Octahedral Tilt Instability of ReO_3-type Crystals

The octahedron tilt transitions of ABX_3 perovskite-structure materials lead to an anti-polar (or antiferroelectric) arrangement of dipoles, with the low temperature structure having six sublattices polarized along various crystallographic directions. It is shown that an important mechanism driving the transition is long range dipole-dipole forces acting on both displacive and induced parts of the anion dipole. This acts in concert with short range repulsion, allowing a gain of electrostatic (Madelung) energy, both dipole-dipole and charge-charge, because the unit cell shrinks when the hard ionic spheres of the rigid octahedron tilt out of linear alignment.Comment: 4 page with 3 figures included; new version updates references and clarifies the argument

Magnetism in Dense Quark Matter

We review the mechanisms via which an external magnetic field can affect the ground state of cold and dense quark matter. In the absence of a magnetic field, at asymptotically high densities, cold quark matter is in the Color-Flavor-Locked (CFL) phase of color superconductivity characterized by three scales: the superconducting gap, the gluon Meissner mass, and the baryonic chemical potential. When an applied magnetic field becomes comparable with each of these scales, new phases and/or condensates may emerge. They include the magnetic CFL (MCFL) phase that becomes relevant for fields of the order of the gap scale; the paramagnetic CFL, important when the field is of the order of the Meissner mass, and a spin-one condensate associated to the magnetic moment of the Cooper pairs, significant at fields of the order of the chemical potential. We discuss the equation of state (EoS) of MCFL matter for a large range of field values and consider possible applications of the magnetic effects on dense quark matter to the astrophysics of compact stars.Comment: To appear in Lect. Notes Phys. "Strongly interacting matter in magnetic fields" (Springer), edited by D. Kharzeev, K. Landsteiner, A. Schmitt, H.-U. Ye

Self-Trapped Excitons in Ionic-Covalent Silver Halide Crystals and Nanostructures: High-Frequency EPR, ESE, ENDOR and ODMR Studies

Silver halides have unique features in solid state physics because their properties are considered to be of borderline nature between ionic and covalent bonding. In AgCl, the self-trapped hole (STH) is centered and partly trapped in the cationic sublattice, forming an Ag2+ ion inside of a (AgCl6)4− complex as a result of the Jahn–Teller distortion. The STH in AgCl can capture an electron from the conduction band forming the self-trapped exciton (STE). Recent results of a study of STE by means of high-frequency electron paramagnetic resonance, electron spin echo, electron–nuclear double resonance (ENDOR) and optically detected magnetic resonance (ODMR) are reviewed. The properties of the STE in AgCl crystals, such as exchange coupling, the ordering of the triplet and singlet sublevels, the dynamical properties of the singlet and triplet states, and the hyperfine interaction with the Ag and Cl (Br) nuclei are discussed. Direct information about the spatial distribution of the wave function of STE unpaired electrons was obtained by ENDOR. From a comparison with the results of an ENDOR study of the shallow electron center and STH, it is concluded that the electron is mainly contained in a hydrogen-like 1s orbital with a Bohr radius of 15.1 ± 0.6 Å, but near its center the electron density reflects the charge distribution of the hole. The hole of the STE is virtually identical to an isolated STH center. For AgCl nanocrystals embedded into the KCl crystalline matrix, the anisotropy of the g-factor of STE and STH was found to be substantially reduced compared with that of bulk AgCl crystals, which can be explained by a considerable suppression of the Jahn–Teller effect in nanoparticles. A study of ODMR in AgBr nanocrystals in KBr revealed spatial confinement effects and allowed estimating the nanocrystal size from the shape of the ODMR spectra

MAGNETIC INTERACTION BETWEEN O-2 MOLECULAR IONS

Dans des cristaux ClK, BrK et IK avec une concentration des centres O-2 de quelques 0/00 nous avons étudié en détail les spectres EPR des centres voisins couplés par échange. Les structures géométriques des paires et les paramètres de leurs spins-hamiltoniens ont été déterminés. Les énergies d'échanges ont très faibles ( | J | k < 2°K)et dépendent en signe et grandeur non seulement de la distance des deux partenaires mais aussi de leurs orientations relatives.We prepared KCl, KBr and KI crystals containing enough O-2 centers (10/00) for a detailed study of the EPR lines belonging to pairs of exchange coupled neighbours. The structures of the pairs and the parameters of their spin-hamiltonian were determined. The exchange energies are small ( | J | k < 2°K) and depend in sign and magnitude not only upon the distance of the partners but also upon their mutual orientations

MOLECULAR AND MAGNETIC ORDER IN ALKALI HYPEROXIDES : A SHORT REVIEW OP RECENT WORK

On donne un aperçu des propriétés et des structures des hyperoxides alcalins NaO2 et KO2, basé sur des mesures magnétiques et caloriques, sur la diffraction et la diffusion des rayons X et des neutrons, sur des expériences de résonance paramagnétique et sur des recherches optiques et spectroscopiques, avec égard spécial aux travaux du groupe de Zürich sur des monocristaux.The properties and the structures of the alkali-hyperoxides NaO2 and KO2 deduced from magnetic and caloric measurements, X-ray and neutron scattering, paramagnetic resonance, optic and spectroscopic investigations are reviewed with special emphasis on the single crystal work done in Zürich