Naslov
Gaps and excitations in fullerides with partially filled bands: NMR study of Na_2C_60 and K_4C_60

Autori
Brouet, Veronique ; Alloul, Henry ; Garaj, Slaven ; Forro, Laszlo

Izvornik
Physical Review B (1098-0121) 66 (2002);

Vrsta, podvrsta i kategorija rada
Radovi u časopisima, članak, znanstveni

Ključne riječi
NMR; Phase transition; Molecular dynamics; K_4C_60; Na_2C_60; Relaxation; C-13 NMR

Sažetak
We present a nuclear magnetic resonance (NMR) study of Na_2C_60 and K_4C_60, two compounds that are related by electron-hole symmetry in the C_60 triply degenerate conduction band. In both systems, it is known that NMR spin-lattice relaxation rate (1/T_1) measurements detect a gap in the electronic structure, most likely related to singlet-triplet excitations of the Jahn-Teller distorted (JTD) C_(60)^(2-) or C_(60)^(4-). However, the extended temperature range of the measurements presented here (10 K to 700 K) allows one to reveal deviations with respect to this general trend, both at high and low temperatures. Above room temperature, 1/T_1 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 by the appearance of quadrupole effects on the 23Na spectra of Na_2C_60. In K_4C_60, modifications of the 13C spectra line shapes 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_1T tends to a constant value for Na_2C_60, both for 13C and 23Na NMR. This indicates a residual metallic character, which emphasizes the proximity of metallic and insulting behaviors in alkali fullerides.

Izvorni jezik
Engleski

Znanstvena područja
Fizika

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