Browsing by Author "Quintero-Castro, DL"

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    Asymmetric thermal line shape broadening in a gapped 3D antiferromagnet: evidence for strong correlations at finite temperature
    (American physical Society, 2012-09-19) Quintero-Castro, DL; Lake, B; Islam, ATMN; Wheeler, EM; Balz, C; Mansson, M; Rule, KC; Gvasaliya, S; Zheludev, A
    It is widely believed that magnetic excitations become increasingly incoherent as the temperature is raised due to random collisions which limit their lifetime. This picture is based on spin-wave calculations for gapless magnets in 2 and 3 dimensions and is observed experimentally as a symmetric Lorentzian broadening in energy. Here, we investigate a three-dimensional dimer antiferromagnet and find unexpectedly that the broadening is asymmetric-indicating that far from thermal decoherence, the excitations behave collectively like a strongly correlated gas. This result suggests that a temperature activated coherent state of quasiparticles is not confined to special cases like the highly dimerized spin-1/2 chain but is found generally in dimerized antiferromagnets of all dimensionalities and perhaps gapped magnets in general. © 2012, American Physical Society.
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    Gains from the upgrade of the cold neutron triple-axis spectrometer FLEXX at the BER-II reactor
    (Elsevier, 2013-11-21) Le, MD; Quintero-Castro, DL; Toft-Petersen, R; Groitl, F; Skoulatos, M; Rule, KC; Habicht, K
    The upgrade of the cold neutron triple-axis spectrometer FLEXX is described. We discuss the characterisation of the gains from the new primary spectrometer, including a larger guide and double focussing monochromator, and present measurements of the energy and momentum resolution and of the neutron flux of the instrument. We found an order of magnitude gain in intensity (at the cost of coarser momentum resolution), and that the incoherent elastic energy widths are measurably narrower than before the upgrade. The much improved count rate should allow the use of smaller single crystals samples and thus enable the upgraded FLEXX spectrometer to continue making leading edge measurements. © 2013, Elsevier Ltd.
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    Lineshape of the singlet-triplet excitations in the dimer system Sr3Cr2O8 to first order in the high-density 1/z expansion
    (Physical Review B, 2014-04-11) Jensen, J; Quintero-Castro, DL; Islam, ATMN; Rule, KC; Månsson, M; Lake, B
    The Cr 5+ ions in Sr 3 Cr 2 O 8 constitute a strongly correlated spin-1/2 dimer system. Experiments show that the collective singlet-triplet excitations in this system are well defined in the zero-temperature limit, but, when heated, the inelastic neutron scattering peaks decrease rapidly in intensity and acquire a nonzero line width. When including the fluctuations to leading order in 1/z , where z is the coordination number, the diagrammatic high-density expansion is found to offer an accurate description of the singlet-triplet excitations. The theory explains not only the temperature dependencies of the intensities and line widths, but also that strong correlation causes the lineshapes to become asymmetric at temperatures comparable to the excitation energies.© 2014, American Physical Society.