Browsing by Author "Graf, Matthias J."

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Hyperfine Fields and Magnetic Structure in the B Phase of CeCoIn(5)
(2010-02-01)
Curro, Nicholas J.
;
Young, Ben-Li
;
Urbano, Ricardo R.
;
Graf, Matthias J.
;
電子物理學系
We re-analyze Nuclear Magnetic Resonance (NMR) spectra observed at low temperatures and high magnetic fields in the field-induced B phase of CeCoIn(5). The NMR spectra are consistent with incommensurate antiferromagnetic order of the Ce magnetic moments. However, we find that the spectra of the In(2) sites depend critically on the direction of the ordered moments, the ordering wavevector and the symmetry of the hyperfine coupling to the Ce spins. Assuming isotropic hyperfine coupling, the NMR spectra observed for H parallel to [100] are consistent with magnetic order with wavevector Q = pi (1+delta/a, 1/a, 1/c) and Ce moments ordered antiferromagnetically along the [100] direction in real space. If the hyperfine coupling has dipolar symmetry, then the NMR spectra require Ce moments along the [ 001] direction. The dipolar scenario is also consistent with recent neutron scattering measurements that find an ordered moment of 0.15 mu(B) along [001] and Q(n) = pi(1+delta/a, 1+delta/a, 1/c) with in-commensuration delta = 0.12 for field H parallel to [1 (1) over bar0]. Using these parameters, we find that a hyperfine field with dipolar contribution is consistent with findings from both experiments. We speculate that the B phase of CeCoIn(5) represents an intrinsic phase of modulated superconductivity and antiferromagnetism that can only emerge in a highly clean system.
Hyperfine Fields and Magnetic Structure in the B Phase of CeCoIn5
(2010-02-01)
Curro, Nicholas J.
;
Young, Ben-Li
;
Urbano, Ricardo R.
;
Graf, Matthias J.
;
電子物理學系
We re-analyze Nuclear Magnetic Resonance (NMR) spectra observed at low temperatures and high magnetic fields in the field-induced B phase of CeCoIn5. The NMR spectra are consistent with incommensurate antiferromagnetic order of the Ce magnetic moments. However, we find that the spectra of the In(2) sites depend critically on the direction of the ordered moments, the ordering wavevector and the symmetry of the hyperfine coupling to the Ce spins. Assuming isotropic hyperfine coupling, the NMR spectra observed for H parallel to [100] are consistent with magnetic order with wavevector Q = pi (1+delta/a, 1/a, 1/c) and Ce moments ordered antiferromagnetically along the [100] direction in real space. If the hyperfine coupling has dipolar symmetry, then the NMR spectra require Ce moments along the [ 001] direction. The dipolar scenario is also consistent with recent neutron scattering measurements that find an ordered moment of 0.15 mu(B) along [001] and Q(n) = pi(1+delta/a, 1+delta/a, 1/c) with in-commensuration delta = 0.12 for field H parallel to [1 (1) over bar0]. Using these parameters, we find that a hyperfine field with dipolar contribution is consistent with findings from both experiments. We speculate that the B phase of CeCoIn5 represents an intrinsic phase of modulated superconductivity and antiferromagnetism that can only emerge in a highly clean system.
NMR studies of field induced magnetism in CeCoIn(5)
(2010-12-01)
Curro, Nicholas J.
;
Young, Ben-Li
;
Urbano, Ricardo R.
;
Graf, Matthias J.
;
電子物理學系
Recent Nuclear Magnetic Resonance and elastic neutron scattering experiments have revealed conclusively the presence of static incommensurate magnetism in the field-induced B phase of CeCoIn(5). We analyze the NMR data assuming the hyperfine coupling to the In(2) nuclei is anisotropic and simulate the spectra for several different magnetic structures. The NMR data are consistent with ordered Ce moments along the [001] direction, but are relatively insensitive to the direction of the incommensurate wavevector. (C) 2009 Published by Elsevier B.V.
NMR studies of field induced magnetism in CeCoIn5
(2010-12-01)
Curro, Nicholas J.
;
Young, Ben-Li
;
Urbano, Ricardo R.
;
Graf, Matthias J.
;
電子物理學系
Recent Nuclear Magnetic Resonance and elastic neutron scattering experiments have revealed conclusively the presence of static incommensurate magnetism in the field-induced B phase of CeCoIn5. We analyze the NMR data assuming the hyperfine coupling to the In(2) nuclei is anisotropic and simulate the spectra for several different magnetic structures. The NMR data are consistent with ordered Ce moments along the [001] direction, but are relatively insensitive to the direction of the incommensurate wavevector. (C) 2009 Published by Elsevier B.V.
Plastic response of dislocation glide in solid helium under dc strain-rate loading
(2013-07-24)
Zhou, Caizhi
;
Su, Jung-Jung
;
Graf, Matthias J.
;
Reichhardt, Charles
;
Balatsky, Alexander V.
We develop a model for the gliding of dislocations and plasticity in solid He-4. This model takes into account the Peierls barrier, multiplication and interaction of dislocations, as well as classical thermally and mechanically activated processes leading to dislocation glide. We specifically examine the dc stress-strain curve and how it is affected by temperature, strain rate, and dislocation density. As a function of temperature and shear strain, we observe plastic deformation and discuss how this may be related to the experimental observation of elastic anomalies in solid hcp He-4 that have been discussed in connection with the possibility of supersolidity or giant plasticity. Our theory gives several predictions for the dc stress strain curves, for example, the yield point and the change in the work-hardening rate and plastic dissipation peak, that can be compared directly to constant strain-rate experiments and thus provide bounds on model parameters.