769 research outputs found
Preparation of bis(4-(3,4 dimethylene pyrrolidyl) phenyl) methane as a high temperature reactive oligomer
A major goal in the field of high temperature polymers was to prepare aromatic polyimides that can be easily fabricated with the required thermal and physical properties for aerospace applications. Recent research was directed to achieve polyimides that are: soluable in a common organic solvent; melt-processable; and thermally curable without the evolution of volatile by-products. A monomer, N-phenyl 3,4-dimethylene pyrrolidine, that can be modified to form a bis (exocyclodiene) I for the preparation of polyimides by the Diels-Alder process was developed. Preparation of high temperature polymeric materials by Diels-Alder polymerization that will maintain their integrity and toughness during long exposure time at elevated temperature is sought
Self-Energy Effects on the Low- to High-Energy Electronic Structure of SrVO3
The correlated electronic structure of SrVO3 has been investigated by
angle-resolved photoemission spectroscopy using in-situ prepared thin films.
Pronounced features of band renormalization have been observed: a sharp kink
~60 meV below the Fermi level (EF) and a broad so-called "high-energy kink"
~0.3 eV below EF as in the high-Tc cuprates although SrVO3 does not show
magnetic fluctuations. We have deduced the self-energy in a wide energy range
by applying the Kramers-Kronig relation to the observed spectra. The obtained
self-energy clearly shows a large energy scale of ~0.7 eV which is attributed
to electron-electron interaction and gives rise to the ~0.3 eV "kink" in the
band dispersion as well as the incoherent peak ~1.5eV below EF. The present
analysis enables us to obtain consistent picture both for the incoherent
spectra and the band renormalization.Comment: 5 pages, 3 figure
Electronic correlations and Hund's coupling effects in SrMoO revealed by photoemission spectroscopy
We investigate the electronic structure of a perovskite-type Pauli paramagnet
SrMoO3 (t2g2) thin film using hard x-ray photoemission spectroscopy and compare
the results to the realistic calculations that combine the density functional
theory within the local-density approximation (LDA) with the dynamical-mean
field theory (DMFT). Despite the clear signature of electron correlations in
the electronic specific heat, the narrowing of the quasiparticle bands is not
observed in the photoemission spectrum. This is explained in terms of the
characteristic effect of Hund's rule coupling for partially-filled t2g bands,
which induces strong quasiparticle renormalization already for values of
Hubbard interaction which are smaller than the bandwidth. The interpretation is
supported by additional model DMFT calculations including Hund's rule coupling,
that show renormalization of low-energy quasiparticles without affecting the
overall bandwidth. The photoemission spectra show additional spectral weight
around -2 eV that is not present in the LDA+DMFT. We interpret this weight as a
plasmon satellite, which is supported by measured Mo, Sr and Oxygen core-hole
spectra that all show satellites at this energy.Comment: 8 pages, 7 figure
Anisotropic spin-density distribution and magnetic anisotropy of strained LaSrMnO thin films: Angle-dependent x-ray magnetic circular dichroism
Magnetic anisotropies of ferromagnetic thin films are induced by epitaxial
strain from the substrate via strain-induced anisotropy in the orbital magnetic
moment and that in the spatial distribution of spin-polarized electrons.
However, the preferential orbital occupation in ferromagnetic metallic
LaSrMnO (LSMO) thin films studied by x-ray linear dichroism
(XLD) has always been found out-of-plane for both tensile and compressive
epitaxial strain and hence irrespective of the magnetic anisotropy. In order to
resolve this mystery, we directly probed the preferential orbital occupation of
spin-polarized electrons in LSMO thin films under strain by angle-dependent
x-ray magnetic circular dichroism (XMCD). Anisotropy of the spin-density
distribution was found to be in-plane for the tensile strain and out-of-plane
for the compressive strain, consistent with the observed magnetic anisotropy.
The ubiquitous out-of-plane preferential orbital occupation seen by XLD is
attributed to the occupation of both spin-up and spin-down out-of-plane
orbitals in the surface magnetic dead layer.Comment: 20 pages, 4 figure
Electronic phase diagram of Cr-doped VO2 epitaxial films studied by in situ photoemission spectroscopy
Through in situ photoemission spectroscopy (PES), we investigated the changes
in the electronic structure of Cr-doped VO2 films coherently grown on TiO2
(001) substrates. The electronic phase diagram of CrxV1-xO2 is drawn by a
combination of electric and spectroscopic measurements. The phase diagram is
similar to that of bulk CrxV1-xO2, while the temperature of metal-insulator
transition (TMIT) is significantly suppressed by the epitaxial strain effect.
In the range of x = 0-0.04, where TMIT remains unchanged as a function of x,
the PES spectra show dramatic change across TMIT, demonstrating the
characteristic spectral changes associated with the Peierls phenomenon. In
contrast, for x > 0.04, the TMIT linearly increases, and the metal-insulator
transition (MIT) may disappear at x = 0.08-0.12. The PES spectra at x = 0.08
exhibit pseudogap behavior near the Fermi level, whereas the characteristic
temperature-induced change remains almost intact, suggesting the existence of
local V-V dimerization. The suppression of V-V dimerization with increasing x
was confirmed by polarization-dependent x-ray absorption spectroscopy. These
spectroscopic investigations reveal that the energy gap and V 3d states are
essentially unchanged with 0 x 0.08 despite the suppression of V-V
dimerization. The invariance of the energy gap with respect to x suggests that
the MIT in CrxV1-xO2 arises primarily from the strong electron correlations,
namely the Peierls-assisted Mott transition. Meanwhile, the pseudogap at x =
0.08 eventually evolves to a full gap (Mott gap) at x = 0.12, which is
consistent with the disappearance of the temperature-dependent MIT in the
electronic phase diagram. These results demonstrate that a Mott insulating
phase without V-V dimerization is stabilized at x > 0.08 as a result of the
superiority of Mott instability over the Peierls one.Comment: 27 pages, 3 main figures, 5 supplementary figures. arXiv admin note:
text overlap with arXiv:2005.0030
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