L F Chen

Self-trapped excitons in pure and Na- and Tl-doped caesium halides and the recombination luminescence

On the basis of a method used earlier to predict the off-centre relaxed triplet self-trapped excitons in alkali halides, similar systems in caesium halides are studied. This work confirms the expected off-centre relaxation along the [100] cubic axes with increasing magnitude in the order CsI, CsBr and CsCl. The calculated emission energies are in reasonable agreement with observed values. The well-known 2.95 eV emission band of CsI:Na has been studied as a tunnelling recombination between a close pair consisting of a VK centre and a Na atom. For a number of close-pair geometries the emission energies are close to 3 eV. The strong emission bands of CsI:Tl at 2.25 eV and 2.55 eV have been interpreted as arising from tunnelling recombination of close pairs each consisting of a Tl0 and a VK centre. The calculated emission energies and polarizations are discussed in conjunction with the experimental data.

First-principles investigation of Ge doping effects on the structural, electronic and magnetic properties in antiperovskite Mn3CuN

We have investigated the structural, electronic, and magnetic properties of Mn(3)Cu(1 - x)Ge(x)N (x = 0, 0.125, 0.25) using first-principles density-functional theory within the generalized gradient approximation (GGA) + U schemes. The crystal structure of the compounds is a tetragonal crystal for x = 0 while it is a cubic crystal for x = 0.125, 0.25. The unit cell volume increases as the Ge doping increases. Our GGA + U calculations give a metallic ground state from x = 0 to 0.25 in agreement with experiments. The magnetic structure for x = 0 is found to be the ferromagnetic state while for x = 0.125, 0.25 it is the Γ(5g)-type antiferromagnetic state. From the density of states (DOS), the coupling between Ge 4p and Mn 3d is the main reason for magnetic transition in Mn(3)Cu(1 - x)Ge(x)N.

First-principles investigations of disorder effects on electronic structure and magnetic properties in Sr2CrMoO6

The electronic structures and magnetic properties are reported for ordered and disordered Sr2CrMoO6 presenting oxygen vacancies or/and antisite defects (ASs). We investigate the stability of an antiparallel (AP) magnetic moment on Cr antisites and the calculations show that these solutions are more stable relative to the parallel solution for AS defects with or without oxygen vacancies. Electronic band structure calculations indicate that the perfect Sr2CrMoO6 is half-metallic, and the half-metallic character is preserved for Sr2CrMoO6 containing only oxygen vacancies, while the half-metallic nature is destroyed when 25% ASs (50% ASs) with or without oxygen vacancies is present. For 25% ASs with two oxygen vacancies, the system possibly shows nonmetallic behavior. The experimentally observed reduction of the magnetic moment mainly arises from an antiferromagnetic coupling of Cr–O–Cr (Cr–Cr) bonds in a disordered sample.

Electronic structure and electron-phonon interaction in CaAl2Si2

We calculate the electronic structure, phonon spectrum and electron–phonon (EP) interaction for CaAl2Si2 using a full-potential, density-functional-based method. It is found that CaAl2Si2 is a semimetal with a small overlap between the conduction and valence bands. The transport properties of CaAl2Si2 are discussed in combination with its special electronic structure. It is shown that in CaAl2Si2 both the electronic density of states at the Fermi level and the EP coupling strength are very small, resulting in vanishing superconductivity.

Decay of the self-trapped exciton near the (001) surface in NaBr and KBr

The adiabatic instability of the self-trapped exciton near the (001) surface in NaBr and KBr leading to Br atom desorption is studied. It is shown that in the first two layers below a (001) surface, the exciton relaxes backward to the bulk; no desorption may result thereby. Only from deeper layers does the instability propel the Br-2 molecule toward the surface as well as backward. The relaxation toward the surface is strong in KBr, making energetic Br desorption likely. Similar strong relaxation is not found in NaBr, by contrast. These results are discussed in comparison with experimental data on energetic halogen atom desorption upon electronic excitation.

First-principles study of the electronic and magnetic properties of Sr2Fe1+xMo1−xO6

We have investigated the electronic and magnetic properties of Sr2Fe1+xMo1−xO6 (−1≤x≤0.25), the composition x = 0 corresponding to the well-known double perovskite system Sr2FeMoO6, using first-principles density functional theory within the generalized gradient approximation (GGA)+U schemes. The crystal structure of the compounds has a cubic lattice for x = −1 and 0.25 while the structure of the compounds has a tetragonal lattice from x = −0.75 to 0.0. The lattice parameters decrease slightly as the Fe content increases and the variation of unit cell volume is linear with the composition x. Our spin-polarized calculations give a metallic ground state for the x<0 regime and a half-metallic ground state for the x≥0 regime. The magnetic structures for x≤0 are found to be the ferromagnetic state while the magnetic structure for x = 0.25 is the ferrimagnetic state where any Fe at the Mo crystallographic site is coupled antiparallel to the Fe moments at the correct site.

Intrinsic defects in KMgF3: the ab initio and the extended-ion study

The intrinsic defects in KMgF3, such as VK centres and self-trapped excitons (STEs), are studied by the ab initio method and the extended-ion method. In the ab initio method, the Madelung potentials are introduced into the Fock operator terms to perform calculations on clusters modelling ionic solids. It is found that the VK centre moves toward the nearby interstitial site, still keeping C2v symmetry; and the STE is unstable in the on-centre symmetry, undergoing a relaxation consisting of an axial translation superimposed with a rotation. Such a translation plus rotation relaxation of the STE in KMgF3 is quite different from those in alkali halides and in alkali-earth halides. The calculated results for the excitation energy of the VK centre and for the emission energy of the STE are in reasonable agreement with experiments.

A study of the self-trapped exciton and F centre in

The self-trapped excitons (STEs) and F centres in are studied within the extended-ion approximation. It is found that the STE is in fact a nearest-neighbour F - H pair with symmetry. The theoretical results for the spectra of the STEs and F centres are in good agreement with experiments. The stable F - H pairs converted from STEs are generated only at the hole excited state of the STEs according to the calculations of the adiabatical potential energy curve of the STE/FH pair. A model of the excited-hole hopping diffusion is proposed to explain the reorientation of the H centre and the formation of the F - H pair under cascade excitation.

Desorption of atoms and excimers upon self-trapping of excitons in rare gas solids

Abstract On the basis of one-electron Hartree-Fock approximation, the extended-ion method has been used to investigate desorption induced by the formation and trapping of two types of excitons on and near the three different faces ((100), (110), (111)) for solid Ne and Kr. Excited Ne atom, Ne excimer and Kr excimer can be ejected upon self-trapping of excitons on and near the surface, while excited Kr atom cannot. After desorption, the excited atom and excimer will decay radiatively, the luminescence energies are in fair agreement with the experiment. For the excimer, the radiative decay terminates at the strongly repulsive ground state, this repulsive energy is converted into kinetic energy of two atoms constituting the excimer, resulting in the desorption of ground state atom. It is also observed that a localized hole on the surface and below is efficiently converted into a molecule-ion R2+ without leading to any desorption, showing that exciton-lattice interaction plays a key role in the process of desorption. The ejected direction is basically along the normal to the surface for the excited particle, but for the ground state atom, it does not show a preferential direction of desorption.

First-principles investigation of the structural, magnetic and electronic properties of perovskite SrRu1―xMnxO3

We have investigated the structural, magnetic and electronic properties of single-crystal SrRu(1-x)Mn(x)O(3), using first-principles density functional theory within the generalized gradient approximation (GGA)+U schemes. The entire series of SrRu(1-x)Mn(x)O(3) (x = 0, 0.25, 0.5 and 1) is stabilized in the single-crystal perovskite structure which is in agreement with experimental findings. Our spin-polarized calculations give a metallic ground state for the x<0.5 regime and an insulator ground state for the x≥0.5 regime. The magnetic structure for x = 0 is found to be the ferromagnetic state while the magnetic structures for 0<x<0.5 are the ferrimagnetic state where any Mn ions are coupled antiparallel to the Ru at the near sites. The magnetic structures for x≥0.5 are found to be the antiferromagnetic states. The substitution of itinerant Ru ions by localized Mn ions enhances the p-d coupling between O and the transition metal. It also strongly drives the system from the ferromagnetic metal to the antiferromagnetic insulator.