P. Wzietek

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

Abstract The pressure vs temperature phase diagram of the layered organic superconducting family κ-(BEDT-TTF)2X is accurately determined by investigating the 1 H NMR and AC susceptibility properties of the anion substituted compound X=Cu[N(CN)2]Cl under helium gas pressure. A first order boundary between antiferromagnetism and superconductivity is established which is found to evolve into a first order metal–insulator transition line in the paramagnetic domain. This latter transition line is in turn ending in a critical point above which a mere crossover is retrieved. The whole phase diagram features a point-like region at which metallic, insulating, antiferromagnetic and unconventional superconducting phases all meet.

Examination of the antiferromagnetic ground state of the bechgaard salts

Abstract Transport and 1H NMR experiments show that the magnetic structure and fluctuation spectrum within the antiferromagnetic phase of (TMTTF)2Br undergoes distinctive changes as the pressure is varied by just 10 kbar. Some of the results are interpreted as a consequence of increasing the effective dimensionality of the system along with the pressure-induced suppression of a Mott-Hubbard charge gap at the Fermi surface.

NMR and EPR Approaches to Magnetic Properties of (TMTTF)2Br

We present 13C spin-lattice relaxation time and EPR susceptibility measurements on selectively enriched single crystals of (TMTTF)2Br over a wide temperature domain. The divergence of T1-1 at TN??15?K is associated with the antiferromagnetic transition and 3D critical effects. The EPR susceptibility and T1-1 are shown to satisfy 1D dynamic scaling for the existence of 1D paramagnons and antiferromagnetic correlations up to room temperature. Applied to bromide salt which undergoes a Mott-Hubbard localization under 100?K, our approach thus confirms the spin-charge separation predicted for the 1D correlated Electron. gas model. These findings are in close similarity with the behavior of T1-1 found previously in the 1D antiferromagnet (TMTTF)2PF6.

NMR in the (BEDT)2X organic superconductors

Abstract We review recent NMR studies of a layered organic superconductor of the (BEDT) 2 X family. First the normal state prop erties are discussed. The central issue is the importance of the interplay between antiferromagnetism and superconductivity in these materials demonstrated by the observation of a pseudogap developing prior to the superconducting transition. The use of high pressure to tune the amplitude of AF fluctuations enables us to draw a general phase diagram of the family. In the second part the NMR experiments in the superconducting state are presented. One of the fundamental questions addressed by these experiments is the symmetry of the order parameter related to the pairing mechanism. The most spectacular NMR measurements are those performed in the lock-in state, which give access to the density of superconducting excitations at low energy. These experiments suggest that the pairing state may be unconventional featuring an anisotropic superconducting gap, possibly with nodes.

C60 doped with organic cations: Magnetic resonance measurements

Abstract “Doped” C60 compounds were synthesized by the electrochemical reduction. We stabilize the reduced state of the fullerene using two organic cations of a different size: N-methyl-N-ethyl-morpholine+ and acridizine+. The products of these two reactions precipitated as a powder on the platinum electrode and in the solution. The electrical conductivity of pressed pellets was below 10−8 S/cm for both compounds. We employed the Electron Paramagnetic Resonance and 13C Nuclear Magnetic Resonance measurements as the first characterization of these materials. The samples showed an ERP signal with the line width of 0.5 mT, which is due to the radical anion C60·−. We used broadband and Magic Angle Spinning techniques to obtain the 13C NMR spectra. The main line in the most of these spectra is the single line of the radical anion of the C60 at about 142 ppm. This line appears approximately at the same position as in the pure C60 but it is broader. It suggests that the reorientation of fullerene molecules is disturbed already at room temperature.

NMR spectroscopy in K3C60 as a function of temperature and pressure

Abstract We report the temperature and pressure dependence of 13 C nuclear spin-lattice relaxation time T 1 on K 3 C 60 in the normal state. Within the BCS electron-phonon pairing mechanism, we estimate the coupling strength and the relevant phonon mode involved to account for superconductivity.

Two-dimensional organic superconductors studied by NMR under pressure

The (BEDT)2X family of layered superconductors is one of the largest among nearly 50 organic superconductors currently known. One of the advantages of the organic compounds as prototype materials for studying the mechanisms of superconductivity is their relatively high sensitivity to hydrostatic pressure. We review recent NMR studies of these compounds using NMR under liquid and gas pressure. We focus on the low temperature part of the phase diagram where the physics is controlled by electronic correlations leading to a competition between magnetism and superconductivity. This interplay between different ground states is shown by the observation of a pseudo-gap and antiferromagnetic fluctuations and can be finely tuned by the application of pressure. Using a gas pressure system gives the unique possibility of sweeping the pressure at low temperature. Recently we used this technique to study the AF-SC boundary and established the existence of a first order transition line between the superconducting and antiferromagnetic states.

Singular behavior in the pressure-tuned competition between Spin-Peierls and antiferromagnetic ground states of (TMTTF)2PF6.

Abstract The ground state of the Mott-Hubbard insulator (TMTTF) 2 PF 6 is known to vary from Spin-Peierls (SP) at ambient pressure to antiferromagnetic (AF) with increasing pressure. Near 0.9GPa, the phase boundary of both SP and AF drops singularly to less than 9K. We interpret this result as the system is pressure-tuned near a quantum critical point.

NMR and EPR analysis of spin and lattice fluctuations in the two-chain family of organic conductors Perylene2 [M(mnt)2 (M= Pt, Au)

Abstract The temperature dependence of the 1 H nuclear spinn lattice relaxation rate of the two-chain organic compounds Per 2 [M(mnt) 2 ] (M=Au,Pt) is presented. For M=Pt, the dominant contribution to relaxation is the interstack dipolar coupling with the localised spins of the Pt(mnt) 2 chains. The scaling relation T 1 −1 α T χ s (T) is obeyed in the entire temperature range. The T 1 −1 temperature dependence for the Au compound is characteristic of itinerant electrons with deviations to the Korringa law in the high temperature range. The previously reported data on EPR for Per 2 [Pt(mnt) 2 ] is also analysed and the possible mechanisms of the spin-Peirls like transition observed at 7 K are discussed.