F. Creuzet

A 13C NMR study of the interplay between the spin-peierls and antiferromagnetic ground states in (TMTTF)2PF6 under pressure

Abstract Using 13 C spin-lattice relaxation rate (T 1 −1 measurements, we are able to explicit the microscopic conditions under which the electron-phonon system of (TMTTF) 2 PF 6 compound presents either a spin-Peierls (SP) ground state at ambient pressure or an antiferromagnetic (AF) phase at 13 kbar. At 1 bar, there are three regimes of relaxation precursor to the SP transition. At T > 65, non-singular uniform correlations give the dominant contribution to T 1 −1 (T 1 −1 ∝ TX s 2 ) and below 65 K, one-dimensional AF correlations give strongly enhanced T 1 −1 ( T 1 −1 ≌ cst ) whereas below 40 K this enhancement is damped by the SP pseudogap. The EPR signal clearly shows a decrease of 35% of the density of states at the Fermi level due to this SP pseudogap. In the presence of a non-magnetic gap below the SP critical point, T 1 −1 and the ER susceptibility show a similar activated behaviour. At 13 kbar, there is no pseudo-gap effect and the relaxation is dominated by strong 1D antiferromagnetic correlations below 75 K T 1 −1 = cst) down to the critical regime where T 1 −1 diverges as we approach the AF critical temperature T N ∼ 17 K following the classical law T 1 −1 ∼|T− T N | −1 2 .

Proton NMR Relaxation in the High-Tc Organic Superconductor β-(BEDT-TTF)2I3

We report the measurement of a striking enhancement of the proton spin-lattice relaxation rate occurring at the critical temperature in the high-Tc superconducting state of β-(BEDT-TTF)2I3. The T1-1 behaviour is attributed to a logarithmic critical singularity whose amplitude is enhanced by the low-dimensional character of these conductors.

The High-Tc Superconducting State of β-(BEDT-TTF)2I3 at Atmospheric Pressure: Bulk Superconductivity and Metastability

The AC susceptibility study of β-(BEDT-TTF)2I3 reveals that the high-Tc metastable superconducting state which can be stabilized at atmospheric pressure after a particular pressure-temperature cycling procedure, exhibits bulk superconductivity resembling very closely that of the high-Tc state, which is stabilized above 1 kbar. Annealing experiments show that the high-Tc state remains stable at low temperature as long as the annealing temperature does not exceed 125 K.

Pressure Induced Magnetic State in (TMTTF)2PF6

Abstract We present 1H NMR relaxation data on (TMTTF)2PF6. The low temperature insulating state is shown to be non magnetic at ambient pressure and antiferromagnetic (SDW) at 13kbar.

Spin-peierls and antiferromagnetic transition temperatures in (TMTTF)2PF6: EPR and NMR measurements and theory

Abstract We report on EPR measurements on (TMTTF) 2 PF 6 which address the evolution of the spin-Peierls transition temperature T c as a function of pressure. The line intensity was used as a probe of the magnetic susceptibility, and therefore of the density of states at the Fermi level. This density drops in the spin-Peierls regime. The extrapolated onset of the gap is used as a measurement of T c . Using these results and previous NMR data on the antiferromagnetic transition temperature, a complete phase diagram is presented. It will be explained using a model involving an interacting electron gas coupled to acoustic phonons. The various parameters are obtained from available experimental results while their pressure and temperature dependence is suggested as a theoretical model. The interchain interactions (exchange and charge) are deduced from a fit of the transition temperature at 0 and 13 kbar. A very satisfactory phase diagram is deduced.

ELECTRON-ELECTRON INTERACTION IN ORGANIC CONDUCTORS : AN NMR APPROACH

Abstract A quantitative analysis of the temperature and pressure profiles of the 77Se and 13C nuclear spin-lattice relaxation time T1 is used to determine the amplitude of the short range electron-electron coupling constant in the organic conductors (TMTSF)2PF6, (TMTTF)2PF6 and (TMTTF)2Br. The starting part of the analysis is the observed 1D relation T1−1 = COTχS2 + C1 between the relaxation rate and the uniform susceptibility s in the presence of spin fluctuations from which one one can extract the temperature dependence of the magnetic spin susceptibility under pressure. The data are corrected in order to fit it with the constant volume theoretical expression for S and an intermediate value of the backward scattering diffusion constant (g1) is found for both families.

TWO SUPERCONDUCTING PHASES IN THE ORGANIC CONDUCTOR: β-(BEDT-TTF)2I3

Abstract We have investigated the nature and the stability of the high-T c (∼8 K) and low-T c (∼1.5 K)superconducting phases (respectively called β-H and β-L) of the organic conductor β-(BEDT-TTF) 2 I 3 , First, by resistivity measurements and following a well-defined temperature-pressure cycling process, we show that the β-H phase is stabilized at ambient pressure and low temperature, exhibiting a very sharp and complete superconducting transition at 8.1 K and 1 bar. Secondly, an AC-susceptibility study reveals that this state is stable and exhibits bulk superconductivity at high T c and ambient P as long as the sample is not warmed above 125 K. Finally, we present NMR results on the 1H spin-lattice relaxation rate behaviour in both the β-H phase and the β-L phase.

A survey of the physics of organic conductors and superconductors

Low dimensional organic conductors exhibit a wide variety of physical properties which is exemplified by the observation of ground states such as Peierls, spin-Peierls, SDW and Superconducting states. We survey the experimental situation with a special emphasis on the family of the one-chain conductors to which the famous (TMTCF)2X series C = S, Se belongs. Although the band picture seems to be a reasonable starting point for the understanding of these materials, interactions such as electron-phonon and electron-electron as well as the amplitude of the Umklapp scattering term and the size of the interchain tunneling coupling must all be taken into account. The amplitude of electron-electron interactions can be derived from a detailed analysis of the temperature dependence of the nuclear spin-lattice relaxation rate data. Coulomb interactions of the order of the bandwidth are found. The interplay between magnetism and superconductivity is governed by the interchain overlap which can be varied under pressure. The possibility of a pairing mechanism based on the interchain exchange of 2kF spin fluctuations is strongly supported by experiments. In addition, a wealth of experimental features suggest the coexistence in the precursor (1-D) regime above long range ordering (magnetic or superconducting) of fluctuations simultaneously magnetic and superconducting. The very fast increase of Tc for organic superconductors which has been observed in the last eight years is very stimulating for the future development of this field.

E.S.R. in the ßH and ßL phases of a β (BEDT-TTF) 2I3 crystal

E.S.R. measurements have been carried out on a single crystal of β (BEDT-TTF) 2I3 where, successively, the sH and sL phases were studied. These phases are essentially characterized by different linewidths at low temperatures. In a large temperature range the spin susceptibility is nearly temperature independent and takes nearly the same values in both phases suggesting that the difference in superconducting Tc is not to be accounted for by different values of the density of states at the Fermi level. Anisotropy effects revealed at low temperature, for both phases, in the angular variation of the ESR signal (lineshape and intensity) are likely to be linked to the marked 2 D anisotropy of the conductivity of this material.

An NMR analysis of magnetic correlations and dimensionality in organic conductors

Abstract We present an analysis of experimental NMR data for a class of quasi one-dimensional (1D) organic conductors, the Bechgaard salts and their sulfur analogs. The temperature dependence of the nuclear relaxation, T 1 −1 , of the sulfur series (TMTTF) 2 X supports the existence of 1D antiferromagnetic fluctuations over a wide range of temperatures. Depending on the relative strength of the coupling to the lattice and on the interchain exchange coupling, these correlations evolve towards either a spin-Peierls or a spin density wave phase. We show that the analysis naturally extrapolates to the (TMTSF) 2 X family. The stabilization of the insulating magnetic phase at low magnetic field is apparently provided by the antiferromagnetic interchain exchange coupling, just as it is in the sulfur series. This provides a unique T-P phase diagram for both systems where the interplay between 1D correlation effects and interchain exchange coupling plays a crucial role.