Magnetooptic coupling coefficients for one- and two-magnon Raman scattering in the rutile-structure antiferromagnets FeF₂, MnF₂, CoF₂ and NiF₂

Inelastic light scattering intensities of magnetic excitations are governed by the magnetooptic coupling coefficients, which have been evaluated, for example, for the ferrimagnet Y₃Fe₅O₁₂ (YIG) and the metamagnets FeC₁₂ and FeBr₂. However, by far the most detailed studies to date have been performed on the classic rutilestructure antiferromagnets, and we summarize here the results obtained from the many experimental and theoretical investigations in these compounds for both one-magnon and two-magnon excitations. A comparison of the magnitudes of the various coupling coefficients for MnF₂, FeF₂, CoF₂, and NiF₂ reveals a surprising similarity in many coefficients. In one-magnon Raman scattering, the in-phase linear magnetooptic coefficient dominates and the main differences between MnF₂, FeF₂, and NiF₂ lie in the relative significance of the in-phase quadratic magnetooptic coefficient. Thus the quadratic coefficients are now seen to be of particular importance in determining the strength of the one-magnon scattering in a variety of magnetic insulators. In two-magnon Raman scattering, one magnetooptic coefficient dominates for all of these antiferromagnets. However, each of the other six coefficients are remarkably similar in magnitude, in general, and not negligible in most cases, indicating some universality in the way light interacts with the pairs of magnons of opposite and equal wave vector in rutile structure antiferromagnets

One-magnon and exciton inelastic light scattering in the antiferromagnet CoF₂

Experimental data are reported for the temperature and polarization dependence of the one-magnon Raman light scattering in the rutile-structure antiferromagnet CoF₂ (Néel temperature TN = 38 K). The low-lying excitons are also investigated at low temperatures and comparisons made with results from earlier Raman, infrared, and neutron scattering work. A detailed analysis of the one-magnon Stokes and anti-Stokes Raman spectra is presented resulting in comprehensive data for the temperature variation up to TN of the one-magnon frequency, line width, and integrated intensity. A theory of the one-magnon scattering and other magnetic transitions is constructed based mainly on a spin S = 3/2 exchange model, extending a simpler effective S = 1/2 approach. The excitation energies and spectral intensities over a broad range of temperatures are obtained using a Green's function equation of motion method that allows for a careful treatment of the single-ion anisotropy. Overall the S = 3/2 theory compares well with the experimental data

Influence of the biquadratic interlayer coupling in the specific heat of Fibonacci magnetic multilayers

A theoretical study of the specific heat C(T) as a function of temperature in Fibonacci magnetic superlattices is presented. We consider quasiperiodic structures composed of ferromagnetic films, each described by the Heisenberg model, with biquadratic and bilinear coupling between them. We have taken the ratios between the biquadratic and bilinear exchange terms according to experimental data recently measured for different regions of their regime. Although some previous properties of the spin wave specific heat are also reproduced here, new features appear in this case, the most important of them being an interesting broken-symmetry related to the interlayer biquadratic term.Comment: 13 pages, 4 ps figures, Submitted to Physica

Effects of color superconductivity on the structure and formation of compact stars

We show that if color superconducting quark matter forms in hybrid or quark stars it is possible to satisfy most of recent observational boundaries on masses and radii of compact stellar objects. An energy of the order of $10^{53}$ erg is released in the conversion from a (metastable) hadronic star into a (stable) hybrid or quark star in presence of a color superconducting phase. If the conversion occurs immediately after the deleptonization of the proto-neutron star, the released energy can help Supernovae to explode. If the conversion is delayed the energy released can power a Gamma Ray Burst. A delay between the Supernova and the subsequent Gamma Ray Burst is possible, in agreement with the delay proposed in recent analysis of astrophysical data.Comment: 4 pages, 2 figures. To be published in Phys.Rev.

Strange Stars with a Density-Dependent Bag Parameter

We have studied strange quark stars in the framework of the MIT bag model, allowing the bag parameter B to depend on the density of the medium. We have also studied the effect of Cooper pairing among quarks, on the stellar structure. Comparison of these two effects shows that the former is generally more significant. We studied the resulting equation of state of the quark matter, stellar mass-radius relation, mass-central-density relation, radius-central-density relation, and the variation of the density as a function of the distance from the centre of the star. We found that the density-dependent B allows stars with larger masses and radii, due to stiffening of the equation of state. Interestingly, certain stellar configurations are found to be possible only if B depends on the density. We have also studied the effect of variation of the superconducting gap parameter on our results.Comment: 23 pages, 8 figs; v2: 25 pages, 9 figs, version to be published in Phys. Rev. (D

Absence of spin superradiance in resonatorless magnets

A spin system is considered with a Hamiltonian typical of molecular magnets, having dipole-dipole interactions and a single-site magnetic anisotropy. In addition, spin interactions through the common radiation field are included. A fully quantum-mechanical derivation of the collective radiation rate is presented. An effective narrowing of the dipole-dipole attenuation, due to high spin polarization is taken into account. The influence of the radiation rate on spin dynamics is carefully analysed. It is shown that this influence is completely negligible. No noticeable collective effects, such as superradiance, can appear in molecular magnets, being caused by electromagnetic spin radiation. Spin superradiance can arise in molecular magnets only when these are coupled to a resonant electric circuit, as has been suggested earlier by one of the authors in Laser Phys. {\bf 12}, 1089 (2002).Comment: Latex file, 14 pages, 5 figure

Anisotropic exchange interaction of localized conduction-band electrons in semiconductor structures

The spin-orbit interaction in semiconductors is shown to result in an anisotropic contribution into the exchange Hamiltonian of a pair of localized conduction-band electrons. The anisotropic exchange interaction exists in semiconductor structures which are not symmetric with respect to spatial inversion, for instance in bulk zinc-blend semiconductors. The interaction has both symmetric and antisymmetric parts with respect to permutation of spin components. The antisymmetric (Dzyaloshinskii-Moriya) interaction is the strongest one. It contributes significantly into spin relaxation of localized electrons; in particular, it governs low-temperature spin relaxation in n-GaAs with the donor concentration near 10^16cm-3. The interaction must be allowed for in designing spintronic devices, especially spin-based quantum computers, where it may be a major source of decoherence and errors

ON THE THEORY OF THE MAGNETIC KAPITZA CONDUCTANCE BETWEEN LIQUID He3 AND AN ORDERED FERROMAGNET

Nous présentons des calculs de la conductance de Kapitza magnétique entre le liquide He3 et un ferromagnétique de Heisenberg ordonné. Les spins électroniques du ferromagnétique sont couplés aux spins nucléaires de He3 par une interaction dipole-dipole. Le transfert d'énergie entre les deux systèmes peut être relié aux propriétés d'ondes de spin du ferromagnétique. Nous avons évalué les contributions dues aux ondes de spin de surface et de volume.Calculations are presented for the magnetic Kapitza conductance between liquid He3 and an ordered Heisenberg ferromagnet. The electronic spins in the ferromagnet are couplet by dipole-dipole interactions to the nuclear spins of the He3. The transfer of energy between tht two systems can be related to the spin wave properties of the ferromagnet, and we evaluate contributions arising from surface spin waves as well as from the bulk spin waves

ENERGY RENORMALISATION AND DAMPING OF SURFACE SPIN WAVES IN HEISENBERG FERROMAGNETS

En utilisant la théorie de fonctions de Green, nous étudions l'effet d'une surface sur les interactions magnon-magnon dans un corps ferromagnétique de Heisenberg. On en déduit l'énergie renormalisée et l'amortissement d'ondes de spin de surface.A Green function theory is employed to study the effect of a surface on magnon-magnon interactions in a Heisenberg ferromagnet. Results are deduced for the renormalised energy and damping of the surface spin waves

Collective spontaneous emission from a Bose-Einstein condensate in the framework of a multi-photon q-deformed Dicke model

Investigations are made for the collective spontaneous emission of a Bose-Einstein condensate consisting of N two-level atoms when s atoms are initially excited in a multi-photon q-deformed Dicke model. The model is based on the generalized deformed oscillator algebra in which the field radiation operators are deformed by an operator-valued function f(n̂) of the photon number operator n̂. The time evolution of the expectation value of the atomic inversion is calculated for s = 1, s = 2 and s = 3. When s = 3 its spectra are characterized by nonequidistant eigenvalues, and the phenomenon of the quantum collapse and revival is demonstrated. In particular, the influences of photon multiplicity and q-deformation on the spontaneous emission of the system are discussed. The results show that the nonlinearities due to the photon multiplicity and q-deformation may lead to the inhibition of collective spontaneous emission