249 research outputs found
Nuclear Magnetic Resonance in High Magnetic Field: Application to Condensed Matter Physics
In this review, we describe the potentialities offered by the nuclear
magnetic resonance (NMR) technique to explore at a microscopic level new
quantum states of condensed matter induced by high magnetic fields. We focus on
experiments realised in resistive (up to 34~T) or hybrid (up to 45~T) magnets,
which open a large access to these quantum phase transitions. After an
introduction on NMR observable, we consider several topics: quantum spin
systems (spin-Peierls transition, spin ladders, spin nematic phases,
magnetisation plateaus and Bose-Einstein condensation of triplet excitations),
the field-induced charge density wave (CDW) in high ~superconductors, and
exotic superconductivity including the Fulde-Ferrel-Larkin-Ovchinnikov
superconducting state and the field-induced superconductivity due to the
Jaccarino-Peter mechanism.Comment: 19 pages, 6 figure
Rational dilation problems associated with constrained algebras
It is shown that rational dilation fails on broad collection of distinguished
varieties associated to constrained subalgebras of the disk algebra of the form
C + B A(D), where B is a finite Blaschke product with two or more zeros. This
is accomplished in part by finding a minimal set of test functions. In
addition, an Agler-Pick interpolation theorem is given and it is proved that
there exist Kaijser-Varopoulos style examples of non-contractive unital
representations where the generators are contractions.Comment: Page proof corrections included in this version
Locally commensurate charge-density wave with three-unit-cell periodicity in YBCO
In order to identify the mechanism responsible for the formation of
charge-density waves (CDW) in cuprate superconductors, it is important to
understand which aspects of the CDW's microscopic structure are generic and
which are material-dependent. Here, we show that, at the local scale probed by
NMR, long-range CDW order in YBa2Cu3Oy is unidirectional with a commensurate
period of three unit cells (lambda = 3b), implying that the incommensurability
found in X-ray scattering is ensured by phase slips (discommensurations).
Furthermore, NMR spectra reveal a predominant oxygen character of the CDW with
an out-of-phase relationship between certain lattice sites but no specific
signature of a secondary CDW with lambda = 6b associated with a putative
pair-density wave. These results shed light on universal aspects of the cuprate
CDW. In particular, its spatial profile appears to generically result from the
interplay between an incommensurate tendency at long length scales, possibly
related to properties of the Fermi surface, and local commensuration effects,
due to electron-electron interactions or lock-in to the lattice.Comment: Original submission (revised version available upon request
Hidden magnetism at the pseudogap critical point of a high temperature superconductor
The mysterious pseudogap phase of cuprate superconductors ends at a critical
hole doping level p* but the nature of the ground state below p* is still
debated. Here, we show that the genuine nature of the magnetic ground state in
La2-xSrxCuO4 is hidden by competing effects from superconductivity: applying
intense magnetic fields to quench superconductivity, we uncover the presence of
glassy antiferromagnetic order up to the pseudogap boundary p* ~ 0.19, and not
above. There is thus a quantum phase transition at p*, which is likely to
underlie highfield observations of a fundamental change in electronic
properties across p*. Furthermore, the continuous presence of quasi-static
moments from the insulator up to p* suggests that the physics of the doped Mott
insulator is relevant through the entire pseudogap regime and might be more
fundamentally driving the transition at p* than just spin or charge ordering.Comment: 26 pages, supplementary info include
Magnetic-field-induced charge-stripe order in the high temperature superconductor YBa2Cu3Oy
Electronic charges introduced in copper-oxide planes generate high-transition
temperature superconductivity but, under special circumstances, they can also
order into filaments called stripes. Whether an underlying tendency of charges
to order is present in all cuprates and whether this has any relationship with
superconductivity are, however, two highly controversial issues. In order to
uncover underlying electronic orders, magnetic fields strong enough to
destabilise superconductivity can be used. Such experiments, including quantum
oscillations in YBa2Cu3Oy (a notoriously clean cuprate where charge order is
not observed) have suggested that superconductivity competes with spin, rather
than charge, order. Here, using nuclear magnetic resonance, we demonstrate that
high magnetic fields actually induce charge order, without spin order, in the
CuO2 planes of YBa2Cu3Oy. The observed static, unidirectional, modulation of
the charge density breaks translational symmetry, thus explaining quantum
oscillation results, and we argue that it is most likely the same 4a-periodic
modulation as in stripe-ordered cuprates. The discovery that it develops only
when superconductivity fades away and near the same 1/8th hole doping as in
La2-xBaxCuO4 suggests that charge order, although visibly pinned by CuO chains
in YBa2Cu3Oy, is an intrinsic propensity of the superconducting planes of high
Tc cuprates.Comment: For a final version, see
http://www.nature.com/nature/journal/v477/n7363/full/nature10345.htm
Mechanism of activation of methyltransferases involved in translation by the Trm112 âhubâ protein
Methylation is a common modification encountered in DNA, RNA and proteins. It plays a central role in gene expression, protein function and mRNA translation. Prokaryotic and eukaryotic class I translation termination factors are methylated on the glutamine of the essential and universally conserved GGQ motif, in line with an important cellular role. In eukaryotes, this modification is performed by the Mtq2-Trm112 holoenzyme. Trm112 activates not only the Mtq2 catalytic subunit but also two other tRNA methyltransferases (Trm9 and Trm11). To understand the molecular mechanisms underlying methyltransferase activation by Trm112, we have determined the 3D structure of the Mtq2-Trm112 complex and mapped its active site. Using site-directed mutagenesis and in vivo functional experiments, we show that this structure can also serve as a model for the Trm9-Trm112 complex, supporting our hypothesis that Trm112 uses a common strategy to activate these three methyltransferases
A thousand-genome panel retraces the global spread and adaptation of a major fungal crop pathogen
Human activity impacts the evolutionary trajectories of many species worldwide. Global trade of agricultural goods contributes to the dispersal of pathogens reshaping their genetic makeup and providing opportunities for virulence gains. Understanding how pathogens surmount control strategies and cope with new climates is crucial to predicting the future impact of crop pathogens. Here, we address this by assembling a global thousand-genome panel of Zymoseptoria tritici, a major fungal pathogen of wheat reported in all production areas worldwide. We identify the global invasion routes and ongoing genetic exchange of the pathogen among wheat-growing regions. We find that the global expansion was accompanied by increased activity of transposable elements and weakened genomic defenses. Finally, we find significant standing variation for adaptation to new climates encountered during the global spread. Our work shows how large population genomic panels enable deep insights into the evolutionary trajectory of a major crop pathogen
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