299 research outputs found
Heterogeneous and anisotropic dynamics of a 2D gel
We report X-ray Photon Correlation Spectroscopy (XPCS) results on a
bidimensional (2D) gel formed by a Langmuir monolayer of gold nanoparticles.
The system allows an experimental determination of the fourth order time
correlation function which is compared to the usual second order correlation
functions and to the mechanical response measured on macroscopic scale. The
observed dynamics is anisotropic, heterogeneous and super-diffusive on the
nanoscale. Different time scales, associated with fast heterogeneous dynamics
inside 2D cages and slower motion of larger parts of the film, can be
identified from the correlation functions
Structural and Magnetic Dynamics in the Magnetic Shape Memory Alloy NiMnGa
Magnetic shape memory Heusler alloys are multiferroics stabilized by the
correlations between electronic, magnetic and structural order. To study these
correlations we use time resolved x-ray diffraction and magneto-optical Kerr
effect experiments to measure the laser induced dynamics in a Heusler alloy
NiMnGa film and reveal a set of timescales intrinsic to the system. We
observe a coherent phonon which we identify as the amplitudon of the modulated
structure and an ultrafast phase transition leading to a quenching of the
incommensurate modulation within 300~fs with a recovery time of a few ps. The
thermally driven martensitic transition to the high temperature cubic phase
proceeds via nucleation within a few ps and domain growth limited by the speed
of sound. The demagnetization time is 320~fs, which is comparable to the
quenching of the structural modulation.Comment: 5 pages, 3 figures. Supplementary materials 5 pages, 5 figure
The glass transition in colloidal suspensions of silica nanoparticles in a water-lutidine mixture: A photon correlation study
We discuss the structural and dynamical properties of a colloidal glass of silica nanoparticles in a water-lutidine mixture probed using photon correlation techniques. We describe the small angle set-up used to perform X-ray photon correlation spectroscopy (XPCS) experiments and the procedure followed to measure the volume fraction of the sample. We describe the structure of the glass using a short range potential model and a theoretical structure factor within the mean spherical approximation. The dynamics finally is characterized by a Gaussian-like intermediate scattering function which is not compatible with the classical picture of an heterogeneous diffusive process
Watching the birth of a charge density wave order: diffraction study on nanometer-and picosecond-scales
Femtosecond time-resolved X-ray diffraction is used to study a photo-induced
phase transition between two charge density wave (CDW) states in 1T-TaS,
namely the nearly commensurate (NC) and the incommensurate (I) CDW states.
Structural modulations associated with the NC-CDW order are found to disappear
within 400 fs. The photo-induced I-CDW phase then develops through a
nucleation/growth process which ends 100 ps after laser excitation. We
demonstrate that the newly formed I-CDW phase is fragmented into several
nanometric domains that are growing through a coarsening process. The
coarsening dynamics is found to follow the universal Lifshitz-Allen-Cahn growth
law, which describes the ordering kinetics in systems exhibiting a
non-conservative order parameter.Comment: 6 pages, 5 figure
Ultrafast relaxation dynamics of the antiferrodistortive phase in Ca doped SrTiO3
The ultrafast dynamics of the octahedral rotation in Ca:SrTiO3 is studied by
time resolved x-ray diffraction after photo excitation over the band gap. By
monitoring the diffraction intensity of a superlattice reflection that is
directly related to the structural order parameter of the soft-mode driven
antiferrodistortive phase in Ca:SrTiO3, we observe a ultrafast relaxation on a
0.2 ps timescale of the rotation of the oxygen octahedron, which is found to be
independent of the initial temperaure despite large changes in the
corresponding soft-mode frequency. A further, much smaller reduction on a
slower picosecond timescale is attributed to thermal effects. Time-dependent
density-functional-theory calculations show that the fast response can be
ascribed to an ultrafast displacive modification of the soft-mode potential
towards the normal state, induced by holes created in the oxygen 2p states
Asymmetrically cut crystals as optical elements for highly collimated x‐ray beams
Asymmetrically cut perfect crystals, in both the Laue and Bragg geometries, are examined as single crystal monochromators for x‐ray beams that are collimated to a small fraction of the Darwin width, as is typical in experiments with coherent x rays. Both the Laue and asymmetric Bragg geometries are plagued by an inherent chromatic aberration that increases the beam divergence much beyond that of the symmetric Bragg geometry. Measurements from a recent experiment at the ESRF are presented to compare Si(220) (symmetric Bragg), diamond(111) (asymmetric Laue), and diamond(111) (symmetric Bragg inclined) geometries. © 1995 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70952/2/RSINAK-66-2-1506-1.pd
Ultrafast structural dynamics of the Fe-pnictide parent compound BaFe2As2
Using femtosecond time-resolved x-ray diffraction we investigate the
structural dynamics of the coherently excited A1g phonon mode in the
Fe-pnictide parent compound BaFe2As2. The fluence dependent intensity
oscillations of two specific Bragg reflections with distinctly different
sensitivity to the pnictogen height in the compound allow us to quantify the
coherent modifications of the Fe-As tetrahedra, indicating a transient increase
of the Fe magnetic moments. By a comparison with time-resolved photoemission
data we derive the electron-phonon deformation potential for this particular
mode. The value of Delta mu/Delta z = -(1.0 - 1.5) eV/A is comparable with
theoretical predictions and demonstrates the importance of this degree of
freedom for the electron-phonon coupling in the Fe pnictides.Comment: 5 pages, 4 figures, Supplementary materia
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