609 research outputs found
Determining the Electron-Phonon Coupling Strength in Correlated Electron Systems from Resonant Inelastic X-ray Scattering
We show that high resolution Resonant Inelastic X-ray Scattering (RIXS)
provides direct, element-specific and momentum-resolved information on the
electron-phonon (e-p) coupling strength. Our theoretical analysis demonstrates
that the e-p coupling can be extracted from RIXS spectra by determining the
differential phonon scattering cross section. An alternative, very direct
manner to extract the coupling is to use the one and two-phonon loss ratio,
which is governed by the e-p coupling strength and the core-hole life-time.
This allows measurement of the e-p coupling on an absolute energy scale.Comment: 4 pages, 3 figure
Magnetic Excitations in La2CuO4 probed by Indirect Resonant Inelastic X-ray Scattering
Recent experiments on LaCuO suggest that indirect resonant inelastic
X-ray scattering (RIXS) might provide a probe for transversal spin dynamics. We
present in detail a systematic expansion of the relevant magnetic RIXS cross
section by using the ultrashort core-hole lifetime (UCL) approximation. We
compute the scattering intensity and its momentum dependence in leading order
of the UCL expansion. The scattering is due to two-magnon processes and is
calculated within a linear spin-wave expansion of the Heisenberg spin model for
this compound, including longer range and cyclic spin interactions. We observe
that the latter terms in the Hamiltonian enhance the first moment of the
spectrum if they strengthen the antiferromagnetic ordering. The theoretical
spectra agree very well with experimental data, including the observation that
scattering intensity vanishes for the transferred momenta and
. We show that at finite temperature there is an
additional single-magnon contribution to the scattering with a spectral weight
proportional to . We also compute the leading corrections to the UCL
approximation and find them to be small, putting the UCL results on a solid
basis. All this univocally points to the conclusion that the observed low
temperature RIXS intensity in LaCuO is due to two-magnon scattering.Comment: 11 pages, 13 figures, Phys. Rev. B 77, 134428 (2008) (v4: corrected
figs 7
Bimagnon studies in cuprates with Resonant Inelastic X-ray Scattering at the O K edge. I - An assessment on La2CuO4 and a comparison with the excitation at Cu L3 and Cu K edges
We assess the capabilities of magnetic Resonant Inelastic X-ray Scattering
(RIXS) at the O edge in undoped cuprates by taking La_{2}CuO_{4} as a
benchmark case, based on a series of RIXS measurements that we present here. By
combining the experimental results with basic theory we point out the
fingerprints of bimagnon in the O edge RIXS spectra. These are a dominant
peak around 450 meV, the almost complete absence of dispersion both with
and polarization and the almost constant intensity vs. the transferred
momentum with polarization. This behavior is quite different from Cu
edge RIXS giving a strongly dispersing bimagnon tending to zero at the
center of the Brillouin zone. This is clearly shown by RIXS measurements at the
Cu edge that we present. The Cu bimagnon spectra and those at Cu
edge - both from the literature and from our data - however, have the same
shape. These similarities and differences are understood in terms of different
sampling of the bimagnon continuum. This panorama points out the unique
possibilities offered by O RIXS in the study of magnetic excitations in
cuprates near the center of the BZ
Momentum dependence of orbital excitations in Mott-insulating titanates
High-resolution resonant inelastic x-ray scattering has been used to
determine the momentum dependence of orbital excitations in Mott-insulating
LaTiO and YTiO over a wide range of the Brillouin zone. The data are
compared to calculations in the framework of lattice-driven and
superexchange-driven orbital ordering models. A superexchange model in which
the experimentally observed modes are attributed to two-orbiton excitations
yields the best description of the data.Comment: to appear in PR
Measurement of magnetic excitations in the two-dimensional antiferromagnetic Sr2CuO2Cl2 insulator using resonant x-ray scattering:Evidence for extended interactions
Using high-resolution resonant inelastic x-ray scattering (RIXS), we
performed a momentum-resolved study of magnetic excitations in the model
spin-1/2 2D antiferromagnetic insulator Sr_2CuCl_2O_2. We identify both a
single-spin-wave feature and a multi-magnon continuum, and show that the X-ray
polarization can be used to distinguish these two contributions in the
cross-section. The spin-waves display a large (70 meV) dispersion between the
zone-boundary points (,0) and (/2,/2). Employing an extended
--- one-band Hubbard model, we find significant electronic
hopping beyond nearest-neighbor Cu ions. We conclude that sizeable extended
magnetic interactions are present in \scoc{} and probably important in all
undoped cuprates.Comment: 4 pages, 4 figure
Effectiveness of private land conservation areas in maintaining natural land cover and biodiversity intactness
CITATION: Shumba, T. et al. 2020. Effectiveness of private land conservation areas in maintaining natural land cover and biodiversity intactness. Global Ecology and Conservation, 22:e00935, doi:10.1016/j.gecco.2020.e00935.The original publication is available at https://www.journals.elsevier.com/global-ecology-and-conservationPrivate land conservation areas (PLCAs) are increasingly looked to for meeting the deficit
left by state-owned protected areas in reaching global conservation targets. However,
despite the increasing extent and recognition of PLCAs as a complementary conservation
strategy, little research has been done to quantify their effectiveness; a critical consideration
if they are to be counted towards international biodiversity conservation targets. The
long history of PLCAs in South Africa provides an interesting case study to address this
knowledge gap. Here, we quantified the effectiveness of South African PLCAs by comparing
losses in natural land cover and biodiversity intactness within PLCAs with different levels
of protection to that of unprotected control points. Points within PLCAs were matched with
unprotected control points to test the prediction that if PLCAs offer effective protection,
losses in natural land cover and biodiversity intactness would be significantly lower within
their boundaries in comparison to unprotected controls exposed to similar conditions.
Consequences of natural land cover loss on biodiversity intactness were thus assessed,
thus advancing standard approaches for quantifying effectiveness. Between 1990 and 2013,
PLCAs lost significantly less natural land cover (3%) and biodiversity intactness (2%) than
matched unprotected areas (6% and 4%, respectively). Of the natural land cover lost within
PLCAs, most was converted to cultivated land. Farms can support more species than other
land uses (e.g. mines), a likely explanation for why losses in biodiversity intactness were
less than losses in natural land cover. Contrary to the predicted pattern, effectiveness did
not increase with level of protection; informal PLCAs with no legal protection had comparable
natural land cover and biodiversity intactness retention to strictly protected PLCAs,
with most losses recorded among PLCAs with moderate protection. This study provides the
first national-scale evidence that PLCAs can be an effective mechanism for conserving
natural land cover and biodiversity intactness, which is highly relevant given current
discussions around their likely long-term biodiversity conservation capacity.Publisher's versio
Dispersive charge density wave excitations and temperature dependent commensuration in Bi2Sr2CaCu2O8+{\delta}
Experimental evidence on high-Tc cuprates reveals ubiquitous charge density
wave (CDW) modulations, which coexist with superconductivity. Although the CDW
had been predicted by theory, important questions remain about the extent to
which the CDW influences lattice and charge degrees of freedom and its
characteristics as functions of doping and temperature. These questions are
intimately connected to the origin of the CDW and its relation to the
mysterious cuprate pseudogap. Here, we use ultrahigh resolution resonant
inelastic x-ray scattering (RIXS) to reveal new CDW character in underdoped
Bi2Sr2CaCu2O8+{\delta} (Bi2212). At low temperature, we observe dispersive
excitations from an incommensurate CDW that induces anomalously enhanced phonon
intensity, unseen using other techniques. Near the pseudogap temperature T*,
the CDW persists, but the associated excitations significantly weaken and the
CDW wavevector shifts, becoming nearly commensurate with a periodicity of four
lattice constants. The dispersive CDW excitations, phonon anomaly, and
temperature dependent commensuration provide a comprehensive momentum space
picture of complex CDW behavior and point to a closer relationship with the
pseudogap state
Resonant Inelastic X-ray Scattering Studies of Elementary Excitations
In the past decade, Resonant Inelastic X-ray Scattering (RIXS) has made
remarkable progress as a spectroscopic technique. This is a direct result of
the availability of high-brilliance synchrotron X-ray radiation sources and of
advanced photon detection instrumentation. The technique's unique capability to
probe elementary excitations in complex materials by measuring their energy-,
momentum-, and polarization-dependence has brought RIXS to the forefront of
experimental photon science. We review both the experimental and theoretical
RIXS investigations of the past decade, focusing on those determining the
low-energy charge, spin, orbital and lattice excitations of solids. We present
the fundamentals of RIXS as an experimental method and then review the
theoretical state of affairs, its recent developments and discuss the different
(approximate) methods to compute the dynamical RIXS response. The last decade's
body of experimental RIXS data and its interpretation is surveyed, with an
emphasis on RIXS studies of correlated electron systems, especially transition
metal compounds. Finally, we discuss the promise that RIXS holds for the near
future, particularly in view of the advent of x-ray laser photon sources.Comment: Review, 67 pages, 44 figure
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