Motivated by recent experimental and theoretical progress on the Er2Ti2O7 pyrochlore XY antiferromagnet, we study the problem of quantum order-by-disorder in pyrochlore XY systems. We consider the most general nearest-neighbor pseudo spin-1/2 Hamiltonian for such a system characterized by anisotropic spin-spin couplings Je ≡{J±, J±±, Jz±, Jzz} and construct zero-temperature phase diagrams. Combining symmetry arguments and spin-wave calculations, we show that the ground state phase boundaries between the two candidate ground states of the Γ5 irreducible representation, the ψ2 and ψ3 (basis) states, are rather accurately determined by a cubic equation in (J±J±±)/J 2 z± . Depending on the value of Jzz, there can be one or three phase boundaries that separate alternating regions of ψ2 and ψ3 states. In particular, we find for sufficiently small Jzz/J± a narrow ψ2 sliver sandwiched between two ψ3 regions in the J±±/J± vs Jz±/J± phase diagram. From our results, one would be able to predict which state (ψ2 or ψ3) may be realized in a real material given its set of Je couplings. Our results further illustrate the very large potential sensitivity of the ground state of XY pyrochlore systems to minute changes in their spin Hamiltonian. Finally, using the experimentally determined Je ≡{J±, J±±, Jz±, Jzz} and g-tensor values for Er2Ti2O7, we show that the heretofore neglected long-range 1/r 3 magnetostatic dipole-dipole interactions do not change the conclusion that Er2Ti2O7 has a ψ2 ground state induced via a quantum order-by-disorder mechanism. As a new avenue of research in XY pyrochlore materials distinct from the rare-earth pyrochlore oxides, we propose that the Cd2Dy2Se4 chalcogenide spinel, in which the Dy 3+ ions form a pyrochlore lattice and may be XY -like, could prove interesting to investigate. I. INTRODUCTIONSimplified Hamiltonian (H) models of magnetic systems with competing or geometrically frustrated interactions often feature a large number of accidentally degenerate classical ground states. Such a degeneracy can typically be lifted energetically by additional perturbations to H, such as further neighbor interactions, magnetic anisotropy as well as spin-lattice coupling 1 . A more exotic mechanism is one in which thermal or quantum fluctuations induce long-range order within the degenerate manifold. This is the thermal or quantum order-by-disorder (ObD) mechanism 2-5 .While thermal 2 and quantum 3 ObD has been proposed to be at play in a number of condensed matter systems, the number of compelling experimental demonstrations of ObD among real materials, as opposed to theoretical models, has remained quite limited. In this context, the Er 2 Ti 2 O 7 insulating magnetic pyrochlore oxide 6 stands as a promising textbook example where ObD is at the origin of the experimentally observed long-range order.In Er 2 Ti 2 O 7 , the magnetic Er 3+ ions form a threedimensional network of corner-sharing tetrahedra, the socalled "pyrochlore" lattice 6 . A free Er 3+ ion has angular momentum J = 15/2. The 4 I 15/2 multiplet...

Abstract:We study the dynamical evolution of strongly coupled field theories, initially in thermal equilibrium, under the influence of an external driving force. We model the field theory holographically using classical gravitational dynamics in an asymptotically AdS spacetime. The system is driven by a source for a (composite) scalar operator. We focus on a scenario where the external source is periodic in time and chart out the response of several observables. We find an interesting phase structure in the response as a function of the amplitude of the source and driving frequency. Specifically the system transitions from a dissipation dominated phase, via a dynamical crossover to a highly resonant amplification phase. The diagnostics of these phases include the response of the operator in question, entropy production, energy fluctuations, and the temporal change of entanglement entropy for small subsystems. We comment on evidence for a potential phase transition in the energy fluctuations of the system.

Individual dark matter halos in cosmological simulations vary widely in their detailed structural properties such as shape, rotation, substructure and degree of internal relaxation. Recent nonparametric (principal component) analyses suggest that a few principal components explain a large fraction of the scatter in halo properties. The main principal component is closely linked with concentration, which in turn is known to be related to the mass accretion history of the halo. Here we examine more generally the connection between mass accretion history and structural parameters. The space of mass accretion histories has principal components of its own. We find that the strongest two can be interpreted as the overall age of the halo and the acceleration or deceleration of growth at late times. These two components only account for ∼ 70% of the scatter in mass accretions histories however, due to the stochastic effect of major mergers. Relating structural parameters to formation history, we find that concentration correlates strongly with the early history of the halo, while relaxation correlates with the late history. We examine the inferences about formation history that can be drawn by splitting haloes into subsamples, based on observable properties such as concentration and shape at some final time. This approach suggests interesting possibilities, such as the possibility of defining young and old samples of galaxy clusters in a rigorous, quantitative way, or testing the dynamical assumptions of galaxy formation models empirically.

Eguchi-Hanson solitons are odd-dimensional generalizations of the four-dimensional Eguchi-Hanson metric and are asymptotic to AdS 5 =Z p when the cosmological constant is either positive or negative. We find soliton solutions to Lovelock gravity in 5 dimensions that are generalizations of these objects.