Magnetoplasmon resonances in polycrystalline bismuth as seen via terahertz spectroscopy

We report the magnetic field-dependent far-infrared reflectivity of polycrystalline bismuth. We observe four distinct absorptions that we attribute to magnetoplasmon resonances, which are collective modes of an electron-hole liquid in magnetic field and become optical and acoustic resonances of the electron-hole system in the small-field limit. The acoustic mode is expected only when the masses of distinct components are very different, which is the case in bismuth. In a polycrystal, where the translational symmetry is broken, a big shift of spectral weight to acoustic plasmon is possible. This enables us to detect an associated plasma edge. Although the polycrystal sample has grains of randomly distributed orientations, our reflectivity results can be explained by invoking only two, clearly distinct, series of resonances. In the limit of zero field, the optical modes of these two series converge onto plasma frequencies measured in monocrystal along the main optical axes.Comment: Accepted in PR

Polarization of macrophages toward M2 phenotype is favored by reduction in iPLA2β (group VIA phospholipase A2)*

Macrophages are important in innate and adaptive immunity. Macrophage participation in inflammation or tissue repair is directed by various extracellular signals and mediated by multiple intracellular pathways. Activation of group VIA phospholipase A2 (iPLA2β) causes accumulation of arachidonic acid, lysophospholipids, and eicosanoids that can promote inflammation and pathologic states. We examined the role of iPLA2β in peritoneal macrophage immune function by comparing wild type (WT) and iPLA2β−/− mouse macrophages. Compared with WT, iPLA2β−/− macrophages exhibited reduced proinflammatory M1 markers when classically activated. In contrast, anti-inflammatory M2 markers were elevated under naïve conditions and induced to higher levels by alternative activation in iPLA2β−/− macrophages compared with WT. Induction of eicosanoid (12-lipoxygenase (12-LO) and cyclooxygenase 2 (COX2))- and reactive oxygen species (NADPH oxidase 4 (NOX4))-generating enzymes by classical activation pathways was also blunted in iPLA2β−/− macrophages compared with WT. The effects of inhibitors of iPLA2β, COX2, or 12-LO to reduce M1 polarization were greater than those to enhance M2 polarization. Certain lipids (lysophosphatidylcholine, lysophosphatidic acid, and prostaglandin E2) recapitulated M1 phenotype in iPLA2β−/− macrophages, but none tested promoted M2 phenotype. These findings suggest that (a) lipids generated by iPLA2β and subsequently oxidized by cyclooxygenase and 12-LO favor macrophage inflammatory M1 polarization, and (b) the absence of iPLA2β promotes macrophage M2 polarization. Reducing macrophage iPLA2β activity and thereby attenuating macrophage M1 polarization might cause a shift from an inflammatory to a recovery/repair milieu

Surface state charge dynamics of a high-mobility three dimensional topological insulator

We present a magneto-optical study of the three-dimensional topological insulator, strained HgTe using a technique which capitalizes on advantages of time-domain spectroscopy to amplify the signal from the surface states. This measurement delivers valuable and precise information regarding the surface state dispersion within <1 meV of the Fermi level. The technique is highly suitable for the pursuit of the topological magnetoelectric effect and axion electrodynamics.Comment: Published version, online Sept 23, 201

Negative Thermal Expansion Near the Precipice of Structural Stability in Open Perovskites

Negative thermal expansion (NTE) describes the anomalous propensity of materials to shrink when heated. Since its discovery, the NTE effect has been found in a wide variety of materials with an array of magnetic, electronic and structural properties. In some cases, the NTE originates from phase competition arising from the electronic or magnetic degrees of freedom but we here focus on a particular class of NTE which originates from intrinsic dynamical origins related to the lattice degrees of freedom, a property we term structural negative thermal expansion (SNTE). Here we review some select cases of NTE which strictly arise from anharmonic phonon dynamics, with a focus on open perovskite lattices. We find that NTE is often present close in proximity to competing structural phases, with structural phase transition lines terminating near T=0 K yielding the most prominent displays of the SNTE effect. We further provide a theoretical model to make precise the proposed relationship among the signature behavior of SNTE, the proximity of these systems to structural quantum phase transitions and the effects of phase fluctuations near these unique regions of the structural phase diagram. The effects of compositional disorder on NTE and structural phase stability in perovskites are discussed

Mott insulating negative thermal expansion perovskite TiF3

We characterize perovskite TiF_3, a material which displays significant negative thermal expansion at elevated temperatures above its cubic-to-rhombohedral structural phase transition at 330 K. We find the optical response favors an insulating state in both structural phases, which we show can be produced in density functional theory calculations only through the introduction of an on-site Coulomb repulsion. Analysis of the magnetic susceptibility data gives a S=1/2 local moment per Ti+3 ion and an antiferromagnetic exchange coupling. Together, these results show that TiF_3 is a strongly correlated electron system, a fact which constrains possible mechanisms of strong negative thermal expansion in the Sc_1-xTi_xF3 system. We consider the relative strength of the Jahn-Teller and electric dipole interactions in driving the structural transition.Comment: 8 pages, 4 figures, in review Physical Review

The Kondo Dynamics of YbIn(1-x)AgxCu4

We present an infrared/optical study of the dynamics of the strongly correlated electron system YbIn(1-x)AgxCu4 as a function of doping and temperature for x ranging from 0 to 1, and T between 20 and 300 K. This study reveals information about the unusual phase transition as well as the phases themselves. Scaling relations emerge from the data and are investigated in detail using a periodic Anderson model based calculation. We also provide a picture in which to view both the low and high-energy x-dependent features of the infrared data, including identification of high energy, temperature dependent features.Comment: 12 pages, 11 figures, submitted Phys. Rev.