72 research outputs found
Charge response function probed by resonant inelastic x-ray scattering: the signature of electronic gaps of YBaCuO
In strongly correlated systems the complete determination of the dynamical
susceptibility is of special relevance because of
the entwinement of the spin and charge components. Although Resonant Inelastic
X-Ray Scattering (RIXS) spectra are directly related to both the charge
() and the spin ()
contributions, only the latter has been extensively studied with RIXS so far.
Here we show how to extract from RIXS spectra of high- superconducting
cuprates relevant properties of , such as the presence of the
superconducting gap and of the pseudogap. In particular, we exploit the
temperature dependence of the Cu L edge RIXS spectra of underdoped
YBaCuO at specific wave-vectors q. The signature of the
two gaps is in the departure of the low energy Bosonic excitation continuum
from the statistical temperature-dependence. This approach can be immediately
used to investigate systematically the nature of the pseudogap in cuprates,
thereby taking advantage of the RIXS technique that does not suffer the
limitations of surface-sensitive electron spectroscopies. Its extension to
other interesting materials is foreseen.Comment: 11 pages, 9 figure
Experimental Determination of Momentum-Resolved Electron-Phonon Coupling
We provide a novel experimental method to quantitatively estimate the
electron-phonon coupling and its momentum dependence from resonant inelastic
x-ray scattering (RIXS) spectra based on the detuning of the incident photon
energy away from an absorption resonance. We apply it to the cuprate parent
compound NdBaCuO and find that the electronic coupling to the
oxygen half-breathing phonon mode is strongest at the Brillouin zone boundary,
where it amounts to eV, in agreement with previous studies. In
principle, this method is applicable to any absorption resonance suitable for
RIXS measurements and will help to define the contribution of lattice
vibrations to the peculiar properties of quantum materials.Comment: 6 pages, 3 figure
Fractional Spin Excitations in the Infinite-Layer Cuprate CaCuO2
We use resonant inelastic x-ray scattering (RIXS) to investigate the magnetic dynamics of the infinite-layer cuprate CaCuO2. We find that close to the (1/2,0) point, the single magnon decays into a broad continuum of excitations accounting for about 80% of the total magnetic spectral weight. Polarization-resolved RIXS spectra reveal the overwhelming dominance of the spin-flip (Delta S = 1) character of this continuum with respect to the Delta S = 0 multimagnon contributions. Moreover, its incident-energy dependence is identical to that of the magnon, supporting a common physical origin. We propose that the continuum originates from the decay of the magnon into spinon pairs, and we relate it to the exceptionally high ring exchange J(c) similar to J(1) of CaCuO2. In the infinite-layer cuprates, long-range and multisite hopping integrals are very important, and they amplify the 2D quantum magnetism effects in spite of the 3D antiferromagnetic Neel order
Strange metal behaviour from charge density fluctuations in cuprates
Besides the mechanism responsible for high critical temperature superconductivity, the grand unresolved issue of the cuprates is the occurrence of a strange metallic state above the so-called pseudogap temperature T-*. Even though such state has been successfully described within a phenomenological scheme, the so-called Marginal Fermi-Liquid theory, a microscopic explanation is still missing. However, recent resonant X-ray scattering experiments identified a new class of charge density fluctuations characterized by low characteristic energies and short correlation lengths, which are related to the well-known charge density waves. These fluctuations are present over a wide region of the temperature-vs-doping phase diagram and extend well above T-*. Here we investigate the consequences of charge density fluctuations on the electron and transport properties and find that they can explain the strange metal phenomenology. Therefore, charge density fluctuations are likely the long-sought microscopic mechanism underlying the peculiarities of the metallic state of cuprates. The strange metallic state of cuprates occurring in a broad region of their phase diagram outside the superconducting and pseudogapped regions remains a mystery. Here the authors consider the charge density fluctuations recently discovered in resonant X-ray experiments as a possible source of scattering and show that these fluctuations can account for the strange metallic behavior
Mobile orbitons in CaCuO: crucial role of the Hund's exchange
We investigate the Cu edge resonant inelastic x-ray scattering (RIXS)
spectra of a quasi-1D antiferromagnet CaCuO. In addition to the
magnetic excitations, which are well-described by the two-spinon continuum, we
observe two dispersive orbital excitations, the and the
orbitons. We carry out a quantitative comparison of the RIXS spectra, obtained
with two distinct incident polarizations, with a theoretical model. We show
that any realistic spin-orbital model needs to include a finite, but realistic,
Hund's exchange eV. Its main effect is an increase in orbiton
velocities, so that their theoretically calculated values match those observed
experimentally. Even though Hund's exchange also mediates some interaction
between spinon and orbiton, the picture of spin-orbit separation remains intact
and describes orbiton motion in this compound.Comment: 13 pages, 10 figure
Three-dimensional dispersion of spin waves measured in NiO by resonant inelastic x-ray scattering
Preocupações de carreira e adaptabilidade : estudo exploratório com uma amostra de candidatos a sargentos e oficiais do Exército Português
Tese de mestrado, Psicologia (Psicologia dos Recursos Humanos, do Trabalho e das Organizações), Universidade de Lisboa, Faculdade de Psicologia, 2010O presente trabalho consiste num estudo exploratório sobre as preocupações de
carreira e as dimensões da adaptabilidade, baseando-se na Perspectiva de
Desenvolvimento da Carreira de Donald Super e na Perspectiva Construtivista de
Mark Savickas. Pretende-se, ainda, contribuir para a investigação acerca do Inventário
sobre Adaptabilidade, que se encontra ainda em estudo no âmbito de um Projecto
Internacional.
Foram aplicados o Inventário das Preocupações de Carreira e o Inventário sobre
Adaptabilidade a uma amostra de 75 adultos candidatos a Sargentos e Oficiais do
Exército Português. Os resultados obtidos tendem a confirmar as hipóteses de
investigação formuladas, apontando para a relação entre a variável idade e as tarefas
de desenvolvimento das fases da carreira Exploração e Estabelecimento, para a
predominância das dimensões Confiança e Cooperação, e para a relação entre
preocupações de carreira e dimensões da adaptabilidade. Apresentam-se as
conclusões do presente estudo, algumas limitações deste e futuras linhas de
investigação. Tecem-se também algumas considerações sobre as implicações do
estudo para as práticas de gestão de carreira.This work is an exploratory study about the career concerns and the dimensions of
adaptability, based on the Perspective of the Career Development of Donald Super
and the Constructivist Perspective of Mark Savickas. It is also intended to contribute
to the research on the Career Adapt-Abilities Inventory, which is still under
investigation as part of an International Project.
The Career Concerns Inventory and the Career Adapt-Abilities Inventory were
applied to a sample of 75 adult candidates to Sergeants and Officers of the Portuguese
Army. The results seem to confirm the research hypotheses, pointing to the relation
between the variable age and the developmental tasks of the Exploration and
Establishment career phases, to the predominance of the dimensions Cooperation, and
Confidence and also to the relation between career concerns and adaptability
dimensions. The findings of this study, some of its limitations and future research
paths are presented. Some reflections are also discussed in what concerns the study's
implications for the career management practices
Charge Transfer and excitations in AgF
Charge transfer (CT) insulators are the parent phase of a large group of
today's unconventional high temperature superconductors. Here we study
experimentally and theoretically the interband excitations of the CT insulator
silver fluoride AgF, which has been proposed as an excellent analogue of
oxocuprates. Optical conductivity and resonant inelastic x-ray scattering
(RIXS) on AgF polycrystalline sample show a close similarity with that
measured on undoped LaCuO. While the former shows a CT gap
3.4~eV, larger than in the cuprate, -excitations are nearly at the
same energy in the two materials. DFT and exact diagonalization cluster
computations of the multiplet spectra show that AgF is more covalent than
the cuprate, in spite of the larger fundamental gap. Furthermore, we show that
AgF is at the verge of a charge transfer instability. The overall
resemblance of our data on AgF to those published previously on
LaCuO suggests that the underlying CT insulator physics is the same,
while AgF could also benefit from a proximity to a charge density wave
phase as in BaBiO. Therefore, our work provides a compelling support to the
future use of fluoroargentates for materials' engineering of novel
high-temperature superconductors.Comment: 13 pages, 9 Figures (including SI
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