127 research outputs found
Unusual polymerization in the Li4C60 fulleride
Li4C60, one of the best representatives of lithium intercalated fullerides,
features a novel type of 2D polymerization. Extensive investigations, including
laboratory x-ray and synchrotron radiation diffraction, 13C NMR, MAS and Raman
spectroscopy, show a monoclinic I2/m structure, characterized by chains of
[2+2]-cycloaddicted fullerenes, sideways connected by single C-C bonds. This
leads to the formation of polymeric layers, whose insulating nature, deduced
from the NMR and Raman spectra, denotes the complete localization of the
electrons involved in the covalent bonds.Comment: 7 pages, 6 figures, RevTex4, submitted to Phys. Rev.
NMR study of the Superconducting gap variation near the Mott transition in CsC
Former extensive studies of superconductivity in the \textit{A}C
compounds, where \textit{A} is an alkali, have led to consider that Bardeen
Cooper Schrieffer (BCS) electron-phonon pairing prevails in those compounds,
though the incidence of electronic Coulomb repulsion has been highly debated.
The discovery of two isomeric fulleride compounds CsC which
exhibit a transition with pressure from a Mott insulator (MI) to a
superconducting (SC) state clearly re-opens that question. Using pressure ()
as a single control parameter of the C balls lattice spacing, one can
now study the progressive evolution of the SC properties when the electronic
correlations are increased towards the critical pressure of the Mott
transition. We have used C and Cs NMR measurements on the cubic
phase A15-CsC just above kbar, where the SC
transition temperature displays a dome shape with decreasing cell
volume. From the dependence below of the nuclear spin lattice
relaxation rate we determine the electronic excitations in the
SC state, that is , the SC gap value. We find that
increases with decreasing towards , where decreases on the
SC dome, so that increases regularly upon approaching the
Mott transition. These results bring clear evidence that the increasing
correlations near the Mott transition are not significantly detrimental to SC.
They rather suggest that repulsive electron interactions might even reinforce
elecron-phonon SC, being then partly responsible for the large values,
as proposed by theoretical models taking the electronic correlations as a key
ingredient.Comment: 5 pages, 4 figures, Supplemental Materia
Spin Dynamics at the Mott Transition and in the Metallic State of the Cs_{3}C_{60} Superconducting Phases
We present here ^{13}C and ^{133}Cs NMR spin lattice relaxation T_{1} data in
the A15 and fcc-Cs_{3}C_{60} phases for increasing hydrostatic pressure through
the transition at p_{c} from a Mott insulator to a superconductor. We evidence
that for p>> p_{c} the (T_{1}T)^{-1} data above T_{c} display metallic like
Korringa constant values which match quantitatively previous data taken on
other A_{3}C_{60} compounds. However below the pressure for which T_{c} goes
through a maximum, (T_{1}T)^{-1} is markedly increased with respect to the
Korringa values expected in a simple BCS scenario. This points out the
importance of electronic correlations near the Mott transition. For p > p_{c}
singular T dependences of (T_{1}T)^{-1} are detected for T >> T_{c}. It will be
shown that they can be ascribed to a large variation with temperature of the
Mott transition pressure p_{c} towards a liquid-gas like critical point, as
found at high T for usual Mott transitions.Comment: 6 pages, 6 figures, submitted to EP
Recovering Metallicity in A4C60: The Case of Monomeric Li4C60
The restoration of metallicity in the high-temperature, cubic phase of Li4C60
represents a remarkable feature for a member of the A4C60 family (A = alkali
metal), invariably found to be insulators. Structural and resonance technique
investigations on Li4C60 at T > 600 K, show that its fcc structure is
associated with a complete (4e) charge transfer to C60 and a sparsely populated
Fermi level. These findings not only emphasize the crucial role played by
lattice symmetry in fulleride transport properties, but also re-dimension the
role of Jahn-Teller effects in band structure determination. Moreover, they
suggest the present system as a potential precursor to a new class of
superconducting fullerides.Comment: 4 pages, 3 figure
Pressure tuning of light-induced superconductivity in K3C60
Optical excitation at terahertz frequencies has emerged as an effective means
to manipulate complex solids dynamically. In the molecular solid K3C60,
coherent excitation of intramolecular vibrations was shown to transform the
high temperature metal into a non-equilibrium state with the optical
conductivity of a superconductor. Here we tune this effect with hydrostatic
pressure, and we find it to disappear around 0.3 GPa. Reduction with pressure
underscores the similarity with the equilibrium superconducting phase of K3C60,
in which a larger electronic bandwidth is detrimental for pairing. Crucially,
our observation excludes alternative interpretations based on a high-mobility
metallic phase. The pressure dependence also suggests that transient, incipient
superconductivity occurs far above the 150 K hypothesised previously, and
rather extends all the way to room temperature.Comment: 33 pages, 17 figures, 2 table
Superconducting fluctuations observed far above T<sub>c</sub> in the isotropic superconductor K<sub>3</sub>C<sub>60</sub>
Alkali-doped fullerides are strongly correlated organic superconductors that exhibit high transition temperatures, exceptionally large critical magnetic fields and a number of other unusual properties. The proximity to a Mott insulating phase is thought to be a crucial ingredient of the underlying physics, and may also affect precursors of superconductivity in the normal state above T. We report on the observation of a sizeable magneto-thermoelectric (Nernst) effect in the normal state of KC, which displays the characteristics of superconducting fluctuations. The anomalous Nernst effect emerges from an ordinary quasiparticle background below a temperature of 80K, far above T = 20K. At the lowest fields and close to T, the scaling of the effect is captured by a model based on Gaussian fluctuations. The temperature up to which we observe fluctuations is exceptionally high for a three-dimensional isotropic system, where fluctuation effects are usually suppressed
Giant resonant enhancement for photo-induced superconductivity in KC
Photo-excitation at terahertz and mid-infrared frequencies has emerged as a
new way to manipulate functionalities in quantum materials, in some cases
creating non-equilibrium phases that have no equilibrium analogue. In
KC, a metastable zero-resistance phase was documented with optical
properties and pressure dependences compatible with non-equilibrium high
temperature superconductivity. Here, we report the discovery of a dominant
energy scale for this phenomenon, along with the demonstration of a giant
increase in photo-susceptibility near 10 THz excitation frequency. At these
drive frequencies a metastable superconducting-like phase is observed up to
room temperature for fluences as low as ~400 . These findings shed
light on the microscopic mechanism underlying photo-induced superconductivity.
They also trace a path towards steady state operation, currently limited by the
availability of a suitable high-repetition rate optical source at these
frequencies.Comment: 35 pages, 13 figures, including supplementar
The structural and electronic evolution of Li4C60 through the polymerâmonomer transformation
In this paper, we combine synchrotron powder x-ray diffraction, 7Li nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) experiments to study the structural evolution of Li4C60 and how its electronic ground state depends on the crystal symmetry. The compound in the two-dimensional polymer phase with mixed interfullerene bonding motifs is a band gap insulator. EPR, however, reveals the presence of intrinsic centers originating from broken C60âC60 bonds and local Li off-stoichiometry that create states in the band gap and account for the complex temperature dependence of the spin susceptibility as well as the residual temperature dependence of the 7Li NMR shift. At low temperatures, the Li+ ions are statically disordered on the 7Li NMR timescale. The observed 7Li NMR line narrowing at T>200 K is ascribed to the Li+ diffusion dynamics and above room temperature the polymer phase is already a good ionic conductor. Heating the sample to temperatures above ~470 K results in gradual depolymerization to the metallic monomer fcc high temperature structure. The transformation is first order and polymer as well as monomer phases coexist over a broad temperature interval (130 K)
Persistence of antiferromagnetic order upon La substitution in the Mott insulator CaRuO
The chemical and magnetic structures of the series of compounds
CaLaRuO [, , , ] have been
investigated using neutron diffraction and resonant elastic x-ray scattering.
Upon La doping, the low temperature S-Pbca space group of the parent compound
is retained in all insulating samples [], but with significant
changes to the atomic positions within the unit cell. These changes can be
characterised in terms of the local RuO octahedral coordination: with
increasing doping the structure, crudely speaking, evolves from an orthorhombic
unit cell with compressed octahedra to a quasi-tetragonal unit cell with
elongated ones. The magnetic structure on the other hand, is found to be
robust, with the basic , -axis antiferromagnetic order of the
parent compound preserved below the critical La doping concentration of
. The only effects of La doping on the magnetic structure are to
suppress the A-centred mode, favouring the B mode instead, and to reduce the
N\'{e}el temperature somewhat. Our results are discussed with reference to
previous experimental reports on the effects of cation substitution on the
Mott insulator CaRuO, as well as with regard to theoretical
studies on the evolution of its electronic and magnetic structure. In
particular, our results rule out the presence of a proposed ferromagnetic
phase, and suggest that the structural effects associated with La substitution
play an important role in the physics of the system.Comment: 10 pages, 9 figure
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