NMR in the (BEDT)2X organic superconductors
Résumé
We review recent NMR studies of a layered organic superconductor of the (BEDT)2X family. First the normal state properties 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.
Mots clés
- Organic conductors
- Antiferromagnetism
- Electronic density of states
- Energy gap
- High pressure effects
- Nuclear magnetic resonance spectroscopy
- Phase transitions
- Phase diagrams
- Superconducting materials
- Superconducting phase transitions
- Organic superconductors
- Superconducting transition temperature
- Salts