Graphene has emerged as a subject of enormous scientific interest due to its exceptional electron transport, mechanical properties, and high surface area. When incorporated appropriately, these atomically thin carbon sheets can significantly improve physical properties of host polymers at extremely small loading. We first review production routes to exfoliated graphite with an emphasis on top-down strategies starting from graphite oxide, including advantages and disadvantages of each method. Then solvent- and melt-based strategies to disperse chemically or thermally reduced graphene oxide in polymers are discussed. Analytical techniques for characterizing particle dimensions, surface characteristics, and dispersion in matrix polymers are also introduced. We summarize electrical, thermal, mechanical, and gas barrier properties of the graphene/polymer nanocomposites. We conclude this review listing current challenges associated with processing and scalability of graphene composites and future perspectives f...

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Graphene/Polymer Nanocomposites

日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

The response of the worldwide scientific community to the discovery in 2008 of superconductivity at T c = 26 K in the Fe-based compound LaFeAsO1−x F x has been very enthusiastic. In short order, ot...

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

Kamihara and coworkers' report of superconductivity at ${T}_{c}=26\text{ }\text{ }\mathrm{K}$ in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled ``conventional'' superconductors. Clearly, superconductivity and magnetism or magnetic fluctuations are intimately related in the FePn/Ch, and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With ${T}_{c}$ values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review provides an overview, using numerous references, with a focus on the materials and their superconductivity.

Superconductivity in iron compounds

Journal of Physics Condensed Matter: Preface

${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$ has been found to be exceptionally sensitive to the application of hydrostatic pressure and can be tuned to reveal all the salient features associated with FeAs superconductivity without introducing any disorder. The ambient pressure, 170 K, structural/magnetic, first-order phase transition is suppressed to 128 K by 3.5 kbar. At 5.5 kbar a new transition is detected at 104 K, increasing to above 300 K by 19 kbar. A low temperature, superconducting dome (${T}_{c}\ensuremath{\sim}12\text{ }\text{ }\mathrm{K}$) is centered around 5 kbar, extending down to 2.3 kbar and up to 8.6 kbar. This superconducting phase appears to exist when the low pressure transition is suppressed sufficiently, but before the high pressure transition has reduced the resistivity too dramatically.

Pressure induced superconductivity in CaFe2As2.

Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that CaFe2As2 undergoes a pressure-induced transition to a nonmagnetic volume "collapsed" tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction. Â© 2008 The American Physical Society.

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Pressure-induced volume-collapsed tetragonal phase of CaFe2As2 as seen via neutron scattering

${\text{CaFe}}_{2}{\text{As}}_{2}$ has been synthesized and found to form in the tetragonal ${\text{ThCr}}_{2}{\text{Si}}_{2}$ structure with lattice parameters $a=3912(68)\text{ }\text{\AA{}}$ and $c=11667(45)\text{ }\text{\AA{}}$ Upon cooling through 170 K, ${\text{CaFe}}_{2}{\text{As}}_{2}$ undergoes a first-order structural phase transition to a low-temperature orthorhombic phase with a 2--3 K range of hysteresis and coexistence This transition is clearly evident in microscopic, thermodynamic, and transport measurements ${\text{CaFe}}_{2}{\text{As}}_{2}$ is the third member of the ${\text{AFe}}_{2}{\text{As}}_{2}$ $(A=\text{Ba},\text{Sr},\text{Ca})$ family to exhibit such a dramatic phase transition and is a promising candidate for studies of doping induced superconductivity

First-order structural phase transition in CaFe2As2

Platelike single crystals of ${\text{SrFe}}_{2}{\text{As}}_{2}$ as large as $3\ifmmode\times\else\texttimes\fi{}3\ifmmode\times\else\texttimes\fi{}0.5\text{ }{\text{mm}}^{3}$ have been grown out of Sn flux. The ${\text{SrFe}}_{2}{\text{As}}_{2}$ single crystals show a structural phase transition from a high-temperature tetragonal phase to a low-temperature orthorhombic phase at ${T}_{o}=198\text{ }\text{K}$, and do not show any sign of superconductivity down to 1.8 K. The structural transition is accompanied by an anomaly in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In an intermediate temperature range from 198 to 160 K, single-crystal x-ray diffraction suggests a coexistence of the high-temperature tetragonal and the low-temperature orthorhombic phases.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1070&context=ameslab_pubs

Structural transition and anisotropic properties of single-crystalline SrFe2As2

The radio frequency penetration depth was measured in the superconductor (Ba$_{0.55}$K$_{0.45}$)Fe$_{2}$As$_{2}$ under pulsed magnetic fields extending to 60 tesla and down to 14 K. Using these data we are able to infer a $H_{c2}(T)$, $H-T$ phase diagram, for applied fields parallel and perpendicular to the crystallographic c-axis. The upper critical field curvature is different for the respective orientations but they each remain positive down to 14 K. The upper critical field anisotropy is moderate, $\approx 3.5$ close to $T_c$, and drops with the decrease of temperature, reaching $\approx 1.2$ at $14 K$. These data and analysis indicate that (i) (Ba$_{0.55}$K$_{0.45}$)Fe$_{2}$As$_{2}$ anisotropy diminishes with temperature and has an unusual temperature dependence, (ii) $H_{c2} (T=0)$ for this compound may easily approach fields of 75 tesla.

N. Ni

Papers

Pressure induced superconductivity in CaFe2As2.

Pressure-induced volume-collapsed tetragonal phase of CaFe2As2 as seen via neutron scattering

First-order structural phase transition in CaFe2As2

Structural transition and anisotropic properties of single-crystalline SrFe2As2

Determination of anisotropic Hc2 up to 60 T in (Ba0.55K0.45)Fe2As2 single crystals