583 research outputs found
Structural dynamics during laser induced ultrafast demagnetization
The mechanism underlying femtosecond laser pulse induced ultrafast
magnetization dynamics remains elusive despite two decades of intense research
on this phenomenon. Most experiments focused so far on characterizing
magnetization and charge carrier dynamics, while first direct measurements of
structural dynamics during ultrafast demagnetization were reported only very
recently. We here present our investigation of the infrared laser pulse induced
ultrafast demagnetization process in a thin Ni film, which characterizes
simultaneously magnetization and structural dynamics. This is achieved by
employing femtosecond time resolved X-ray resonant magnetic reflectivity
(tr-XRMR) as probe technique. The experimental results reveal unambiguously
that the sub-picosecond magnetization quenching is accompanied by strong
changes in non-magnetic X-ray reflectivity. These changes vary with reflection
angle and changes up to 30 have been observed. Modeling the X-ray
reflectivity of the investigated thin film, we can reproduce these changes by a
variation of the apparent Ni layer thickness of up to 1. Extending these
simulations to larger incidence angles we show that tr-XRMR can be employed to
discriminate experimentally between currently discussed models describing the
ultrafast demagnetization phenomenon
Hadrons in the Nuclear Medium
Quantum Chromodynamics, the microscopic theory of strong interactions, has
not yet been applied to the calculation of nuclear wave functions. However, it
certainly provokes a number of specific questions and suggests the existence of
novel phenomena in nuclear physics which are not part of the the traditional
framework of the meson-nucleon description of nuclei. Many of these phenomena
are related to high nuclear densities and the role of color in nucleonic
interactions. Quantum fluctuations in the spatial separation between nucleons
may lead to local high density configurations of cold nuclear matter in nuclei,
up to four times larger than typical nuclear densities. We argue here that
experiments utilizing the higher energies available upon completion of the
Jefferson Laboratory energy upgrade will be able to probe the quark-gluon
structure of such high density configurations and therefore elucidate the
fundamental nature of nuclear matter. We review three key experimental
programs: quasi-elastic electro-disintegration of light nuclei, deep inelastic
scattering from nuclei at , and the measurement of tagged structure
functions. These interrelated programs are all aimed at the exploration of the
quark structure of high density nuclear configurations.
The study of the QCD dynamics of elementary hard processes is another
important research direction and nuclei provide a unique avenue to explore
these dynamics. We argue that the use of nuclear targets and large values of
momentum transfer at would allow us to determine whether the physics of the
nucleon form factors is dominated by spatially small configurations of three
quarks.Comment: 52 pages IOP style LaTex file and 20 eps figure
Irreversible transformation of ferromagnetic ordered stripe domains in single-shot IR pump - resonant X-ray scattering probe experiments
The evolution of a magnetic domain structure upon excitation by an intense,
femtosecond Infra-Red (IR) laser pulse has been investigated using single-shot
based time-resolved resonant X-ray scattering at the X-ray Free Electron laser
LCLS. A well-ordered stripe domain pattern as present in a thin CoPd alloy film
has been used as prototype magnetic domain structure for this study. The
fluence of the IR laser pump pulse was sufficient to lead to an almost complete
quenching of the magnetization within the ultrafast demagnetization process
taking place within the first few hundreds of femtoseconds following the IR
laser pump pulse excitation. On longer time scales this excitation gave rise to
subsequent irreversible transformations of the magnetic domain structure. Under
our specific experimental conditions, it took about 2 nanoseconds before the
magnetization started to recover. After about 5 nanoseconds the previously
ordered stripe domain structure had evolved into a disordered labyrinth domain
structure. Surprisingly, we observe after about 7 nanoseconds the occurrence of
a partially ordered stripe domain structure reoriented into a novel direction.
It is this domain structure in which the sample's magnetization stabilizes as
revealed by scattering patterns recorded long after the initial pump-probe
cycle. Using micro-magnetic simulations we can explain this observation based
on changes of the magnetic anisotropy going along with heat dissipation in the
film.Comment: 16 pages, 6 figure
Moments of the Neutron \u3cem\u3eg\u3c/em\u3e\u3csub\u3e2\u3c/sub\u3e Structure Function at Intermediate \u3cem\u3eQ\u3c/em\u3e\u3csup\u3e2\u3c/sup\u3e
We present new experimental results for the 3He spin structure function g2 in the resonance region at Q2 values between 1.2 and 3.0(GeV/c)2. Spin dependent moments of the neutron were extracted. Our main result, the inelastic contribution to the neutron d2 matrix element, was found to be small at ⟨Q2⟩=2.4(GeV/c)2 and in agreement with the lattice QCD calculation. The Burkhardt-Cottingham sum rule for 3He and the neutron was tested with the measured data and using the Wandzura-Wilczek relation for the low x unmeasured region
The f_LT Response Function of D(e,e'p)n at Q^2=0.33(GeV/c)^2
The interference response function f_LT (R_LT) of the D(e,e'p)n reaction has
been determined at squared four-momentum transfer Q^2 = 0.33 (GeV/c)^2 and for
missing momenta up to p_miss= 0.29 (GeV/c). The results have been compared to
calculations that reproduce f_LT quite well but overestimate the cross sections
by 10 - 20% for missing momenta between 0.1 (GeV/c) and 0.2 (GeV/c) .Comment: 12 Pages, 10 figure
Phenomenology of the Deuteron Electromagnetic Form Factors
A rigorous extraction of the deuteron charge form factors from tensor
polarization data in elastic electron-deuteron scattering, at given values of
the 4-momentum transfer, is presented. Then the world data for elastic
electron-deuteron scattering is used to parameterize, in three different ways,
the three electromagnetic form factors of the deuteron in the 4-momentum
transfer range 0-7 fm^-1. This procedure is made possible with the advent of
recent polarization measurements. The parameterizations allow a
phenomenological characterization of the deuteron electromagnetic structure.
They can be used to remove ambiguities in the form factors extraction from
future polarization data.Comment: 18 pages (LaTeX), 2 figures Feb. 25: minor changes of content and in
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