189 research outputs found
The electronic structure of LaSrMnO thin films and its dependence as studied by angle-resolved photoemission
We present angle-resolved photoemission spectroscopy results for thin films
of the three-dimensional manganese perovskite LaSrMnO. We
show that the transition temperature () from the paramagnetic insulating
to ferromagnetic metallic state is closely related to details of the electronic
structure, particularly to the spectral weight at the -point, where
the sharpest step at the Fermi level was observed. We found that this -point is the same for all the samples, despite their different . The
change of is discussed in terms of kinetic energy optimization. Our ARPES
results suggest that the change of the electronic structure for the samples
having different transition temperatures is different from the rigid band
shift.Comment: Accepted by Journal of Physics: Condensed Matte
Direct reaction measurements with a 132Sn radioactive ion beam
The (d,p) neutron transfer and (d,d) elastic scattering reactions were
measured in inverse kinematics using a radioactive ion beam of 132Sn at 630
MeV. The elastic scattering data were taken in a region where Rutherford
scattering dominated the reaction, and nuclear effects account for less than 8%
of the cross section. The magnitude of the nuclear effects was found to be
independent of the optical potential used, allowing the transfer data to be
normalized in a reliable manner. The neutron-transfer reaction populated a
previously unmeasured state at 1363 keV, which is most likely the
single-particle 3p1/2 state expected above the N=82 shell closure. The data
were analyzed using finite range adiabatic wave calculations and the results
compared with the previous analysis using the distorted wave Born
approximation. Angular distributions for the ground and first excited states
are consistent with the previous tentative spin and parity assignments.
Spectroscopic factors extracted from the differential cross sections are
similar to those found for the one neutron states beyond the benchmark
doubly-magic nucleus 208Pb.Comment: 22 pages, 7 figure
Spin Wave Theory of Double Exchange Ferromagnets
We construct the 1/S spin-wave expansion for double exchange ferromagnets at
T=0. It is assumed that the value of Hund's rule coupling, J_H, is sufficiently
large, resulting in a fully saturated, ferromagnetic half-metallic ground
state. We evaluate corrections to the magnon dispersion law, and we also find
that, in contrast to earlier statements in the literature, magnon-electron
scattering does give rise to spin wave damping. We analyse the momentum
dependence of these quantities and discuss the experimental implications for
colossal magnetoresistance compounds.Comment: 4 pages, Latex-Revtex, 2 PostScript figures. Minor revisions,
references added. See also cond-mat/990921
The Single-Particle Structure of Neutron-Rich Nuclei of Astrophysical Interest at the Ornl Hribf
The rapid nuetron-capture process (r process) produces roughly half of the
elements heavier than iron. The path and abundances produced are uncertain,
however, because of the lack of nuclear strucure information on important
neutron-rich nuclei. We are studying nuclei on or near the r-process path via
single-nucleon transfer reactions on neutron-rich radioactive beams at ORNL's
Holifield Radioactive Ion Beam Facility (HRIBF). Owing to the difficulties in
studying these reactions in inverse kinematics, a variety of experimental
approaches are being developed. We present the experimental methods and initial
results.Comment: Proceedings of the Third International Conference on Fission and
Properties of Neutron-Rich Nucle
The magic nature of 132Sn explored through the single-particle states of 133Sn
Atomic nuclei have a shell structure where nuclei with 'magic numbers' of
neutrons and protons are analogous to the noble gases in atomic physics. Only
ten nuclei with the standard magic numbers of both neutrons and protons have so
far been observed. The nuclear shell model is founded on the precept that
neutrons and protons can move as independent particles in orbitals with
discrete quantum numbers, subject to a mean field generated by all the other
nucleons. Knowledge of the properties of single-particle states outside nuclear
shell closures in exotic nuclei is important for a fundamental understanding of
nuclear structure and nucleosynthesis (for example the r-process, which is
responsible for the production of about half of the heavy elements). However,
as a result of their short lifetimes, there is a paucity of knowledge about the
nature of single-particle states outside exotic doubly magic nuclei. Here we
measure the single-particle character of the levels in 133Sn that lie outside
the double shell closure present at the short-lived nucleus 132Sn. We use an
inverse kinematics technique that involves the transfer of a single nucleon to
the nucleus. The purity of the measured single-particle states clearly
illustrates the magic nature of 132Sn.Comment: 19 pages, 5 figures and 4 table
Nesting properties and anisotropy of the Fermi surface of LuNiBC
The rare earth nickel borocarbides, with the generic formula
NiBC, have recently been shown to display a rich variety of
phenomena. Most striking has been the competition between, and even coexistence
of, antiferromagnetism and superconductivity. We have measured the Fermi
surface (FS) of LuNiBC, and shown that it possesses nesting
features capable of explaining some of the phenomena experimentally observed.
In particular, it had previously been conjectured that a particular sheet of FS
is responsible for the modulated magnetic structures manifest in some of the
series. We report the first direct experimental observation of this sheet.Comment: 4 pages, 4 PS figure
Changes in Lysozyme Flexibility upon Mutation Are Frequent, Large and Long-Ranged
We investigate changes in human c-type lysozyme flexibility upon mutation via a Distance Constraint Model, which gives a statistical mechanical treatment of network rigidity. Specifically, two dynamical metrics are tracked. Changes in flexibility index quantify differences within backbone flexibility, whereas changes in the cooperativity correlation quantify differences within pairwise mechanical couplings. Regardless of metric, the same general conclusions are drawn. That is, small structural perturbations introduced by single point mutations have a frequent and pronounced affect on lysozyme flexibility that can extend over long distances. Specifically, an appreciable change occurs in backbone flexibility for 48% of the residues, and a change in cooperativity occurs in 42% of residue pairs. The average distance from mutation to a site with a change in flexibility is 17–20 Å. Interestingly, the frequency and scale of the changes within single point mutant structures are generally larger than those observed in the hen egg white lysozyme (HEWL) ortholog, which shares 61% sequence identity with human lysozyme. For example, point mutations often lead to substantial flexibility increases within the β-subdomain, which is consistent with experimental results indicating that it is the nucleation site for amyloid formation. However, β-subdomain flexibility within the human and HEWL orthologs is more similar despite the lowered sequence identity. These results suggest compensating mutations in HEWL reestablish desired properties
New Constraints on the 18F(p,alpha) 15O Rate in Novae from the (d,p) Reaction
The degree to which the (p,gamma) and (p,alpha) reactions destroy 18F at
temperatures 1-4x10^8 K is important for understanding the synthesis of nuclei
in nova explosions and for using the long-lived radionuclide 18F, a target of
gamma-ray astronomy, as a diagnostic of nova mechanisms. The reactions are
dominated by low-lying proton resonances near the 18F+p threshold (E_x=6.411
MeV in 19Ne). To gain further information about these resonances, we have used
a radioactive 18F beam from the Holifield Radioactive Ion Beam Facility to
selectively populate corresponding mirror states in 19F via the inverse
d(18F,p)19F neutron transfer reaction. Neutron spectroscopic factors were
measured for states in 19F in the excitation energy range 0-9 MeV. Widths for
corresponding proton resonances in 19Ne were calculated using a Woods-Saxon
potential. The results imply significantly lower 18F(p,gamma)19Ne and
18F(p,alpha)15O reaction rates than reported previously, thereby increasing the
prospect of observing the 511-keV annihilation radiation associated with the
decay of 18F in the ashes ejected from novae.Comment: Error involving sum rule was corrected. Proton widths were
recalculated using a Woods-Saxon potential. Both low-lying resonances (8- and
38-keV) are now included in the rate band. 12 pages, 4 figures, 1 table.
Submitted to Phys. Rev.
First proton-transfer study of 18F+p resonances relevant for novae
The 18F(p,α)15O reaction is the predominant destruction mechanism in novae of the radionuclide F18, a target of γ-ray observatories. Thus, its rate is important for understanding F18 production in novae. We have studied resonances in the 18F+p system by making a measurement of a proton-transfer reaction 18F(d,n). We have observed 15 Ne19 levels, 5 of which are below the proton threshold, including a subthreshold state, which has significant l p=0 strength. Our data provide a direct determination of the spectroscopic strength of these states and new constraints on their spins and parities, thereby resolving a controversy, which involves the 8- and 38-keV resonances. The 18F(p,α)15O reaction rate is reevaluated, which takes the subthreshold resonance and other new information determined in this experiment into account. © 2011 American Physical Society
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