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Analytical calculation of the resolution correction function for X-ray surface structure analysis
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Magnetic properties of transition metal dichloride - graphite intercalation compounds
No full textThe magnetic properties of four transition metal dichloridegraphite
intercalation compounds (GIC's) have been studied by neutron
scattering and static magnetization measurements. Because of the
large spatial anisotropy introduced by intercalation, the compounds
studied represent good approximations to magnetic systems in two
dimensions. Magnetization measurements as a function of applied field
and temperature have been used to determine the dominant coupling
terms and anisotropies in the spin Hamiltonian, and magnetic neutron
scattering is used to measure dynamic spin correlations above and
below the critical temperatures. In addition, the atomic positions
and structural dynamics of these compounds have been probed through
x-ray and neutron scattering.
Stage-2 CoC12 -GIC approximates a two-dimensional easy-plane
ferromagnet on a triangular lattice. It is found to order in two
iv
steps, the higher-temperature phase showing no three-dimensional
static correlations, and the lower phase exhibiting a weak
antiferromagnetic correlation between neighboring intercalate layers .
We have probed the wave vector and temperature dependence of the
static and dynamic spin correlations in detail. Some of the
predictions for a Kosterlitz-Thouless type transition are met in this
compound, at least qualitatively , including an apparent jump in the
spin stiffness at the critical point and a diffusive central peak in
the scattering function originating from vortex autocorrelations.
However, there are substantive inconsistencies that prevent
unambiguous assignment of the upper critical temperature as a
vortex-binding transition .
Intercalation compounds of NiC12 , MnC12 , and CuC12 have also been
studied to a lesser extent. The magnetism in NiC12 -GIC is found to be
nearly identical to that in CoC12 -GIC . . MnC12 - and CuC12 -GIC's
approximate two-dimensional antiferromagnets. MnC12 -GIC shows an
easy-plane anisotropy and orders at Tc - 1.1 K to a 2/3 X 2/3
superstructure, indicating strong exchange coupling with far
neighbors. In CuC12 -GIC the triangular lattice is deformed, making
the spin correlations stronger in one direction than the other. We
find no evidence by neutron scattering of magnetic ordering above 9 K.Submitted by Carolyn Mead ([email protected]) on 2011-05-18T16:02:30Z
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The equilibrium shape of NaCl single crystal studied by x-ray reflection on its surfaces
No full textThe equilibrium shape of NaCl single crystals has been studied by x-ray reflectivity measurements at high symmetric surfaces. The equilibrium shape of a crystal is related to the properties of its surfaces through its surface free energy by means of the Wulff construction. Glancing angle x-ray reflectivity is used to measure the properties of the surfaces. X-ray reflectivities from the NaCl (100), (110) and (111) surfaces are measured during annealing of the samples at high temperatures ranging from 550°C to 700°C. A new method to calculate the x-ray reflectivity has been derived in matrix form for a given arbitrary varying surface layer. With this method, the surface mean width can be obtained by fitting the experimental data of the reflected x-ray intensity to the calculated result for a given surface model. Therefore, the mean width of NaCl surface as a function of annealing time and temperature can be obtained from the experiment. The expe~imental results show that NaCl (100) surface is a perfect, smooth surface, while NaCl (110) and (111) surfaces are stable surfaces when annealing at high temperatures. From the experiments it must be concluded that in the experimental temperature range from 550°C to 700°C the shape of NaCl single crystals is a cube with round edges and blunted corners.Submitted by Megan O'Donnell ([email protected]) on 2012-02-01T17:53:23Z
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Hydrogen in niobium-tantalum superlattices
No full textHydrogen has been dissolved in Nb/Ta superlattices for
which the structural and thermodynamic properties of this
new H-metal system were studied, in situ, by x-ray
diffraction. It is found that H induces a strain modulation
exhibiting a Curie-Weiss type temperature dependence, thus,
providing a first example of a strained layer superlattice
which can be strained after the growth of the structure. In
addition, a lattice-gas lattice-liquid phase transition was
observed where critical fluctuations exist only for
wavelengths longer than the superlattice periodicity. The
absence of short wavelength fluctuations prohibit phase
separation and constitutes a· novel manifestation of a
coherent phase transition. A mean field lattice gas model
is presented, which, combined with x-ray data from the
superlattice satellites, provides insight to the fundamental
properties of a lattice gas in the presence of modulated
interactions. Motivated by these experiments, a generalized
theoretical framework from which to view coherent phase
transitions is discussed.
Due to the epitaxial constraints imposed by the
substrate on the film, the lattice expansion due to H occurs
only normal to the plane of the film, up to a maximum
lattice expansion of Delta d / d ~ .016, and beyond which,
precipitation of oriented phases occurs. Also observed is
an enhanced solubility for H where it takes two orders of
magnitude less gas pressure to obtain the same H
concentration as compared to bulk Nb. It is demonstrated
that this enhancement is directly related to the one
dimensional lattice expansion.Submitted by Carolyn Mead ([email protected]) on 2011-06-20T15:29:50Z
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Lattice anharmonicity in alkali-graphite intercalation compounds
No full textSubmitted by Carolyn Mead ([email protected]) on 2011-06-03T17:46:52Z
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X-ray studies of gallium arsenide-silicon heterostructures
No full textGallium arsenide-silicon heterostructures combine complementary electronic and
structural properties to greater technological potential. The large 4% mismatch in lattice
parameter between the two semiconductors strongly affects disorder within the film
and at the interface. We employ x-ray scattering to explore the structural properties of
these layered materials grown by molecular beam epitaxy.
The thermal expansion of thick GaAs films on nominal Si(001) is measured with x-ray
diffraction. The gallium arsenide film exhibits an anisotropy between the in-plane and outof-
plane lattice parameters due to the large thermal mismatch and the interfacial constraint.
Thick films are found to be strain-free at the growth temperature and expanded in-plane at
room temperature. This contrasts with the in-plane room-temperature contraction shown
in thin films due to the residual strain present during growth.
Interface roughness in thin gallium arsenide films (a few hundred angstroms thick) on
silicon substrates is measured with x-ray reflectivity. The roughness at the GaAs/Si interface
is found to be approximately 9A for a (001) orientation miscut by 4°. Ex situ in vacuo
annealing just below the growth temperature, while reducing both film disorder and the
roughness of the GaAs surface, does not substantially reduce interface roughness. Slow
heating just above the growth temperature degrades the overall film quality.
We also study interface roughness in GaAs films grown on very thin films of Si on
a GaAs substrate. The silicon thickness ranges from pseudomorphic (that is, in-plane
lattice parameter matching the underlying GaAs) to non-pseudomorphic. The GaAs/Si
interface roughness increases from approximately 5A for the thinnest Si interlayer. The
GaAs surface roughness increases dramatically at about the critical film thickness. But the onset of increased surface roughness occurs well before the Si thickness approaches critical
and before the onset of increased interface roughness.Submitted by Carolyn Rauber ([email protected]) on 2012-01-25T22:33:26Z
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Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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