Nature of Magnetic Excitations in the High-Field Phase of α-RuCl_{3}.

We present comprehensive electron spin resonance (ESR) studies of in-plane oriented single crystals of α-RuCl_{3}, a quasi-two-dimensional material with honeycomb structure, focusing on its high-field spin dynamics. The measurements were performed in magnetic fields up to 16 T, applied along the [110] and [100] directions. Several ESR modes were detected. Combining our findings with recent inelastic neutron- and Raman-scattering data, we identified most of the observed excitations. Most importantly, we show that the low-temperature ESR response beyond the boundary of the magnetically ordered region is dominated by single- and two-particle processes with magnons as elementary excitations. The peculiarities of the excitation spectrum in the vicinity of the critical field are discussed.

Proximate Kitaev quantum spin liquid behaviour in a honeycomb magnet.

Quantum spin liquids (QSLs) are topological states of matter exhibiting remarkable properties such as the capacity to protect quantum information from decoherence. Whereas their featureless ground states have precluded their straightforward experimental identification, excited states are more revealing and particularly interesting owing to the emergence of fundamentally new excitations such as Majorana fermions. Ideal probes of these excitations are inelastic neutron scattering experiments. These we report here for a ruthenium-based material, α-RuCl3, continuing a major search (so far concentrated on iridium materials) for realizations of the celebrated Kitaev honeycomb topological QSL. Our measurements confirm the requisite strong spin-orbit coupling and low-temperature magnetic order matching predictions proximate to the QSL. We find stacking faults, inherent to the highly two-dimensional nature of the material, resolve an outstanding puzzle. Crucially, dynamical response measurements above interlayer energy scales are naturally accounted for in terms of deconfinement physics expected for QSLs. Comparing these with recent dynamical calculations involving gauge flux excitations and Majorana fermions of the pure Kitaev model, we propose the excitation spectrum of α-RuCl3 as a prime candidate for fractionalized Kitaev physics.

Destabilization of Magnetic Order in a Dilute Kitaev Spin Liquid Candidate.

The insulating honeycomb magnet α-RuCl_{3} exhibits fractionalized excitations that signal its proximity to a Kitaev quantum spin liquid state; however, at T=0, fragile long-range magnetic order arises from non-Kitaev terms in the Hamiltonian. Spin vacancies in the form of Ir^{3+} substituted for Ru are found to destabilize this long-range order. Neutron diffraction and bulk characterization of Ru_{1-x}Ir_{x}Cl_{3} show that the magnetic ordering temperature is suppressed with increasing x, and evidence of zizag magnetic order is absent for x>0.3. Inelastic neutron scattering demonstrates that the signature of fractionalized excitations is maintained over the full range of x investigated. The depleted lattice without magnetic order thus hosts a spin-liquid-like ground state that may indicate the relevance of Kitaev physics in the magnetically dilute limit of RuCl_{3}.

Antiferromagnetic Resonance and Terahertz Continuum in α-RuCl_{3}.

We report measurements of optical absorption in the zigzag antiferromagnet α-RuCl_{3} as a function of temperature T, magnetic field B, and photon energy ℏω in the range ∼0.3-8.3 meV, using time-domain terahertz spectroscopy. Polarized measurements show that threefold rotational symmetry is broken in the honeycomb plane from 2 to 300 K. We find a sharp absorption peak at 2.56 meV upon cooling below the Néel temperature of 7 K at B=0 that we identify as the magnetic-dipole excitation of a zero-wave-vector magnon, or antiferromagnetic resonance (AFMR). With the application of B, the AFMR broadens and shifts to a lower frequency as long-range magnetic order is lost in a manner consistent with transitioning to a spin-disordered phase. From a direct, internally calibrated measurement of the AFMR spectral weight, we place an upper bound on the contribution to the dc susceptibility from a magnetic excitation continuum.

The impact of carbon coating on the synthesis and properties of α''-Fe16N2 powders.

This paper presents the preparation of carbon composite Fe16N2 powders, and the influence of a protective carbon coating on the yield and magnetic properties of Fe16N2. Nanoparticle precursors with and without carbon were reacted under ammonia gas flow to produce Fe16N2. Neutron and X-ray powder diffraction indicate that the powders contain typically 40-60% Fe16N2, with the remaining phases being unreacted iron, Fe4N or Fe3N. Transmission electron microscopy demonstrates that the carbon coating is effective at reducing the level of sintering of Fe nanoparticles during the reduction stage prior to ammonolysis. XPS results support the retention of a carbon coating on the surface after ammonolysis, and that there is Fe-C bonding present at the particle surface. In situ TEM was used to observe loss of ordering in the nitrogen sublattice of carbon composite Fe16N2 powders in the range of 168 °C to 200 °C. Magnetic susceptibility measurements show maximum values for saturation magnetization in the range of 232 emu g(-1), and for coercivity near 930 Oe, for different samples measured up to 2 T applied field at 300 K.

Dynamics of Emim+ in [Emim][TFSI]/LiTFSI Solutions as Bulk and under Confinement in a Quasi-liquid Solid Electrolyte.

Quasi-liquid solid electrolytes are a promising alternative for next-generation Li batteries. These systems combine the safety of solid electrolytes with the desired properties of liquids and are typically formed by solutions of Li salts in ionic liquids incorporated into solid matrices. Here, we present a fundamental understanding of the transport properties in solutions of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim][TFSI]), either in bulk form or incorporated in a boron nitride (BN) matrix. We performed a series of quasi-elastic neutron scattering experiments that, given the high incoherent neutron scattering cross section of hydrogen, allowed us to focus on the Emim+ dynamics. First, [Emim][TFSI]/LiTFSI solutions (0.5 and 2.5 mol·kg-1) were investigated and we show how the increase in the concentration reduces the Emim+ mobility and increases the activation energy of their long-range motions. Then, the 0.5 mol·kg-1 solution was incorporated into the BN matrix and we report that the diffusivities of the Emim+ cations that remain mobile under confinement are highly accelerated in comparison with the bulk sample and the activation energy of these motions is drastically reduced. We present the experimental evidence that this effect is related to the content of the Emim+ cations immobilized near the surfaces of the BN pores.

Atomic-scale observation of structural and electronic orders in the layered compound α-RuCl3.

A pseudospin-1/2 Mott phase on a honeycomb lattice is proposed to host the celebrated two-dimensional Kitaev model which has an elusive quantum spin liquid ground state, and fascinating physics relevant to the development of future templates towards topological quantum bits. Here we report a comprehensive, atomically resolved real-space study by scanning transmission electron and scanning tunnelling microscopies on a novel layered material displaying Kitaev physics, α-RuCl3. Our local crystallography analysis reveals considerable variations in the geometry of the ligand sublattice in thin films of α-RuCl3 that opens a way to realization of a spatially inhomogeneous magnetic ground state at the nanometre length scale. Using scanning tunnelling techniques, we observe the electronic energy gap of ≈0.25 eV and intra-unit cell symmetry breaking of charge distribution in individual α-RuCl3 surface layer. The corresponding charge-ordered pattern has a fine structure associated with two different types of charge disproportionation at Cl-terminated surface.

Matrix-assisted laser desorption ionization mass spectrometry for the analysis of monosulfated oligosaccharides.

Sulfated oligosaccharides are an important class of compounds in the field of glycobiology. Mass spectrometric analysis of these molecules is challenging due to their readiness to dissociate in sample preparation and their tendency to fragment during ionization. Moreover, their presence in small quantity in biological systems poses additional problems. We report the development of a mass spectrometric method based on matrix-assisted laser desorption ionization (MALDI) in a time-lag focusing time-of-flight mass spectrometer for the analysis of monosulfated oligosaccharides. It is found that coumarin 120 is an excellent matrix for the analysis of monosulfated disaccharides, whereas the use of a mixture of coumarin 120 and 6-aza-2-thiothymine is very effective for the ionization of sulfated trisaccharides and tetrasaccharides including those containing N-acetylneuraminic acid. Molecular ions for a series of synthetic sulfo/sialo beta Gal(1-->3)GlcNAc and beta Gal(1-->4)GlcNAc structures can thus be observed with subpicomole detection sensitivity using a uniform microcrystal matrix/sample preparation procedure. It is demonstrated that, with this matrix formulation, the presence of a high amount of sodium chloride or sodium phosphate buffer, which is often the case for the HPLC fractionated samples, does not deteriorate the MALDI performance. The analysis of mixtures containing different types of oligosaccharides is also examined. It is found that different classes of oligosaccharides require different matrix preparation methods.