8/3/2019 T. Itou et al- Spin liquid state and its instability in the organic triangular 1/2-spin system EtMe3Sb[Pd(dmit)2]2

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Abstract ID T066

Spin liquid state and its instability in the organic triangular1/2-spin system EtMe3Sb[Pd(dmit)2]2

T. Itou,a T. Kubota,a K. Yamashita,a M. Nishiyama,a A. Oyamada,a S. Maegawa,a

K. Kubo,b,c H. M. Yamamoto,b,d and R. Kato,baGraduated School of Human and Environmental Studies, Kyoto University, Japan

bCondensed Molecular Materials Laboratory, RIKEN, JapancDepartment of Chemistry, Graduated School of Science, Tohoku University, Japan

dPRESTO, Japan Science and Technology Agency, Japan

Quantum liquidsknown to be realized in 3He, 4He, and electrons in metalsgenerallyexhibit instabilities unforeseen under classical Newtonian dynamics such as the superfluid/ superconducting transition. Recently, a new quantum liquid has been discovered in or-ganic frustrated antiferromagnetic spin-1/2 systems, called the quantum spin liquid. The

fundamental question is whether instabilities other than classical ordering occur in such aquantum spin liquid, as well as in the typical well-known quantum liquids. Indeed, theoristshave proposed several possible instabilities, such as spinon pairing and chiral ordering. In themost studied organic spin-liquid material -(BEDT-TTF)2Cu2(CN)3, however, experimen-tal reports on its low-temperature nature are controversial and this question has remainedan open issue. We report the discovery of spin-liquid instability found in a further organicspin-liquid material, EtMe3Sb[Pd(dmit)2]2, which is a 2D triangular-lattice spin system withantiferromagnetic interactions J = 220-250 K. It has previously been found that this spinsystem maintains a gapless spin-liquid state down to at least 1.37 K [1]. We performed13C-NMR measurements at ultra-low temperatures and found an obvious kink at around 1.0K in the temperature dependence of T11 . This strongly suggests that a continuous phase

transition occurs at this temperature. Since continuous transitions always involve essen-tial changes of states, that is, symmetry breaking and/or topological ordering, our resultindicates that the gapless spin liquid changes to an essentially different spin state with sym-metry breaking and/or topological ordering. The low-temperature state clearly differs fromthe classical magnetic-ordered phase, because the 13C-NMR spectra do not broaden at all.The steep decrease in T11 in the low-temperature phase suggests the appearance of the spingap. We point out that the decrease does not follow an exponential law but a power law oftemperature at sufficiently low temperatures, which implies that this gap may be nodal one.We will discuss possible scenarios to explain the instability at around 1.0 K and the natureof the low-temperature phase, which is possibly a new quantum state of matter.

This work was supported by Grant-In-Aid for Scientific Research from MEXT, Japan (num-bers 16GS0219, 18740199, 19052005, 20110003, and 21740255).

[1] T. Itou et al., Phys. Rev. B 77, 104413 (2008).