Selective observation of spin and charge dynamics in an organic superconductor λ−(BETS)2GaCl4 using Ga

Abstract

For the unconventional organic superconductor λ-(BETS) 2 GaCl 4 [BETS = bis(ethylenedithio) tetraselenafulvalene], the importance of both spin and charge degrees of freedom has been discussed based on the broadening of the NMR linewidth due to charge disproportionation and the occurrence of a spin-density-wave phase near the superconducting phase. NMR with the nuclear spin of 1/2, previously used for studying organic conductors, is an effective method for revealing electronic states microscopically; however, it cannot distinguish between charge and spin anomalies. To resolve this problem, in this paper, we performed 69,71 Ga-NMR measurements, which enabled us to study both charge and spin dynamics using different gyromagnetic ratios and quadrupole moments between two isotopes. The spin-lattice relaxation rate is dominated by electric-field gradient fluctuations originating from molecular dynamics above 150 K and, below this temperature, it is dominated by spin fluctuations derived from the π electrons of BETS layers. This change in the relaxation mechanism is considered to be due to the development of interactions between GaCl 4 ions and BETS layers upon freezing of the molecular motion by cooling. Below 150 K, the contribution of spin fluctuations monotonically increases, and no increase in the charge fluctuation was observed, suggesting that the spin degree of freedom plays a major role in low-temperature physical properties. Our findings will aid theoretical studies on superconducting properties.

Charge imbalance in $λ$-(BETS)$_2$GaCl$_4$ and their interplay with superconductivity

“…Finally, we want to point out that the ν 27 (b 1u ) molecular vibrations involve the inner C=C double bonds. These should not be significantly affected by the freezingout of the ethylene end-groups motion, that was suggested as a source of charge imbalance by Kobayashi et al [28]. Varying the cooling rate is a common approach for investigating freezing effects.…”

“…On the other hand, 77 Se-NMR measurement of λ-(BETS) 2 FeCl 4 and λ-(BETS) 2 GaCl 4 showed linewidth broadening at low temperature that is assigned to charge disproportionation [26,27], which can also contribute to the mechanism of superconductivity. In order to elucidate this point, comprehensive NMR investigations have been performed on λ-(BETS) 2 GaCl 4 probing different isotopes 69 Ga and 71 Ga in order to separate contributions of spin and charge dynamics [28]. While there is no enhancement of charge fluctuations below T = 150 K, spin fluctuations become dominant at low temperature.…”