Gaps and excitations in fullerides with partially filled bands : NMR study of Na2C60 and K4C60

Abstract

We present an NMR study of Na2C60 and K4C60, two compounds that are related by electron-hole symmetry in the C60 triply degenerate conduction band. In both systems, it is known that NMR spin-lattice relaxation rate (1/T1) measurements detect a gap in the electronic structure, most likely related to singlet-triplet excitations of the Jahn-Teller distorted (JTD) C60^{2-} or C60^{4-}. However, the extended temperature range of the measurements presented here (10 K to 700 K) allows to reveal deviations with respect to this general trend, both at high and low temperatures. Above room temperature, 1/T1 deviates from the activated law that one would expect from the presence of the gap and saturates. In the same temperature range, a lowering of symmetry is detected in Na2C60 by the appearance of quadrupole effects on the 23Na spectra. In K4C60, modifications of the 13C spectra lineshapes also indicate a structural modification. We discuss this high temperature deviation in terms of a coupling between JTD and local symmetry. At low temperatures, 1/T${}_1$T tends to a constant value for Na2C60, both for 13C and 23Na NMR. This indicates a residual metallic character, which emphasizes the proximity of metallic and insulting behaviors in alkali fullerides.

12 pages, 13 figures

References in corpus (4)

• Role of dynamic Jahn-Teller distortions in Na2C60 and Na2CsC60 studied by NMR
• Persistence of molecular excitations in metallic fullerides and their role in a possible metal to insulator transition at high temperatures
• Coexistence of spin-singlets and metallic behavior in simple cubic CsC60
• Ordered low-temperature structure in K4C60 detected by infrared spectroscopy

Cited by in corpus (22)

• Modeling the Unconventional Superconducting Properties of Expanded A${}_3$C${}_{60}$ Fullerides
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• NMR study of the Mott transitions to superconductivity in the two Cs_3C_60 phases
• Enhancing superconductivity in A3C60 fullerides
• Static and dynamic Jahn-Teller effect in the alkali metal fulleride salts A4C60 (A = K, Rb, Cs)
• Rotationally-invariant slave-bosons for Strongly Correlated Superconductors
• NMR study of the Superconducting gap variation near the Mott transition in Cs${}_3$C${}_{60}$
• Spin Dynamics at the Mott Transition and in the Metallic State of the Cs_{3}C_{60} Superconducting Phases
• Persistence of molecular excitations in metallic fullerides and their role in a possible metal to insulator transition at high temperatures
• Coexistence of spin-singlets and metallic behavior in simple cubic CsC60
• Ultralong spin lifetime in light alkali atom doped graphene
• Phase segregation on the nanoscale in Na2C60
• Reversible phase transformation and doubly-charged anions at the surface of simple cubic RbC60
• Multipurpose High Frequency Electron Spin Resonance Spectrometer for Condensed Matter Research
• Mott transition in the A15 phase of Cs${}_3$C${}_{60}$: absence of pseudogap and charge order
• Dynamical Jahn-Teller effect of fullerene anions
• Metal-to-insulator evolution in (NH3)xNaK2C60: an NMR study
• Multiplet Structures of Charged Fullerenes
• The degenerate 3-band Hubbard model with "anti-Hund's rule" interactions; a model for AxC60
• Influence of local fullerene orientation on the electronic properties of A3C60 compounds
• Jahn-Teller effect versus Hund's rule coupling in C60N-
• HTSC Cuprate Phase Diagram Using a Modified Boson-Fermion-Gossamer Model Describing Competing Orders, a Quantum Critical Point and Possible Resonance Complex