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We consider magnetic graphene quantum dots (MGQDs) and study the impact of the Aharonov–Bohm (AB) flux and gap on the scattering process of electrons. Our emphasis is on the finite lifetimes of quasi-bound states arising from the interaction between electrons and the magnetic field within MGQDs. Initially, we calculate the scattering coefficients, scattering efficiency, and probability density by ensuring the continuity of eigenspinors at the boundary of MGQDs. The results indicate that as the gap increases, the quasi-bound states reach higher maxima. We show that an increase in AB-flux leads to a generation of quasi-bound states requiring less magnetic field, and the scattering efficiency starts to take non-zero values at smaller MGQD sizes. The analysis of probability density shows that the quasi-bound states, corresponding to non-resonantly excited scattering modes, exhibit a significant improvement in the concentrated density at MGQDs. The improvement is a result of reducing the diffraction phenomenon and suppressing the Klein effect through an increase in AB-flux and gap. This increases the probability of retaining the electron for a longer period of time. [ABSTRACT FROM AUTHOR]

A quantum particle constrained between two high potential barriers provides a paradigmatic example of a system sustaining quasi-bound (or resonance) states. When the system is prepared in one of such quasi-bound states, the wave function approximately maintains its shape but decays in time in a nearly exponential manner radiating into the surrounding space, the lifetime being of the order of the reciprocal of the width of the resonance peak in the transmission spectrum. Naively, one could think that adding more lateral barriers would preferentially slow down or prevent the quantum decay since tunneling is expected to become less probable and due to quantum backflow induced by multiple scattering processes. However, this is not always the case and in the early stage of the dynamics quantum decay can be accelerated (rather than decelerated) by additional lateral barriers, even when the barrier heights are arbitrarily large. The decay acceleration originates from resonant tunneling effects and is associated to large deviations from an exponential decay law. We discuss such a counterintuitive phenomenon by considering the hopping dynamics of a quantum particle on a tight-binding lattice with on-site potential barriers. [ABSTRACT FROM AUTHOR]

We study quasi-bound states of two electrons that arise in two-dimensional materials with a Mexican-hat dispersion (MHD) at an energy above its central maximum. The width of the resonance of the local density of states created by pairs is determined by the hybridization of atomic orbitals, due to which the MHD is formed. The mechanism of the quasi-bound state formation is due to the fact that effective reduced mass of electrons near the MHD top is negative. An unusual feature of quasi-bound states is that the resonance width can vanish and then they transform into bound states in continuum. We study in detail the quasi-bound states for topological insulators, when the MHD is due to the hybridization of inverted electron and hole bands. In this case, the resonance width is extremely small at weak hybridization. The highest binding energy is achieved for singlet quasi-bound pairs with zero angular number. • Quasi-bound pairs of electrons can arise in materials with a Mexican-hat dispersion. • The resonance width of the quasi-bound state can be extremely narrow. • Quasi-bound pair states can transform into bound states in a continuum. • The hybridization of atomic orbitals plays a key role in the formation and properties of quasi-bound pairs. [ABSTRACT FROM AUTHOR]

Mexican-hat dispersion of band electrons in two-dimensional materials attracts a lot of interest, mainly due to the Van Hove singularity of the density of states near the band edge. In this paper, we show that there is one more feature of such a dispersion, which also leads to nontrivial effects. It consists in the fact that the sign of the effective mass in the momentum space near the central extremum is opposite to the sign of the mass outside this region. For this reason, any localized potential that repels quasiparticles in the outer region attracts quasiparticles in the central region and thereby creates quasi-bound states. We study these states in the case when the Mexican-hat dispersion is formed due to the hybridization of the inverted electron and hole bands, and the potential is created by a point defect. The energy and width of the resonance of the local density of states corresponding to a quasi-bound state are found, and it is shown that, under certain conditions, a quasi-bound state can transform into a bound state in a continuum of band states. The presence of quasi-bound states leads to nontrivial effects in the spin-dependent scattering of electrons. Due to the quasi-bound state, the skew scattering is strongly enhanced for electrons with energy near the resonance, and the skewness angle varies over a wide range depending on the energy. In addition, in a certain energy range, a nontrivial effect of scattering suppression appears in the direction opposite to the skewness angle. • Repulsive impurity creates quasi-bound states in the band spectrum with a mexican-hat dispersion. • The quasi-bound states can transform into bound states in continuum. • Arising quasi-bound states give rise to resonances of strongly enhanced skew scattering. • Spin-dependent anti-resonances of the skew scattering appear in some direction. [ABSTRACT FROM AUTHOR]

We report an ab initio simulation and theoretical study of collective dynamics in liquid Sb at 973 K. An application of the GCM (generalized collective modes) theoretical approach to analysis of simulation-derived time correlation functions resulted in two types of propagating eigenmodes. We found that the almost flat dispersion of the high-frequency branch of propagating modes can be explained by out-of-phase oscillations of nearest neighbors which form quasi-bound atomic pairs for at least 30 ps. We discuss the features of collective dynamics in non-simple metallic melts containing quasi-bound pairs., (© 2023 IOP Publishing Ltd.)

The energy dependence of the tunneling coefficient for a cylindrical semiconductor nanowire, i.e., a one-dimensional electron gas, with two constrictions is studied. Using the R-matrix formalism the localization probabilities at the resonant energies can be computed. They give decisive information about the physical meaning of the resonant peaks and dips that appear. The nanowire with two constrictions yields a well-defined system for the experimental evidence of the quasi-bound states of the evanescent channels. Clearly marked dips due to them should appear in the linear conductance at low temperatures. Tunneling coefficient [ABSTRACT FROM AUTHOR]

The problem of quasi-bound states for ultra-relativistic Dirac electrons and holes in electric and magnetic quantum dots in graphene is discussed. It is shown that these states with a rather long lifetime appear in an electric quantum dot in the case of a large orbital momentum, and in a magnetic quantum dot if its dimensions exceed the Larmor radius of the electron. The quasibound state properties are analysed by using the local density of states technique the application of which is demonstrated by a simple one-dimensional model of the decaying state. In addition, the analogy between two-dimensional graphene and onedimensional polymers is discussed, which helps in understanding and interpreting the sophisticated features of the electron spectrum. [ABSTRACT FROM AUTHOR]

Abstract: The effects of molecular rotation on the dynamics of high-temperature nonequilibrium flows is investigated in this work, with an application to nitrogen shocked flows. The overall manifold of rovibrational levels for the ground electronic state of N2 is firstly obtained through the solution of the radial Schrödinger equation over a high-fidelity potential curve. The obtained solutions predict the existence of 9510 states, including quasi-bound states which may spontaneously dissociate through tunneling effects. The lifetimes of such quasi-bound states are determined using a semiclassical method, and most of them are found to be long-lived. A semi-empirical rate for state-specific rotational translations has then been applied so as to examine the importance of rotational dissociation against vibrational dissociation, in equilibrium conditions. Rotational dissociation is found to be one to two orders of magnitude below vibrational dissociation, from low temperatures up to 100,000K. The contribution of quasi-bound states to the overall dissociation rate is also found to be essentially negligible (increase of the rotational dissociation rate by a factor of 1.5–2 in the whole temperature range). Finally, as the full state-specific modeling of the interactions of this large number of rovibrational levels remains out of reach of the state of the art theories and computational resources, a simplified state-specific model for the simulation of shocked flows is proposed, treating vibration-translation and rotation-translation interactions in an uncoupled fashion. A Boltzmann equilibrium distribution is considered for each subset of rotational levels, for each of the molecular vibrational levels, at a characteristic rotational temperature . Rotational collisional numbers are calculated according to the method recently proposed by Park and confirm the rapid increase of the number of collisions needed for establishing rotational–translational equilibrium, as the gas temperature rises. The results of our simulations also confirm the claims by Park that for high temperatures (roughly above 12,000K), rotational–translational equilibration actually occurs after vibrational-translational equilibration, contrary to the popular belief. [Copyright &y& Elsevier]

Abstract: The ANKE spectrometer (Apparatus for studies of Nucleon and Kaon Ejectiles) situated at the COSY of the Froschungszentrum Jülich is well suited to investigate meson production in deuteron–proton collisions, especially close to threshold. By studying the reaction , differential and total cross sections as well as the meson mass have been determined with high accuracy. The very rapid rise of the total cross section to its maximum value within the first 0.5 MeV above the threshold implies a very large scattering length, i.e., a very strong final state interaction, and might indicate the presence of a quasi-bound state extremely close to threshold. In order to clarify the unambiguous meson mass situation, a new measurement investigating the reaction at 12 fixed energies at COSY-ANKE was conducted. Using this two-body reaction, the meson mass can be extracted from pure kinematics through the determination of the production threshold. This requires to measure the final state momenta and the associated beam momenta with highest precision. [Copyright &y& Elsevier]

In analogy with the definition of resonant or quasi-bound states used in three-dimensional quantal scattering, we define the quasi-bound states that occur in one-dimensional transmission generated by twin symmetric potential barriers and evaluate their energies and widths using two typical examples: (i) twin rectangular barrier and (ii) twin Gaussian-type barrier. The energies at which reflectionless transmission occurs correspond to these states and the widths of the transmission peaks are also the same as those of quasi-bound states. We compare the behaviour of the magnitude of wave functions of quasi-bound states with those for bound states and with the above-barrier state wave function. We deduce a Breit-Wigner-type resonance formula which neatly describes the variation of transmission coefficient as a function of energy at below-barrier energies. Similar formula with additional empirical term explains approximately the peaks of transmission coefficients at above-barrier energies as well. Further, we study the variation of tunnelling time as a function of energy and compare the same with transmission, reflection time and Breit-Wigner delay time around a quasi-bound state energy. We also find that tunnelling time is of the same order of magnitude as lifetime of the quasi-bound state, but somewhat larger. [ABSTRACT FROM AUTHOR]

Abstract: The quantum conductance for electrons scattering from a uniform scatterer in a narrow-wire semiconductor is calculated. Instead of getting the conductance directly from the calculation of transmission coefficient, we calculate the reflection coefficient instead. The transmission coefficient is then calculated by using the conservation law, . This alternative method can avoid the instability of the conductance obtained by including more evanescent modes for a finite-range scatterer in a narrow-wire semiconductor. This method is applied to a semi-infinite strip potential barrier and a rectangular potential barrier in a narrow wire. The quantum stepwise conductance is obtained in both cases. For a repulsive rectangular potential barrier, there are oscillations in each stepwise conductance. For an attractive rectangular potential barrier, there exist multiple quasi-bound states below the sub-band energies which can cause the drop of the quantum conductance. The effect of the continuum quasi-bound states diminishes as the energy of the incident electron increases, but the influence of the discrete quasi-bound states still persists. [Copyright &y& Elsevier]

Abstract: A full quantum-mechanical description of the quasi-bound states in parabolic quantum wells and barriers is presented. The GaAs/AlGaAs heterostructures are considered within the framework of the semi-empirical spin dependent tight-binding model and the surface Green function matching method (SGFM). Series of equidistant energy quasi-bound states are observed in the continuous spectrum and their FWHM and mean life time are calculated. Strong energy and spatial localizations are found in correlation with the large mean life times. [Copyright &y& Elsevier]

Abstract: In our previous work [S.J. Vlaev, V.M. González Robles, Phys. Stat. Sol.(c) 2 (2005) 3653.] we have developed a numerical scheme to calculate the mean lifetimes of virtual energy levels. In the present study, this scheme is applied to the quasi-bound electronic states in rectangular GaAs/AlGaAs quantum barriers. We have considered rectangular barriers of different widths and heights. The numerical calculations of the energy spectrum, the line shape and the mean lifetime are performed in a semi-empirical sp3s* tight-binding model including the spin. The SGFM method is applied to treat the interface problem [S. Vlaev, V.R. Velasco, F. García-Moliner, Phys. Rev. B 49 (1994) 11222]. We found that the mean lifetime increases significantly for thin barriers reaching values of nanoseconds and even more in some cases. The strong spatial localizations of the quasi-bound states correspond to their strong energy localizations and large mean lifetimes. The relation between the de Broglie wavelength of the barrier material and the barrier width determines the characteristics of the quasi-bound states for electrons and holes. A qualitative interpretation in terms of electron wave diffraction is suggested. [Copyright &y& Elsevier]

Abstract: The observed variation of the total cross section for the reaction near threshold means that the magnitude of the s-wave amplitude falls very rapidly with the η centre-of-mass momentum. It is shown here that recent measurements of the momentum dependence of the angular distribution imply a strong variation also in the phase of this amplitude. Such a behaviour is that expected from a quasi-bound or virtual state. The interpretation can be investigated further through measurements of the deuteron or proton analysing powers and/or spin-correlations. [Copyright &y& Elsevier]

Abstract: In this work, we consider a Rashba-type quantum channel (RQC) consisting of one AC-biased finger-gates (FG) that orient perpendicularly and located above the RQC. Such an AC-biased FG gives rise to a local time-modulation in the Rashba coupling parameter, and is shown recently to generate a DC spin current [L.Y. Wang, C.S. Tang, C.S. Chu, Cond-mat/0409291, 2004]. No charge current, however, is generated in this configuration. We explore the robustness of such DC spin current generation against elastic scattering in the RQC. The effect of backscattering is studied by introducing a static barrier that is uniform in the transverse dimension. The effects of both backscattering and subband mixing is studied by introducing a static partial-barrier that is spatially localized and non-uniform in the transverse dimension. In addition, we compare the cases of attractive and repulsive partial-barriers. It is found that attractive partial-barrier gives rise to additional DC spin current structures due to resonant inter-subband and inter-sideband transition to quasi-bound states formed just beneath subband thresholds. [Copyright &y& Elsevier]

Abstract: A theoretical study is reported on adiabatic processes involving an initially unbound two-body system. The system consists of a positron and an antiproton in a magnetic field. For parameters associated with recent antihydrogen production experiments, computer simulations indicate that it is possible for quasi-bound states to form adiabatically. An expression for the cross section of the formation of quasi-bound states is obtained, and reasonable agreement with simulation results is found. [Copyright &y& Elsevier]

A method based on a generic small-signal equivalent circuit for field-effect transistors is proposed for predicting the unity-current-gain frequency fT for carbon-nanotube devices. The key to the useful implementation of the method is the rigorous estimation of the values for the components of the equivalent circuit. This is achieved by numerical differentiation of the charges and currents resulting from self-consistent solutions to the equations of Schrodinger and Poisson. Sample results are presented, which show that fT can have a very unusual dependence on the gate-source bias voltage. This behavior is due mainly to the voltage dependence of the transconductance and capacitance in the presence of quasi-bound states in the nanotube. [ABSTRACT FROM PUBLISHER]

The α parameters and extinction ratios of InGaAsP-InP electroabsorption (EA) modulators and Mach-Zehnder (MZ) modulators are theoretically investigated. The bound states of excitons in quantum wells (QWs) under electric field are calculated through the finite-difference method, and quasi-bound states are obtained by the transfer-matrix method. Reducing the heights of the potential barriers of the QWs is prerequisite to achieving small values of the α parameter for EA modulators and low driving voltages for MZ modulators. Bulk EA modulators are shown to inherently have relatively small α parameters; however, they also require a tradeoff between the extinction ratio and the insertion loss. The α parameters of symmetrical and π-phase-shifted MZ modulators in the single- and dual-drive cases are also discussed. [ABSTRACT FROM PUBLISHER]

Four potential energy surfaces (PESs) for the ground electronic state of the LiFH system were constructed by fitting the energies obtained in coupled-cluster (CCSD(T)) and multi-reference configuration interaction (MRCI) calculations. Two potentials were constructed by fitting the CCSD(T) energies corrected for the effects of the basis set superposition error (BSSE) and monomer deformation. The third potential was constructed by fitting the BSSE-uncorrected CCSD(T) data. The fourth potential was constructed by fitting the energies obtained with the MRDCI variant of the MRCI method. All four PESs were used to calculate bound and quasi-bound ro-vibrational states of the Li⋯FH complex. The calculations of the bound-state energies, energy positions and widths of the quasi-bound states, and densities of the scattering states provided useful insights into the effects of BSSE, ab initio method employed, and fitting procedure on spectroscopic and dynamical characteristics of the Li⋯FH complex. It was demonstrated that the MRDCI and BSSE-corrected CCSD(T) potentials are similar from the spectroscopic and dynamical point of view, while there is a substantial difference between the MRDCI and BSSE-uncorrected CCSD(T) potentials. Information about the Li⋯FH ro-vibrational resonances was used to provide new insights into the dynamics of the photo-induced Li+HF→LiF+H reaction, in which products are obtained by exciting the HF fragment in the Li⋯FH complex with the infrared radiation. [Copyright &y& Elsevier]

Abstract: We present a theoretical study of the mean life times of the electron and hole quasi-bound states in δ-doped GaAs quantum wells. The full width at half-maxima and the mean life time are found for a series of donor and acceptor concentrations. The spatial localizations of these states are calculated and discussed in relation with theirs line shapes. The numerical calculations are performed within the framework of the semi-empirical sp3s* tight-binding model taking into account the spin. [Copyright &y& Elsevier]

Measurements on the η meson production in proton-deuteron collisions have been performed using the COSY-11 facility at COSY (Jülich). Here we present preliminary results on total and differential cross sections for the pd→3Heη reaction at five excess energies between Q=5.1 and Q=40.6 MeV. The obtained angular distributions for the emitted η mesons in the center of mass system expose a transition from an almost isotropic emission to a highly anisotropic distribution. The extracted total cross sections support a strong η-3He final state interaction and will be compared with model predictions. [ABSTRACT FROM AUTHOR]