Magnets with strong geometric frustration

A non-technical introduction to the theory of magnets with strong geometric frustration is given, concentrating on magnets on corner-sharing (kagome, pyrochlore, SCGO and GGG) lattices. Their rich behaviour is traced back to a large ground-state degeneracy in model systems, which renders them highly unstable towards perturbations. A systematic classification according to properties of their ground states is discussed. Other topics addressed in this overview article include a general theoretical framework for thermal order by disorder; the dynamics of how the vast regions of phase space accessible at low temperature are explored; the origin of the featureless magnetic susceptibility fingerprint of geometric frustration; the role of perturbations; and spin ice. The rich field of quantum frustrated magnets is also touched on.Comment: Key-note theory talk of Conference on Highly Frustrated Magnetism (HFM-2000) in Waterloo, Canada, June 2000; 8 page

Statistics of Peculiar Velocities from Cosmic Strings

We calculate the probability distribution of a single component of peculiar velocities due to cosmic strings, smoothed over regions with a radius of several $h^{-1}$ Mpc. The probability distribution is shown to be Gaussian to good accuracy, in agreement with the distribution of peculiar velocities deduced from the 1.9 Jy IRAS redshift survey. Using the normalization of parameters of the cosmic string model from CMB measurements, we show that the rms values for peculiar velocities inferred from IRAS are consistent with the cosmic string model provided that long strings have some small-scale structure.Comment: 17 pages, uses Latex, to appear in MNRAS, 1 Postscript figure available on reques

Optimal monetary policy with uncertainty about financial frictions

This paper studies optimal discretionary monetary policy in the presence of uncertainty about the degree of financial frictions. Changes in the degree of financial frictions are modelled as changes in parameters of a hybrid New-Keynesian model calibrated for the UK, following Bean, Larsen and Nikolov (2002). Uncertainty about the degree of financial frictions is modelled as Markov switching between regimes without and with strong financial frictions. Optimal monetary policy is determined for different scenarios of permanent and temporary regime shifts in financial frictions, as well as for variations in financial frictions over the business cycle. Optimal monetary policy is found to be state-dependent. In each state, optimal monetary policy depends on the transition probabilities between the different regimes. JEL Classification: E52, E58, E61, E44financial frictions, monetary policy, uncertainty

Dynamics of the (spin-) Hall effect in topological insulators and graphene

A single two-dimensional Dirac cone with a mass gap produces a quantized (spin-) Hall step in the absence of magnetic field. What happens in strong electric fields? This question is investigated by analyzing time evolution and dynamics of the (spin-) Hall effect. After switching on a longitudinal electric field, a stationary Hall current is reached through damped oscillations. The Hall conductivity remains quantized as long as the electric field (E) is too weak to induce Landau-Zener transitions, but quantization breaks down for strong fields and the conductivity decreases as 1/sqrt{E}. These apply to the (spin-) Hall conductivity of graphene and the Hall and magnetoelectric response of topological insulators.Comment: 4 pages, 3 figure

Optimal discretionary policy and uncertainty about inflation persistence

This paper studies optimal discretionary policy with parameter uncertainty about inflation inertia. Optimal policy rules and impulse responses are presented within a hybrid New-Keynesian model estimated for the euro area by Smets (2003). We find that it may be optimal for policy to respond more aggressively to cost-push shocks and real interest rate shocks in the presence of uncertainty about inflation inertia, depending on the form of the central bank’s objective function. Moreover, in the cases where optimal policy is not certainty equivalent, we find that inflation returns slightly more gradually to equilibrium following a shock when the degree of inflation inertia is uncertain. JEL Classification: E52, E58Inflation persistence, monetary policy, uncertainty

Formation of High Redshift Objects in a Cosmic String Theory with Hot Dark Matter

Using a modification of the Zel'dovich approximation adapted to hot dark matter, the accretion of such matter onto moving cosmic string loops is studied. It is shown that a large number of $10^{12}M_\odot$ nonlinear objects will be produced by a redshift of $z=4$. These objects could be the hosts of high redshift quasars.Comment: 21 pages, 1 figure, uses phyzzx and epsf macro

Maxwell electromagnetism as an emergent phenomenon in condensed matter

The formulation of a complete theory of classical electromagnetism by Maxwell is one of the milestones of science. The capacity of many-body systems to provide emergent mini-universes with vacua quite distinct from the one we inhabit was only recognised much later. Here, we provide an account of how simple systems of localised spins manage to emulate Maxwell electromagnetism in their low-energy behaviour. They are much less constrained by symmetry considerations than the relativistically invariant electromagnetic vacuum, as their substrate provides a non-relativistic background with even translational invariance broken. They can exhibit rich behaviour not encountered in conventional electromagnetism. This includes the existence of magnetic monopole excitations arising from fractionalisation of magnetic dipoles; as well as the capacity of disorder, by generating defects on the lattice scale, to produce novel physics, as exemplified by topological spin glassiness or random Coulomb magnetism.Comment: Talk at Royal Society Symposium, "Unifying Physics and Technology in the Light of Maxwell's Equations", November 201