Nature of the spin liquid ground state in a breathing kagome compound studied by NMR and series expansion

Orateur : Jean-Christophe ORAIN (PSI Villigen Suisse)

The search for quantum spin liquid states such as the resonant valence bond state (RVB) formed by the macroscopic r esonance between the various spin singlet coverings of the lattice, is a major challenge in both experimental and theoretical condensed matter research [1]. The prime candidate to host this state of matter is the S = 1/2 Kagome antiferromagnet (KAFM) in two dimensions. Indeed, on the theoretical side, various studies pointed out the stabilization of a quantum spin liquid state on such lattice. Nevertheless, the nature of this ground state, with gapped or gappless excitation spectrum, is still subject to debate. On the experimental side, only few candidates exist. Various experiments on the most studied one, the herbertsmithite, have pointed out a gapless excitation spectrum, but recent NMR investigations on a single crystal revealed a gapped excitation spectrum and challenged this conclusion. Further, the influence of the different deviations to the ideal Heisenberg Hamiltonian on the ground state nature remains a difficult issue [2]. Among the rare experimental realizations of the KAFM model the recently synthesized compound, [NH4]2[C7H14N][V7O6F18] (DQVOF) [3], is the first one to host magnetically active V4+ (d1) ions rather than more common Cu2+ (d9) thus allowing to investigate the effects of different perturbations to the ideal Heisenberg Hamiltonian. Further, this compound seems to be the first experimental realization of the breathing Kagome model, formed by two different equilaterals triangles with two different interactions, J and J, which seems to stabilize a gapped ground state [4]. Our low temperature magnetization and specific heat results suggest that DQVOF is a good candidate for the S=1/2 KAFM physics despite a complex structure with mixed V4+ (S = 1/2) in the kagome planes and nearly free interlayer V3+ (S = 1) [5]. Besides, the low temperature specific heat and μSR studies evidence a gapless spin liquid behavior down to 20 mK [6]. Our 17O NMR studies unveil the intrinsic susceptibility of the Kagome layers. The high temperature series analysis of this latter reveals a ratio J/J ≃ 0.5 but can not be used to discriminate among a gapped or gapless excitation spectrum. However, our spin lattice time relaxation measurements points out a gapless or lightly gapped (∆ ≤ 0.007 J) [7] in contradiction with the recent theoretical study [4]. Our NMR results call for more calculations on the breathing kagome lattice. Further, the next dominant perturbation in DQVOF is likely the interaction between the interlayer V3+ and the kagome V4+ which is difficult to estimate. Some studies on compounds presenting different interlayer planes could help us to understand its influence on the low temperature physics [8].

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