Magnetoabsorption Spectra Research Articles

Odnovremennoe nablyudenie tsiklotronnogo rezonansa dyrok i elektronov v dvoynoy kvantovoy yame HgTe/CdHgTe v usloviyakh effekta “opticheskogo zatvora”

Spectral studies of the photoconductivity in the temperature range of T = 5–70 K, as well as studies of the magneto-absorption and magnetotransport at T = 4.2 K, have been performed in a HgTe/CdHgTe heterostructure with a double quantum well under an “optical gate” effect. Studies of magneto-absorption spectra under the controlled optical exposure have made it possible to observe absorption lines caused by both the cyclotron resonances of electrons and holes simultaneously. The coexistence of electrons and holes in the HgTe/CdHgTe double quantum well with a relatively large bandgap (~80 meV) indicates the appearance of a strongly inhomogeneous light-induced distribution of charge carriers in the plane of the structure. Experimental results obtained clearly demonstrate disadvantages of the control of the Fermi level positions in heterostructures with HgTe/CdHgTe quantum wells by means of the optical gate.

Simultaneous Observation of the Cyclotron Resonances of Electrons and Holes in a HgTe/CdHgTe Double Quantum Well under “Optical Gate” Effect

Spectral studies of the photoconductivity in the temperature range of T = 5–70 K, as well as studies of the magneto-absorption and magnetotransport at T = 4.2 K, have been performed in a HgTe/CdHgTe heterostructure with a double quantum well under an “optical gate” effect. Studies of magneto-absorption spectra under the controlled optical exposure have made it possible to observe absorption lines caused by both the cyclotron resonances of electrons and holes simultaneously. The coexistence of electrons and holes in the HgTe/CdHgTe double quantum well with a relatively large bandgap (~80 meV) indicates the appearance of a strongly inhomogeneous light-induced distribution of charge carriers in the plane of the structure. Experimental results obtained clearly demonstrate disadvantages of the control of the Fermi level positions in heterostructures with HgTe/CdHgTe quantum wells by means of the optical gate.

Zeeman and Davydov splitting of Frenkel excitons in the antiferromagnet CuB2O4

The optical spectra of antiferromagnetic copper metaborate ${\mathrm{CuB}}_{2}{\mathrm{O}}_{4}$ are characterized by an exceptionally rich structure of narrow absorption lines due to electronic transitions within the magnetic ${\mathrm{Cu}}^{2+}$ ions, but their unambiguous identification and behavior in magnetic fields have remained far from being fully understood. We study the polarized magnetoabsorption spectra of this tetragonal antiferromagnet with high spectral resolution across the energy range of $1.4055--1.4065$ eV in magnetic fields up to 9.5 T for temperatures from 1.6 up to the N\'eel temperature ${T}_{N}=20$ K. We observe a set of eight absorption lines at $T=1.6$ K in magnetic fields exceeding 1.4 T, which we identify as arising from Frenkel excitons related to the ground and first excited states of the ${\mathrm{Cu}}^{2+}$ ions. The number of these excitons is defined by the presence of the four ${\mathrm{Cu}}^{2+}$ ions with doubly degenerate spin state $S=1/2$ at the $4b$ positions in the crystallographic unit cell. The energies of these excitons are determined by the exchange interaction of 0.5 meV of the ${\mathrm{Cu}}^{2+}$ ions in the excited state with the surrounding ions and by the Davydov splitting of 0.12 meV. In large magnetic field the observed Zeeman splitting is controlled by the anisotropic $g$-factors of both the ground and excited states. We develop a theoretical model of Frenkel excitons in the magnetic field that accounts for specific features of the spin structure and exchange interactions in ${\mathrm{CuB}}_{2}{\mathrm{O}}_{4}$. The model is used for fitting the experimental data and evaluation of the Frenkel exciton parameters, such as the Davydov splitting, the molecular exchange energy, and the $g$-factors of the ground and excited states of the ${\mathrm{Cu}}^{2+}$ ions.

Landau quantization in tilted Weyl semimetals with broken symmetry

Broken symmetry and tilting effects are ubiquitous in Weyl semimetals (WSMs). Therefore, it is crucial to understand their impacts on the materials’ electronic and optical properties. Here, using a realistic four-band model for WSMs, which incorporates both the symmetry breaking and tilting effects, we study its Landau quantization and the associated magneto-absorption spectrum. We show that the Landau bands in tilted WSMs can be obtained by considering a non-tilt Hamiltonian through Lorentz boost. However, broken symmetry effects can generate an additional term in the Hamiltonian, which equivalently lead to band reconstruction. Our work provides a more realistic view of the magnetic field response of WSMs that shall be taken into account in relevant future device applications.

Rydberg magnetoexcitons in Cu2O quantum wells

We present a theoretical approach that allows for calculation of optical functions for a ${\mathrm{Cu}}_{2}\mathrm{O}$ quantum well (QW) with Rydberg excitons, in an external magnetic field of an arbitrary strength. Both Faraday and Voigt configurations are considered in the energetic region of $p$ excitons. We use the real density matrix approach and an effective electron-hole potential, which enable us to derive analytical expressions for the QW magneto-optical functions. For both configurations, all three field regimes---weak, intermediate, and high field---are considered and treated separately. With the help of the developed approximation method we are able to estimate the limits between the field regimes. The obtained theoretical magnetoabsorption spectra show a good agreement with available experimental data.

Effects of the Electron—Electron Interaction in the Magneto-Absorption Spectra of HgTe/CdHgTe Quantum Wells with an Inverted Band Structure

Magneto-absorption in HgTe/CdHgTe quantum wells with an inverted band structure in magnetic fields up to 30 T has been studied. It has been shown that the positions of magneto-absorption lines for transitions from the zero-mode Landau levels in a wide range of magnetic field cannot in principle be described in the “single-electron” approximation using the eight-band Kane model even taking into account effects associated with the absence of spatial inversion symmetry. The revealed dependence of the energies of optical transitions on the magnetic field can be qualitatively explained within a “many-body” picture, where transitions between Landau levels are considered as collective modes hybridized by the electron-electron interaction.

Magnetoabsorption in HgCdTe/CdHgTe Quantum Wells in Tilted Magnetic Fields

Absorption spectra in a 30-nm Hg0.075Cd0.925Te/Cd0.74Hg0.26Te quantum well in tilted magnetic fields up to 13 T have been studied. Energy spectra in the conduction band (in the absence of the normal field component), as well as electron Landau levels spectra in the presence of the magnetic field component in the quantum well plane, have been calculated within the eight-band Kane model. In classical magnetic fields, the broadening of cyclotron resonance lines has been detected; this broadening is due both to the removal of the Kramers degeneracy of the lower quantum-confinement subband and to difference of cyclotron masses in split subbands. The longitudinal field component in quantizing magnetic fields is responsible for the hybridization of Landau levels, which is enhanced with an increase in their energy (on the scale of the quantum confinement energy), manifested in the appearance of “forbidden” transitions in magnetoabsorption spectra.

Terahertz Spectroscopy of Two-Dimensional Semimetal in Three-Layer InAs/GaSb/InAs Quantum Well

The features of terahertz photoluminescence and magnetoabsorption in magnetic fields up to 16 T in threelayer InAs/GaSb/InAs quantum wells with a band structure corresponding to a “two-dimensional semimetal” are studied. The comparison of the positions and amplitudes of the photoluminescence lines with the theoretical calculations of oscillator strengths for interband and intraband transitions performed using the eight-band Kane Hamiltonian indicates the existence of a nonradiative recombination channel associated with the overlap of the conduction and valence bands. The energies of interband and intraband Landau level transitions observed in the magnetoabsorption spectra are in good agreement with theoretical calculations, which confirms the predicted band structure.

The diverse magneto-optical selection rules in bilayer black phosphorus

The magneto-optical properties of bilayer phosphorene is investigated by the generalized tight-binding model and the gradient approximation. The vertical inter-Landau-level transitions, being sensitive to the polarization directions, are mainly determined by the spatial symmetries of sub-envelope functions on the distinct sublattices. The anisotropic excitations strongly depend on the electric and magnetic fields. A uniform perpendicular electric field could greatly diversify the selection rule, frequency, intensity, number and form of symmetric absorption peaks. Specifically, the unusual magneto-optical properties appear beyond the critical field as a result of two subgroups of Landau levels with the main and side modes. The rich and unique magnetoabsorption spectra arise from the very close relations among the geometric structures, multiple intralayer and interlayer hopping integrals and composite external fields.

Magneto-absorption spectra of hydrogen-like yellow exciton series in cuprous oxide: excitons in strong magnetic fields

We study the absorption spectra of the yellow excitons in Cu2O in high magnetic fields using polarization-resolved optical absorption measurements with a high frequency resolution. We show that the symmetry of the yellow exciton results in unusual selection rules for the optical absorption of polarized light and that the mixing of ortho- and para- excitons in magnetic field is important. The calculation of the energies of the yellow exciton series in strong and weak magnetic field limits suggests that a broad n = 2 line is comprized by two closely overlapping lines, gives a good fit to experimental data and allows to interpret the complex structure of excitonic levels.

Diverse magnetic quantization in bilayer silicene

The generalized tight-binding model is developed to investigate the rich and unique electronic properties of AB-bt (bottom-top) bilayer silicene under uniform perpendicular electric and magnetic fields. The first pair of conduction and valence bands, with an observable energy gap, displays unusual energy dispersions. Each group of conduction/valence Landau levels (LLs) is further classified into four subgroups, that is, there exist the sublattice- and spin-dominated LL subgroups. The magnetic-field-dependent LL energy spectra exhibit irregular behavior corresponding to the critical points of the band structure. Moreover, the electric field can induce many LL anti-crossings. The main features of the LLs are uncovered with many van Hove singularities in the density-of-states and non-uniform delta-function-like peaks in the magneto-absorption spectra. The feature-rich magnetic quantization directly reflects the geometric symmetries, intra-layer and inter-layer atomic interactions, spin-orbital couplings, and the field effects. The results of this work can be applied to novel designs of $Si$-based nano-electronics and nano-devices with enhanced mobilities.

On the band spectrum in p-type HgTe/CdHgTe heterostructures and its transformation under temperature variation

The magnetoabsorption and interband photoconductivity spectra of HgTe/CdHgTe quantum wells exhibiting p-type conductivity are studied at different temperatures. It is shown that, for a sample with a normal band structure, the long-wavelength edge of the spectra shifts to higher energies with temperature increase, indicating an increase of the band gap in the quantum well. For a sample with an inverted band structure, it is for the first time found that the long-wavelength cut-off shifts to lower energies due to the topological phase transition from the inverted band structure to the normal structure with temperature increase. The experimental data are in agreement with the results of theoretical band-structure calculations based on the Kane model.

Optical properties of graphene nanocones under electric and magnetic fields

Here we present a theoretical study of the optical properties of graphene nanocones tuned by external electric and magnetic fields. We investigate the effects of the size and topology of the carbon nanostructures on the density of states and on the electro- and magneto-absorption of linearly polarized electromagnetic radiation in different nanocone geometries. We find that the electric field induces changes in the electric charge distribution mainly at the cone edges. In the infrared range the absorption coefficient shows a peculiar dependence on the electric field (magnitude and direction) and on the photon polarization for all investigated structures. Our results suggest that the electric field may be used to control the electric charge at the apex and for a selective light absorption. The presence of an axial magnetic field induces new features in the nanocone density of states due to the induced localization effects. For high fields the density of states exhibits a sequence of peaks resembling the graphene Landau spectra. The magneto-absorption spectra present a series of resonances strongly sensitive to the photon polarization opening routes for manipulation of the optical responses.

Probing the semiconductor to semimetal transition in InAs/GaSb double quantum wells by magneto-infrared spectroscopy

We perform a magnetoinfrared spectroscopy study of the semiconductor to semimetal transition of InAs/GaSb double quantum wells from the normal to the inverted state. We show that owing to the low carrier density of our samples, the magnetoabsorption spectra evolve from a single cyclotron resonance peak in the normal state to multiple absorption peaks in the inverted state with distinct magnetic field dependence. Using an eight-band Pidgeon-Brown model, we explain all the major absorption peaks observed in our experiment. We demonstrate that the semiconductor to semimetal transition can be realized by manipulating the quantum confinement, the strain, and the magnetic field. Our work paves the way for band engineering of optimal InAs/GaSb structures for realizing novel topological states as well as for device applications in the terahertz regime.

Magnetospectroscopy of double HgTe/CdHgTe quantum wells

The magnetoabsorption spectra in double HgTe/CdHgTe quantum wells (QWs) with normal and inverted band structures are investigated. The Landau levels in symmetric QWs with a rectangular potential profile are calculated based on the Kane 8 × 8 model. The presence of a tunnel-transparent barrier is shown to lead to the splitting of states and “doubling” of the main magnetoabsorption lines. At a QW width close to the critical one the presence of band inversion and the emergence of a gapless band structure, similar to bilayer graphene, are shown for a structure with a single QW. The shift of magnetoabsorption lines as the carrier concentration changes due to the persistent photoconductivity effect associated with a change in the potential profile because of trap charge exchange is detected. This opens up the possibility for controlling topological phase transitions in such structures.

Interband magnetoabsorption spectra of Mn-doped ZnO nanowires

The electronic structures of Mn-doped ZnO nanowires in the magnetic field are calculated based on the six-band k·p effective-mass theory. Through the calculation, it is found that the first two valence subbands with positive spin angular momentum Jh will reverse when the magnetic field is greater than a critical value, while they will not reverse when Jh is negative. Several lowest transitions are shown and the interband magnetoabsorption spectra of Mn-doped ZnO nanowires at the Γ point are also presented in the magnetic field, and it is found that the lowest optical transition is left circular polarized light. Meanwhile, red shifts of the absorption peaks will occur if the magnetic field is applied, and the absorption peaks will show blue shifts with the increase of the temperature. In addition, quasi-Fermi level of the valence subbands of Mn-doped ZnO nanowires increase with the increase of the temperature, and more absorption peaks will arise when the carrier density and the temperature increase.

Magneto-optical Kramers-Kronig analysis.

We describe a simple magneto-optical experiment and introduce a magneto-optical Kramers-Kronig analysis (MOKKA) that together allow extracting the complex dielectric function for left- and right-handed circular polarizations in a broad range of frequencies without actually generating circularly polarized light. The experiment consists of measuring reflectivity and Kerr rotation, or alternatively transmission and Faraday rotation, at normal incidence using only standard broadband polarizers without retarders or quarter-wave plates. In a common case, where the magneto-optical rotation is small (below ∼0.2 rad), a fast measurement protocol can be realized, where the polarizers are fixed at 45(∘) with respect to each other. Apart from the time-effectiveness, the advantage of this protocol is that it can be implemented at ultra-high magnetic fields and in other situations, where an in-situ polarizer rotation is difficult. Overall, the proposed technique can be regarded as a magneto-optical generalization of the conventional Kramers-Kronig analysis of reflectivity on bulk samples and the Kramers-Kronig constrained variational analysis of more complex types of spectral data. We demonstrate the application of this method to the textbook semimetals bismuth and graphite and also use it to obtain handedness-resolved magneto-absorption spectra of graphene on SiC.

Electric-field-induced rich magneto-absorption spectra of ABC-stacked trilayer graphene

The magneto-optical spectra of ABC-stacked trilayer graphene are enriched by an electric field, providing a way to experimentally identify the stacking order of few layer graphene systems.

One-dimensional massless Dirac bands in semiconductor superlattices

Semiconductor superlattices may display dispersions that are degenerate either at the zone center or zone boundary [G. Bastard, Wave Mechanics Applied to Semiconductor Heterostructures, Monographies de Physique (Les \'Editions de Physique, Les Ulis, 1988); C. Sirtori, F. Capasso, D. L. Sivco, and A. Y. Cho, Appl. Phys. Lett. 64, 2982 (1994)]. We show that they are linear upon the wave vector in the vicinity of the crossing point. This establishes a realisation of massless Dirac bands within semiconductor materials. We show that the eigenstates and the corresponding Wannier functions of these superlattices have peculiar symmetry properties. We discuss the stability of the properties of such superlattices versus the electron in-plane motion. As a distinct fingerprint, the inter-sub-band magnetoabsorption spectrum for such superlattices is discussed.

Magnetoelectronic and optical properties of a <span class="aps-inline-formula"><math><msub><mrow><mi>MoS</mi></mrow><mn>2</mn></msub></math></span> monolayer

A generalized tight-binding model is utilized to study Landau-level spectra of monolayer molybdenum disulfide. The intrinsic spin-orbit coupling effectively gives rise to multiple splitting of Landau levels. With a close inspection of wave-function characteristics, these levels can be classified into specific groups in terms of their orbital, spin, and valley signatures. In the calculation of magnetoabsorption spectra, the physical origin of the optical selection rules is clearly resolved. In particular, crossings of absorption lines are available and serve as a unique feature of the spin-orbit coupling. Our numerical results clearly demonstrate the magnetic control of spin and valley charge carriers and provide a basis for future experiments.