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

TL;DR

This study investigates a disordered quantum spin state in a stripe lattice organic conductor, revealing quantum criticality and a potential quantum spin liquid state without magnetic order down to very low temperatures.

Contribution

It provides experimental evidence of a quantum disordered state with critical spin fluctuations in a stripe lattice system, highlighting a novel quantum state with no magnetic order at low temperatures.

Findings

Development of antiferromagnetic spin fluctuations with decreasing temperature

Absence of long-range magnetic order down to 1.63 K despite critical slowing down

Saturation of spin-lattice relaxation rate below 3.5 K indicating a quantum disordered state

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 $\lambda$-(STF)$_2$GaCl$_4$, 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 $1/T_1$ 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 $J/k_{\rm B} \simeq$ 194 K. Moreover, $1/T_1$ 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.