High field NMR in strongly correlated low-dimensional electronic systems
Rather than increasing the resolution and sensitivity in NMR spectroscopy, the use of high-field NMR in Solid State Physics at the LNCMI is aimed at studying magnetic field induced phenomena. Examples are :
1) Low dimensional (D) quantum spin systems in which the magnetic field can induce phase transitions by closing the singlet-triplet gap : molecular rings (zero-dimensional systems), spin-chains (spin-Peierls chain CuGeO3, Haldane chain Y2BaNiO5, diamond chain azurite, Ising chain BaCo2V2O8) and spin-ladders (Cu(Hp)Cl, BPCB, DIMPY, BCPO) and their equivalents (DTN), 2D spin-systems (Shastry-Sutherland compound SrCu2(BO3)2, “Han purple” BaCuSi2O6 and “Pinwheel” Kagome compound Rb2Cu3SnF12).
2) While the principal effect of magnetic field is to suppress superconductivity, field can also induce superconductivity by e.g. the Jaccarino-Peter compensation effect, or modify it as in the Fulde-Ferrel-Larkin-Ovchinnikov phase. A detailed microscopic characterization of these states by NMR remains to be done. As regards high temperature superconductors and cobaltates, our studies are rather focused on characterizing phase diagrams of these compounds and revealing their antiferromagnetism ; in these studies high field is used to facilitate NMR technique.
3) The quantum-Hall effect where magnetic field ensures the quantization of kinetic energy of 2D electrons.
4) Dimensionality cross-over in low-D organic conductors, e.g., in Bechgaard salts, where strong magnetic field reduces the transversal motion of electrons and converts the dimensionality from 2D to 1D. Furthermore, at low temperature it induces a cascade of “field induced spin density waves”.
In all these cases we study strongly interacting fermionic systems in which the magnetic field is the pertinent parameter, revealing rich physics.