Magnetoabsorption Spectra Research Articles

Magneto-optical selection rules of curved graphene nanoribbons and carbon nanotubes

We derive the generalized magneto-absorption spectra for curved graphene nanoribbons and carbon nanotubes by using the Peierls tight-binding model. The main spectral characteristics and the optical selection rules result from the cooperative or competitive relationships between the geometric structure and a magnetic field. In curved ribbons, the dominant selection rule remains unchanged during the variation of the curvature. When the arc angle increases, the prominent peaks are split, with some even vanishing as the angle exceeds a critical value. In carbon nanotubes, the angular-momentum coupling induces extra selection rules, of which more are revealed due to the increase of either (both) of the factors: tube diameter and field strength. Particularly once the two factors exceed certain critical values, the optical spectra could reflect the quasi-Landau-level structures. The identifying features of the spectra provide insight into optical excitations for curved systems with either open or closed boundary condition.

Magnetoabsorption in narrow-gap HgCdTe epitaxial layers in the terahertz range

The magnetoabsorption and photoconductivity spectra are investigated in the terahertz (THz) range at a temperature of T = 4.2 K for n-Hg1 − xCdxTe bulk epitaxial layers of various compositions (both semiconductor and semimetallic) grown by molecular-beam epitaxy. Within the framework of the Kane 8 · 8 model, the electron and hole Landau levels are calculated. It is shown that, in contrast to the results of previous investigations, all observed resonance lines are related to transitions between the Landau levels of free carriers (the cyclotron resonance in the conduction band and the transitions between heavy-hole and electron Landau levels), which is evidence of the high purity and structural perfection of the samples. The possibility of using zero-gap Hg1 − xCdxTe solid solutions as THz photodetectors tunable by magnetic field is shown.

Magnetoelectronic and Optical Properties of Monolayer and AB-Stacked Bilayer Graphenes

The low-frequency magneto-absorption spectra of monolayer and bilayer graphene are evaluated within the gradient approximation. On the basis of Peierls tight-binding model, we can draw the wave function distribution over its sublattices and bring them into the calculation of optical spectra. The optical selection rules and the absorption rate can be accurately determined. The interlayer interactions in bilayers effectively alter the Landau level structures and thus the absorption spectra, such as the field dependence of absorption lines and the appearance of double-peaked absorptions.

Controlling Magneto-Absorption Spectra of a Graphene Ribbon by a Modulated Electric Field

This study shows that a spatially modulated sinusoidal electric field can significantly tune the magneto-absorption spectra of a graphene ribbon. The pattern of absorption spectra evolves with the amplitude of electric potential (V0). When V0 is weaker than the state energy of the first Landau level, E1, the modulated electric field not only changes the peak intensity and peak position of the absorption lines, but also generates new peaks. These additional peaks follow the unusual optical selection rule. When V0 is stronger than E1, the intensity of original peaks, induced by the uniform magnetic field, shrinks, and an anomalous lowest peak appears. Most importantly, the energy position of the first peak strongly depends on both the amplitude of the electric potential and the strength of the magnetic field. These characteristics can be used to modulate the threshold frequency of the magneto-absorption spectra.

Landau levels and magnetopolaron effect in dilute GaAs:N

The magnetic-field dependence of the energy spectrum of GaAs doped with nitrogen impurities is investigated. Our theoretical model is based on the phenomenological band anticrossing model (BAC) which we extended in order to include the magnetic field and electron–phonon interaction. Due to the highly localized nature of the nitrogen state, we find that the energy levels are very different from those of pure GaAs. The polaron correction results in a lower cyclotron resonance energy as compared to pure GaAs. The magneto-absorption spectrum exhibits series of asymmetric peaks close to the cyclotron energy ħ ω c .

Magneto-Absorption Spectra from Selected Chirality of Single-Walled Carbon Nanotubes

Magneto-absorption spectra of SWNTs (Single-Walled Carbon Nanotubes) in the region of near infrared were measured under high magnetic fields up to 54 T. We succeeded to observe clear peak shifts and splittings for four different chiralities of SWNTs, (7,5), (7,6), (8,6), and (8,7). We employed PFO (poly(9,9-dioctylfluorenyl-2,7-diyl))-SWNTs. Absorption peaks are very sharp and we could observe well-defined peaks of each chirality with less mixture of the other peaks of different chirality. Energy positions of band-edge bright and dark excitons were determined distinctly in each chirality.

Magnetoabsorption spectra of bilayer graphene ribbons with Bernal stacking

Magnetoabsorption spectra of bilayer graphene ribbons with Bernal stacking are studied by the Peierls-coupling tight-binding method. When the magnetic confinement prevails over the quantum confinement, low-energy spectra chiefly exhibit many Landau peaks, which are strongly modified by the inter-ribbon interactions and the magnetic-field magnitude $(B)$. The spectra show denser Landau peaks in bilayer graphene ribbon than in a monolayer ribbon with the same ribbon width. The absorption frequencies of Landau peaks of a wide monolayer ribbon show the $\sqrt{B}$ dependence, while those of a bilayer ribbon exhibit a varying $B$-field dependence. In the spectra region $\ensuremath{\omega}\ensuremath{\le}100\text{ }\text{meV}$, the absorption frequencies of Landau peaks are linearly dependent on the magnetic-field magnitude. At $\ensuremath{\omega}\ensuremath{\ge}100\text{ }\text{meV}$, they evolve from the $B$ dependence to the $\sqrt{B}$ dependence with the increase in the field strength. The absorption frequencies of Landau peaks exhibit $\sqrt{B}$ dependence at $B\ensuremath{\ge}20\text{ }\text{T}$. The relationship between the magneto-optical properties and electronic structures (the state energies and wave functions) are explored. The Landau wave functions are illustrated and used to identify the optical selection rule.

Landau levels and magneto-optical properties of graphene ribbons

On the basis of Peierl coupling tight-binding model, we study the low energy magnetoelectronic properties of zigzag graphene ribbons by changing the ribbon width from the nanometer to the mesoscopic scale. The evolution of the Landau levels with the ribbon width shows that the number and the range of Landau levels are chiefly dominated by the ribbon width and the magnetic field (B). The Landau-level energies abide by the simple relation ∣E∣∝∣n∣B at low energy, not at the high energy (n subband index). However, a scaling law between the number of Landau levels and the ribbon width cannot be figured out. The Landau states occur only when the ribbon width is close to or greater than the distribution width of the Landau wave function and more Landau levels are generated with the increase in width. The low-frequency magnetoabsorption spectra reveal electronic properties and, thus, exhibit distinguishable delta-function-like peaks (Landau peaks). The peak height runs higher when the width increases, for more Landau states are allowed in a wider ribbon. The spectral frequencies of the Landau peaks are not determined by the ribbon width but by the field strength. The transition channels of the Landau peaks are identified and the selection rule is analyzed and discussed. The bearded defects can hardly affect the magnetoabsorption peaks due to the Landau levels.

Electronic properties and the quantum Hall effect in bilayer graphene

In this paper, I review the quantum Hall effect (QHE) and far-infra red (FIR) absorption properties of bilayer graphene in a strong magnetic field. This includes a derivation of the effective low-energy Hamiltonian for this system and the consequences of this Hamiltonian for the sequencing of the Landau levels in the material: the form of this effective Hamiltonian gives rise to the presence of a level with doubled degeneracy at zero energy. The effect of a potential difference between the layer of a bilayer is also investigated. It is found that there is a density-dependent gap near the K points in the band structure. The consequences of this gap on the QHE are then described. Also, the magneto-absorption spectrum is investigated and an experiment proposed to distinguish between model groundstates of the bilayer QHE system based on the different absorption characteristics of right- and left-handed polarization of FIR light. Finally, the effects of trigonal warping are taken into account in the absorption picture.

QHE and far infra-red properties of bilayer graphene in a strong magnetic field

We describe the quantum Hall effect (QHE) and far infra-red (FIR) absorption properties of bilayer graphene in a strong magnetic field and contrast them with the weak field regime. This includes a derivation of the effective low energy Hamiltonian for this system and the consequences of this Hamiltonian for the sequencing of the Landau levels in the material: The form of this effective Hamiltonian gives rise to the presence of a level with doubled degeneracy at zero energy. The effect of a potential difference between the layer of a bilayer is also investigated. It is found that there is a density-dependent gap near the K points in the band structure. The consequences of this gap on the QHE are then described. Also, the magneto-absorption spectrum is investigated and an experiment proposed to distinguish between model ground states of the bilayer quantum Hall effect system based on the different absorption characteristics of right-handed and left-handed polarisation of FIR light. Finally, the effects of trigonal warping are taken into account in the absorption picture.

Far-infrared absorption of vertically coupled self-assembled quantum rings

We investigate theoretically the far-infrared absorption of two vertically coupled quantum rings. The obtained spectrum can be described as a superimposition of inter-ring (bonding-antibonding) and intraring transitions. The two kinds of resonances have different responses to external magnetic fields, depend on different structural parameters and can be switched on and off independently by using polarized light. Structural information on the coupled quantum rings can be obtained by monitoring the magnetoabsorption spectrum of the bonding-antibonding transitions. We also show that direct imaging of the electron energy structure in a single quantum ring is possible by measuring the absorption spectrum of the ring vertically coupled to a quantum dot of similar volume.

Shallow Acceptor Levels in Ge∕GeSi Heterostructures with Quantum Wells in a Magnetic Field

Shallow acceptors in Ge/GeSi heterostructures with quantum wells are studied theoretically and experimentally in the presence of a magnetic field. It is shown that, in addition to the cyclotron resonance lines, magnetoabsorption spectra reveal transitions from the acceptor ground state to excited states related to Landau levels from the first and second confinement subbands, as well as the resonances caused by ionization of A+ centers.

Microwave EPR spectroscopy of cobalt-doped germanium cuprate

Resonance magnetoabsorption spectra of CuGeO3 single crystals containing 2% Co impurity have been studied in the frequency range 60–360 GHz in magnetic fields of up to 16 T and in the temperature interval 2–60 K with the magnetic field B aligned parallel to the a crystallographic axis. In addition to the Cu2+ chain resonance, a new absorption line (unobserved previously in doped CuGeO3 and deriving apparently from the Co2+ ions) was detected in EPR spectra. Quantitative analysis of the spectra suggests that the spin-Peierls transition occurs in about 10% Cu2+ chains, while the spin-Peierls state in the remaining 90% chains is completely destroyed by cobalt doping. The results obtained reveal considerable deviations from the universally accepted scenario of CuGeO3 doping and are discussed within alternative theoretical models, namely, the quantum critical behavior (based on the EPR theory for quasi-one-dimensional systems) and a three-dimensional antiferromagnet with the Neel temperature lowered by disorder.

Shallow-impurity-assisted transitions in the course of submillimeter magnetoabsorption of strained Ge/GeSi(111) quantum-well heterostructures

The submillimeter (f=130–1250 GHz) magnetoabsorption spectra of strained Ge/GeSi(111) multilayer heterostructures with quantum wells are investigated at T=4.2 K upon band-gap optical excitation. It is found that the magnetoabsorption spectra contain lines associated with the excitation of residual shallow acceptors. The resonance absorption observed can be initiated by optical transitions between the impurity states belonging to two pairs of Landau levels of holes in germanium quantum-well layers.

Magnetoelectronic properties of a graphite sheet

Magnetoelectronic structures of a two-dimensional (2D) graphite sheet are calculated by the tight-binding model. They are very sensitive to the magnitude of perpendicular magnetic field ( B). B imposes the periodical boundary condition on the Bloch functions in the real and momentum spaces. Thus, B changes energy dispersions, energy spacing, bandwidth, and oscillation period of Landau levels. B could reduce the dimensionality of a graphite sheet. Energy dispersions mainly exhibit zero-dimensional (or 1D) characteristics. A lot of delta-function-like peaks (or square-root peaks) in the density of states can be clearly found. The magnetic field dramatically changes the joint density of states and the magnetoabsorption spectra. So, many peaks with different structures are produced.

Intersubband cyclotron resonance of holes in strained Ge/GeSi(111) heterostructures with germanium wide quantum wells and cyclotron resonance of 1L electrons in GeSi layers

The submillimeter (ℏω=0.5–5 meV) magnetoabsorption spectra of strained Ge/Ge1−xSix(111) multilayer heterostructures with thick Ge layers (dGe=300–850 A, dGeSi≈200 A, x≈0.1) are investigated at T=4.2 K upon band-gap optical excitation. It is revealed that the absorption spectra contain cyclotron resonance lines of 1L electrons localized in GeSi solid solution layers (unlike the previously studied structures with thin Ge layers as quantum wells for 3L electrons). The absorption spectra of the samples with thick Ge layers (dGe=800–850 A) exhibit cyclotron resonance lines of holes due to transitions from the lower Landau levels in the first quantum-well subband to the Landau levels belonging to the third and fifth higher subbands.

THz spectroscopy of diluted magnetic semiconductors and semiconductor quantum structures in ultrahigh magnetic fields

Magneto-absorption spectra in ferromagnetic semiconductor In 1− x Mn x As films and self-organized PbSe/PbEuTe quantum dot superlattices have been studied in the terahertz range at very high magnetic fields up to 500 T . Both heavy hole (HH) and light hole (LH) cyclotron resonance (CR) have been observed in bulk In 1− x Mn x As thin films with different Mn concentrations. The detailed Landau level calculation in terms of the effective mass approximation well explained the CR peak positions, line shapes and the dependence of the circular polarization of the incident light on the CR spectra. In InMnAs/GaSb heterostructures that have higher ferromagnetic transition temperature ( T c) than the bulk samples, the observed HH and LH cyclotron masses are larger than that in the bulk thin films. We found that the CR peak position and its line shape suddenly change in the vicinity of the ferromagnetic transition temperature, suggesting the change in the electronic structure due to the ferromagnetic transition. Electron CR in PbSe/PbEuTe quantum dots has been observed and it was found that the effective mass of the electrons is considerably modified by the quantum confinement potential and the lattice strain around the dots. A large wavelength dependence of the absorption intensity was observed due to the interference effect of the radiation inside the sample.

Far IR magnetoabsorption in Ge/GeSi multiple-quantum-well heterostructures

Shallow acceptor-related absorption lines have been discovered in the far IR magnetoabsorption spectra in strained Ge/GeSi multiple-quantum-well heterostructures under band-gap photoexcitation at T=4.2K. The lines are referred to as optical transitions between shallow acceptor states bound with two pairs of two-dimensional hole Landau levels.

Magneto-optical effects observed for GaSe in megagauss magnetic fields

Magnetoabsorption spectra of a layered compound GaSe have been measured both for the Faraday and for the Voigt configurations with magnetic fields up to 150 T. Interband transitions between the Landau subbands have been clearly observed for the Faraday configuration. The exciton exhibited a diamagnetic shift, from which we estimated the reduced mass of the exciton, ${\ensuremath{\mu}}_{\ensuremath{\perp}}=0.15$ and ${\ensuremath{\mu}}_{\ensuremath{\Vert}}=0.13.$ The anisotropy of the reduced mass is surprisingly small in spite of the layered structure of the crystal. We have also observed the anticrossing effect between the $2s$ and the ${3d}_{0}$ states of the hydrogen-atom-like excitonic states. We discussed the correspondence between the excitonic states in a relatively low magnetic field and the Landau states in a high magnetic field.

Comparative study on the excitons in lead-halide-based perovskite-type crystals CH 3NH 3PbBr 3 CH 3NH 3PbI 3

Optical absorption and magnetoabsorption spectra of the lead-halide-based perovskite-type crystals, CH 3NH 3PbX 3 (X=Br, I) have been investigated. The lowest-energy excitons in these crystals are normal three-dimensional Wannier-type excitons. Bohr radii, binding energies, reduced masses, effective g factors, and oscillator strengths of the excitons have been determined with satisfactory accuracy. A larger bandgap and more tightly bound nature of the excitons in CH 3NH 3PbBr 3 compared to those in CH 3NH 3PbI 3 are a natural consequence of the halogen substitution.