The role of high growth temperature GaAs spacer layers in 1.3-/spl mu/m In(Ga)As quantum-dot lasers

We investigate the mechanisms by which high growth temperature spacer layers (HGTSLs) reduce the threshold current of 1.3-/spl mu/m emitting multilayer quantum-dot lasers. Measured optical loss and gain spectra are used to characterize samples that are nominally identical except for the HGTSL. We find that the use of the HGTSL leads to the internal optical mode loss being reduced from 15 /spl plusmn/ 2 to 3.5 /spl plusmn/ 2 cm/sup -1/, better defined absorption features, and more absorption at the ground state resulting from reduced inhomogenous broadening and a greater dot density. These characteristics, together with a reduced defect density, lead to greater modal gain at a given current density

A spinor approach to Walker geometry

A four-dimensional Walker geometry is a four-dimensional manifold M with a neutral metric g and a parallel distribution of totally null two-planes. This distribution has a natural characterization as a projective spinor field subject to a certain constraint. Spinors therefore provide a natural tool for studying Walker geometry, which we exploit to draw together several themes in recent explicit studies of Walker geometry and in other work of Dunajski (2002) and Plebanski (1975) in which Walker geometry is implicit. In addition to studying local Walker geometry, we address a global question raised by the use of spinors.Comment: 41 pages. Typos which persisted into published version corrected, notably at (2.15

Pressure Induced Change in the Magnetic Modulation of CeRhIn5

We report the results of a high pressure neutron diffraction study of the heavy fermion compound CeRhIn5 down to 1.8 K. CeRhIn5 is known to order magnetically below 3.8 K with an incommensurate structure. The application of hydrostatic pressure up to 8.6 kbar produces no change in the magnetic wave vector qm. At 10 kbar of pressure however, a sudden change in the magnetic structure occurs. Although the magnetic transition temperature remains the same, qm increases from (0.5, 0.5, 0.298) to (0.5, 0.5, 0.396). This change in the magnetic modulation may be the outcome of a change in the electronic character of this material at 10 kbar.Comment: 4 pages, 3 figures include

Magnetically mediated superconductivity: Crossover from cubic to tetragonal lattice

We compare predictions of the mean-field theory of superconductivity for nearly antiferromagnetic and nearly ferromagnetic metals for cubic and tetragonal lattices. The calculations are based on the parametrization of an effective interaction arising from the exchange of magnetic fluctuations and assume that a single band is relevant for superconductivity. The results show that for comparable model parameters, the robustness of magnetic pairing increases gradually as one goes from a cubic structure to a more and more anisotropic tetragonal structure either on the border of antiferromagnetism or ferromagnetism.Comment: 16 pages 14 figure

Singularity, complexity, and quasi--integrability of rational mappings

We investigate global properties of the mappings entering the description of symmetries of integrable spin and vertex models, by exploiting their nature of birational transformations of projective spaces. We give an algorithmic analysis of the structure of invariants of such mappings. We discuss some characteristic conditions for their (quasi)--integrability, and in particular its links with their singularities (in the 2--plane). Finally, we describe some of their properties {\it qua\/} dynamical systems, making contact with Arnol'd's notion of complexity, and exemplify remarkable behaviours.Comment: Latex file. 17 pages. To appear in CM

Pressure-Temperature Phase Diagram of Antiferromagnetism and Superconductivity in CeRhIn5 and CeIn3 : In-NQR Study under Pressure

We report the novel pressure($P$) - temperature($T$) phase diagram of antiferromagnetism and superconductivity in CeRhIn$_5$ and CeIn$_3$ revealed by the $^{115}$In nuclear-spin-lattice-relaxation ($T_1$) measurement. In the itinerant magnet CeRhIn$_5$, we found that the N\'eel temperature $T_N$ is reduced at $P \geq$ 1.23 GPa with an emergent pseudogap behavior. In CeIn$_3$, the localized magnetic character is robust against the application of pressure up to $P \sim$ 1.9 GPa, beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations.Comment: 4 pages, 3 figures, to be published in Phys.Rev.B. Rapid

Density Fluctuation Mediated Superconductivity

We conpare predictions of the mean-field theory of supercnductivity for metallic systems on the border of a density instability for cubic and tetragonal lattices. The calculations are based on a parametrisation of an effective interaction arising from the exchange of density fluctuations and assume that a single band is relevant for superconductivity. The results show that for comparable model parameters, desnity fluctuation mediated pairing is more robust in quasi-two dimensions than in three dimensions, and that the robustness of pairing increases gradually as one goes from a cubic to a more and more anisotropic tetragonal structure. We also find that the robustness of density fluctuation mediated pairing can depend sensitively on the incipient ordering wavevector. We discuss the similarities and differences between the mean-field theories of superconductivity for density and magnetically mediated pairing

The Self Model and the Conception of Biological Identity in Immunology

The self/non-self model, first proposed by F.M. Burnet, has dominated immunology for sixty years now. According to this model, any foreign element will trigger an immune reaction in an organism, whereas endogenous elements will not, in normal circumstances, induce an immune reaction. In this paper we show that the self/non-self model is no longer an appropriate explanation of experimental data in immunology, and that this inadequacy may be rooted in an excessively strong metaphysical conception of biological identity. We suggest that another hypothesis, one based on the notion of continuity, gives a better account of immune phenomena. Finally, we underscore the mapping between this metaphysical deflation from self to continuity in immunology and the philosophical debate between substantialism and empiricism about identity

Magnetic structure of CeRhIn_5 as a function of pressure and temperature

We report magnetic neutron-diffraction and electrical resistivity studies on single crystals of the heavy-fermion antiferromagnet CeRhIn$_{5}$ at pressures up to 2.3 GPa. These experiments show that the staggered moment of Ce and the incommensurate magnetic structure change weakly with applied pressure up to 1.63 GPa, where resistivity, specific heat and NQR measurements confirm the presence of bulk superconductivity. This work places new constraints on an interpretation of the relationship between antiferromagnetism and unconventional superconductivity in CeRhIn$_{5}$.Comment: 6 pages, 6 figures, submitted to Phys. Rev.

Searching for star-planet magnetic interaction in CoRoT observations

Close-in massive planets interact with their host stars through tidal and magnetic mechanisms. In this paper, we review circumstantial evidence for star-planet interaction as revealed by the photospheric magnetic activity in some of the CoRoT planet-hosting stars, notably CoRoT-2, CoRoT-4, and CoRoT-6. The phenomena are discussed in the general framework of activity-induced features in stars accompanied by hot Jupiters. The theoretical mechanisms proposed to explain the activity enhancements possibly related with hot Jupiter are also briefly reviewed with an emphasis on the possible effects at photospheric level. The unique advantages of CoRoT and Kepler observations to test these models are pointed out.Comment: Invited review paper accepted by Astrophysics and Space Science, 13 pages, 5 figure