72 research outputs found
Origin of the reduced exchange bias in epitaxial FeNi(111)/CoO(111) bilayer
We have employed Soft and Hard X-ray Resonant Magnetic Scattering and
Polarised Neutron Diffraction to study the magnetic interface and the bulk
antiferromagnetic domain state of the archetypal epitaxial
NiFe(111)/CoO(111) exchange biased bilayer. The combination of
these scattering tools provides unprecedented detailed insights into the still
incomplete understanding of some key manifestations of the exchange bias
effect. We show that the several orders of magnitude difference between the
expected and measured value of exchange bias field is caused by an almost
anisotropic in-plane orientation of antiferromagnetic domains. Irreversible
changes of their configuration lead to a training effect. This is directly seen
as a change in the magnetic half order Bragg peaks after magnetization
reversal. A 30 nm size of antiferromagnetic domains is extracted from the width
the (1/2 1/2 1/2) antiferromagnetic magnetic peak measured both by neutron and
x-ray scattering. A reduced blocking temperature as compared to the measured
antiferromagnetic ordering temperature clearly corresponds to the blocking of
antiferromagnetic domains. Moreover, an excellent correlation between the size
of the antiferromagnetic domains, exchange bias field and frozen-in spin ratio
is found, providing a comprehensive understanding of the origin of exchange
bias in epitaxial systems.Comment: 8 pages, 5 figures, submitte
Spectroscopy of stripe order in La1.8Sr0.2NiO4 using resonant soft x-ray diffraction
Strong resonant enhancements of the charge-order and spin-order
superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when
x-ray energies in the vicinity of the Ni L2,3 absorption edges are used. The
pronounced photon-energy and polarization dependences of these diffraction
intensities allow for a critical determination of the local symmetry of the
ordered spin and charge carriers. We found that not only the antiferromagnetic
order but also the charge-order superstructure resides within the NiO2 layers;
the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with
the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the
cuprates.Comment: 4 pages, 3 figure
Ground State of the Quasi-1D \bvs\ resolved by Resonant Magnetic X-ray Scattering
Resonant-magnetic x-ray scattering (RMXS) near the vanadium
-absorption edges has been used to investigate the low temperature
magnetic structure of high quality \bvs\ single crystals. Below = 31 K,
the strong resonance revealed a triple-incommensurate magnetic ordering at wave
vector (0.226 0.226 ) in the hexagonal notation, with = 0.033. The
simulations of the experimental RMXS spectra with a time-dependent density
functional theory indicate an antiferromagnetic order with the spins polarized
along in the monoclinic structure.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Strongly coupled charge, orbital, and spin order in TbTe3
We report a ground state with strongly coupled magnetic and charge density wave orders mediated via orbital ordering in the layered compound TbTe3. In addition to the commensurate antiferromagnetic (AFM) and charge density wave (CDW) orders, new magnetic peaks are observed whose propagation vector equals the sum of the AFM and CDW propagation vectors, revealing an intricate and highly entwined relationship. This is especially interesting given that the magnetic and charge orders lie in different layers of the crystal structure where the highly localized magnetic moments of the Tb3+ ions are netted in the Tb-Te stacks, while the charge order is formed by the conduction electrons of the adjacent Te-Te layers. Our results, based on neutron diffraction and resonant x-ray scattering, reveal that the charge and magnetic subsystems mutually influence each other via the orbital ordering of Tb3+ ions. © 2020 American Physical Society.S. C. thanks K. Prokes for the helpful insights regarding the heavy-fermion physics. This work has been partially supported by the Ministry of Education and Science of the Russian Federation, Contracts No. 02.A03.21.0006 and No. 02.A03.21.0011
Nonmagnetic band gap at the Dirac point of the magnetic topological insulator Bi1 xMnx 2 Se3
Magnetic doping is expected to open a band gap at the Dirac point of
topological insulators by breaking time-reversal symmetry and to enable novel
topological phases. Epitaxial (BiMn)Se is a
prototypical magnetic topological insulator with a pronounced surface band gap
of meV. We show that this gap is neither due to ferromagnetic order
in the bulk or at the surface nor to the local magnetic moment of the Mn,
making the system unsuitable for realizing the novel phases. We further show
that Mn doping does not affect the inverted bulk band gap and the system
remains topologically nontrivial. We suggest that strong resonant scattering
processes cause the gap at the Dirac point and support this by the observation
of in-gap states using resonant photoemission. Our findings establish a novel
mechanism for gap opening in topological surface states which challenges the
currently known conditions for topological protection.Comment: 26 pages, 7 figure
Laboratory Evolution of Fast-Folding Green Fluorescent Protein Using Secretory Pathway Quality Control
Green fluorescent protein (GFP) has undergone a long history of optimization to become one of the most popular proteins in all of cell biology. It is thermally and chemically robust and produces a pronounced fluorescent phenotype when expressed in cells of all types. Recently, a superfolder GFP was engineered with increased resistance to denaturation and improved folding kinetics. Here we report that unlike other well-folded variants of GFP (e.g., GFPmut2), superfolder GFP was spared from elimination when targeted for secretion via the SecYEG translocase. This prompted us to hypothesize that the folding quality control inherent to this secretory pathway could be used as a platform for engineering similar ‘superfolded’ proteins. To test this, we targeted a combinatorial library of GFPmut2 variants to the SecYEG translocase and isolated several superfolded variants that accumulated in the cytoplasm due to their enhanced folding properties. Each of these GFP variants exhibited much faster folding kinetics than the parental GFPmut2 protein and one of these, designated superfast GFP, folded at a rate that even exceeded superfolder GFP. Remarkably, these GFP variants exhibited little to no loss in specific fluorescence activity relative to GFPmut2, suggesting that the process of superfolding can be accomplished without altering the proteins' normal function. Overall, we demonstrate that laboratory evolution combined with secretory pathway quality control enables sampling of largely unexplored amino-acid sequences for the discovery of artificial, high-performance proteins with properties that are unparalleled in their naturally occurring analogues
Two distinct β-sheet structures in Italian-mutant amyloid-beta fibrils: a potential link to different clinical phenotypes
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