Quantum Phase Transitions in the Itinerant Ferromagnet ZrZn2_2

We report a study of the ferromagnetism of ZrZn$_{2}$, the most promising material to exhibit ferromagnetic quantum criticality, at low temperatures $T$ as function of pressure $p$. We find that the ordered ferromagnetic moment disappears discontinuously at $p_c$=16.5 kbar. Thus a tricritical point separates a line of first order ferromagnetic transitions from second order (continuous) transitions at higher temperature. We also identify two lines of transitions of the magnetisation isotherms up to 12 T in the $p-T$ plane where the derivative of the magnetization changes rapidly. These quantum phase transitions (QPT) establish a high sensitivity to local minima in the free energy in ZrZn$_{2}$, thus strongly suggesting that QPT in itinerant ferromagnets are always first order

No-go theorem for bimetric gravity with positive and negative mass

We argue that the most conservative geometric extension of Einstein gravity describing both positive and negative mass sources and observers is bimetric gravity and contains two copies of standard model matter which interact only gravitationally. Matter fields related to one of the metrics then appear dark from the point of view of an observer defined by the other metric, and so may provide a potential explanation for the dark universe. In this framework we consider the most general form of linearized field equations compatible with physically and mathematically well-motivated assumptions. Using gauge-invariant linear perturbation theory, we prove a no-go theorem ruling out all bimetric gravity theories that, in the Newtonian limit, lead to precisely opposite forces on positive and negative test masses.Comment: 19 pages, no figures, journal versio

Multimetric extension of the PPN formalism: experimental consistency of repulsive gravity

Recently we discussed a multimetric gravity theory containing several copies of standard model matter each of which couples to its own metric tensor. This construction contained dark matter sectors interacting repulsively with the visible matter sector, and was shown to lead to cosmological late-time acceleration. In order to test the theory with high-precision experiments within the solar system we here construct a simple extension of the parametrized post-Newtonian (PPN) formalism for multimetric gravitational backgrounds. We show that a simplified version of this extended formalism allows the computation of a subset of the PPN parameters from the linearized field equations. Applying the simplified formalism we find that the PPN parameters of our theory do not agree with the observed values, but we are able to improve the theory so that it becomes consistent with experiments of post-Newtonian gravity and still features its promising cosmological properties.Comment: 19 pages, no figures, journal versio

Geometry for the accelerating universe

The Lorentzian spacetime metric is replaced by an area metric which naturally emerges as a generalized geometry in quantum string and gauge theory. Employing the area metric curvature scalar, the gravitational Einstein-Hilbert action is re-interpreted as dynamics for an area metric. Without the need for dark energy or fine-tuning, area metric cosmology explains the observed small acceleration of the late Universe.Comment: 4 pages, 1 figur

How the prospect of judicial review shapes bureaucratic decision making

Many observers view the judiciary as the weakest branch of American government due to its inability to unilaterally create policies and enforce change in domestic politics without the action of Congress and the president. Perhaps for this reason, existing research has often overlooked how federal agencies might strategically anticipate the influence of courts in the policymaking process. This study develops a general theory outlining the incentives that agencies possess to avoid costly litigation and the potential for unfavorable court judgments on the merits. Agencies, as a result, should make regulatory enforcement decisions based, in part, on how they expect relevant courts to view their actions. In order to test this theory, this study examines the regulatory behavior of three separate federal agencies--the Environmental Protection Agency, National Labor Relations Board, and the Occupational Safety & Health Review Commission. Using original data sets measuring each agency's regulatory enforcement decisions, the empirical analyses demonstrate how the ideological composition of federal courts exhibit a signicant impact on bureaucratic decision making. Most notably, the results highlight the U.S. Supreme Court as a consistent political constraint on each agency examined in the study. Thus, the judicial environment exhibits a meaningful impact on a diverse set of regulatory decision making in the American political system. This research provides scholars with new evidence on the extent of judicial influence in the policymaking process and interinstitutional politics more generally

The tenth justice?: consequences of politicization in the solicitor general's office

Previous analyses of the solicitor general's influence on the Supreme Court emphasize one of three explanations: expertise, an independent agent, or an ideological signal. Overall, the solicitor general is widely considered to serve two masters, both the President and Court. This creates an inherent tension between duties as a political advocate and independent legal agent. I propose a theory attempting to explain the interaction between these two roles. I argue that the political advocacy of the solicitor general has a direct impact on this independent status and success. I test my assertion using time series data from 1953-1985 and an individual-level analysis spanning the 1953-1993 Court terms. My results show a relationship between politicization and solicitor general success conditioned by the party supported in amici, the historical time period under investigation, and political issue salience. The data suggest a solicitor general-Court relationship dependent on the context surrounding litigation

Supreme Court Opinions and Audiences

This Article evaluates different rhetorical strategies Supreme Court justices employ in writing their opinions for specific audiences. Black, Owens, Wedeking, and Wohlfarth suggest justices keep lower federal courts, state governments, federal bureaucratic agencies, and the public in mind when crafting decisions, particularly to ensure compliance with the decision and avoid non-compliance. The Article identifies opinion clarity as a means of ensuring lower federal courts will follow precedent, as well as a way for smaller and less sophisticated bureaucratic agencies to avoid shirking the Court’s rulings. The Article concludes judicial clarity is only one of an arsenal of rhetorical devices used by the Supreme Court justices, and further evaluation and research may be helpful

Massive motion in Brans-Dicke geometry and beyond

Gravity theories that can be viewed as dynamics for area metric manifolds, for which Brans-Dicke theory presents a recently studied example, require for their physical interpretation the identification of the distinguished curves that serve as the trajectories of light and massive matter. Complementing previous results on the propagation of light, we study effective massive point particle motion. We show that the relevant geometrical structure is a special Finsler norm determined by the area metric, and that massive point particles follow Finsler geodesics.Comment: 12 page

Gravitational dynamics for all tensorial spacetimes carrying predictive, interpretable and quantizable matter

Only a severely restricted class of tensor fields can provide classical spacetime geometries, namely those that can carry matter field equations that are predictive, interpretable and quantizable. These three conditions on matter translate into three corresponding algebraic conditions on the underlying tensorial geometry, namely to be hyperbolic, time-orientable and energy-distinguishing. Lorentzian metrics, on which general relativity and the standard model of particle physics are built, present just the simplest tensorial spacetime geometry satisfying these conditions. The problem of finding gravitational dynamics---for the general tensorial spacetime geometries satisfying the above minimum requirements---is reformulated in this paper as a system of linear partial differential equations, in the sense that their solutions yield the actions governing the corresponding spacetime geometry. Thus the search for modified gravitational dynamics is reduced to a clear mathematical task.Comment: 47 pages, no figures, minor update