We demonstrate the use of functionalized graphene sheets (FGSs) as multifunctional nanofillers to improve mechanical properties, lower gas permeability, and impart electrical conductivity for several distinct elastomers. FGS consists mainly of single sheets of crumbled graphene containing oxygen functional groups and is produced by the thermal exfoliation of oxidized graphite (GO). The present investigation includes composites of FGS and three elastomers: natural rubber (NR), styrene–butadiene rubber, and polydimethylsiloxane (PDMS). All of these elastomers show similar and significant improvements in mechanical properties with FGS, indicating that the mechanism of property improvement is inherent to the FGS and not simply a function of chemical crosslinking. The decrease in gas permeability is attributed to the high aspect ratio of the FGS sheets. This creates a tortuous path mechanism of gas diffusion; fitting the permeability data to the Nielsen model yields an aspect ratio of ∼1000 for the FGS. Electrical conductivity is demonstrated at FGS loadings as low as 0.08% in PDMS and reaches 0.3 S/m at 4 wt % loading in NR. This combination of functionalities imparted by FGS is shown to result from its high aspect ratio and carbon‐based structure. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012
We report that the lowest energy transverse-optic phonon in metallic SnTe softens to near zero energy at the structural transition at T-C = 75 K and importantly show that the energy of this mode below T-C increases as the temperature decreases. Since the mode is a polar displacement this proves unambiguously that SnTe undergoes a ferroelectric displacement below T-C. Concentration gradients and imperfect stoichiometry in large crystals may explain why this was not seen in previous inelastic neutron scattering studies. Despite SnTe being metallic we find that the ferroelectric transition is similar to that in ferroelectric insulators, unmodified by the presence of conduction electrons: we find that (i) the damping of the polar mode is dominated by coupling to acoustic phonons rather than electron-phonon coupling, (ii) the transition is almost an ideal continuous transition, and (iii) comparison with density functional calculations identifies the importance of dipolar-dipolar screening for understanding this behavior
The transition between paramagnetism and ferromagnetism is the paradigm for a continuous phase transition at finite temperature. When such a transition is tuned to zero temperature in clean materials, the growth of low-energy zero-point fluctuations potentially drives an array of phenomena, including the formation of novel states such as nonconventional superconductivity. Experimentally, the growth of the fluctuations, however, is curtailed and the transition becomes discontinuous as its temperature is reduced. This is understood to arise from non-analytic corrections to the free energy that always occur 1 . In a recent theory 2,3 , changes of the excitation spectrum are self-consistently considered alongside the ground state. This analysis reveals that a transition to a new state may be an alternative outcome. As the excitation spectrum (the 'disorder') is pivotal to promoting the new 'order' this mechanism is referred to as 'order by disorder'. Here, we report the discovery of modulated order in PrPtAl, consistent with complex spirals, at the boundary between paramagnetism and ferromagnetism, giving the first clear experimental realization of such a state.In our theoretical model, deformations of the Fermi surface in the modulated state enlarge the phase space available for low-energy particle-hole fluctuations and this self-consistently lowers the free energy relative to a uniform ferromagnetic state. Although previous theory predicting spiral formation 4,5 based on this mechanism has considered isotropic magnets, easy-plane systems are better candidate materials, as a hard magnetic axis provides a natural orientation for the spiral wavevector and suppresses 'unwanted' moment fluctuations along the spiral direction. The anisotropy can be introduced with local moments 6 , although the theoretical description close to a ferromagnetic quantum critical point 7 has only recently been extended to include the coupling of these moments to the conduction electrons 8 . Here we describe our findings for PrPtAl. This material is close to being an easy-plane ferromagnet, but has an additional magnetic anisotropy between the two easy axes in the plane. The electronic levels of the praseodymium 4f 2 Pr 3+ ions are split in the crystal environment (PrPtAl has an orthorhombic TiNiSn structure) into nine non-magnetic singlet states. Inelastic neutron studies 9 reveal clear crystal field excitations between these. Choosing a system with only singlets simplifies the theory considerably, avoiding Kondo lattice physics, while still introducing magnetic anisotropy.As there are no preformed moments, ferromagnetic order is achieved by mixing singlets via an inter-site exchange interaction 10 , a process referred to as induced-moment magnetism. Our theoretical approach to analysing the magnetic interactions that bring about magnetic order in PrPtAl differs from the standard treatment 11 by keeping the full frequency dependence of the fermion-mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. This is the key element for a descr...
A better understanding of the synthesis conditions, composition and physical properties of UTe2 are required to interpret previously reported unconventional superconductivity. Here we report how the superconducting properties of single crystals depend on the ratio of elements present in their synthesis by chemical vapour transport. We have obtained crystals with the highest reported ambient pressure T c and a larger superconducting heat capacity jump from a growth with a U:Te ratio different from that widely used in the literature. For these crystals, the ratio of residual heat capacity in the superconducting state to that of the normal state, γ*/γ N, is significantly lower than 0.5, reported elsewhere. An upturn in the heat capacity below 200 mK is also reduced compared to other studies and is well described by a Schottky anomaly and residual Sommerfeld term rather than quantum critical behaviour.
Broadband frequency-doubling properties of c-axis oriented zinc oxide (ZnO) nanorod arrays grown by low-temperature chemical bath method on glass substrate were studied. The maximum effective nonlinearity was found to be about 7.5 times higher than that of a type-I beta-barium borate crystal for a pump intensity of 5.5×1010 W/cm2. The angular dependence of second harmonic generation (SHG) was determined experimentally. The measured spectral profile of SHG was found to be in good agreement with theoretical simulations.
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