Spin-density wave in the vicinity of superconducting state in λ−(BETS)2GaBrxCl4−x probed by

Abstract

13 C NMR and electrical resistivity of λ-(BETS) 2 GaBr 0.75 Cl 3.25 [BETS: bis(ethylenedithio) tetraselenafulvalene] were simultaneously measured to clarify the electronic state of the insulating phase near the boundary of the superconducting (SC) phase of λ-(BETS) 2 GaCl 4. We found the divergent peak in the nuclear spin-lattice relaxation rate divided by temperature 1/T 1 T with the metal-insulator transition at 13 K, below which the appearance of internal fields in NMR spectra was confirmed. Results of this study indicate that λ-(BETS) 2 GaBr 0.75 Cl 3.25 undergoes spin-density-wave (SDW) ordering. We suggest that the existence of the SDW phase in the vicinity of the SC phase can explain the unexplainable enhancement of 1/T 1 T at low temperatures observed in λ-(BETS) 2 GaCl 4 , and SDW fluctuation plays an important role in the pairing mechanism of λ-type BETS superconductors.

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

“…Recently, the divergence of ( 13 T1 T ) −1 with a metalinsulator transition was observed in λ-(BETS) 2 GaBr 0.75 Cl 3.25 , indicating that the SDW phase is in the vicinity of the SC phase [23]. Considering the neighboring SDW phase and the present finding of magnetic fluctuations at temperatures immediately above T c , a possibility of SDW fluctuationmediated superconductivity is suggested.…”

“…As discussed in the literature [26], this behavior can be understood as the suppression of antiferromagnetic fluctuations due to the development of the coherence of conduction electrons. The enhancement of ( 69,71 T1 T ) −1 below 20 K is considered to represent the spin fluctuations derived from the adjacent SDW phase [23].…”

“…Regarding the physical properties derived from the spin degree of freedom, λ-(BETS) 2 GaBr x Cl 4−x with x = 0.75 exhibits divergence of the spin-lattice relaxation rate divided by temperature (T 1 T ) −1 with a metal-insulator transition at 13 K, indicating spin-density wave (SDW) ordering [23]. The salt with x = 0.75 is located much closer to the SC salt λ-(BETS) 2 GaCl 4 (x = 0) in the phase diagram of λ-(BETS) 2 GaBr x Cl 4−x [24,25]; in the latter, (T 1 T ) −1 increases with a decrease in temperature below 10 K when the superconductivity is suppressed by the magnetic field [21,26,27].…”

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

“…In λ-type salts the Fermi surface comprises cylindrical portions with one-dimensional flat sections [49,50], thus nesting effects and possible instabilities are expected. Indeed, the spin-density-wave (SDW) state accompanied by the metal-insulator transition was recently observed in λ-(BETS) 2 GaBr x Cl 4−x series with x = 0.75 [25]. No doubt that this state of itinerant antiferromagnetism certainly affects electronic properties of adjacent λ-(BETS) 2 GaCl 4 , where the increase of 1/T 1 T just above the superconducting transition was ascribed to the SDW fluctuations [23], and as a result, magnetic fluctuations were considered as a pairing mechanism of superconductivity.…”

“…In addition, another increase in spinlattice relaxation rate of λ-(BETS) 2 GaCl 4 was observed below 10 K and interpreted as sign of Fermi surface nesting probably related to spin-density wave formation [23]. More light is shed on this issue by considering the series λ-(BETS) 2 GaBr x Cl 4−x , where the Br substitution acts as negative pressure, driving the system insulating with rising x [24,25]. 13 C-NMR spectroscopy infers that spin degree of freedom plays an important role in the pairing mechanism of λ-type organic superconductors.…”