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In spinels ACr2O4 (A=Mg, Zn) realisation of the classical pyrochlore Heisenberg antiferromagnet model is complicated by a strong spin-lattice coupling: the extensive degeneracy of the ground state is lifted by a magneto-structural transition at TN=12.5 K. We study the resulting low-temperature low-symmetry crystal structure by synchrotron x-ray diffraction. The consistent features of x-ray low-temperature patterns are explained by the tetragonal model of Ehrenberg et. al (Pow. Diff. 17, 230( 2002)), while other features depend on sample or cooling protocol. Complex partially ordered magnetic state is studied by neutron diffraction and spherical neutron polarimetry. Multiple magnetic domains of configuration arms of the propagation vectors k1=(1/2 1/2 0), k2=(1 0 1/2) appear. The ordered moment reaches 1.94(3) muB/Cr3+ for k1 and 2.08(3) muB/Cr3+ for k2, if equal amount of the k1 and k2 phases is assumed. The magnetic arrangements have the dominant components along the [110] and [1-10] diagonals and a smaller c-component. By inelastic neutron scattering we investigate the spin excitations, which comprise a mixture of dispersive spin waves propagating from the magnetic Bragg peaks and resonance modes centered at equal energy steps of 4.5 meV. We interpret these as acoustic and optical spin wave branches, but show that the neutron scattering cross sections of transitions within a unit of two corner-sharing tetrahedra match the observed intensity distribution of the resonances. The distinctive fingerprint of cluster-like excitations in the optical spin wave branches suggests that propagating excitations are localized by the complex crystal structure and magnetic orders.

When the energy eigenvalues of two coupled quantum states approach each other in a certain parameter space, their energy levels repel each other and level crossing is avoided. Such level repulsion, or avoided level crossing, is commonly used to describe the dispersion relation of quasiparticles in solids. However, little is known about the level repulsion when more than two quasiparticles are present; for example, in an open quantum system where a quasiparticle can spontaneously decay into many particle continuum. Here we show that even in this case level repulsion exists between a long-lived quasiparticle state and a continuum. In our fine resolution neutron spectroscopy study of magnetic quasiparticles in a frustrated quantum magnet BiCu2PO6, we observe a renormalization of quasiparticle dispersion relation due to the presence of the continuum of multi-quasiparticle states. Our results have a broadimplication for understanding open quantum systems described by non-hermitian Hamiltonian., Comment: main text - 16 pages, 4 figures; supplementary information - 3 pages, 1 figure

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 nearest- neighbour 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]., Comment: 5 pages, 4 figures

The pressure dependence of a large number of phonon modes in CaFe2As2 with energies covering the full range of the phonon spectrum has been studied using inelastic x-ray and neutron scattering. The observed phonon frequency changes are in general rather small despite the sizable changes of the lattice parameters at the phase transition. This indicates that the bonding properties are not profoundly altered by the phase transition. The transverse acoustic phonons propagating along the c-direction are an exception because they stiffen very significantly in response to the large contraction of the c-axis. The lattice parameters are found to change significantly as a function of pressure before, during and after the first-order phase transition. However, the frequencies change nearly uniformly with the change in the lattice parameters due to pressure, with no regard specifically to the first-order phase transition. Density functional theory describes the frequencies in both the zero pressure and in the collapsed phase in a satisfactory way if based on the respective crystal structures.

We present experiments on the magnetic field-dependent thermal transport in the spin-1/2 ladder system (C$_5$H$_{12}$N)$_2$CuBr$_4$. The thermal conductivity $\kappa(B)$ is only weakly affected by the field-induced transitions between the gapless Luttinger-liquid state realized for $B_{c1}< B < B_{c2}$ and the gapped states, suggesting the absence of a direct contribution of the spin excitations to the heat transport. We observe, however, that the thermal conductivity is strongly suppressed by the magnetic field deeply within the Luttinger-liquid state. These surprising observations are discussed in terms of localization of spinons within finite ladder segments and spinon-phonon umklapp scattering of the predominantly phononic heat transport., Comment: 4 pages, 3 figures

We present high-resolution measurements of the $c^\star$-axis thermal expansion and magnetostriction of piperidinium copper bromide \hp. The experimental data at low temperatures is well accounted for by a two-leg spin-ladder Hamiltonian. The thermal expansion shows a complex behaviour with various sign changes and approaches a $1/\sqrt{T}$ divergence at the critical fields. All low-temperature features are semi-quantitatively explained within a free fermion model; full quantitative agreement is obtained with Quantum Monte Carlo simulations., Comment: 4 pages, 5 figures; version 2 is slightly shortened and typos are corrected