We have synthesized and characterized different stable phases of sodium cobaltates Na$_{x}$CoO$_{2}$ with sodium content $0.65<x<0.80$. We demonstrate that $^{23}$Na NMR allows to determine the difference in the susceptibility of the phases and revea
ls the presence of Na order in each phase. $^{59}$Co NMR experiments give clear evidence that Co charge disproportionation is a dominant feature of Na cobaltates. Only a small fraction ($approx$ 25%) of cobalts are in a non-magnetic Co$^{3+}$ charge state whereas electrons delocalize on the other cobalts. The magnetic and charge properties of the different Co sites are highly correlated with each other as their magnetic shift $K_{ZZ}$ scales linearly with their quadrupolar frequency $nu_Q$. This reflects the fact that the hole content on the Co orbitals varies from site to site. The unusual charge differentiation found in this system calls for better theoretical understanding of the incidence of the Na atomic order on the electronic structures of these compounds.
Large pulsed magnetic fields up to 60 Tesla are used to suppress the contribution of superconducting fluctuations (SCF) to the ab-plane conductivity above Tc in a series of YBa2Cu3O6+x single crystals. The fluctuation conductivity is found to vanish
nearly exponentially with temperature, allowing us to determine precisely the field Hc(T) and the temperature Tc above which the SCFs are fully suppressed. Tc is always found much smaller than the pseudogap temperature. A careful investigation near optimal doping shows that Tc is higher than the pseudogap T*, which indicates that the pseudogap cannot be assigned to preformed pairs. For nearly optimally doped samples, the fluctuation conductivity can be accounted for by gaussian fluctuations following the Ginzburg-Landau scheme. A phase fluctuation contribution might be invoked for the most underdoped samples in a T range which increases when controlled disorder is introduced by electron irradiation. Quantitative analysis of the fluctuating magnetoconductance allows us to determine the critical field Hc2(0) which is found to be quite similar to Hc(0) and to increase with hole doping. Studies of the incidence of disorder on both Tc and T* enable us to propose a three dimensional phase diagram including a disorder axis, which allows to explain most observations done in other cuprate families.
Former extensive studies of superconductivity in the textit{A}$_{3}$C$_{60}$ 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 Cs$_{3}$C$_{60}$ which exhibit a transition with pressure from a Mott insulator (MI) to a superconducting (SC) state clearly re-opens that question. Using pressure ($p$) as a single control parameter of the C$_{60}$ balls lattice spacing, one can now study the progressive evolution of the SC properties when the electronic correlations are increased towards the critical pressure $p_{c}$ of the Mott transition. We have used $^{13}$C and $^{133}$Cs NMR measurements on the cubic phase A15-Cs$_{3}$C$_{60}$ just above $p_{c}=5.0(3)$ kbar, where the SC transition temperature $T_{c}$ displays a dome shape with decreasing cell volume. From the $T$ dependence below $T_{c}$ of the nuclear spin lattice relaxation rate $(T_{1})^{-1}$ we determine the electronic excitations in the SC state, that is $2Delta$, the SC gap value. We find that $2Delta $ increases with decreasing $p$ towards $p_{c}$, where $T_{c}$ decreases on the SC dome, so that $2Delta /k_{B}T_{c}$ 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 $T_{c}$ values, as proposed by theoretical models taking the electronic correlations as a key ingredient.
The experimental investigations done in our paper Phys.Rev.B84,014522(2011) allowed us to establish that the superconducting fluctuations (SCF) always die out sharply with increasing T. But contrary to the claim done in the comment of Ramallo et al.,
this sharp cutoff of SCF measured in YBa2Cu3O{6+x} depends on hole doping and/or disorder. So our data cannot be used to claim for a universality of the extended gaussian Ginzburg Landau theory proposed by the authors of the comment. Furthermore, to explain quantitatively our data near optimal doping using this model they need to consider that fluctuations in the two CuO2 planes of a bilayer are totally decoupled, which is not physically well justified. On the contrary a consistent interpretation of all our data (paraconductivity, Nernst effect and magnetoresistance) has been done by considering that the coupling between the two layers of the unit cell is dominant at least up to 1.1Tc.
Large pulsed magnetic fields up to 60 Tesla are used to suppress the contribution of superconducting fluctuations (SCF) to the ab-plane conductivity above Tc in a series of YBa2Cu3O(6+x). These experiments allow us to determine the field Hc(T) and th
e temperature Tc above which the SCFs are fully suppressed. A careful investigation near optimal doping shows that Tc is higher than the pseudogap temperature T*, which is an unambiguous evidence that the pseudogap cannot be assigned to preformed pairs. Accurate determinations of the SCF contribution to the conductivity versus temperature and magnetic field have been achieved. They can be accounted for by thermal fluctuations following the Ginzburg-Landau scheme for nearly optimally doped samples. A phase fluctuation contribution might be invoked for the most underdoped samples in a T range which increases when controlled disorder is introduced by electron irradiation. Quantitative analysis of the fluctuating magnetoconductance allows us to determine the critical field Hc2(0) which is found to be be quite similar to Hc(0) and to increase with hole doping. Studies of the incidence of disorder on both Tc and T* allow us to propose a three dimensional phase diagram including a disorder axis, which allows to explain most observations done in other cuprate families.
We report a complete set of $^{59}$Co NMR data taken on the $x=2/3$ phase of sodium cobaltates Na$_{x}$CoO$_{2}$, for which we have formerly established the in plane Na ordering and its three dimensional stacking from a combination of symmetry argume
nts taken from Na and Co NQR/NMR data. Here we resolve all the parameters of the Zeeman and quadrupolar Hamiltonians for all cobalt sites in the unit cell and report the temperature dependencies of the NMR shift and spin lattice relaxation $T_{1}$ data for these sites. We confirm that three non-magnetic Co$^{3+}$ (Co1) are in axially symmetric positions and that the doped holes are delocalized on the nine complementary magnetic cobalt sites (Co2) of the atomic unit cell. The moderately complicated atomic structure resumes then in a very simple electronic structure in which the electrons delocalize on the Co2 kagome sublattice of the triangular lattice of Co sites. The observation of a single temperature dependence of the spin susceptibilities indicates that a single band picture applies, and that the magnetic properties are dominated by the static and dynamic electronic properties at the Co2 sites. We evidence that they display a strong in plane electronic anisotropy initially unexpected but which accords perfectly with an orbital ordering along the kagome sublattice organization. These detailed data should now permit realistic calculations of the electronic properties of this compound in order to determine the incidence of electronic correlations.
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.
We have used pulsed magnetic fields up to 60Tesla to suppress the contribution of superconducting fluctuations(SCF)to the conductivity above Tc in a series of YBa2Cu3O6+x from the deep pseudogapped state to slight overdoping. Accurate determinations
of the SCF conductivity versus temperature and magnetic field have been achieved. Their joint quantitative analyses with respect to Nernst data allow us to establish that thermal fluctuations following the Ginzburg-Landau(GL) scheme are dominant for nearly optimally doped samples. The deduced coherence length xi(T) is in perfect agreement with a gaussian (Aslamazov-Larkin) contribution for 1.01Tc<T<1.2Tc. A phase fluctuation contribution might be invoked for the most underdoped samples in a T range which increases when controlled disorder is introduced by electron irradiation. For all dopings we evidence that the fluctuations are highly damped when increasing T or H. The data permits us to define a field Hc^prime and a temperature Tc^prime above which the SCF are fully suppressed. The analysis of the fluctuation magnetoconductance in the GL approach allows us to determine the critical field Hc2(0). The actual values of Hc^prime(0) and Hc2(0) are found quite similar and both increase with hole doping. These depairing fields, which are directly connected to the magnitude of the SC gap, do therefore follow the Tc variation which is at odds with the sharp decrease of the pseudogap T* with increasing hole doping. This is on line with our previous evidence that T* is not the onset of pairing. We finally propose a three dimensional phase diagram including a disorder axis, which allows to explain most peculiar observations done so far on the diverse cuprate families.
We report 13C nuclear magnetic resonance measurements on single wall carbon nanotube (SWCNT) bundles. The temperature dependence of the nuclear spin-lattice relaxation rate, 1/T1, exhibits a power-law variation, as expected for a Tomonage-Luttinger l
iquid (TLL). The observed exponent is smaller than that expected for the two band TLL model. A departure from the power law is observed only at low T, where thermal and electronic Zeeman energy merge. Extrapolation to zero magnetic field indicates gapless spin excitations. The wide T range on which power-law behavior is observed suggests that SWCNT is so far the best realization of a one-dimensional quantum metal.
Using 23Na NMR measurements on sodium cobaltates at intermediate dopings (0.44<=x<=0.62), we establish the qualitative change of behavior of the local magnetic susceptibility at x*=0.63-0.65, from a low x Pauli-like regime to the high x Curie-Weiss r
egime. For 0.5<=x<=0.62, the presence of a maximum T* in the temperature dependence of the susceptibility shows the existence of an x-dependent energy scale. T_1 relaxation measurements establish the predominantly antiferromagnetic character of spin correlations for x<x*. This contradicts the commonly assumed uncorrelated Pauli behavior in this x range and is at odds with the observed ferromagnetic correlations for x>x*. It is suggested that at a given x the ferromagnetic correlations might dominate the antiferromagnetic ones above T*. From 59Co NMR data, it is shown that moving towards higher x away from x=0.5 results in the progressive appearance of nonmagnetic Co3+ sites, breaking the homogeneity of Co states encountered for x<=0.5. The main features of the NMR-detected 59Co quadrupolar effects, together with indications from the powder x-ray diffraction data, lead us to sketch a possible structural origin for the Co3+ sites. In light of this ensemble of new experimental observations, a new phase diagram is proposed, taking into account the systematic presence of correlations and their x-dependence.
