Non-Hermitian topology of spontaneous magnon decay

Spontaneous magnon decay is a generic feature of the magnetic excitations of anisotropic magnets and isotropic magnets with non-collinear order. In this paper, we argue that the effect of interactions on one-magnon states can, under many circumstances, be treated in terms of an effective, energy independent, non-Hermitian Hamiltonian for the magnons. In the vicinity of Dirac or Weyl touching points, we show that the spectral function has a characteristic anisotropy arising from topologically protected exceptional points or lines in the non-Hermitian spectrum. Such features can, in principle, be detected using inelastic neutron scattering or other spectroscopic probes. We illustrate this physics through a concrete example: a honeycomb ferromagnet with Dzyaloshinskii-Moriya exchange. We perform interacting spin wave calculations of the structure factor and spectral function of this model, showing good agreement with results from a simple effective non-Hermitian model for the splitting of the Dirac point. Finally, we argue that the zoo of known topological protected magnon band structures may serve as a nearly ideal platform for realizing and exploring non-Hermitian physics in solid-state systems.Comment: 4+epsilon page

A Tariff-Growth Paradox? Protection's Impact the World Around 1875-1997

This paper uses a new database to establish two findings covering the first globalization boom before World War I, the second since World War II, and the autarkic interlude in between. First, there is strong evidence supporting a Tariff-Growth Paradox: protection was associated with fast growth before World War II, while it was associated with slow growth thereafter. Second, there is strong evidence supporting regional asymmetry: while the tariff-growth association was powerful and positive in the Core and rich New World before World War II, it was typically weak and negative in the poor Periphery. The paper offers explanations for the Paradox by controlling for a changing world economic environment. It shows how the oft-quoted Sachs-Warner results for 1970-1989 are significantly revised when one controls for trading partners' growth, trading partners' tariffs and the effective distance between them over the longer half-century 1950-1997. Falling partners' tariffs was the most important force accounting for the switch in sign on the tariff-growth connection after 1950. An increase in own tariffs after 1950 hurt growth, but it would not have hurt growth in a world where partners' tariffs were much higher, trading partners' growth much slower, and the world less closely connected by transportation. World environment matters. Leader-country reaction to big world events (like the Great Depression) matter. Followers take notice.

Evaluation of true interlamellar spacing from microstructural observations

A method for evaluating true interlamellar spacing from micrographs is proposed for a multidomained lamellar structure. The microstructure of these materials is assumed to be composed of many domains with the lamellae aligned roughly parallel to each other within each domain and with the domains themselves randomly oriented relative to one another. An explicit expression for the distribution of apparent interlamellar spacing is derived assuming that the distribution of the true interlamellar spacing is Gaussian. The average interlamellar spacing is close to the peak interlamellar spacing observed in the distribution. The theoretical distributions are compared with experimental ones obtained by analyzing micrographs of PbTe–Sb2Te3 lamellar composites

Where did British Foreign Capital Go? Fundamentals, Failures and the Lucas Paradox: 1870-1913

A decade has passed since Robert Lucas asked why capital does not flow from rich to poor countries. Lucas used a contemporary example to illustrate his Paradox, the very modest flow of capital from the United States to India during the second great global capital market boom, after 1970. Had he paid more attention to the first great global capital market boom, after 1870, he might have been less surprised. Very little of British capital exports went to poor, labor-abundant countries. Indeed, about two-thirds of it went to the labor-scarce New World where only a tenth of the world's population lived, and only about a quarter of it went to labor-abundant Asia and Africa where almost two-thirds of the world's population lived. Why? Was it caused by some international market failure, or was it due to some shortfall in underlying economic, demographic or geographic fundamentals that made capital's productivity low in poor countries? This paper constructs a panel data set for 34 countries who as a group got 92 percent of British capital, and uses it to conclude that international capital market failure (including whether the country was on or off the Gold Standard) was not involved. It then ranks the three big fundamentals that mattered schooling, natural resources and demography.

Closed Jaguar, Open Dragon: Comparing Tariffs in Latin America and Asia before World War II

Despite an enormous literature that has analyzed the comparative experiences of Latin America and Asia in post-World War II trade policy, almost no attention has been paid to the comparative experience prior to the wars. Even a cursory look at the best available empirical evidence reveals tremendous contrasts between the two regions. Latin America had the highest tariff barriers on earth before 1914; Asia had the lowest. Protected Latin America's belle ‚poque also boasted some of the most explosive growth performance on earth, while Asia registered some of the worst. What brought the two regions to the opposite ends of the tariff policy spectrum? And why are these quantum differences in economic performance so at odds with postwar conventional wisdom? We begin by describing a novel tariff database we have constructed from largely original sources. We explore the impact of colonial rule and unequal treaties' on Asian tariffs, as well as the impact of geography and political economy on Latin American tariffs. Limits to tariff policy autonomy explain one third of the vast difference between the two regions' tariffs before 1914; differences in the extent and structure of internal markets as well as the world tariff environment explain much of the rest. We conclude with an agenda for the future.