Anisotropic Quantum Dots Research Articles

Influence of Temperature and Spin–Orbit Interaction on the Effective Mass of Polaron in an Anisotropic Quantum Dot

Influence of Temperature and Spin–Orbit Interaction on the Effective Mass of Polaron in an Anisotropic Quantum Dot

Polaronic Effects on the Electron Raman Scattering in a Spherical Anisotropic Quantum Dot

We study the polaronic effect on the differential cross section (DCS) for an electron Raman scattering process in a spherical anisotropic semiconductor quantum dot in the presence of a homogeneous external electric field. The Fröhlich coupling of electrons with confined and surface polar optical phonon modes for resonance Raman scattering is considered. The influence of electron-phonon coupling on the energy spectrum is taken into account in the framework of perturbation theory. The emission spectra are discussed for different anisotropy cases. Resonant peaks in the spectra are found and interpreted.

Self-assembled InAs/GaAs single quantum dots with suppressed InGaAs wetting layer states and low excitonic fine structure splitting for quantum memory

Abstract Epitaxial semiconductor quantum dots (QDs) have been demonstrated as on-demand entangled photon sources through biexciton–exciton (XX-X) cascaded radiative processes. However, perfect entangled photon emitters at the specific wavelengths of 880 nm or 980 nm, that are important for heralded entanglement distribution by absorptive quantum memories, remain a significant challenge. We successfully extend the QD emission wavelength to 880 nm via capping Stranski–Krastanow grown In(Ga)As/GaAs QDs with an ultra-thin Al x Ga1−x As layer. After carefully investigating the mechanisms governing the vanishing of wetting-layer (WL) states and the anisotropy of QDs, we optimize the growth conditions and achieve a strong suppression of the WL emission as well as a measured minor fine structure splitting of only ∼(3.2 ± 0.25) μeV for the exciton line. We further extend this method to fabricate In(Ga)As QDs emitted at 980 nm via introducing InGaAs capping layer, and demonstrate a two-photon resonant excitation of the biexciton without any additional optical or electrical stabilized source. These QDs with high symmetry and stability represent a highly promising platform for the generation of polarization entanglement and experiments on the interaction of photons from dissimilar sources, such as rare-earth-ion-doped crystals for solid quantum memory.

Dynamics of Hole Singlet-Triplet Qubits with Large g-Factor Differences.

The spin-orbit interaction permits to control the state of a spin qubit via electric fields. For holes it is particularly strong, allowing for fast all electrical qubit manipulation, and yet an in-depth understanding of this interaction in hole systems is missing. Here we investigate, experimentally and theoretically, the effect of the cubic Rashba spin-orbit interaction on the mixing of the spin states by studying singlet-triplet oscillations in a planar Ge hole double quantum dot. Landau-Zener sweeps at different magnetic field directions allow us to disentangle the effects of the spin-orbit induced spin-flip term from those caused by strongly site-dependent and anisotropic quantum dot g tensors. Our work, therefore, provides new insights into the hole spin-orbit interaction, necessary for optimizing future qubit experiments.

Influence of Rashba effect and Zeeman effect on properties of bound magnetopolaron in an anisotropic quantum dot

The influence of Rashba effect and Zeeman effect on the properties of bound magnetopolaron in an anisotropic quantum dot are studied with Pekar variational method. The expression of the ground state energy of the bound magnetopolaron is obtained through theoretical derivation. The relationship of the ground state energy of the polaron with the transverse effective confinement length, the longitudinal effective confinement length, the magnetic field cyclotron resonance frequency, and the Coulomb bound potential are discussed, respectively. Owing to the crystal structural inversion asymmetry and the time inversion asymmetry, the polaron energy experiences Rashba spin-orbit splitting and Zeeman splitting. Under the strong and weak magnetic field, we discuss the dominant position of Zeeman effect and Rashba effect, respectively. Owing to the presence of phonons and impurities, the polaron is more stable than the bare electron state.

Solutions to the generalized Eshelby conjecture for anisotropic media: Proofs of the weak version and counter-examples to the high-order and the strong versions

The Eshelby formalism for an inclusion in a solid is of significant theoretical and practical implications in mechanics and other fields of heterogeneous media. Eshelby’s finding that a uniform eigenstrain prescribed in a solitary ellipsoidal inclusion in an infinite isotropic medium results in a uniform elastic strain field in the inclusion leads to the conjecture that the ellipsoid is the only inclusion that possesses the so-called Eshelby uniformity property. Previously, only the weak version of the conjecture has been proved for the isotropic medium, whereas the general validity of the conjecture for anisotropic media in three dimensions is yet to be explored. In this work, firstly, we present proofs of the weak version of the generalized Eshelby conjecture for anisotropic media that possess cubic, transversely isotropic, orthotropic, and monoclinic symmetries, which substantiates that only the ellipsoidal shape can transform all uniform eigenstrains into uniform elastic strain fields in a solitary inclusion in infinite media possessing these symmetries. Secondly, we prove that in these anisotropic media, there exist non-ellipsoidal inclusions that can transform particular polynomial eigenstrains of even degrees into polynomial elastic strain fields of the same even degrees in them. These results constitute counter-examples, in the strong sense, to the generalized high-order Eshelby conjecture (inverse problem of Eshelby’s polynomial conservation theorem) for polynomial eigenstrains in both anisotropic media and the isotropic medium (quadratic eigenstrain only). In addition, we also show that there are counter-examples to the strong version of the generalized Eshelby conjecture for uniform eigenstrains in these anisotropic media. A sufficient condition for the existence of those counter-example inclusions is provided. These findings reveal striking richness of the uniformity between the eigenstrains and the correspondingly induced elastic strains in inclusions in anisotropic media beyond the canonical ellipsoidal inclusions. Since the strain fields in embedded and inherently anisotropic quantum dot crystals are effective tuning knobs of the quality of the emitted photons by the quantum dots, the results may have implications in the technology of quantum information, in addition to in mechanics and materials science.

The decoherence time of anisotropic quantum dot qubit: Effects of electric field, electron–phonon interactions, and impurities

In the field of quantum computation , one of the key issues is how to prolong the decoherence time of qubits. In the present work, the decoherence time is calculated in the framework of the Landau-Pekar variational approach for an anisotropic quantum dot (QD) under external electric fields including electron–phonon interactions. We obtained the parameterized eigenenergies of the ground state (GS) and the first excited-state (ES) by constructing the electronic wave functions for the GS and the first ES. We demonstrate that the calculation can explain consistently the dependence of decoherence time on the electric field strength, longitudinal-optical phonons and impurities. Our results should be useful for experimental investigation of QD qubit. • This paper studied investigated decoherence time of anisotropic quantum dot qubit. • A Landau-Pekar method is employed to obtain the GS and first ES energies. • An analysis of how the decoherence time depend on the electric field is presented. • We plotted the parameter phase diagram between the confinement length in the z-direction and the electric field strength.

Influence of Rashba effect and Zeeman effect on the properties of bound magnetopolaron in an Anisotropic Quantum Dot

Influence of Rashba effect and Zeeman effect on the properties of bound magnetopolaron in an Anisotropic Quantum Dot

Influence of a magnetic field on Rashba spin–orbit interaction in an anisotropic quantum dot

The influence of a magnetic field on the Rashba spin–orbit interaction in an anisotropic quantum dot is theoretically studied, and the expression of the ground state energy of a magnetopolaron is obtained with the Pekar variational method. The ground state energy of the magnetopolaron splits into two branches due to the Rashba effect, and the splitting appears saturated phenomenon with increasing the transverse and longitudinal effective confinement lengths. Because the contribution of the magnetic field cyclotron resonance frequency to the Rashba spin–orbit splitting is a positive value, the energy spacing becomes larger as the magnetic field cyclotron resonance frequency increases. Due to the spin–orbit coupling interaction, the energy splits at zero magnetic field. The total energy is reduced due to the presence of phonons. Therefore, the polaron state is more stable than the bare electron state, and the polaron energy splitting is more stable.

First-excited-state energy of hydrogenic impurity bound polaron in an anisotropic quantum dot in an electric field

The first-excited-state (ES) binding energy of hydrogenic impurity bound polaron in an anisotropic quantum dot (QD) is obtained by constructing a variational wavefunction under the action of a uniform external electric field. As for a comparison, the ground-state (GS) binding energy of the system is also included. We apply numerical calculations to KBr QD with stronger electron–phonon (E–P) interaction in which the new variational wavefunction is adopted. We analyzed specifically the effects of electric field and the effects of both the position of the impurity and confinement lengths in the xy-plane and the [Formula: see text] direction on the ground and the first-ES binding energies (BEs). The results show that the selected trial wavefunction in the ES is appropriate and effective for the current research system.

Effective tuning of isotropic and anisotropic properties of quantum dots and rings by external fields

Abstract The combined effects of intense THz laser field, magnetic field and in-plane electric field on electronic states and intraband optical properties of single isotropic quantum dots and rings are investigated using the non-perturbative Floquet theory in the high-frequency limit. Here we show that the change of the electric field direction for fixed values of intense THz laser field parameter can recover the degeneracy of the excited states and the dipole-allowed transition intensities in a quantum dot. Additionally, it is shown that in isotropic quantum rings the intense THz laser and the in-plane electric field create unusual Aharonov-Bohm oscillations. For fixed values of the intense THz laser field parameter, the amplitudes of Aharonov-Bohm oscillations can be effectively tuned by changing the electric field direction. Furthermore, for fixed values of the electric field strength and the laser field parameter the intraband optical properties can be effectively tuned by changing the electric field direction. Therefore, it can be asserted that circular quantum dots or circular rings in the intense THz laser field are equivalent to anisotropic quantum dots or rings respectively, and the electric field direction effectively tunes the single-electron energy spectrum and intraband optical properties of these structures.

Influence of Rashba Effect on the Bound Magnetopolaron in an Anisotropic Quantum Dot

Using Pekar variational method, we studied the influence of Rashba effect on the bound magnetopolaron in an anisotropic quantum dot. The expression of the ground state energy of the bound magnetopolaron is obtained by theoretical derivation. Due to the influence of the Rashba effect, the ground state energy of the bound magnetopolaron splits into two branches. This phenomenon fully demonstrates that the influence of orbit and spin interaction in different directions on the energy of the polaron is not negligible. Because the contribution of the magnetic field cyclotron resonance frequency to the Rashba spin-orbit splitting is a positive value, the energy spacing becomes larger as the magnetic field cyclotron resonance frequency increases. Due to the presence of phonons and impurities, the polaron is more stable than the bare electron state, and the energy splitting is more stable.

Readout of a dopant spin in the anisotropic quantum dot with a single magnetic ion

Owing to exchange interaction between the exciton and magnetic ion, a quantum dot embedding a single magnetic ion is a great platform for optical control of individual spin. In particular, a quantum dot provides strong and sharp optical transitions, which give experimental access to spin states of an individual magnetic ion. We show, however, that physics of quantum dot excitons also complicate spin readout and optical spin manipulation in such a system. This is due to electron-hole exchange interaction in anisotropic quantum dots, which affects the polarisation of the emission lines. One of the consequences is that the intensity of spectral lines in a single spectrum are not simply proportional to the population of various spin states of magnetic ion. In order to provide a solution of the above problem, we present a method of extracting both the spin polarisation degree of a neutral exciton and magnetic dopant inside a semiconductor quantum dot in an external magnetic field. Our approach is experimentally verified on a system of CdSe/ZnSe quantum dot containing a single Fe2+ ion. Both the resonant and non-resonant excitation regimes are explored resulting in a record high optical orientation efficiency of dopant spin in the former case. The proposed solutions can be easily expanded to any other system of quantum dots containing magnetic dopants.

Optical Properties, Synthesis, and Potential Applications of Cu-Based Ternary or Quaternary Anisotropic Quantum Dots, Polytypic Nanocrystals, and Core/Shell Heterostructures.

This review summaries the optical properties, recent progress in synthesis, and a range of applications of luminescent Cu-based ternary or quaternary quantum dots (QDs). We first present the unique optical properties of the Cu-based multicomponent QDs, regarding their emission mechanism, high photoluminescent quantum yields (PLQYs), size-dependent bandgap, composition-dependent bandgap, broad emission range, large Stokes’ shift, and long photoluminescent (PL) lifetimes. Huge progress has taken place in this area over the past years, via detailed experimenting and modelling, giving a much more complete understanding of these nanomaterials and enabling the means to control and therefore take full advantage of their important properties. We then fully explore the techniques to prepare the various types of Cu-based ternary or quaternary QDs (including anisotropic nanocrystals (NCs), polytypic NCs, and spherical, nanorod and tetrapod core/shell heterostructures) are introduced in subsequent sections. To date, various strategies have been employed to understand and control the QDs distinct and new morphologies, with the recent development of Cu-based nanorod and tetrapod structure synthesis highlighted. Next, we summarize a series of applications of these luminescent Cu-based anisotropic and core/shell heterostructures, covering luminescent solar concentrators (LSCs), bioimaging and light emitting diodes (LEDs). Finally, we provide perspectives on the overall current status, challenges, and future directions in this field. The confluence of advances in the synthesis, properties, and applications of these Cu-based QDs presents an important opportunity to a wide-range of fields and this piece gives the reader the knowledge to grasp these exciting developments.

Binding energies and chemical potential of neutral and charged exciton-complexes of transition metal dichalcogenides/anisotropic 2-D quantum dots in magnetic field by exact multi-pole expansion of coulomb correlations

Anisotropy is inevitable fate of charged excitonic complexes due to difference in dielectric constants, effective masses, charges of carriers, magnetic field, applied gate voltage etc along in-plane and out-of-plane directions of transition metal dichalcogenides (TMDCs-MX2; M = Mo, W; X = S, Se, Te) as well as in anisotropic quantum dots. In the present paper, the multi-dynamic attractive and scattering interactions among neutral and charged exciton-complexes (N-excitonic, N = 2,3,4, ….) amongst the various charge carriers, i.e., e − h, e − e, h − h has been elegantly accounted for by the use of Lauricella functions and the subsequent application of Chu-vandermonde identity. An exact variational treatment for such complex and intertwined interactions in presence of anisotropy has been achieved by employing multi-pole expansion. The result of which emerges as a unique single-summed, finite, exact and absolutely terminating integrals. The calculated binding energies for various N-excitonic systems have been found to be consistent with the previous experimental and computational studies. A discussion of energy spectra, chemical potential (μ) and addition energy (Δμ) are presented. The results thus obtained indicate increase in binding energy and bosonic character of the multi-excitonic complexes with magnetic field. The effects of anisotropy are manifested through the interplay between many-body coulomb interactions resulting in the dissolution of degeneracy of excitonic states.

Effect of electric field and temperature on the binding energy of bound polaron in an anisotropic quantum dot

The ratio between the confinement lengths in the xy-plane and the z direction plays an important role in determining the properties of anisotropic quantum dot. Within a variational approach of Pekar type, we investigated theoretically the effects of electric field and temperature on the ground-state binding energies of hydrogenic impurity polarons in KBr anisotropic quantum dot. The obtained results illustrate that the binding energies increase with the electric field strength and temperature but decrease with the position of the impurity when considering different confinement lengths in the xy-plane and the z direction and present the properties of the anisotropic quantum dot.

Influence of surface nano-patterning on the placement of InAs quantum dots

We have examined the influence of spontaneous nano-patterning on the placement of InAs quantum dots (QDs) on (Al)GaAs surfaces using an experimental-computational approach. Both atomically flat and mounded surfaces, generated via a surface instability induced by the Ehrlich-Schwoebel barrier, are employed as templates for the subsequent deposition of InAs QDs. Using height profiles from atomic-force micrographs, we simulate QD deposition with a 2D phase field model, which describes the time evolution of the InAs layer driven by a chemical potential gradient. For flat surfaces, phase-field simulations result in QD densities comparable to experimental observations. For mounded surfaces, the simulations reveal QDs preferentially positioned in regions of positive curvature (substrate valleys), e.g., at the edge of surface mounds, consistent with the anisotropic QD placement observed experimentally. We discuss the role of curvature-driven diffusion in the spontaneous ordering of QDs, demonstrating the applicability of this mechanism to AlGaAs mounds.

Shell structure and paramagnetism of 3-DN-eanisotropic (ellipsoidal) quantum dots: Exact multi-pole expansion of coulomb interaction under fermionic exchange symmetry

As the density of electrons (N=2, 3, 4, 5, 6, 7, 8,.) increases, complexity arises due to coulomb interactions, inclusion of fermionic exchange symmetry and anisotropy. Consequently, Schrodinger equations of anisotropic quantum dots become non-trivial. Recasting such non-relativistic quantum equations into Whittaker-M basis functions unifies coulomb (exchange) correlation and antisymmetric nature of electrons in two-centered integrals of exact, finite, single-summed, terminated and simplest Lauricella functions (F2) via multi-pole expansion and the subsequent Chu-Vandermonde identity. Coulomb correlations interplay with in-plane and out-of-plane electrical and magnetic confinements in their moderate fields. On the other hand, full-scale fermionic exchange symmetry is incorporated through multiple number of Slater determinants, unlike Hartree-Fock method. In this context, both open-shell and restricted closed-shell configurations are considered for trial Hartree-products which are composed of the basis set of oscillator spin-orbitals. Thus optimized bound states can easily capture large number of mixed term symbols. Consequently, level clustering/accidental degeneracies occur in energy level diagram due to competition among strength of anisotropy in electrical confinements, magnetic field, mass of the carrier and dielectric constant. It brings about orbital induced paramagnetism (T∼(0-1)K), signature of fractional quantum Hall effect (FQHE) in chemical potential cusps (μ) and formation of different ′shell structures′ in capacitive energy respectively, spanning over wide dielectric range of materials (atomic like quantum dot, ZnO, GaAs, CdSe (Cadmium Chalcogenide) and PbSe (Lead Chalcogenide) etc).As the density of electrons (N=2, 3, 4, 5, 6, 7, 8,.) increases, complexity arises due to coulomb interactions, inclusion of fermionic exchange symmetry and anisotropy. Consequently, Schrodinger equations of anisotropic quantum dots become non-trivial. Recasting such non-relativistic quantum equations into Whittaker-M basis functions unifies coulomb (exchange) correlation and antisymmetric nature of electrons in two-centered integrals of exact, finite, single-summed, terminated and simplest Lauricella functions (F2) via multi-pole expansion and the subsequent Chu-Vandermonde identity. Coulomb correlations interplay with in-plane and out-of-plane electrical and magnetic confinements in their moderate fields. On the other hand, full-scale fermionic exchange symmetry is incorporated through multiple number of Slater determinants, unlike Hartree-Fock method. In this context, both open-shell and restricted closed-shell configurations are considered for trial Hartree-products which are composed of the basis set...

Addition energy and magnetization of 3-D multicarrier anisotropic quantum dots in magnetic field by exact multi-pole expansion of coulomb correlations

Coulomb interactions replicates in N(N − 1)∕2 terms for interacting 3-D N − e confined quantum dots leading to non-trivial Schrödinger equations. For the systems having N > 3, e − e interaction reciprocating to both lateral (electrical) and transverse (magnetic) confinements in moderate fields causes acute interplay in anisotropic quantum dots. As DFT, Hartree-Fock and other perturbative methods have certain limitations, recasting Schrödinger equations into self-adjoint Whittaker-M functions addresses coulomb (exchange) integrals as simplest, finite and single-summed analytical Lauricella functions (F2) via Chu-Vandermonde identity. In case of higher ′N′, expansion of coulomb correlations in terms of multi-pole expansion facilitates the convergences for bound states upto quadrapole and octopole terms. Although, fermionic exchange symmetry of many electron systems could be included in terms of various two-electron integrals, for the sake of brevity we have aimed to reproduce experimental results without spin. Level clustering/accidential degeneracies manifest on energy level spectra due to competitive role among anisotropic confinement strength, magnetic field, mass of the carrier and dielectric constant of the medium. It triggers orbital induced paramagnetism (T ∼ (0 − 1)K), signature of fractional quantum Hall effect (FQHE) in chemical potential cusps (μ) and formation of different ′shell structures′ in capacitive energy respectively, spanning over wide dielectric range of materials (atomic like quantum dots, ZnO, GaAs and PbSe (Lead Chalcogenide) etc).

Hydrogenic impurity bound polaron in an anisotropic quantum dot

The effect of the electron–phonon interaction on an electron bound to a hydrogenic impurity in a three-dimensional (3D) anisotropic quantum dot (QD) is studied theoretically. We use the Landau–Pekar variational approach to calculate the binding energy of ground state (GS) and first-excited state (ES) with considering electron–phonon interaction. The expressions of the GS and ES energies under investigation depict a rich variety of dependent relationship with the variational parameters in three different limiting cases. Numerical calculations were performed for ZnSe QDs with different confinement lengths in the xy-plane and the z-direction, respectively. It is illustrated that binding energies of impurity polarons corresponding to each level are larger in small QDs. Furthermore, the contribution to binding energy from phonon is about 15% of the total binding energy.