Contrasting spin dynamics between underdoped and overdoped Ba(Fe1-xCox)2As2.

We report the first NMR investigation of spin dynamics in the overdoped nonsuperconducting regime of Ba(Fe1-xCox)2As2 up to x=0.26. We demonstrate that the absence of interband transitions with large momentum transfer Q{AF} approximately (pi/a,0) between the hole and electron Fermi surfaces results in complete suppression of antiferromagnetic spin fluctuations for x greater than or approximately 0.

Why does undoped FeSe become a high-Tc superconductor under pressure?

Unlike the parent phases of the iron-arsenide high-Tc superconductors, undoped FeSe is not magnetically ordered and exhibits superconductivity with Tc approximately 9 K. Equally surprising is the fact that applied pressure dramatically enhances the modest Tc to approximately 37 K. We investigate the electronic properties of FeSe using 77Se NMR to search for the key to the superconducting mechanism.

59Co NMR evidence for charge ordering below T_(CO) approximately 51 K in Na0.5CoO2.

The CoO2 layers in NaxCoO2 may be viewed as a spin S=1/2 triangular-lattice doped with charge carriers. The underlying physics of the cobaltates is very similar to that of the high T_(c) cuprates. We will present unequivocal 59Co NMR evidence that below T_(CO) approximately 51 K, the insulating ground state of the itinerant antiferromagnet Na0.

Superconductivity on the border of itinerant-electron ferromagnetism in UGe2.

The absence of simple examples of superconductivity adjoining itinerant-electron ferromagnetism in the phase diagram has for many years cast doubt on the validity of conventional models of magnetically mediated superconductivity. On closer examination, however, very few systems have been studied in the extreme conditions of purity, proximity to the ferromagnetic state and very low temperatures required to test the theory definitively. Here we report the observation of superconductivity on the border of ferromagnetism in a pure system, UGe2, which is known to be qualitatively similar to the classic d-electron ferromagnets.