Yu. E. Kitaev

Optical Selection Rules for Hexagonal GaN

We determined the possible symmetries for band (Bloch) states at various points of the Brillouin zone. The spin–orbit interaction has been taken into account. Free excitons offer larger sets of possible state symmetries than single carriers do. The selection rules for the optical transitions have been established. Experiments with polarized light are shown to be relevant to study the nature (s or p) of A, B, and C excitons.

Ferromagnetic and antiferromagnetic ordering in the wurtzite-type diluted magnetic semiconductors

We determined the magnetic symmetry groups (Shubnikov groups) of the wurtzite-type crystals doped with magnetic atoms periodically distributed at cation sites. The magnetic groups of doped crystals with ferro- or antiferromagnetic properties arise from the P63mc (C6v4), P3m1 (C3v1), Cs3 (Cm) or C11 (P1) ordinary space groups. Those arising from the P63mc or P3m1 group can present macroscopic ferromagnetic polarization only along the wurtzite symmetry axis whereas those arising from the Cs3 (Cm) group can present ferromagnetic polarization only in the symmetry plane and those arising from the C11 (P1) group can present polarization along any direction. Actual structures do not keep in general long-range order for magnetic atom distribution but the samples with macroscopic polarization should present local symmetries close to those listed above. Therefore, orientation of easy magnetization can provide information about the most frequent local symmetry.

SELECTION RULES FOR OPTICAL TRANSITIONS INVOLVING IMPURITIES AND DEFECTS IN HEXAGONAL GAN

We have performed a complete group-theoretical analysis of state symmetries for electrons and excitons bound to impurities or defects in hexagonal GaN. The spin–orbit interaction has been taken into account. The selection rules for the optical transitions involving bound states have been established. Transitions with light polarized along and perpendicular to the symmetry axis, respectively, are shown to generally obey different selection rules.

Electron state symmetries and optical transitions in semiconductor superlattices: II. grown along the [110] and [111] directions

Using the method of induced band representations of space groups, we have performed a complete group-theory analysis of electron state symmetries in superlattices grown along the [110] and [111] directions. The spin - orbit interaction has been taken into account. The formulae giving, for arbitrary numbers of monolayers m and n, the arrangements of atoms over the Wyckoff positions have been obtained. The selection rules for both direct and phonon-assisted optical transitions have been derived. Using results of the group-theory analysis, we predict some variations in the optical transitions when m and/or n are varied. We also propose polarized-light optical experiments to establish the detailed electron-band structures of the superlattices.

Phonon-assisted optical transitions in GaN with impurities and defects

Abstract We have classified all the possible types of phonon-assisted optical processes involving bound or extended initial, final, and virtual (intermediate) electron states and formulated, for the first time, in terms of site symmetry, the selection rules for corresponding transitions. We apply this theory to phonon-assisted transitions in hexagonal GaN involving substitutional impurities and vacancies with C 3v site-symmetry as well as interstitial impurities and molecular point defects (paired impurities, double vacancies, and vacancy-impurity complexes) occupying sites with C 3v , C s , and C 1 ones. We show that phonon-assisted optical recombination is allowed in any polarization for free and bound carriers and excitons whatever is the number of involved phonons. Just, the nature of virtual state(s) and phonon(s) can depend on the polarization of the emitted light. We discuss, in particular, the case of excitons bound to neutral donors or acceptors. Our predictions are in good agreement with experimental optical spectra published in the literature which exhibit numerous lines assigned to one- and multi-phonon-assisted transitions.

Photoreflectance studies of optical transitions in type II (GaAs)m(AlAs)n superlattices

Photoreflectance (PR) spectra of two type II [001](GaAs)m(AlAs)n superlattices (SLs) have been measured at 77 K. In the conventional picture of the envelope-function approximation, the lowest conduction state originates, in the first sample, from the Xz point of the AlAs Brillouin zone (z being the growth direction) whereas it originates from the Xx,y point in the second sample. Our spectra exhibit Franz–Keldysh oscillation (FKO) features and interband transition lines. The origin of the built-in electric field within the samples is discussed and its strength calculated from FKOs. For interpreting our spectra of interband optical transitions, a least-squares fit of the data to the Aspnes third-derivative functional form has been performed as well as computation of the optical transition energies. From the energy and amplitude of the interband transition lines in PR spectra, we showed that the two SLs are pseudo-direct, i.e. the ground optical transition in any of them is direct in the k space and takes place at the Γ point of the SL Brillouin zone. All the other interband transitions appearing in the SL spectra are also direct in k space.

Generalized symmetry and vibrational states in halogene - fullerene compounds

Abstract We present an experimental and theoretical study of phonon subsystem of different fullerites and halogene-fullerenes such as: C 60 , C 70 , C 60 I 4−x , C 70 I 2 , C 60 Vr 24 , and C 70 Cl x . Two types of chemical bonding in these materials have been established: (i) C 60 I 4−x and C 70 I 2 compounds are formed by C 60 or C 70 molecule sublattice, and I 2 molecule sublattice which weakly interact via van-der-Waals forces; (ii) C 60 Br 24 , C 60 Cl 24 , and C 70 Cl 17 compounds are characterized by a strong covalent bonding CBr, CCl. In C 60 Cl 24 , we discovered the effect of resonance Raman scattering. This effect is attributed to interaction of phonon subsystem and electron states with energies 2.3 – 2.5 eV.

Layer and bulk phonon modes in the YBa2Cu3O6 compound

Abstract The layer description of the phonon subsystem of the YBa 2 Cu 3 O 6 lattice has been performed. A group-theory analysis and model dynamical calculations were carried out both for isolated layers and for the bulk YBa 2 Cu 3 O 6 structure. The correspondence between layer and bulk modes has been established. It has been shown that acoustic modes of isolated layers give rise to acoustic bulk modes Γ ac and interlayer bulk modes Γ inter , and optical vibrations of isolated layers induce intralayer bulk modes Γ intra . The set of vibrations in the Brillouin zone center is given by the expression Γ = 3 Γ ac + 15 Γ inter + 18 Γ intra .

Quasi two dimensionality of phonon subsystems of perovskite-like high-Tc superconductors

Abstract The results of group-theory analysis, lattice dynamical calculations, and experimental data on the vibrational spectra of perovskite-like HTSC are considered together. A conclusion on quasi-2D nature of the phonon subsystems is made.

Phonon symmetry and Raman spectra of fullerites

Abstract A group-theory analysis of phonon symmetries in C 60 : I 4 crystals is performed. The Raman spectra of various fullerites and shungites are investigated.