We report the observation of a stepwise "melting" of the low-temperature Na-vacancy order in the layered transition-metal oxide Na0.7CoO2. High-resolution neutron powder diffraction analysis indicates the existence of two first-order structural transitions, one at T1≈290  K followed by a second at T2≈400  K. Detailed analysis strongly suggests that both transitions are linked to changes in the Na mobility. Our data are consistent with a two-step disappearance of Na-vacancy order through the successive opening of first quasi-1D (T1>T>T2) and then 2D (T>T2) Na diffusion paths. These results shed new light on previous, seemingly incompatible, experimental interpretations regarding the relationship between Na-vacancy order and Na dynamics in this material. They also represent an important step towards the tuning of physical properties and the design of tailored functional materials through an improved control and understanding of ionic diffusion.

Neutron inelastic scattering has been used to probe the spin dynamics of the quantum (S=1/2) ferromagnet on the pyrochlore lattice Lu2V2O7. Well-defined spin waves are observed at all energies and wavevectors, allowing us to determine the parameters of the Hamiltonian of the system. The data are found to be in excellent overall agreement with a minimal model that includes a nearestneighbour Heisenberg exchange J = 8.22(2) meV and a Dzyaloshinskii-Moriya interaction (DMI) D = 1.5(1) meV. The large DMI term revealed by our study is broadly consistent with the model developed by Onose et al. to explain the magnon Hall effect they observed in Lu2V2O7 [1], although our ratio of D/J = 0.18(1) is roughly half of their value and three times larger than calculated by ab initio methods [2]. PACS numbers:The highly frustrated nature of the pyrochlore lattice leads to a rich diversity of fascinating properties when the lattice sites are decorated with "classical" (large S) spins [3]. Arguably the most celebrated example is ferromagnetically coupled Ising spins which give rise to spinice [4,5] and the emergence of magnetic monopoles [6,7]. While many examples of classical pyrochlores exist, there are few examples of pyrochlores where the spins of the magnetic ions are explicitly in the quantum (S=1/2) limit. Quantum effects can, however, play a decisive role even in classical pyrochlores if their low-energy physics maps onto an effective spin 1/2 model [3]. In either case, quantum effects may produce a range of novel phenomena including the realisation of a three-dimensional quantum spin-liquid ground state, emergent electromagnetism supporting photon-like excitations, etc. [8][9][10][11]. Interest in itinerant pyrochlore magnets is also motivated by the various anomalous transport properties they exhibit [12].Lu 2 V 2 O 7 is a ferromagnetic, small-gap Mott insulator that crystallises in the pyrochlore structure and displays a number of exceptional properties. Fig. 1(a) shows the V 4+ (S=1/2) sites in the pyrochlore lattice, which form a three-dimensionally coordinated network of corner sharing tetrahedra. Bulk measurements have established that the spins order ferromagnetically at T C =70 K [1,13]. Measurements of the thermal conductivity in Lu 2 V 2 O 7 by Onose et al. have been interpreted in terms of a magnon Hall effect [1], based on the observation that the thermal conductivity has a distinctive dependence on applied magnetic fields for temperatures below T C . This highly unusual and previously unreported phenomenon was shown to be consistent with a model in which the Dzyaloshinskii-Moriya interaction (DMI) between nearest neighbour spins deflects magnon wavepackets propagating from the hot to the cold side of the sample [1].Further evidence of the exceptional properties of Lu 2 V 2 O 7 was provided by Zhou et al. [13] who discovered that it displays a large (50%) magneto-resistance around 70 K in an applied magnetic field. Additionally, polarised neutron diffraction found evidence for an orbital ordered ground state ...

We have used neutron spectroscopy to investigate the spin dynamics of the quantum (S ¼ 1=2) antiferromagnetic Ising chains in RbCoCl 3 . The structure and magnetic interactions in this material conspire to produce two magnetic phase transitions at low temperatures, presenting an ideal opportunity for thermal control of the chain environment. The high-resolution spectra we measure of two-domain-wall excitations therefore characterize precisely both the continuum response of isolated chains and the "Zeeman-ladder" bound states of chains in three different effective staggered fields in one and the same material. We apply an extended Matsubara formalism to obtain a quantitative description of the entire dataset, Monte Carlo simulations to interpret the magnetic order, and finite-temperature density-matrix renormalization-group calculations to fit the spectral features of all three phases.