Optimized unconventional superconductivity in a molecular Jahn-Teller metal

Understanding the relationship between the superconducting, the neighboring insulating, and the normal metallic state above Tc is a major challenge for all unconventional superconductors. The molecular A3C60 fulleride superconductors have a parent antiferromagnetic insulator in common with the atom-based cuprates, but here, the C603– electronic structure controls the geometry and spin state of the structural building unit via the on-molecule Jahn-Teller effect. We identify the Jahn-Teller metal as a fluctuating microscopically heterogeneous coexistence of both localized Jahn-Teller–active and itinerant electrons that connects the insulating and superconducting states of fullerides. The balance between these molecular and extended lattice features of the electrons at the Fermi level gives a dome-shaped variation of Tc with interfulleride separation, demonstrating molecular electronic structure control of superconductivity

Low Temperature Magnetic Instabilities in Triply Charged Fulleride Polymers

The electronic properties of the C603- polymer in Na2Rb0.3Cs0.7C60 were studied by X-band and high field (109.056 GHz) ESR. They are characteristic of a strongly correlated quasi-one-dimensional metal down to 45 K. On further cooling, a pseudogap of magnetic origin opens at the Fermi level below 45 K with three-dimensional magnetic ordering occurring below TN≈15K, as confirmed by the observation of an antiferromagnetic resonance mode. The Na2Rb1-xCsxC60 family of polymers offers a unique way to chemically control the electronic properties, as the opening of the gap in this system of predominantly itinerant electrons is an extremely sensitive function of the interchain separation

Magnetic Ordering in the Ammoniated Fulleride (ND)K 3 C 60

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Temperature dependence of antiferromagnetic resonance mode in two-dimensional system Ni-5(TeO3)(4)Br-2

Antiferromangentic resonance (AFMR) in layered Ni-5(TeO3)(4)Br-2 system with a geometrically frustrated Ni2+ (S =1) spin arrangement has been investigated. Temperature dependence of the lowest mode resonant field could be described with a simple two sublattice model, considering different magnetic susceptibilities. The AFMR linewidth follows a power law T-2.8, due to the magnon-magnon scattering processes. (c) 2007 Elsevier B.V. All rights reserved