Disordered quantum spin state in the stripe lattice system consisting of triangular and square tilings

Abstract

Quantum fluctuations originating phase competition or geometrical frustration of spins lead to novel states such as a quantum critical point and a quantum spin liquid where the strong quantum fluctuations suppress any ordered states even at 0 K. Utilizing site-selective NMR for a quasi-two dimensional organic conductor λ-(STF)2GaCl4, we investigate the non-magnetic insulating phase of the stripe lattice system consisting of triangular and square tilings. We found development of AF spin fluctuations with decreasing temperature. Regardless of large enhancement of spin-lattice relaxation rate 1/T1 owing to critical slowing down below 10 K, no long-range magnetic ordering was observed down to 1.63 K two orders of magnitude less than the exchange interaction J/kB 194 K. Moreover, 1/T1 saturated below 3.5 K. These results are in stark contrast to observed behaviors so far in other non-magnetic ground states discussed in terms of spin liquids, demonstrating realization of an exotic quantum state accompanying quantum criticality.

Antiferromagnetic ordering of organic Mott insulator $λ$-(BEDSe-TTF)$_2$GaCl$_4$

“…As for λ-ET and λ-STF, the temperature dependencies of χ spin have been discussed using the two-dimensional (2D) Heisenberg AF spin model [55,56]. Interestingly, χ spin of λ-ET and λ-STF have been explained by the square and triangular lattice models, respectively [25][26][27]. Here, we applied these analyses to λ-BEDSe.…”

“…However, the difference in the network of J between λ-ET and λ-BETS suggests that the network of J in λ-STF is also different. In addition, because line broadening of the 13 C-NMR spectra has been observed in λ-STF [27], there is a possibility of charge disproportionation, which can modify the transfer integrals between the molecules. These features may be responsible for the temperature dependence of the triangular lattice-like χ spin in λ-STF.…”

“…pressure, and superconductivity emerges at a pressure of ∼ 1.3 GPa [23,25], which confirms the universal phase diagram. λ-(ET) 2 GaCl 4 (abbreviated as λ-ET), which is located at a more negative pressure side than λ-STF, undergoes a transition from a Mott insulator to an antiferromagnet at 13 K [26], whereas magnetic ordering is not observed in λ-STF [25,27]. The reason for the absence of magnetic order may be the quantum critical effect of λ-STF being located between the AF and superconducting phases [27], or the disorder effect originating from the asymmetric BEDT-STF molecules.…”

“…λ-(ET) 2 GaCl 4 (abbreviated as λ-ET), which is located at a more negative pressure side than λ-STF, undergoes a transition from a Mott insulator to an antiferromagnet at 13 K [26], whereas magnetic ordering is not observed in λ-STF [25,27]. The reason for the absence of magnetic order may be the quantum critical effect of λ-STF being located between the AF and superconducting phases [27], or the disorder effect originating from the asymmetric BEDT-STF molecules. In contrast, AF order has been observed in λ-(BETS) 2 FeCl 4 [28,29] and λ-(STF) 2 FeCl 4 [30][31][32], although there is a contribution of 3d spins from Fe ions.…”

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

“…In this context, the series λ-D 2 GaCl 4 is of particular interest, as the electronic bandwidth W is varied by modifying the organic molecule D = BEDT-TTF, BEDT-STF or BETS [Fig. does not exhibit magnetic order down to lowest temperatures but rather unconventional behavior assigned to a disordered quantum spin state [22]. λ-(BETS) 2 GaCl 4 , on the other side, remains metallic all the way down to the superconducting transition at T c = 4.7 K. The picture is confirmed by pressure-dependent studies [19,20].…”