91 research outputs found
Superconductivity in epitaxial thin films of NaxCoO2 y D2O
The observation of superconductivity in the layered transition metal oxide
NaxCoO2 y H2O (K. Takada et al., Nature 422, 53 (2003)) has caused a tremendous
upsurge of scientific interest due to its similarities and its differences to
the copper based high-temperature superconductors. Two years after the
discovery, we report the fabrication of single-phase superconducting epitaxial
thin films of Na0.3CoO2 x 1.3 D2O grown by pulsed laser deposition technique.
This opens additional roads for experimental research exploring the
superconducting state and the phase diagram of this unconventional material.Comment: 3 pages, 5 figure
Resolving the nature of electronic excitations in resonant inelastic x-ray scattering
The study of elementary bosonic excitations is essential toward a complete
description of quantum electronic solids. In this context, resonant inelastic
X-ray scattering (RIXS) has recently risen to becoming a versatile probe of
electronic excitations in strongly correlated electron systems. The nature of
the radiation-matter interaction endows RIXS with the ability to resolve the
charge, spin and orbital nature of individual excitations. However, this
capability has been only marginally explored to date. Here, we demonstrate a
systematic method for the extraction of the character of excitations as
imprinted in the azimuthal dependence of the RIXS signal. Using this novel
approach, we resolve the charge, spin, and orbital nature of elastic
scattering, (para-)magnon/bimagnon modes, and higher energy dd excitations in
magnetically-ordered and superconducting copper-oxide perovskites (Nd2CuO4 and
YBa2Cu3O6.75). Our method derives from a direct application of scattering
theory, enabling us to deconstruct the complex scattering tensor as a function
of energy loss. In particular, we use the characteristic tensorial nature of
each excitation to precisely and reliably disentangle the charge and spin
contributions to the low energy RIXS spectrum. This procedure enables to
separately track the evolution of spin and charge spectral distributions in
cuprates with doping. Our results demonstrate a new capability that can be
integrated into the RIXS toolset, and that promises to be widely applicable to
materials with intertwined spin, orbital, and charge excitations
Comment on "Raman spectroscopy study of NaxCoO2 and superconducting NaxCoO2 yH2O"
The effect of surface degradation of the thermolectric cobaltite on Raman
spectra is discussed and compared to experimental results from Co3O4 single
crystals. We conclude that on NaCl flux grown NaxCoO2 crystals a surface layer
of Co3O4 easily forms that leads to the observation of an intense phonon around
700 cm-1 [Phys. Rev. B 70, 052502 (2004)]. Raman spectra on freshly cleaved
crystals from optical floating zone ovens do not show such effects and have a
high frequency phonon cut-off at approximately 600 cm -1 [Phys. Rev. Lett 96,
167204 (2006)]. We discuss the relation of structural dimensionality,
electronic correlations and the high frequency phonon cut-off of the
thermolectric cobaltite.Comment: 2 pages, 1 figure, to be published in Phys. Rev. B (2007
Raman light scattering study and microstructural analysis of epitaxial films of the electron-doped superconductor La_{2-x}Ce_{x}CuO_{4}
We present a detailed temperature-dependent Raman light scattering study of
optical phonons in molecular-beam-epitaxy-grown films of the electron-doped
superconductor La_{2-x}Ce_{x}CuO_{4} close to optimal doping (x ~ 0.08, T_c =
29 K and x ~ 0.1, T_c = 27 K). The main focus of this work is a detailed
characterization and microstructural analysis of the films. Based on
micro-Raman spectroscopy in combination with x-ray diffraction,
energy-dispersive x-ray analysis, and scanning electron microscopy, some of the
observed phonon modes can be attributed to micron-sized inclusions of Cu_{2}O.
In the slightly underdoped film (x ~ 0.08), both the Cu_{2}O modes and others
that can be assigned to the La_{2-x}Ce_{x}CuO_{4} matrix show pronounced
softening and narrowing upon cooling below T ~ T_c. Based on control
measurements on commercial Cu_{2}O powders and on a comparison to prior Raman
scattering studies of other high-temperature superconductors, we speculate that
proximity effects at La_{2-x}Ce_{x}CuO_{4}/Cu_{2}O interfaces may be
responsible for these anomalies. Experiments on the slightly overdoped
La_{2-x}Ce_{x}CuO_{4} film (x ~ 0.1) did not reveal comparable phonon
anomalies.Comment: 7 pages, 8 figure
New class of T-prime-structure cuprate superconductors
High-temperature superconductivity has been discovered in La2-xBaxCuO4 [1], a
compound that derives from the undoped La2CuO4 crystallizing in the perovskite
T-structure. In this structure oxygen octahedra surround the copper ions. It is
common knowledge that charge carriers induced by doping in such an undoped
antiferromagnetic Mott-insulator lead to high-temperature superconductivity [2-
4]. The undoped material La2CuO4 is also the basis of the electron-doped
cuprate superconductors [5] of the form La2-xCexCuO4+y [6,7] which however
crystallize in the so called T-prime-structure, i.e. without apical oxygen
above or below the copper ions of the CuO2-plane. It is well known that for
La2-xCexCuO4+y the undoped T-prime-structure parent compound cannot be prepared
due to the structural phase transition back into the T-structure occuring
around x ~ 0.05. Here, we report that if La is substituted by RE = Y, Lu, Sm,
Eu, Gd, or Tb, which have smaller ionic radii but have the same valence as La,
nominally undoped La2-xRExCuO4 can be synthesized by molecular beam epitaxy in
the T-prime-structure. The second important result is that all these new
T-prime-compounds are superconductors with fairly high critical temperatures up
to 21 K. For this new class of cuprates La2-xRExCuO4, which forms the
T-prime-parent compounds of the La-based electron doped cuprates, we have not
been able to obtain the Mott-insulating ground state for small x before the
structural phase transition into the T-structure takes place.Comment: 17 pages, 7 figure
Angle-resolved photoemission spectroscopy of the low-energy electronic structure of superconducting PrCuO driven by oxygen non-stoichiometry
Bulk crystals of electron-doped cuprates with the T'-type structure require
both Ce substitutions and reduction annealing for the emergence of
superconductivity while the reduction annealing alone can induce
superconductivity in thin films of the T'-type cuprates. In order to reveal
low-energy electronic states which are responsible for the superconductivity,
we have conducted angle-resolved photoemission spectroscopy measurements on
thin films of the superconducting Ce-free T'-type cuprate PrCuO. The
results indicate that the overall band structure and the Fermi surface area of
the superconducting PrCuO are similar to those of superconducting
Ce-doped bulk single crystals, highlighting the importance of the actual
electron concentration rather than the Ce concentration when discussing the
physical properties of the T'-type cuprates
Gamma-rays from millisecond pulsars in Globular Clusters
Globular clusters (GCs) with their ages of the order of several billion years
contain many final products of evolution of stars such as: neutron stars, white
dwarfs and probably also black holes. These compact objects can be at present
responsible for the acceleration of particles to relativistic energies.
Therefore, gamma-ray emission is expected from GCs as a result of radiation
processes occurring either in the inner magnetosperes of millisecond pulsars or
in the vicinity of accreting neutron stars and white dwarfs or as a result of
interaction of particles leaving the compact objects with the strong radiation
field within the GC. Recently, GeV gamma-ray emission has been detected from
several GCs by the new satellite observatory Fermi. Also Cherenkov telescopes
reported interesting upper limits at the TeV energies which start to constrain
the content of GCs. We review the results of these gamma-ray observations in
the context of recent scenarios for their origin.Comment: 20 pages, 9 figures, will be published in Astrophysics and Space
Science Series (Springer), eds. N. Rea and D.F. Torre
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