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Glasco, D.M.,Sittaramane, V.,Bryant, W.,Fritzsch, B.,Sawant, A.,Paudyal, A.,Stewart, M.,Andre, P.,Cadete Vilhais-Neto, G.,Yang, Y.,Song, M.R.,Murdoch, J.N.,Chandrasekhar, A. Academic Press 2012 Developmental Biology Vol.369 No.2
During development, facial branchiomotor (FBM) neurons, which innervate muscles in the vertebrate head, migrate caudally and radially within the brainstem to form a motor nucleus at the pial surface. Several components of the Wnt/planar cell polarity (PCP) pathway, including the transmembrane protein Vangl2, regulate caudal migration of FBM neurons in zebrafish, but their roles in neuronal migration in mouse have not been investigated in detail. Therefore, we analyzed FBM neuron migration in mouse looptail (Lp) mutants, in which Vangl2 is inactivated. In Vangl2<SUP>Lp/+</SUP> and Vangl2<SUP>Lp/Lp</SUP> embryos, FBM neurons failed to migrate caudally from rhombomere (r) 4 into r6. Although caudal migration was largely blocked, many FBM neurons underwent normal radial migration to the pial surface of the neural tube. In addition, hindbrain patterning and FBM progenitor specification were intact, and FBM neurons did not transfate into other non-migratory neuron types, indicating a specific effect on caudal migration. Since loss-of-function in some zebrafish Wnt/PCP genes does not affect caudal migration of FBM neurons, we tested whether this was also the case in mouse. Embryos null for Ptk7, a regulator of PCP signaling, had severe defects in caudal migration of FBM neurons. However, FBM neurons migrated normally in Dishevelled (Dvl) ½ double mutants, and in zebrafish embryos with disrupted Dvl signaling, suggesting that Dvl function is essentially dispensable for FBM neuron caudal migration. Consistent with this, loss of Dvl2 function in Vangl2<SUP>Lp/+</SUP> embryos did not exacerbate the Vangl2<SUP>Lp/+</SUP> neuronal migration phenotype. These data indicate that caudal migration of FBM neurons is regulated by multiple components of the Wnt/PCP pathway, but, importantly, may not require Dishevelled function. Interestingly, genetic-interaction experiments suggest that rostral FBM neuron migration, which is normally suppressed, depends upon Dvl function.
Superconducting properties of the <i>s</i> <sup>?</sup>-wave state: Fe-based superconductors
Bang, Yunkyu,Stewart, G R IOP 2017 Journal of Physics, Condensed Matter Vol.29 No.12
<P>Although the pairing mechanism of Fe-based superconductors (FeSCs) has not yet been settled with consensus with regard to the pairing symmetry and the superconducting (SC) gap function, the vast majority of experiments support the existence of spin-singlet sign-changing <I>s</I>-wave SC gaps on multi-bands (<img ALIGN='MIDDLE' ALT='${{s}^{\pm }}$ ' SRC='http://ej.iop.org/images/0953-8984/29/12/123003/cmaa564bieqn003.gif'/>-wave state). This multi-band <img ALIGN='MIDDLE' ALT='${{s}^{\pm }}$ ' SRC='http://ej.iop.org/images/0953-8984/29/12/123003/cmaa564bieqn004.gif'/>-wave state is a very unique gap state <I>per se</I> and displays numerous unexpected novel SC properties, such as a strong reduction of the coherence peak, non-trivial impurity effects, nodal-gap-like nuclear magnetic resonance signals, various Volovik effects in the specific heat (SH) and thermal conductivity, and anomalous scaling behaviors with a SH jump and condensation energy versus <I>T</I> <SUB>c</SUB>, etc. In particular, many of these non-trivial SC properties can easily be mistaken as evidence for a nodal-gap state such as a <I>d</I>-wave gap. In this review, we provide detailed explanations of the theoretical principles for the various non-trivial SC properties of the <img ALIGN='MIDDLE' ALT='${{s}^{\pm }}$ ' SRC='http://ej.iop.org/images/0953-8984/29/12/123003/cmaa564bieqn005.gif'/>-wave pairing state, and then critically compare the theoretical predictions with experiments on FeSCs. This will provide a pedagogical overview of to what extent we can coherently understand the wide range of different experiments on FeSCs within the <img ALIGN='MIDDLE' ALT='${{s}^{\pm }}$ ' SRC='http://ej.iop.org/images/0953-8984/29/12/123003/cmaa564bieqn006.gif'/>-wave gap model.</P>
Single-crystal growth and superconducting properties of LiFeAs
Lee, Bumsung,Khim, Seunghyun,Kim, Jung Soo,Stewart, G. R.,Kim, Kee Hoon Editions de Physique 2010 Europhysics letters Vol.91 No.6
<P>We report the successful growth of high-quality single crystals of LiFeAs with lateral sizes up to 5×5 mm<SUP>2</SUP> by the Sn-flux method. Electrical resistivity studies reveal that the superconducting onset temperature is 18.2 K with a transition width less than 1.1 K and the ratio of room temperature to residual resistivity is about 24. Bulk superconductivity is supported by perfect shielding in the magnetic susceptibility and a clear jump in the specific heat <I>C</I><SUB>p</SUB>, resulting in Δ<I>C</I><SUB>p</SUB>/<I>T</I> ≈ 20.0 mJ/mol K<SUP>2</SUP>. Upper-critical-field slopes of d<I>H</I><SUB>c2</SUB><SUP><I>c</I></SUP>/d<I>T</I> ≈−1.39 and d<I>H</I><SUB>c2</SUB><SUP><I>ab</I></SUP>/d<I>T</I> ≈−2.99 T/K near <I>T</I><SUB>c</SUB> predict zero-temperature upper critical fields of <I>H</I><SUB>c2</SUB><SUP><I>c</I></SUP>(0) ≈ 17.2 and <I>H</I><SUB>c2</SUB><SUP><I>ab</I></SUP>(0) ≈ 36.9 T and coherence lengths of ξ<SUB><I>ab</I></SUB>=4.4 and ξ<SUB><I>c</I></SUB>=2.0 nm in a single-band model. This result points to a modest superconducting anisotropy about 2.3 in LiFeAs.</P>
Current-voltage Characteristics of NdFeAsO0.85F0.15 and NdFeAsO0.85 Superconductors
Yong Liu,YiSheng Chai,Hyeong-Jin Kim,G.R. Stewart,김기훈,Zhi-An Ren,Zhong-Xian Zhao 한국물리학회 2009 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.55 No.2
The vortex phase diagrams of NdFeAsO0.85F0.15 and NdFeAsO0.85 superconductors are determined from an analysis of resistivity and current-voltage (I-V ) measurements in magnetic fields up to 9 T. A vortex glass to liquid transition can be identified only in the oxygen-deficient NdFeAsO0.85, for which the I-V curves can be well scaled onto liquid and glass branches consistent with the vortex glass theory. With increasing magnetic field, the activation energy, U0, deduced from the Arrhenius plots of the resistivity based on the thermally-activated flux-flow model decays more quickly for NdFeAsO0.85F0.15 than for NdFeAsO0.85. Moreover, the irreversibility field, Hirr, of NdFeAsO0.85 increases more rapidly than that of NdFeAsO0.85F0.15 with decreasing temperature. These observations imply strong vortex pinning effects in the oxygen-deficient NdFeAsO0.85, presumably caused by enhanced defects and disorders. We infer that the observation of a vortex glass to liquid transition in NdFeAsO0.85 may be also related to the enhanced defects and disorder in the specimen.
Terzioglu, M.,Ruzzenente, B.,Harmel, J.,Mourier, A.,Jemt, E.,Lopez, M.,Kukat, C.,Stewart, James B.,Wibom, R.,Meharg, C.,Habermann, B.,Falkenberg, M.,Gustafsson, Claes M.,Park, C.,Larsson, N.G. Cell Press 2013 Cell metabolism Vol.17 No.4
Mitochondrial transcription termination factor 1, MTERF1, has been reported to couple rRNA gene transcription initiation with termination and is therefore thought to be a key regulator of mammalian mitochondrial ribosome biogenesis. The prevailing model is based on a series of observations published over the last two decades, but no in vivo evidence exists to show that MTERF1 regulates transcription of the heavy-strand region of mtDNA containing the rRNA genes. Here, we demonstrate that knockout of Mterf1 in mice has no effect on mitochondrial rRNA levels or mitochondrial translation. Instead, loss of Mterf1 influences transcription initiation at the light-strand promoter, resulting in a decrease of de novo transcription manifested as reduced 7S RNA levels. Based on these observations, we suggest that MTERF1 does not regulate heavy-strand transcription, but rather acts to block transcription on the opposite strand of mtDNA to prevent transcription interference at the light-strand promoter.
Kim, Jun Sung,Khim, Seunghyun,Yan, Liqin,Manivannan, N,Liu, Yong,Kim, Ingyu,Stewart, G R,Kim, Kee Hoon IOP Pub 2009 Journal of Physics, Condensed Matter Vol.21 No.10
<P>In order to investigate whether magnetism and superconductivity coexist in Co-doped SrFe<SUB>2</SUB>As<SUB>2</SUB>, we have prepared single crystals of SrFe<SUB>2−<I>x</I></SUB>Co<SUB><I>x</I></SUB>As<SUB>2</SUB>, <I>x</I> = 0 and 0.4, and characterized them via x-ray diffraction, electrical resistivity in zero and applied field up to 9 T as well as at ambient and applied pressure up to 1.6 GPa, and magnetic susceptibility. At <I>x</I> = 0.4, there is both magnetic and resistive evidence for a spin density wave transition at 120 K, while <I>T</I><SUB>c</SUB> = 19.5 K—indicating coexistent magnetism and superconductivity. A discussion of how these results compare with reported results, both in SrFe<SUB>2−<I>x</I></SUB>Co<SUB><I>x</I></SUB>As<SUB>2</SUB> and in other doped 122 compounds, is given. </P>
Lee, B.,Khim, S.,Jeon, B.J.,Park, J.Y.,Lee, S.H.,Choi, K.Y.,Stewart, G.R.,Kim, K.H. North-Holland 2013 Physica. C, Superconductivity Vol.495 No.-
Single crystals of Li(Fe<SUB>1-x</SUB>Co<SUB>x</SUB>)As (x=0.00, 0.02, 0.04, 0.06, 0.08 and 0.1) were grown by the Sn-flux method and their resistivity behaviors were investigated. As the Co concentration x increased, the superconducting transition temperature T<SUB>c</SUB> as well as the residual resistivity ratio systematically decreased, indicating a monotonic doping of electron carriers. Upon the analysis of the Fermi-liquid behavior of the resistivity and by using the Kadowaki-Woods relation, the T<SUP>2</SUP> coefficient of the resistivity A and the Sommerfeld coefficient γ<SUB>N</SUB> were found to increase abruptly near x=0.06. This is the point in the phase diagram at which the density of states at the Fermi energy (proportional to γ<SUB>N</SUB>) is expected to be enhanced due to the collapse of the small hole-pockets. This result suggests that the Li(Fe<SUB>1-x</SUB>Co<SUB>x</SUB>)As series at least up to x=0.08 remain as compensated metals and their transport properties are determined by multi-band effects.