3,583 research outputs found
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 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 1990β2003 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 firmsβ 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 β irrespective of ownership arrangements β 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 β firms from both financially integrated and segmented markets β 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 firmsβ 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 firmsβ 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
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