387 research outputs found

    Unconventional magnetic phase separation in γ\gamma-CoV2_2O6_6

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    We have explored the magnetism in the non-geometrically frustrated spin-chain system γ\gamma-CoV2_{2}O6_{6} which possesses a complex magnetic exchange network. Our neutron diffraction patterns at low temperatures (TT ⩽\leqslant TNT_{\mathrm{N}} = 6.6 K) are best described by a model in which two magnetic phases coexist in a volume ratio 65(1) : 35(1), with each phase consisting of a single spin modulation. This model fits previous studies and our observations better than the model proposed by Lenertz etet alal in J. Phys. Chem. C 118, 13981 (2014), which consisted of one phase with two spin modulations. By decreasing the temperature from TNT_{\mathrm{N}}, the minority phase of our model undergoes an incommensurate-commensurate lock-in transition at T∗T^{*} = 5.6 K. Based on these results, we propose that phase separation is an alternative approach for degeneracy-lifting in frustrated magnets

    Exploring the fragile antiferromagnetic superconducting phase in CeCoIn5

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    CeCoIn5 is a heavy fermion Type-II superconductor which exhibits clear indications of Pauli-limited superconductivity. A variety of measurements give evidence for a transition at high magnetic fields inside the superconducting state, when the field is applied either parallel to or perpendicular to the c axis. When the field is perpendicular to the c axis, antiferromagnetic order is observed on the high-field side of the transition, with a magnetic wavevector of (q q 0.5), where q = 0.44 reciprocal lattice units. We show that this order remains as the magnetic field is rotated out of the basal plane, but the associated moment eventually disappears above 17 degrees, indicating that the anomalies seen with the field parallel to the c axis are not related to this magnetic order. We discuss the implications of this finding.Comment: Accepted Physical Review Letters, September 2010. 4 pages, 4 figure

    The Absence of Vortex Lattice Melting in a Conventional Superconductor

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    The state of the vortex lattice extremely close to the superconducting to normal transition in an applied magnetic field is investigated in high purity niobium. We observe that thermal fluctuations of the order parameter broaden the superconducting to normal transition into a crossover but no sign of a first order vortex lattice melting transition is detected in measurements of the heat capacity or the small angle neutron scattering (SANS) intensity. Direct observation of the vortices via SANS always finds a well ordered vortex lattice. The fluctuation broadening is considered in terms of the Lowest Landau Level theory of critical fluctuations and scaling is found to occur over a large H_{c2}(T) range

    Square vortex lattice at anomalously low magnetic fields in electron-doped Nd1.85_{1.85}Ce0.15_{0.15}CuO4_{4}

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    We report here on the first direct observations of the vortex lattice in the bulk of electron-doped Nd1.85_{1.85}Ce0.15_{0.15}CuO4_{4} single crystals. Using small angle neutron scattering, we have observed a square vortex lattice with the nearest-neighbors oriented at 45∘^{\circ} from the Cu-O bond direction, which is consistent with theories based on the d-wave superconducting gap. However, the square symmetry persists down to unusually low magnetic fields. Moreover, the diffracted intensity from the vortex lattice is found to decrease rapidly with increasing magnetic field.Comment: 4 pages, 4 Figures, accepted for publication in Phys. Rev. Let

    Glass phases of flux lattices in layered superconductors

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    We study a flux lattice which is parallel to superconducting layers, allowing for dislocations and for disorder of both short wavelength and long wavelength. We find that the long wavelength disorder has a significant effect on the phase diagram -- it produces a first order transition within the Bragg glass phase and leads to melting at strong disorder. This then allows a Friedel scenario of 2D superconductivity.Comment: 5 pages, 1 eps figure, Revte

    Observation of a Triangular to Square Flux Lattice Phase Transition in YBCO

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    We have used the technique of small-angle neutron scattering to observe magnetic flux lines directly in an YBCO single crystal at fields higher than previously reported. For field directions close to perpendicular to the CuO2 planes, we find that the flux lattice structure changes smoothly from a distorted triangular co-ordination to nearly perfectly square as the magnetic induction approaches 11 T. The orientation of the square flux lattice is as expected from recent d-wave theories, but is 45 deg from that recently observed in LSCO

    High magnetic field studies of the Vortex Lattice structure in YBa2Cu3O7

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    We report on small angle neutron scattering measurements of the vortex lattice in twin-free YBa2Cu3O7, extending the previously investigated maximum field of 11~T up to 16.7~T with the field applied parallel to the c axis. This is the first microscopic study of vortex matter in this region of the superconducting phase. We find the high field VL displays a rhombic structure, with a field-dependent coordination that passes through a square configuration, and which does not lock-in to a field-independent structure. The VL pinning reduces with increasing temperature, but is seen to affect the VL correlation length even above the irreversibility temperature of the lattice structure. At high field and temperature we observe a melting transition, which appears to be first order, with no detectable signal from a vortex liquid above the transition

    Polarized Neutron Laue Diffraction on a Crystal Containing Dynamically Polarized Proton Spins

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    We report on a polarized-neutron Laue diffraction experiment on a single crystal of neodynium doped lanthanum magnesium nitrate hydrate containing polarized proton spins. By using dynamic nuclear polarization to polarize the proton spins, we demonstrate that the intensities of the Bragg peaks can be enhanced or diminished significantly, whilst the incoherent background, due to proton spin disorder, is reduced. It follows that the method offers unique possibilities to tune continuously the contrast of the Bragg reflections and thereby represents a new tool for increasing substantially the signal-to-noise ratio in neutron diffraction patterns of hydrogenous matter.Comment: 5 pages, 3 figure
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