The crossover between lasing and polariton condensation in optical microcavities

We study a model of a photon mode dipole-coupled to a medium of two-level oscillators in a microcavity in the presence of dephasing processes introduced by coupling to external baths. Decoherence processes can be classified as pair-breaking or non-pair-breaking in analogy with magnetic or non-magnetic impurities in superconductors. In the absence of dephasing, the ground state of the model is a polariton condensate with a gap in the excitation spectrum. Increase of the pair-breaking parameter $\gamma$ reduces the gap, which becomes zero at a critical value $\gamma_{C1}$; for large $\gamma$, the conventional laser regime is obtained in a way that demonstrates its close analogy to a gapless superconductor. In contrast, weak non-pair-breaking processes have no qualitative effect on the condensate or the existence of a gap, although they lead to inhomogeneous broadening of the excitations

Extreme Electron-Phonon Coupling in Boron-based Layered Superconductors

The phonon-mode decomposition of the electron-phonon coupling in the MgB2-like system Li_{1-x}BC is explored using first principles calculations. It is found that the high temperature superconductivity of such systems results from extremely strong coupling to only ~2% of the phonon modes. Novel characteristics of E_2g branches include (1) ``mode lambda'' values of 25 and greater compared to a mean of $\sim 0.4$ for other modes, (2) a precipitous Kohn anomaly, and (3) E_2g phonon linewidths within a factor of ~2 of the frequency itself, indicating impending breakdown of linear electron-phonon theory. This behavior in borne out by recent inelastic x-ray scattering studies of MgB2 by Shukla et al.Comment: 4 two-column pages, 4 figures. Equations simplified. Figure 4 changed. Comparison with new data include

Preparation and characterization of diamond–silicon carbide–silicon composites by gaseous silicon vacuum infiltration process

Diamond–SiC–Si composites have been prepared using gaseous silicon vacuum infiltration. The evolution of the phases and microstructures of the composites have been analyzed using X-ray diffraction technique and scanning electron microscopy. It has been found that the diamond–SiC–Si composite is composed of β-SiC, diamond, and residual Si. The diamond particles were distributed homogeneously in the dense matrix of the composites. Besides, the effects of particle size and content of diamond on the properties of diamond–SiC–Si composites have been analyzed. The thermal conductivity of the composites increases with particle size and content of diamond. When the particle size and content of diamond are 300 µm and 80 wt %, respectively, the thermal conductivity of the composites approaches the value of 280 W·m⁻¹·K⁻¹.Проведен анализ эволюции фаз и микроструктуры композитов алмаз–SiC–Si, изготовленных с использованием процесса вакуумной инфильтрации газообразного кремния. Исследование выполнено с помощью дифракции рентгеновских лучей и сканирующей электронной микроскопии. Установлено, что композит алмаз–SiC–Si состоит из β-SiC, алмаза и остаточного Si. Алмазные частицы распределены однородно в плотной матрице композитов. Также проанализировано влияние размера частиц и содержания алмазов на свойства композитов алмаз–SiC–Si. Показано, что теплопроводность композитов возрастает с увеличением размера частиц и содержания алмазов. Теплопроводность композитов приближается к значению 280 Вт∙м⁻¹∙K⁻¹ при размере частиц и содержании алмаза 300 мкм и 80 % (по массе), соответственно.Проведено аналіз еволюції фаз і мікроструктури композитів алмаз–SiC–Si, виготовлених з використанням процесу вакуумної інфільтрації газоподібного кремнію. Дослідження виконано за допомогою дифракції рентгенівських променів і скануючої електронної мікроскопії. Встановлено, що композит алмаз–SiC–Si складається з β-SiC, алмазу і залишкового Si. Алмазні частки розподілені однорідно в щільній матриці композитів. Також проаналізовано вплив розміру частинок і вмісту алмазів на властивості композитів алмаз–SiC–Si. Показано, що теплопровідність композитів зростає зі збільшенням розміру частинок і вмісту алмазів. Теплопровідність композитів наближається до значення 280 Вт∙м⁻¹∙K⁻¹ при розмірі частинок і вмісту алмазу 300 мкм і 80 % (за масою) відповідно.This work was financially supported by the National Natural Science Foundation of China (grant no. 51102282) and Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province

On the existence of traveling waves in the 3D Boussinesq system

We extend earlier work on traveling waves in premixed flames in a gravitationally stratified medium, subject to the Boussinesq approximation. For three-dimensional channels not aligned with the gravity direction and under the Dirichlet boundary conditions in the fluid velocity, it is shown that a non-planar traveling wave, corresponding to a non-zero reaction, exists, under an explicit condition relating the geometry of the crossection of the channel to the magnitude of the Prandtl and Rayleigh numbers, or when the advection term in the flow equations is neglected.Comment: 15 pages, to appear in Communications in Mathematical Physic

Electronic structure and dynamics of optically excited single-wall carbon nanotubes

We have studied the electronic structure and charge-carrier dynamics of individual single-wall carbon nanotubes (SWNTs) and nanotube ropes using optical and electron-spectroscopic techniques. The electronic structure of semiconducting SWNTs in the band-gap region is analyzed using near-infrared absorption spectroscopy. A semi-empirical expression for $E_{11}^{\rm S}$ transition energies, based on tight-binding calculations is found to give striking agreement with experimental data. Time-resolved PL from dispersed SWNT-micelles shows a decay with a time constant of about 15 ps. Using time-resolved photoemission we also find that the electron-phonon ({\it e-ph}) coupling in metallic tubes is characterized by a very small {\it e-ph} mass-enhancement of 0.0004. Ultrafast electron-electron scattering of photo-excited carriers in nanotube ropes is finally found to lead to internal thermalization of the electronic system within about 200 fs.Comment: 10 pages, 10 figures, submitted to Applied Physics

Comparison of structural transformations and superconductivity in compressed Sulfur and Selenium

Density-functional calculations are presented for high-pressure structural phases of S and Se. The structural phase diagrams, phonon spectra, electron-phonon coupling, and superconducting properties of the isovalent elements are compared. We find that with increasing pressure, Se adopts a sequence of ever more closely packed structures (beta-Po, bcc, fcc), while S favors more open structures (beta-Po, simple cubic, bcc). These differences are shown to be attributable to differences in the S and Se core states. All the compressed phases of S and Se considered are calculated to have weak to moderate electron-phonon coupling strengths consistent with superconducting transition temperatures in the range of 1 to 20 K. Our results compare well with experimental data on the beta-Po --> bcc transition pressure in Se and on the superconducting transition temperature in beta-Po S. Further experiments are suggested to search for the other structural phases predicted at higher pressures and to test theoretical results on the electron-phonon interaction and superconducting properties

Faster linearizability checking via PP-compositionality

Linearizability is a well-established consistency and correctness criterion for concurrent data types. An important feature of linearizability is Herlihy and Wing's locality principle, which says that a concurrent system is linearizable if and only if all of its constituent parts (so-called objects) are linearizable. This paper presents $P$-compositionality, which generalizes the idea behind the locality principle to operations on the same concurrent data type. We implement $P$-compositionality in a novel linearizability checker. Our experiments with over nine implementations of concurrent sets, including Intel's TBB library, show that our linearizability checker is one order of magnitude faster and/or more space efficient than the state-of-the-art algorithm.Comment: 15 pages, 2 figure

Polaronic Signatures in Mid-Infrared Spectra: Prediction for LaMnO3 and CaMnO3

Hole-doped LaMnO3 and electron-doped CaMnO3 form self-trapped electronic states. The spectra of these states have been calculated using a two orbital (Mn eg Jahn-Teller) model, from which the non-adiabatic optical conductivity spectra are obtained. In both cases the optical spectrum contains weight in the gap region, whose observation will indicate the self-trapped nature of the carrier states. The predicted spectra are proportional to the concentration of the doped carriers in the dilute regime, with coefficients calculated with no further model parameters.Comment: 6 pages with 3 figures imbedde

Transition Spectra for a BCS Superconductor with Multiple Gaps: Model Calculations for MgB_2

We analyze the qualitative features in the transition spectra of a model superconductor with multiple energy gaps, using a simple extension of the Mattis-Bardeen expression for probes with case I and case II coherence factors. At temperature T = 0, the far infrared absorption edge is, as expected, determined by the smallest gap. However, the large thermal background may mask this edge at finite temperatures and instead the secondary absorption edges found at Delta_i+Delta_j may become most prominent. At finite T, if certain interband matrix elements are large, there may also be absorption peaks at the gap difference frequencies | Delta_i-Delta_j | . We discuss the effect of sample quality on the measured spectra and the possible relation of these predictions to the recent infrared absorption measurement on MgB_2