A Large Iron Isotope Effect in SmFeAsO1-xFx and Ba1-xKxFe2As2

The recent discovery of superconductivity in oxypnictides with the critical temperature (TC) higher than McMillan limit of 39 K (the theoretical maximum predicted by Bardeen-Cooper-Schrieffer (BCS) theory) has generated great excitement. Theoretical calculations indicate that the electron-phonon interaction is not strong enough to give rise to such high transition temperatures, while strong ferromagnetic/antiferromagnetic fluctuations have been proposed to be responsible. However, superconductivity and magnetism in pnictide superconductors show a strong sensitivity to the lattice, suggesting a possibility of unconventional electron-phonon coupling. Here we report the effect of oxygen and iron isotopic mass on Tc and the spin-density wave (SDW) transition temperature (TSDW) in SmFeAsO1-xFx and Ba1-xKxFe2As2 systems. The results show that oxygen isotope effect on TC and TSDW is very little, while the iron isotope exponent alpha=-dlnTc/dlnM is about 0.35, being comparable to 0.5 for the full isotope effect. Surprisingly, the iron isotope exchange shows the same effect on TSDW as TCc These results indicate that electron-phonon interaction plays some role in the superconducting mechanism, but simple electron-phonon coupling mechanism seems to be rather unlikely because a strong magnon-phonon coupling is included. Sorting out the interplay between the lattice and magnetic degrees of freedom is a key challenge for understanding the mechanism of high-TC superconductivity.Comment: 22 pages, 7 figur

Niche stiffness underlies the ageing of central nervous system progenitor cells.

Ageing causes a decline in tissue regeneration owing to a loss of function of adult stem cell and progenitor cell populations1. One example is the deterioration of the regenerative capacity of the widespread and abundant population of central nervous system (CNS) multipotent stem cells known as oligodendrocyte progenitor cells (OPCs)2. A relatively overlooked potential source of this loss of function is the stem cell 'niche'-a set of cell-extrinsic cues that include chemical and mechanical signals3,4. Here we show that the OPC microenvironment stiffens with age, and that this mechanical change is sufficient to cause age-related loss of function of OPCs. Using biological and synthetic scaffolds to mimic the stiffness of young brains, we find that isolated aged OPCs cultured on these scaffolds are molecularly and functionally rejuvenated. When we disrupt mechanical signalling, the proliferation and differentiation rates of OPCs are increased. We identify the mechanoresponsive ion channel PIEZO1 as a key mediator of OPC mechanical signalling. Inhibiting PIEZO1 overrides mechanical signals in vivo and allows OPCs to maintain activity in the ageing CNS. We also show that PIEZO1 is important in regulating cell number during CNS development. Thus we show that tissue stiffness is a crucial regulator of ageing in OPCs, and provide insights into how the function of adult stem and progenitor cells changes with age. Our findings could be important not only for the development of regenerative therapies, but also for understanding the ageing process itself.The work was supported by European Research Council (ERC) grant 772798 (to K.J.C.) and 772426 (to K.F.); the UK Multiple Sclerosis Society (to R.J.M.F.); Biotechnology and Biological Sciences Research Council (BBSRC) grant BB/M008827/1 (to K.J.C and R.J.M.F.) and BB/N006402/1 (to K.F.); the Adelson Medical Research Foundation (R.J.M.F. and D.H.R.); an EMBO Long-Term Fellowship ALTF 1263-2015 and European Commission FP7 actions LTFCOFUND2013, GA-2013-609409 (to I.P.W.); and a core support grant from the Wellcome Trust and Medical Research Council (MRC) to the Wellcome Trust–MRC Cambridge Stem Cell Institute

Structural inhomogeneities in oxygen-deficient ErBa2Cu3O6+x associated with the tetragonal-to-orthorhombic transition: Evidence of first-order behavior.

Neutron powder diffraction and ac susceptibility data on a series of ErBa2Cu3O6+x samples prepared to achieve thermal equilibrium at low temperature provide evidence of structural inhomogeneities near the tetragonal-to- orthorhombic (T'O) transition. In the composition range 0.25x0.45, the data can best be described as a mixture of orthorhombic and tetragonal phases. This observation, together with the presence of discontinuities in the order parameters, leads to the conclusion that the T'O transition is first order in the temperature range where these samples were prepared. This interpretation provides a simple explanation for the differences between oxygen-deficient RBa2Cu3O6+x (R=Y, rare earths) samples prepared by different techniques. © 1992 The American Physical Society

TRANSVERSE SOUND IN A MAGNETIC-FIELD IN UPT3

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Structure of superconducting Sr0.9La0.1CuO2 (Tc=42 K) from neutron powder diffraction.

We have refined the crystal structure of the electron-doped infinite-layer superconducting compound Sr0.9La0.1CuO2 (Tc=42 K) from neutron-powder-diffraction data for an 82-mg sample synthesized at high pressure. The metal- and oxygen-atom lattices are perfectly stoichiometric and there is no excess (interstitial) oxygen in the Sr(La) layer. Thus, neither oxygen vacancies nor interstitial oxygen play a role in the doping of this compound. © 1993 The American Physical Society

Structure, doping and superconductivity in HgBa2CaCu2O6+δ (Tc ≤ 128 K)

We have studied the defect structure and superconducting properties of HgBa2CaCu2O6+δ by neutron powder diffraction, AC susceptibility and DC resistivity. An as-synthesized sample has an onset Tc of 128 K, the highest yet reported for this material. The critical temperature decreases slightly (126 K) after oxygenation, and drops to 92 K after reduction in flowing argon. Neutron diffraction data give evidence that the primary doping mechanism in this material is provided by oxygen atoms in an interstitial position, similar to the case of the one-layer compound HgBa2CaO4+δ. The occupancy of this site varies from 0.08 (1) for the argon-reduced sample to 0.22(1) for the oxygenated sample. Unlike the case of the one-layer compound, no evidence of an additional defect site was found in these samples, HgBa2CaCu2O6+δ has remarkably long copper-apical oxygen distances and almost perfectly flat CuO2 planes

Metastable behavior of the superconducting phase in the BaBi1-xPbxO3 system.

The structural phases of the BaBi1-xPbxO3 system have been studied using neutron powder diffraction. As lead is substituted on the bismuth site, the room-temperature structure is monoclinic for 0.00x0.20, orthorhombic for 0.25x0.65, diphasic (orthorhombic and tetragonal) for 0.70x0.80, and monoclinic for x=1.0. Superconductivity is observed for samples in the composition range 0.70x0.80. The superconducting phase is the tetragonal phase. In situ neutron-powder-diffraction measurements versus temperature show that it is not stable at temperatures below about 425 K. All superconducting samples consist of the metastable tetragonal phase and a semiconducting orthorhombic phase because the tetragonal-to-orthorhombic transition does not proceed to completion. Samples of Ba1-yKyBi1-xPbxO3 have Tcs that are inversely proportional to the total dopant concentration, x+y, in the composition region (0.5x+y0.9), where the material is metallic. © 1992 The American Physical Society

Multiple defects in overdoped Tl2Ba2CuO6+delta: Effects on structure and superconductivity

It is shown that the superconducting and structural properties of Tl2Ba2CuO6+δ depend on at least two defects. The Tl/Cu ratio used during synthesis determines the concentration of a defect that is likely to be Cu substitution on the Tl site, based on previously published work that proves the existence of such a defect. The structure exhibits a smaller orthorhombic strain, or tetragonal symmetry, for high concentrations of this defect. Tc tends to be lowered by this defect. An oxygen interstitial defect in the Tl-O double layer has a more pronounced effect on Tc, allowing the compound to be overdoped as the concentration of this defect increases. Features in the diffraction data also suggest the existence of a third defect which may involve the filling of oxygen vacancies at an oxygen site in the structure during early stages of oxygenation