissipation due to electron–phonon and electron–electron interactions in these systems is negligible. Although the electrons only interact with the walls of the waveguide, they can have a complicated spectrum including both positive energy bound states and quasibound states which appear as complex energy poles of the scattering S-matrix or energy Green's function. The quasibound states can give rise to zeros in the waveguide conductance as the energy of the electrons is varied. The width of the conduction zeros is determined by the lifetimes of the quasibound states. The complex energy spectrum associated with the quasibound states also governs the survival probability of electrons placed in the waveguide cavities.     

The quasibound state model for self-consistent characteristics of semiconductor intersubband devices

The quasibound state model (QBSM) for determining the self-consistent conduction band profile and space charge density of semiconductor intersubband devices is presented. This new method is based on the quasibound (QB) state resonances of quantum structures. For heterostructures, the traditional self-consistent energy continuum model (ECM) calculates space charge by integration over the entire energy continuum, weighted by Fermi–Dirac statistics. In the present approach, the continuum of energy states of the heterostructure is accurately represented by a small number of QB states, and the space charge calculations are performed only at these eigen-energies. This approach significantly reduces the computational burden associated with all self-consistent algorithms. Theoretical formulation of QBSM is compared with the traditional ECM approach. The bound (B) and QB eigenenergies of the structure are obtained by solving the single-band effective-mass Schrödinger equation using the argument principle method. The performance and the accuracy of the QBSM are evaluated for a double-barrier resonant structure and an asymmetric Fabry–Perot electron-wave interference filter. The self-consistent electron density and potential profiles calculated by the present method are shown to be in excellent agreement with the results obtained from the traditional ECM model. In addition to requiring less computational time, the present method is easily implemented and may be applied equally well to biased/unbiased, symmetric/asymmetric heterostructures.     

Transmission resonance via quantum bound states in confined arrays of antidots

We have studied the transmission resonances for a confined array of antidots, using the lattice Green's function method. Two kinds of resonant peaks via quasibound states are found. One kind of resonant peak corresponds to the split quasibound states. The split states originate from the superposition of quasibound states respectively localized in different (T or crossed) junctions, while the number of quasibound states in each junction is associated with the arm-width of the junction. Electrons in these split states are mainly localized in the junctions. The other kind of resonant peaks correspond to the high quasibound states which exist in (transverse and longitude) multi-period confined arrays of antidots. It is interesting to note that electrons in some of the high quasibound states are mainly localized in the intersection of the junctions rather than in the junctions themselves.     

Quasibound states in the continuum in a two channel quantum wire with an adatom

We report the prediction of quasibound states (resonant states with very long lifetimes) that occur in the eigenvalue continuum of propagating states for certain systems in which the continuum is formed by two overlapping energy bands. We illustrate this effect using a quantum wire system with two channels and an attached adatom. When the energy bands of the two channels overlap, a would-be bound state that lays just below the upper energy band is slightly destabilized by the lower energy band and thereby becomes a resonant state with a very long lifetime (a second such state lays above the lower energy band). Unlike the bound states in continuum predicted by von Neumann and Wigner, these states occur for a wide region of parameter space.     

(K)-nucleus dynamics: from quasibound states to kaon condensation

Coupled-channel (K)N dynamics near threshold and its repercussions in few-body (K)-nuclear systems are briefly reviewed, highlighting studies of a K-pp quasibound state. In heavier nuclei, the extension of mean-field calculations to multi-(K) nuclear and hypernuclear quasibound states is discussed. It is concluded that strangeness in finite self-bound systems is realized through hyperons, with no room for kaon condensation.     

Aharonov–Bohm effect in Kane-type semiconductor quantum wire

In the present study, the Aharonov–Bohm effect for bound states in Kane-type quantum wire is analysed. It is demonstrated that the wave function and the energy spectra of carriers depend on the magnetic flux. The energy and the g factor of electrons are of a periodic function of the magnetic flux. The statistical property of the orbital magnetism of electrons at low temperatures and strong magnetic fields is presented. It is shown that the magnetisation of electron oscillations depends on the magnetic flux.     

Resonant transport and quantum bound states in Z-shaped graphene nanoribbons

We study the transport properties of a Z-shaped graphene nanoribbon (GNR). It is found that the quasibound states in the Z-shaped junction induce resonant peaks around the Dirac point in the conductance profile. The resonant transmission via the quantum bound state is very sensitive to the size of the junction. The number and also the lifetimes of the quasibound states increase with the size of the Z-shaped junction. Long lifetime bound states which do not induce obvious resonant peaks exist in the junction with a wider or longer zigzag edged GNR. The resonant characteristics of the Z-shaped GNR can be tuned by the variation of the geometrical size.     

Fano resonance in graphene anti-dot and periodic graphene anti-dot arrays

We study the electron transport properties of graphene anti-dot and periodic graphene anti-dot arrays using the nonequilibrium Green?s function method and Landauer–Büttiker formula. Fano resonant peaks are observed in the vicinity of Fermi energy, because discrete states coexist with continuum energy states. These peaks move closer to Fermi energy with increasing the width of anti-dots, but move away from the Fermi energy with increasing the length of anti-dots. When N periodic anti-dots exist in the longitude direction, a rapid fluctuation appears in the conductance with varying resonance peaks, which is mainly from the local resonances created by quasibound state. When P periodic anti-dots exist in the transverse direction, P-fold resonant splitting peaks are observed around the Fermi energy, owing to the symmetric and antisymmetric superposition of quasibound states.     

Resonance-enhanced group delay times in an asymmetric single quantum barrier

It is shown that transmission and reflection group delay times in an asymmetric single quantum barrier are greatly enhanced by the transmission resonance when the energy of incident particles is larger than the height of the barrier. The resonant transmission group delay is of the order of the quasibound state lifetime in the barrier region. The reflection group delay can be either positive or negative, depending on the relative height of the potential energies on the two sides of the barrier. Its magnitude is much larger than the quasibound state lifetime. These predictions have been observed in microwave experiments.     

一氟化碳电子态的光谱性质和预解离机理的理论研究

采用考虑Davidson修正的内收缩多参考组态相互作用(icMRCI+Q)方法结合相关一致基组aug-cc-pV5Z和aug-cc-pV6Z首次计算了一氟化碳(CF)11个Λ-S 态(X²Π , a⁴Σ^-, A²Σ⁺, B²Δ, 1⁴Π, 1²Σ^-, 2⁴Π, 1^{4}Δ , 1⁴Σ⁺, 2²Σ^-和2⁴Σ^-) 所产生的25个Ω 态的势能曲线. 计算中考虑了旋轨耦合效应、核价相关和标量相对论修正以及将参考能和相关能分别外推至完全基组极限. 基于得到的势能曲线, 获得了束缚和准束缚的Λ-S态和Ω 态的光谱常数, 与已有的实验结果非常符合. 分析了束缚和准束缚的Λ-S态在各自平衡核间距R_e处的主要电子组态. 由于1⁴Π 和2⁴Π态的避免交叉, 发现准束缚态2⁴Π. 由Λ-S态势能曲线的交叉现象, 借助于计算的旋轨耦合矩阵元, 分析了a⁴Σ^-和B²Δ 态的预解离机理. 计算了25个Ω 态的离解关系, 给出了它们的离解极限. 最后研究了A²Σ⁺-X²Π 跃迁特性, 本文计算得到的A²Σ⁺-X²Π跃迁的Frank-Condon 因子和辐射寿命与已有实验值也符合得非常好.     

Avalanche dynamics of magnetic flux in a two-dimensional discrete superconductor

The critical state of a two-dimensional discrete superconductor in an external magnetic field is studied. This state is found to be self-organized in the generalized sense, i.e., is a set of metastable states that transform to each other by means of avalanches. An avalanche is characterized by the penetration of a magnetic flux to the system. The sizes of the occurring avalanches, i.e., changes in the magnetic flux, exhibit the power-law distribution. It is also shown that the size of the avalanche occurring in the critical state and the external magnetic field causing its change are statistically independent quantities.     

Coherent electron transport in a helical nanotube

The quantum dynamics of carriers bound to helical tube surfaces is investigated in a thin-layer quantization scheme. By numerically solving the open-boundary Schrödinger equation in curvilinear coordinates, geometric effect on the coherent transmission spectra is analysed in the case of single propagating mode as well as multimode. It is shown that, the coiling endows the helical nanotube with different transport properties from a bent cylindrical surface. Fano resonance appears as a purely geometric effect in the conductance, the corresponding energy of quasibound state is obviously influenced by the torsion and length of the nanotube. We also find new plateaus in the conductance. The transport of double-degenerate mode in this geometry is reminiscent of the Zeeman coupling between the magnetic field and spin angular momentum in quasi-one-dimensional structure.     

Possibility of Studying Neutron–Neutron Correlations in Halo Nuclei

An experimental investigation of the reaction of core pickup from ⁶He and ¹¹Li two-neutronhalo nuclei is proposed. In such experiments, neutron–neutron correlations in a halo nucleus will be assessed on the basis of the energy of a neutron–neutron quasibound state. A detailed kinematical simulation of the reaction ⁶He + ²H → ⁶Li + (nn) →⁶ Li + n + n is performed. It is shown that the energy of the quasibound state in question can determined from the shape of the energy spectrum of neutrons originating from the breakup of this state. In the proposed exclusive experiment, a beam of ⁶He (¹¹Li) nuclei with an energy of about 5 to 10 MeV per nucleon interacts with a deuterated-polyethylene target. This will permit detecting charged particles (⁶Li and ¹¹Be) and a neutron. On the basis of determining the energy of the neutron–neutron quasibound state, it will become possible to estimate the effective attraction between the valence neutrons in the field of the third particle (core).     

Theory of equilibrium flux lattice in UPt3 under magnetic field parallel to hexagonal crystal axis

We investigate Abrikosov lattice structures in the unconventional superconductor UPt (3) under magnetic field parallel to the hexagonal crystal axis. Only the two-dimensional E2 superconducting state among the many other states of different symmetry is compatible with the recent observation [A. Huxley et al., Nature (London) 406, 160 (2000)] of the flux lattice in the A phase misaligned with crystallographic directions. It is shown that the inequality of the London penetration depths in the basal plane directions resulting from the superposition of hexagonal crystal and superconducting state anisotropies leads for E2 to a slightly distorted triangular flux lattice.     

Approximate energies and thermal properties of a position-dependent mass charged particle under external magnetic fields

We solve the Schr?dinger equation with a position-dependent mass(PDM) charged particle interacted via the superposition of the Morse-plus-Coulomb potentials and is under the influence of external magnetic and Aharonov–Bohm(AB) flux fields. The nonrelativistic bound state energies together with their wave functions are calculated for two spatially-dependent mass distribution functions. We also study the thermal quantities of such a system. Further, the canonical formalism is used to compute various thermodynamic variables for second choosing mass by using the Gibbs formalism. We give plots for energy states as a function of various physical parameters. The behavior of the internal energy, specific heat, and entropy as functions of temperature and mass density parameter in the inverse-square mass case for different values of magnetic field are shown.     

扶手椅型石墨烯介观环中的持续电流

在紧束缚近似下, 解析求解了扶手椅型边界石墨烯介观环的能量本征值问题, 计算和讨论了不同大小尺寸的介观环中持续电流随Aharonov-Bohm (A-B)磁通的变化, 并证明了能级和持续电流关于磁通变化的周期性和特殊对称性. 研究表明, 持续电流显著地依赖于介观环的几何结构; 零能量附近的能级可以承载较大的持续电流, 而远离零能量的其他能级对持续电流的贡献很小.