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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.


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.


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.


Williams T.J.,McMaster University | Aczel A.A.,McMaster University | Baggio-Saitovitch E.,Brazilian Center for Research in Physics (CBPF) | Bud'Ko S.L.,Iowa State University | And 16 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We report muon spin rotation (μSR) measurements of single-crystal Ba (Fe1-x Cox) 2 As2 and Sr (Fe1-x Cox) 2 As2. From measurements of the magnetic field penetration depth λ we find that for optimally and overdoped samples, 1/λ ( T→0 ) 2 varies monotonically with the superconducting transition temperature TC. Within the superconducting state we observe a positive shift in the muon precession signal, likely indicating that the applied field induces an internal magnetic field. The size of the induced field decreases with increasing doping but is present for all Co concentrations studied. © 2010 The American Physical Society.


Amole C.,York University | Ashkezari M.D.,Simon Fraser University | Baquero-Ruiz M.,University of California at Berkeley | Bertsche W.,University of Swansea | And 41 more authors.
Nature Communications | Year: 2013

Physicists have long wondered whether the gravitational interactions between matter and antimatter might be different from those between matter and itself. Although there are many indirect indications that no such differences exist and that the weak equivalence principle holds, there have been no direct, free-fall style, experimental tests of gravity on antimatter. Here we describe a novel direct test methodology; we search for a propensity for antihydrogen atoms to fall downward when released from the ALPHA antihydrogen trap. In the absence of systematic errors, we can reject ratios of the gravitational to inertial mass of antihydrogen >75 at a statistical significance level of 5%; worst-case systematic errors increase the minimum rejection ratio to 110. A similar search places somewhat tighter bounds on a negative gravitational mass, that is, on antigravity. This methodology, coupled with ongoing experimental improvements, should allow us to bound the ratio within the more interesting near equivalence regime. © 2013 Macmillan Publishers Limited. All rights reserved.


Timusk T.,McMaster University | Timusk T.,Canadian Institute of Advanced Research | Carbotte J.P.,McMaster University | Homes C.C.,Brookhaven National Laboratory | And 3 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

The optical conductivity of quasicrystals is characterized by two features not seen in ordinary metallic systems. There is an absence of the Drude peak and the interband conductivity rises linearly from a very low value up to normal metallic levels over a wide range of frequencies. The absence of a Drude peak has been attributed to a pseudogap at the Fermi surface but a detailed explanation of the linear behavior has not been found. Here we show that the linear conductivity, which seems to be universal in all Al based icosahedral quasicrystal families, as well as their periodic approximants, follows from a simple model that assumes that the entire Fermi surface is gapped except at a finite set of Dirac points. There is no evidence of a semiconducting gap in any of the materials suggesting that the Dirac spectrum is massless, protected by topology leading to a Weyl semimetal. This model gives rise to a linear conductivity with only one parameter, the Fermi velocity. This picture suggests that decagonal quasicrystals should, like graphene, have a frequency independent conductivity, without a Drude peak. This is in accord with the experimental data as well. © 2013 American Physical Society.


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.


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.


Aczel A.A.,McMaster University | Williams T.J.,McMaster University | Goko T.,McMaster University | Goko T.,TRIUMF Laboratory Particle and Nuclear Physics | And 10 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We have searched for time-reversal symmetry-breaking fields in the noncentrosymmetric superconductor Mg10 Ir19 B16 via muon spin relaxation (μSR) in zero applied field. We also measured the temperature dependence of the superfluid density by muon spin rotation in transverse field to investigate the superconducting pairing symmetry in two polycrystalline samples of significantly different purities. In the high-purity sample, we detected no time-reversal symmetry-breaking fields greater than 0.05 G. The superfluid density was also found to be exponentially flat as T→0 and so can be fit to a single-gap BCS model. In contrast, the lower purity sample showed an increase in the zero-field μSR relaxation rate below Tc corresponding to a characteristic field strength of 0.6 G. While the temperature dependence of the superfluid density was also found to be consistent with a single-gap BCS model, the magnitude as T→0 was found to be much lower for a given applied field than in the case of the high-purity sample. These findings suggest that Mg10 Ir19 B16 is a superconductor characterized by a single-gap function with no nodes and sample quality drastically affects the superconducting properties of this system. © 2010 The American Physical Society.


Radtke G.,CNRS Institute Materials Microelectronics nanosciences of Provence | Saul A.,CNRS Interdisciplinary Nanoscience Centre of Marseille | Dabkowska H.A.,McMaster University | Luke G.M.,McMaster University | And 2 more authors.
Physical Review Letters | Year: 2010

We present a theoretical investigation of the orbital ordering occurring in Sr3Cr2O8 based on density functional theory calculations. We demonstrate that the strong electron correlation arising within the Cr-3d shell can clearly explain both the phase transition leading to the stabilization of its monoclinic C2/c space-group symmetry and its spin-singlet magnetic ground state. The relevance of the electronic structure determined theoretically is further established by comparison to high-resolution electron energy loss spectroscopy measurements. © 2010 The American Physical Society.

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