286 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
Sr 2 IrO 4 magnetic phase diagram, from resistivity
International audienceWe show that the transition to the antiferromagnetic state in zero magnetic field does show up in the transverse resistivity, for which we point out the possibility for a direct spin orientation effect. In an applied field, we propose that the transition is split into two lines, corresponding to in-plane and out-of-plane magnetic ordering. This picture is corroborated by transverse magnetization measurements. The magnetic phase diagram for Sr2IrO4 was investigated, using the angular dependence of the resistivity transverse to the IrO2 planes
Role of dynamic Jahn-Teller distortions in Na2C60 and Na2CsC60 studied by NMR
Through 13C NMR spin lattice relaxation (T1) measurements in cubic Na2C60, we
detect a gap in its electronic excitations, similar to that observed in
tetragonal A4C60. This establishes that Jahn-Teller distortions (JTD) and
strong electronic correlations must be considered to understand the behaviour
of even electron systems, regardless of the structure. Furthermore, in metallic
Na2CsC60, a similar contribution to T1 is also detected for 13C and 133Cs NMR,
implying the occurence of excitations typical of JT distorted C60^{2-} (or
equivalently C60^{4-}). This supports the idea that dynamic JTD can induce
attractive electronic interactions in odd electron systems.Comment: 3 figure
Mn local moments prevent superconductivity in iron-pnictides Ba(Fe 1-x Mn x)2As2
75As nuclear magnetic resonance (NMR) experiments were performed on
Ba(Fe1-xMnx)2As2 (xMn = 2.5%, 5% and 12%) single crystals. The Fe layer
magnetic susceptibility far from Mn atoms is probed by the75As NMR line shift
and is found similar to that of BaFe2As2, implying that Mn does not induce
charge doping. A satellite line associated with the Mn nearest neighbours
(n.n.) of 75As displays a Curie-Weiss shift which demonstrates that Mn carries
a local magnetic moment. This is confirmed by the main line broadening typical
of a RKKY-like Mn-induced staggered spin polarization. The Mn moment is due to
the localization of the additional Mn hole. These findings explain why Mn does
not induce superconductivity in the pnictides contrary to other dopants such as
Co, Ni, Ru or K.Comment: 6 pages, 7 figure
Evidence for distinct polymer chain orientations in KC60 and RbC60
The KC60 and RbC60 polymer phases exhibit contrasting electronic properties
while powder diffraction studies have revealed no definite structural
difference. We have performed single crystal X-ray diffraction and diffuse
scattering studies of these compounds. It is found that KC60 and RbC60 possess
different chain orientations about their axes, which are described by distinct
space groups Pmnn and I2/m, respectively. Such a structural difference will be
of great importance to a complete understanding of the physical properties.Comment: To be published in Phys. Rev. Let
Orientation-dependent C60 electronic structures revealed by photoemission
We observe, with angle-resolved photoemission, a dramatic change in the
electronic structure of two C60 monolayers, deposited respectively on Ag (111)
and (100) substrates, and similarly doped with potassium to half-filling of the
C60 lowest unoccupied molecular orbital. The Fermi surface symmetry, the
bandwidth, and the curvature of the dispersion at Gamma point are different.
Orientations of the C60 molecules on the two substrates are known to be the
main structural difference between the two monolayers, and we present new
band-structure calculations for some of these orientations. We conclude that
orientations play a key role in the electronic structure of fullerides.Comment: 4 pages, 4 figure
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
Detection by NMR of a "local spin-gap" in quenched CsC60
We present a 13C and 133Cs NMR investigation of the CsC60 cubic quenched
phase. Previous ESR measurements suggest that this phase is metallic, but NMR
reveals contrasting electronic behavior on the local scale. The 13C
spin-lattice relaxation time (T1) exhibits a typical metallic behavior down to
50 K, but indicates that a partial spin-gap opens for T<50 K. Unexpectedly,
133Cs NMR shows that there are two inequivalent Cs sites. For one of these
sites, the NMR shift and (T1T)^{-1} follow an activated law, confirming the
existence of a spin-gap. We ascribe this spin-gap to the occurrence of
localized spin-singlets on a small fraction of the C60 molecules.Comment: 4 figure
Orbitally resolved lifetimes in Ba(Fe0.92Co0.08)2As2 measured by ARPES
Despite many ARPES investigations of iron pnictides, the structure of the
electron pockets is still poorly understood. By combining ARPES measurements in
different experimental configurations, we clearly resolve their elliptic shape.
Comparison with band calculation identify a deep electron band with the dxy
orbital and a shallow electron band along the perpendicular ellipse axis with
the dxz/dyz orbitals. We find that, for both electron and hole bands, the
lifetimes associated with dxy are longer than for dxz/dyz. This suggests that
the two types of orbitals play different roles in the electronic properties and
that their relative weight is a key parameter to determine the ground state
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