Electronic transport through a Majorana bound state coupled to a T-shaped quantum-dot system with Coulomb interaction

Using the Green’s function technique, we respectively investigate the electron transport properties of two spin components through the system of a T-shaped double quantum dot structure coupled to a Majorana bound state, in which only one quantum dot is connected with two metallic leads. We explore the interplay between the Fano effect and the MBSs for different dot-MBS coupling strength λ, dot-dot coupling strength t, and MBS-MBS coupling strength ε_M in the noninteracting case. Then the Coulomb interaction and magnetic field effect on the conductance spectra are investigated. Our results indicate that G_↓(ω) is not affected by the Majorana bound states, but a “0.5” conductance signature occurs in the vicinities of Fermi level of G_↑(ω). This robust property persists for a wide range of dot-dot coupling strength and dot-MBS coupling strength, but it can be destroyed by Coulomb interaction in quantum dots. By adjusting the size and direction of magnetic field around the quantum dots, the “0.5” conductance signature damaged by U can be restored. At last, the spin magnetic moments of two dots by applying external magnetic field are also predicted.     

Magnetpolaron effect in two-dimensional anisotropic parabolic quantum dot in a perpendicular magnetic field

We study the two-dimensional weak-coupling Fr o¨hlich polaron in a completely anisotropic quantum dot in a perpendicular magnetic field. By performing a unitary transformation, we first transform the Hamiltonian into a new one which describes an anisotropic harmonic oscillator with new mass and trapping frequencies interacting with the same phonon bath but with different interaction form and strength. Then employing the second-order Rayleigh–Schr o¨dinger perturbation theory, we obtain the polaron correction to the ground-state energy. The magnetic field and anisotropic effects on the polaron correction to the ground-state energy are discussed.     

Two interacting electrons in a Gaussian confining potential quantum dot

Two interacting electrons in a Gaussian confining potential quantum dot are considered under the influence of a perpendicular homogeneous magnetic field. The energy levels of the low-lying states are calculated as a function of magnetic field. Calculations are made by using the method of few-body physics within the effective-mass approximation. A ground state behavior (singlet→triplet state transitions) as a function of the strength of a magnetic field has been found in the weak confinement case as a two-electron quantum dot with parabolic confining potential.     

Quantum dot with spin-orbit interaction in noncommutative phase space and analog Landau levels

We have studied a quantum dot with Rashba spin-orbit interaction in noncommutative phase space. The energy eigenvalues are analogous to Landau energy levels. It is shown that this system is related with a physically realizable model of a quantum dot with Rashba spin-orbit interaction in a magnetic field whereby a relation is derived among the noncommutative parameters, spin-orbit coupling strength and magnetic field.     

Analytical Ground-State of an Exciton Trapped in a Circular Parabolic Quantum Dot in the Presence of a Magnetic Field

The quasi-exact properties of an exciton are investigated theoretically in the presence of an external magnetic field using the effective-mass approach in GaAs parabolic quantum dot. The energy spectrum is obtained analytically as a function of the dot radius, interaction strength and magnetic field. It is established that, a steady bound state of an exciton in the ground state exists under the effect of a strong magnetic field; also I noticed that the exciton binding energy decreases by increasing both the radius of the dot and the magnetic field strength and the reduction becomes pronounced for larger dots. As expected, it has been found that the exciton total energy decreases with increasing the size of the dot and it enhances by increasing the magnetic field. It appears that the exciton total energy strongly depends on the magnetic field for dots with big size. The magnetic field effect on the exciton size also has been studied. It is shown that the increase in the magnetic field leads to a reduction in the exciton size; due to magnetic field confinement, while the size of an exciton reach its bulk limit as the dot size increases. Moreover, it is shown that, if the dot radius is sufficiently large the oscillator strength saturates and it becomes insensitive to the magnetic field while the increase in the magnetic field gradually weakened the oscillator strength. I have calculated the ground-state distribution for both the electron and the hole. It is found that the localization of the electron/hole increases in the presence of a magnetic field. Moreover, the ground-state optical-absorption intensity is investigated. Finally, the dependence of the lowest five states of an exciton on both the dot radius and the magnetic field are discussed.

     

Entanglement and Zeeman interaction in diluted magnetic semiconductor quantum dot

We present theoretically the Zeeman coupling and exchange-induced swap action in spin-based quantum dot quantum computer models in the presence of magnetic field. We study the valence and conduction band states in a double quantum dots made in diluted magnetic semiconductor. The latter have been proven to be very useful in building an all-semiconductor platform for spintronics. Due to a strong p–d exchange interaction in diluted magnetic semiconductor (Cd_0.57Mn_0.43Te), the relative contribution of this component is strongly affected by an external magnetic field, a feature that is absent in nonmagnetic double quantum dots. We determine the energy spectrum as a function of magnetic field within the Hund–Mulliken molecular-orbit approach and by including the Coulomb interaction. Since we show that the ground state of the two carriers confined in a vertically coupled quantum dots provide a possible realization for a gate of a quantum computer, the crossing between the lowest states, caused by the giant spin splitting, can be observed as a pronounced jump in the magnetization of small magnetic field amplitude. Finally, we determine the swap time as a function of magnetic field and the inter dot distance. We estimate quantitatively swap errors caused by the field, establishing that error correction would, in principle, be possible in the presence of nonuniform magnetic field in realistic structures.     

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.     

Magnetic field and symmetry effects in small quantum dots

Shell phenomena in small quantum dots with a few electrons under a perpendicular magnetic field are discussed within a simple model. It is shown that various kinds of shell structures, which occur at specific values for the magnetic field lead to a disappearance of the orbital magnetization for particular magic numbers for noninteracting electrons in small quantum dots. Including the Coulomb interaction between two electrons, we found that the magnetic field gives rise to dynamical symmetries of a three-dimensional axially symmetric two-electron quantum dot with a parabolic confinement. These symmetries manifest themselves as near-degeneracy in the quantum spectrum at specific values of the magnetic field and are robust at any strength of the electron-electron interaction. A remarkable agreement between experimental data and calculations exhibits the important role of the thickness for the two-electron quantum dot for analysis of ground state transitions in a perpendicular magnetic field. The text was submitted by the author in English.     

NEGATIVE DONOR CENTER QUANTUM DOTS IN MAGNETIC FIELDS

The method of few-body physics is applied to treat a D^{-<\sup> center quantum dot system in a magnetic field. The magnetic field is applied in the z direction. Using this method, we investigate the energy spectra of low-lying states of D-<\sup> center quantum dots as a function of magnetic field. The dependence of the binding energies of the ground-state of the D-<\sup> center are calculated as a function of the dot radius with a few values of the magnetic field strength and compared with other results.}     

隧穿量子点分子模型与光场相互作用的量子信息保真度

利用全量子理论和数值计算方法,研究隧穿量子点分子模型与光场相互作用的量子信息保真度,通过计算机模拟出量子点分子、光场和系统的保真度随时间的演化图像,分析了压缩系数和失谐量对量子信息保真度的影响.结果表明：改变初始光场的压缩系数和失谐量能调节相互作用系统的量子信息保真度.     

Properties of strong-coupling magneto-bipolaron qubit in quantum dot under magnetic field

Based on the variational method of Pekar type, we study the energies and the wave-functions of the ground and the first-excited states of magneto-bipolaron, which is strongly coupled to the LO phonon in a parabolic potential quantum dot under an applied magnetic field, thus built up a quantum dot magneto-bipolaron qubit. The results show that the oscillation period of the probability density of the two electrons in the qubit decreases with increasing electron–phonon coupling strength α, resonant frequency of the magnetic field ω_c, confinement strength of the quantum dot ω_0, and dielectric constant ratio of the medium η; the probability density of the two electrons in the qubit oscillates periodically with increasing time t, angular coordinate φ_2, and dielectric constant ratio of the medium η; the probability of electron appearing near the center of the quantum dot is larger, and the probability of electron appearing away from the center of the quantum dot is much smaller.     

Thermally driven pure spin current through an interacting quantum dot

We study spin-dependent electron transport properties of a thermally driven interacting quantum dot. When an external magnetic field is applied to the quantum dot, the effective transmissions of spin-up and spin-down electrons are separated from each other and have a perfect mirror symmetry with respect to the incident energy at a certain gate voltage. A pure spin current can be induced in the system and modulated by a magnetic field. Under certain magnetic field strengths, a larger pure spin current can be obtained at gate voltages with the values in a range, not just at a specific voltage. These results indicate that the system can be worked as a pure spin current generator.     

Transport spectroscopy of an impurity spin in a carbon nanotube double quantum dot

We make use of spin selection rules to investigate the electron spin system of a carbon nanotube double quantum dot. Measurements of the electron transport as a function of the magnetic field and energy detuning between the quantum dots reveal an intricate pattern of the spin state evolution. We demonstrate that the complete set of measurements can be understood by taking into account the interplay between spin-orbit interaction and a single impurity spin coupled to the double dot. The detection and tunability of this coupling are important for quantum manipulation in carbon nanotubes.     

Singlet-Triplet Transitions of a Pöschl-Teller Quantum Dot

We study the energy spectra of a two-dimensional two-electron quantum dot (QD) with Pöschl-Teller confining potential under the influence of perpendicular homogeneous magnetic field. Calculations are made by using the method of numerical diagonalization of Hamiltonian matrix within the effective-mass approximation. A ground-state behavior (spin singlet-triplet transitions) as a function of the strength of a magnetic field is found. We find that the dot radius R of a Pöschl-Teller potential is important for the ground-state transition and the feature of ground-state for a Pöschl-Teller QD and a parabolic QD is similar when R is larger. The larger the well depth, the higher the magnetic field for the singlet-triplet transition of the ground-state of two interacting electrons in a Pöschl-Teller QD.     

Magnetization of interacting electrons in anisotropic quantum dots with Rashba spin–orbit interaction

Magnetization of anisotropic quantum dots in the presence of the Rashba spin–orbit interaction has been studied for three and four interacting electrons in the dot for non-zero values of the applied magnetic field. We observe unique behaviors of magnetization that are direct reflections of the anisotropy and the spin–orbit interaction parameters independently or concurrently. In particular, there are saw-tooth structures in the magnetic field dependence of the magnetization, as caused by the electron–electron interaction, that are strongly modified in the presence of large anisotropy and high strength of the spin–orbit interactions. We also report the temperature dependence of magnetization that indicates the temperature beyond which these structures due to the interactions disappear. Additionally, we found the emergence of a weak sawtooth structure in magnetization for three electrons in the high anisotropy and large spin–orbit interaction limit that was explained as a result of merging of two low-energy curves when the level spacings evolve with increasing values of the anisotropy and the spin–orbit interaction strength.     

The nonlinear optical properties of a magneto-exciton in a strained Ga_(0.2)In_(0.8)As/GaAs quantum dot

The magnetic field-dependent heavy hole excitonic states in a strained Ga0.2In0.8As/GaAs quantum dot are investigated by taking into account the anisotropy,non-parabolicity of the conduction band,and the geometrical confinement.The strained quantum dot is considered as a parabolic dot of InAs embedded in a GaAs barrier material.The dependence of the effective excitonic g-factor as a function of dot radius and the magnetic field strength is numerically measured.The interband optical transition energy as a function of geometrical confinement is computed in the presence of a magnetic field.The magnetic field-dependent oscillator strength of interband transition under the geometrical confinement is studied.The exchange enhancements as a function of dot radius are observed for various magnetic field strengths in a strained Ga0.2In0.8As/GaAs quantum dot.Heavy hole excitonic absorption spectra,the changes in refractive index,and the third-order susceptibility of third-order harmonic generation are investigated in the Ga0.2In0.8As/GaAs quantum dot.The result shows that the effect of magnetic field strength is more strongly dependent on the nonlinear optical property in a low-dimensional semiconductor system.     

Thermodynamic behaviour of Rashba quantum dot in the presence of magnetic field

The thermodynamic properties of an In Sb quantum dot have been investigated in the presence of Rashba spin–orbit interaction and a static magnetic field. The energy spectrum and wave-functions for the system are obtained by solving the Schrodinger wave-equation analytically. These energy levels are employed to calculate the specific heat, entropy,magnetization and susceptibility of the quantum dot system using canonical formalism. It is observed that the system is susceptible to maximum heat absorption at a particular value of magnetic field which depends on the Rashba coupling parameter as well as the temperature. The variation of specific heat shows a Schottky-like anomaly in the low temperature limit and rapidly converges to the value of 2kB with the further increase in temperature. The entropy of the quantum dot is found to be inversely proportional to the magnetic field but has a direct variation with temperature. The substantial effect of Rashba spin–orbit interaction on the magnetic properties of quantum dot is observed at low values of magnetic field and temperature.     

Implementation of Kohn's theorem for the ellipsoidal quantum dot in the presence of external magnetic field

An electron gas in a strongly oblated ellipsoidal quantum dot with impenetrable walls in the presence of external magnetic field is considered. Influence of the walls of the quantum dot is assumed to be so strong in the direction of the minor axis (the OZ axis) that the Coulomb interaction between electrons in this direction can be neglected and considered as two-dimensional. On the basis of geometric adiabaticity we show that in the case of a few-particle gas a powerful repulsive potential of the quantum dot walls has a parabolic form and localizes the gas in the geometric center of the structure. Due to this fact, conditions occur to implement the generalized Kohn theorem for this system. The parabolic confinement potential depends on the geometry of the ellipsoid, which allows, together with the magnetic field to control resonance frequencies of transitions by changing the geometric dimensions of the QD.     

Gate adjustable coherent three and four level mixing in a vertical quantum dot molecule

We study level mixing in the single-particle energy spectrum of one of the constituent quantum dots in a vertical double quantum dot by performing magneto-resonant-tunneling spectroscopy. The device used in this study differs from previous vertical double quantum dot devices in that the single side gate is now split into four separate gates. Because of the presence of natural perturbations caused by anharmonicity and anisotropy, applying different combinations of voltages to these gates allows us to alter the effective potential landscape of the two dots and hence influence the level mixing. We present here preliminary results from one three level crossing and one four level crossing high up in the energy spectrum of one of the probed quantum dots, and demonstrate that we are able to change significantly the energy dispersions with magnetic field in the vicinity of the crossing regions.     

串型耦合双量子点处于自旋阻塞区时磁输运性质的研究

使用双杂质安德森模型的哈密顿量,从理论上研究了串型耦合双量子点系统处于自旋阻塞区时的磁输运性质,并用主方程近似方法求解了哈密顿量.结果表明,自旋轨道耦合作用导致的双量子点间的自旋反转隧穿能够解除系统的自旋阻塞.同时也研究了超精细相互作用导致的在量子点内自旋反转和双量子点之间的自旋关联对系统的磁输运性质的影响,取得了一些有价值的结果,并对相关的物理问题进行了讨论.