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ABSTRACT: Neutron scattering in strong magnetic fields is used to show the spin resonance in superconducting CeCoIn_{5} (T_{c}=2.3 K) is a doublet. The underdamped resonance (ℏΓ=0.069±0.019 meV) Zeeman splits into two modes at E_{±}=ℏΩ_{0}±αμ_{B}μ_{0}H with α=0.96±0.05. A linear extrapolation of the lower peak reaches zero energy at 11.2±0.5 T, near the critical field for the incommensurate "Q phase." Kenzelmann et al. [Science 321, 1652 (2008)] This, taken with the integrated weight and polarization of the low-energy mode (E_{-}), indicates that the Q phase can be interpreted as a Bose condensate of spin excitons.
Physical Review Letters 10/2012; 109(16):167207. · 7.37 Impact Factor
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ABSTRACT: Spin fluctuations are reported near the magnetic field driven quantum
critical point in YbRh2Si2. On cooling, ferromagnetic fluctuations evolve into
incommensurate correlations located at q0=+/- (delta,delta) with delta=0.14 +/-
0.04 r.l.u. At low temperatures, an in plane magnetic field induces a sharp
intra doublet resonant excitation at an energy E0=g muB mu0 H with g=3.8 +/-
0.2. The intensity is localized at the zone center indicating precession of
spin density extending xi=6 +/- 2 A beyond the 4f site.
07/2012;
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ABSTRACT: Neutron scattering in strong magnetic fields is used to show the
spin-resonance in superconducting CeCoIn5 (Tc=2.3 K) is a doublet. The
underdamped resonance (\hbar \Gamma=0.069 \pm 0.019 meV) Zeeman splits into two
modes at E_{\pm}=\hbar \Omega_{0}\pm g\mu_{B} \mu_{0}H with g=0.96 \pm 0.05. A
linear extrapolation of the lower peak reaches zero energy at 11.2 \pm 0.5 T,
near the critical field for the incommensurate "Q-phase" indicating that the
Q-phase is a bose condensate of spin excitons.
03/2012;
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ABSTRACT: Whether the phase relationship (phase shift) between cerebral blood flow velocity as assessed by transcranial Doppler ultrasound and blood pressure at 0.1 Hz can be used to assess cerebral autoregulation (CA) in patients with severe traumatic brain injury (TBI).
In 33 healthy volunteers (mean age, SD; 37+/-17 years, range 17-65) middle cerebral artery (MCA) blood velocity (V) was recorded simultaneously with finger blood pressure (BP) over a period of 10 minutes under normocapnic and hypocapnic conditions to generate normative data. In 27 patients with severe TBI (Glasgow Coma scale score < or =8) serial close in time investigations of cranial computed tomography (CT) scanning and phase shift assessment were performed on days 1, 3, 5, and 8 after trauma. Phase shift in the MCA was compared to brain parenchyma lesion size in the MCA territory on CT scanning. Lesion size was classified into 0, normal; 1, presence of a small lesion (diameter <3 cm); 2, presence of a large lesion (>3 cm).
Compared to normocapnia, hypocapnia significantly increased phase shift at 0.1 Hz from 78+/-28 degrees to 101+/-25 degrees (p < 0.001). In the TBI patients, 115 comparisons between CT findings and CA results were possible. Phase shift detected a pathological CA in 31 instances, which were more frequent in CT lesion type 2 (19/42) than in group 0 (7/44) and group 1 (5/29).
When CA is intended to be assessed by use of phase shift, the hyperventilation setting needs its own reference values. In MCA territories containing a traumatic lesion greater than 3 cm in diameter phase shift at 0.1 Hz will detect a high frequency (44%) of a disturbed state of CA.
Acta Neurochirurgica 08/2003; 145(8):643-7; discussion 647-8. · 1.52 Impact Factor
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ABSTRACT: We sought to describe the dynamic changes in the cerebrovascular system after traumatic brain injury by transfer function estimation and coherence.
In 42 healthy volunteers (mean+/-SD age, 37+/-17 years; range, 17 to 65 years), spontaneous fluctuations of middle cerebral artery blood flow velocity and of finger blood pressure (BP) were simultaneously recorded over a period of 10 minutes under normocapnic and hypocapnic conditions to generate normative spectra of coherence, phase shift, and gain over the frequency range of 0 to 0.25 Hz. Similar recordings were performed in 24 patients with severe traumatic brain injury (Glasgow Coma Scale score <or=8; mean+/-SD age, 50+/-20 years) serially on days 1, 3, 5, and 8 after trauma. Cranial perfusion pressure was kept at >70 mm Hg. Each blood flow velocity/BP recording was related to the presence or absence of middle cerebral artery territory brain parenchyma lesions on cranial CT performed within a close time frame.
In controls, hypocapnia decreased coherence (0.0 to 0.20 Hz), increased phase shift (0.0 to 0.17 Hz), and decreased gain in the frequency range of 0.0 to 0.11 Hz but increased gain at frequencies of 0.20 to 0.25 Hz (P<0.01 for all frequency ranges reported). In patients with traumatic brain injury, 102 investigations were possible. Compared with controls, coherence was increased in the frequency range <0.03 Hz and between 0.13 and 0.25 Hz in both normocapnia and hypocapnia, irrespective of the CT findings. Gain was unchanged in normocapnia and in the absence of a CT lesion. Gain was decreased in hypocapnia at frequencies >0.12 Hz irrespective of the presence/absence of a CT lesion. Phase shift decreased rapidly between 0.06 and 0.13 Hz under hypocapnic conditions and under normocapnic conditions in the presence of a CT lesion (P< 0.01).
Use of spontaneous fluctuations of blood flow velocity and BP to assess the cerebrovascular system dynamically requires consideration of the Paco2 level. In different conditions, including severe traumatic brain injury, the cerebrovascular system behaves linearly only in parts of the investigated frequency range.
Stroke 05/2003; 34(5):1197-202. · 5.73 Impact Factor
