Magneto-optical Absorption Spectra Research Articles

Electro- and magneto- optical properties of edge states in bilayer black phosphorene quantum dots

We theoretically investigate the electronic and optical properties of stacked bilayer black phosphorus (BBPs) quantum dots (QDs) with three typical BBPs configurations and two AB-stacking with different edge structures, using the tight-binding (TB) method. Our calculation results show that the energy levels can be affected by the different stacking configurations of BBPs. Unlike the monolayer black phosphorus (MBP), the strong inter-layer coupling in AC-stacked BBP and unsymmetrical distribution in AB-stacked BBP will enhance the optical transitions and realize a dark-to-bright transition in the electro and magneto-optical absorption spectra. The coupling strength of the edge states of different configurations is different, resulting in the difference of the properties of the three stacking configurations, which provides a feasible idea for modulating the electronic and optical properties of BBPs.

Effects of tilted Dirac cones and in-plane electric field on the valley-dependent magneto-optical absorption spectra in monolayer 8–Pmmn borophene

We investigate the optical absorption properties in a 8-Pmmn borophene monolayer with anisotropic tilted Dirac cones, subjected to a perpendicular magnetic field and in-plane dc electric field (EF). Numerical results for the magneto-optical absorption coefficient (MOAC) show that the tilt of the Dirac cones causes the redshift of absorption peaks. More interestingly, there appears minor absorption peaks corresponding to the transitions between the Landau levels (LLs) n and n±2 beside the principal maxima corresponding to the transitions between LLs n and n±1. These minor peaks are absent in other two-dimensional Dirac materials with non-titled Dirac cones. The simultaneous presence of in-plane EF and the tilt parameter in 8–Pmmn borophene lifts the valley degeneracy in the LLs and causes the valley polarization in the MOAC, that do not occur in the absence of in-plane EF and/or the tilt parameter. The valley-polarized MOAC opens up potential applications in valleytronics.

Geometry tailored magneto-optical absorption spectra of elliptically deformed double quantum rings

ABSTRACT We theoretically investigated the effect of the geometry change on the electronic and magneto-optical absorption spectra of GaAs/Ga0.7Al0.3As double quantum rings. The inner ring shape was changed continuously from circular to elliptic. Our numerical results revealed the decrement of the levels energies with the augments of the eccentricity of the internal/external ellipses that limit the inner ring, leading to a blue shift of the maxima in the absorption spectra in the magnetic field absence. Analysing separately the electronic spectra of the inner/outer single rings and of the double quantum rings structure in the magnetic field, we have found that, in the electronic spectra of the double ring, only the energies and the Aharonov–Bohm periodicity of the outer ring are altered by interference, while the Aharonov–Bohm oscillations of the inner ring levels propagate unchanged in the spectra. The magneto-optical spectra of elliptically deformed double quantum rings cover a wider range of energies than those of circular double ring and, at higher magnetic fields, present new peaks corresponding to transitions to higher states. Also, they show superposed peaks of increased absorption for a particular elliptic geometry and a red-shift of low energy maxima at the augment of the magnetic field which is the more pronounced the eccentricity increases. Moreover, the influence of the temperature is stronger than for the symmetrical structure.

Anisotropic magneto-optical absorption and linear dichroism in two-dimensional semi-Dirac electron systems

We present a theoretical study on the Landau levels (LLs) and magneto-optical absorption of a two-dimensional semi-Dirac electron system under a perpendicular magnetic field. Based on an effective k.p Hamiltonian, we find that the LLs are proportional to the two-thirds power law of the magnetic field and level index, which can be understood as a hybridization of the LL of Schrodinger and Dirac electrons with new features. With the help of Kubo formula, we find the selection rule for interband (intraband) magneto-optical transition is anisotropic (isotropic). Specifically, the selection rules for interband magneto-optical transitions are $\Delta n$=0, $\pm2$ ($\pm2$, $\pm4$) for linearly polarized light along the linear (parabolic) dispersion direction, while the selection rules for the intraband transition are $\Delta n$=$\pm1$, $\pm3$ regardless of the polarization direction of the light. Further, the magneto-optical conductivity for interband (intraband) transition excited by linearly polarized light along the linear dispersion direction is two (one) orders of magnitude larger than that along the parabolic dispersion direction. This anisotropic magneto-optical absorption spectrum clearly reflects the structure of the LLs, and results in a strong linear dichroism. Importantly, a perfect linear dichroism with magnetic-field tunable wavelength can be achieved by using the interband transition between the two lowest LLs, i.e, from Ev0 to Ec0. Our results shed light on the magneto-optical property of the two dimensional semi-Dirac electron systems and pave the way to design magnetically controlled optical devices.

Electronic and optical properties of monolayer InSe quantum dots

The novel two-dimensional (2D) semiconductor, InSe, with tunable band gap and high electron mobility, has attracted increasing research interest. In this work, we demonstrate theoretically the strong geometry confinement in InSe quantum dots (QD) and manipulate their electronic and optical properties using QD shape and external field. The electronic energy levels, density of states, probability density of states and magneto-optical absorption spectra, are calculated by utilizing the tight-binding method with Coulomb interaction in the Hubbard model. In contrast to other 2D materials, e.g. phosphorene, InSe-QDs exhibit distinct features as (a) edge states appear in the bottom of the conduction band regardless of the shapes of InSe-QDs; (b) the edge states are mainly provided by the In atoms at the both armchair and zigzag edges in InSe-QDs; (c) optical gap is distinct from band gap. The rectangular InSe-QDs produce anisotropic optical absorption spectra, whereas hexagonal and triangular InSe-QDs produce isotropic absorption spectra. The applied magnetic field would weaken this anisotropy.

Peculiar optical properties of bilayer silicene under the influence of external electric and magnetic fields

We conduct a comprehensive investigation of the effect of an applied electric field on the optical and magneto-optical absorption spectra for AB-bt (bottom-top) bilayer silicene. The generalized tight-binding model in conjunction with the Kubo formula is efficiently employed in the numerical calculations. The electronic and optical properties are greatly diversified by the buckled lattice structure, stacking configuration, intralayer and interlayer hopping interactions, spin-orbital couplings, as well as the electric and magnetic fields ({E}_{z}hat{z}& {B}_{z}hat{z}). An electric field induces spin-split electronic states, a semiconductor-metal phase transitions and the Dirac cone formations in different valleys, leading to the special absorption features. The Ez-dependent low-lying Landau levels possess lower degeneracy, valley-created localization centers, peculiar distributions of quantum numbers, well-behaved and abnormal energy spectra in Bz-dependencies, and the absence of anti-crossing behavior. Consequently, the specific magneto-optical selection rules exist for diverse excitation categories under certain critical electric fields. The optical gaps are reduced as Ez is increased, but enhanced by Bz, in which the threshold channel might dramatically change in the former case. These characteristics are in sharp contrast with those for layered graphene.

Linear and nonlinear magneto-optical absorption in a triangular quantum well

In this work, we study the linear and nonlinear magneto-optical absorption spectrum in a triangular quantum well (TrQW) created by the applied electric field via investigating the phonon-assisted cyclotron resonance (PACR) effect. The results are calculated for a specific Ga[Formula: see text]Al[Formula: see text]As/GaAs quantum well. The magneto-optical absorption coefficient (MOAC) and the full width at half maximum (FWHM) are found to be significantly dependent on the magnetic field, the electric field and the temperature. Our results showed that the MOAC and FWHM increase with the magnetic, electric fields and temperature. The obtained results also suggest a useful way to control the magneto-optical properties of TrQW by changing these parameters.

Valley- and spin-polarized oscillatory magneto-optical absorption in monolayer MoS2 quantum rings

Besides optical valley selectivity, strong spin-orbit interaction along with Berry curvature effects also leads to unconventional valley- and spin-polarized Landau levels in monolayer transition metal dichalcogenides (TMDCs) under a perpendicular magnetic field. We find that these unique properties are inherited to the magneto-optical absorption spectrum of the TMDC quantum rings (QRs). In addition, it is robust against variation of the magnetic flux and of the QR geometry. In stark contrast to the monolayer bulk material, the ${\mathrm{MoS}}_{2}$ QRs manifest themselves in both the optical valley selectivity and unprecedented size tunability of the frequency of the light absorbed. We also find that when the magnetic field setup is changed, the phase transition from Aharonov-Bohm (AB) quantum interference to aperiodic oscillation of magneto-optical absorption spectrum takes place. The exciton spectrum in a realistic finite thickness ${\mathrm{MoS}}_{2}$ QR is also discussed.

Magneto-optical manifestation of bilayer silicene

AB stacking bilayer silicene is a two-dimensional material which is predicted to be a chiral topological superconductor. In contrast to monolayer silicene and bilayer graphene, bilayer silicene hosts more complexity due to the respective buckled nature of monolayer silicene, thus leading to a mechanism of optical manifestation. Here, we investigate Landau levels and magnetooptical absorption spectrum of bilayer silicene with different stacking sequences under the application of electric and magnetic fields. It is found that the Landau levels and magnetooptical absorption spectrum of bilayer silicene are sensitive to electric fields and exhibit the distinct profiles for different stacking sequences. Especially, there is a split of absorption peaks for bilayer silicene with different stacking sequences in the longitudinal magnetooptical conductivity, facilitating the possibility of identifying the stacking sequences of bilayer silicene. The manipulation of the magnetooptical absorption spectrum in AB stacking bilayer silicene might be crucial for enhancing the understandings of these materials as well as the possible applications.

Inelastic light and electron scattering in parabolic quantum dots in magnetic field: Implications of generalized Kohn's theorem

We investigate a one-component, quasi–zero-dimensional, quantum plasma exposed to a parabolic potential and an applied magnetic field in the symmetric gauge. If the size of such a system as can be realized in the semiconducting quantum dots is on the order of the de Broglie wavelength, the electronic and optical properties become highly tunable. Then the quantum size effects challenge the observation of many-particle phenomena such as the magneto-optical absorption, Raman intensity, and electron energy loss spectrum. An exact analytical solution of the problem leads us to infer that these many-particle phenomena are, in fact, dictated by the generalized Kohn's theorem in the long-wavelength limit. Maneuvering the confinement and/or the magnetic field furnishes the resonance energy capable of being explored with the FIR, Raman, or electron energy loss spectroscopy. This implies that either of these probes should be competent in observing the localized magnetoplasmons in the system. A deeper insight into the physics of quantum dots is paving the way for their implementation in diverse fields such as quantum computing and medical imaging.

Observability of localized magnetoplasmons in quantum dots: Scrutinizing the eligibility of far-infrared, Raman, and electron-energy-loss spectroscopies

We investigate a one-component, quasi-zero dimensional, quantum plasma exposed to a parabolic potential and an applied magnetic field in the symmetric gauge. If the size of such a system as can be realized in the semiconducting quantum dots is on the order of the de-Broglie wavelength, the electronic and optical properties become highly tunable. Then the quantum size effects challenge the observation of many-particle phenomena such as the magneto-optical absorption, Raman intensity, and electron energy-loss spectrum. An exact analytical solution of the problem leads us to infer that these many-particle phenomena are, in fact, dictated by the generalized Kohn’s theorem (GKT) in the long-wavelength limit. Maneuvering the confinement and/or the magnetic field furnishes the resonance energies capable of being explored with the FIR, Raman, and/or electron-energy-loss spectroscopy. This implies that either of these probes is competent in observing the localized magnetoplasmons in the system. As an application of the rigorous analytical diagnosis of the system, we have presented various pertinent single-particle, such as Fock-Darwin spectrum, Fermi energy, zigzag excitation spectrum, and magneto-optical transitions, and the many-particle phenomena, such as magneto-optical absorption, Raman intensity, and electron energy-loss probability. In the latter, the energy position of the resonance peaks is observed to be independent of the electron-electron interactions and hence of the number of electrons in the quantum dot in compliance with the GKT. It is found that both confinement potential and magnetic field play a decisive role in influencing the aforementioned many-particle phenomena. Specifically, increasing (decreasing) the strength of the confining potential is found to be analogous to shrinking (expanding) the size of the quantum dots and results into a blue (red) shift in the respective spectra. Intensifying the magnetic field has two-fold effects in the resonance spectra associated with all three spectroscopies considered here: the lower resonance peak observes a red shift, whereas the higher one experiences a blue shift. This is a unique and intriguing behavior observed in the quantum dots with complete confinement. A deeper insight into the physics of the quantum dots is paving the way for their implementation in such diverse fields as quantum computing and medical imaging.

Magneto-optical properties of ABC-stacked trilayer graphene.

The generalized tight-binding model is developed to investigate the magneto-optical absorption spectra of ABC-stacked trilayer graphene. The absorption peaks can be classified into nine categories of inter-Landau-level optical excitations, including three intra-group and six inter-group ones. Most of them belong to the twin-peak structures because of the asymmetric Landau level spectrum. The threshold absorption peak alone comes from a certain excitation channel, and its frequency is associated with a specific interlayer atomic interaction. The Landau-level anticrossings cause extra absorption peaks. Moreover, a simple relationship between the absorption frequency and the field strength is absent. The magneto-optical properties of ABC-stacked trilayer graphene are totally different from those of AAA- and ABA-stacked ones, such as the number, intensity and frequency of absorption peaks.

Electric modulation effect on magneto-optical spectrum of monolayer graphene

The generalized tight-binding model is developed to largely reduce the numerical computation time in calculating optical properties. Modulated electric potentials can control the low-frequency magneto-optical absorption spectra of monolayer graphene. They induce the oscillatory energy dispersions of Landau subbands and the spatial symmetry breaking of the wave function; therefore, the original peaks and extra peaks of different selection rules come into existence simultaneously. Such peaks mainly arise from the band-edge states and/or the middle states. Their number, intensities, frequencies and structures are dominated by the modulation strength and period. More absorption peaks appear with an increase in potential. The extra peaks can relatively easily be observed for higher frequencies and stronger potentials. However, the absorption spectra remain unchanged for a fixed ratio of strength over period.

Plasmonic enhancement of the magneto-optical response of MnP nanoclusters embedded in GaP epilayers

We report on the magneto-optical activity of MnP nanoclusters embedded in GaP epilayers and MnP thin film as a function of temperature, magnetic field, and wavelength in the near infrared and visible. The measured Faraday rotation originates from the ferromagnetic magnetization of the metallic MnP phase and exhibits a hysteretic behavior as a function of an externally applied magnetic field closely matching that of the magnetization. The Faraday rotation spectrum of MnP shows a magnetoplasmonic resonance whose energy depends on the MnP filling factor and surrounding matrix permittivity. At resonance, the measured rotary power for the epilayer systems increases by a factor of 2 compared to that of the MnP film in terms of degrees of rotation per MnP thickness for an applied magnetic field of 410 mT. We propose an effective medium model, which qualitatively reproduces the Faraday rotation and the magnetocircular dichroism spectra, quantitatively determines the spectral shift induced by variations in the MnP volume fraction, and demonstrates the influence of the shape and orientation distributions of ellipsoidal MnP nanoclusters on the magneto-optical activity and absorption spectra.

Exact solution of the spectrum and magneto-optics of multilayer hexagonal graphene

We demonstrate that by decoupling the interlayer interactions, N-layer hexagonal graphene is decomposed into the N independent subsystems. Each subsystem, exactly described by a 2 × 2 matrix, is treated as a renormalized graphene with the renormalized site energy and intralayer interaction. The analytical form of the energy dispersions and wave functions of each renormalized graphene is easily obtained. The study reveals the origin of electron-hole asymmetry, and how it is caused by the interlayer interaction between different sublattices at adjacent layers. The monolayer-graphene-like characteristics allow us to describe Landau-level energies and magneto-optical absorption spectra of each renormalized graphene based on the effective mass model. There are N sets of Landau levels in the energy spectra of the N-layer hexagonal graphene. The magneto-optical spectra exhibit N groups of Landau-peaks. Each group of Landau-peaks follows the same optical selection as that of a monolayer graphene.

Effects of Transverse Electric Fields on Quasi-Landau Levels in Zigzag Graphene Nanoribbons

The magnetoelectronic properties of one-dimensional zigzag graphene nanoribbons are investigated using the Peierls tight-binding model with uniform magnetic and electric fields. They are mainly determined by external fields and quantum confinement effects. Magnetic fields lead to quasi-Landau levels (QLLs), enhance partial flat bands, and result in Landau wave functions. Electric fields significantly distort dispersionless QLLs, change the band symmetry, induce more band-edge states, split partial flat bands, and drastically alter the distribution of wave functions. The density of states directly reflects the main features of energy bands, such as the numbers, frequencies, heights, and divergence forms of prominent peaks, which can be confirmed experimentally. Magneto-optical absorption spectra are predicted to be markedly changed under the influence of external electric fields.

Influence of electron-electron interaction on the cyclotron resonance spectrum of magnetic quantum dots containing few electrons

The configuration interaction method is used to obtain the magneto-optical absorption spectrum of a few-electron (${N}_{e}=1,2,\dots{},5$) quantum dot containing a single magnetic ion. We find that the IR spectrum (the position, the number, and the oscillator strength of the cyclotron resonance peaks) depends on the strength of the Coulomb interaction, the number of electrons, and the position of the magnetic ion. We find that the Kohn theorem is no longer valid as a consequence of the electron-spin-magnetic-ion-spin-exchange interaction.

Optical transitions between Landau levels: AA-stacked bilayer graphene

The magneto-optical absorption spectra of AA-stacked bilayer graphene (AABG) exhibit two kinds of absorption peaks resulting from two groups of Landau levels (LLs). Only intragroup excitations that follow a single selection rule take place. The excitation channels are altered as the field strength approaches a critical strength. These optical properties can be comprehended by the characteristics of the LL wave functions. A comparison of AABG and AB-stacked bilayer graphene (BBG) demonstrates that the optical properties are dominated by the stacking symmetry. The presented results could offer a way to distinguish AABG from BBG and monolayer graphene.

The breakdown of Kohn's theorem in few-electron parabolic quantum dots doped with a single magnetic impurity Mn2+

The cyclotron resonance (CR) absorption spectrum is calculated for a II-VI parabolic quantum dot (QD) containing few electrons and a single magnetic dopant (Mn2+). We find that Kohn's theorem no longer holds for this system and that the CR spectrum depends on the number of electrons inside the QD. The electron-Mn-ion interaction strength can be tuned for example by the magnetic field and by moving the Mn-ion to different positions inside the QD. We demonstrate that due to the presence of the Mn-ion the relative motion of the electrons couple with their center-of-mass motion through the electron-Mn-ion spin-spin exchange term resulting in an electron-electron interaction dependence of the magneto-optical absorption spectrum. At the ferromagnetic-antiferromagnetic transition we observe significant discontinuities in the CR lines.

Destruction of quasi-Landau levels in nanographene ribbons by the external electric fields

Our study reveals that the magneto-electronic properties of the zigzag nanographene ribbons are tuned by the external electric fields. The spatially modulated electric field (the uniform transverse electric field) converts the dispersionless Landau levels into subbands with the sinusoidal (linear) energy dispersions. The energies of the band-edge states are affected by the electric potentials. The localization positions and the spatial symmetry of the Landau wave functions are altered. The modulated electric potential with shorter periodic length has stronger influence on the quasi-Landau levels. The magneto-optical absorption spectra are expected to be changed by the external electric fields.