Longitudinal excitations in quantum antiferromagnets

By extending our recently proposed magnon-density-waves to low dimensions, we investigate, using a microscopic many-body approach, the longitudinal excitations of the quasi-one-dimensional (quasi-1d) and quasi-2d Heisenberg antiferromagnetic systems on a bipartite lattice with a general spin quantum number. We obtain the full energy spectrum of the longitudinal mode as a function of the coupling constants in the original lattice Hamiltonian and find that it always has a non-zero energy gap if the ground state has a long-range order and becomes gapless for the pure isotropic 1d model. The numerical value of the minimum gap in our approximation agrees with that of a longitudinal mode observed in the quasi-1d antiferromagnetic compound KCuF${}_3$ at low temperature. It will be interesting to compare values of the energy spectrum at other momenta if their experimental results are available.Comment: 19 pages, 4 figure

Financial market integration and the value of global diversification: evidence from US acquirers in cross-border mergers and acquisitions

Using theories of internal capital markets, this paper examines the link between financial market integration and the value of global diversification. Based on a sample of 1,491 completed cross-border mergers and acquisitions (M&As) conducted by US acquirers during the 1990–2003 period, we find that, in general, US shareholders gain significant positive abnormal returns following the announcement of the merger/acquisition. Specifically, firms that acquire/merge with targets from countries with financially segmented markets experience significantly higher positive abnormal returns than those that acquire/merge with targets from countries with financially integrated capital markets. We find that the significantly higher positive returns are driven particularly by deals between firms from unrelated industries. These firms with higher announcement returns are also characterized by positive and significant post-merger operating performance. This finding is consistent with our event study results and suggests that the overall improvement in the merged firms’ performance is likely due to the influx of internal capital from wholly integrated acquirers to segmented targets, firms that, on average are usually faced with higher capital constraints.financial market integration; global diversification; internal capital markets; mergers; acquisitions

Political connections and the process of going public: evidence from China

We examine how political connections impact the process of going public. Specifically, we test how political connections impact the pricing of newly offered shares, the magnitude of underpricing, and the fixed cost of going public. Based on experiences of the new public firms in the Chinese security markets and using multiple measures of political connections, we find robust evidence that issuing firms with political connections reap significant preferential benefits from going public. To be specific, we find that firms – irrespective of ownership arrangements – with greater political connections have higher offering prices, less underpricing, and lower fixed costs during the going-public process.political connections; IPO; emerging markets

The signalling hypothesis revisited: Evidence from foreign IPOs

While the signalling hypothesis has played a prominent role as the economic rationale associated with the initial public offering (IPO) underpricing puzzle (Welch, 1989), the empirical evidence on it has been mixed at best (Jegadeesh, Weinstein and Welch, 1993; Michaely and Shaw, 1994). This paper revisits the issue from the vantage point of close to two decades of additional experience by examining a sample of foreign IPOs – firms from both financially integrated and segmented markets – in US markets. The evidence indicates that signalling does matter in determining IPO underpricing, especially for firms domiciled in countries with segmented markets, which as a result face higher information asymmetry and lack access to external capital markets. We find a significant positive and robust relationship between the degree of IPO underpricing and segmented-market firms’ seasoned equity offering activities. For firms from integrated markets, in contrast, the analyst-coverage-purchase hypothesis appears to matter more in explaining IPO underpricing and the aftermarket price appreciation explains these firms’ seasoned equity offering activities. The evidence, therefore, clearly supports the notion that some firms are willing to leave money on the table voluntarily to get a more favorable price at seasoned offerings when they are substantially wealth constrained, a prediction embedded in the signalling hypothesis.IPO underpricing; seasoned equity offering; cross-listing; signalling hypothesis; financial market integration; market-feedback hypothesis

Electro-diffusion in a plasma with two ion species

Electric field is a thermodynamic force that can drive collisional inter-ion-species transport in a multicomponent plasma. In an inertial confinement fusion (ICF) capsule, such transport causes fuel ion separation even with a target initially prepared to have equal number densities for the two fuel ion species. Unlike the baro-diffusion driven by ion pressure gradient and the thermo-diffusion driven by ion and electron temperature gradients, electro-diffusion has a critical dependence on the charge-to-mass ratio of the ion species. Specifically, it is shown here that electro-diffusion vanishes if the ion species have the same charge-to-mass ratio. An explicit expression for the electro-diffusion ratio is obtained and used to investigate the relative importance of electro- and baro-diffusion mechanisms. In particular, it is found that electro-diffusion reinforces baro-diffusion in the deuterium and tritium mix, but tends to cancel it in the deuterium and helium-3 mix.Comment: Submitted to Phys. Plasmas on 2012-03-06 (revised version 05/13/2012

Bet hedging in the underworld

Under starvation conditions, the soil bacterium Sinorhizobium meliloti divides into two types of daughter cell: one suited to short-term and the other to long-term starvation

Characterizing mixed mode oscillations shaped by noise and bifurcation structure

Many neuronal systems and models display a certain class of mixed mode oscillations (MMOs) consisting of periods of small amplitude oscillations interspersed with spikes. Various models with different underlying mechanisms have been proposed to generate this type of behavior. Stochastic versions of these models can produce similarly looking time series, often with noise-driven mechanisms different from those of the deterministic models. We present a suite of measures which, when applied to the time series, serves to distinguish models and classify routes to producing MMOs, such as noise-induced oscillations or delay bifurcation. By focusing on the subthreshold oscillations, we analyze the interspike interval density, trends in the amplitude and a coherence measure. We develop these measures on a biophysical model for stellate cells and a phenomenological FitzHugh-Nagumo-type model and apply them on related models. The analysis highlights the influence of model parameters and reset and return mechanisms in the context of a novel approach using noise level to distinguish model types and MMO mechanisms. Ultimately, we indicate how the suite of measures can be applied to experimental time series to reveal the underlying dynamical structure, while exploiting either the intrinsic noise of the system or tunable extrinsic noise.Comment: 22 page

Representation Class and Geometrical Invariants of Quantum States under Local Unitary Transformations

We investigate the equivalence of bipartite quantum mixed states under local unitary transformations by introducing representation classes from a geometrical approach. It is shown that two bipartite mixed states are equivalent under local unitary transformations if and only if they have the same representation class. Detailed examples are given on calculating representation classes.Comment: 11 page