The frustrated pyrochlore magnet Yb2Ti2O7 has the remarkable property that it orders magnetically, but has no propagating magnons over wide regions of the Brillouin zone. Here we use inelastic neutron scattering to follow how the spectrum evolves in cubic-axis magnetic fields. At high fields we observe in addition to dispersive magnons also a two-magnon continuum, which grows in intensity upon reducing the field and overlaps with the one-magnon states at intermediate fields leading to strong renormalization of the dispersion relations, and magnon decays. Using heat capacity measurements we find that the low and high field regions are smoothly connected with no sharp phase transition, with the spin gap increasing monotonically in field. Through fits to an extensive data set we re-evaluate the spin Hamiltonian finding dominant quantum exchange terms, which we propose are responsible for the anomalously strong fluctuations and quasiparticle breakdown effects observed at low fields. The lattice of corner-shared tetrahedra realized in cubic A 2 B 2 O 7 pyrochlores and AB 2 O 4 spinels, is a canonical lattice to explore correlated magnetism in the presence of strong geometric frustration effects. In the strongly spin-orbit coupled rare earth pyrochlores, experiment has uncovered materials offering a tremendously rich spectrum of magnetic behavior. Notable examples include classical spin ice physics as in the rare-earth titanates (Ho/Dy) 2 Ti 2 O 7 where Ising antiferromagnetism leads to an emergent classical electrostatics at low temperatures [1] and "order-by-disorder" in XY antiferromagnets where thermal and quantum fluctuations lift a large frustration-induced degeneracy resulting in unconventional magnetic order as in Er 2 Ti 2 O 7 [2-5]. Currently, much of the interest in this field concentrates on a handful of materials that seem to fall outside a semiclassical understanding of these systems. The pyrochlore Yb 2 Ti 2 O 7 [6-25], where Kramers Yb 3+ ions behave as effective spin 1/2 moments, is quite unique in its behavior: in high applied magnetic fields dispersive magnons were observed [12], which are apparently replaced by a broad continuum of scattering at zero field [16] despite the presence of ferromagnetic order. This exotic behavior is not yet understood. To make progress one would like to know i) how the broad scattering continuum in zero field originates from quantum fluctuations, whether those fluctuations are also present at high field and, if so, how they manifest themselves, ii) how the sharp magnons "disappear" over a wide range of the Brillouin zone as the field is lowered. Here we experimentally answer those questions by studying the behavior in a magnetic field applied along the cubic [001] direction, which has not been explored in detail before and which, we will show, allows for a transparent interpretation of the phase diagram and evolution of the spectrum in a magnetic field. The experiment also allows us to re-visit the parametrization of the magnetic exchange, which is a critical ingredien...

Paramagnetic correlations in the magnetic material Yb2Ti2O7 have been investigated via neutron scattering, revealing a [111] rod of scattering intensity. Assuming interactions between the Yb 3+ ions composed of all symmetry-allowed nearest neighbor exchange interactions and long-range dipolar interactions, we construct a model Hamiltonian that allows for an excellent description of the neutron scattering data. Our results provide compelling evidence for significant anisotropic exchange interactions in an insulating magnetic pyrochlore oxide. We also compute the real space correlations leading to the [111] rod of scattering.In geometrically frustrated magnetic materials there exists no configuration of magnetic moments that simultaneously satisfies all the pairwise magnetic interactions. Experimental and theoretical research over the past twenty years has shown that frustrated magnetic systems are prone to exhibit novel and intriguing collective thermodynamic phenomena [1].Among frustrated three dimensional systems, the A 2 B 2 O 7 pyrochlores have attracted much attention [2]. In these compounds, A is a trivalent rare earth ion (Ho, Dy, Tb, Gd, Yb) or yttrium (Y) and B is a tetravalent transition metal ion (Ti, Sn, Mo, Mn). Both A and B reside on two distinct lattices of corner-sharing tetrahedra. Theory predicts that classical [3] and quantum [4] Heisenberg spins on a pyrochlore lattice interacting via an isotropic antiferromagnetic nearest neighbor exchange Hamiltonian, H H , fail to develop conventional LRO down to zero temperature. In real pyrochlore compounds, however, there generally exists some combination of other perturbing magnetic interactions (e.g. single ion anisotropy, dipolar interactions, etc) beyond H H . Since H H alone does not produce LRO, the low temperature magnetic correlations of these materials are strongly influenced by the competition between materialspecific perturbations. This is the origin of the richness of phenomena observed in the A 2 B 2 O 7 pyrochlores [2] including spin liquid [5], spin glass [6], spin ice [7], and LRO with persistent low-temperature spin dynamics [8,9]. In this article, we consider the Yb 2 Ti 2 O 7 pyrochlore which does not apparently exhibit any of the aforementioned phenomena and has some unique and unusual features of its own which have heretofore remained unexplained.Yb 2 Ti 2 O 7 has a ferromagnetic character with a Curie-Weiss temperature, θ CW = +0.65 ± 0.15 K [10,11]. The Yb 3+ ∼ 3 µ B magnetic moments predominantly lie perpendicular to the local [111] cubic unit cell diagonals, making this system the only known local [111] XY pyrochlore with a ferromagnetic θ CW [2]. Magnetic specific heat (C m ) measurements reveal a sharp first order transition at T c ≈ 240 mK [12], suggesting the onset of LRO. While a single crystal elastic neutron scattering (NS) study suggested ferromagnetic order below T c [13], a subsequent polarized NS study [14] did not confirm such ordering. Furthermore, powder NS shows no LRO down to 110 mK [15] and very recent NS on a si...

The quasi-one-dimensional (1D) Ising ferromagnet CoNb2O6 has recently been driven via applied transverse magnetic fields through a continuous quantum phase transition from spontaneous magnetic order to a quantum paramagnet, and dramatic changes were observed in the spin dynamics, characteristic of weakly perturbed 1D Ising quantum criticality. We report here extensive single-crystal inelastic neutron scattering measurements of the magnetic excitations throughout the three-dimensional (3D) Brillouin zone in the quantum paramagnetic phase just above the critical field to characterize the effects of the finite interchain couplings. In this phase, we observe that excitations have a sharp, resolution-limited line shape at low energies and over most of the dispersion bandwidth, as expected for spin-flip quasiparticles. We map the full bandwidth along the strongly dispersive chain direction and resolve clear modulations of the dispersions in the plane normal to the chains, characteristic of frustrated interchain couplings in an antiferromagnetic isosceles triangular lattice. The dispersions can be well parametrized using a linear spin-wave model that includes interchain couplings and further neighbor exchanges. The observed dispersion bandwidth along the chain direction is smaller than that predicted by a linear spin-wave model using exchange values determined at zero field, and this effect is attributed to quantum renormalization of the dispersion beyond the spin-wave approximation in fields slightly above the critical field, where quantum fluctuations are still significant.

A quantitative analysis is made of the range of operations for which conventional categorical weather forecasts are useful. It is shown that, because individual forecasts must of necessity be designed for only a small range of operations, their utility may be severely limited when the operating risks are much different from those to which the forecasts apply. This deficiency may be minimized, either by making categorical forecasts so that the optimum operating decision is provided for each user, or by providing an estimate of the probability of occurrence of critical weather and thus permitting the user to make his own operating decision.

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