Magneto-Raman scattering of graphene on graphite: electronic and phonon excitations

Magneto-Raman-scattering experiments from the surface of graphite reveal novel features associated to purely electronic excitations which are observed in addition to phonon-mediated resonances. Graphene-like and graphite domains are identified through experiments with ～1 μm spatial resolution performed in magnetic fields up to 32 T. Polarization resolved measurements emphasize the characteristic selection rules for electronic transitions in graphene. Graphene on graphite displays the unexpected hybridization between optical phonon and symmetric across the Dirac point inter Landau level transitions. The results open new experimental possibilities--to use light scattering methods in studies of graphene under quantum Hall effect conditions.     

Carrier relaxation in epitaxial graphene photoexcited near the Dirac point

We study the carrier dynamics in epitaxially grown graphene in the range of photon energies from 10 to 250 meV. The experiments complemented by microscopic modeling reveal that the carrier relaxation is significantly slowed down as the photon energy is tuned to values below the optical-phonon frequency; however, owing to the presence of hot carriers, optical-phonon emission is still the predominant relaxation process. For photon energies about twice the value of the Fermi energy, a transition from pump-induced transmission to pump-induced absorption occurs due to the interplay of interband and intraband processes.     

Photoluminescence of n-doped double quantum well—electron subbands under influence of in-plane magnetic fields

We report on photoluminescence (PL) measurements of a GaAs/AlGaAs double quantum well (DQW) in high magnetic fields. Measurements were carried out on a selectively contacted symmetric p-δn-DQW-δn-p structure, which allows a variation of the electron density in DQW by a p–n bias and simultaneously a tilting of DQW, when a p–p bias is applied. Attention was paid to phenomena in in-plane magnetic fields, theoretically studied by Huang and Lyo (HL), [Phys. Rev. B 59, (1999) 7600]. In this paper, we compare our results for both symmetric and asymmetric DQWs with the theoretical model made by HL. Whereas the spectra from a symmetric DQW fully confirmed the theoretical predictions, the results gained from DQW with an electric-field-induced asymmetry did not allow a proper study of anticipated effects. The reasons for that are discussed.     

Magnetospectroscopy of double HgTe/CdHgTe QWs with inverted band structure in high magnetic fields up to 30 T

Magnetoabsorption in far and mid IR ranges in double HgTe/CdHgTe quantum wells with inverted band structure has been studied in high magnetic fields up to 30 T. Numerous intraband and interband transitions have been revealed in the spectra and interpreted within axial 8 × 8 k·p model. Splitting of dominant magnetoabsorption lines resulting from optical transitions from hole-like zero-mode Landau level has been discovered and discussed in terms of a built-in electric field and collective phenomena.     

Origin of Structure Inversion Asymmetry in Double HgTe Quantum Wells

JETP Letters - The density dependences of the splitting values of the dominant magneto-absorption lines in undoped p-type double HgTe/CdHgTe quantum wells are studied. An analysis of the splitting...     

Simplex Optimized LC Analysis of Plant Coumarins and Furanocoumarins

Sequential simplex methods are common and efficient optimization techniques applied in analytical chemistry. This study reports on the optimization of LC separation of coumarins using a variable-size simplex algorithm. The solvent systems consisted of methanol, water and tetrahydrofuran. Using an optimized method, seven standard simple coumarins and furanocoumarins derived from plants were successfully separated in one chromatographic run. The mobile phase at the point corresponding to the optimum consisted of 34% MeOH, 59% H₂O and 7% THF. Applying a solvent mixture in this proportion permitted separation of all critical pairs, such as esculetin/scopoletin, scopoletin/umbelliferone, umbelliferone/coumarin and xanthotoxin/psoralen. The retention ratio factors k for coumarins at the optimum of the simplex algorithm lay in the range 1 < k < 4. The optimal conditions assigned to the coumarin standards were then applied to the plant matter: herb of the rue (Ruta graveolens L., Rutae herba), anthodium of the camomile (Chamomilla recutita L., Chamomillae anthodium), herb of the southernwood (Artemisia abrotanum L., Abrotani herba), and radix of the lovage (Levisticum officinale K., Levistici radix). The validity of the method was confirmed with respect to these samples.     

High-energy limit of massless Dirac fermions in multilayer graphene using magneto-optical transmission spectroscopy

We have investigated the absorption spectrum of multilayer graphene in high magnetic fields. The low-energy part of the spectrum of electrons in graphene is well described by the relativistic Dirac equation with a linear dispersion relation. However, at higher energies (>500 meV) a deviation from the ideal behavior of Dirac particles is observed. At an energy of 1.25 eV, the deviation from linearity is approximately 40 meV. This result is in good agreement with the theoretical model, which includes trigonal warping of the Fermi surface and higher-order band corrections. Polarization-resolved measurements show no observable electron-hole asymmetry.     

Comparison of extraction techniques for gas chromatographic determination of volatile carbonyl compounds in alcohols

Two extraction procedures, i.e. conventional liquid-liquid extraction (LLE) and liquid solid-phase microextraction (SPME) for extraction of the oximes formed after derivatization of carbonyl compounds with o-(2,3, 4,5,6-pentafluorobenzyl) hydroxylamine (PFBHA) in alcoholic solutions have been compared. The limit of detection for LLE followed by GC-ECD determination of C₁–C₆ was in the range of 0.23–3.3 μg/L, whereas for liquid SPME 0.005–0.33 μg/L. Both methods elaborated can be applied to the determination of carbonyl compounds present in spirits and alcoholic beverages. Received: 26 September 2000 / Revised: 14 January 2001 / Accepted: 16 January 2001     

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

JETP Letters - 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...