Detailed magnetization, specific heat, and 7 Li nuclear magnetic resonance (NMR) measurements on single crystals of the hyperhoneycomb Kitaev magnet β-Li2IrO3 are reported. At high temperatures, anisotropy of the magnetization is reflected by the different Curie-Weiss temperatures for different field directions, in agreement with the combination of a ferromagnetic Kitaev interaction (K) and a negative off-diagonal anisotropy (Γ) as two leading terms in the spin Hamiltonian. At low temperatures, magnetic fields applied along a or c have only a weak effect on the system and reduce the Néel temperature from 38 K at 0 T to about 35.5 K at 14 T, with no field-induced transitions observed up to 58 T on a powder sample. In contrast, the field applied along b causes a drastic reduction in the TN that vanishes around Hc = 2.8 T giving way to a crossover toward a quantum paramagnetic state. 7 Li NMR measurements in this field-induced state reveal a gradual line broadening and a continuous evolution of the line shift with temperature, suggesting the development of local magnetic fields. The spin-lattice relaxation rate shows a peak around the crossover temperature 40 K and follows power-law behavior below this temperature. arXiv:1906.09089v2 [cond-mat.str-el]

Using thermodynamic measurements, neutron diffraction, nuclear magnetic resonance, and muon spin relaxation, we establish putative quantum spin liquid behavior in Ba3InIr2O9, where unpaired electrons are localized on mixed-valence Ir2O9 dimers with Ir 4.5+ ions. Despite the antiferromagnetic Curie-Weiss temperature on the order of 10 K, neither long-range magnetic order nor spin freezing are observed down to at least 20 mK, such that spins are short-range-correlated and dynamic over nearly three decades in temperature. Quadratic power-law behavior of both spin-lattice relaxation rate and specific heat indicates gapless nature of the ground state. We envisage that this exotic behavior may be related to an unprecedented combination of the triangular and buckled honeycomb geometries of nearest-neighbor exchange couplings in the mixed-valence setting. *

Li nuclear magnetic resonance and terahertz (THz) spectroscopies are used to probe magnetic excitations and their field dependence in the hyperhoneycomb Kitaev magnet β-Li 2 IrO 3. Spin-lattice relaxation rate (1/T 1) measured down to 100 mK indicates the gapless nature of the excitations at low fields (below H c 2.8 T), in contrast to the gapped magnon excitations found in the honeycomb Kitaev magnet α-RuCl 3 at zero applied magnetic field. At higher temperatures in β-Li 2 IrO 3 , 1/T 1 passes through a broad maximum without any clear anomaly at the Néel temperature T N 38 K, suggesting the abundance of low-energy excitations that are indeed observed as two peaks in the THz spectra; both correspond to zone-center magnon excitations. At higher fields (above H c), an excitation gap opens, and a redistribution of the THz spectral weight is observed without any indication of an excitation continuum, in contrast to α-RuCl 3 where an excitation continuum was reported.

We have investigated the thermodynamic and local magnetic properties of the Mott insulating system Ag3LiRu2O6 containing Ru 4+ (4d 4 ) for novel magnetism. The material crystallizes in a monoclinic C2/m structure with RuO6 octahedra forming an edge-shared two-dimensional honeycomb lattice with limited stacking order along the c-direction. The large negative Curie-Weiss temperature (θCW = −57 K) suggests antiferromagnetic interactions among Ru 4+ ions though magnetic susceptibility and heat capacity show no indication of magnetic long-range order down to 1.8 K and 0.4 K, respectively. 7 Li nuclear magnetic resonance (NMR) shift follows the bulk susceptibility between 120-300 K and levels off below 120 K. Together with a power-law behavior in the temperature dependent spin-lattice relaxation rate between 0.2 and 2 K, it suggest dynamic spin correlations with gapless excitations. Electronic structure calculations suggest an S = 1 description of the Ru-moments and the possible importance of further neighbour interactions as also bi-quadratic and ring-exchange terms in determining the magnetic properties. Analysis of our µSR data indicates spin freezing below 5 K but the spins remain on the borderline between static and dynamic magnetism even at 20 mK.

Here we present the structural and magnetic properties of a new honeycomb material Ag3LiMn2O6. The system Ag[Li 1/3 Mn 2/3 ]O2 belongs to a quaternary 3R-delafossite family and crystallizes in a monoclinic symmetry with space group C 2/m and the magnetic Mn 4+ (S = 3/2) ions form a honeycomb network in the ab-plane. An anomaly around 50 K and the presence of antiferromagnetic (AFM) coupling (Curie-Weiss temperature θCW ∼ −51 K) were inferred from our magnetic susceptibility data. The magnetic specific heat clearly manifests the onset of magnetic ordering in the vicinity of 48 K and the recovered magnetic entropy, above the ordering temperature, falls short of the expected value, implying the presence of short-range magnetic correlations. An asymmetric Bragg peak (characteristic of two dimensional order), seen in neutron diffraction, gains intensity even above the ordering temperature, thus showing the existence of short-range spin correlations. Our electron spin resonance ESR experiments corroborate the bulk magnetic data. Additionally, the (ESR) line broadening on approaching the ordering temperature T N could be described in terms of a Berezinski-Kosterlitz-Thouless (BKT) scenario with T KT = 40(1) K. 7 Li NMR line-shift probed as a function of temperature tracks the static susceptibility (Kiso) of magnetically coupled Mn 4+ ions. The 7 Li spin-lattice relaxation rate (1/T 1) exhibits a sharp decrease below about 50 K. A critical divergence is absent at the ordering temperature perhaps because of the filtering out of the antiferromagnetic fluctuations at the Li site, i.e., at the centers of the hexagons in the honeycomb network. Combining our bulk and local probe measurements, we establish the presence of an ordered ground state for the honeycomb system Ag3LiMn2O6. Our ab initio electronic structure calculations suggest that in the ab-plane, the nearest neighbor (NN) exchange interaction is strong and AFM, while the next NN and the third NN exchange interactions are FM and AFM respectively. The interplanar exchange interaction is found to be relatively small. In the absence of any frustration the system is expected to exhibit long-range, AFM order, in agreement with experiment. arXiv:1903.08366v2 [cond-mat.str-el]