Enhancement of the ¹H NMR relaxation rate at low temperatures and development of antiferromagnetic spin fluctuations of π-electrons in superconducting λ-(BETS)₂GaCl₄

Extending previous ⁷⁷Se NMR studies [J. Phys. Soc. Jpn. 72 (2003) 483], the microscopic magnetic properties of superconducting (Tc ≅ 5-6K) λ-(BETS)₂GaCl₄ [BETS = bis(ethylenedithio) tetraselenafulvalene] have been investigated by ¹H NMR in the normal state between 7.1 and 140 K on randomly oriented single crystals. Contrary to almost T-independent 1/(⁷⁷T₁T) of the ⁷⁷Se NMR, 1/(¹T₁T) of the ¹H NMR shows a significant increase below about 25 K. This quite contrasting low-T behavior in 1/(T ₁T) between ⁷⁷Se and ¹H is shown to be explained consistently in terms of the development of antiferromagnetic spin fluctuations of π-electrons, which has already been indicated by the ⁷⁷Se NMR as a significant low-T decrease in the intrinsic uniform susceptibility χ_q=0(T) and no concomitant decrease in 1/( ⁷⁷T₁T). In the explanation, it is pointed out that the inter-atomic dipolar interaction with the extended spin polarization of the HOMO plays an important role in the ¹H nuclear magnetic relaxation, which has been disregarded in previous ¹H NMR studies on related organic conductors.