169 research outputs found
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
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
Structure of self-organized Fe clusters grown on Au(111) analyzed by Grazing Incidence X-Ray Diffraction
We report a detailed investigation of the first stages of the growth of
self-organized Fe clusters on the reconstructed Au(111) surface by grazing
incidence X-ray diffraction. Below one monolayer coverage, the Fe clusters are
in "local epitaxy" whereas the subsequent layers adopt first a strained fcc
lattice and then a partly relaxed bcc(110) phase in a Kurdjumov-Sachs epitaxial
relationship. The structural evolution is discussed in relation with the
magnetic properties of the Fe clusters.Comment: 7 pages, 6 figures, submitted to Physical Review B September 200
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
Relaxation and reconstruction on (111) surfaces of Au, Pt, and Cu
We have theoretically studied the stability and reconstruction of (111)
surfaces of Au, Pt, and Cu. We have calculated the surface energy, surface
stress, interatomic force constants, and other relevant quantities by ab initio
electronic structure calculations using the density functional theory (DFT), in
a slab geometry with periodic boundary conditions. We have estimated the
stability towards a quasi-one-dimensional reconstruction by using the
calculated quantities as parameters in a one-dimensional Frenkel-Kontorova
model. On all surfaces we have found an intrinsic tensile stress. This stress
is large enough on Au and Pt surfaces to lead to a reconstruction in which a
denser surface layer is formed, in agreement with experiment. The
experimentally observed differences between the dense reconstruction pattern on
Au(111) and a sparse structure of stripes on Pt(111) are attributed to the
details of the interaction potential between the first layer of atoms and the
substrate.Comment: 8 pages, 3 figures, submitted to Physical Review
Asymmetrically cut crystals as optical elements for highly collimated xâray beams
Asymmetrically cut perfect crystals, in both the Laue and Bragg geometries, are examined as single crystal monochromators for xâray 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
Temperature-dependent electron-phonon coupling in LaSrCuO probed by femtosecond X-ray diffraction
The strength of the electron-phonon coupling parameter and its evolution
throughout a solid's phase diagram often determines phenomena such as
superconductivity, charge- and spin-density waves. Its experimental
determination relies on the ability to distinguish thermally activated phonons
from those emitted by conduction band electrons, which can be achieved in an
elegant way by ultrafast techniques. Separating the electronic from the
out-of-equilibrium lattice subsystems, we probed their re-equilibration by
monitoring the transient lattice temperature through femtosecond X-ray
diffraction in LaSrCuO single crystals with =0.1 and 0.21.
The temperature dependence of the electron-phonon coupling is obtained
experimentally and shows similar trends to what is expected from the
\textit{ab-initio} calculated shape of the electronic density-of-states near
the Fermi energy. This study evidences the important role of band effects in
the electron-lattice interaction in solids, in particular in superconductors
- âŠ