Mott Transition in the A15 Phase of Cs_{3}C_{60}: Absence of a Pseudogap and Charge Order.

We present a detailed NMR study of the insulator-to-metal transition induced by an applied pressure p in the A15 phase of Cs_{3}C_{60}. We evidence that the insulating antiferromagnetic (AFM) and superconducting (SC) phases coexist only in a narrow p range. At fixed p, in the metallic state above the SC transition T_{c}, the ^{133}Cs and ^{13}C NMR spin-lattice relaxation data are seemingly governed by a pseudogaplike feature. We prove that this feature, also seen in the ^{133}Cs NMR shift data, is rather a signature of the Mott transition which broadens and smears out progressively for increasing (p,T). The analysis of the variation of the quadrupole splitting ν_{Q} of the ^{133}Cs NMR spectrum precludes any cell symmetry change at the Mott transition and only monitors a weak variation of the lattice parameter. These results open an opportunity to consider theoretically the Mott transition in a multiorbital three-dimensional system well beyond its critical point.

NMR study of the superconducting gap variation near the Mott transition in Cs3C60.

Former extensive studies of superconductivity in the A3C60 compounds, where A is an alkali metal, have led one to consider that Bardeen-Cooper-Schrieffer 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 Cs3C60 which exhibit a transition with pressure from a Mott insulator (MI) to a superconducting (SC) state clearly reopens that question. Using pressure (p) as a single control parameter of the C60 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 13C and 133Cs NMR measurements on the cubic phase A15-Cs3C60 just above p(c)=5.0(3) kbar, where the SC transition temperature Tc displays a dome shape with decreasing cell volume. From the T dependence below T(c) of the nuclear spin lattice relaxation rate (T1)(-1) we determine the electronic excitations in the SC state, that is 2Δ, the gap value. The latter is found to be largely enhanced with respect to the Bardeen-Cooper-Schrieffer value established in the case of dense A3C60 compounds. It even increases slightly with decreasing p towards p(c), where T(c) decreases on the SC dome, so that 2Δ/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 superconductivity. They rather suggest that repulsive electron interactions might even reinforce elecron-phonon superconductivity, being then partly responsible for the large T(c) values, as proposed by theoretical models taking the electronic correlations as a key ingredient.

Enhanced NMR relaxation of Tomonaga-Luttinger liquids and the magnitude of the carbon hyperfine coupling in single-wall carbon nanotubes.

Recent transport measurements [Churchill et al. Nature Phys. 5, 321 (2009)] found a surprisingly large, 2-3 orders of magnitude larger than usual (13)C hyperfine coupling (HFC) in (13)C enriched single-wall carbon nanotubes. We formulate the theory of the nuclear relaxation time in the framework of the Tomonaga-Luttinger liquid theory to enable the determination of the HFC from recent data by Ihara et al. [Europhys. Lett. 90, 17,004 (2010)]. Though we find that 1/T(1) is orders of magnitude enhanced with respect to a Fermi-liquid behavior, the HFC has its usual, small value. Then, we reexamine the theoretical description used to extract the HFC from transport experiments and show that similar features could be obtained with HFC-independent system parameters.

NMR study of the Mott transitions to superconductivity in the two Cs3C60 phases.

We report a NMR and magnetometry study on the expanded intercalated fulleride Cs3C60 in both its A15 and face centered cubic structures. NMR allowed us to evidence that both exhibit a first-order Mott transition to a superconducting state, occurring at distinct critical pressures p{c} and temperatures T{c}. Though the ground state magnetism of the Mott phases differs, their high T paramagnetic and superconducting properties are found similar, and the phase diagrams versus unit volume per C60 are superimposed. Thus, as expected for a strongly correlated system, the interball distance is the relevant parameter driving the electronic behavior and quantum transitions of these systems.

Inequivalent routes across the Mott transition in V2O3 explored by X-ray absorption.

The changes in the electronic structure of V2O3 across the metal-insulator transition induced by temperature, doping, and pressure are identified using high resolution x-ray absorption spectroscopy at the V pre-K edge. Contrary to what has been taken for granted so far, the metallic phase reached under pressure is shown to differ from the one obtained by changing doping or temperature. Using a novel computational scheme, we relate this effect to the role and occupancy of the a{1g} orbitals. This finding unveils the inequivalence of different routes across the Mott transition in V2O3.

Asymmetric addition of pyridine to 1,4-bis(ethynyl)bicyclo[2.2.2]octane: dissymmetrical rotator ligands illustrated in dpppPt(II) and [Re6Se8]2+ complexes and an amphidynamic hydrogen bonded framework solid.

A polar function like pyridine increases the polarity difference between mono- and disubstituted 1,4-bis(trimethylsilylethynyl)bicyclo[2.2.2]octane cores allowing for their ready separation by chromatography, affording two dissymmetrical ligands illustrated in metal-rotor hybrids like a corner-shaped mononuclear complex, an octahedral hexanuclear metal complex, and a framework solid with a rather slow rotator.

NMR evidence for gapped spin excitations in metallic carbon nanotubes.

We report on the spin dynamics of 13C isotope enriched inner walls in double-wall carbon nanotubes using 13C nuclear magnetic resonance. Contrary to expectations, we find that our data set implies that the spin-lattice relaxation time (T1) has the same temperature (T) and magnetic field (H) dependence for most of the inner-wall nanotubes detected by NMR. In the high-temperature regime (T approximately > or = 150 K), we find that the T and H dependence of 1/T1T is consistent with a 1D metallic chain. For T approximately < or = 150 K we find a significant increase in 1/T1T with decreasing T, followed by a sharp drop below approximately = 20 K. The data clearly indicate the formation of a gap in the spin excitation spectrum, where the gap value 2delta approximately = 40 K (congruent to 3.7 meV) is H independent.

Giant spin canting in the S=1/2 antiferromagnetic chain [CuPM(NO3)2(H2O)2]n observed by 13C-NMR.

We present a combined experimental and theoretical study on copper pyrimidine dinitrate [CuPM(NO3)2(H2O)2]n, a one-dimensional S=1/2 antiferromagnet with alternating local symmetry. From the local susceptibility measured by NMR at the three inequivalent carbon sites in the pyrimidine molecule we deduce a giant spin canting, i.e., an additional staggered magnetization perpendicular to the applied external field at low temperatures. The magnitude of the transverse magnetization, the spin canting of (52+/-4) degrees at 10 K and 9.3 T, and its temperature dependence are in excellent agreement with exact diagonalization calculations.

Universality and critical behavior at the Mott transition.

We report conductivity measurements of Cr-doped V2O3 using a variable pressure technique. The critical behavior of the conductivity near the Mott insulator to metal critical endpoint is investigated in detail as a function of pressure and temperature. The critical exponents are determined, as well as the scaling function associated with the equation of state. The universal properties of a liquid-gas transition are found. This is potentially a generic description of the Mott critical endpoint in correlated electron materials.

Mott transition, antiferromagnetism, and unconventional superconductivity in layered organic superconductors.

The phase diagram of the organic superconductor kappa-(ET)2Cu[N(CN)2]Cl has been accurately measured from 1H NMR and ac susceptibility techniques under helium gas pressure. The domains of stability of antiferromagnetic and superconducting orders in the pressure vs temperature plane have been determined. Both phases overlap through a first-order boundary that separates two regions of inhomogeneous phase coexistence. The boundary curve merges with the first-order line of the metal-insulator transition which ends with a critical point at higher temperature. The whole phase diagram features a point-like region where metallic, insulating, antiferromagnetic, and non-s-wave superconducting phases all meet.

Mott transition and transport crossovers in the organic compound kappa-(BEDT-TTF)2Cu[N(CN)2]Cl.

We have performed in-plane transport measurements on the two-dimensional organic salt kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl. A variable (gas) pressure technique allows for a detailed study of the changes in conductivity through the insulator-to-metal transition. We identify four different transport regimes as a function of pressure and temperature (corresponding to insulating, semiconducting, "bad metal," and strongly correlated Fermi-liquid behaviors). Marked hysteresis is found in the transition region, which displays complex physics that we attribute to strong spatial inhomogeneities. Away from the critical region, good agreement is found with a dynamical mean-field calculation of transport properties using the numerical renormalization group technique.