Browsing by Author "Lake, B"

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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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    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.
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    Magnetic soft modes in the distorted triangular antiferromagnet alpha-CaCr(2)O(4)
    (American Physical Society, 2012-09-19) Toth, S; Lake, B; Hradil, K; Guidi, T; Rule, KC; Stone, MB; Islam, ATMN
    In this Letter, we explore the phase diagram and excitations of a distorted triangular lattice anti-ferromagnet. The unique two-dimensional distortion considered here is very different from the "isosceles''- type distortion that has been extensively investigated. We show that it is able to stabilize a 120 degrees spin structure for a large range of exchange interaction values, while new structures are found for extreme distortions. A physical realization of this model is alpha-CaCr(2)O(4), which has a 120 degrees structure but lies very close to the phase boundary. This is verified by inelastic neutron scattering which reveals unusual rotonlike minima at reciprocal space points different from those corresponding to the magnetic order. © 2012 The American Physical Society
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    Magnetic soft modes in the distorted triangular antiferromagnet α-CaCr2O4
    (American Physical Society, 2012-09-19) Toth, S; Lake, B; Hradil, K; Guidi, T; Rule, KC; Stone, MB; Islam, ATMN
    In this Letter, we explore the phase diagram and excitations of a distorted triangular lattice anti-ferromagnet. The unique two-dimensional distortion considered here is very different from the "isosceles''- type distortion that has been extensively investigated. We show that it is able to stabilize a 120 degrees spin structure for a large range of exchange interaction values, while new structures are found for extreme distortions. A physical realization of this model is alpha-CaCr2O4, which has a 120 degrees structure but lies very close to the phase boundary. This is verified by inelastic neutron scattering which reveals unusual rotonlike minima at reciprocal space points different from those corresponding to the magnetic order. © 2012, American Physical Society.
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    Magnetic-field-induced instability of the cooperative paramagnetic state in ZnxCo(4-x)(OD)6Cl2
    (American Physcical Society, 2012-05-29) Dissanayake, SE; Chan, C; Ji, S; Lee, J; Qiu, Y; Rule, KC; Lake, B; Green, MA; Hagihala, M; Zheng, XG; Ng, TK; Lee, SH
    Using elastic and inelastic neutron scattering techniques with and without application of an external magnetic field H, the magnetic ground states of ZnxCo(4-x)(OD)6Cl2 (x = 0,1) were studied. Our results show that for x = 0, the ground state is a magnetic long-range ordered (LRO) state where each tetrahedron forms an "umbrella"-type structure. On the other hand, for x = 1, no static ordering was observed down to 1.5 K, which resembles the behavior found in the isostructural quantum system ZnxCo(4-x)(OD)6Cl2. When H field is applied, however, the x = 1 system develops the same LRO state as x = 0. This indicates that the x = 1 disordered state is in the vicinity of the x = 0 ordered state. © 2012, American Physical Society.