221 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
Orbit spaces of free involutions on the product of two projective spaces
Let be a finitistic space having the mod 2 cohomology algebra of the
product of two projective spaces. We study free involutions on and
determine the possible mod 2 cohomology algebra of orbit space of any free
involution, using the Leray spectral sequence associated to the Borel fibration
. We also
give an application of our result to show that if has the mod 2 cohomology
algebra of the product of two real projective spaces (respectively complex
projective spaces), then there does not exist any -equivariant
map from for (respectively ), where
is equipped with the antipodal involution.Comment: 14 pages, to appear in Results in Mathematic
Fermi surface of the colossal magnetoresistance perovskite La_{0.7}Sr_{0.3}MnO_{3}
Materials that exhibit colossal magnetoresistance (CMR) are currently the
focus of an intense research effort, driven by the technological applications
that their sensitivity lends them to. Using the angular correlation of photons
from electron-positron annihilation, we present a first glimpse of the Fermi
surface of a material that exhibits CMR, supported by ``virtual crystal''
electronic structure calculations. The Fermi surface is shown to be
sufficiently cubic in nature that it is likely to support nesting.Comment: 5 pages, 5 PS figure
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
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
Ab-initio electronic and magnetic structure in La_0.66Sr_0.33MnO_3: strain and correlation effects
The effects of tetragonal strain on electronic and magnetic properties of
strontium-doped lanthanum manganite, La_{2/3}Sr_{1/3}MnO_3 (LSMO), are
investigated by means of density-functional methods. As far as the structural
properties are concerned, the comparison between theory and experiments for
LSMO strained on the most commonly used substrates, shows an overall good
agreement: the slight overestimate (at most of 1-1.5 %) for the equilibrium
out-of-plane lattice constants points to possible defects in real samples. The
inclusion of a Hubbard-like contribution on the Mn d states, according to the
so-called "LSDA+U" approach, is rather ineffective from the structural point of
view, but much more important from the electronic and magnetic point of view.
In particular, full half-metallicity, which is missed within a bare
density-functional approach, is recovered within LSDA+U, in agreement with
experiments. Moreover, the half-metallic behavior, particularly relevant for
spin-injection purposes, is independent on the chosen substrate and is achieved
for all the considered in-plane lattice constants. More generally, strain
effects are not seen to crucially affect the electronic structure: within the
considered tetragonalization range, the minority gap is only slightly (i.e. by
about 0.1-0.2 eV) affected by a tensile or compressive strain. Nevertheless, we
show that the growth on a smaller in-plane lattice constant can stabilize the
out-of-plane vs in-plane e_g orbital and significatively change their relative
occupancy. Since e_g orbitals are key quantities for the double-exchange
mechanism, strain effects are confirmed to be crucial for the resulting
magnetic coupling.Comment: 16 pages, 7 figures, to be published on J. Phys.: Condensed Matte
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
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
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
Hydrogen bond networks determine emergent mechanical and thermodynamic properties across a protein family
<p>Abstract</p> <p>Background</p> <p>Gram-negative bacteria use periplasmic-binding proteins (bPBP) to transport nutrients through the periplasm. Despite immense diversity within the recognized substrates, all members of the family share a common fold that includes two domains that are separated by a conserved hinge. The hinge allows the protein to cycle between open (apo) and closed (ligated) conformations. Conformational changes within the proteins depend on a complex interplay of mechanical and thermodynamic response, which is manifested as an increase in thermal stability and decrease of flexibility upon ligand binding.</p> <p>Results</p> <p>We use a distance constraint model (DCM) to quantify the give and take between thermodynamic stability and mechanical flexibility across the bPBP family. Quantitative stability/flexibility relationships (QSFR) are readily evaluated because the DCM links mechanical and thermodynamic properties. We have previously demonstrated that QSFR is moderately conserved across a mesophilic/thermophilic RNase H pair, whereas the observed variance indicated that different enthalpy-entropy mechanisms allow similar mechanical response at their respective melting temperatures. Our predictions of heat capacity and free energy show marked diversity across the bPBP family. While backbone flexibility metrics are mostly conserved, cooperativity correlation (long-range couplings) also demonstrate considerable amount of variation. Upon ligand removal, heat capacity, melting point, and mechanical rigidity are, as expected, lowered. Nevertheless, significant differences are found in molecular cooperativity correlations that can be explained by the detailed nature of the hydrogen bond network.</p> <p>Conclusion</p> <p>Non-trivial mechanical and thermodynamic variation across the family is explained by differences within the underlying H-bond networks. The mechanism is simple; variation within the H-bond networks result in altered mechanical linkage properties that directly affect intrinsic flexibility. Moreover, varying numbers of H-bonds and their strengths control the likelihood for energetic fluctuations as H-bonds break and reform, thus directly affecting thermodynamic properties. Consequently, these results demonstrate how unexpected large differences, especially within cooperativity correlation, emerge from subtle differences within the underlying H-bond network. This inference is consistent with well-known results that show allosteric response within a family generally varies significantly. Identifying the hydrogen bond network as a critical determining factor for these large variances may lead to new methods that can predict such effects.</p
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