We report on the upper critical field B c2 of a superconducting LiFeAs single crystal with T c $ 16 K, determined from magnetic torque measurements in dc-magnetic fields up to 35 T and at temperatures down to 0.3 K. B c2 at 0.3 K is obtained to be 26.4 and 15.5 T for the applied field B a k ab and B a k c, respectively. The anisotropy parameter À ¼ B ab c2 =B c c2 is $3 at T c and decreases to 1.7 as T ! 0, showing rather isotropic superconductivity. While B c2 is orbitally-limited for B a k c, the spin-paramagnetic effect is evident in the temperature dependence of B c2 for B a k ab. KEYWORDS: iron-based superconductor, LiFeAs, upper critical field, anisotropySince the discovery of superconductivity in LaFeAs(O,F) with T c ¼ 26 K, 1) a variety of related compounds containing FeAs-layers has been found to exhibit superconductivity.2)The parent compounds RFeAsO (R = rare earth, ''1111'' system) and AFe 2 As 2 (A = alkaline earth or Eu, ''122'' system) with the ZrCuSiAs-and ThCr 2 Si 2 -type structures, respectively, undergo antiferromagnetic and structural transitions. The transitions can be suppressed by several kinds of doping effects 1,[3][4][5][6] or application of pressure, [7][8][9] and T c reaches $56 K in some compounds. [10][11][12] The magnetic long range order usually competes with superconductivity, but the fluctuation likely plays a crucial role in the pairing mechanism of the Fe-based high-T c systems. It is also of interest that, around the optimal condition where T c shows its maximum, deviation from conventional Fermi-liquid behavior has been observed such as $ T , 13,14) anomalous Hall angle, 13) an enhancement of effective masses, 15) etc. These superconducting and normal-state features bear resemblance with those widely reported for strongly-correlated electron systems including cuprates and heavy fermion compounds.The title compound LiFeAs, categorized as the ''111'' system with the CeFeSi-type structure, has distinctive characteristics: (i) the stoichiometric superconductivity with T c as high as $17 K, 16) (ii) no experimental evidence for the magnetic/structural transitions, 17,18) and (iii) single crystals with high quality (residual resistivity ratio up to 50). 19,20) Therefore, LiFeAs provides a unique opportunity to probe the intrinsic properties of Fe-based high-T c superconductivity.Up to now, there are few reports on systematic measurements of the upper critical field B c2 of Fe-based superconductors to address the issue of the pair-breaking mechanism. This is mainly due to the fact that high-T c superconductors including Fe-based systems generally have extremely high B c2 . In many cases, accordingly, the lowtemperature behavior is extrapolated from the high temperature data around T c , which may lead to misleading conclusions. A precise determination of B c2 over the whole temperature range could provide important clues to the pairbreaking mechanism of high-T c superconductivity.Here, we present the first report on the whole temperature dependence of B c2 and its angular variatio...
Single-walled carbon nanotubes were dispersed in an aqueous medium using surfactants and filtered to make entangled networks, called buckypaper (BP), and the Raman spectra of BP samples revealed the degree of entanglement and residual surfactant content. The temperature dependence of the G-band peak shift in the BP was found to depend on the reduction in residual surfactant and nanotube oxidation. The electrical conductivity was improved after removing the surfactant and increasing the nanotube alignment, although the temperature dependence of electrical resistivity still followed a variable range hopping conduction behavior. The mechanical properties were affected by the degree of entanglement, alignment, and residual surfactant content, and tensile properties were found to improve with the reduction in surfactant and enhancement of alignment.
We report a comparative investigation of the magnetostrictive and magnetoconductivity effects in the low temperature ferrimagnetic phases of the spinel vanadate structure AV 2 O 4 where A = Fe, Mn, and Co. In temperature and magnetic field exhibit distinctive physical properties in each case. These include the magneto-caloric signatures of magnetization reversal (Fe and Co), separable isotropic and anisotropic magnetostrictive effects (Mn), glass-like dynamics (Co), and evidence in the fielddependent dielectric response for the alteration of the ferrimagnetic spin structure with increasing 2 magnetic field (Fe). These findings can guide future dielectric-and magnetoconductance-based spinel studies with a focus on the low temperature and high magnetic field properties of canted ferrimagnetic spin configurations and orbital-lattice ordering effects. 75.25.+z, 75.50.Gg, 75.80.+q, 71.70 PACS numbers
A homopolymer iPP and a series of propylene‐ethylene random copolymers with a content of ethylene from 7 to 21 mol % were used as matrices to prepare single‐walled carbon nanotube (SWCNT) nanocomposites in a range of SWCNT concentration from 0.15 to 1 wt %. The solution blending and melt‐ compression molding procedures were kept identical for all nanocomposites. The poly(propylenes) have crystallinities ranging from 70 to 10%, and serve to test the role of SWCNTs acting as nucleants to preserve in the nanocomposites the uniform dispersion of SWCNTs after sonication. The major role of polymer crystallinity is to mediate toward a more open and more connected SWCNT network structure. Fast nucleation and growth of high crystalline matrices on multiple sites along the surface of the nanotubes prevents SWCNT clustering, and entraps the SWCNT network between the semicrystalline structure reducing the driving force of nanotubes to curl and twist. Depletion of crystallites in the less crystalline matrices (<35% crystallinity) leads to curled and poorly connected nanotubes. A consequence of the gradual loss of SWCNT connectivity is a decreased electrical conductivity; however, the change with crystallinity is not linear. Conductivity decreases sharply with decreasing crystallinity for SWCNT contents near the percolation region, while for contents approaching the plateau region the electrical conductivity is less sensitive to matrix crystallinity. The percolation threshold decreases rapidly for polymers with <∼30% crystallinity and slowly levels off at crystallinities >∼40%. At SWCNT concentrations of 0.15 wt %, SEM images of nanocomposites with the highest crystallinity matrix indicate debundled and interconnected nanotubes, whereas more disconnected and curled SWCNTs remain in the lowest crystallinity nanocomposites. Electrical conductivity in the former is relatively high, whereas the latter are insulators. Also discussed is the nucleating effect of nanotubes and restrictions of the filler to polymer chain diffusion in the crystallization of the polymers. SEM images and Raman spectra in the radial breathing modes region (100–400 cm−1) are complementary tools to extract the quality and details of the SWCNT dispersion in the nanocomposites. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2084–2096, 2010
The mechanical stability of metallic nanomaterials has been intensively studied due to their unique structures and promising applications. Although extensive investigations have been carried out on the deformation behaviors of metallic nanomaterials, the atomic-scale deformation mechanism of metallic nanomaterials with unconventional hexagonal structures remains unclear because of the lack of direct experimental observation. Here, we conduct an atomic-resolution in situ tensile-straining transmission electron microscopy investigation on the deformation mechanism of gold nanoribbons with the 4H (hexagonal) phase. Our results reveal that plastic deformation in the 4H gold nanoribbons comprises three stages, in which both full and partial dislocations are involved. At the early deformation stage, plastic deformation is governed by full dislocation activities. Partial dislocations are subsequently activated in regions that have undergone full dislocation gliding, leading to phase transformation from the 4H phase to the face-centered cubic (FCC) phase. At the last stage of the deformation process, the volume fraction of the FCC phase increases, and full dislocation activities in the FCC regions also play an important role.
Dielectric relaxation is universal in characterizing polar liquids and solids, insulators, and semiconductors, and the theoretical models are well developed. However, in high magnetic fields, previously unknown aspects of dielectric relaxation can be revealed and exploited. Here, we report low temperature dielectric relaxation measurements in lightly doped silicon in high dc magnetic fields B both parallel and perpendicular to the applied ac electric field E. For B//E, we observe a temperature and magnetic field dependent dielectric dispersion ε(ω) characteristic of conventional Debye relaxation where the free carrier concentration is dependent on thermal dopant ionization, magnetic freeze-out, and/or magnetic localization effects. However, for B⊥E, anomalous dispersion emerges in ε(ω) with increasing magnetic field. It is shown that the Debye formalism can be simply extended by adding the Lorentz force to describe the general response of a dielectric in crossed magnetic and electric fields. Moreover, we predict and observe a new transverse dielectric response E H ⊥ B ⊥ E not previously described in magnetodielectric measurements. The new formalism allows the determination of the mobility and the ability to discriminate between magnetic localization/freeze out and Lorentz force effects in the magneto-dielectric response.
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