396 research outputs found
Evidence for Lattice Effects at the Charge-Ordering Transition in (TMTTF)X
High-resolution thermal expansion measurements have been performed for
exploring the mysterious "structureless transition" in (TMTTF)X (X =
PF and AsF), where charge ordering at coincides with the
onset of ferroelectric order. Particularly distinct lattice effects are found
at in the uniaxial expansivity along the interstack
-direction. We propose a scheme involving a charge
modulation along the TMTTF stacks and its coupling to displacements of the
counteranions X. These anion shifts, which lift the inversion symmetry
enabling ferroelectric order to develop, determine the 3D charge pattern
without ambiguity. Evidence is found for another anomaly for both materials at
0.6 indicative of a phase transition
related to the charge ordering
Low temperature structural effects in the (TMTSF)PF and AsF Bechgaard salts
We present a detailed low-temperature investigation of the statics and
dynamics of the anions and methyl groups in the organic conductors
(TMTSF)PF and (TMTSF)AsF (TMTSF :
tetramethyl-tetraselenafulvalene). The 4 K neutron scattering structure
refinement of the fully deuterated (TMTSF)PF-D12 salt allows locating
precisely the methyl groups at 4 K. This structure is compared to the one of
the fully hydrogenated (TMTSF)PF-H12 salt previously determined at the
same temperature. Surprisingly it is found that deuteration corresponds to the
application of a negative pressure of 5 x 10 MPa to the H12 salt. Accurate
measurements of the Bragg intensity show anomalous thermal variations at low
temperature both in the deuterated PF and AsF salts. Two different
thermal behaviors have been distinguished. Low-Bragg-angle measurements reflect
the presence of low-frequency modes at characteristic energies {\theta} =
8.3 K and {\theta} = 6.7 K for the PF-D12 and AsF-D12 salts,
respectively. These modes correspond to the low-temperature methyl group
motion. Large-Bragg-angle measurements evidence an unexpected structural change
around 55 K which probably corresponds to the linkage of the anions to the
methyl groups via the formation of F...D-CD2 bonds observed in the 4 K
structural refinement. Finally we show that the thermal expansion coefficient
of (TMTSF)PF is dominated by the librational motion of the PF
units. We quantitatively analyze the low-temperature variation of the lattice
expansion via the contribution of Einstein oscillators, which allows us to
determine for the first time the characteristic frequency of the PF6
librations: {\theta} = 50 K and {\theta} = 76 K for the PF-D12 and
PF-H12 salts, respectively
Resonant inelastic x-ray scattering probes the electron-phonon coupling in the spin-liquid kappa-(BEDT-TTF)2Cu2(CN)3
Resonant inelastic x-ray scattering at the N K edge reveals clearly resolved
harmonics of the anion plane vibrations in the kappa-(BEDT-TTF)2Cu2(CN)3
spin-liquid insulator. Tuning the incoming light energy at the K edge of two
distinct N sites permits to excite different sets of phonon modes. Cyanide CN
stretching mode is selected at the edge of the ordered N sites which are more
strongly connected to the BEDT-TTF molecules, while positionally disordered N
sites show multi-mode excitation. Combining measurements with calculations on
an anion plane cluster permits to estimate the sitedependent electron-phonon
coupling of the modes related to nitrogen excitation
Purely antiferromagnetic frustrated Heisenberg model in spin ladder compound BaFeSe
The spin dynamics in the block magnetic phase of the iron-based ladder
compound \bfs\ has been studied by means of single crystal inelastic neutron
scattering. Using linear spin wave theory and Monte-Carlo simulations, our
analysis points to a magnetic Heisenberg model with effective frustrated
antiferromagnetic couplings only, able to describe both the exotic block order
and its dynamics. This new and purely antiferromagnetic picture offers a
fruitful perspective to describe multiferroic properties but also understand
the origin of the stripe-like magnetic instability observed under pressure as
well as in other parent compounds with similar crystalline structure
Recent high-magnetic-field studies of unusual groundstates in quasi-two-dimensional crystalline organic metals and superconductors
After a brief introduction to crystalline organic superconductors and metals,
we shall describe two recently-observed exotic phases that occur only in high
magnetic fields. The first involves measurements of the non-linear electrical
resistance of single crystals of the charge-density-wave (CDW) system
(Per)Au(mnt) in static magnetic fields of up to 45 T and temperatures
as low as 25 mK. The presence of a fully gapped CDW state with typical CDW
electrodynamics at fields higher that the Pauli paramagnetic limit of 34 T
suggests the existence of a modulated CDW phase analogous to the
Fulde-Ferrell-Larkin-Ovchinnikov state. Secondly, measurements of the Hall
potential of single crystals of -(BEDT-TTF)KHg(SCN), made using
a variant of the Corbino geometry in quasistatic magnetic fields, show
persistent current effects that are similar to those observed in conventional
superconductors. The longevity of the currents, large Hall angle, flux
quantization and confinement of the reactive component of the Hall potential to
the edge of the sample are all consistent with the realization of a new state
of matter in CDW systems with significant orbital quantization effects in
strong magnetic fields.Comment: SNS 2004 Conference presentatio
Near-Edge X‐ray Absorption Fine Structure Investigation of the Quasi-One-Dimensional Organic Conductor (TMTSF)2PF6
We present high-resolution near-edge X-ray absorption fine structure (NEXAFS) measurements at the P L2/3 edges, F K edge, C K edge, and Se M2/3 edges of the quasi-one-dimensional (1D) conductor and superconductor (TMTSF)2PF6. NEXAFS allows probing the donor and acceptor moieties separately; spectra were recorded between room temperature (RT) and 30 K at normal incidence. Spectra taken around RT were also studied as a function of the angle (θ) between the electric field of the X-ray beam and the 1D conducting direction. In contrast with a previous study of the S L2/3-edges spectra in (TMTTF)2AsF6, the Se M2/3 edges of (TMTSF)2PF6 do not exhibit a well-resolved spectrum. Surprisingly, the C K-edge spectra contain three well-defined peaks exhibiting strong and nontrivial θ and temperature dependence. The nature of these peaks as well as those of the F K-edge spectra could be rationalized on the basis of first-principles DFT calculations. Despite the structural similarity, the NEXAFS spectra of (TMTSF)2PF6 and (TMTTF)2AsF6 exhibit important differences. In contrast with the case of (TMTTF)2AsF6, the F K-edge spectra of (TMTSF)2PF6 do not change with temperature despite stronger donor−anion interactions. All these features reveal subtle differences in the electronic structure of the TMTSF and TMTTF families of salts
Cellular, molecular and functional characterisation of YAC transgenic mouse models of Friedreich Ataxia
Copyright © 2014 Anjomani Virmouni et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.This article has been made available through the Brunel Open Access Publishing Fund.Background - Friedreich ataxia (FRDA) is an autosomal recessive neurodegenerative disorder, caused by a GAA repeat expansion mutation within intron 1 of the FXN gene. We have previously established and performed preliminary characterisation of several human FXN yeast artificial chromosome (YAC) transgenic FRDA mouse models containing GAA repeat expansions, Y47R (9 GAA repeats), YG8R (90 and 190 GAA repeats) and YG22R (190 GAA repeats).
Methodology/Principal Findings - We now report extended cellular, molecular and functional characterisation of these FXN YAC transgenic mouse models. FXN transgene copy number analysis of the FRDA mice demonstrated that the YG22R and Y47R lines each have a single copy of the FXN transgene while the YG8R line has two copies. Single integration sites of all transgenes were confirmed by fluorescence in situ hybridisation (FISH) analysis of metaphase and interphase chromosomes. We identified significant functional deficits, together with a degree of glucose intolerance and insulin hypersensitivity, in YG8R and YG22R FRDA mice compared to Y47R and wild-type control mice. We also confirmed increased somatic GAA repeat instability in the cerebellum and brain of YG22R and YG8R mice, together with significantly reduced levels of FXN mRNA and protein in the brain and liver of YG8R and YG22R compared to Y47R.
Conclusions/Significance - Together these studies provide a detailed characterisation of our GAA repeat expansion-based YAC transgenic FRDA mouse models that will help investigations of FRDA disease mechanisms and therapy.European Union, Ataxia UK and FARA
Effects of rearing system on performance, animal welfare and meat quality in two pig genotypes
Abstract The effects of an alternative rearing system (O) for growing-finishing pigs (sawdust-shave bedding with free outdoor access, 2.4 m²/pig) compared to a conventional (C) one (slatted floor, 0.65 m²/pig) were evaluated for performance, animal welfare and meat quality in two (Duroc or synthetic line crossbreds) genotypes. Trials were conducted in spring and winter, each involving one pen of 10 pigs / genotype / system (a total of 40 pigs / season). No significant interactions between rearing system and genotype were observed on any of the traits evaluated. On the whole, the O pigs spent 40% more time on exploratory activities, in particular towards the bedding, suggesting an improved animal welfare with the O system. Urine levels of cortisol and catecholamines in the O were similar with those in C pigs at 70kg. The O pigs exhibited a 6% increase in growth rate and were 5kg heavier at slaughter at the same age. Back fat depth and lean meat content, as well as plasma ACTH and cortisol, and urine cortisol and catecholamines levels at slaughter were not significantly affected by the rearing system. The O pigs exhibited similar pH 1 and pHu values, higher drip losses, but also higher intramuscular fat contents. The O system improved loin juiciness, but did not influence other eating quality traits
Roles of Pif1-like helicases in the maintenance of genomic stability
The Pif1p family of DNA helicases is conserved from yeast to humans. To date, four members of this family have been analyzed in some detail by in vitro and in vivo assays: the two baker's yeast helicases, ScPif1p and Rrm3p, the fission yeast Pfh1p and the human enzyme hPif1p. In vitro, these enzymes are 5′ to 3′ DNA helicase and show little processivity. In vivo, ScPif1p, Rrm3p and probably Pfh1p, function in both the nucleus at specific genomic loci and in mitochondria, where they are needed for the stable maintenance of the genome as accessory helicases to the replication machinery. Interestingly, they act on common DNA substrates but appear to have largely non-overlapping cellular functions, ranging from Okazaki fragment processing, telomerase inhibition, to helping the replication fork progress through non-nucleosomal protein–DNA complexes. For example, both ScPif1p and Rrm3p affect the replication of telomeres, but in a different way: Pif1p inhibits telomerase-mediated telomere elongation by directly removing telomerase from a DNA end, whereas Rrm3p facilitates replication through telomeric DNA. Here we review the current knowledge on the Pif1-like helicases, as a first step towards understanding the basis of their functional specialization and mechanism of action
A new quantum fluid at high magnetic fields in the marginal charge-density-wave system -(BEDT-TTF)Hg(SCN) (where ~K and Rb)
Single crystals of the organic charge-transfer salts
-(BEDT-TTF)Hg(SCN) have been studied using Hall-potential
measurements (K) and magnetization experiments ( = K, Rb). The data show
that two types of screening currents occur within the high-field,
low-temperature CDW phases of these salts in response to time-dependent
magnetic fields. The first, which gives rise to the induced Hall potential, is
a free current (), present at the surface of the sample.
The time constant for the decay of these currents is much longer than that
expected from the sample resistivity. The second component of the current
appears to be magnetic (), in that it is a microscopic,
quasi-orbital effect; it is evenly distributed within the bulk of the sample
upon saturation. To explain these data, we propose a simple model invoking a
new type of quantum fluid comprising a CDW coexisting with a two-dimensional
Fermi-surface pocket which describes the two types of current. The model and
data are able to account for the body of previous experimental data which had
generated apparently contradictory interpretations in terms of the quantum Hall
effect or superconductivity.Comment: 13 pages, 11 figure
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