142 research outputs found
Gaps and excitations in fullerides with partially filled bands : NMR study of Na2C60 and K4C60
We present an NMR study of Na2C60 and K4C60, two compounds that are related
by electron-hole symmetry in the C60 triply degenerate conduction band. In both
systems, it is known that NMR spin-lattice relaxation rate (1/T1) measurements
detect a gap in the electronic structure, most likely related to
singlet-triplet excitations of the Jahn-Teller distorted (JTD) C60^{2-} or
C60^{4-}. However, the extended temperature range of the measurements presented
here (10 K to 700 K) allows to reveal deviations with respect to this general
trend, both at high and low temperatures. Above room temperature, 1/T1 deviates
from the activated law that one would expect from the presence of the gap and
saturates. In the same temperature range, a lowering of symmetry is detected in
Na2C60 by the appearance of quadrupole effects on the 23Na spectra. In K4C60,
modifications of the 13C spectra lineshapes also indicate a structural
modification. We discuss this high temperature deviation in terms of a coupling
between JTD and local symmetry. At low temperatures, 1/TT tends to a
constant value for Na2C60, both for 13C and 23Na NMR. This indicates a residual
metallic character, which emphasizes the proximity of metallic and insulting
behaviors in alkali fullerides.Comment: 12 pages, 13 figure
Transfer of spectral weight across the gap of Sr2IrO4 induced by La doping
We study with Angle Resolved PhotoElectron Spectroscopy (ARPES) the evolution
of the electronic structure of Sr2IrO4, when holes or electrons are introduced,
through Rh or La substitutions. At low dopings, the added carriers occupy the
first available states, at bottom or top of the gap, revealing an anisotropic
gap of 0.7eV in good agreement with STM measurements. At further doping, we
observe a reduction of the gap and a transfer of spectral weight across the
gap, although the quasiparticle weight remains very small. We discuss the
origin of the in-gap spectral weight as a local distribution of gap values
Fermi Surface reconstruction in the CDW state of CeTe3 observed by photoemission
CeTe3 is a layered compound where an incommensurate Charge Density Wave (CDW)
opens a large gap (400 meV) in optimally nested regions of the Fermi Surface
(FS), whereas other sections with poorer nesting remain ungapped. Through
Angle-Resolved Photoemission, we identify bands backfolded according to the CDW
periodicity. They define FS pockets formed by the intersection of the original
FS and its CDW replica. Such pockets illustrate very directly the role of
nesting in the CDW formation but they could not be detected so far in a CDW
system. We address the reasons for the weak intensity of the folded bands, by
comparing different foldings coexisting in CeTe3
New electronic orderings observed in cobaltates under the influence of misfit periodicities
We study with ARPES the electronic structure of CoO2 slabs, stacked with
rock-salt (RS) layers exhibiting a different (misfit) periodicity. Fermi
Surfaces (FS) in phases with different doping and/or periodicities reveal the
influence of the RS potential on the electronic structure. We show that these
RS potentials are well ordered, even in incommensurate phases, where STM images
reveal broad stripes with width as large as 80\AA. The anomalous evolution of
the FS area at low dopings is consistent with the localization of a fraction of
the electrons. We propose that this is a new form of electronic ordering,
induced by the potential of the stacked layers (RS or Na in NaxCoO2) when the
FS becomes smaller than the Brillouin Zone of the stacked structure
The degenerate 3-band Hubbard model with "anti-Hund's rule" interactions; a model for AxC60
We consider the orbitally degenerate 3-band Hubbard model with on-site
interactions which favor low spin and low orbital angular momentum using
standard second order perturbation theory in the large Hubbard-U limit. At even
integer filling this model is a Mott insulator with a non-degenerate ground
state that allows for a simple description of particle-hole excitations as well
as gapped spin and orbital modes. We find that the Mott gap is generally
indirect and that the single particle spectrum at low doping reappears close to
even filling but rescaled by a factor 2/3 or 1/3. The model captures the basic
phenomenology of the Mott insulating and metallic fullerides AxC60. This
includes the existence of a smaller spin gap and larger charge gap at even
integer filling, the fact that odd integer stoichiometries are generally
metallic while even are insulating, as well as the rapid suppression of the
density of states and superconducting transition temperatures with doping away
from x=3.Comment: Revised with additional reference
Interplay of Superconductivity and Fermi-Liquid Transport in Rh-Doped CaFe2As2 with Lattice-Collapse Transition
Ca(FeRh)As undergoes successive phase transitions with
increasing Rh doping in the 0 limit. The antiferromagnetic-metal phase
with orthorhombic structure at 0.00 0.020 is driven to a
superconducting phase with uncollapsed-tetragonal (ucT) structure at 0.020
0.024; a non-superconducting collapsed-tetragonal (cT) phase
takes over at 0.024. The breakdown of Fermi-liquid transport is
observed in the ucT phase above . In the adjacent cT phase,
Fermi-liquid transport is restored along with a disappearance of
superconductivity. This interplay of superconductivity and Fermi-liquid
transport suggests the essential role of magnetic fluctuations in the emergence
of superconductivity in doped CaFeAs.Comment: 11 pages, 4 figure
Coexistence of Superconductivity and Charge Density Wave in SrPt2As2
SrPt2As2 is a novel arsenide superconductor, which crystallizes in the
CaBe2Ge2-type structure as a different polymorphic form of the ThCr2Si2-type
structure. SrPt2As2 exhibits a charge-density-wave (CDW) ordering at about 470
K and enters into a superconducting state at Tc = 5.2 K. The coexistence of
superconductivity and CDW refers to Peierls instability with a moderately
strong electron-phonon interaction. Thus SrPt2As2 can be viewed as a
nonmagnetic analog of iron-based superconductors, such as doped BaFe2As2, in
which superconductivity emerges in close proximity to spin-density-wave
ordering.Comment: 4 pages, 5 figure
Experimental study of the incoherent spectral weight in the photoemission spectra of the misfit cobaltate [Bi2Ba2O4][CoO2]2
Previous ARPES experiments in NaxCoO2 reported both a strongly renormalized
bandwidth near the Fermi level and moderately renormalized Fermi velocities,
leaving it unclear whether the correlations are weak or strong and how they
could be quantified. We explain why this situation occurs and solve the problem
by extracting clearly the coherent and incoherent parts of the band crossing
the Fermi level. We show that one can use their relative weight to estimate
self-consistently the quasiparticle weight Z, which turns out to be very small
Z=0.15 +/- 0.05. We suggest this method could be a reliable way to study the
evolution of correlations in cobaltates and for comparison with other strongly
correlated systems
Valence band electronic structure of V2O3: identification of V and O bands
We present a comprehensive study of the photon energy dependence of the
valence band photoemission yield in the prototype Mott-Hubbard oxide V2O3. The
analysis of our experimental results, covering an extended photon energy range
(20-6000 eV) and combined with GW calculations, allow us to identify the nature
of the orbitals contributing to the total spectral weight at different binding
energies, and in particular to locate the V 4s at about 8 eV binding energy.
From this comparative analysis we can conclude that the intensity of the
quasiparticle photoemission peak, observed close to the Fermi level in the
paramagnetic metallic phase upon increasing photon energy, does not have a
significant correlation with the intensity variation of the O 2p and V 3d
yield, thus confirming that bulk sensitivity is an essential requirement for
the detection of this coherent low energy excitation
The Thermal, Mechanical, Structural, and Dielectric Properties of Cometary Nuclei After Rosetta
The physical properties of cometary nuclei observed today relate to their complex history and help to constrain their formation and evolution. In this article, we review some of the main physical properties of cometary nuclei and focus in particular on the thermal, mechanical, structural and dielectric properties, emphasising the progress made during the Rosetta mission. Comets have a low density of 480±220 kgmâ3 and a low permittivity of 1.9â2.0, consistent with a high porosity of 70â80%, are weak with a very low global tensile strength â1mâ2sâ1/2 that allowed them to preserve highly volatiles species (e.g. CO, CO2, CH4, N2) into their interior since their formation. As revealed by 67P/Churyumov-Gerasimenko, the above physical properties vary across the nucleus, spatially at its surface but also with depth. The broad picture is that the bulk of the nucleus consists of a weakly bonded, rather homogeneous material that preserved primordial properties under a thin shell of processed material, and possibly covered by a granular material; this cover might in places reach a thickness of several meters. The properties of the top layer (the first meter) are not representative of that of the bulk nucleus. More globally, strong nucleus heterogeneities at a scale of a few meters are ruled out on 67Pâs small lobe
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