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Magnetic moments arranged at the corners of a honeycomb lattice are predicted to form a novel state of matter, the Kitaev quantum spin liquid, under the influence of frustration effects between bond-dependent Ising interactions. Some layered honeycomb iridates and related materials, such as Na 2 IrO 3 and α-RuCl 3 , are proximate to the Kitaev quantum spin liquid, but bosonic spin-wave excitations associated with undesirable antiferromagnetic long-range order mask the inherent properties of the Kitaev Hamiltonian. Here, we use 63 Cu nuclear quadrupole resonance to uncover the low-energy spin excitations in the nearly ideal honeycomb lattice of effective spin S ¼ 1=2 at the Ir 4þ sites in Cu 2 IrO 3 . We demonstrate that, unlike Na 2 IrO 3 , Ir spin fluctuations exhibit no evidence for critical slowing-down toward magnetic long-range order in zero external magnetic field. Moreover, the low-energy spin excitation spectrum is dominated by a mode that has a large excitation gap comparable to the Ising interactions, a signature expected for Majorana fermions of the Kitaev quantum spin liquid.

The presence of charge and spin stripe order in the La2CuO4-based family of superconductors continues to lead to new insight on the unusual ground state properties of high Tc cuprates. Soon after the discovery of charge stripe order at T charge 65 K in Nd 3+ co-doped La1.48Nd0.4Sr0.12CuO4 (Tc 6 K) [Tranquada et al., Nature 375 (1995) 561], Hunt, Singer et al. demonstrated that La1.48Nd0.4Sr0.12CuO4 and superconducting La2−xSrxCuO4 with x ∼ 1/8 (Tc 30 K) share nearly identical NMR anomalies near T charge of the former [Phys. Rev. Lett. 82 (1999) 4300]. Their inevitable conclusion that La1.885Sr0.115CuO4 also undergoes charge order at a comparable temperature became controversial, because diffraction measurements at the time were unable to detect Bragg peaks associated with charge order. Recent advances in x-ray diffraction techniques finally led to definitive confirmations of the charge order Bragg peaks in La1.885Sr0.115CuO4 with an onset at as high as T charge 80 K. Meanwhile, improved instrumental technology has enabled routine NMR measurements that were not feasible two decades ago. Motivated by these new developments, we revisit the charge order transition of a La1.885Sr0.115CuO4 single crystal based on 63 Cu NMR techniques. We demonstrate that 63 Cu NMR properties of the nuclear spin Iz = − 1 2 to + 1 2 central transition below T charge exhibit unprecedentedly strong dependence on the measurement time scale set by the separation time τ between the 90 • and 180 • radio frequency pulses; a new kind of anomalous, very broad wing-like 63 Cu NMR signals gradually emerge below T charge only for extremely short τ 4 µs, while the spectral weight I N ormal of the normal NMR signals is progressively wiped out. The NMR linewidth and relaxation rates depend strongly on τ below T charge , and their enhancement in the charge ordered state indicates that charge order turns on strong but inhomogeneous growth of Cu spin-spin correlations. I.arXiv:1712.07777v1 [cond-mat.supr-con]

139 La NMR is suited for investigations into magnetic properties of La2CuO4-based cuprates in the vicinity of their magnetic instabilities, owing to the modest hyperfine interactions between 139 La nuclear spins and Cu electron spins. We report comprehensive 139 La NMR measurements on a single crystal sample of high Tc superconductor La1.885Sr0.115CuO4 in a broad temperature range across the charge and spin order transitions (T charge 80 K, T neutron spin

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