Scattering of surface magnetic polaritons rom order-parameter fluctuations near second-order magnetic phase transitions

A study is reported of the scattering of surface magnetic polaritons in the vicinity of second-order spin-reorientational phase transitions, which is caused by spatial fluctuations of the order parameter. A four-sublattice antiferromagnet α-Fe₂O₃ is used as an example to derive expressions for the vacuum-scattered radiation near exchange magnetic-vibration frequencies. The scattering of surface magnetic polaritons is shown to be resonantly enhanced in characteristic regions of their spectrum. Estimates made for α-Fe₂O₃ indicate a possibility of experimental observation of surface magnetic-polariton scattering from fluctuations of the refractive index near second-order spin-reorientational phase transitions.     

Dispersionless surface polaritons in the vicinity of different sections of optically uniaxial crystals

The sectors of propagation directions of dispersionless polaritons localized at the optically uniaxial crystal-optically isotropic medium interface are determined. The region of possible orientations of the crystal surface containing these sectors is revealed. The boundaries of the sector of possible propagation directions are analytically described as functions of the crystal surface orientation with respect to the optical axis of the crystal. Consideration is given to the optimum ratios of permittivities at which the width of the propagation sectors appears to be maximum. It is found that the boundaries of the sectors of polariton propagation along different surfaces change drastically in the limit of small dielectric anisotropy of the crystal.     

Spontaneous coherent phase transition of polaritons in CdTe microcavities

We report on evidence for polariton condensation out of a reservoir of incoherent polaritons. Polariton population and first-order coherence are investigated by spectroscopic imaging of the far-field emission of a CdTe-based microcavity under nonresonant pumping. With increasing pumping power, stimulated emission with thresholdlike behavior and spectral narrowing is observed in the strong exciton-photon coupling regime. We show that it comes from a narrow ring in k space, exhibiting enhanced spatial and angular coherence at the stimulation onset.     

Enhanced decoherence in the vicinity of a phase transition

We study the decoherence of a spin-1/2 induced by an environment which is on the verge of a continuous phase transition. We consider spin environments described by the ferromagnetic and antiferromagnetic Heisenberg models on a square lattice. As is well known, these two-dimensional systems undergo a continuous phase transition at zero temperature, where the spins order spontaneously. For weak coupling of the central spin to these baths, we find that as one approaches the transition temperature, critical fluctuations make the central spin decohere faster. Furthermore, the decoherence is maximal at zero temperature as signaled by the divergence of the Markovian decoherence rate.     

Resonant magnetoresistance in the vicinity of a phase transition

The change in the electrical conductivity of manganite films upon microwave pumping in the magnetic resonance conditions is investigated. The temperature dependence of the effect correlates with the temperature variation of colossal magnetoresistance (CMR), passing through a maximum at the Curie point. The results are interpreted using a model that assumes a decrease in the absolute value |M| of the magnetic moment of the sample under the action of magnetoresonant saturation, which leads to an increase in resistance in accordance with the CMR mechanism. Theoretical analysis based on the Landau-Lifshitz-Bloch equation confirms the correctness of this model and ensures good agreement with experiment.     

The study of surface polaritons by transition radiation

An experimental method for the study of surface polaritons by the use of of transition radiation is proposed. It is found that the placement of a prism close to the surface of a crystal allows one to study the decaying surface modes that are excited by a geam of charged particles travelling perpendicular to the surface of the crystal. Analytic and numerical results for the energy radiated per unit solid angle per unit frequency interval by a single charged particle over its complete trajectory are presented for the phonon form of the dielectric onstant for the crystal GaP. It is found that the predicted effects should be observable experimentally.     

Organization of ecosystems in the vicinity of a novel phase transition

It is shown that an ecosystem in equilibrium is generally organized in a state which is poised in the vicinity of a novel phase transition.     

Quasiparticle dynamics in the vicinity of metal-insulator phase transition in NaxCoO2

Layered cobaltates embody novel realizations of correlated matter on a spin-1/2 triangular lattice. We report a high-resolution systematic photoemission study of the insulating cobaltates. The observation of a single-particle gap opening and band folding provides direct evidence of anisotropic particle-hole instability on the Fermi surface due to its unique topology. Overlap of the measured Fermi surface is observed with the square root 3xsquare root 3 charge-order Brillouin zone near x=1/3 but not at x=1/2 where the insulating transition is actually observed. Unlike conventional density waves, charge stripes, or band insulators, the onset of the gap depends on the quasiparticle's quantum coherence which is found to occur well below the disorder-order symmetry breaking temperature of the crystal (the first known example of its kind).     

Magnetoresistance of organic conductors in the vicinity of a topological phase transition

We analyze the effect of magnetic breakdown on the resistance of layered organic conductors with a multisheet Fermi surface consisting of a cylinder and two slightly corrugated planes along the projection of the momentum onto the normal to the layers. Analytic expressions are derived for the charge carrier distribution function, and the dependences of the interlayer and intralayer conductivities on the magnitude and orientation of the external magnetic field in the immediate vicinity of a topological phase transition are determined when the distance between the different sheets of the Fermi surface is quite small, but the topological structure of the Fermi surface is still intact.     

Electrical resistivity anomaly in the vicinity of structural phase transition of p-SnTe

Electrical resistivity of p-SnTe is calculated taking into account the second-order scattering processes through the inter-band electron-phonon interaction based on the Kristoffel-Konsin model. The experimental results can be reasonably explained by our numerical results.     

Light scattering near phase transition points in solids

The present state of theoretical and experimental investigations of the light scattering near phase transition points in solids is reviewed. It is emphasized that the results available here may be understood on the ground of a simple but rather general phenomenological or semiphenomenological theory. From this point of view the anomaly of the total (integrated) light scattering intensity is considered with account taken of a coupling of the order parameter to other quantities including elastic strains and long-range electric fields. The temperature dependence of the scattered light spectral distribution is discussed in detail, the main attention being given to the so-called central peak problem. The influence of defects both on the total intensity and on the spectral distribution of the scattered light is also examined.     

Scattering of surface plasmon polaritons by a nanoparticle with the inclusion of the magnetic dipole contribution

Using the method of the tensor Green’s function of the wave equation, the differential and total cross sections for the scattering of surface optical electromagnetic waves (surface plasmon polaritons) by a spherical nanoparticle into surface plasmon polaritons and light have been obtained with the inclusion of the magnetic dipole contribution. Using the example of noble metal systems, it has been shown that the magnetic dipole contribution may significantly affect the angular dependence of the differential cross sections and increase their anisotropy as the plasmon wavelength increases.     

On the type of a phase transition in the system of exciton polaritons in an optical microcavity

The type of a phase transition in the quasi-equilibrium system of exciton polaritons in a two-dimensional optical microcavity has been analyzed. It has been shown that, although the system contains two types of bosons undergoing mutual transformations into each other, only one phase transition to the superfluid state with the quasilong-range order occurs in the two-dimensional system. This phase transition is a Kosterlitz-Thouless phase transition. A new physical implementation—excitons in a photon crystal—has been proposed for the Bose condensation of exciton polaritons. The superfluid properties of the ordered phase are discussed, and the superfluid density and Kosterlitz-Thouless transition temperature have been calculated in the low-density approximation.     

Resonant Raman Scattering by excitonic polaritons in semiconductors

We report results on Resonant Raman Scattering (RRS) mediated by excitonic polaritons in a high-purity semiconductor (CdTe) at low temperatures. For the first time ingoing and outgoing resonances at the n = 1, 2, 3 exciton states are detected on the one and two LO-phonon RRS. The transformation at resonance of sharp Raman peaks into well-thermalized exciton luminescence bands is observed for every ingoing, outgoing or intermediate state resonance.     

Orientational order in the vicinity of a second order smectic-A to nematic phase transition

Measurements of the quadrupolar splitting of the central N¹⁴ are reported for the liquid crystal NBOA. The splitting is a measure of the orientational order parameter. At the nematic to smectic-A transition, the orientational order parameter varies continuously as a function of temperature, but with a discontinuous slope, as expected of a second order phase transition. The results are interpreted in terms of the Kobayashi-McMillan theory.