Semiconductor Quantum Rings Research Articles

The magnetization in the lattice of quantum rings with rashba spin-orbit interaction

The magnetic properties of the lattice formed by noninteracting semiconductor quantum rings in a longitudinal magnetic field are investigated. It was shown that when the value of the Rashba parameter ξ = 1.5, the magnetization varies from positive to negative with changes in the Aharonov–Bohm flux and amplitude oscillations decrease.

Case study of the validity of truncation schemes of kinetic equations of motion: Few magnetic impurities in a semiconductor quantum ring

We carry out a study on the validity and limitations of truncation schemes customarily employed to treat the quantum kinetic equations of motion of complex interacting systems. Our system of choice is a semiconductor quantum ring with one electron interacting with few magnetic impurities via a Kondo-like Hamiltonian. This system is an interesting prototype which displays the necessary complexity when suitably scaled (large number of magnetic impurities) but can also be solved exactly when few impurities are present. The complexity in this system comes from the indirect electron-mediated impurity–impurity interaction and is reflected in the Heisenberg equations of motion, which form an infinite hierarchy. For the cases of two and three magnetic impurities, we solve for the quantum dynamics of our system both exactly and following a truncation scheme developed for diluted magnetic semiconductors in the bulk. We find an excellent agreement between the two approaches when physical observables like the impurities’ spin angular momentum are computed for times that well exceed the time window of validity of perturbation theory. On the other hand, we find that within time ranges of physical interest, the truncation scheme introduces negative populations which represents a serious methodological drawback.

Floquet engineering of electronic states and optical absorption in laterally-coupled quantum rings under a magnetic field.

Tuning electronic and optical properties of low-dimensional quantum systems in a flexible way is of particular importance in designing semiconductor-based devices. Semiconductor quantum rings (QRs) are nanoscopic structures that have become promising systems for physical and technological applications due to their unique electronic and optical properties. Here, we explore the fundamental electronic and optical properties of laterally-coupled QRs by taking into account the combined effects of applied magnetic and non-resonant terahertz intense laser fields. The laser-dressed electronic states are solved using the Floquet theory of periodically driven quantum systems in high-frequency limits within the framework of effective mass approximation. We demonstrate that increasing the laser field parameter leads to reduced tunnel splitting in the energy spectrum and more pronounced Aharonov-Bohm oscillations for the ground state energy, due to the attenuation of the tunnel coupling. Furthermore, depending on the position of the avoided crossings in Aharonov-Bohm oscillations of laterally-coupled quantum rings, the evolution of the avoided crossings with increasing the laser field parameter can be elucidated as the attenuated tunnel coupling or the competition between the attenuated tunnel coupling and the strengthening anisotropy of the laser-dressed QR potential well. The intensity of the intraband optical transition of laterally-coupled QRs can be effectively tuned and reaches a larger value by manipulating the laser parameter and magnetic field. Optical Aharonov-Bohm oscillations in laterally-coupled QRs are still exhibited by changing laser field parameters. Our findings offer a novel approach to manipulate the electronic and optical performances as well as the Aharonov-Bohm oscillations based on the laterally-coupled QRs by using an intense terahertz laser field.

Study on the effects of anisotropic effective mass on electronic properties, magnetization and persistent current in semiconductor quantum ring with conical geometry

We study a 2D mesoscopic ring with an anisotropic effective mass considering surface quantum confinement effects. Consider that the ring is defined on the surface of a cone, which can be controlled topologically and mapped to the 2D ring in flat space. We demonstrate through numerical analysis that the electronic properties, the magnetization, and the persistent current undergo significant changes due to quantum confinement and non-isotropic mass. We investigate these changes in the direct band gap semiconductors SiC, ZnO, GaN, and AlN. There is a plus (or minus) shift in the energy sub-bands for different values of curvature parameter and anisotropy. Manifestations of this nature are also seen in the Fermi energy profile as a function of the magnetic field and in the ring width as a function of the curvature parameter. Aharonov–Bohm (AB) and de Haas van-Alphen (dHvA) oscillations are also studied, and we find that they are sensitive to variations in curvature and anisotropy.

Elliptical Quantum Rings with Variable Heights and under Spin–Orbit Interactions

We investigate the electronic properties of a semiconductor quantum ring with an elliptical shape and non-uniform height, allowing for distributed quantum-dot-like bulges along its perimeter. The adiabatic approximation and the finite element method are combined to calculate the allowed electron states in the structure under the effective mass approximation, considering the contributions from Rashba and Dresselahaus spin–orbit interactions and the Zeeman effect in the presence of an applied magnetic field. We discuss the features of the calculated spectra for two different ring geometries: a symmetric one with four dot-like bulges, and an asymmetric one with three hilled protuberances. The information about those states allows us to evaluate the linear optical absorption response associated with interlevel transitions between the ground and lowest excited states. This phenomenon takes place at resonant energies of only a few milielectronvolts. It is observed that spin–orbit interactions tend to quench this response under zero-field conditions in the case of symmetric confinement.

The influence of off-centre donor impurity on the tuning of the quantum ring magnetic susceptibility

The 1/N expansion method has been used to investigate the problem of an electron moving in a two-dimensional semiconductor quantum ring with an off-center impurity. The energy spectra, magnetization, and magnetic susceptibility have been calculated as a function of the magnetic field, impurity position, temperature, and quantum ring parameters. Interesting energy-level crossings attributed to QR confinement manifest themselves as low-temperature oscillations in the magnetization and magnetic susceptibility curves. On the contrary, the presence of impurity at the QR center eliminates these level crossings. This work shows that changing the impurity position can manipulate the magnetic field value correspondence to these diamagnetic-paramagnetic phase transitions.

Thermodynamic behaviour of a D 2 + molecular complex in quantum rings under the combined effects of nonresonant intense laser field radiation and electric field

ABSTRACT A methodical study of the thermal properties of a hydrogen molecular ion strongly confined in a flat semiconductor quantum ring under external probes: intense laser-field radiation and static electric fields were carried out. The corresponding Schrödinger equation for these two coupled donors system was numerically solved by using the finite element method within the effective mass approximation. The effects of the donor angular and radial positions on the entropy, heat capacity and internal energy were analysed. Results show that the entropy of the system is maintained low as long as the molecule preserves its integrity but if the molecule is fragmented, the randomness of the system is substantially increased. Nevertheless, applying an intense laser or an external electric field can reduce this property in real time.

Fabrication of quantum dot and ring arrays by direct laser interference patterning for nanophotonics

Abstract Epitaxially grown semiconductor quantum dots (QDs) and quantum rings (QRs) have been demonstrated to be excellent sources of single photons and entangled photon pairs enabling applications within quantum photonics. The emerging field of QD-based nanophotonics requires the deterministic integration of single or multiple QD structures into photonic architectures. However, the natural inhomogeneity and spatial randomness of self-assembled QDs limit their potential, and the reliable formation of homogeneous and ordered QDs during epitaxy still presents a challenge. Here, we demonstrate the fabrication of regular arrays of single III–V QDs and QRs using molecular beam epitaxy assisted by in situ direct laser interference patterning. Both droplet epitaxy (DE) GaAs/AlGaAs QDs and QRs and Stranski–Krastanov (SK) InAs/GaAs QDs are presented. The resulting QD structures exhibit high uniformity and good optical quality, in which a record-narrow photoluminescence linewidth of ∼17 meV from patterned GaAs QD arrays is achieved. Such QD and QR arrays fabricated through this novel optical technique constitute a next-generation platform for functional nanophotonic devices and act as useful building blocks for the future quantum revolution.

Interplay between optical vortices and condensed matter

The interaction between light and matter is a subject of ongoing fundamental interest at the intersection of optics and condensed-matter physics. This review discusses how the structuring of light into optical vortices leads to distinctive optical selection rules and coupling to sample geometry, requiring reformulation of the standard theory for interaction of plane-wave light with matter. The interactions of optical vortices with semiconductor nanostructures, quantum rings, and 2D materials are presented. The results suggest further avenues for fundamental investigation as well as current and prospective applications in quantum control, communications, and sensing.

Intense laser field effect on the [formula omitted] molecular complex strongly confined in a semiconductor quantum ring: Electronic and optical properties

The present theoretical study is concerned with the behavior of a D2+ molecular complex strongly confined in a semiconductor quantum ring under external probes: intense laser-field radiation and orthogonal electric and magnetic fields. The theoretical calculations were made using finite element method within the effective mass approximation. The effects of the external probes, donor-donor separation and quantum ring geometrical parameters on the D2+ total energy, D2+ binding energy, and D2+ optical response are studied. We show that the laser-field radiation has a great influence on the two fragmentation processes of D2+ and on their essential features: equilibrium length and dissociation energy. The D2+ optical response defined through adsorption coefficient is studied. Laser field radiation may cause red or blue shifts in the adsorption coefficient peak depending on the quantum ring size and donor-donor positions while the magnetic field can induce an optical transparency effect.

Dynamics of the angular momentum in narrow quantum rings with Rashba and Dresselhaus spin-orbit interactions

The quantum dynamics of the electron's spin and orbital angular momenta in semiconductor quantum rings is analyzed. Both Rashba and Dresselhaus spin-orbit interactions (SOIs) in their quasi-two-dimensional forms are taken into account. The narrow quantum rings are treated with models including one and two radial modes. We find that when either Rashba or Dresselhaus SOI acts alone, the different angular momentum states are coupled in blocks of two (for a single radial mode) or four (for two radial modes). We also show that the full Hilbert space splits into two disjoint subspaces, which are not coupled by either of the two SOIs, thereby decoupling accordingly the state evolution. When both SOI mechanisms are present, in principle infinitely many states are coupled, but we find by numerical computation of the quantum dynamics that for typical evolution times in practice only a few neighboring states are involved in the dynamics. Thus the exchange of angular momenta proceeds only via very few states. Furthermore, we find a trend that when initially high orbital momenta are prepared, the time evolution of spin and orbital momenta is almost unaffected by the availability of a second radial mode, in sharp contrast to the case of preparing the system in low orbital angular momentum states. The implications of our findings for the coherent control of angular momentum in quantum rings are pointed out.

A Thin Semiconductor Quantum Ring as an Analog of a Magnetically Controlled Bohr Atom

A Thin Semiconductor Quantum Ring as an Analog of a Magnetically Controlled Bohr Atom

Spectral Parameters of an Exciton in Double Semiconductor Quantum Rings in an Electric Field

Spectral Parameters of an Exciton in Double Semiconductor Quantum Rings in an Electric Field

Electronic and optical properties of an electro-magnetic non-uniform narrow quantum ring under repulsive scattering centre

ABSTRACT Non-uniform height semiconductor quantum rings are studied in order to determine their electronic and optical absorption properties. Theoretical modelling of the structure includes an analytical description of the non-regular multi-hilled confining potential as well as the presence of repulsive scattering centre and external crossing electric and magnetic fields. We have discussed the features of localised and extended (rotational, Aharonov–Bohm-like) states in the presence of the magnetic field. A modification of the spectrum, with the appearance of a Stark-like behaviour, and its corresponding modification related to the repulsive potential is analysed when the electric field effect is considered. In double-hilled structures, these properties of the energy spectrum are of main importance in explaining the apparent optical transparency induced within a certain range of the electric field strength. The presence of the repulsive centre is found to cause a moderate redshift of the light absorption response.

Narrow quantum rings with general Rashba and Dresselhaus spin-orbit interactions

We solve analytically the energy eigenvalue problem of narrow semiconductor quantum rings with a general spin-orbit term that includes as a special case the Rashba and Dresselhaus interactions acting simultaneously. The eigenstates and eigenenergies of the system are found for arbitrary values of the spin-orbit coupling constants without making use of approximations. The general eigenstates are expressed as products of a scalar Mathieu function and a spinor factor which is periodic or pseudo-periodic on the ring. Our general solution reduces to the previously found solutions for particular combinations of the Rashba and Dresselhaus couplings, like the well-studied cases of Rashba-only and of equal coupling constants.

Rashba Spin-orbit Interaction in Semiconductor Nanostructures (Review)

In this work I review of the theoretical and experimental issue related to the Rashba Spin-Orbit interaction in semiconductor nanostructures. The Rashba spin-orbit interaction has been a promising candidate for controlling the spin of electrons in the field of semiconductor spintronics. In this work, I focus study of the electrons spin and holes in isolated semiconductor quantum dots and rings in the presence of magnetic fields. Spin-dependent thermodynamic properties with strong spin-orbit coupling inside their band structure in systems are investigated in this work. Additionally, specific heat and magnetization in two dimensional, one-dimensional quantum ring and dot nanostructures with Spin Orbit Interaction are discussed.

Magnetoresistance of electrons in diluted magnetic semiconductor Volcano ring

The influence of exchange interaction on the transport properties of a two-dimensional diluted magnetic semiconductor quantum ring with finite width has been investigated in the presence of a uniform perpendicular magnetic field. The dependence of magnetoresistance on the magnetic field, Mn concentration, and quantum ring width are investigated. In the absence of exchange interaction, a typical beating pattern with well-defined node-positions in the oscillating magnetoresistance is observed. It was shown that in the present exchange interactions the beating pattern is destroyed.

Uniaxial transition dipole moments in semiconductor quantum rings caused by broken rotational symmetry

Semiconductor quantum rings are topological structures that support fascinating phenomena such as the Aharonov–Bohm effect and persistent current, which are of high relevance in the research of quantum information devices. The annular shape of quantum rings distinguishes them from other low-dimensional materials, and enables topologically induced properties such as geometry-dependent spin manipulation and emission. While optical transition dipole moments (TDMs) in zero to two-dimensional optical emitters have been well investigated, those in quantum rings remain obscure despite their utmost relevance to the quantum photonic applications of quantum rings. Here, we study the dimensionality and orientation of TDMs in CdSe quantum rings. In contrast to those in other two-dimensional optical emitters, we find that TDMs in CdSe quantum rings show a peculiar in-plane linear distribution. Our theoretical modeling reveals that this uniaxial TDM originates from broken rotational symmetry in the quantum ring geometries.

Wave packet propagation through branched quantum rings under applied magnetic fields

We investigate the effect of opening and closing pathways on the dynamics of electron wave packets in semiconductor quantum rings with different geometries. Our analysis is based on the time evolution of an electron wave packet, within the effective-mass approximation. We demonstrate that opening an extra channel in the quantum ring does not necessarily improve the electron transmission and, depending on the extra channel width, may even reduce it, either due to enhancement of quantum scattering or due to interference. In the latter case, transmission reduction can be controlled through the Aharonov-Bohm phase of the wave function, via an applied magnetic field. It is also shown that, closing one of the channels of the quantum ring, system improves the transmission probability under specific conditions, an effect which is a quantum analog of the Braess paradox.

Spin-Orbit Interactions in Semiconductor Quantum Ring in the Presence of Magnetic Field

Energy levels of electrons in the semiconductor circular quantum ring are obtained within the framework of perturbation theory in the presence of the Rashba and Dresselhaus spin-orbit interactions and external uniform constant magnetic field. The confinement effect is simulated by the realistic potential well of a finite depth.