A neutron scattering study of the interplay between structure and magnetism in Ba(Fe1−x_{1-x}Cox_{x})2_2As2_2

Single crystal neutron diffraction is used to investigate the magnetic and structural phase diagram of the electron doped superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. Heat capacity and resistivity measurements have demonstrated that Co doping this system splits the combined antiferromagnetic and structural transition present in BaFe$_2$As$_2$ into two distinct transitions. For $x$=0.025, we find that the upper transition is between the high-temperature tetragonal and low-temperature orthorhombic structures with ($T_{\mathrm{TO}}=99 \pm 0.5$ K) and the antiferromagnetic transition occurs at $T_{\mathrm{AF}}=93 \pm 0.5$ K. We find that doping rapidly suppresses the antiferromagnetism, with antiferromagnetic order disappearing at $x \approx 0.055$. However, there is a region of co-existence of antiferromagnetism and superconductivity. The effect of the antiferromagnetic transition can be seen in the temperature dependence of the structural Bragg peaks from both neutron scattering and x-ray diffraction. We infer from this that there is strong coupling between the antiferromagnetism and the crystal lattice

Taming Charge Transport in Semiconducting Polymers with Branched Alkyl Side Chains

National Research Fund of Luxembourg. Grant Number: 6932623; Croucher Foundation; Kodak Graduate Fellowship; Office of Naval Research. Grant Number: N00014-17-1-2214; U.S. Department of Energy. Grant Number: DE-AC02-76SF0051

Enhanced superconducting pairing interaction in indium-doped tin telluride

The ferroelectric degenerate semiconductor Sn$_{1-\delta}$Te exhibits superconductivity with critical temperatures, $T_c$, of up to 0.3 K for hole densities of order 10$^{21}$ cm$^{-3}$. When doped on the tin site with greater than $x_c$ $= 1.7(3)$ indium atoms, however, superconductivity is observed up to 2 K, though the carrier density does not change significantly. We present specific heat data showing that a stronger pairing interaction is present for $x > x_c$ than for $x < x_c$. By examining the effect of In dopant atoms on both $T_c$ and the temperature of the ferroelectric structural phase transition, $T_{SPT}$, we show that phonon modes related to this transition are not responsible for this $T_c$ enhancement, and discuss a plausible candidate based on the unique properties of the indium impurities.Comment: 7 page

Pressure-induced quenching of the charge-density-wave state observed by x-ray diffraction

We report an x-ray diffraction study on the charge-density-wave (CDW) LaTe$_3$ and CeTe$_3$ compounds as a function of pressure. We extract the lattice constants and the CDW modulation wave-vector, and provide direct evidence for a pressure-induced quenching of the CDW phase. We observe subtle differences between the chemical and mechanical compression of the lattice. We account for these with a scenario where the effective dimensionality in these CDW systems is dependent on the type of lattice compression and has a direct impact on the degree of Fermi surface nesting and on the strength of fluctuation effects

Thermal stability and spectroscopic properties of erbium-doped niobic-tungsten-tellurite glasses for laser and amplifier devices

Er3+ doped niobic-tungsten-tellurite glasses doped with concentration of Er3+ ion up to 3 wt% were fabricated. The effect of Er3+ doping concentration on thermal stability and optical properties was investigated in order to obtain the most suitable rare earth content for developing 1.5 micron compact fiber amplifier pumped with a commercial telecom 980 nm laser diode. The maximum doping concentration allowed was found to be around 1.77 x 10^20 ions/cm^3, for which a broad 1.5 micron emission spectra of 65 nm FWHM and a lifetime of 3.4 ms for the 4I13/2 level was measure

Spectroscopic and Mechanistic Studies of Heterodimetallic Forms of Metallo-β-lactamase NDM-1

In an effort to characterize the roles of each metal ion in metallo-β-lactamase NDM-1, heterodimetallic analogues (CoCo-, ZnCo-, and CoCd-) of the enzyme were generated and characterized. UV–vis, 1H NMR, EPR, and EXAFS spectroscopies were used to confirm the fidelity of the metal substitutions, including the presence of a homogeneous, heterodimetallic cluster, with a single-atom bridge. This marks the first preparation of a metallo-β-lactamase selectively substituted with a paramagnetic metal ion, Co(II), either in the Zn1 (CoCd-NDM-1) or in the Zn2 site (ZnCo-NDM-1), as well as both (CoCo-NDM-1). We then used these metal-substituted forms of the enzyme to probe the reaction mechanism, using steady-state and stopped-flow kinetics, stopped-flow fluorescence, and rapid-freeze-quench EPR. Both metal sites show significant effects on the kinetic constants, and both paramagnetic variants (CoCd- and ZnCo-NDM-1) showed significant structural changes on reaction with substrate. These changes are discussed in terms of a minimal kinetic mechanism that incorporates all of the data

Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution

Solution-processed semiconductors are in demand for presentandnext-generation optoelectronic technologies ranging from displaysto quantum light sources because of their scalability and ease ofintegration into devices with diverse form factors. One of the centralrequirements for semiconductors used in these applications is a narrowphotoluminescence (PL) line width. Narrow emission line widths areneeded to ensure both color and single-photon purity, raising thequestion of what design rules are needed to obtain narrow emissionfrom semiconductors made in solution. In this review, we first examinethe requirements for colloidal emitters for a variety of applicationsincluding light-emitting diodes, photodetectors, lasers, and quantuminformation science. Next, we will delve into the sources of spectralbroadening, including "homogeneous" broadening fromdynamical broadening mechanisms in single-particle spectra, heterogeneousbroadening from static structural differences in ensemble spectra,and spectral diffusion. Then, we compare the current state of theart in terms of emission line width for a variety of colloidal materialsincluding II-VI quantum dots (QDs) and nanoplatelets, III-VQDs, alloyed QDs, metal-halide perovskites including nanocrystalsand 2D structures, doped nanocrystals, and, finally, as a point ofcomparison, organic molecules. We end with some conclusions and connections,including an outline of promising paths forward