281 research outputs found
Resonant Elastic Soft X-Ray Scattering
Resonant (elastic) soft x-ray scattering (RSXS) offers a unique element,
site, and valence specific probe to study spatial modulations of charge, spin,
and orbital degrees of freedom in solids on the nanoscopic length scale. It
cannot only be used to investigate single crystalline materials. This method
also enables to examine electronic ordering phenomena in thin films and to zoom
into electronic properties emerging at buried interfaces in artificial
heterostructures. During the last 20 years, this technique, which combines
x-ray scattering with x-ray absorption spectroscopy, has developed into a
powerful probe to study electronic ordering phenomena in complex materials and
furthermore delivers important information on the electronic structure of
condensed matter. This review provides an introduction to the technique, covers
the progress in experimental equipment, and gives a survey on recent RSXS
studies of ordering in correlated electron systems and at interfaces
Ecological implications of motorboat noise on coral reef fish communities
Anthropogenic noise is recognised as a major pollutant of international concern. Motorised vessels are the dominant source of anthropogenic noise in the marine environment. Small motorboats are widespread among coastal regions, exposing shallow marine ecosystems to noise disturbance. Short-term exposure to motorboat noise can have a profound impact on fish physiology and behaviour. However, it remains unclear how such impacts translate to wild fish assemblages. No study has yet investigated the community-wide implications of motorboat noise. The aim of this thesis is to review the current understanding of motorboat noise on fish ecology; provide the first assessment of chronic motorboat noise on a wild fish community; and consider future directions in research, management and mitigation. Using existing spatial variation in motorboat traffic across coral reefs in French Polynesia we carried out visual census techniques to investigate the effect of chronic motorboat noise on a coral reef fish community. In addition, we conducted a month-long motorboat manipulation of a coral reef with minimal disturbance history to test whether community responses can be experimentally induced. There was no difference in the overall fish abundance, species richness and diversity on coral reefs exposed to chronic motorboat noise. Yet, 5 species had significantly lower abundances, whilst 8 species had significantly greater abundances on reef exposed to chronic motorboat noise, resulting in a significant difference in the overall community composition. In addition, the month-long motorboat manipulation replicated the same response in two species as the previous study; though this was not enough to significantly alter the community composition. This study demonstrates that fish species respond differently to chronic motorboat noise, and community implications are more complex than previously predicted. Future studies should consider the diversity of functional traits, noise tolerance and interspecific interactions when investigating the ecological implications of motorboat noise on fishes
Magnetic domain fluctuations in an antiferromagnetic film observed with coherent resonant soft x-ray scattering
We report the direct observation of slow fluctuations of helical
antiferromagnetic domains in an ultra-thin holmium film using coherent resonant
magnetic x-ray scattering. We observe a gradual increase of the fluctuations in
the speckle pattern with increasing temperature, while at the same time a
static contribution to the speckle pattern remains. This finding indicates that
domain-wall fluctuations occur over a large range of time scales. We ascribe
this non-ergodic behavior to the strong dependence of the fluctuation rate on
the local thickness of the film.Comment: to appear in Phys. Rev. Let
Observation of Devil's Staircase in the Novel Spin Valve System SrCoO
Using resonant soft x-ray scattering as a function of both temperature and
magnetic field, we reveal a large number of almost degenerate magnetic orders
in SrCo6O11. The Ising-like spins in this frustrated material in fact exhibit a
so-called magnetic devil's staircase. It is demonstrated how a magnetic field
induces transitions between different microscopic spin configurations, which is
responsible for the magnetoresistance of SrCo6O11. This material therefore
constitutes a unique combination of a magnetic devil's staircase and spin valve
effects, yielding a novel type of magnetoresistance system.Comment: 5 pages, 5 figure
Tunable Charge and Spin Order in PrNiO Thin Films and Superlattices
We have used polarized Raman scattering to probe lattice vibrations and
charge ordering in 12 nm thick, epitaxially strained PrNiO films, and in
superlattices of PrNiO with the band-insulator PrAlO. A carefully
adjusted confocal geometry was used to eliminate the substrate contribution to
the Raman spectra. In films and superlattices under tensile strain, which
undergo a metal-insulator transition upon cooling, the Raman spectra reveal
phonon modes characteristic of charge ordering. These anomalous phonons do not
appear in compressively strained films, which remain metallic at all
temperatures. For superlattices under compressive strain, the Raman spectra
show no evidence of anomalous phonons indicative of charge ordering, while
complementary resonant x-ray scattering experiments reveal antiferromagnetic
order associated with a modest increase in resistivity upon cooling. This
confirms theoretical predictions of a spin density wave phase driven by spatial
confinement of the conduction electrons.Comment: PRL, in pres
Spin-polarized electronic structure of the core-shell ZnO/ZnO:Mn nanowires probed by x-ray absorption and emission spectroscopy
The combination of x-ray spectroscopy methods complemented with theoretical
analysis unravels the coexistence of paramagnetic and antiferromagnetic phases
in the Zn_0.9Mn_0.1O shell deposited onto array of wurtzite ZnO nanowires. The
shell is crystalline with orientation toward the ZnO growth axis, as
demonstrated by X-ray linear dichroism. EXAFS analysis confirmed that more than
90% of Mn atoms substituted Zn in the shell while fraction of secondary phases
was below 10%. The value of manganese spin magnetic moment was estimated from
the Mn K{\beta} X-ray emission spectroscopy to be 4.3{\mu}B which is close to
the theoretical value for substitutional Mn_Zn. However the analysis of L_2,3
x-ray magnetic circular dichroism data showed paramagnetic behaviour with
saturated spin magnetic moment value of 1.95{\mu}B as determined directly from
the spin sum rule. After quantitative analysis employing atomic multiplet
simulations such difference was explained by a coexistence of paramagnetic
phase and local antiferromagnetic coupling of Mn magnetic moments. Finally,
spin-polarized electron density of states was probed by the spin-resolved Mn
K-edge XANES spectroscopy and consequently analyzed by band structure
calculations.Comment: Supplementary information available at
http://www.rsc.org/suppdata/ja/c3/c3ja50153a/c3ja50153a.pdf J. Anal. At.
Spectrom., 201
Electronic structure, magnetic and dielectric properties of the edge-sharing copper-oxide chain compound NaCuO
We report an experimental study of \nco, a Mott insulator containing chains
of edge-sharing CuO plaquettes, by polarized x-ray absorption spectroscopy
(XAS), resonant magnetic x-ray scattering (RMXS), magnetic susceptibility, and
pyroelectric current measurements. The XAS data show that the valence holes
reside exclusively on the Cu sites within the copper-oxide spin chains
and populate a -orbital polarized within the CuO plaquettes. The RMXS
measurements confirm the presence of incommensurate magnetic order below a
N\'eel temperature of K, which was previously inferred from
neutron powder diffraction and nuclear magnetic resonance data. In conjunction
with the magnetic susceptibility and XAS data, they also demonstrate a new
"orbital" selection rule for RMXS that is of general relevance for magnetic
structure determinations by this technique. Dielectric property measurements
reveal the absence of significant ferroelectric polarization below , which
is in striking contrast to corresponding observations on the isostructural
compound \lco. The results are discussed in the context of current theories of
multiferroicity.Comment: 7 pages, 7 figure
Momentum-dependent charge correlations in YBaCuO superconductors probed by resonant x-ray scattering: Evidence for three competing phases
We have used resonant x-ray scattering to determine the momentum dependent
charge correlations in YBaCuO samples with highly ordered
chain arrays of oxygen acceptors (ortho-II structure). The results reveal
nearly critical, biaxial charge density wave (CDW) correlations at in-plane
wave vectors (0.315, 0) and (0, 0.325). The corresponding scattering intensity
exhibits a strong uniaxial anisotropy. The CDW amplitude and correlation length
are enhanced as superconductivity is weakened by an external magnetic field.
Analogous experiments were carried out on a YBaCuO crystal with
a dilute concentration of spinless (Zn) impurities, which had earlier been
shown to nucleate incommensurate magnetic order. Compared to pristine crystals
with the same doping level, the CDW amplitude and correlation length were found
to be strongly reduced. These results indicate a three-phase competition
between spin-modulated, charge-modulated, and superconducting states in
underdoped YBaCuO.Comment: 6 pages, 3 figures revised version, to appear in Phys. Rev. Let
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