111 research outputs found
Damping of antiferromagnetic spin waves by valence fluctuations in the double layer perovskite YBaFe2O5
Inelastic neutron scattering experiments show that spin dynamics in the
charge ordered insulating ground state of the double-layer perovskite YBaFe2O5
is well described in terms of eg superexchange interactions. Above the Verwey
transition at TV = 308 K, t2g double exchange-type conduction within
antiferromagnetic FeO2--BaO--FeO2 double layers proceeds by an electron hopping
process that requires a spin flip of the five-fold coordinated Fe ions, costing
an energy 5S^2 of approximately 0.1 eV. The hopping process disrupts
near-neighbor spin correlations, leading to massive damping of zone-boundary
spin waves.Comment: RevTeX, 4 pages, 4 figures, submitted to Phys. Rev. Let
Stabilization of charge ordering in La_(1/3)Sr_(2/3)FeO_(3-d) by magnetic exchange
The magnetic exchange energies in charge ordered La_(1/3)Sr_(2/3)FeO_(3-d)
(LSFO) and its parent compound LaFeO_(3) (LFO) have been determined by
inelastic neutron scattering. In LSFO, the measured ratio of ferromagnetic
exchange between Fe3+ - Fe5+ pairs (J_F) and antiferromagnetic exchange between
Fe3+ - Fe3+ pairs (J_AF) fulfills the criterion for charge ordering driven by
magnetic interactions (|J_F/J_AF| > 1). The 30% reduction of J_AF as compared
to LFO indicates that doped holes are delocalized, and charge ordering occurs
without a dominant influence from Coulomb interactions.Comment: 18 pages, 4 color figure
Crystal fields, disorder, and antiferromagnetic short-range order in Yb0.24Sn0.76Ru
We report extensive measurements on a new compound (Yb0.24Sn0.76)Ru that
crystallizes in the cubic CsCl structure. Valence band photoemission and L3
x-ray absorption show no divalent component in the 4f configuration of Yb.
Inelastic neutron scattering (INS) indicates that the eight-fold degenerate
J-multiplet of Yb3+ is split by the crystalline electric field (CEF) into a
{\Gamma}7 doublet ground state and a {\Gamma}8 quartet at an excitation energy
20 meV. The magnetic susceptibility can be fit very well by this CEF scheme
under the assumption that a {\Gamma}6 excited state resides at 32 meV; however,
the {\Gamma}8/{\Gamma}6 transition expected at 12 meV was not observed in the
INS. The resistivity follows a Bloch- Gr\"uneisen law shunted by a parallel
resistor, as is typical of systems subject to phonon scattering with no
apparent magnetic scattering. All of these properties can be understood as
representing simple local moment behavior of the trivalent Yb ion. At 1 K,
there is a peak in specific heat that is too broad to represent a magnetic
phase transition, consistent with absence of magnetic reflections in neutron
diffraction. On the other hand, this peak also is too narrow to represent the
Kondo effect in the {\Gamma}7 ground state doublet. On the basis of the
field-dependence of the specific heat, we argue that antiferromagnetic
shortrange order (possibly co-existing with Kondo physics) occurs at low
temperatures. The long-range magnetic order is suppressed because the Yb site
occupancy is below the percolation threshold for this disordered compound
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Active Creation of Instrinsically Localized Vibrations in Uranium Using X-Ray and Neutron Scattering
In real materials, nonlinear forces cause the frequencies of vibrating atoms to depend on amplitude. As a consequence, a large-amplitude fluctuation on the scale of the atom spacing can develop a frequency that does not resonate with the normal modes, causing energy to become trapped in an intrinsically localized mode (ILM)--also called 'discrete breather' or 'lattice soliton'. As temperature is increased, entropy is expected to stabilize increased concentrations of these random hotspots. This mechanism, which spontaneously concentrates energy, has been observed in analogous systems on a larger scale, but direct sightings at the atomic scale have proved difficult. Two challenges have hampered progress: (1) the need to separate ILMs from modes associated with crystal imperfections, and (2) complications that arise at high temperatures, including feature broadening and multiphonon processes. Here we solve both of these problems by actively creating ILMs at low temperatures in {alpha}-uranium using high-energy inelastic x-ray and neutron scattering. The ILM creation excitation occurs at energies ten times higher than conventional lattice excitations, cleanly separating it from modes associated with crystal imperfections. The discovery of this excitation not only proves the existence of ILMs in uranium but also opens up a new route for finding ILMs in other materials and, in the process, a new area for spectroscopy
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Non-Equilibrium Creation of Intrinsically Localized Vibrations in Uranium Using X-Ray and Neutron Scattering
In an anharmonic crystal, large-amplitude vibrational fluctuations on the scale of the lattice spacing can develop frequencies that do not resonate with the normal modes, causing energy to become trapped in intrinsically localized modes (ILMs)--also called 'discrete breathers' or 'lattice solitons'. This mechanism has been observed in analogous systems on a larger scale, but unambiguous sightings in atomic lattice vibrations, where quantum mechanics may play a role, have proved difficult. Two challenges have hampered progress: (1) the need to separate ILMs from defect modes, and (2) complications that arise at high temperatures, including feature broadening and multiphonon processes. Here we solve these problems by using x-ray and neutron scattering to induce ILM-forming amplitude fluctuations in uranium at low temperatures, thereby creating nonequilibrium ILMs. Creation of ILMs occurs at a discrete energy, indicating an unexpected quantum character to ILM formation and greatly simplifying detection
Role of Magnetic Exchange Energy on Charge Ordering in RSFO (R=La, Pr, and Nd)
Inelastic neutron scattering is applied to study the role of magnetism in
stabilizing the charge ordered state in \emph{R}SrFeO
(\emph{R}SFO) (\emph{R} = La, Pr, and Nd). The ratio of the ferromagnetic
exchange energy (J) and antiferromagnetic exchange energy (J),
|J /J_{1/3}_{2/3}_{3}_{1/3}_{2/3}_{3}_{1/3}_{2/3}_{3}_{1/3}_{2/3}_{3}$ which indicates a gradual destabilization of the
magnetic exchange mechanism for charge ordering in correspondence with the
observed reduction in the ordering temperature
Crystalline Electric Field Excitations in the Heavy Fermion Superconductor CeCoIn_5
The crystalline electric field (CEF) energy level scheme of the heavy fermion
superconductor CeCoIn_5 has been determined by means of inelastic neutron
scattering (INS). Peaks observed in the INS spectra at 8 meV and 27 meV with
incident neutron energies between E_i=30-60 meV and at a temperature T = 10 K
correspond to transitions from the ground state to the two excited states,
respectively. The wavevector and temperature dependence of these peaks are
consistent with CEF excitations. Fits of the data to a CEF model yield the CEF
parameters B^0_2=-0.80 meV, B^0_4=0.059 meV, and |B^4_4|= 0.137 meV
corresponding to an energy level scheme: Gamma_7^(1) (0)[=0.487|+/-5/2> -
0.873|-/+3/2>], Gamma_7^(2) (8.6 meV, 100 K), and Gamma_6 (24.4 meV, 283 K).Comment: uses latex packages revtex4,amsmath,graphicx,natbib, 9th Annual
MMM-Intermag Conference, (Accepted for publication in J. Appl. Phys.) 7
pages, 2 figure
Presence of anticitrullinated protein antibodies in a large population-based cohort from the Netherlands
Objectives To determine the prevalence of anticitrullinated protein antibodies (ACPAs) and their association with known rheumatoid arthritis (RA) risk factors in the general population. Methods Lifelines is a multidisciplinary prospective population-based cohort study in the Netherlands. Cross-sectional data from 40 136 participants were used. The detection of ACPA was performed by measuring anti-CCP2 on the Phadia-250 analyser with levels >= 6.2 U/mL considered positive. An extensive questionnaire was taken on demographic and clinical information, including smoking, periodontal health and early symptoms of musculoskeletal disorders. RA was defined by a combination of self-reported RA, medication use for the indication of rheumatism and visiting a medical specialist within the last year. Results Of the total 40 136 unselected individuals, 401 (1.0%) had ACPA level >= 6.2 U/mL. ACPA positivity was significantly associated with older age, female gender, smoking, joint complaints, RA and first degree relatives with rheumatism. Of the ACPA-positive participants, 22.4% had RA (15.2% had defined RA according to our criteria and 7.2% self-reported RA only). In participants without RA, 311 (0.8%) were ACPA-positive. In the non-RA group, older age, smoking and joint complaints remained significantly more frequently present in ACPA-positive compared with ACPA-negative participants. Conclusions In this large population-based study, the prevalence of ACPA levels >= 6.2 U/mL was 1.0% for the total group and 0.8% when excluding patients with RA. Older age, smoking and joint complaints were more frequently present in ACPA-positive Lifelines participants. To our knowledge, this study is the largest study to date on ACPA positivity in the general, mostly Caucasian population
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