280 research outputs found
Damping of spin waves and singularity of the longitudinal modes in the dipolar critical regime of the Heisenberg-ferromagnet EuS
By inelastic scattering of polarized neutrons near the (200)-Bragg
reflection, the susceptibilities and linewidths of the spin waves and the
longitudinal spin fluctuations were determined separately. By aligning the
momentum transfers q perpendicular to both \delta S_sw and the spontaneous
magnetization M_s, we explored the statics and dynamics of these modes with
transverse polarizations with respect to q. In the dipolar critical regime,
where the inverse correlation length kappa_z(T) and q are smaller than the
dipolar wavenumber q_d, we observe:(i) the static susceptibility of \delta
S_sw^T(q) displays the Goldstone divergence while for \delta S_z^T(q) the
Ornstein-Zernicke shape fits the data with a possible indication of a
thermal(mass-)renormalization at the smallest q-values, i.e. we find
indications for the predicted 1/q divergence of the longitudinal
susceptibility; (ii) the spin wave dispersion as predicted by the
Holstein-Primakoff theory revealing q_d=0.23(1)\AA^{-1}in good agreement with
previous work in the paramagnetic and ferromagnetic regime of EuS; (iii) within
experimental error, the (Lorentzian) linewidths of both modes turn out to be
identical with respect to the q^2-variation, the temperature independence and
the absolute magnitude. Due to the linear dispersion of the spin waves they
remain underdamped for q<q_d. These central results differ significantly from
the well known exchange dominated critical dynamics, but are quantitatively
explained in terms of dynamical scaling and existing data for T>=T_C. The
available mode-mode coupling theory, which takes the dipolar interactions fully
into account, describes the gross features of the linewidths but not all
details of the T- and q-dependencies. PACS: 68.35.Rh, 75.40.GbComment: 10 pages, 7 figure
New Magnetic Excitations in the Spin-Density-Wave of Chromium
Low-energy magnetic excitations of chromium have been reinvestigated with a
single-Q crystal using neutron scattering technique. In the transverse
spin-density-wave phase a new type of well-defined magnetic excitation is found
around (0,0,1) with a weak dispersion perpendicular to the wavevector of the
incommensurate structure. The magnetic excitation has an energy gap of E ~ 4
meV and at (0,0,1) exactly corresponds to the Fincher mode previously studied
only along the incommensurate wavevector.Comment: 4 pages, 4 figure
Characterization of low-energy magnetic excitations in chromium
The low-energy excitations of Cr, i.e. the Fincher-Burke (FB) modes, have
been investigated in the transversely polarized spin-density-wave phase by
inelastic neutron scattering using a single-(Q+-) crystal with a propagation
vector (Q+-) parallel to [0,0,1]. The constant-momentum-transfer scans show
that the energy spectra consist of two components, namely dispersive FB modes
and an almost energy-independent cross section. Most remarkably, we find that
the spectrum of the FB modes exhibits one peak at 140 K near Q = (0,0,0.98) and
two peaks near Q = (0,0,1.02), respectively. This is surprising because Cr
crystallizes in a centro-symmetric bcc structure. The asymmetry of those energy
spectra decreases with increasing temperature. In addition, the observed
magnetic peak intensity is independent of Q suggesting a transfer of
spectral-weight between the upper and lower FB modes. The energy-independent
cross section is localized only between the incommensurate peaks and develops
rapidly with increasing temperature.Comment: 6 pages, 8 figure
Neutron scattering study of soft phonons and diffuse scattering in insulating LaSrCuO
Soft phonons and diffuse scattering in insulating LaSrCuO
() have been studied by the neutron scattering technique. The X-point
phonon softens from high temperature towards the structural transition
temperature K, and the Z-point phonon softens again below 200 K.
The Z-point phonon softening persists to low temperature, in contrast to the
behavior observed in the superconducting compound, in which the
Z-point phonon hardens below . The diffuse scattering associated with the
structural phase transition at 410 K appears at commensurate positions. These
results highlight interesting differences between superconducting and
insulating samples.Comment: 5 pages, 5 figure
Soft phonons and structural phase transitions in LaBaCuO
Soft phonon behavior associated with a structural phase transition from the
low-temperature-orthorhombic (LTO) phase ( symmetry) to the
low-temperature-tetragonal (LTT) phase ( symmetry) was investigated
in LaBaCuO using neutron scattering. As temperature
decreases, the TO-mode at -point softens and approaches to zero energy
around K, where the LTO -- LTT transition occurs. Below , the phonon hardens quite rapidly and it's energy almost saturates below
50 K. At , the energy dispersion of the soft phonon along in-plane
direction significantly changes while the dispersion along out-of-plane
direction is almost temperature independent. Coexistence between the LTO phase
and the LTT phase, seen in both the soft phonon spectra and the peak profiles
of Bragg reflection, is discussed in context of the order of structural phase
transitions.Comment: 6 pages, 8 figure
Renormalization of Commensurate Magnetic Peak in Ni-doped LaSrCuO
We have studied the magnetic excitations in impurity doped
LaSrCuAO (A=Ni or Zn) by neutron
scattering. The dispersion for Zn: is similar to that for the impurity
free sample: incommensurate peaks with the incommensurability
(rlu) do not change their positions up to 21 meV. On the
other hand, for Ni:, two incommensurate peaks observed at low energies
suddenly change into a broad commensurate peak at meV.
Compared to the impurity free sample with a similar Sr-concentration ,
[B. Vignolle {\it et al.} Nature Physics {\bf 3} (2007) 163],
for Ni: is decreased by nearly the same factor for
the reduction in . This is very similar to the shift of the resonance
energy () in Ni-doped YBaCuO.[Y. Sidis {\it
et al.}: Phys. Rev. Lett. {\bf 84} (2000) 5900]. These common impurity effects
on the shift of and suggest the same
magnetic origin for the resonance peak in YBaCuO and
that for a crossing point of upward and downward dispersions in the
LaSrCuO. We propose that the sudden change in the
dispersion is better described by a crossover from incommensurate spin
fluctuations to a gapped spin wave rather than a hourglass-like dispersion.Comment: 11 pages, 5 figure
One year follow-up of physical performance and quality of life in patients surviving COVID-19: a prospective cohort study
INTRODUCTION: The coronavirus disease (COVID-19) continues to affect many countries globally, with the long-term impact of the disease now being recognized. According to the latest research, some of the affected individuals continue to experience functional limitations, reduced physical performance and impaired health-related quality of life (HRQoL) even after eight months. This prospective cohort study aimed to describe the longer-term recovery of physical performance and HRQoL in COVID-19 survivors over one year.
METHOD: A cohort (n = 43; 32-84 years old) hospitalized with COVID-19 between March and June 2020 was followed over one year and assessed at three time points: hospital discharge, 3 months and 12 months post-admission. Participants experienced mild (10/43) to critical (6/43) pneumonia and stayed in the hospital for a median of 10 days (IQR 9). Participants were assessed for physical performance (six-minute walk test), HRQoL (EQ-5D-5L), COVID-19 related limitations in functionality (PCFS), hospital-related anxiety and depression (HADS-A/-D), lung function (FEV1, FVC) and dyspnea during activity (mMRC). All assessments were conducted by physiotherapists trained in cardio-respiratory rehabilitation.
RESULTS: After discharge, 8/34 showed reduced physical performance, 9/42 had lower HRQoL and 14/32 had COVID-19 induced limitations in functionality on the PCFS scale. Physical performance did not change significantly between discharge and 12-month follow-up, but 15/34 participants showed clinically relevant improvements in walking distance (>30 m). However, 16/34 had a decreased walking distance >30 m when comparing 3-month to 12-month follow-up. At 12 months, 12/41 of participants still perceived COVID-19 related limitations in daily life on the PCFS scale. For HRQoL, 12/41 participants still perceived moderate-to-severe symptoms of pain and discomfort and 13/41 slight-to-severe symptoms of anxiety and depression.
CONCLUSION: This cohort of adult patients hospitalized for mild to severe COVID-19 in Switzerland was generally mildly affected but still reported some limitations after one year. These results offer preliminary indications for ongoing support after hospitalization and point towards the need for specific, individualized follow-up to support their recovery.
ClinicalTrials.gov (NCT04375709
Ferromagnetism in the Strong Hybridization Regime of the Periodic Anderson Model
We determine exactly the ground state of the one-dimensional periodic
Anderson model (PAM) in the strong hybridization regime. In this regime, the
low energy sector of the PAM maps into an effective Hamiltonian that has a
ferromagnetic ground state for any electron density between half and three
quarters filling. This rigorous result proves the existence of a new magnetic
state that was excluded in the previous analysis of the mixed valence systems.Comment: Accepted in Phys. Rev.
Structural instability associated with the tilting of CuO6 octahedra in La2-xSrxCuO4
Comprehensive inelastic neutron-scattering measurements were performed to
study the soft optical phonons in La2-xSrxCuO4 at x=0.10, 0.12 and 0.18. We
found at x=0.18 that the softening of Z-point phonon, suggesting incipient
structural transition from the low-temperature orthorhombic (LTO) to
low-temperature tetragonal (LTT) phase, breaks at Tc, which is consistent with
the previous report by Lee et al. for the optimally doped x=0.15 sample. As for
x=0.10 and 0.12, on the other hand, the softening continues even below Tc. It
is thus clarified that the breaking of soft phonon is characteristic of
La2-xSrxCuO4 in the optimally and overdoped regions. In the course of studying
the soft phonons, we discovered that a central peak remains above the LTO to
high-temperature tetragonal (HTT) phase transition at Ts1 and splits into
incommensurate components along the (1 1 0)HTT direction at higher
temperatures. This is a common feature for both x=0.12 and 0.18 and their
temperature dependences of the splitting 2d can be scaled by using a
renormalized temperature T/Ts1. In the high temperature limit, d saturates
around d ~ 0.12 r.l.u., which value is close to the splitting of incommensurate
magnetic signals. This implies that the incipient lattice modulation starts
appearing at very high temperature. Details of this modulation and its
relations with other properties are, however, not yet clarified.Comment: 7 pages, 5 eps figure
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