Selected aspects of the quantum dynamics and electronic structure of atoms in magnetic microtraps

We analyze the quantum properties of atoms in a magnetic quadrupole field. The quantum dynamics of ground state atoms in this field configuration is studied firstly. We formulate the Hamiltonian and perform a symmetry analysis. Due to the particular shape of the quadrupole field in general there exist no stable states. We provide resonance energies, lifetimes and calculate the density of states and investigate under what conditions quasi-bound states occur that possess long lifetimes. An effective scalar Schrödinger equation describing such states is derived. As a next step we explore the influence of a high gradient quadrupole field on the electronic structure of excited atoms. An effective one-body approach together with the fixed nucleus approximation is employed in order to derive the electronic Hamiltonian. We present the energy spectrum and discuss peculiar features such as non-trivial spin densities and magnetic field induced electric dipole moments.     

Inelastic electron tunneling spectroscopy of a single nuclear spin

Detection of a single nuclear spin constitutes an outstanding problem in different fields of physics such as quantum computing or magnetic imaging. Here we show that the energy levels of a single nuclear spin can be measured by means of inelastic electron tunneling spectroscopy (IETS). We consider two different systems, a magnetic adatom probed with scanning tunneling microscopy and a single Bi dopant in a silicon nanotransistor. We find that the hyperfine coupling opens new transport channels which can be resolved at experimentally accessible temperatures. Our simulations evince that IETS yields information about the occupations of the nuclear spin states, paving the way towards transport-detected single nuclear spin resonance.     

Electronic properties of telescoping carbon nanotubes under external fields

In this work, we use the tight-binding model to study the low-energy electronic properties of telescoping double-walled carbon nanotubes subject to the influences of a transverse electric field and a parallel magnetic field. The state energy and energy spacings are found to oscillate significantly with the overlapping length. External fields would modify the state energies, alter the energy gaps, and destroy the state degeneracy. Complete energy gap modulations can be accomplished either by varying the overlapping length, or by applying an electric field or a magnetic field. The variations of state energies with the external fields will be directly reflected in the density of states. The numbers, heights, and frequencies of the density of states peaks are strongly dependent on the external fields.     

Magnetic field induced incommensurate resonance in cuprate superconductors

The influence of a uniform external magnetic field on the dynamical spin response of cuprate superconductors in the superconducting state is studied based on the kinetic energy driven superconducting mechanism. It is shown that the magnetic scattering around low and intermediate energies is dramatically changed with a modest external magnetic field. With increasing the external magnetic field, although the incommensurate magnetic scattering from both low and high energies is rather robust, the commensurate magnetic resonance scattering peak is broadened. The part of the spin excitation dispersion seems to be an hourglass-like dispersion, which breaks down at the heavily low energy regime. The theory also predicts that the commensurate resonance scattering at zero external magnetic field is induced into the incommensurate resonance scattering by applying an external magnetic field large enough.     

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.     

外场下压力及屏蔽对无限深量子阱中施主结合能的影响

考虑外加磁场、压力及屏蔽效应,利用变分方法数值计算GaN/AlxGa1-xN无限深量子阱系统中的杂质态结合能。给出结合能随磁场和阱宽的变化关系,同时讨论了有无屏蔽时的区别。结果表明:在磁场和压力作用下,结合能随阱宽的增大而减小;阱宽和压力一定时,结合能随磁场的增大而增大。屏蔽效应使得有效库仑吸引作用减弱而导致杂质态结合能显著下降。屏蔽效应对结合能的影响随压力增大而增强,随磁场强度增大而减弱。     

Control of the binding energy by tuning the single dopant position,magnetic field strength and shell thickness in ZnS/CdSe core/shell quantum dot

Recently, the new tunable optoelectronic devices associated to the inclusion of the single dopant are in continuous emergence. Combined to other effects such as magnetic field, geometrical confinement and dielectric discontinuity, it can constitute an approach to adjusting new transitions. In this paper, we present a theoretical investigation of magnetic field, donor position and quantum confinement effects on the ground state binding energy of single dopant confined in ZnS/CdSe core/shell quantum dot. Within the framework of the effective mass approximation, the Schrödinger equation was numerically been solved by using the Ritz variational method under the finite potential barrier. The results show that the binding energy is very affected by the core/shell sizes and by the external magnetic field. It has been shown that the single dopant energy transitions can be controlled by tuning the dopant position and/or the field strength.     

Donor-bound electron states in a two-dimensional quantum ring under uniform magnetic field

The electron states in a two-dimensional GaAs/AlGaAs quantum ring are theoretically studied in effective mass approximation. On-centre donor impurity and uniform magnetic field perpendicular to the ring plane are taken into account. The energy spectrum with different angular momentum changes dramatically with the geometry of the ring. The donor impurity reduces the energies with an almost fixed value; however, the magnetic field alters energies in a more complex way. For example, energy levels under magnetic field will cross each other when increasing the inner radius and outer radius of the ring, leading to the fact that the arrangement of energy levels is distinct in certain geometry of the ring. Moreover, energy levels with negative angular momentum exhibit the non-monotonous dependence on the increasing magnetic field.     

Magnetic field effect on state energies and transition frequency of a strong-coupling polaron in an anisotropic quantum dot

By employing a variational method of the Pekar-type, which has different variational parameters in the x–y plane and the z-direction, we study the ground and the first excited state energies and transition frequency between the ground and the first excited states of a strong-coupling polaron in an anisotropic quantum dot (AQD) under an applied magnetic field along the z-direction. The effects of the magnetic field and the electron–phonon coupling strength are taken into account. It is found that the ground and the first excited state energies and the transition frequency are increasing functions of the external applied magnetic field. The ground state and the first excited state energies are decreasing functions, whereas transition frequency is an increasing function of the electron–phonon coupling strength. We find two ways of tuning the state energies and the transition frequency: by adjusting (1) the magnetic field and (2) the electron–phonon coupling strength.     

Marginal Fermi liquid resonance induced by a quantum magnetic impurity in d-wave superconductors

We consider a model of an Anderson impurity embedded in a d(x(2)-y(2))--wave superconducting state to describe the low-energy excitations of cuprate superconductors doped with a small amount of magnetic impurities. Because of the Dirac-like energy dispersion, a sharp localized resonance above the Fermi energy, showing a marginal Fermi liquid behavior ( omega ln omega as omega-->0), is predicted for the impurity states. The same logarithmic dependence of self-energy and a linear frequency dependence of the relaxation rate are also derived for the conduction electrons, characterizing a new universality class for the strong coupling fixed point. At the resonant energies, the spatial distribution of the electron density of states around the magnetic impurity is also calculated.     

Squeezed magnetobipolarons in two-dimensional quantum dot

In this Letter, a different method was given for calculating the energies of the magnetobipolarons confined in a parabolic QD (quantum dot). We introduced single-mode squeezed states transformation, which are based on the Lee-Low-Pines and Huybrechts (LLP-H) canonical transformations. This method can provide results not only for the ground state energy but also for the excited states energies. Moreover, it can be applied to the entire range of the electron-phonon coupling strength. Comparing with the results of the LLP-H transformations, we have obtained more accurate results for the ground state energy, excited states energies and binding energy of the bipolarons. It shows that the magnetic field and the quantum dot can facilitate the formation of the bipolarons when η is smaller than some value.     

A Positronium Molecule Confined in a Two-Dimensional Space Under a Magnetic Field

Making use of hyperspherical coordinates, we investigate the qualitative features of the ground and lowlying states of a positronium molecule confined in a two-dimensional (2D) space under a magnetic field. We find that a positronium molecule has more bound states in 2D than in 3D. With the increase of the magnetic field, the second bound state experiences a transition in angular momentum. The result shows that symmetry plays an essential role in the energy spectrum of low-lying states.     

Optical pumping and a nondestructive readout of a single magnetic impurity spin in an InAs/GaAs quantum dot

We report on the resonant optical pumping of the | ± 1? spin states of a single Mn dopant in an InAs/GaAs quantum dot which is embedded in a charge tunable device. The experiment relies on a W scheme of transitions reached when a suitable longitudinal magnetic field is applied. The optical pumping is achieved via the resonant excitation of the central Λ system at the neutral exciton X(0) energy. For a specific gate voltage, the redshifted photoluminescence of the charged exciton X- is observed, which allows a nondestructive readout of the spin polarization. An arbitrary spin preparation in the | + 1? or |-1? state characterized by a polarization near or above 50% is evidenced.     

Barrier D centers in a quantum disc

The bound states of the barrier D⁻ center, which consists of a positive ion located on the z-axis at a distance λ from the two-dimensional quantum disc plane with a confined parabolic potential and two electrons in the disc plane bound by the ion, are studied under a perpendicular homogeneous magnetic field. The binding energies of the three lowest bound states are calculated as a function of the applied magnetic field strength γ. Discontinuous ground state transitions induced by an external magnetic field have been obtained. We have investigated the effect of the impurity position and found that the transition of the ground-state occurs for finite λ with increasing γ.     

二氢化钚分子激发态结构的外场效应

采用密度泛函(DFT)方法B3LYP/Gen，在Pu为SDD基组、H为6-311++G^**基组水平上优化得到了分子轴方向不同电偶极场(-0.005—0.005a.u.)作用下，二氢化钚的基态电子状态、几何结构、电偶极矩和分子总能量.在优化构型下用同样的基组采用含时密度泛函(TDDFT)方法(TD-B3LYP)研究了同样外电场条件下对二氢化钚的激发能和振子强度的影响.计算结果表明，分子几何构型与电场大小和方向呈现较强的依赖，电场强度增加基态偶极矩随电场强度线性增加，H-Pu-H的角度线性减小，分子总能量线性减小；激发能随电场强度增加而减小，且对电场方向的依赖呈现近似对称性，满足Grozema关系.电场对振子强度的影响比较复杂，但仍满足跃迁选择定则. 关键词： 二氢化钚 激发态 电偶极场 TD-DFT     

Surface-state localization at adatoms

Low-temperature scanning tunneling spectroscopy of magnetic and nonmagnetic metal atoms on Ag(111) and on Cu(111) surfaces reveals the existence of a common electronic resonance at an energy below the binding energies of the surface states. Using an extended Newns-Anderson model, we assign this resonance to an adsorbate-induced bound state, split off from the bottom of the surface-state band, and broadened by the interaction with bulk states. A line shape analysis of the bound state indicates that Ag and Cu adatoms on Ag(111) and Cu(111), respectively, decrease the surface-state lifetime, while a cobalt adatom causes no significant change.     

Quasi-bound states, resonance tunnelling, and tunnelling times generated by twin symmetric barriers

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.     

Kinetics of precessing ball solitons in ferromagnets at the first-order transition

The fundamentals of precessing ball solitons (PBS) arising as a result of the energy fluctuations at the first-order phase transition induced by a magnetic field in ferromagnets with uniaxial anisotropy are presented. When the external magnetic field is anti-parallel to the magnetization direction of the crystal, PBS states are possible in a wide range of amplitudes and energies, including the positive and negative energies relative to an initial condition. PBS are born with the greatest probability at near-zero energy, i.e. near the bifurcation point. Evolution of the PBS, at which they transform into macroscopic domains of a new magnetic phase or into a quasi-equilibrium solitonic state, is analysed.     

Ordering of Ground State Energy Levels of Two-Electron Quantum Dot in a Magnetic Field

The energy levels structure of two interacting electrons in a parabolic quantum dot under an external magnetic field of arbitrary strength is studied via the asymptotic iteration method. The method gives accurate results over the full range of quantum dot parameters. A crossings between spin-singlet and spin-triplet ground states energies as a function of the magnetic field is predicted.     

STM/STS measurements of two-dimensional electronic states trapped around surface defects in magnetic fields

The local density of states (LDOS) near point defects on a surface of highly oriented pyrolytic graphite (HOPG) was studied at very low temperatures in magnetic fields up to 6 T. We observed localized electronic states over a distance of the magnetic length around the defects in differential tunnel conductance images at the valley energies of the Landau levels (LLs) as well as relatively extended states at the peak ones of LLs. These states appear mainly at energies above the Fermi energy corresponding to the electron LL bands. The data suggest that the quantum Hall state is realized in the quasi two dimensional electron system in HOPG. At the peak energy associated with the n=0 (electron) and -1 (hole) LLs characteristic of the graphite structure, a reduced LDOS around the defects is observed. The spatial distribution is almost field independent, which indicates that it represents the potential shape produced by the defects.