Band Mixing Effects Research Articles

Maximally localized Wannier functions, interaction models, and fractional quantum anomalous Hall effect in twisted bilayer MoTe2

We investigate the moiré band structures and the strong correlation effects in twisted bilayer MoTe[Formula: see text] for a wide range of twist angles, employing a combination of various techniques. Using large-scale first-principles calculations, we pinpoint realistic continuum modeling parameters, subsequently deriving the maximally localized Wannier functions for the top three moiré bands. Simplifying our model with reasonable assumptions, we obtain a minimal two-band model, encompassing Coulomb repulsion, correlated hopping, and spin exchange. Our minimal interaction models pave the way for further exploration of the rich many-body physics in twisted MoTe[Formula: see text]. Furthermore, we explore the phase diagrams of the system through Hartree-Fock approximation and exact diagonalization (ED). Our two-band ED analysis underscores significant band-mixing effects in this system, which enlarge the optimal twist angle for fractional quantum anomalous Hall states.

Impact of stacking sequence on the tight-binding electronic band structures of (BeX)m/(ZnX)m, X = S, Se and Te superlattices

By using a semi-empirical tight-binding sp3s* method, the results of comprehensive electronic band structures are reported for the zinc-blende beryllium- and zinc-chalcogenides (BeX and ZnX; X = S, Se, Te) as well as their representative (BeX)m/(ZnX)m (001) superlattices (SLs). For the bulk BeX and ZnX materials, the simulations of energy band dispersions Ejk→ have offered the correct band gaps in very good agreement with the first-principles calculations. The band-mixing effect through the interfaces of two constituent compounds (BeX indirect- and ZnX direct-band gap) has played an important role for determining the overall band lineup in the (BeX)m /(ZnX)m SLs over the entire Brillouin zone. Based on the quantum confinement effects, the impact of stacking sequence m ≤ 10 is carefully examined for assessing the band structures of SLs. The results have clearly revealed that the nature of energy bandgaps is quite sensitive to the choice of well (BeX) and barrier (ZnX) layer thickness. Obviously, this intuition has implied that controlling m to achieve direct bandgaps in novel (BeX)m/(ZnX)m SLs is probably an effective way of assessing their potential use in technologically important optoelectronic devices.

Investigation of orientation dependence of piezoelectric effects in strained GaAs/InGaAs quantum well laser

In this work, we report a theoretical investigation of the influence of piezoelectric on the optical transition and laser-gain in [111]-oriented GaAs/InGaAs quantum well (QW). A k.p multi-bands Hamiltonian, taking into account the band mixing and strain effect has been introduced and factorized into two 4×4 real matrices via a unitary transformation. The impact of the piezoelectric on the band structure and optical transition and gain is visualized under inclusion of the strain in the [111]-oriented QWs. It has been shown that the piezoelectric changes the electronic band structure and as a result the optical transition and gain decreases with the piezoelectric field by about 50% for transverse electric and magnetic modes. Spinor and wave functions distributions of the band states in the QW are also calculated. It is found that the wave function and spinor exhibit very different distribution patterns in the QWs oriented along [001]- and [111]-directions. Finally, we can predict that GaAs/InGaAs QW realized in the [001]-orientation are more convenient than QWs of [111]-orientation in most applications involving optical transitions and laser-gain for optoelectronics devices.

Numerical investigation into optical and electronic performance of crystal orientation-dependent InGaAs/InP near-infrared laser

The energy band dispersion profile and their dependence on crystal orientation for III-V zinc-blende compound quantum wells (QW) have earned significant attention in recent years due to reduced band mixing effects emerging from increased energy separation between valence subbands. So QW like InGaAs grown in non-conventional orientation seems to be quite optimistic to serve as active region in near-infrared optoelectronic applications due to small piezoelectric polarization, improved confinement of high energy states and increased favorable spectral range than traditional (100)-oriented growth. Here, crystal orientation-dependent electronic and optical performance of InGaAs-InP laser subjected to 1.60% compressive strain and emitting around 2 µm is studied numerically after working out an eight-band k.p Hamiltonian with the help of finite difference method and Tensor rotation technique. An equivalent circuit model using three-level laser rate equations is employed here to reveal optical output power and steady state frequency response. It is noticed that the wave functions of hole are more confined in (111) orientation than (100) orientation. Also, there is a noteworthy reliance of the energy band gap, optical gain spectra and output lasing power profile on change in crystal orientation. The estimated gains are found to be 4250, 3900, 3555, 3210 and 2950 cm−1 in (111), (100), (131), (110), and (113) orientations. The topmost lasing power and lowest threshold current are noted to be 56.4mW and 4.5 mA, respectively, in the (111) crystal orientation. Further, the highest optical emission point is observed to be moved towards longer wavelength for the alteration in crystal orientation from (131) to (111).

Ge1\u2212xSnx alloys: Consequences of band mixing effects for the evolution of the band gap \u0393-character with Sn concentration

In this work we study the nature of the band gap in GeSn alloys for use in silicon-based lasers. Special attention is paid to Sn-induced band mixing effects. We demonstrate from both experiment and ab-initio theory that the (direct) Γ-character of the GeSn band gap changes continuously with alloy composition and has significant Γ-character even at low (6%) Sn concentrations. The evolution of the Γ-character is due to Sn-induced conduction band mixing effects, in contrast to the sharp indirect-to-direct band gap transition obtained in conventional alloys such as Al1−xGaxAs. Understanding the band mixing effects is critical not only from a fundamental and basic properties viewpoint but also for designing photonic devices with enhanced capabilities utilizing GeSn and related material systems.

Influence of quantum well design on light polarization switching in AlGaN ultraviolet emitters

Polarization properties from AlGaN quantum well (QW) strongly determine the efficiency of deep ultraviolet (UV) light-emitting diodes (LEDs), hence knowing the critical Al-content at which the light polarization switches is essential for high-efficiency deep UV LED designs. This work theoretically investigates the influence of QW design on the light polarization switching in AlGaN-based UV LEDs. The physics analysis by using the self-consistent 6-band k·p model shows that the Al-content for valence subbands crossover presents an increasing trend as AlGaN QW thickness increases with consideration of polarization electric field, carrier screening effect and strain state. On the other hand, the critical Al-content where the transverse-electric-polarized spontaneous emission recombination rate (Rsp) is equal to the transverse-magnetic-polarized Rsp has the maximum value at the QW thickness of ∼1.5 nm. The difference between the two types of critical Al-contents can be explained by the quantum confined stark effect and the band mixing effect. The light polarization properties from reported AlGaN-based UV emitters show a similar trend to our theoretical results on critical Al-contents, indicating the importance on the understanding of QW design for high-efficiency deep-UV emitters.

Symmetric analysis, categorization, and optical spectrum of ideal pyramid quantum dots

Self-assembled quantum dots possess an intrinsic geometric symmetry. Applying group representation theory, we systematically analyze the symmetric properties of the bound states for ideal pyramid quantum dots, which neglect band mixing and strain effects. We label each bound state by its symmetry group’s corresponding irreducible representation and define a concept called the quantum dots’ symmetry category. A class of quantum dots with the same irreducible representation sequence of bound states are characterized as belonging to a specific symmetry category. This category concept generally describes the symmetric order of Hilbert space or wavefunction space. We clearly identify the connection between the symmetry category and the geometry of quantum dots by the symmetry category graph or map. The symmetry category change or transition corresponds to an accidental degeneracy of the bound states. The symmetry category and category transition are observable from the photocurrent spectroscopy or optical spectrum. For simplicity’s sake, in this paper, we only focus on inter-subband transition spectra, but the methodology can be extended to the inter-band transition cases. We predict that from the spectral measurements, the quantum dots’ geometric information may be inversely extracted.

Finite valence nucleon number and rotation-vibration interactions

Background: A characteristic observable of nuclear collective motion is the relative $B(E2)$ values from the $\ensuremath{\gamma}$ band to the ground band in even-even deformed nuclei. The Alaga rules provide an idealized set of benchmarks for these observables. However, deviations from the Alaga rules are universally observed and have been traditionally and successfully interpreted in terms of parameterized $\ensuremath{\gamma}$-band--ground-band bandmixing. An alternate approach, partial dynamical symmetries, has no bandmixing whatsoever and is parameter free, yet mimics closely the effects of bandmixing, due solely to the effects of finite valence nucleon number.Purpose: To investigate the relation between these two seemingly contradictory approaches to understand how they can produce such similar results.Method: To derive approximate relations between the two formalisms.Results: A consistent relationship is found linking bandmixing to finite valence nucleon number effects on interband $\ensuremath{\gamma}$ to ground-band $B(E2)$ values.Conclusions: Two disparate approaches to one of the iconic characteristics of deformed nuclei are shown to be intimately related. Moreover, a systematic difference in their predictions also emerges naturally from the derivation. The qualitative linkage of valence nucleon number and the separation of vibrational and rotational degrees of freedom has long been assumed but never before explicitly demonstrated through complementary models.

Theoretical analysis of influence of random alloy fluctuations on the optoelectronic properties of site-controlled (111)-oriented InGaAs/GaAs quantum dots

We use an $sp^3d^5s^* $ tight-binding model to investigate the electronic and optical properties of realistic site-controlled (111)-oriented InGaAs/GaAs quantum dots. Special attention is paid to the impact of random alloy fluctuations on key factors that determine the fine-structure splitting in these systems. Using a pure InAs/GaAs quantum dot as a reference system, we show that the combination of spin-orbit coupling and biaxial strain effects can lead to sizeable spin-splitting effects in these systems. Then, a realistic alloyed InGaAs/GaAs quantum dot with 25\% InAs content is studied. Our analysis reveals that the impact of random alloy fluctuations on the electronic and optical properties of (111)-oriented InGaAs/GaAs quantum dots reduces strongly as the lateral size of the dot increases and approaches realistic sizes. For instance the optical matrix element shows an almost vanishing anisotropy in the (111)-growth plane. Furthermore, conduction and valence band mixing effects in the system under consideration are strongly reduced compared to standard (100)-oriented InGaAs/GaAs systems. All these factors strongly indicate a reduced fine structure splitting in site-controlled (111)-oriented InGaAs/GaAs quantum dots. Thus, we conclude that quantum dots with realistic (50-80~nm) base length represent promising candidates for polarization entangled photon generation, consistent with recent experimental data.

Renormalization of optical transition strengths in semiconductor nanoparticles due to band mixing

Unique optical properties of semiconductor nanoparticles (SN) make them very promising in the multitude of applications including lasing, light emission and photovoltaics. In many of these applications it is imperative to understand the physics of interaction of electrons in a SN with external electromagnetic fields on the quantitative level. In particular, the strength of electron–photon coupling determines such important SN parameters as the radiative lifetime and absorption cross section. This strength is often assumed to be fully encoded by the so called Kane momentum matrix element. This parameter, however, pertains to a bulk semiconductor material and, as such, is not sensitive to the quantum confinement effects in SNs. In this work we demonstrate that the quantum confinement, via the so called band mixing, can result in a significant suppression of the strength of electron interaction with electromagnetic field. Within the envelope function formalism we show how this suppression can be described by introducing an effective energy-dependent Kane momentum. Then, the effect of band mixing on the efficiencies of various photoinduced processes can be fully captured by the conventional formulae (e.g., spontaneous emission rate), once the conventional Kane momentum is substituted with the renormalized energy-dependent Kane momentum introduced in here. As an example, we evaluate the energy-dependent Kane momentum for spherical PbSe and PbS SNs (i.e., quantum dots) and show that neglecting band mixing in these systems can result in the overestimation of absorption cross sections and emission rates by a factor of ∼2.

Characterization of InN-In0.25Ga0.75N Quantum Well Laser with In0.4Al0.6N Layers for 1300 nm Band

ABSTRACTA wurtzite-strained nitride Quantum Well Laser has been characterized for short distance communication wavelength. InN and In0.25Ga0.75N have been chosen as well material and barrier material respectively with In0.4Al0.6N SCH layers at the end of barrier layers to improve the carrier and photon confinement within the active region. This structure shows less compressive strain (7.33%) with respect to previously proposed structure which makes the structure more suitable for fabrication. To obtain the electronic band structure, self-consistent method with k.p formalism has been performed where valence band mixing effect, strain effect and spontaneous and piezoelectric polarization effect has been included. From the electronic characteristics, the optical properties have been performed with numerical model. From the optical properties, the structure has been found as TE polarized with C1-HH1, C1-LH1, C2-HH1 and C2-LH1 dominating transition elements. From the performance of the numerical model, 4731.98 cm−1 optical gain for TE polarization at 1315.5 nm emission wavelength and 8.017×1027 cm−3s−1eV−1 spontaneous emission rate at 1301.7nm wavelength have been found for 12Å well width, 17Å barrier width and 52Å SCH layer width at 5×1019 cm−3 carrier density. The obtained properties have been shown a good agreement with previously published works.

Electronic and optical properties of InN nanowires

We have employed a multiband envelope function method to study wurtzite [0001] InN nanowires of cylindrical cross section. The electronic subband structure and optical absorption spectrum are calculated as a function of the nanowire radius. The energies of the Γ-point conduction band states show a monotonous increase with decreasing radius. On the other hand, the size dispersion of the valence band states is more involved, showing various crossing and anticrossing effects due to the interplay between the confinement and band mixing effects. Thus, for small (R<2nm) nanowires the highest valence band state has dominant pz-symmetry, but for R>2nm the highest state has dominant p⊥-symmetry, although it is optically inactive. The optical absorption spectra polarized along or perpendicular to the nanowire axis are different on account of the different transitions involved. The spectra of the smaller nanowires display a distinguishable sequence of peaks reflecting the one-dimensional density-of-states. The absorption peaks become denser for larger nanowires, and for radii larger than 10nm the spectrum starts to ressemble that of bulk InN.

Characterization of InN - In0.25Ga0.75N Quantum Well Laser Structure for 1330 nm Wavelength

In this work, a nitride based wurtzite-strained single quantum well laser has been designed and characterized at 1330 nm wavelength. Here, 12Å InN layer has been used as well material and 15Å In0.25Ga0.75N layer has been used as barrier material along with GaN as separate confinement heterostructure for better carrier and optical confinement. To determine the electronic properties of the designed structure, a developed self-consistent method has been solved. Conduction subband calculation has been performed with the solution of Schrodinger equation formed by single band Hamiltonian followed by Poisson's equation. 6-bands k . p Hamiltonian for wurtzite semiconductor has been solved for valence band Schrodinger's equation with Poisson's equation considering band mixing effect, strain due to lattice mismatch and spontaneous and piezoelectric polarization. The interband momentum matrix elements, optical gain, spontaneous emission rate, spontaneous radiative recombination rate and radiative current density have been calculated to analyze the optical properties of the designed laser and a good agreement with previously published works is obtained. All analysis has been performed using injection carrier density of 6×10-19 cm-3 at T = 300 K. Due to the strain effect, the wave-function overlap integral has been obtained as 43.27%. From the electronic properties analysis, it has been found that, the designed structure is TE polarized with C1-HH1 and C1-LH1 dominating transitions. From the analysis of optical properties, the obtained spontaneous emission rate is 5945.2 s-1 at 1329.55 nm and spontaneous emission rate per energy interval per unit volume is 7.21×1027 s-1cm-3eV-1 at same wavelength. The obtained radiative recombination rate is 7.77×1029 s-1cm-3 and radiative current density is 149.19 Acm-2. The optical gain for TE polarization of the designed QW structure is 5261.52cm-1 at 1336.7 nm. Figure 1

Optical Gain Optimization of Al0.8Ga0.2N-Delta-GaN Quantum Well Laser in Ultraviolet Spectra Using Genetic Algorithm

In this work, AlGaN based quantum well laser has been designed with delta layer and characterized the designed laser structure. After the characterization, Genetic Algorithm process has been performed over optical gain to optimize it. In this design, AlN has been used as barrier material, Al0.8Ga0.2N has been utilized as active region material and GaN layer has been used as delta layer. For optimization, well width, delta layer width and barrier width have been taken as the variable for which genetic algorithm has performed the optimization process. Characterization of the designed laser shows good agreement with previously published works. Optimization of optical gain for both TE and TM gain also exhibits good improved result than previously published works. From characterization, it is found that the designed laser structure gives peak optical gain of 3589.5 cm−1 and peak spontaneous emission rate of 12.94×1028 s-1cm-3eV-1 at 246.89 nm for 5×1019 cm-3 carrier density. From optimization, it is obtained that optimum TE optical gain is found 6479.1 cm-1 for 11 Å of QW along with 7 Å delta layer at 279 nm wavelength and optimum TM gain is found found 116.95 cm-1 for 20 Å of QW along with 3 Å delta layer at 289.8 nm wavelength. In this work, electronic characterization has been performed with 6 band k.p Hamiltonian for valence subband calculation including valence band mixing effect, strain effect due to lattice mismatch, spontaneous and piezoelectric polarization. For conduction subband calculation, single band Hamiltonian has been solved with strain and polarization effect. Figure 1

Structure of the quantum spin Hall states in HgTe/CdTe and InAs/GaSb/AlSb quantum wells

A solution of the $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ model is presented for bulk and quantum spin hall (QSH) edge states in semiconductor topological insulator (TI) quantum wells (QWs), bounded at the edge by an infinite wall potential. The edge states are exponentially localized, with a nonzero amplitude at the QW edge, and obey standard boundary conditions for the wave function and its derivative. Single helical edge states with spin locked to the direction of motion are found in the TI band gap $({E}_{\mathrm{TI}})$ of QWs with both strong (HgTe/CdTe) and weak (InAs/GaSb/AlSb) $s\text{\ensuremath{-}}p$ hybridization, but in the second case only below a small critical band gap, ${E}_{\mathrm{crit}}\ensuremath{\sim}1.6\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$. For ${E}_{\mathrm{TI}}&gt;{E}_{\mathrm{crit}}$, there appear to be two degenerate states for each spin direction. It is suggested that ${Z}_{2}$-like topological properties can still be maintained if one of these states is spurious or suppressed by disorder. The effect of interface band mixing, and band mixing due to structural inversion asymmetry and bulk inversion asymmetry is also considered. Simple model Hamiltonians are developed for the bulk and edge states which are calibrated against a bulk eight-band $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ calculation close to the TI transition. At the transition, the zero gap bulk states exhibit a spin splitting, essentially changing the Dirac point to a circle. In the TI phase, there is a small change in the dispersion of the QSH edge states. These results confirm the robustness of the QSH edge states to spatial symmetry breaking interactions.

Dispersions of hole and electron g-factors in single InAs quantum rings evaluated using optically induced nuclear spin polarization

The electron and hole g-factors in individual InAs/GaAs quantum rings were evaluated using the bistable responses of the optically induced nuclear spin polarization. Although the dispersions of the hole and exciton g-factors were larger than that of the electron g-factor in the individual quantum rings, a strong correlation between the hole and exciton g-factors was clearly observed. Part of the dispersion of the measured hole g-factor was explained well by considering the effect of strain-induced valence band mixing.

A theoretical analysis of the optical absorption properties in one-dimensional InAs/GaAs quantum dot superlattices

We present theoretical investigations of miniband structures and optical properties of InAs/GaAs one-dimensional quantum dot superlattices (1D-QDSLs). The calculation is based on the multi-band k·p theory, including the conduction and valence band mixing effects, the strain effect, and the piezoelectric effect; all three effects have periodic boundary conditions. We find that both the electronic and optical properties of the 1D-QDSLs show unique states which are different from those of well known single quantum dots (QDs) or quantum wires. We predict that the optical absorption spectra of the 1D-QDSLs strongly depend on the inter-dot spacing because of the inter-dot carrier coupling and changing strain states, which strongly influence the conduction and valence band potentials. The inter-miniband transitions form the absorption bands. Those absorption bands can be tuned from almost continuous (closely stacked QD case) to spike-like shape (almost isolated QD case) by changing the inter-dot spacing. The polarization of the lowest absorption peak for the 1D-QDSLs changes from being parallel to the stacking direction to being perpendicular to the stacking direction as the inter-dot spacing increases. In the case of closely stacked QDs, in-plane anisotropy, especially [110] and [11¯0] directions also depend on the inter-dot spacing. Our findings and predictions will provide an additional degree of freedom for the design of QD-based optoelectronic devices.

Extracting the Chern Number from the Dynamics of a Fermi Gas: Implementing a Quantum Hall Bar for Cold Atoms

We propose a scheme to measure the quantized Hall conductivity of an ultracold Fermi gas initially prepared in a topological Chern insulating phase and driven by a constant force. We show that the time evolution of the center of mass, after releasing the cloud, provides a direct and clear signature of the topologically invariant Chern number. We discuss the validity of this scheme, highlighting the importance of driving the system with a sufficiently strong force to displace the cloud over measurable distances while avoiding band-mixing effects. The unusual shapes of the driven atomic cloud are qualitatively discussed in terms of a semiclassical approach.

Strain and band-mixing effects on the excitonic Aharonov-Bohm effect in In(Ga)As/GaAs ringlike quantum dots

Neutral excitons in strained axially symmetric In(Ga)As/GaAs quantum dots with ringlike shape are investigated. Similar to experimental self-assembled quantum rings, the analyzed quantum dots have volcano-like shapes. The continuum mechanical model is employed to determine the strain distribution, and the single-band envelope function approach is adopted to compute the electron states. The hole states are determined by the axially symmetric multiband Luttinger-Kohn Hamiltonian, and the exciton states are obtained from an exact diagonalization. We found that the presence of the inner layer covering the ring opening enhances the excitonic Aharonov-Bohm (AB) oscillations. The reason is that the hole becomes mainly localized in the inner part of the quantum dot due to strain, whereas the electron resides mainly inside the ring-shaped rim. Interestingly, larger AB oscillations are found in the analyzed quantum dot than in a fully opened quantum ring of the same width. Comparison with the unstrained ring-like quantum dot shows that the amplitude of the excitonic Aharonov-Bohm oscillations are almost doubled in the presence of strain. The computed oscillations of the exciton energy levels are comparable in magnitude to the oscillations measured in recent experiments.

The effects of carrier dependant nonlinear gain on quantum well VCSEL characteristics

In this paper, we present a numerical opto-electro-thermal model for studying vertical cavity surface emitting lasers operation. The model is applied to an index-guided structure with an oxide aperture and multiple quantum-wells in active layer. The interdependent process of carrier transport, heat generation and optical field are solved self-consistently using finite difference time domain in cylindrical system. The gain of quantum wells (QWs) is calculated based on the solution of Schrodinger equation considering heavy hole-light hole band-mixing effect. The calculated maximum gain versus injected carriers is fitted by a 3th order polynomial function and used in opto-electro-thermal model. The inclusion of QW maximum gain calculation for constant wavelength in the model allows us to study threshold current value and higher order transverse modes as well as their dependencies on variation of gain and refractive index induced by carrier and heat more accurately than linear gain approximation. The results show a lower threshold current compared with linear gain approximation. For injection current above the threshold, we consider the spatial hole burning, thermal lensing and self focusing effects.