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Stock C.,Center for Neutron Research | Stock C.,Indiana University | Cowley R.A.,Rutherford Appleton Laboratory | Cowley R.A.,Clarendon Laboratory | And 11 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We have measured the spin fluctuations in the YBa2 Cu 3 O6.5 (YBCO6.5, Tc =59 K) superconductor at high-energy transfers above ∼100 meV. Within experimental error, the momentum dependence is isotropic at high energies, similar to that measured in the insulator for two-dimensional spin waves, and the dispersion extrapolates back to the incommensurate wave vector at the elastic position. This result contrasts with previous expectations based on measurements around 50 meV which were suggestive of a softening of the spin-wave velocity with increased hole doping. Unlike the insulator, we observe a significant reduction in the intensity of the spin excitations for energy transfers above ∼100 meV similar to that observed above ∼200 meV in the YBCO6.35 (T c =18 K) superconductor as the spin waves approach the zone boundary. We attribute this high-energy scale with a second gap and find agreement with measurements of the pseudogap in the cuprates associated with electronic anomalies along the antinodal positions. In addition, we observe a sharp peak at around 400 meV whose energy softens with increased hole doping. We discuss possible origins of this excitation including a hydrogen-related molecular excitation and a transition of electronic states between d levels. © 2010 The American Physical Society. Source

Yamani Z.,National Research Council Canada | Buyers W.J.L.,National Research Council Canada | Cowley R.A.,Canadian Institute of Advanced Research | Prabhakaran D.,University of Oxford
Canadian Journal of Physics | Year: 2010

Magnetic and phonon excitations in the antiferromagnet CoO with an unquenched orbital angular momentum are studied by neutron scattering. Results of energy scans in several Brillouin zones in the (HHL) plane for energy transfers up to 16 THz are presented. The measurements were performed in the antiferromagnetic ordered state at 6 K (well below TN ∼290 K) as well as in the paramagnetic state at 450 K. Several magnetic excitation modes are identified from the dependence of their intensity on wavevector and temperature. Within a Hund's rule model, the excitations correspond to fluctuations of coupled orbital and spin degrees of freedom, whose bandwidth is controlled by interionic superexchange. The different <111> ordering domains give rise to several magnetic peaks at each wavevector transfer. Source

Munevar J.,Brazilian Center for Research in Physics (CBPF) | Sanchez D.R.,Brazilian Center for Research in Physics (CBPF) | Sanchez D.R.,Federal University of Fluminense | Alzamora M.,Brazilian Center for Research in Physics (CBPF) | And 12 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Static magnetic order of quasi-two-dimensional FeAs compounds Sr 4A2O6-xFe2As2, with A= Sc and V, has been detected by Fe57 Mössbauer and muon spin relaxation (μSR) spectroscopies. The nonsuperconducting stoichiometric (x=0) A= Sc system exhibits a static internal/hyperfine magnetic field both at the Fe57 and μ+ sites, indicating antiferromagnetic order of Fe moments below TN = 35 K with ~0.1 bohr magneton per Fe at T= 2 K. The superconducting and oxygen deficient (x=0.4) A= V system exhibits a static internal field only at the μ+ site below TN~40 K, indicating static magnetic order of V moments coexisting with superconductivity without freezing of Fe moments. These results suggest that the 42622 FeAs systems belong to the same paradigm with the 1111 and 122 FeAs systems with respect to magnetic behavior of Fe moments. © 2011 American Physical Society. Source

Munevar J.,Brazilian Center for Research in Physics (CBPF) | Micklitz H.,Brazilian Center for Research in Physics (CBPF) | Alzamora M.,Brazilian Center for Research in Physics (CBPF) | Arguello C.,Columbia University | And 14 more authors.
Solid State Communications | Year: 2014

We have studied the magnetism in superconducting single crystals of EuFe2As1.4P0.6 by using the local probe techniques of zero-field muon spin rotation/relaxation and 151Eu/ 57Fe Mössbauer spectroscopy. All of these measurements reveal magnetic hyperfine fields below the magnetic ordering temperature TM=18K of the Eu2+ moments. The analysis of the data shows that there is a coexistence of antiferromagnetism, resulting from Eu 2+ moments ordered along the crystallographic c-axis, and superconductivity below TSC≈ 10 K. We find indications for a change in the dynamics of the small Fe magnetic moments (∼0.07 μB) at T*≲15K that may be triggering the onset of superconductivity: below T* the Fe magnetic moments seem to be "frozen" within the ab-plane. © 2014 Elsevier Ltd. Source

Amole C.,York University | Ashkezari M.D.,Simon Fraser University | Baquero-Ruiz M.,University of California at Berkeley | Bertsche W.,University of Manchester | And 45 more authors.
Nature Communications | Year: 2014

The properties of antihydrogen are expected to be identical to those of hydrogen, and any differences would constitute a profound challenge to the fundamental theories of physics. The most commonly discussed antiatom-based tests of these theories are searches for antihydrogen-hydrogen spectral differences (tests of CPT (charge-parity-time) invariance) or gravitational differences (tests of the weak equivalence principle). Here we, the ALPHA Collaboration, report a different and somewhat unusual test of CPT and of quantum anomaly cancellation. A retrospective analysis of the influence of electric fields on antihydrogen atoms released from the ALPHA trap finds a mean axial deflection of 4.1±3.4mm for an average axial electric field of 0.51Vmm-1. Combined with extensive numerical modelling, this measurement leads to a bound on the charge Qe of antihydrogen of Q=(-1.3±1.1±0.4) × 10-8. Here, e is the unit charge, and the errors are from statistics and systematic effects. © 2014 Macmillan Publishers Limited. All rights reserved. Source

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